Biomimetic Liquid-Repellent Surfaces by Ultrafast Laser Processing

Elena Fadeeva; Boris Chichkov

doi:10.3390/app8091424

Fadeeva, Elena;Chichkov, Boris

2018-08-21 00:00:00

applied sciences Review Biomimetic Liquid-Repellent Surfaces by Ultrafast Laser Processing Elena Fadeeva * and Boris Chichkov Institute of Quantum Optics, Leibniz University of Hanover, Welfengarten 1, D-30167 Hanover, Germany; chichkov@iqo.uni-hannover.de * Correspondence: fadeeva@iqo.uni-hannover.de; Tel.: +49-511-532-1344 Received: 20 July 2018; Accepted: 14 August 2018; Published: 21 August 2018 Abstract: This review is focused on the realization of liquid-repellent surfaces, inspired by two biological models: “dry” superhydrophobic leaves and “slippery” liquid-repellent carnivorous plants using ultrafast laser processing. After a short introduction to a biomimetic development process, an overview of the laser-fabricated structures, which were intensively used for the realization of biomimetic “dry” and “slippery” liquid-repellent surfaces, is given. The inﬂuence of process parameters on the structure morphology is discussed. A summary of superhydrophobic and liquid-repellent modiﬁcations of different materials (metals, semiconductors, and polymers), including wettability characteristics and processing details, is provided. The technological applications of laser-structured liquid-repellent surfaces are discussed. Keywords: ultrafast laser processing; biomimetics; liquid-repellent surfaces 1. Introduction This article gives an overview of the biomimetic liquid-repellent surfaces produced by ultrafast laser radiation. During the last few decades, both of these ﬁelds: biomimetics and ultrafast laser technologies have been developed very intensively. The exponential growth of biomimetics is reﬂected in a steadily rising number of scientiﬁc publications in this ﬁeld, from tens of papers in the mid-1990s to nearly 2500 papers per year as of 2017 [Web of Science]. As an introduction to this topic, the following books can be named [1,2]. The subject area of biomimetics doubles every 2–3 years, exceeding the modest expansion of about 6% per year for science in general [3]. Progress in technological transfer of biomimetic structures can be clearly seen in the rapid growth in the number of patents granted in this ﬁeld [4,5]. However, the term “biomimetics” is not always used unambiguously and correctly in numerous scientiﬁc publications. Therefore, ﬁrstly a brief remark on terminology in biomimetics is given. Originally, the word “biomimetics” was introduced by an American inventor, engineer, and biophysicist Otto Schmitt in the 1950s. He made a distinction between physics and engineering approaches to the biological sciences, which was deﬁned as “biophysics”, and a biological approach to engineering, which was deﬁned as “biomimetics.” Recently, the ISO/TC 266 Biomimetics committee has given the following deﬁnition for the term Biomimetics: “Interdisciplinary cooperation of biology and technology or other ﬁelds of innovation with the goal of solving practical problems through the function analysis of biological systems, their abstraction into models and the transfer into and application of these models to the solution” [6]. Based on the guidelines of the German Association of Engineers (Verband Deutscher Ingenieure, VDI) [7], a technical development can be referred to as biomimetic if it fulﬁlls the following criteria: Appl. Sci. 2018, 8, 1424; doi:10.3390/app8091424 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1424 2 of 22 1. There is a biological model (which is linked to a technical question). Appl. Sci. 2018, 8, x FOR PEER REVIEW 2 of 22 2. The properties of the biological model are analyzed and abstracted. (Analysis means a systematic examination of the system by its decomposition into elementary components using a suitable 1. There is a biological model (which is linked to a technical question). method following their subsequent evaluation and reorganization. Abstraction can be understood 2. The properties of the biological model are analyzed and abstracted. (Analysis means a systematic as an inductive process in which a general conclusion is made in the form of physical relationships examination of the system by its decomposition into elementary components using a suitable based on the observation of the biological model.) method following their subsequent evaluation and reorganization. Abstraction can be understood as an inductive process in which a general conclusion is made in the form of physical 3. Abstracted biological results are transferred to a technical application. relationships based on the observation of the biological model.) In this context, both biological species with particular properties (materials, structures, functions, 3. Abstracted biological results are transferred to a technical application. processes) and the process of evolution itself could be considered as biological models. There are In this context, both biological species with particular properties (materials, structures, functions, many reasons to use ideas “made by nature” for technical applications. Nature has already invested processes) and the process of evolution itself could be considered as biological models. There are 3.5 billion years of evolution in the development of highly optimized biological models. The result many reasons to use ideas “made by nature” for technical applications. Nature has already invested of this development is some 10 million living prototypes (i.e., the estimated number of existing plants 3.5 billion years of evolution in the development of highly optimized biological models. The result of this development is some 10 million living prototypes (i.e., the estimated number of existing plants and animals) [8]. Last but not least, the same physical laws and constants are valid in biology and and animals) [8]. Last but not least, the same physical laws and constants are valid in biology and in in technology. technology. The essence of biomimetics is interdisciplinarity and transdisciplinarity. It requires a high level The essence of biomimetics is interdisciplinarity and transdisciplinarity. It requires a high level of cooperation between experts from different ﬁelds of research such as biology, chemistry, physics, of cooperation between experts from different fields of research such as biology, chemistry, physics, and engineering, leading to innovative, cross-disciplinary methods and tools. The typical methodology and engineering, leading to innovative, cross-disciplinary methods and tools. The typical methodology used in biomimetics is presented in Figure 1 [7]. used in biomimetics is presented in Figure 1 [7]. Figure 1. Simplified flow chart of a biomimetic development process, according to [7] (Copyright: Figure 1. Simpliﬁed ﬂow chart of a biomimetic development process, according to [7] (Copyright: Fraunhofer UMSICHT/BIOKON). Fraunhofer UMSICHT/BIOKON). The starting point for new development in biomimetics can be either initiated from basic The biolog starting ical respoint earch or for fro new m a spec development ific technica in biomimetics l problem. The f can be irseither t approach initiated is refe from rred t basic o as t biological he biology push bottom-up process. The second one is referred to as the technology pull top-down research or from a speciﬁc technical problem. The ﬁrst approach is referred to as the biology push process. bottom-up process. The second one is referred to as the technology pull top-down process. As a next stage, the biological model is analyzed using methods from the natural sciences as well As a next stage, the biological model is analyzed using methods from the natural sciences as well as engineering sciences and abstracted. The phases between planning and invention involve typical as engineering sciences and abstracted. The phases between planning and invention involve typical steps of product or process development. The result of biomimetic process is an invention, which can steps of product or process development. The result of biomimetic process is an invention, which can be a new product or process. The flow chart in Figure 1 represents an ideal case with a linear and be a new product or process. The ﬂow chart in Figure 1 represents an ideal case with a linear and sequential procedure [7]. sequential procedure [7]. On the way from a new idea to an invention, a stage of prototype construction or manufacturing On requ tire hesway a sufr itable t om ae new chnoidea logy t to hat an en invention, ables the re a al stage ization of of t prototype he biolog constr icallyuction based or solmanufacturing ution in a technical way. At this point, laser-based manufacturing methods can meaningfully contribute to the requires a suitable technology that enables the realization of the biologically based solution in a realization of novel biomimetic prototypes. As will be discussed in this review, particularly in the technical way. At this point, laser-based manufacturing methods can meaningfully contribute to the realization of novel biomimetic prototypes. As will be discussed in this review, particularly in the Appl. Sci. 2018, 8, 1424 3 of 22 development of functional liquid-repellent biomimetic surfaces, where speciﬁc surface structure at the micro- and nanoscale is one of the key components providing the desired function, we can beneﬁt Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 22 greatly from ultrafast laser micromachining. Ultrafast means picosecond and femtosecond lasers with a pulse width from several tens development of functional liquid-repellent biomimetic surfaces, where specific surface structure at of femtoseconds to tens of picoseconds, where a pulse width shorter than picoseconds is usually the micro- and nanoscale is one of the key components providing the desired function, we can benefit utilized for fundamental research, while longer pulses are used for industrial applications due to greatly from ultrafast laser micromachining. their higher Ultrafast means picose output power andcon better d and r femtos eliability econd lase [9]. Historically rs with a p , u the lse era width from of machining several tens o by ultrafast f femtoseconds to tens of picoseconds, where a pulse width shorter than picoseconds is usually utilized laser radiation began to develop intensively after the invention of the post-ampliﬁed solid state for fundamental research, while longer pulses are used for industrial applications due to their higher Ti:Sapphire femtosecond laser. Since that time, the number of publications on ultrafast laser processing output power and better reliability [9]. Historically, the era of machining by ultrafast laser radiation has progressively increased, reaching almost half of the papers presented at major specialized began to develop intensively after the invention of the post-amplified solid state Ti:Sapphire conference on laser technologies, including the SPIE LAMOM (Lasers Applications in Microelectronic femtosecond laser. Since that time, the number of publications on ultrafast laser processing has and Optoelectronic Manufacturing), the Conference on Laser Ablation (COLA), the International progressively increased, reaching almost half of the papers presented at major specialized conference Symposium on Laser Precision Microfabrication (LPM), and the European Conference on Lasers on laser technologies, including the SPIE LAMOM (Lasers Applications in Microelectronic and and Electro-Optics (CLEO Europe) [9]. Compared to conventional processing using continuous or Optoelectronic Manufacturing), the Conference on Laser Ablation (COLA), the International nanosecond-pulsed lasers, ultrafast laser pulses provide the advantages of precise energy deposition Symposium on Laser Precision Microfabrication (LPM), and the European Conference on Lasers and into the Electro-Optic processed s (CLEO region Europe) and rapid [9]. Co material mpared removal to conventional with theprocessin suppressed g using contin heat diffusi uous or on to the nanosecond-pulsed lasers, ultrafast laser pulses provide the advantages of precise energy deposition surrounding area, resulting in a signiﬁcantly smaller heat-affected zone and, therefore, better precision into the processed region and rapid material removal with the suppressed heat diffusion to the and a higher resolution of the generated structures. surrounding area, resulting in a significantly smaller heat-affected zone and, therefore, better Ultrafast laser processing offers many benefits in comparison with other structuring techniques used precision and a higher resolution of the generated structures. for fabrication surface structures, such as photolithography, electron or ion beam, and mechanical methods: Ultrafast laser processing offers many benefits in comparison with other structuring techniques used for fabrication surface structures, such as photolithography, electron or ion beam, and This technique is applicable to almost all solid materials, irrespective of whether they are opaque mechanical methods: or transparent, brittle or hard, heat-sensitive or magnetic; • This technique is applicable to almost all solid materials, irrespective of whether they are opaque It enables ﬂexible and controllable structuring of 3D objects with complex geometries; or transparent, brittle or hard, heat-sensitive or magnetic; Machining is contact-free and can be realized in different environments, such as normal • It enables flexible and controllable structuring of 3D objects with complex geometries; atmospheric conditions, in a vacuum, in different gases or liquids; • Machining is contact-free and can be realized in different environments, such as normal There is no need for special expensive facilities; atmospheric conditions, in a vacuum, in different gases or liquids; It enables the fabrication of a large variety of surface structures. • There is no need for special expensive facilities; • It enables the fabrication of a large variety of surface structures. Physical processes underlying the interaction between ultrafast laser radiation and different types Physical processes underlying the interaction between ultrafast laser radiation and different of materials have been described in several key publications in recent years [10–15]. Surface structures types of materials have been described in several key publications in recent years [10–15]. Surface generated with ultrafast laser radiation can be roughly divided into two main groups: laser-inscribed structures generated with ultrafast laser radiation can be roughly divided into two main groups: and laser-irradiated structures (Figure 2) [16]. The structures that were intensively used for the laser-inscribed and laser-irradiated structures (Figure 2) [16]. The structures that were intensively fabrication of biomimetic liquid-repellent surfaces are marked in gray. used for the fabrication of biomimetic liquid-repellent surfaces are marked in gray. Figure 2. Classification of laser-patterned surface structures; structures that were intensively used for Figure 2. Classiﬁcation of laser-patterned surface structures; structures that were intensively used for the fabrication of biomimetic liquid-repellent surfaces are marked in gray, modified according to [16]. the fabrication of biomimetic liquid-repellent surfaces are marked in gray, modiﬁed according to [16]. Appl. Sci. 2018, 8, 1424 4 of 22 Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 22 As will be seen below, through the fabrication of laser-inscribed structures a nearly any structure As will be seen below, through the fabrication of laser-inscribed structures a nearly any structure can be realized (see Section 2.1). The structural characteristics of periodic nanostructure (spatial period) can be realized (see Section 2.1). The structural characteristics of periodic nanostructure (spatial and columnar structure (average column height and average distance between the neighboring period) and columnar structure (average column height and average distance between the neighboring columns) can be controlled by the adjustment of process parameters (see Sections 2.2.1 and 2.2.2). columns) can be controlled by the adjustment of process parameters (see Sections 2.2.1 and 2.2.2). Moreover, the unique hierarchical morphology of columnar structures produced by ultrafast laser Moreover, the unique hierarchical morphology of columnar structures produced by ultrafast laser processing resembles the natural surfaces of the biological model (superhydrophobic plants). processing resembles the natural surfaces of the biological model (superhydrophobic plants). 2. 2. Ov Overview erview of of the the Str Structures uctures th that at W Were ere Intensively Intensively Used for the Used for the Fabrication Fabrication of Bi of Biomimetic omimetic Liquid-Repellent Surfaces Liquid-Repellent Surfaces 2.1. Laser-Inscribed Structures 2.1. Laser-Inscribed Structures Laser-inscribed structures are also referred to as deterministic or geometrically deﬁned. In this Laser-inscribed structures are also referred to as deterministic or geometrically defined. In this case the resulting ablation geometry is determined by the intensity distribution of laser radiation case the resulting ablation geometry is determined by the intensity distribution of laser radiation on on the target surface, taking into account the speciﬁc ablation threshold. Ultrafast laser ablation the target surface, taking into account the specific ablation threshold. Ultrafast laser ablation enables enables the realization of nearly any user-deﬁned structure shapes with resolution down to 1 m, the realization of nearly any user-defined structure shapes with resolution down to 1 µm, independent of material properties. An example of a laser-inscribed structure (15 m wide grooves) is independent of material properties. An example of a laser-inscribed structure (15 µm wide grooves) shown in Figure 3 (left). It is known that with ultrafast pulses the diffraction limit can be overcome by is shown in Figure 3 (left). It is known that with ultrafast pulses the diffraction limit can be overcome choosing a peak laser ﬂuence slightly above the ablation threshold. In that case, only the central part by choosing a peak laser fluence slightly above the ablation threshold. In that case, only the central of the beam, which is characterized by Gaussian intensity distribution, can ablate the material, and it part of the beam, which is characterized by Gaussian intensity distribution, can ablate the material, becomes possible to produce sub-diffraction structures with sub-micrometer resolution (see Figure 3, and it becomes possible to produce sub-diffraction structures with sub-micrometer resolution (see right) [17]. Figure 3, right) [17]. Figure 3. Laser-inscribed structures by ultrafast laser processing: confocal laser microscope image of Figure 3. Laser-inscribed structures by ultrafast laser processing: confocal laser microscope image grooves on titanium (left); scanning electron microscope (SEM)-picture of nano-holes on a sapphire of grooves on titanium (left); scanning electron microscope (SEM)-picture of nano-holes on a sapphire crystal (right) (reprinted by permission from Springer: [17], copyright 2003). crystal (right) (reprinted by permission from Springer: [17], copyright 2003). 2.2. Laser-Irradiated Structures 2.2. Laser-Irradiated Structures Laser-irradiated structures represent both random and periodic nanostructures, as well as Laser-irradiated structures represent both random and periodic nanostructures, as well as various various microstructures. The size of the single structures in this case is considerably smaller than the microstructures. The size of the single structures in this case is considerably smaller than the laser spot laser spot width (see Figure 4). width (see Figure 4). Remarkable overviews on laser-irradiated structure type can be found, for example, in [16,18–20]. Remarkable overviews on laser-irradiated structure type can be found, for example, in [16,18–20]. Periodic nanostructures and columnar microstructures, which have predominantly been used for the Periodic nanostructures and columnar microstructures, which have predominantly been used for fabrication of biomimetic liquid-repellent functional surfaces in the past, will be discussed in detail the fabrication of biomimetic liquid-repellent functional surfaces in the past, will be discussed in below. detail below. Appl. Sci. 2018, 8, 1424 5 of 22 Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 22 Figure 4. Titanium surfaces after irradiation at 2.5 J/cm² with 100 laser pulses (stationary conditions): Figure 4. Titanium surfaces after irradiation at 2.5 J/cm with 100 laser pulses (stationary conditions): columnar structure (in the center of the ablation crater), periodic nanostructure (in the outer region of columnar structure (in the center of the ablation crater), periodic nanostructure (in the outer region of the ablation crater). the ablation crater). 2.2.1. Periodic Nanostructures 2.2.1. Periodic Nanostructures Laser-induced periodic surface structures (LIPSS, ripples) can be generated on different Laser-induced periodic surface structures (LIPSS, ripples) can be generated on different materials materials (metals, semiconductors, and dielectrics) by linearly polarized laser radiation at fluences (metals, semiconductors, and dielectrics) by linearly polarized laser radiation at ﬂuences slightly slightly above the ablation threshold. By ultrafast laser radiation two different types of LIPSS can be above the ablation threshold. By ultrafast laser radiation two different types of LIPSS can be fabricated: low-spatial-frequency LIPPS (LSFL) and high-spatial-frequency LIPSS (HSFL). LSFL on fabricated: low-spatial-frequency LIPPS (LSFL) and high-spatial-frequency LIPSS (HSFL). LSFL on strong absorbing materials, such as metals and semiconductors are characterized by a spatial period strong absorbing materials, such as metals and semiconductors are characterized by a spatial period close to or less than the wavelength of applied laser radiation and an orientation perpendicular to the close to or less than the wavelength of applied laser radiation and an orientation perpendicular to the polarization of laser beam. The formation of LSFL is explained by interference of the incident laser polarization of laser beam. The formation of LSFL is explained by interference of the incident laser beam with surface electromagnetic waves generated at the surface roughness including excitation of beam with surface electromagnetic waves generated at the surface roughness including excitation surface plasmon polaritons. LSFL on dielectrics can be generated with orientation perpendicular or of surface plasmon polaritons. LSFL on dielectrics can be generated with orientation perpendicular (for dielectrics with large band gap) parallel to the laser beam polarization. In case of dielectrics, the or (for dielectrics with large band gap) parallel to the laser beam polarization. In case of dielectrics, absorption of high intensity ultrafast laser radiation leads to a metal-like behavior of ionized material. the absorption of high intensity ultrafast laser radiation leads to a metal-like behavior of ionized Therefore, the ripple formation on dielectrics can be explained by analogy to metals. More detailed material. Therefore, the ripple formation on dielectrics can be explained by analogy to metals. information can be found, for example, in [21]. LSFL on dielectrics are characterized by spatial More detailed information can be found, for example, in [21]. LSFL on dielectrics are characterized by periods near the wavelength of laser radiation (λ) or nearly λ/n, where n is the refractive index of the spatial periods near the wavelength of laser radiation () or nearly /n, where n is the refractive index dielectric material. Detailed pictures of LSFL made by scanning electron microscopy show that LSFL of the dielectric material. Detailed pictures of LSFL made by scanning electron microscopy show that generated by ultrafast laser radiation are densely covered by nanostructures [22,23], in contrast to LSFL generated by ultrafast laser radiation are densely covered by nanostructures [22,23], in contrast smooth LIPSS produced by long-pulsed lasers (Figure 5). to smooth LIPSS produced by long-pulsed lasers (Figure 5). Appl. Sci. 2018, 8, 1424 6 of 22 Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 22 Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 22 Figure 5. Detailed pictures of LIPPS on shows that LSFL generated by ultrafast laser radiation are Figure 5. Detailed pictures of LIPPS on shows that LSFL generated by ultrafast laser radiation are densely covered by nanostructures (reprinted from [22] by permission from Springer, copyright 2007). densely covered by nanostructures (reprinted from [22] by permission from Springer, copyright 2007). Figure 5. Detailed pictures of LIPPS on shows that LSFL generated by ultrafast laser radiation are High-spatial-frequency LIPSS (HSFL) have a significantly shorter spatial period compared to the densely covered by nanostructures (reprinted from [22] by permission from Springer, copyright 2007). High-spatial-frequency LIPSS (HSFL) have a signiﬁcantly shorter spatial period compared to irradiation wavelength (<λ/2), and orientation often perpendicular but occasionally parallel to the thela irradiation ser be High am p -spa wavelength otial-frequenc larization. T (< h ye form LI /2), PSS and at (HSF ion orient L) ha of Hation S ve FL a si isoften gnif stilli unde cperpendicular antly shorter r discussi spa on but tand iaoccasionally l peri explain od compa ed by parallel r d ed to ifferthe to ent the laser irr mechanisms such as seco abeam diation w polarization. avelength ( The <λnd-harmonic /2formation ), and orie n of generation, se tat HSFL ion oft isestill n plef-or under rpendicu ganiz disc at laion, and int r but occa ussion and sieona ract explained llion y pa w ra itll by hel to the specif differ ic ent laser beam polarization. The formation of HSFL is still under discussion and explained by different types of plasmon modes. For the practical application of LIPPS in biomimetics, it is important to mechanisms such as second-harmonic generation, self-organization, and interaction with speciﬁc types mechanisms such as second-harmonic generation, self-organization, and interaction with specific control their structural properties (spatial period). It can be realized by variation of laser wavelength of plasmon modes. For the practical application of LIPPS in biomimetics, it is important to control types of pla (smaller wav smon modes. elength leadsFor the pra to a smallec r tispat cal iaapplica l perio tio d) [ n 2 of LIPP 4], the S i ann gle o biomimet f incideic nce (by ch s, it is import anging t ant h to e their structural properties (spatial period). It can be realized by variation of laser wavelength (smaller control their structural pro angle of incidence from 0° t po erties 80° sp (spa atia tia l p l period). It ca eriod could bn be rea e controll liz ab ed by variation ly increased, fo of laser w r exampla e f velength rom 0.6 wavelength leads to a smaller spatial period) [24], the angle of incidence (by changing the angle of (sma to 3.7 ll µm er wav for e pl lengt atinum h lead tsa trg o a sma et [25]), ll an er spat d proces ial per sing iod e ) [n 2vironment 4], the angle o (processin f incidegnce (by ch in liquidan leging t ads toh e a incidence from 0 to 80 spatial period could be controllably increased, for example from 0.6 to 3.7 m angle of incidence from 0° to 80° spatial period could be controllably increased, for example from 0.6 decrease in spatial period compared to processing under normal ambient conditions [26]). Detailed for platinum target [25]), and processing environment (processing in liquid leads to a decrease in to 3.7 µm for platinum target [25]), and processing environment (processing in liquid leads to a information can be found in the following key publications: [19,27]. spatial period compared to processing under normal ambient conditions [26]). Detailed information decrease in spatial period compared to processing under normal ambient conditions [26]). Detailed can be found in the following key publications: [19,27]. 2.2.2. Columnar Microstructures information can be found in the following key publications: [19,27]. 2.2.2. Columnar Microstructures Columnar microstructures—conical, quasi-ordered micrometer-sized structures—were reported 2.2.2. Columnar Microstructures to be generated on semiconductors as well as on metals. These structures are also referred to in the Columnar microstructures—conical, quasi-ordered micrometer-sized structures—were reported literat Columnar microstruct ure as pillars, bumpy, u and res—conical, spikes. Micro quasi-or -sized dered micrometer-sized struc columns can be specified by ture ths e —were re average cporte olumn d to be generated on semiconductors as well as on metals. These structures are also referred to in to be generated on semiconductors as well as on metals. These structures are also referred to in the height and the average distance between the neighboring columns, or by the density of the columns. the literature as pillars, bumpy, and spikes. Micro-sized columns can be speciﬁed by the average literature as pillars, bumpy, and spikes. Micro-sized columns can be specified by the average column The effects of different processing parameters on the morphology of columnar structures were column height and the average distance between the neighboring columns, or by the density of the height studied in and de the ave tail in t rage d he pas istance between t (see the refe the ne rences ighbor summ ing co arized lumn in Tab s, or by the le 1). It is den imsity o portant f the to columns. note that columns. The effects of different processing parameters on the morphology of columnar structures The effect the fabricat s o iof n of differ laser- entirr processin adiated co g par luma nmet ar micro ers on t structure does not ha he morphology of co ve alum univ nar st ersalruct char ure actse were r and were studied in detail in the past (see the references summarized in Table 1). It is important to note st cannot be fab udied in det ra icated il in t, for he pas example, on cop t (see the refep rences er [28]. F summ igure 6 show arized in Tab s ablation cr le 1). It is aters import on steel ant t and c o not o epper, that that the fabrication of laser-irradiated columnar microstructure does not have a universal character tgenera he fabric ted at by 1 ion of 00 l la aser ser-pu irrlses adiat (30 ed f co s) lum at 10 n J ar /cm². micro structure does not have a universal character and and cannot be fabricated, for example, on copper [28]. Figure 6 shows ablation craters on steel and cannot be fabricated, for example, on copper [28]. Figure 6 shows ablation craters on steel and copper, copper, generated by 100 laser pulses (30 fs) at 10 J/cm . generated by 100 laser pulses (30 fs) at 10 J/cm². Figure 6. SEM images of ablation craters on steel (left) and copper (right), generated by 100 laser pulses (30 fs) at fluence of 10 J/cm². Figure 6. SEM images of ablation craters on steel (left) and copper (right), generated by 100 laser Figure 6. SEM images of ablation craters on steel (left) and copper (right), generated by 100 laser pulses (30 fs) at fluence of 10 J/cm². pulses (30 fs) at ﬂuence of 10 J/cm . Appl. Sci. 2018, 8, 1424 7 of 22 As can be seen in Figure 6, at the same experimental conditions for the laser ablation of copper are more efﬁcient compared to for steel. In both cases a speciﬁc surface structure in the middle of the ablation crater can be observed, which is formed from melted and re-solidiﬁed material. Therefore, the height of the individual structures is in the range of 20 to 30 m for steel and in the range of 2 to 3 m for copper. This difference can be explained by the different physical characteristics of these materials and their melts. However, columns on copper, if required, can be produced only as laser-inscribed structures, as was reported for the fabrication of biomimetics liquid-repellent surfaces, for example, in [29]. As distinguished from smooth surfaces of columnar structures produced with long laser pulses or under stationary conditions (without sample translation with respect to the laser beam) with ultrafast laser radiation, columns produced by non-stationary ultrafast laser radiation are superimposed with LIPSS or nanoroughness [30]. The nanostructures develop on the microcolumn surfaces due to the irradiation at ﬂuences near ablation threshold at the trailing edge of the moving laser beam. Due to their unique morphology, which resembles the naturally superhydrophobic surfaces of plants, columnar structures were intensively used for the fabrication of biomimetic surfaces and have great potential for future applications. Although the arbitrary change of column morphology cannot be realized, some ﬁne tuning of their shape can be done by adjusting process parameters enabling better ﬁt to potential application. Main tendencies of inﬂuence of process parameters on morphology of columnar microstructures are summarized in Table 1. Table 1. Inﬂuence of process parameters on morphology of columnar microstructures. Process Parameters Inﬂuence on Morphology of Columnar Structures Material References Columns start to develop after irradiation with a deﬁned Silicon (Si) [31] number of laser pulses. Titanium (Ti) [16,28,32,33] Average column height and distance between neighboring Number of laser pulses columns grow with increasing number of laser pulses. Nickel (Ni) [34] After exceeding a certain number of laser pulses, the column Steel [28,35,36] morphology does not change any more. There are low and upper ﬂuence thresholds for the formation Silicon (Si) [37] of columnar structures. Aluminum (Al) [28] At laser ﬂuences below the low threshold ﬂuence, Titanium (Ti) [30] column structures cannot be generated independent of the applied number of laser pulses. Laser ﬂuence Nickel (Ni) [34] At ﬂuences higher than the upper threshold ﬂuence, column structures can either be destroyed or different structure type starts to develop. Steel [18,28,38] Average column height and distance between neighboring columns grow with increasing ﬂuence. Linear polarization leads to elliptical shape of the Silicon (Si) [37] column bases. Polarization Circular polarization leads to circular shape of the Nickel (Ni) [39] column bases. Processing with laser pulses at shorter wavelengths leads to Silicon (Si) [37] reduction of average column height and distance between Wavelength Steel [40] neighboring columns. Processing with nanosecond laser pulses leads to the generation of columnar structures with larger column height and average distance between neighboring columns Pulse duration Silicon (Si) [41–43] compared to processing with ultrafast laser pulses. Columnar structures generated by nanosecond pulses are much smoother than those generated by ultrafast lasers. Laser processing of silicon in SF or Cl atmosphere leads to Silicon (Si) [37,44,45] 6 2 the formation of sharp columnar structures; processing of Aluminum (Al) [28] silicon in SF leads to higher density of columns. Laser processing of different materials in liquids leads to Titanium (Ti) [28,32,46] Environment much smaller column height and average distance between neighboring columns compared to processing in gasses or air. Increasing of gas pressure leads to decreasing of column Steel [28,47] height and average distance between neighboring columns in laser processing of metals. Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 22 Laser processing of silicon in SF6 or Cl2 atmosphere leads to Aluminum [28] the formation of sharp columnar structures; processing of (Al) silicon in SF6 leads to higher density of columns. Titanium [28,32,46] Laser processing of different materials in liquids leads to (Ti) much smaller column height and average distance between neighboring columns compared to processing in gasses or Appl. Sci. 2018, 8, 1424 8 of 22 air. Steel [28,47] Increasing of gas pressure leads to decreasing of column height and average distance between neighboring columns Table 1. Cont. in laser processing of metals. Process Parameters Inﬂuence on Morphology of Columnar Structures Material References Average column height and distance between neighboring Silicon (Si) [48] Average column height and distance between neighboring columns decrease by decreasing the collimated beam Silicon (Si) [48] Collimated columns decrease by decreasing the collimated beam diameter diameter at a fixed fluence. Collimated beam diameter at a ﬁxed ﬂuence. beam diameter Steel [18] Threshold fluence for the formation of columnar structures Steel [18] Threshold ﬂuence for the formation of columnar structures decreases with increasing beam diameter. decreases with increasing beam diameter. By variation of angle of incidence inclined columns can Angle of By variation of angle of incidence inclined columns can be Angle of incidence Nickel (Ni) [39] Nickel (Ni) [39] be generated. incidence generated. 3. Superhydrophobic, Self-Cleaning, and Antifouling Surfaces 3. Superhydrophobic, Self-Cleaning, and Antifouling Surfaces According to the impressive estimation from [8] superhydrophobic plant leaves (e.g., grasses) According to the impressive estimation from [8] superhydrophobic plant leaves (e.g., grasses) comprise in total an area of around 250 million km , which is about 50% of the total surface of our comprise in total an area of around 250 million km , which is about 50% of the total surface of our planet. The surfaces of many plant leaves are known to be superhydrophobic and water-repellent for planet. The surfaces of many plant leaves are known to be superhydrophobic and water-repellent for a long time. However, only since the 1970s, with the introduction of scanning electron microscope a long time. However, only since the 1970s, with the introduction of scanning electron microscope studies, has it been revealed that their unique properties are related to the speciﬁc surface structure studies, has it been revealed that their unique properties are related to the specific surface structure of the leaf surfaces. Since then, the surface structures and wetting properties of almost 20,000 of the leaf surfaces. Since then, the surface structures and wetting properties of almost 20,000 biological superhydrophobic species have been discovered and investigated [8,49–52]. One of the biological superhydrophobic species have been discovered and investigated [8,49–52]. One of the most famous biological superhydrophobic species is Nelumbo nucifera (Lotus). It was found that most famous biological superhydrophobic species is Nelumbo nucifera (Lotus). It was found that the the speciﬁc surface structure of lotus leaves is built up of microstructures (distinctively convex to specific surface structure of lotus leaves is built up of microstructures (distinctively convex to papillose micrometer-sized epidermal cells), which are covered by nanoroughness (a very dense layer papillose micrometer-sized epidermal cells), which are covered by nanoroughness (a very dense of epicuticular waxes) (Figure 7). layer of epicuticular waxes) (Figure 7). Figure 7. A flowering plant of the Lotus family (Nelumbo nucifera) (a); a spherical water droplet on a Figure 7. A ﬂowering plant of the Lotus family (Nelumbo nucifera) (a); a spherical water droplet on a superhydrophobic leaf (b); SEM image of the lotus leaf surface (c). superhydrophobic leaf (b); SEM image of the lotus leaf surface (c). Biological research initiated technical implementation of self-cleaning functional surfaces and, Biological research initiated technical implementation of self-cleaning functional surfaces and, therefore, in this case, the biomimetic development process can be clearly referred to as biology push therefore, in this case, the biomimetic development process can be clearly referred to as biology or bottom-up. Since the beginning of the 1990s, a large number of scientific papers have been push or bottom-up. Since the beginning of the 1990s, a large number of scientiﬁc papers have been dedicated to the analysis and abstraction of superhydrophobic surfaces. Below, the basics of wettability phenomena, which are required for the abstraction of superhydrophobicity, are brieﬂy summarized [Box 1]. For deeper insight, the following works are recommended: [1,53,54]. Appl. Sci. 2018, 8, 1424 9 of 22 Box 1. Basics of wettability phenomena. The wettability of the solid surface by the liquid depends on interfacial tensions , , and —solid–air sa sl la ( ), liquid–solid ( ), and solid–air ( )—and is characterized by the measurement of the contact angle. sa sl la The correlation between the contact angle and the interfacial tensions is the famous Young equation: sa sl cos = . la The Young equation describes the case of an ideal solid surface, which is assumed to be smooth, rigid, chemically homogeneous, insoluble, and non-reactive. Surfaces that are characterized by a contact angle <91 are referred to as wetting (if the liquid is water, they are called hydrophilic); those with a contact angle 91 as non-wetting (or hydrophobic in the case of water). On smooth, low-energy surfaces, a maximum water contact angle of approx. 120 can be achieved [55]. By the interaction of liquids with a structured surface, different wetting states can be realized. In the ﬁrst state, the liquid wets the structured surface completely, forming a homogeneous solid/liquid interface. In this case the contact angle of liquid on the structured surface is given by the Wenzel equation, introducing the structure parameter r (r > 1 for rough surface), deﬁned as the ratio of the structured enlarged surface area to its ﬂat projected area [56]: cos = r cos . W Y Due to the realization of this wetting state, the interface and therefore adhesion between the solid and liquid is increased compared to that on a ﬂat surface of the same material. In the second state, the liquid wets the structured surface incompletely; a composite solid/liquid–air interface is formed. In this case the contact angle of liquid on the structured surface is given by the following equation, introducing the structure parameter f (0 < f < 1, ideally, close to 0), deﬁned as the ratio of the solid surface in sl sl contact with liquid to the ﬂat surface projected area [57]: cos = f (cos + 1) 1 . CB sl V This equation is a particular case of the general Cassie–Baxter expression cos = f cos q + f cos q . CB 1 Y1 2 Y2 This expression was developed for application to a chemically heterogeneous surface consisting of two materials, each of them characterized by its own q and its own portion of the total area f ( f + f = 1). Y 1 2 The superhydrophobic stage is a special case where a chemically heterogeneous surface is formed from solid and air. Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 22 As a result, the interface between liquid droplets and solid can be strongly reduced. An important characteristic of liquid-repellent surfaces is small contact angle hysteresis (∆θ). Hysteresis is defined as the difference between the advanced and receding angles of a liquid droplet. Shortly before a droplet starts to move over the inclined solid surface, the advancing contact angle can be observed at the front droplet side and receding at the back side. It is important to note that retentive force, which describes drop retention on a tilted surface, is directly proportional to cos θ − cos θ [58]. Another parameter often used for characterization of liquid- repellency is tilt angle α (also known as the tilting or sliding angle), which is required for the initiation of droplet sliding from the surface. The droplet mobility on the surface is characterized by contact angle hysteresis and tilt angle. The low values of contact angle hysteresis and tilt angle are intrinsic characteristics of a nearly defect-free surface that enables liquid repellency. Dry superhydrophobic liquid-repelling surfaces (if the liquid is water, they are referred to as water-repelled or superhydrophobic) is a special case of realization of the state, which is described by the Cassie–Baxter equation. In the case of superhydrophobic plants, low-energy wax makes the surface hydrophobic, and the hierarchical structure of leaves makes them superhydrophobic. Water drops on superhydrophobic leaves are supported by a hierarchical structure on the composite solid– air interface. Studies with AFM and confocal light microscopy found that hierarchical roughness results in a reduction of the contact area by more than 95% compared to the projected area of a water droplet [59]. The superhydrophobic plant surfaces are characterized by a static contact angle of ≥150° and contact angle hysteresis and tilt angle ≤10° [50,51]. Inspired by biomimetic models, superhydrophobic surfaces were fabricated on different materials by ultrafast laser ablation. The first works demonstrated superhydrophobic modification of silicon using a columnar structure superimposed with LIPSS or nanoroughness in combination with low-energy surface coating [60,61], following the application of this functionalization technique to other materials. Superhydrophobic modification of metals was realized analogous to silicon solely by structuring [62–66] or in combination with a coating [38,67]. The study of Kietzig et al. (2009) [64] has shown that after some time superhydrophobicity develops on intrinsically hydrophilic structured metal surfaces due to the formation of a carbonaceous layer that alters the chemistry of the surface. The timescale of this process was shown to be dependent on both the structured material and the environmental conditions and to be in the range of hours to days [68]. In [69] the increase of hydrophobicity occurring over time on steel and aluminum after structuring with ultrafast radiation was studied as well. This study explained hydrophobicity increases on steel by removing a thin film of water that was initially present on the metal surface due to laser processing and on aluminum by Appl. Sci. 2018, 8, 1424 10 of 22 An important characteristic of liquid-repellent surfaces is small contact angle hysteresis (D). Hysteresis is deﬁned as the difference between the advanced and receding angles of a liquid droplet. Shortly before a droplet starts to move over the inclined solid surface, the advancing contact angle can be observed at the front droplet side and receding at the back side. It is important to note that retentive force, which describes drop retention on a tilted surface, is directly proportional to cos cos [58]. Another parameter often used for advancing receding characterization of liquid-repellency is tilt angle (also known as the tilting or sliding angle), which is required for the initiation of droplet sliding from the surface. The droplet mobility on the surface is characterized by contact angle hysteresis and tilt angle. The low values of contact angle hysteresis and tilt angle are intrinsic characteristics of a nearly defect-free surface that enables liquid repellency. Dry superhydrophobic liquid-repelling surfaces (if the liquid is water, they are referred to as water-repelled or superhydrophobic) is a special case of realization of the state, which is described by the Cassie–Baxter equation. In the case of superhydrophobic plants, low-energy wax makes the surface hydrophobic, and the hierarchical structure of leaves makes them superhydrophobic. Water drops on superhydrophobic leaves are supported by a hierarchical structure on the composite solid–air interface. Studies with AFM and confocal light microscopy found that hierarchical roughness results in a reduction of the contact area by more than 95% compared to the projected area of a water droplet [59]. The superhydrophobic plant surfaces are characterized by a static contact angle of 150 and contact angle hysteresis and tilt angle 10 [50,51]. Inspired by biomimetic models, superhydrophobic surfaces were fabricated on different materials by ultrafast laser ablation. The ﬁrst works demonstrated superhydrophobic modiﬁcation of silicon using a columnar structure superimposed with LIPSS or nanoroughness in combination with low-energy surface coating [60,61], following the application of this functionalization technique to other materials. Superhydrophobic modiﬁcation of metals was realized analogous to silicon solely by structuring [62–66] or in combination with a coating [38,67]. The study of Kietzig et al. (2009) [64] has shown that after some time superhydrophobicity develops on intrinsically hydrophilic structured metal surfaces due to the formation of a carbonaceous layer that alters the chemistry of the surface. The timescale of this process was shown to be dependent on both the structured material and the environmental conditions and to be in the range of hours to days [68]. In [69] the increase of hydrophobicity occurring over time on steel and aluminum after structuring with ultrafast radiation was studied as well. This study explained hydrophobicity increases on steel by removing a thin ﬁlm of water that was initially present on the metal surface due to laser processing and on aluminum by the appearance of new functional groups (especially methyl group–CH and graphitic carbon) after laser processing. Superhydrophobic modiﬁcation of glass was done through fabrication of laser-inscribed grooves superimposed with LIPSS or nanoroughness in combination with s low-energy surface coating [70,71]. Realization of superhydrophobic polymers was shown through two different approaches: fabrication of laser-irradiated roughness [72] or transfer laser-generated structures in polymer by replication or injection molding [72–74]. An interesting review of laser structuring of polymers for biomedical applications can be found in [75]. An overview of the superhydrophobic modiﬁcation of different materials by ultrafast laser processing is given in Table 2. Appl. Sci. 2018, 8, 1424 11 of 22 Table 2. Overview of superhydrophobic modiﬁcation of different materials by ultrafast laser processing. Laser System Wave Material Length/Pulse Superhydrophobic Modiﬁcation , D, Reference CB Duration/Repetition Rate columnar structure superimposed > 160 CB an ampliﬁed Ti:Sapphire laser Silicon (Si) with LIPSS or nanoroughness D < 3 [60] 800 nm, 100 fs, 1 kHz + silanization < 2 columnar structure superimposed an ampliﬁed Ti:Sapphire laser = 156 CB Silicon (Si) with LIPSS or nanoroughness [61] 800 nm, 180 fs, 1 kHz < 10 + silanization columnar structure superimposed an ampliﬁed Ti:Sapphire laser = 154 1 CB Silicon (Si) with LIPSS or nanoroughness [76] 800 nm, 180 fs, 1 kHz D = 5 2 + silanization an ampliﬁed Ti:Sapphire laser columnar structure superimposed Steel (different alloy) 800 nm, 150 fs, 1 kHz (seed: = 153 CB with LIPSS or nanoroughness [64] Titanium alloy Coherent Mira HP, ampliﬁer: D < 3 without coating Coherent Legend) columnar structure superimposed an ampliﬁed Ti:Sapphire laser = 166.3 CB Steel with LIPSS or nanoroughness [38] 800 nm, 130 fs, 1 kHz = 4.2 + silanization an ampliﬁed Ti:Sapphire laser columnar structure superimposed = 166 4 CB Titanium 800 nm, 30 fs, 1 kHz with LIPSS or nanoroughness [62] D = 10 4.5 (Femtopower Compact Pro) without coating Ti-6Al-4V: picosecond laser (Trumpf = 152 3 CB Laser-irradiated structures: LIPSS Steel Ti-6Al-4V alloy TruMicro), frequency-tripled D < 5 [67] and nanoroughness + silanization 343 nm, 6.7 ps, 400 kHz Steel: = 140 3 CB laser-inscribed structure (trench an ampliﬁed Ti:Sapphire laser Steel and matrix) superimposed with > 150 [66] CB 800 nm, 130 fs, 1 kHz LIPSS without coating picosecond laser (an Edgewave) Laser-irradiated structures: LIPSS = 153.9 3.2 CB Copper [65] 1064 nm, 10 ps, 203.6 kHz without coating = 11 3 an ampliﬁed Ti:Sapphire laser Different laser-induced structures Platinum (Pt) = 158 [63] CB 800 nm, 65 fs, 1 kHz without coating laser-inscribed structure (column) superimposed with LIPSS or an ampliﬁed Ti:Sapphire laser = 165 CB Polypropylene nanoroughness on steel were [73] (Coherent RegA) 200 fs, 250 kHz D very small transferred in polypropylene by injection molding an ampliﬁed Ti:Sapphire laser direct fabrication or replication of > 170 CB Polydimethyl-siloxane [72] 810 nm, 150 fs, 1 kHz laser-induced structure < 3 replication of the laser-inscribed picosecond laser (High-Q, structures superimposed with = 164.5 CB Polydimethyl-siloxane [74] Austria) 532 nm, 10 ps, 1 kHz LIPPS and nanoroughness from = 8.44 steel in polydimethylsiloxane laser-inscribed structure (grooves) an ampliﬁed Ti:Sapphire laser 800 = 152.3 1.5 CB K9 Glass superimposed with LIPSS or [71] nm, 130 fs, 1 kHz = 4.6 0.8 nanoroughness + silanization an ampliﬁed Ti:Sapphire laser laser-inscribed structure (grooves) Glass (Cyber Laser Inc., IFRIT-LH-C031) superimposed with LIPSS or ~152–155 [70] CB 786 nm, 183 fs, 1 kHz nanoroughness + silanization As can be seen from Table 2, hierarchical structures are preferably used for the fabrication of superhydrophobic surfaces. As was shown in [77], the occurrence of the second (nanoscale) level of topography on the microscale topography leads to increasing the contact angle ( ) and eliminating CB the contact angle hysteresis (D). As mentioned in Table 1, processing with ultrafast laser radiation enables a simpler fabrication of hierarchical structures compared to the nanosecond laser. For example, columnar structures generated by nanosecond pulses are much smoother and do not have the nanoscale level typically generated by ultrafast lasers [41–43]. An essential element of biomimetic development is transfer to a technical application. For example, ultrafast laser-processed superhydrophobic water-repellent surfaces were proposed as anti-icing coating for application in aviation [78]. In this case an industrial suited 25 W laser system operating at Appl. Sci. 2018, 8, 1424 12 of 22 400 kHz was used for structure fabrication. The surface of stainless steel underwent superhydrophobic modiﬁcation by laser-inscribed structuring (column) superimposed with LIPSS or nanoroughness and silanization. Rime testing showed that the superhydrophobic surfaces only slightly slowed down the formation of rime on the coating surface, but it was demonstrated that the adhesion of frozen droplets was dramatically reduced during a ice rain test. In [68] the structured steel was proposed for reduction of ice friction. The reduction of ice friction is relevant for many applications, starting with sports and outdoor activity equipment and ending with sleds and vehicles for transport over snow and ice. Ice friction experiments with a structured slider showed a continuous decrease in the friction coefﬁcient with increasing sliding speed in comparison to the polished slider (demonstrating initial decrease and substantial increase after passing through a minimum value). However, the main drawback of that application of superhydrophobic modiﬁcation was the poor durability of the structure under the load and frictional motion. In [29], ultrafast laser-processed hierarchical superhydrophobic structures on copper were applied for drag reduction. Tests of these surfaces showed considerably higher drag Appl. Sci. 2018, 8, x FOR PEER REVIEW 13 of 22 reduction than what was predicted from theoretical and experimental models on non-hierarchical strexperimental uctures. Even models on better no drag n-hier reduction archical struct was ures achieved . Even better dr after silanization ag reduction of was ach structur ieved ed surfaces. after silanization of structured surfaces. Another application was related to the antifouling properties of Another application was related to the antifouling properties of superhydrophobic surfaces for superhydrophobic surfaces for prevention of bacterial attachment. Nature itself evolved prevention of bacterial attachment. Nature itself evolved superhydrophobicity in plants, predominantly superhydrophobicity in plants, predominantly as a defense mechanism against the attachment of as a defense mechanism against the attachment of pathogens (e.g., fungal spores) [79]. In [62,80] pathogens (e.g., fungal spores) [79]. In [62,80] superhydrophobic titanium surfaces (columnar superhydrophobic titanium surfaces (columnar structures superimposed with nanoroughness without structures superimposed with nanoroughness without coating) were tested for their ability to retain coating) were tested for their ability to retain different bacterial strains. It was shown that all tested different bacterial strains. It was shown that all tested bacteria strains were retained by the structured bacteria strains were retained by the structured surface to different degrees. It is noteworthy that each surface to different degrees. It is noteworthy that each strain was found to preferentially attach to strain was found to preferentially attach to crevices located between the columnar surface features. crevices located between the columnar surface features. The upper regions of the microscale columns The upper regions of the microscale columns remained essentially bacteria-free. It was hypothesized remained essentially bacteria-free. It was hypothesized that air trapped by the topographical features that air trapped by the topographical features initially inhibited contact between the bacteria and the initially inhibited contact between the bacteria and the titanium substratum. Attachment of bacteria titanium substratum. Attachment of bacteria (S. aureus) was shown to increase substantially over (S. aureus) was shown to increase substantially over a period of 1 h, which corresponds to the period a period of 1 h, which corresponds to the period of maximum replacement of air trapped between of maximum replacement of air trapped between microscale columns by the incubation medium microscale columns by the incubation medium (Figure 8). (Figure 8). Figure 8. Dynamic wettability of the lotus-mimicked superhydrophobic titanium surfaces immersed Figure 8. Dynamic wettability of the lotus-mimicked superhydrophobic titanium surfaces immersed in water. Air replacement by water (top); graph (bottom) showing the percentage of air trapped in in water. Air replacement by water (top); graph (bottom) showing the percentage of air trapped superhydrophobic titanium surfaces after 1.5-h immersion in water. (Reprinted with permission from in superhydrophobic titanium surfaces after 1.5-h immersion in water. (Reprinted with permission [62]. Copyright 2011 American Chemical Society.) from [62]. Copyright 2011 American Chemical Society.) Replacement of air increases the surface area available for cells to make contact with the Ti surface. Notably, the quantification of air trapped by in situ small-angle X-ray scattering showed that nano-sized air bubbles trapped within nanoroughness still covered 45% of the titanium surface, even after immersion for 50 min [80]. The replacement of the entrapped air by incubation medium described above is an example of wetting transition. Generally, transition from a superhydrophobic state, described by the Cassie–Baxter equation, to a Wenzel state or a mixed Cassie–Baxter/Wenzel state is accompanied by a loosening of antifouling properties. These transitions were observed under evaporation, pressing, vibration, and bouncing of droplets [81] and are the reason, for example, for failing the rime test in [78]. The same structured surface can demonstrate excellent superhydrophobic properties under certain conditions while failing in other environments. Therefore, careful planning of a technical application of superhydrophobic surfaces, taking into consideration all operation conditions, is vital. Particularly for biomedical applications, a comprehensive overview of utilization of superhydrophobic surfaces for control over protein absorption, adhesion of tissue cells and bacteria, blood compatibility, drug delivery, and diagnostic applications can be found in [82]. Briefly Appl. Sci. 2018, 8, 1424 13 of 22 Replacement of air increases the surface area available for cells to make contact with the Ti surface. Notably, the quantiﬁcation of air trapped by in situ small-angle X-ray scattering showed that nano-sized air bubbles trapped within nanoroughness still covered 45% of the titanium surface, even after immersion for 50 min [80]. The replacement of the entrapped air by incubation medium described above is an example of wetting transition. Generally, transition from a superhydrophobic state, described by the Cassie–Baxter equation, to a Wenzel state or a mixed Cassie–Baxter/Wenzel state is accompanied by a loosening of antifouling properties. These transitions were observed under evaporation, pressing, vibration, and bouncing of droplets [81] and are the reason, for example, for failing the rime test in [78]. The same structured surface can demonstrate excellent superhydrophobic properties under certain conditions while failing in other environments. Therefore, careful planning of a technical application of superhydrophobic surfaces, taking into consideration all operation conditions, is vital. Particularly for biomedical applications, a comprehensive overview of utilization of superhydrophobic surfaces for control over protein absorption, adhesion of tissue cells and bacteria, blood compatibility, Appl. Sci. 2018, 8, x FOR PEER REVIEW 14 of 22 drug delivery, and diagnostic applications can be found in [82]. Brieﬂy summarized, it concludes that superhydrophobic surfaces can limit the surface area available for protein, tissue, and blood cells, summarized, it concludes that superhydrophobic surfaces can limit the surface area available for as well as bacteria to bind due to the presence of a protective air barrier. Taking into account the limited protein, tissue, and blood cells, as well as bacteria to bind due to the presence of a protective air barrier. lifetime of the underwater entrapped air layer, superhydrophobic surfaces can be used as a temporary Taking into account the limited lifetime of the underwater entrapped air layer, superhydrophobic barrier to biofouling. surfaces can be used as a temporary barrier to biofouling. 4. Slippery Liquid-Infused Porous Surfaces (SLIPS) 4. Slippery Liquid-Infused Porous Surfaces (SLIPS) A biological model of the considered here biomimetic development is carnivorous plants. A biological model of the considered here biomimetic development is carnivorous plants. Compared to superhydrophobicity, carnivorousness in plants is a relatively rare phenomenon in Compared to superhydrophobicity, carnivorousness in plants is a relatively rare phenomenon in nature resulting from the adaptation to nutrient-poor habitats. About 600 plant species were identiﬁed nature resulting from the adaptation to nutrient-poor habitats. About 600 plant species were to having developed interesting morphological and anatomical features related to carnivorousness [83]. identified to having developed interesting morphological and anatomical features related to One of the most studied pitcher plants is the genus Nepenthes, containing approx. 120 species carnivorousness [83]. One of the most studied pitcher plants is the genus Nepenthes, containing (Figure 9). approx. 120 species (Figure 9). Figure 9. Nepenthes pitcher organization (left), SEM image of peristome and slippery surface (middle), Figure 9. Nepenthes pitcher organization (left), SEM image of peristome and slippery surface (middle), detailed image of slippery zone (right). detailed image of slippery zone (right). These plants evolve special properties of the flower peristome to promote the falling of insects These plants evolve special properties of the ﬂower peristome to promote the falling of insects into the flower pitcher. The peristome surrounds the opening of the pitcher and has a unique slippery into the ﬂower pitcher. The peristome surrounds the opening of the pitcher and has a unique surface that prevents the escape of prey. The flower peristome is characterized by a regular slippery surface that prevents the escape of prey. The ﬂower peristome is characterized by a regular microstructure [84] whose surface is completely wetted, so that a homogeneous liquid film covers the microstructure [84] whose surface is completely wetted, so that a homogeneous liquid ﬁlm covers the surface. Only when wet, the peristome surface is slippery for insects, so that most insects coming into surface. Only when wet, the peristome surface is slippery for insects, so that most insects coming into contact become trapped [84,85]. contact become trapped [84,85]. Inspired by this biological model, liquid-repellent synthetic surfaces named “slippery liquid- Inspired by this biological model, liquid-repellent synthetic surfaces named “slippery liquid- infused porous surface(s)” (SLIPS) were developed by Aizenberg’s group [86]. Instead of using the infused porous surface(s)” (SLIPS) were developed by Aizenberg’s group [86]. Instead of using the dry dry structuring surface to repel liquids, the surface structure was used for immobilization of a lubricating liquid that forms a continuous overlying film on the surface. It was presumed that such an intrinsically smooth and defect-free liquid surface can repel immiscible liquids of virtually any surface tension. The design of SLIPS is based on the three following criteria: (1) the lubricating liquid must be stable, immobilized on the substrate surface; (2) the solid must be preferentially wetted by the lubricating liquid rather than by the liquid one wants to repel, and (3) the lubricating liquid and the liquid one wants to repel must be immiscible (Figure 10) [86]. Figure 10. Fabrication of slippery liquid-infused porous surfaces (SLIPS) by infiltrating a functionalized textured solid with a lubricating liquid to form a physically smooth and chemically homogeneous lubricating film on the surface, enabling the test liquid to easily slide down on the tilted SLIPS. Appl. Sci. 2018, 8, x FOR PEER REVIEW 14 of 22 summarized, it concludes that superhydrophobic surfaces can limit the surface area available for protein, tissue, and blood cells, as well as bacteria to bind due to the presence of a protective air barrier. Taking into account the limited lifetime of the underwater entrapped air layer, superhydrophobic surfaces can be used as a temporary barrier to biofouling. 4. Slippery Liquid-Infused Porous Surfaces (SLIPS) A biological model of the considered here biomimetic development is carnivorous plants. Compared to superhydrophobicity, carnivorousness in plants is a relatively rare phenomenon in nature resulting from the adaptation to nutrient-poor habitats. About 600 plant species were identified to having developed interesting morphological and anatomical features related to carnivorousness [83]. One of the most studied pitcher plants is the genus Nepenthes, containing approx. 120 species (Figure 9). Figure 9. Nepenthes pitcher organization (left), SEM image of peristome and slippery surface (middle), detailed image of slippery zone (right). These plants evolve special properties of the flower peristome to promote the falling of insects into the flower pitcher. The peristome surrounds the opening of the pitcher and has a unique slippery surface that prevents the escape of prey. The flower peristome is characterized by a regular microstructure [84] whose surface is completely wetted, so that a homogeneous liquid film covers the surface. Only when wet, the peristome surface is slippery for insects, so that most insects coming into contact become trapped [84,85]. Appl. Sci. 2018, 8, 1424 14 of 22 Inspired by this biological model, liquid-repellent synthetic surfaces named “slippery liquid- infused porous surface(s)” (SLIPS) were developed by Aizenberg’s group [86]. Instead of using the structuring surface to repel liquids, the surface structure was used for immobilization of a lubricating dry structuring surface to repel liquids, the surface structure was used for immobilization of a liquid that forms a continuous overlying ﬁlm on the surface. It was presumed that such an intrinsically lubricating liquid that forms a continuous overlying film on the surface. It was presumed that such smooth and defect-free liquid surface can repel immiscible liquids of virtually any surface tension. an intrinsically smooth and defect-free liquid surface can repel immiscible liquids of virtually any The design of SLIPS is based on the three following criteria: (1) the lubricating liquid must be stable, surface tension. The design of SLIPS is based on the three following criteria: (1) the lubricating liquid immobilized on the substrate surface; (2) the solid must be preferentially wetted by the lubricating must be stable, immobilized on the substrate surface; (2) the solid must be preferentially wetted by liquid rather than by the liquid one wants to repel, and (3) the lubricating liquid and the liquid one the lubricating liquid rather than by the liquid one wants to repel, and (3) the lubricating liquid and wants to repel must be immiscible (Figure 10) [86]. the liquid one wants to repel must be immiscible (Figure 10) [86]. Figure 10. Fabrication of slippery liquid-infused porous surfaces (SLIPS) by infiltrating a functionalized Figure 10. Fabrication of slippery liquid-infused porous surfaces (SLIPS) by inﬁltrating a functionalized textured solid with a lubricating liquid to form a physically smooth and chemically homogeneous textured solid with a lubricating liquid to form a physically smooth and chemically homogeneous lubricating film on the surface, enabling the test liquid to easily slide down on the tilted SLIPS. lubricating ﬁlm on the surface, enabling the test liquid to easily slide down on the tilted SLIPS. Similar to the development of dry superhydrophobic surfaces, this biomimetic development can also be referred to as biology push or bottom-up. The ﬁrst SLIPS were fabricated from periodically ordered nanostructured epoxy resin-based and silanized surfaces and a random network of Teﬂon nanoﬁbrous membranes in combination with low-surface-tension perﬂuorinated oil. Synthetic SLIPS exhibited extreme liquid repellence, characterized by low contact angle hysteresis <2.5 and low tilt 1 1 angle 5 against liquids with surface tension from 17.2 mNm (n-pentane) to 72.4 mNm (water). These slippery surfaces maintained their repellent properties even under high pressure (~676 atm). Moreover, the lubricating ﬁlm was shown to have a self-healing effect; following damage of the material surface, lubricating liquid reﬁlled the damaged area (voids). It was demonstrated that by matching the refractive indices of substrate and lubricant, SLIPS with enhanced optical transparency in visible and near-infrared wavelengths can be constructed. Nowadays, SLIPS are fabricated on different materials by ultrafast laser ablation. In [87,88], laser-irradiated porous network microstructures were generated on various polymer surfaces, including poly(ethylene terephthalate) (PET), poly(methyl methacrylate), polyamide, polycarbonate, polyethylene, and polylactic acid. Ultrafast laser-generated structures consisting of protrusions and pores with diameters of several hundred nanometres were subsequently silanized, and infused with silicon oil. Contact angle measurements demonstrated an advancing contact angle of 27.8 0.1 and receding contact angle of 27.2 0.1 , resulting in contact angle hysteresis of 0.6 against a water droplet on PET-SLIPS. A tilting angle of 4 was measured for water slowly sliding downwards and oil droplets. Additionally, PET-SLIPS were shown to have high resistance to bending and friction. In [89], the fabrication of cauliﬂower-like slippery liquid-infused surfaces on steel using ultrafast laser processing was reported (Figure 11). After subsequent UV/ozone treatment, which was applied to remove organic contamination and generate hydroxyl groups, surfaces were silanized. Steel-SLIPS were prepared by surface infusion with Krytox 103 (DuPont) perﬂuorinated oil. By wettability characterization of steel-SLIPS the following data were obtained: water contact angle: 116.6 1.3 , contact angle hysteresis: 0.6 0.2 , and tilting angle: 2 . Appl. Sci. 2018, 8, x FOR PEER REVIEW 15 of 22 Similar to the development of dry superhydrophobic surfaces, this biomimetic development can also be referred to as biology push or bottom-up. The first SLIPS were fabricated from periodically ordered nanostructured epoxy resin-based and silanized surfaces and a random network of Teflon nanofibrous membranes in combination with low-surface-tension perfluorinated oil. Synthetic SLIPS exhibited extreme liquid repellence, characterized by low contact angle hysteresis <2.5° and low tilt −1 −1 angle ≤5° against liquids with surface tension from 17.2 mNm (n-pentane) to 72.4 mNm (water). These slippery surfaces maintained their repellent properties even under high pressure (~676 atm). Moreover, the lubricating film was shown to have a self-healing effect; following damage of the material surface, lubricating liquid refilled the damaged area (voids). It was demonstrated that by matching the refractive indices of substrate and lubricant, SLIPS with enhanced optical transparency in visible and near-infrared wavelengths can be constructed. Nowadays, SLIPS are fabricated on different materials by ultrafast laser ablation. In [87,88], laser-irradiated porous network microstructures were generated on various polymer surfaces, including poly(ethylene terephthalate) (PET), poly(methyl methacrylate), polyamide, polycarbonate, polyethylene, and polylactic acid. Ultrafast laser-generated structures consisting of protrusions and pores with diameters of several hundred nanometres were subsequently silanized, and infused with silicon oil. Contact angle measurements demonstrated an advancing contact angle of 27.8 ± 0.1° and receding contact angle of 27.2 ± 0.1°, resulting in contact angle hysteresis of 0.6° against a water droplet on PET-SLIPS. A tilting angle of 4° was measured for water slowly sliding downwards and oil droplets. Additionally, PET-SLIPS were shown to have high resistance to bending and friction. In [89], the fabrication of cauliflower-like slippery liquid-infused surfaces on steel using ultrafast laser processing was reported (Figure 11). After subsequent UV/ozone treatment, which was applied to remove organic contamination and generate hydroxyl groups, surfaces were silanized. Steel-SLIPS were prepared by surface infusion with Krytox 103 (DuPont) perfluorinated oil. By wettability characterization of steel-SLIPS the following data were obtained: water contact angle: 116.6° ± 1.3°, Appl. Sci. 2018, 8, 1424 15 of 22 contact angle hysteresis: 0.6° ± 0.2°, and tilting angle: 2°. Figure 11. SEM images of native (A,B) laser-irradiated structure on steel (C,D), which was used for Figure 11. SEM images of native (A,B) laser-irradiated structure on steel (C,D), which was used for SLIPS fabrication. (Reprinted with permission from [89]. Copyright 2017 American Chemical Society.) SLIPS fabrication. (Reprinted with permission from [89]. Copyright 2017 American Chemical Society.) In [90], different laser-fabricated structures (hierarchical columnar and ripple structures, as well In [90], different laser-fabricated structures (hierarchical columnar and ripple structures, as well as laser-inscribed groove structures, Figure 12) and five lubricants with different viscosities (Krytox as laser-inscribed groove structures, Figure 12) and ﬁve lubricants with different viscosities (Krytox GPL 100 (4 cSt at 40 °C), Krytox 143 AZ (18 cSt at 38 °C), Krytox GPL 104 (60 cSt at 40 °C), Krytox GPL 100 (4 cSt at 40 C), Krytox 143 AZ (18 cSt at 38 C), Krytox GPL 104 (60 cSt at 40 C), Krytox GPL GPL 105 (160 cSt at 40 °C), and Krytox GPL 106 (240 cSt at 40 °C)) were tested in view of their SLIPS 105 (160 cSt at 40 C), and Krytox GPL 106 (240 cSt at 40 C)) were tested in view of their SLIPS performance on titanium surfaces. performance on titanium surfaces. Appl. Sci. 2018, 8, x FOR PEER REVIEW 16 of 22 Figure 12. SEM of different titanium surfaces (with laser-irradiated hierarchical columnar structure Figure 12. SEM of different titanium surfaces (with laser-irradiated hierarchical columnar structure (A) with laser-inscribed grooves structure (B), with laser-irradiated ripples structures (C), native (D)), (A) with laser-inscribed grooves structure (B), with laser-irradiated ripples structures (C), native (D)), which was tested in [90] for fabrication of SLIPS. (Reprinted with permission from [90]. Copyright which was tested in [90] for fabrication of SLIPS. (Reprinted with permission from [90]. Copyright 2017 2017 American Chemical Society.) American Chemical Society.) Before the spin-coating with lubricants, titanium surfaces were coated with a fluorinated polymer. On all titanium-SLIPS, regardless of the structure and lubricant applied, a static water contact angle of 100° was measured. The water contact angle hysteresis below 5° on columnar- titanium-SLIPS for all tested lubricants was observed. For groove-, ripple-, and unstructured- titanium-SLIPS, contact angle hysteresis was also lower than 5° for more viscous lubricants (GPL 104, GPL 105, and GPL 106), and could not be measured for GPL 100 and 143 AZ due to pinning of the water droplet by tilting the surface. For a closer investigation, different titanium structures were infused with 143 AZ or GPL 104 and analyzed by optical coherence tomography. In all cases, the lubricant’s thickness is approximately 60–85 µm but varies according to the underlying structure and lubricant. It was found that the thickness of 143 AZ lubricant layers was ~20 µm lower than the corresponding thickness of GPL 104 layer on all samples. It is noteworthy that the lubricant’s thickness on columnar-titanium-SLIPS was approximately 10 µm greater than that on the corresponding groove-, ripple-, or unstructured-titanium-SLIPS samples. The durability of different titanium-SLIPS structures infused with 143 AZ or GPL 104 was tested using optical coherence tomography after incubation in an aqueous environment (biofilm cultivation medium) for 18 h. It was found that the thickness of the lubricant layer on columnar-titanium-SLIPS was reduced by 20–30% due to immersion, leaving a film of 60 µm thickness on the surface. On SLIPS fabricated on grooves, ripples, and unstructured titanium surface, complete removal of the lubricant film was observed. This is the result of lubricant separation and building of spheres in hydrophilic environment, such as the biofilm cultivation medium or PBS, to decrease the overall free energy [91]. It was assumed that only the columnar structure thermodynamically favors lubricant immobilization on the surface against hydrophobic sphere formation in a hydrophilic environment, probably due to the specific hierarchical nature of the structure. Therefore, among all tested titanium surface structures only the columnar-structured titanium possesses the potential for stable SLIPS. Finally, durability of columnar-titanium-SLIPS coated with different lubricants under rotational forces (aqueous shear stress for 5 h at 150 rpm) was tested. Optical coherence tomography showed that lubricant layers could only be detected for the lubricants of intermediate viscosity (143 AZ, GPL 104, GPL 105). It has already been shown that physical forces, such as dynamic flow or centrifugal forces, may remove lubricants from SLIPS [92–94]. A study [90] experimentally demonstrated that there exists a definite range of surface structure dimensions and lubricant viscosities leading to stable Appl. Sci. 2018, 8, 1424 16 of 22 Before the spin-coating with lubricants, titanium surfaces were coated with a ﬂuorinated polymer. On all titanium-SLIPS, regardless of the structure and lubricant applied, a static water contact angle of 100 was measured. The water contact angle hysteresis below 5 on columnar-titanium-SLIPS for all tested lubricants was observed. For groove-, ripple-, and unstructured-titanium-SLIPS, contact angle hysteresis was also lower than 5 for more viscous lubricants (GPL 104, GPL 105, and GPL 106), and could not be measured for GPL 100 and 143 AZ due to pinning of the water droplet by tilting the surface. For a closer investigation, different titanium structures were infused with 143 AZ or GPL 104 and analyzed by optical coherence tomography. In all cases, the lubricant’s thickness is approximately 60–85 m but varies according to the underlying structure and lubricant. It was found that the thickness of 143 AZ lubricant layers was ~20 m lower than the corresponding thickness of GPL 104 layer on all samples. It is noteworthy that the lubricant’s thickness on columnar-titanium-SLIPS was approximately 10 m greater than that on the corresponding groove-, ripple-, or unstructured-titanium-SLIPS samples. The durability of different titanium-SLIPS structures infused with 143 AZ or GPL 104 was tested using optical coherence tomography after incubation in an aqueous environment (bioﬁlm cultivation medium) for 18 h. It was found that the thickness of the lubricant layer on columnar-titanium-SLIPS was reduced by 20–30% due to immersion, leaving a ﬁlm of 60 m thickness on the surface. On SLIPS fabricated on grooves, ripples, and unstructured titanium surface, complete removal of the lubricant ﬁlm was observed. This is the result of lubricant separation and building of spheres in hydrophilic environment, such as the bioﬁlm cultivation medium or PBS, to decrease the overall free energy [91]. It was assumed that only the columnar structure thermodynamically favors lubricant immobilization on the surface against hydrophobic sphere formation in a hydrophilic environment, probably due to the speciﬁc hierarchical nature of the structure. Therefore, among all tested titanium surface structures only the columnar-structured titanium possesses the potential for stable SLIPS. Finally, durability of columnar-titanium-SLIPS coated with different lubricants under rotational forces (aqueous shear stress for 5 h at 150 rpm) was tested. Optical coherence tomography showed that lubricant layers could only be detected for the lubricants of intermediate viscosity (143 AZ, GPL 104, GPL 105). It has already been shown that physical forces, such as dynamic ﬂow or centrifugal forces, may remove lubricants from SLIPS [92–94]. A study [90] experimentally demonstrated that there exists a deﬁnite range of surface structure dimensions and lubricant viscosities leading to stable SLIPS. Lubricants with very low viscosity are not inert enough to withstand physical and energetic forces, whereas lubricants with very high viscosity are not fully trapped by the surface structures. Technical applications of SLIPS fabricated by ultrafast laser processing include self-cleaning and antifouling. For example, in [87,88] polymer-SLIPS were shown to repel a broad range of liquids, such as water, ink, glycerol, coffee, milk, fruit juice, egg white, and egg yolk from tilted surfaces under normal ambient conditions. In 89] the anti-fouling properties of steel SLIPS were tested in a real industrial pasteurization scenario in view of their ability for dairy fouling mitigation. In the dairy sector thermal treatments are mandatory to ensure product safety. The result of the thermal treatment is heat-induced fouling, forming an essential part (up to 80%) of the total production costs. Steel-SLIPS in industrial-like conditions demonstrated signiﬁcant antifouling performance, with a contamination weight reduction of 63 4 wt % compared to native stainless steel. Moreover, tests showed that a 20-min rinsing step with water was sufﬁcient to eliminate all traces of fouling from the steel-SLIPS surface. In comparison, a multiple-step procedure consisting of a pre-rinse, followed by an alkali cleaning, an intermediate rinsing step, an acid cleaning, and a ﬁnal rinsing to achieve the same result on native stainless steel. For durability tests, steel-SLIPS were tested and rinsed repeatedly with and without oil reimpregnation in between. Re-impregnated steel-SLIPS showed the same performance as at their ﬁrst use. Samples without reimpregnation exhibited reduced contamination weight reduction of only 26 12 wt %, which was attributed to oil shedding from the surface. To sum up, this study clearly demonstrated the potential of steel-SLIPS against heat-induced dairy fouling; however, the problems related to dairy products’ contamination with oil and the improvement of oil retention should be addressed in further investigations. Appl. Sci. 2018, 8, x FOR PEER REVIEW 17 of 22 SLIPS. Lubricants with very low viscosity are not inert enough to withstand physical and energetic forces, whereas lubricants with very high viscosity are not fully trapped by the surface structures. Technical applications of SLIPS fabricated by ultrafast laser processing include self-cleaning and antifouling. For example, in [87,88] polymer-SLIPS were shown to repel a broad range of liquids, such as water, ink, glycerol, coffee, milk, fruit juice, egg white, and egg yolk from tilted surfaces under normal ambient conditions. In 89] the anti-fouling properties of steel SLIPS were tested in a real industrial pasteurization scenario in view of their ability for dairy fouling mitigation. In the dairy sector thermal treatments are mandatory to ensure product safety. The result of the thermal treatment is heat-induced fouling, forming an essential part (up to 80%) of the total production costs. Steel- SLIPS in industrial-like conditions demonstrated significant antifouling performance, with a contamination weight reduction of 63 ± 4 wt % compared to native stainless steel. Moreover, tests showed that a 20-min rinsing step with water was sufficient to eliminate all traces of fouling from the steel-SLIPS surface. In comparison, a multiple-step procedure consisting of a pre-rinse, followed by an alkali cleaning, an intermediate rinsing step, an acid cleaning, and a final rinsing to achieve the same result on native stainless steel. For durability tests, steel-SLIPS were tested and rinsed repeatedly with and without oil reimpregnation in between. Re-impregnated steel-SLIPS showed the same performance as at their first use. Samples without reimpregnation exhibited reduced contamination weight reduction of only 26 ± 12 wt %, which was attributed to oil shedding from the surface. To sum up, this study clearly demonstrated the potential of steel-SLIPS against heat-induced Appl. Sci. 2018, 8, 1424 17 of 22 dairy fouling; however, the problems related to dairy products’ contamination with oil and the improvement of oil retention should be addressed in further investigations. Another important application tested on titanium-SLIPS fabricated using ultrafast laser processing Another important application tested on titanium-SLIPS fabricated using ultrafast laser is the prevention of biofouling in medicine—for example, inhibition of bacterial colonization. In [25], processing is the prevention of biofouling in medicine—for example, inhibition of bacterial columnar-titanium-SLIPS coated with lubricants of intermediate viscosity (143 AZ, GPL 104, GPL 105) colonization. In [25], columnar-titanium-SLIPS coated with lubricants of intermediate viscosity (143 showed statistically signiﬁcant reduction in adhering bacteria (S. oralis) 10-fold compared to that of AZ, GPL 104, GPL 105) showed statistically significant reduction in adhering bacteria (S. oralis) ∼10- the unstructured, uncoated control (Figure 13). fold compared to that of the unstructured, uncoated control (Figure 13). Figure 13. Strong biofilm-repellent properties of SLIPS on titanium structured by ultrafast laser processing. Figure 13. Strong biofilm-repellent properties of SLIPS on titanium structured by ultrafast laser (Reprinted (adapted) with permission from [90]. Copyright 2017 American Chemical Society.) processing. (Reprinted (adapted) with permission from [90]. Copyright 2017 American Chemical Society.) For live/dead bacteria distribution, no statistical differences could be detected compared to the For live/dead bacteria distribution, no statistical differences could be detected compared to the control. This supports the hypothesis that SLIPS prevent initial cell attachment rather than inhibit control. This supports the hypothesis that SLIPS prevent initial cell attachment rather than inhibit biofilm growth. Tests with human fibroblasts and osteoblasts demonstrated that both cell types were bioﬁlm growth. Tests with human ﬁbroblasts and osteoblasts demonstrated that both cell types not able to adhere to slippery surfaces. Also in this case, live/dead fluorescence staining has indicated were not able to adhere to slippery surfaces. Also in this case, live/dead ﬂuorescence staining has that cells were viable [90]. In [88], C6 glioma cells were tested on flat, structured, and slippery PET indicated that cells were viable [90]. In [88], C6 glioma cells were tested on ﬂat, structured, and slippery surfaces (silanized and infused with silicon oil). The number of adhered cells was significantly PET surfaces (silanized and infused with silicon oil). The number of adhered cells was signiﬁcantly −2 −2 increased on structured (≈3469 mm ) compared to flat (≈1349 mm ) PET surfaces. In contrast, nearly 2 2 increased on structured (3469 mm ) compared to ﬂat (1349 mm ) PET surfaces. In contrast, no detectable adhered cells were observed on PET-SLIPS [88]. These findings narrow the range of nearly no detectable adhered cells were observed on PET-SLIPS [88]. These ﬁndings narrow the range possible applications of SLIPS for biomedical products requiring cell- and microorganism-free of possible applications of SLIPS for biomedical products requiring cell- and microorganism-free surfaces, like catheters or endoscopes. surfaces, like catheters or endoscopes. 5. Conclusions Two different approaches towards biomimetic biology push developments, based on ultrafast laser processing, demonstrate the realization of a desired property—liquid repellency—by fabrication of dry “lotus-like” and slippery “nepenthes-like” surfaces. The application of ultrafast lasers for the realization of “lotus-like” surfaces on different materials, including semiconductors, metals, glasses, and polymers, has a relatively long history that goes back to 2006, with an accompanying increase in the understanding of processes underlying the structure generation, including chemical modiﬁcation and process optimization. Although surfaces fabricated by ultrafast laser processing demonstrated characteristics similar to those of the biological models, some inherent limitations restrict their broad applications. The dry “lotus-like” surfaces have air trapped within the surface structure, which strongly reduces the contact between liquid and solid, enabling droplets to roll off easily. The escaping of trapped air under harsh conditions of speciﬁc applications is a weak point of these surfaces. However, “lotus-like” surfaces can be successfully applied for diagnostic applications where the handling of microliter-scale droplets is often required, especially for miniaturized devises. Moreover, these surfaces can be used to control liquid ﬂow in microﬂuidics or lab-on-chip devices. Developing a new class of biomimetic liquid-repellent materials—slippery “nepenthes-like” surfaces—in which a lubricant is immobilized within the surface structure to create a slippery surface, promises broader applications. Slippery “nepenthes-like” surfaces were found to repel different liquids with a tilting angle of approx. 5 and to be stable under high pressure. Since 2017 SLIPS have been Appl. Sci. 2018, 8, 1424 18 of 22 reported to be fabricated using ultrafast laser processing. As can be seen from the examples of steel and titanium-SLIPS, their fabrication using ultrafast laser processing can beneﬁt from knowledge gained due to the development of structuring techniques for the realization of dry “lotus-like” surfaces. Author Contributions: Conceptualization, E.F. and B.C.; Methodology, E.F. and B.C.; Writing-Original Draft Preparation, X.E.F. and B.C.; Writing-Review & Editing, E.F. and B.C.; Visualization, E.F. and B.C. Funding: This research was funded by [the Ministry of Lower Saxony] and [the Volkswagen Foundation in frame of the BIOFABRICATION FOR NIFE initiative]. Acknowledgments: The authors gratefully acknowledge the support of T. Melnyk during the SEM investigations. Conﬂicts of Interest: The authors declare no conﬂict of interest. References 1. Bhushan, B. Biomimetics; Springer: Berlin/Heidelberg, Germany, 2012. 2. Nosonovsky, M.; Rohatgi, P.K. Biomimetics in Materials Science; Springer: New York, NY, USA, 2012. 3. Lepora, N.F.; Verschure, P.; Prescott, T.J. The state of the art in biomimetics. Bioinspir. Biomim. 2013, 8, 013001. [CrossRef] [PubMed] 4. Bonser, R.H.C. Patented Biologically-inspired technological innovations: A twenty year view. J. Bionic Eng. 2006, 3, 39–41. [CrossRef] 5. Bonser, R.H.C.; Vincent, J.F.V. Technology trajectories, innovation, and the growth of biomimetics. Proc. Inst. Mech. Eng. Part C J. Mech. Eng. Sci. 2007, 221, 1177–1180. [CrossRef] 6. International Organization for Standardization. Biomimetics—Terminology, Concepts and Methodology; ISO/TC266; International Organization for Standardization: Geneva, Switzerland, 2015. 7. Verein Deutscher Ingenieure (VDI). Guideline 6220: Biomimetics, Conception and strategy; Differences between Biomimetic and Conventional Method/Producs; VDI: Düsseldorf, Germany, 2012. 8. Barthlott, W.; Mail, M.; Bhushan, B.; Koch, K. Plant Surfaces: Structures and Functions for Biomimetic Innovations. Nano-Micro Lett. 2017, 9, 23. [CrossRef] 9. Sugioka, K. Progress in ultrafast laser processing and future prospects. Nanophotonics 2017, 6, 393–413. [CrossRef] 10. Gamaly, E.G.; Rode, A.V.; Luther-Davies, B. Ablation of solids by femtosecond lasers: Ablation mechanism and ablation thresholds for metals and dielectrics. Phys. Plasmas 2002, 9, 949–957. [CrossRef] 11. Shimotsuma, Y.; Kazansky, P.G.; Qiu, J.; Hirao, K. Self-Organized Nanogratings in Glass Irradiated by Ultrashort Light Pulses. Phys. Rev. Lett. 2003, 91, 247405. [CrossRef] [PubMed] 12. Rethfeld, B.; Sokolowski-Tinten, K.; von der Linde, D.; Anisimov, S. Timescales in the response of materials to femtosecond laser excitation. Appl. Phys. A 2004, 79, 767–769. [CrossRef] 13. Zhigilei, L.V.; Lin, Z.; Ivanov, D.S. Atomistic Modelling of Short Pulse Laser Ablation of Metals: Connections between Melting, Spallation, and Phase Explosion. J. Phys. Chem. C 2009, 113, 11892–11906. [CrossRef] 14. Bulgakova, N.; Stoian, R.; Rosenfeld, A.; Hertel, I. Chapter Continuum Models of Ultrafast Pulsed Laser Ablation. In Laser-Surface Interactions for New Materials Production; Springer: Berlin/Heidelberg, Germany, 2010; pp. 81–97. 15. Athanasiou, C.-E.; Hongler, M.-O.; Bellouard, Y. Unraveling Brittle-Fracture Statistics from Intermittent Patterns Formed During Femtosecond Laser Exposure. Phys. Rev. Appl. 2017, 8, 054013. [CrossRef] 16. Ahmmed, K.M.T.; Grambow, C.; Kietzig, A.-M. Fabrication of Micro/Nano Structures on Metals by Femtosecond Laser Micromachining. Micromachines 2014, 5, 1219–1253. [CrossRef] 17. Korte, F.; Serbin, J.; Koch, J.; Egbert, A.; Fallnich, C.; Ostendorf, A.; Chichkov, B. Towards nanostructuring with femtosecond laser pulses. Appl. Phys. A 2003, 77, 229–235. 18. Kurselis, K.; Kiyan, R.; Chichkov, B.N. Formation of corrugated and porous steel surfaces by femtosecond laser irradiation. Appl. Surf. Sci. 2012, 258, 8845–8852. [CrossRef] 19. Vorobyev, A.; Guo, C. Direct femtosecond laser surface nano/microstructuring and its applications. Laser Photonics Rev. 2013, 7, 385–407. [CrossRef] 20. Fadeeva, E.; Schlie-Wolter, S.; Chichkov, B.; Paasche, G.; Lenarz, T. 5-Structuring of biomaterial surfaces with ultrashort pulsed laser radiation. In Laser Surface Modiﬁcation of Biomaterials; Vilar, R., Ed.; Woodhead Publishing: Sawston/Cambridge, UK, 2016; pp. 145–172. Appl. Sci. 2018, 8, 1424 19 of 22 21. Buividas, R.; Mikutis, M.; Juodkazis, S. Surface and bulk structuring of materials by ripples with long and short laser pulses: Recent advances. Prog. Quantum Electron. 2014, 38, 119–156. [CrossRef] 22. Vorobyev, A.; Guo, C. Effects of nanostructure-covered femtosecond laser-induced periodic surface structures on optical absorptance of metals. Appl. Phys. A 2007, 86, 321–324. [CrossRef] 23. Vorobyev, A.Y.; Makin, V.S.; Guo, C. Periodic ordering of random surface nanostructures induced by femtosecond laser pulses on metals. J. Appl. Phys. 2007, 101, 034903. [CrossRef] 24. Vorobyev, A.Y.; Guo, C. Femtosecond laser-induced periodic surface structure formation on tungsten. J. Appl. Phys. 2008, 104, 063523. [CrossRef] 25. Hwang, T.Y.; Guo, C. Angular effects of nanostructure-covered femtosecond laser induced periodic surface structures on metals. J. Appl. Phys. 2010, 108, 073523. [CrossRef] 26. Shen, M.; Carey, J.; Crouch, C.; Kandyla, M.; Stone, H.; Mazur, E. High-Density Regular Arrays of Nanometer-Scale Rods Formed on Silicon Surfaces via Femtosecond Laser Irradiation in Water. Nano Lett. 2008, 8, 2087–2091. [CrossRef] [PubMed] 27. Bonse, J.; Krüger, J.; Höhm, S.; Rosenfeld, A. Femtosecond laser-induced periodic surface structures. J. Laser Appl. 2012, 24, 042006. [CrossRef] 28. Nayak, B.K.; Gupta, M.C. Self-organized micro/nano structures in metal surfaces by ultrafast laser irradiation. Opt. Lasers Eng. 2010, 48, 940–949. [CrossRef] 29. Ahmmed, T.K.M.; Kietzig, A.-M. Drag reduction on laser-patterned hierarchical superhydrophobic surfaces. Soft Matter 2016, 12, 4912–4922. [CrossRef] [PubMed] 30. Oliveira, V.; Ausset, S.; Vilar, R. Surface micro/nanostructuring of titanium under stationary and non-stationary femtosecond laser irradiation. Appl. Surf. Sci. 2009, 255, 7556–7560. [CrossRef] 31. Tull, B.R.; Carey, J.E.; Mazur, E.; McDonald, J.P.; Yalisove, S.M. Silicon Surface Morphologies after Femtosecond Laser Irradiation. MRS Bull. 2006, 31, 626–633. [CrossRef] 32. Nayak, B.K.; Gupta, M.C.; Kolasinski, K.W. Formation of nano-textured conical microstructures in titanium metal surface by femtosecond laser irradiation. Appl. Phys. A 2008, 90, 399–402. [CrossRef] 33. Tsukamoto, M.; Asuka, K.; Nakano, H.; Hashida, M.; Katto, M.; Abe, N.; Fujita, M. Periodic microstructures produced by femtosecond laser irradiation on titanium plate. Vacuum 2006, 80, 1346–1350. [CrossRef] 34. Zuhlke, C.A.; Anderson, T.P.; Alexander, D.R. Formation of multiscale surface structures on nickel via above surface growth and below surface growth mechanisms using femtosecond laser pulses. Opt. Express 2013, 21, 8460–8473. [CrossRef] [PubMed] 35. Kam, D.H.; Bhattacharya, S.; Mazumder, J. Control of the wetting properties of an AISI 316L stainless steel surface by femtosecond laser-induced surface modiﬁcation. J. Micromech. Microeng. 2012, 22, 105019. [CrossRef] 36. Ahmmed, K.T.; Ling, E.J.Y.; Servio, P.; Kietzig, A.-M. Introducing a new optimization tool for femtosecond laser-induced surface texturing on titanium, stainless steel, aluminum and copper. Opt. Lasers Eng. 2015, 66, 258–268. [CrossRef] 37. Carey, J.E. Femtosecond-laser Microstructuring of Silicon for Novel Optoelectronic Devices. Ph.D. Thesis, Harvard University, Cambridge, MA, USA, 2004. 38. Wu, B.; Zhou, M.; Li, J.; Ye, X.; Li, G.; Cai, L. Superhydrophobic surfaces fabricated by microstructuring of stainless steel using a femtosecond laser. Appl. Surf. Sci. 2009, 256, 61–66. [CrossRef] 39. Hwang, T.; Vorobyev, A. Polarization and angular effects of femtosecond laser-induced conical microstructures on Ni. J. Appl. Phys. 2012, 111, 083518. [CrossRef] 40. Fraggelakis, F.; Mincuzzi, G.; Manek-Honninger, I.; Lopez, J.; Kling, R. Generation of micro- and nano-morphologies on a stainless steel surface irradiated with 257 nm femtosecond laser pulses. RSC Adv. 2018, 8, 16082–16087. [CrossRef] 41. Crouch, C.H.; Carey, J.E.; Warrender, J.M.; Aziz, M.J.; Mazur, E.; Genin, F.Y. Comparison of structure and properties of femtosecond and nanosecond laser-structured silicon. Appl. Phys. Lett. 2004, 84, 1850–1852. [CrossRef] 42. Skantzakis, E.; Zorba, V.; Papazoglou, D.; Zergioti, I.; Fotakis, C. Ultraviolet laser microstructuring of silicon and the effect of laser pulse duration on the surface morphology. Appl. Surf. Sci. 2006, 252, 4462–4466. [CrossRef] Appl. Sci. 2018, 8, 1424 20 of 22 43. Zorba, V.; Boukos, N.; Zergioti, I.; Fotakis, C. Ultraviolet femtosecond, picosecond and nanosecond laser microstructuring of silicon: Structural and optical properties. Appl. Opt. 2008, 47, 1846–1850. [CrossRef] [PubMed] 44. Her, T.-H.; Finlay, R.J.; Wu, C.; Deliwala, S.; Mazur, E. Microstructuring of silicon with femtosecond laser pulses. Appl. Phys. Lett. 1998, 73, 1673–1675. [CrossRef] 45. Shen, M.; Crouch, C.; Carey, J.; Mazur, E. Femtosecond laser-induced formation of submicrometer spikes on silicon in water. Appl. Phys. Lett. 2004, 85, 5694. [CrossRef] 46. Yang, Y.; Yang, J.; Liang, C.; Wang, H.; Zhu, X.; Zhang, N. Surface microstructuring of Ti plates by femtosecond lasers in liquid ambiences: A new approach to improving biocompatibility. Opt. Express 2009, 17, 21124–21133. [CrossRef] [PubMed] 47. Rajab, F.H.; Whitehead, D.; Liu, Z.; Li, L. Characteristics of hierarchical micro/nano surface structure formation generated by picosecond laser processing in water and air. Appl. Phys. B 2017, 123, 282. [CrossRef] 48. Schuetz, V.; Horn, A.; Nagel, H.; Stute, U. Black Silicon Solar Cell Processing with High Repetitive Laser Systems. In Proceedings of the EU PVSEC Proceedings, Frankfurt, Germany, 24–28 September 2012; pp. 1864–1866. 49. Barthlott, W.; Mail, M.; Neinhuis, C. Superhydrophobic hierarchically structured surfaces in biology: Evolution, structural principles and biomimetic applications. Phil. Trans. R. Soc. Lond. A Math. Phys. Eng. Sci. 2016, 374. [CrossRef] [PubMed] 50. Koch, K.; Bhushan, B.; Barthlott, W. Multifunctional surface structures of plants: An inspiration for biomimetics. Prog. Mater. Sci. 2009, 54, 137–178. [CrossRef] 51. Koch, K.; Bhushan, B.; Barthlott, W. Diversity of structure, morphology and wetting of plant surfaces. Soft Matter 2008, 4, 1943–1963. [CrossRef] 52. Neinhuis, C.; Barthlott, W. Characterization and Distribution of Water-repellent, Self-cleaning Plant Surfaces. Ann. Bot. 1997, 79, 667–677. [CrossRef] 53. Israelachvili, J. Intermolecular and Surface Forces; Academic Press: London, UK, 1992. 54. De Gennes, P.; Brochard-Wyart, F.; Quere, D. Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves; Springer: New York, NY, USA, 2004. 55. Genzer, J.; Eﬁmenko, K. Recent developments in superhydrophobic surfaces and their relevance to marine fouling: A review. Biofouling 2006, 22, 339–360. [CrossRef] [PubMed] 56. Wenzel, R.N. Resistance of solid surfaces to wetting by water. Ind. Eng. Chem. 1936, 28, 988–994. [CrossRef] 57. Cassie, A.; Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 1944, 40, 546–551. [CrossRef] 58. Extrand, C.W. Super Repellency. In Encyclopedia of Suface and Colloid Science; Taylor & Francis: Abingdon, UK, 2006; pp. 5846–5859. 59. Wagner, P.; Färstner, R.; Barthlott, W.; Neinhuis, C. Quantitative assessment to the structural basis of water repellency in natural and technical surfaces. J. Exp. Bot. 2003, 54, 1295–1303. [CrossRef] [PubMed] 60. Baldacchini, T.; Carey, J.E.; Zhou, M.; Mazur, E. Superhydrophobic Surfaces Prepared by Microstructuring of Silicon Using a Femtosecond Laser. Langmuir 2006, 22, 4917–4919. [CrossRef] [PubMed] 61. Zorba, V.; Stratakis, E.; Barberoglou, M.; Spanakis, E.; Tzanetakis, P.; Fotakis, C. Tailoring the wetting response of silicon surfaces via fs laser structuring. Appl. Phys. A Mater. Sci. Process. 2008, 93, 819–825. [CrossRef] 62. Fadeeva, E.; Truong, V.K.; Stiesch, M.; Chichkov, B.N.; Crawford, R.J.; Wang, J.; Ivanova, E.P. Bacterial Retention on Superhydrophobic Titanium Surfaces Fabricated by Femtosecond Laser Ablation. Langmuir 2011, 27, 3012–3019. [CrossRef] [PubMed] 63. Fadeeva, E.; Schlie, S.; Koch, J.; Chichkov, B.N.; Vorobyev, A.Y.; Guo, C.; Mittal, K.L. (Eds.) Chapter Femtosecond Laser-Induced Surface Structures on Platinum and Their Effects on Surface Wettability and Fibroblast Cell Proliferation. In Contact Angle, Wettability and Adhesion; Vision Service Plan: Rancho Cordova, CA, USA, 2009; Volume 6, pp. 163–172. 64. Kietzig, A.-M.; Hatzikiriakos, S.G.; Englezos, P. Patterned Superhydrophobic Metallic Surfaces. Langmuir 2009, 25, 4821–4827. [CrossRef] [PubMed] 65. Long, J.; Fan, P.; Zhong, M.; Zhang, H.; Xie, Y.; Lin, C. Superhydrophobic and colorful copper surfaces fabricated by picosecond laser induced periodic nanostructures. Appl. Surf. Sci. 2014, 311, 461–467. [CrossRef] Appl. Sci. 2018, 8, 1424 21 of 22 66. Martinez-Calderon, M.; Rodriguez, A.; Dias-Ponte, A.; Morant-Minana, M.; Gomez-Aranzadi, M.; Olaizola, S. Femtosecond laser fabrication of highly hydrophobic stainless steel surface with hierarchical structures fabricated by combining ordered microstructures and LIPSS. Appl. Surf. Sci. 2016, 374, 81–89. [CrossRef] 67. Jagdheesh, R.; Pathiraj, B.; Karatay, E.; Roemer, G.R.B.E.; Huis in’t Veld, A.J. Laser-Induced Nanoscale Superhydrophobic Structures on Metal Surfaces. Langmuir 2011, 27, 8464–8469. [CrossRef] [PubMed] 68. Kietzig, A.-M.; Mirvakili, M.N.; Kamal, S.; Englezos, P.; Hatzikiriakos, S.G. Nanopatterned Metallic Surfaces: Their Wettability and Impact on Ice Friction. J. Adhes. Sci. Technol. 2011, 25, 1293–1303. [CrossRef] 69. Bizi-bandoki, P.; Valette, S.; Audouard, E.; Benayoun, S. Time dependency of the hydrophilicity and hydrophobicity of metallic alloys subjected to femtosecond laser irradiations. Appl. Surf. Sci. 2013, 273, 399–407. [CrossRef] 70. Ahsan, M.S.; Dewanda, F.; Lee, M.S.; Sekita, H.; Sumiyoshi, T. Formation of superhydrophobic soda-lime glass surface using femtosecond laser pulses. Appl. Surf. Sci. 2013, 265, 784–789. [CrossRef] 71. Zhou, M.; Yang, H.F.; Li, B.J.; Dai, J.; Di, J.K.; Zhao, E.L.; Cai, L. Forming mechanisms and wettability of double-scale structures fabricated by femtosecond laser. Appl. Phys. A 2009, 94, 571–576. [CrossRef] 72. Yoon, T.O.; Shin, H.J.; Jeoung, S.C.; Park, Y.-I. Formation of superhydrophobic poly(dimethysiloxane) by ultrafast laser-induced surface modiﬁcation. Opt. Express 2008, 16, 12715–12725. [CrossRef] [PubMed] 73. Groenendijk, M. Fabrication of Super Hydrophobic Surfaces by fs Laser Pulses. Laser Tech. J. 2008, 5, 44–47. [CrossRef] 74. Liu, B.; Wang, W.; Jiang, G.; Mei, X.; Wang, Z.; Wang, K.; Cui, J. Study on hierarchical structured PDMS for surface super-hydrophobicity using imprinting with ultrafast laser structured models. Appl. Surf. Sci. 2016, 364, 528–538. [CrossRef] 75. Riveiro, A.; Macon, A.L.B.; Del Val, J.; Comesana, R.; Pou, J. Laser Surface Texturing of Polymers for Biomedical Applications. Front. Phys. 2018, 6, 16. [CrossRef] 76. Barberoglou, M.; Zorba, V.; Stratakis, E.; Spanakis, E.; Tzanetakis, P.; Anastasiadis, S.; Fotakis, C. Bio-inspired water repellent surfaces produced by ultrafast laser structuring of silicon. Appl. Surf. Sci. 2009, 255, 5425–5429. [CrossRef] 77. Gao, L.; McCarthy, T.J. The “Lotus Effect” Explained: Two Reasons Why Two Length Scales of Topography Are Important. Langmuir 2006, 22, 2966–2967. [CrossRef] [PubMed] 78. Roemer, G.; Del Gerro, D.; Sipkema, R.; Groenendijk, M.; Huis in’t Veld, A. Ultra short pulse laser generated surface textures for anti-ice applications in aviation. In Proceedings of the ICALEO, Orlando, FL, USA, 2–5 November 2009. 79. Schwab, M.; Noga, G.; Barthlott, W. Bedeutung der Epicuticularwachse fÃ r die Pathogenabwehr am Beispiel von Botrytis cinerea-Infektionen bei Kohlrabi und Erbse/The Signiﬁcance of Epicuticular Waxes for Defence of Pathogens as Shown for Botrytis cinerea Infections in Kohlrabi and Pea Plants. Die Gartenbauwissenschaft 1995, 60, 102–109. 80. Truong, V.; Webb, H.; Fadeeva, E.; Chichkov, B.; Wu, A.; Lamb, R.; Wang, J.; Crawford, R.; Ivanova, E. Air-directed attachment of coccoid bacteria to the surface of superhydrophobic lotus-like titanium. Biofouling 2012, 28, 539–550. [CrossRef] [PubMed] 81. Bormashenko, E.; Nosonovsky, M.; Whyman, G.; Bhushan, B. (Eds.) Chapter 6: Towards Undersanding Wetting Transitions on Biomimetic Surfaces: Scaling Arguments and Physical Mechanisms. In Green Tribology: Biomimetic, Energy Conservation and Sustainability; Springer: Berlin, Germany, 2012; pp. 127–147. 82. Falde, E.J.; Yohe, S.T.; Colson, Y.L.; Grinstaff, M.W. Superhydrophobic materials for biomedical applications. Biomaterials 2016, 104, 87–103. [CrossRef] [PubMed] 83. Miguel, S.; Hehn, A.; Bourgaud, F. Nepenthes: State of the art of an inspiring plant for biotechnologists. J. Biotechnol. 2018, 265, 109–115. [CrossRef] [PubMed] 84. Bohn, H.; Federle, W. Insect aquaplaning: Nepenthes pitcher plants capture prey with the peristome, a fully wettable water-lubricated anisotropic surface. Proc. Natl. Acad. Sci. USA 2004, 101, 14138–14143. [CrossRef] [PubMed] 85. Bauer, U.; Grafe, T.U.; Federle, W. Evidence for alternative trapping strategies in two forms of the pitcher plant, Nepenthes rafﬂesiana. J. Exp. Bot. 2011, 62, 3683–3692. [CrossRef] [PubMed] 86. Wong, T.-S.; Kang, S.H.; Tang, S.K.Y.; Smythe, E.J.; Hatton, B.D.; Grinthal, A.; Aizenberg, J. Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity. Nature 2011, 477, 443–447. [CrossRef] [PubMed] Appl. Sci. 2018, 8, 1424 22 of 22 87. Yong, J.; Chen, F.; Yang, Q.; Fang, Y.; Huo, J.; Zhang, J.; Hou, X. Nepenthes Inspired Design of Self-Repairing Omniphobic Slippery Liquid Infused Porous Surface (SLIPS) by Femtosecond Laser Direct Writing. Adv. Mater. Interfaces 2017, 4, 1700552. [CrossRef] 88. Yong, J.; Huo, J.; Yang, Q.; Chen, F.; Fang, Y.; Wu, X.; Liu, L.; Lu, X.; Zhang, J.; Hou, X. Femtosecond Laser Direct Writing of Porous Network Microstructures for Fabricating Super-Slippery Surfaces with Excellent Liquid Repellence and Anti-Cell Proliferation. Adv. Mater. Interfaces 2018, 5, 1701479. [CrossRef] 89. Zouaghi, S.; Six, T.; Bellayer, S.; Moradi, S.; Hatzikiriakos, S.G.; Dargent, T.; Thomy, V.; Cofﬁnier, Y.; Andre, C.; Delaplace, G.; et al. Antifouling Biomimetic Liquid-Infused Stainless Steel: Application to Dairy Industrial Processing. ACS Appl. Mater. Interfaces 2017, 9, 26565–26573. [CrossRef] [PubMed] 90. Doll, K.; Fadeeva, E.; Schaeske, J.; Ehmke, T.; Winkel, A.; Heisterkamp, A.; Chichkov, B.N.; Stiesch, M.; Stumpp, N.S. Development of Laser-Structured Liquid-Infused Titanium with Strong Bioﬁlm-Repellent Properties. ACS Appl. Mater. Interfaces 2017, 9, 9359–9368. [CrossRef] [PubMed] 91. Blokzijl, W.; Engberts, J.B.F.N. Hydrophobic Effects. Opinions and Facts. Angew. Chem. Int. Ed. Engl. 1993, 32, 1545–1579. [CrossRef] 92. Kim, P.; Kreder, M.J.; Alvarenga, J.; Aizenberg, J. Hierarchical or Not? Effect of the Length Scale and Hierarchy of the Surface Roughness on Omniphobicity of Lubricant-Infused Substrates. Nano Lett. 2013, 13, 1793–1799. [CrossRef] [PubMed] 93. Wexler, J.S.; Grosskopf, A.; Chow, M.; Fan, Y.; Jacobi, I.; Stone, H.A. Robust liquid-infused surfaces through patterned wettability. Soft Matter 2015, 11, 5023–5029. [CrossRef] [PubMed] 94. Wexler, J.S.; Jacobi, I.; Stone, H.A. Shear-Driven Failure of Liquid-Infused Surfaces. Phys. Rev. Lett. 2015, 114, 168301. [CrossRef] [PubMed] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png

Applied Sciences Multidisciplinary Digital Publishing Institute

http://www.deepdyve.com/lp/multidisciplinary-digital-publishing-institute/biomimetic-liquid-repellent-surfaces-by-ultrafast-laser-processing-GeuWiOR5Vv

Biomimetic Liquid-Repellent Surfaces by Ultrafast Laser Processing

Loading next page...

Publisher: Multidisciplinary Digital Publishing Institute
ISSN: 2076-3417
DOI: 10.3390/app8091424
Publisher site: See Article on Publisher Site

Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Biomimetic Liquid-Repellent Surfaces by Ultrafast Laser Processing

Biomimetic Liquid-Repellent Surfaces by Ultrafast Laser Processing

Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Biomimetic Liquid-Repellent Surfaces by Ultrafast Laser Processing

Biomimetic Liquid-Repellent Surfaces by Ultrafast Laser Processing

Abstract

Journal

Recommended Articles

There are no references for this article.

Our policy towards the use of cookies