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Permeability and wettability of bioresorbable nanofiber nonwoven membranes

Permeability and wettability of bioresorbable nanofiber nonwoven membranes DE GRUYTER Current Directions in Biomedical Engineering 2020;6(3): 20203119 Swen Großmann*, Sabine Illner, Jan Oldenburg, Robert Ott, Michael Stiehm, Niels Grabow, Klaus-Peter Schmitz, and Stefan Siewert Permeability and wettability of bioresorbable nanofiber nonwoven membranes Abstract: Nanofiber nonwoven membranes produced by to new functionalized or optimized devices. Furthermore, an electrospinning provide the possibility to adjust mechanical adjustment of the wettability and the water permeability of the material parameters as well as simultaneously the biologically nonwovens is also indispensable, for instance in biodegradable relevant properties - a fundamental aspect in developing new implants, nanoparticle- and drug-delivery systems or implants and medical devices. Wettability and permeability corrosion-resistant coatings which have to seal and are also of great importance, as they have a decisive influence simultaneously be water permeable to a certain amount [1, 2]. on the release of drugs, cell attachment, degradability and According to Darcy’s law the water permeability finally the nutrient supply of the surrounding tissue. Within represents a constant which connects the pressure gradient this work the wettability and permeability of several over a structure with the fluid velocity: electrospun poly-L-lactide nonwovens, including different 𝝁 𝝏𝒑 𝐮 = − ∙ (1) additives, were investigated and a correlation to membrane 𝜼 𝝏𝒛 morphology was found. A potential modification of the permeability by the fluid viscosity was also investigated. The Here 𝑢 denotes the fluid velocity in z-direction, 𝜂 the dynamic results form a fundamental building block in the development ⁄ viscosity of the fluid, 𝜕𝑝 𝜕𝑧 the pressure gradient over the of permeable biodegradable implants and medical devices. structure in z-direction and 𝜇 the permeability of the structure. In this context, permeability can be considered as a material parameter of the permeable structure. Depending on the Keywords: permeability, wettability, nanofiber, nonwoven, porosity of the investigated structure the flow can also be biodegradable, electrospinning, membrane turbulent, leading to a more complex relation between fluid velocity and pressure gradient. Furthermore, for nanofiber https://doi.org/10.1515/cdbme-2020-3119 membranes porosity depends on the morphology which can be modified during applying a pressure to the membrane. In consequence, it needs to be investigated whether the water 1 Introduction permeability of nanofiber membranes depends on the actual conditions like viscosity and the applied pressure gradient. Electrospun nanofiber nonwovens represent a milestone in In this study the wettability and water permeability of developing new materials. Their mechanical and biological bioresorbable nanofiber nonwoven membranes were properties can be modified by the polymer components and the investigated for different types of additives in the poly-L- spinning parameters [1]. Especially for medical applications lactide (PLLA) nonwovens. In addition, a potential matching the mechanical properties like Young’s modulus or modification of the permeability by the applied pressure maximum tensile strength to desired functionality and even gradient or the fluid viscosity was investigated. introduce anisotropies by adjusting the fiber direction can lead ______ 2 Methods * Corresponding author: Swen Großmann: Institute for ImplantTechnology and Biomaterials e.V., Friedrich- Barnewitz-Str.4, 18119 Rostock, Germany, 2.1 Preparation of nanofiber membranes e-mail: swen.grossmann@uni-rostock.de Jan Oldenburg, Robert Ott, Michael Stiehm, Klaus-Peter by electrospinning Schmitz, and Stefan Siewert: Institute for ImplantTechnology and Biomaterials e.V., 18119 Polymer solutions of 4 to 5 wt.% were obtained by dissolving Rostock, Germany PLLA (RESOMER L210, Evonik, Germany) in a mixture of Sabine Illner, Klaus-Peter Schmitz, and Niels Grabow: chloroform and 2,2,2-trifluoroethanol (TFE) (1:4 v/v) at Institute for Biomedical Engineering, Rostock University Medical 37 °C. The 4 wt.% polymer solutions were loaded with Center, 18119 Rostock, Germany Open Access. © 2020 Swen Großmann et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License. Swen Großmann et al., Permeability and wettability of bioresorbable nanofiber nonwoven membranes — 2 12.5 wt.% of Triton X-100 (TX 100) or 1 wt.% of lecithin or 2.4 Water contact angle 1 wt.% of lecithin and 1 wt.% of tetraethylammonium chloride Measurements of the initial water contact angle were (TEAC), respectively. Here the concentrations are with respect performed by sessile drop method on the nonwoven polymer to the polymer weight. The 5 wt.% polymer solution was kept surface using a mobile surface analyser (MSA, KRÜSS pure. The process of electrospinning was performed using the GmbH, Hamburg, Germany) with ADVANCE 1.9.2 software. 4SPIN C4S LAB2 device (Contipro, Dolní Dobrouč, Czech The contact angles of water were determined against air, Republic) which includes a custom-built rotating continual whereby droplets compromising a volume of 2 µL were collector as well as a multi-jet capillary emitter with six or deposited and measured within one second. eight cannulas. All trials were performed at an emitter- collector distance of 15 cm, the applied voltage and the feed rate were chosen to be 18 to 30 kV and 100 μL/min or 220 μL/min, depending on the number of cannulas. The 3 Results and discussion spinning process was performed using ambient temperature of 22 °C ± 2 °C and relative humidity of 25 % ± 5 %. 3.1 Wetting the membranes 2.2 Electron microscopy To investigate the wetting by the test fluid a pressure gradient was applied to the mounted membranes. The corresponding The morphology of the fabricated nonwoven membranes was fluid velocities for the applied pressure gradients are plotted in investigated using a field emission scanning electron Figure 1 using pure water as test fluid. As the pressure gradient microscope (Quattro S, Thermo Fisher Scientific Inc., USA). increases, the fluid velocity suddenly changes its dependence The membranes were covered with a gold layer of a few on the pressure gradient. The observed changes occur nanometers (A2008, Agar Scientific Ltd, United Kingdom) to instantaneously and mark the points at which the accessible reduce charging during electron beam irradiation and area of the membranes is completely wetted, except for the subsequently investigated at identical magnification in low membrane including lecithin and TEAC. This structure could vacuum mode at 50 Pa with an acceleration voltage of 15 kV. not be wetted at all, resulting in zero fluid velocity for all applied pressure gradients. Figure 1 shows the observed -1 changes occurring at 73, 92, and 137 Pa·µm for membranes without additives, with TX 100 and with lecithin, respectively. 2.3 Permeability setup To investigate the origin of this pressure gradient dependent wetting, the water contact angle was measured for all The permeability was experimentally determined using a membranes (cf. Table 1). Within the obtained uncertainty the previously described setup [3]. In short, a hydraulic pump (Corio CD, JULABO GmbH, Germany) generates a constant PLLA L210 flow and simultaneously controls the temperature of the fluid. PLLA L210, TX 100 The pressure drop over the membrane is determined using a PLLA L210, lecithin PLLA L210, lecithin & TEAC pressure gauge (86A 3R–000000–005P G, Measurement Specialties Inc., USA) and the volumetric flow is measured using a Doppler sonography flow sensor (Leviflow LFS-008, Levitronix GmbH, Switzerland) in front of the sample chamber. The fluid temperature was set to 37 °C. As test fluid a mixture of deionized water and glycerol was chosen and the concentration was modified to achieve different dynamic viscosities. For correlation between viscosity and concentration, the viscosity was determined 0 100 200 300 400 500 -1 using a commercial rheometer (RheoStress 1, Thermo Fisher pressure gradient (Pa µm ) Scientific Inc., USA). For the evaluation of the membrane Figure 1: Fluid velocity during wetting of the membranes measured using pure water. After the complete wetting of the permeability, three glycerol concentrations 0 wt.%, 20 wt.%, membranes the slope of the fluid velocity changes for each and 40 wt.% were chosen, which correspond to a dynamic membrane. Point of complete wetting is indicated by the arrows. viscosity of 0.937 mPa·s, 1.397 mPa·s, and 2.401 mPa·s, The membrane containing lecithin and TEAC was not wetted for the applied pressure gradients. respectively. -1 fluid velocity (mm s ) Swen Großmann et al., Permeability and wettability of bioresorbable nanofiber nonwoven membranes — 3 calculating the permeability according to Equation 𝐮 = Table 1: Mean values of the water contact angle and fiber 𝒛 𝝁 𝝏𝒑 thickness of the investigated PLLA nanofiber membranes. − ∙ (1. The exact values of the permeability 𝜼 𝝏𝒛 can be found in Table 2. It should be noted that no permeability Polymer Contact angle Fiber thickness could be determined for the membranes containing lecithin additives (°) (nm) and TEAC as well as for the membrane with lecithin at a none 132.5 ± 13.5 1129 ± 375 viscosity of 1.397 mPa·s due to the lack of wetting. Apart from TX 100 12.7 ± 2.8 531 ± 116 a distinct difference for all types of membranes, the permeability tends to increase with increasing fluid viscosity lecithin 135.3 ± 1.9 325 ± 96 for each membrane. lecithin & TEAC 139.1 ± 72.8 165 ± 32 One possible reason for this effect could be that Darcy’s 𝝁 𝝏𝒑 contact angle does not differ for the investigated membranes, law (cf. Equation 𝐮 = − ∙ (1) is not valid 𝜼 𝝏𝒛 except for the membrane including the surfactant TX 100. for the investigated membranes using the present experimental Accordingly, the differences in the wettability are probably not parameters. Potentially due to turbulent flow within the correlated to the initial water contact angle. membranes during the entire measurement or because of Next the membranes morphology was analyzed by means deformation initiated by the applied pressure gradients also of electron microscopy. A clear difference in the fiber diameter causing turbulent flow or just a change in fiber fraction. and thus also in the effective interface roughness was found An indication of this is provided by the partly non-linear (cf. Error! Reference source not found.2). When comparing characteristic of the fluid velocities shown in Figure 3 a-c. the pressure gradients required to wet the membranes and the However, since the porosity of electrospun membranes as well averaged fiber diameter of the corresponding membrane, it as the fiber morphology can only be estimated, no clear becomes apparent that a change in the wetting at low pressure statement can be made whether laminar or turbulent flow is gradients is probably correlated to a large fiber diameter (cf. present in the conducted experiments. Table 1). Similar effects were recently found by comparing Nonetheless, Figure 3 d-f shows the thicknesses for the membranes consisting of different polymers with larger fiber investigated membranes. Within the existing uncertainties, no dimensions [4]. Note that, a smaller fiber diameter is irreversible change in thickness can be detected. Thus, the commonly associated with smaller vacancies between the observed non-linearity of the fluid velocity and the associated fibers. variation of the permeability for different viscosities is probably caused by a change in the thickness and morphology of the membranes during the measurement [5]. Table 2: Water permeability µ for the investigated membranes with respect to the dynamic viscosity η of the test fluid. µ (m²) Polymer η = 0.937 η = 1.397 η = 2.401 additives (mPa s) (mPa s) (mPa s) -14 -14 -14 none 23.98 ·10 81.02 ·10 95.80 ·10 -14 -14 -14 TX 100 1.90 ·10 3.51 ·10 13.35 ·10 -14 -14 lecithin 2.86 ·10 - 4.78 ·10 Figure 2: Electron micrograph sections of the investigated PLLA nanofiber nonwovens a without additives, b with TX 100, c with lecithin, and d with lecithin and TEAC. 4 Conclusion Within this study nanofiber membranes consisting of biodegradable PLLA nonwovens were investigated with 3.2 Permeability of the membranes respect to their wettability and water permeability. The nanofiber membranes showed a distinct correlation of the After wetting the membranes, the fluid velocity measurements pressure gradient needed for wetting and the fiber dimensions were repeated (cf. Figure 3 a-c). Subsequently, the slope of of the corresponding membrane [4], whereas no difference in each dataset was determined by linear regression followed by Swen Großmann et al., Permeability and wettability of bioresorbable nanofiber nonwoven membranes — 4 the initial water contact angle for the investigated membranes was observed. a b c PLLA L210 PLLA L210 0.937 mPa s PLLA L210 0.937 mPa s 0.937 mPa s without additives with TX 100 1.397 mPa s with lecithin 1.397 mPa s 1.397 mPa s 2.401 mPa s 2.401 mPa s 2.401 mPa s 0 0 0 0 50 100 150 0 50 100 150 200 250 300 0 50 100 150 200 -1 -1 -1 pressure gradient (Pa·µm ) pressure gradient (Pa·µm ) pressure gradient (Pa·µm ) before measurement after measurement d e f 150 150 100 100 50 50 0 0 0.937 1.397 2.401 0.937 1.397 2.401 0.937 1.397 2.401 η (mPa·s) η (mPa·s) η (mPa·s) Figure 3: Fluid velocity as a function of the applied pressure gradient for a PLLA L210 without additives, b with TX 100, and c with lecithin. The slope was determined by linear regression and used to calculate the permeability. For each type of membrane, the shown data includes the measurements for a fluid viscosity of 0.937 mPa·s, 1.397 mPa·s, and 2.401 mPa·s. Through the membrane consisting of PLLA L210 with lecithin and TEAC no fluid velocity could been measured. After performing the measurements no distinct variation in the thickness was found for the membranes made of PLLA d without additives, e with TX 100, and f with lecithin. Furthermore, it was shown that besides the different (BMBF) within RESPONSE "Partnership for Innovation in wettability there is also a clear variation in water permeability. Implant Technology" is gratefully acknowledged. Therefore, for the investigated electrospun membranes the Conflict of interest: Authors state no conflict of interest. permeability cannot be considered as a constant parameter according to Darcy's law. Since no irreversible change in thickness was observed, it is supposed that the membrane References changes its permeability during the presence of a pressure [1] Greiner A, Wendorff JH. Electrospinning: A Fascinating gradient. It has also been shown that the permeability varies Method for the Preparation of Ultrathin Fibers. Angewandte with fluid viscosity. Especially, for their degradability and Chemie International Edition 2007;46:5670–703. [2] Bakhsheshi-Rad HR, Akbari M, Ismail AF, Aziz M, Hadisi Z, when used as drug- and nanoparticle-delivery systems [1] or Pagan E, et al. Coating biodegradable magnesium alloys corrosion resistance coating [2], the wettability as well as the with electrospun poly-L-lactic acid-åkermanite-doxycycline viscosity-dependent permeability of the nonwovens are of nanofibers for enhanced biocompatibility, antibacterial decisive importance. Thus, the results presented here form the activity, and corrosion resistance. Surface and Coatings substantial basis for the development of new permeable and Technology 2019;377:124898. [3] Grossmann S, Siewert S, Ott R, Schmitz K-P, Kohse S, semi-permeable implants and medical devices consisting of Schmidt W, et al. Standardized technique of water biodegradable nanofiber membranes. permeability measurement for biomedical applications. Current Directions in Biomedical Engineering 2018;4:633–6. Author Statement [4] Szewczyk P, Ura D, Metwally S, Knapczyk-Korczak J, Gajek M, Marzec M, et al. Roughness and Fiber Fraction Research funding: Partial financial support by the European Dominated Wetting of Electrospun Fiber-Based Porous Regional Development Fund (ERDF) and the European Social Meshes. Polymers 2018;11:34. Fund (ESF) within the collaborative research between [5] Choong LT, Khan Z, Rutledge GC. Permeability of economy and science of the state Mecklenburg-Vorpommern electrospun fiber mats under hydraulic flow. Journal of and by the Federal Ministry of Education and Research Membrane Science 2014;451:111–6. -1 thickness (µm) fluid velocity ) (mm·s thickness (µm) -1 fluid velocity (mm·s ) thickness (µm) -1 fluid velocity (mm·s ) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Current Directions in Biomedical Engineering de Gruyter

Permeability and wettability of bioresorbable nanofiber nonwoven membranes

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de Gruyter
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© 2020 by Walter de Gruyter Berlin/Boston
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2364-5504
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10.1515/cdbme-2020-3119
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Abstract

DE GRUYTER Current Directions in Biomedical Engineering 2020;6(3): 20203119 Swen Großmann*, Sabine Illner, Jan Oldenburg, Robert Ott, Michael Stiehm, Niels Grabow, Klaus-Peter Schmitz, and Stefan Siewert Permeability and wettability of bioresorbable nanofiber nonwoven membranes Abstract: Nanofiber nonwoven membranes produced by to new functionalized or optimized devices. Furthermore, an electrospinning provide the possibility to adjust mechanical adjustment of the wettability and the water permeability of the material parameters as well as simultaneously the biologically nonwovens is also indispensable, for instance in biodegradable relevant properties - a fundamental aspect in developing new implants, nanoparticle- and drug-delivery systems or implants and medical devices. Wettability and permeability corrosion-resistant coatings which have to seal and are also of great importance, as they have a decisive influence simultaneously be water permeable to a certain amount [1, 2]. on the release of drugs, cell attachment, degradability and According to Darcy’s law the water permeability finally the nutrient supply of the surrounding tissue. Within represents a constant which connects the pressure gradient this work the wettability and permeability of several over a structure with the fluid velocity: electrospun poly-L-lactide nonwovens, including different 𝝁 𝝏𝒑 𝐮 = − ∙ (1) additives, were investigated and a correlation to membrane 𝜼 𝝏𝒛 morphology was found. A potential modification of the permeability by the fluid viscosity was also investigated. The Here 𝑢 denotes the fluid velocity in z-direction, 𝜂 the dynamic results form a fundamental building block in the development ⁄ viscosity of the fluid, 𝜕𝑝 𝜕𝑧 the pressure gradient over the of permeable biodegradable implants and medical devices. structure in z-direction and 𝜇 the permeability of the structure. In this context, permeability can be considered as a material parameter of the permeable structure. Depending on the Keywords: permeability, wettability, nanofiber, nonwoven, porosity of the investigated structure the flow can also be biodegradable, electrospinning, membrane turbulent, leading to a more complex relation between fluid velocity and pressure gradient. Furthermore, for nanofiber https://doi.org/10.1515/cdbme-2020-3119 membranes porosity depends on the morphology which can be modified during applying a pressure to the membrane. In consequence, it needs to be investigated whether the water 1 Introduction permeability of nanofiber membranes depends on the actual conditions like viscosity and the applied pressure gradient. Electrospun nanofiber nonwovens represent a milestone in In this study the wettability and water permeability of developing new materials. Their mechanical and biological bioresorbable nanofiber nonwoven membranes were properties can be modified by the polymer components and the investigated for different types of additives in the poly-L- spinning parameters [1]. Especially for medical applications lactide (PLLA) nonwovens. In addition, a potential matching the mechanical properties like Young’s modulus or modification of the permeability by the applied pressure maximum tensile strength to desired functionality and even gradient or the fluid viscosity was investigated. introduce anisotropies by adjusting the fiber direction can lead ______ 2 Methods * Corresponding author: Swen Großmann: Institute for ImplantTechnology and Biomaterials e.V., Friedrich- Barnewitz-Str.4, 18119 Rostock, Germany, 2.1 Preparation of nanofiber membranes e-mail: swen.grossmann@uni-rostock.de Jan Oldenburg, Robert Ott, Michael Stiehm, Klaus-Peter by electrospinning Schmitz, and Stefan Siewert: Institute for ImplantTechnology and Biomaterials e.V., 18119 Polymer solutions of 4 to 5 wt.% were obtained by dissolving Rostock, Germany PLLA (RESOMER L210, Evonik, Germany) in a mixture of Sabine Illner, Klaus-Peter Schmitz, and Niels Grabow: chloroform and 2,2,2-trifluoroethanol (TFE) (1:4 v/v) at Institute for Biomedical Engineering, Rostock University Medical 37 °C. The 4 wt.% polymer solutions were loaded with Center, 18119 Rostock, Germany Open Access. © 2020 Swen Großmann et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License. Swen Großmann et al., Permeability and wettability of bioresorbable nanofiber nonwoven membranes — 2 12.5 wt.% of Triton X-100 (TX 100) or 1 wt.% of lecithin or 2.4 Water contact angle 1 wt.% of lecithin and 1 wt.% of tetraethylammonium chloride Measurements of the initial water contact angle were (TEAC), respectively. Here the concentrations are with respect performed by sessile drop method on the nonwoven polymer to the polymer weight. The 5 wt.% polymer solution was kept surface using a mobile surface analyser (MSA, KRÜSS pure. The process of electrospinning was performed using the GmbH, Hamburg, Germany) with ADVANCE 1.9.2 software. 4SPIN C4S LAB2 device (Contipro, Dolní Dobrouč, Czech The contact angles of water were determined against air, Republic) which includes a custom-built rotating continual whereby droplets compromising a volume of 2 µL were collector as well as a multi-jet capillary emitter with six or deposited and measured within one second. eight cannulas. All trials were performed at an emitter- collector distance of 15 cm, the applied voltage and the feed rate were chosen to be 18 to 30 kV and 100 μL/min or 220 μL/min, depending on the number of cannulas. The 3 Results and discussion spinning process was performed using ambient temperature of 22 °C ± 2 °C and relative humidity of 25 % ± 5 %. 3.1 Wetting the membranes 2.2 Electron microscopy To investigate the wetting by the test fluid a pressure gradient was applied to the mounted membranes. The corresponding The morphology of the fabricated nonwoven membranes was fluid velocities for the applied pressure gradients are plotted in investigated using a field emission scanning electron Figure 1 using pure water as test fluid. As the pressure gradient microscope (Quattro S, Thermo Fisher Scientific Inc., USA). increases, the fluid velocity suddenly changes its dependence The membranes were covered with a gold layer of a few on the pressure gradient. The observed changes occur nanometers (A2008, Agar Scientific Ltd, United Kingdom) to instantaneously and mark the points at which the accessible reduce charging during electron beam irradiation and area of the membranes is completely wetted, except for the subsequently investigated at identical magnification in low membrane including lecithin and TEAC. This structure could vacuum mode at 50 Pa with an acceleration voltage of 15 kV. not be wetted at all, resulting in zero fluid velocity for all applied pressure gradients. Figure 1 shows the observed -1 changes occurring at 73, 92, and 137 Pa·µm for membranes without additives, with TX 100 and with lecithin, respectively. 2.3 Permeability setup To investigate the origin of this pressure gradient dependent wetting, the water contact angle was measured for all The permeability was experimentally determined using a membranes (cf. Table 1). Within the obtained uncertainty the previously described setup [3]. In short, a hydraulic pump (Corio CD, JULABO GmbH, Germany) generates a constant PLLA L210 flow and simultaneously controls the temperature of the fluid. PLLA L210, TX 100 The pressure drop over the membrane is determined using a PLLA L210, lecithin PLLA L210, lecithin & TEAC pressure gauge (86A 3R–000000–005P G, Measurement Specialties Inc., USA) and the volumetric flow is measured using a Doppler sonography flow sensor (Leviflow LFS-008, Levitronix GmbH, Switzerland) in front of the sample chamber. The fluid temperature was set to 37 °C. As test fluid a mixture of deionized water and glycerol was chosen and the concentration was modified to achieve different dynamic viscosities. For correlation between viscosity and concentration, the viscosity was determined 0 100 200 300 400 500 -1 using a commercial rheometer (RheoStress 1, Thermo Fisher pressure gradient (Pa µm ) Scientific Inc., USA). For the evaluation of the membrane Figure 1: Fluid velocity during wetting of the membranes measured using pure water. After the complete wetting of the permeability, three glycerol concentrations 0 wt.%, 20 wt.%, membranes the slope of the fluid velocity changes for each and 40 wt.% were chosen, which correspond to a dynamic membrane. Point of complete wetting is indicated by the arrows. viscosity of 0.937 mPa·s, 1.397 mPa·s, and 2.401 mPa·s, The membrane containing lecithin and TEAC was not wetted for the applied pressure gradients. respectively. -1 fluid velocity (mm s ) Swen Großmann et al., Permeability and wettability of bioresorbable nanofiber nonwoven membranes — 3 calculating the permeability according to Equation 𝐮 = Table 1: Mean values of the water contact angle and fiber 𝒛 𝝁 𝝏𝒑 thickness of the investigated PLLA nanofiber membranes. − ∙ (1. The exact values of the permeability 𝜼 𝝏𝒛 can be found in Table 2. It should be noted that no permeability Polymer Contact angle Fiber thickness could be determined for the membranes containing lecithin additives (°) (nm) and TEAC as well as for the membrane with lecithin at a none 132.5 ± 13.5 1129 ± 375 viscosity of 1.397 mPa·s due to the lack of wetting. Apart from TX 100 12.7 ± 2.8 531 ± 116 a distinct difference for all types of membranes, the permeability tends to increase with increasing fluid viscosity lecithin 135.3 ± 1.9 325 ± 96 for each membrane. lecithin & TEAC 139.1 ± 72.8 165 ± 32 One possible reason for this effect could be that Darcy’s 𝝁 𝝏𝒑 contact angle does not differ for the investigated membranes, law (cf. Equation 𝐮 = − ∙ (1) is not valid 𝜼 𝝏𝒛 except for the membrane including the surfactant TX 100. for the investigated membranes using the present experimental Accordingly, the differences in the wettability are probably not parameters. Potentially due to turbulent flow within the correlated to the initial water contact angle. membranes during the entire measurement or because of Next the membranes morphology was analyzed by means deformation initiated by the applied pressure gradients also of electron microscopy. A clear difference in the fiber diameter causing turbulent flow or just a change in fiber fraction. and thus also in the effective interface roughness was found An indication of this is provided by the partly non-linear (cf. Error! Reference source not found.2). When comparing characteristic of the fluid velocities shown in Figure 3 a-c. the pressure gradients required to wet the membranes and the However, since the porosity of electrospun membranes as well averaged fiber diameter of the corresponding membrane, it as the fiber morphology can only be estimated, no clear becomes apparent that a change in the wetting at low pressure statement can be made whether laminar or turbulent flow is gradients is probably correlated to a large fiber diameter (cf. present in the conducted experiments. Table 1). Similar effects were recently found by comparing Nonetheless, Figure 3 d-f shows the thicknesses for the membranes consisting of different polymers with larger fiber investigated membranes. Within the existing uncertainties, no dimensions [4]. Note that, a smaller fiber diameter is irreversible change in thickness can be detected. Thus, the commonly associated with smaller vacancies between the observed non-linearity of the fluid velocity and the associated fibers. variation of the permeability for different viscosities is probably caused by a change in the thickness and morphology of the membranes during the measurement [5]. Table 2: Water permeability µ for the investigated membranes with respect to the dynamic viscosity η of the test fluid. µ (m²) Polymer η = 0.937 η = 1.397 η = 2.401 additives (mPa s) (mPa s) (mPa s) -14 -14 -14 none 23.98 ·10 81.02 ·10 95.80 ·10 -14 -14 -14 TX 100 1.90 ·10 3.51 ·10 13.35 ·10 -14 -14 lecithin 2.86 ·10 - 4.78 ·10 Figure 2: Electron micrograph sections of the investigated PLLA nanofiber nonwovens a without additives, b with TX 100, c with lecithin, and d with lecithin and TEAC. 4 Conclusion Within this study nanofiber membranes consisting of biodegradable PLLA nonwovens were investigated with 3.2 Permeability of the membranes respect to their wettability and water permeability. The nanofiber membranes showed a distinct correlation of the After wetting the membranes, the fluid velocity measurements pressure gradient needed for wetting and the fiber dimensions were repeated (cf. Figure 3 a-c). Subsequently, the slope of of the corresponding membrane [4], whereas no difference in each dataset was determined by linear regression followed by Swen Großmann et al., Permeability and wettability of bioresorbable nanofiber nonwoven membranes — 4 the initial water contact angle for the investigated membranes was observed. a b c PLLA L210 PLLA L210 0.937 mPa s PLLA L210 0.937 mPa s 0.937 mPa s without additives with TX 100 1.397 mPa s with lecithin 1.397 mPa s 1.397 mPa s 2.401 mPa s 2.401 mPa s 2.401 mPa s 0 0 0 0 50 100 150 0 50 100 150 200 250 300 0 50 100 150 200 -1 -1 -1 pressure gradient (Pa·µm ) pressure gradient (Pa·µm ) pressure gradient (Pa·µm ) before measurement after measurement d e f 150 150 100 100 50 50 0 0 0.937 1.397 2.401 0.937 1.397 2.401 0.937 1.397 2.401 η (mPa·s) η (mPa·s) η (mPa·s) Figure 3: Fluid velocity as a function of the applied pressure gradient for a PLLA L210 without additives, b with TX 100, and c with lecithin. The slope was determined by linear regression and used to calculate the permeability. For each type of membrane, the shown data includes the measurements for a fluid viscosity of 0.937 mPa·s, 1.397 mPa·s, and 2.401 mPa·s. Through the membrane consisting of PLLA L210 with lecithin and TEAC no fluid velocity could been measured. After performing the measurements no distinct variation in the thickness was found for the membranes made of PLLA d without additives, e with TX 100, and f with lecithin. Furthermore, it was shown that besides the different (BMBF) within RESPONSE "Partnership for Innovation in wettability there is also a clear variation in water permeability. Implant Technology" is gratefully acknowledged. Therefore, for the investigated electrospun membranes the Conflict of interest: Authors state no conflict of interest. permeability cannot be considered as a constant parameter according to Darcy's law. Since no irreversible change in thickness was observed, it is supposed that the membrane References changes its permeability during the presence of a pressure [1] Greiner A, Wendorff JH. Electrospinning: A Fascinating gradient. It has also been shown that the permeability varies Method for the Preparation of Ultrathin Fibers. Angewandte with fluid viscosity. Especially, for their degradability and Chemie International Edition 2007;46:5670–703. [2] Bakhsheshi-Rad HR, Akbari M, Ismail AF, Aziz M, Hadisi Z, when used as drug- and nanoparticle-delivery systems [1] or Pagan E, et al. Coating biodegradable magnesium alloys corrosion resistance coating [2], the wettability as well as the with electrospun poly-L-lactic acid-åkermanite-doxycycline viscosity-dependent permeability of the nonwovens are of nanofibers for enhanced biocompatibility, antibacterial decisive importance. Thus, the results presented here form the activity, and corrosion resistance. Surface and Coatings substantial basis for the development of new permeable and Technology 2019;377:124898. [3] Grossmann S, Siewert S, Ott R, Schmitz K-P, Kohse S, semi-permeable implants and medical devices consisting of Schmidt W, et al. Standardized technique of water biodegradable nanofiber membranes. permeability measurement for biomedical applications. Current Directions in Biomedical Engineering 2018;4:633–6. Author Statement [4] Szewczyk P, Ura D, Metwally S, Knapczyk-Korczak J, Gajek M, Marzec M, et al. Roughness and Fiber Fraction Research funding: Partial financial support by the European Dominated Wetting of Electrospun Fiber-Based Porous Regional Development Fund (ERDF) and the European Social Meshes. Polymers 2018;11:34. Fund (ESF) within the collaborative research between [5] Choong LT, Khan Z, Rutledge GC. Permeability of economy and science of the state Mecklenburg-Vorpommern electrospun fiber mats under hydraulic flow. Journal of and by the Federal Ministry of Education and Research Membrane Science 2014;451:111–6. -1 thickness (µm) fluid velocity ) (mm·s thickness (µm) -1 fluid velocity (mm·s ) thickness (µm) -1 fluid velocity (mm·s )

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Current Directions in Biomedical Engineering – de Gruyter

Published: Sep 1, 2020

Keywords: permeability; wettability; nanofiber; nonwoven; biodegradable; electrospinning; membrane

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