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All-soft multiaxial force sensor based on liquid metal for electronic skin

All-soft multiaxial force sensor based on liquid metal for electronic skin Electronic skin (E-skin) capable of detecting various physical stimuli is required for monitoring external environments accurately. Here, we report an all-soft multiaxial force sensor based on liquid metal microchannel array for electronic skin applications. The proposed sensor is composed of stretchable elastomer and Galinstan, a eutectic gallium-indium alloy, providing a high mechanical flexibility and electro-mechanical durability. Liquid metal microchannel arrays are fabricated in multilayer and positioned along a dome structure to detect multi-directional forces, supported by numerical simulation results. By adjusting the height of the dome, we could control the response of the multiaxial sensor with respect to the deflection. As a demonstration of multiaxial force sensing, we were able to monitor the direction of multidirectional forces using a finger by the response of liquid metal microchannel arrays. This research could be applied to various fields including soft robotics, wearable devices, and smart prosthetics for artificial intel- ligent skin applications. Keywords: Electronic skin, Force sensor, Multiaxial sensor, Liquid metal, 3D printing understanding of the dynamics at the interfaces, the Introduction monitoring of multidirectional forces is essential. Human skin can detect and understand various mechani- Currently, various multiaxial force sensors have been cal stimuli using multilayered and array structures of reported for E-skin applications using MEMS-based mechanoreceptors [1, 2]. Many researchers have recently metal strain gauges [14, 15]. However, these sensors are reported various types of electronics skins (E-skin) mim- limited by their rigid silicon substrate and their adapt- icking versatile functions of skins such as pressure sens- ability on the soft human skin. Thus, there have been ing [3–6], tactile sensing [7, 8], and temperature sensing multiaxial force sensors using conductive yarns [16, 17], [9]. To understand the external environments or stimuli carbon nanotubes [18, 19] and metal nanowires [20, 21] in an accurate manner, E-skin should perceive various with flexible substrates like polydimethylsiloxane (PDMS) mechanical information emulating human perception and Ecoflex. However, solid-state electronics employing and natural touch. Previously many flexible tactile sen - metal film, carbon nanotubes, etc. have a limited stretch - sors that solely measure normal [10, 11] or shear forces ability and signal drifting in their long-term use. Also, [12, 13] have been reported. At the interface of the skin, the signals are easily affected by external environmental however, not only normal force but also tangential force conditions like temperature and humidity, limiting their is experienced, resulting multiaxial forces to the skin. applications in real-life. For the dexterous manipulation of objects and thorough In this research, we introduce a liquid metal (LM)- based soft multiaxial force sensor for E-skin applications. Detection of multiaxial force was achieved by the imple- *Correspondence: inkyu@kaist.ac.kr mentation of multiple liquid microchannels with 3D Department of Mechanical Engineering, Korea Advanced Institute dome structure (Fig.  1a). Four LM microchannels were of Science and Technology (KAIST ), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea fabricated on the top elastomer layer and the other four Full list of author information is available at the end of the article © The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. Kim et al. Micro and Nano Syst Lett (2021) 9:2 Page 2 of 8 Fig. 1 Liquid metal-based multiaxial force sensor with dome structure. a Schematic design of the proposed sensor. b Images of the proposed sensor channels were made on the bottom layer in perpendicu- matrix for spatial detection of applied forces. Each lar direction, along the 3D dome structure for the detec- microchannel has a width of 500 μm and a thickness of tion of multidirectional forces (Fig.  1b). The responses 200  μm. Furthermore, the multilayer LM microchan- to the normal force with respect to the sensor substrate nels are shaped into the 3D dome structure. When were investigated for the adjustment of sensitivity. As the morphology of the sensor is a plane or in 2D, it is a demonstration, various multidirectional forces were difficult to detect the tangential loadings and cause a applied with the finger and its corresponding responses severe shear stress inside. On the other hand, the 3D were monitored. dome structure can translate the tangential loading to change of cross-sectional area of LM microchan- Design and working principle nel along the dome. Unlike previous multiaxial force The LM-based multiaxial force sensor is composed sensors with dome structure [22], the dome structure of two parts: LM microchannel array with multilayer does not have to be rigid our proposed sensor. Pre- structure and the dome structure. Multi arrays of liq- vious multiaxial sensors used a rigid half-spherical uid metal microchannels are designed for the dis- structure to deliver external forces to the strain gauges crimination of loading location. It is not possible to or sensing structures beneath it by torque induced understand the loaded point or region with only one by the tangential force. When LM microchannels are LM microchannel. Thus, four straight microchannels located along the surface of the dome, however, they on the top elastomer layer and another four micro- are directly compressed by the external forces. This channels on the bottom layer were designed. The enables the sensor to become all-soft structure that number of microchannel in a unit area means the spa- can conformally adapted to arbitrary substrates. tial resolution of the sensor. It is possible to measure All-soft liquid metal-based multiaxial force sen- more accurate signals from various loading angles with sor detects the forces by the change of resistances of more microchannels. In the E-skin application in our multi LM microchannel array. Galinstan, which is research, the area of interest is within 5  mm × 5  mm an eutectic alloy of gallium, indium, and tin attracts for the measurement of finger force. Since the mini- increasing attentions for its excellent mechanical and mum feature size of the microchannel using FDM 3D electrical properties as a component of stretchable elec- printing process is limited as 500  µm, the maximum tronic applications [23, 24]. Due to its intrinsic prop- number of microchannel was determined as four. Since erties as a liquid metal, the microchannel filled with the top channels and the bottom channels are per- Galinstan can easily deformed into 3D dome shape with- pendicular to each other, they comprise four by four out any mechanical or electrical failure. When there is K im et al. Micro and Nano Syst Lett (2021) 9:2 Page 3 of 8 an external force applied to the LM microchannel, the of the sensor would shift from the center to the cross-sectional area of the microchannel decreases, side, which can be visualized as in Fig.  2b: 1(θ = 0° resulting the increase of the resistance. The responses of ) → 2(θ = 15°) → 3(θ = 30°). The deformations of the each LM microchannel differ with respect to the loading dome structure with respect to the loading angle θ condition. were simulated using finite element method (FEM) Liquid metal microchannels of the sensor on the (Fig . 2b). bottom layer were labeled as × 1, x2, x3, and x4 and those on the top layer were as y1, y2, y3, and Fabrication process y4 (Fig.  2a). In cross-sectional view (x–z plane), The LM-based multiaxial force sensor is fabricated by multidirectional load could be applied to the sensor the following procedures (Fig.  3). First, a master mold with the angle θ. When the normal force is loaded is designed for the liquid metal microchannel using (θ = 0°), more force would be delivered to the those 3D CAD (Fusion 360, Autodesk). Four microchannels LM microchannels at the center: x2, x3, y2, and y3 are sharing a single reservoir structure at the end. The than those on the side. As θ increases, the response reservoir provides a room for the liquid metal injection Fig. 2 Working principle of the proposed sensor. a Multi-layer liquid metal microchannels in x (x1, x2, x3, and x4) and y (y1, y2, y3, and y4) direction and different loading directions in x(y)-z plane. Different responses of the sensor array with respect to the loading direction (①, ②, and ③). b Finite element analysis (FEA) simulation results showing the corresponding deformations of the liquid metal microchannels Kim et al. Micro and Nano Syst Lett (2021) 9:2 Page 4 of 8 Fig. 3 Fabrication process of the proposed sensor. a 3D Printing of microchannel mold. b Placement of the 3D-printed PVA mold on the elastomer film. c Embedment of the mold by spin coating and curing. d Placement of the 2nd mold on the top. e Embedment of the mold. f Removal of the mold. g Filling of liquid metal inside microchannel using syringe injection or vacuum filling. h Fabrication of dome structure using the vacuum chamber. i Curing of additional elastomer for the fixation of dome structure. j Fabrication of the proposed sensor and stores remaining liquid metals after filling the microchannels using a syringe injection or vacuum fill - microchannel. The mold is 3D-printed with poly(vinyl ing method (Fig.  3g). 3D dome structure is fabricated alcohol) (PVA) filament which is a water-soluble mate - with a vacuum chamber. Vacuum was applied to the rial (Fig.  3a). The dimension of the microchannel mold crossing area of the liquid metal-filled plane sensor at is 500 μm width and 200 μm thickness which is a mini- the hole with a diameter of 5  mm (Fig.  3h). The height mum feature size of fused deposition modeling (FDM) of the dome was controlled by the amplitude of applied 3D printing. The 3D printed mold is positioned on the pressure. When the thickness of the elastomer was elastomer sheet (Fig.  3b). Due to their intrinsic stic- 0.8  mm, the height of the dome (H) was 0.9  mm with tion between the mold and the elastomer, the mold is Δp = −10  kPa and H = 2.2  mm with Δp = −30  kPa. To not move during the spin coating process. The mold fix the shape of the dome, an additional elastomer was is then embedded into the elastomer (Dragonskin10, filled and cured on the opposite side of the dome, fill - Smooth-on) by spin coating (300  rpm, 60  s), followed ing the concave hills (Fig. 3i). After the curing, the pres- by the additional mold embedment (Fig.  3c). The mold sure is released (Fig.  3j). Electric wires are then directly on the top layer is aligned perpendicular to that on the inserted into each reservoir of the channels for the inte- bottom so that the microchannels on the top and the gration with the circuit system. bottom can cross each other in a perpendicular direc- tion (Fig.  3d–e). Then, the PVA molds are dissolved by Performance characterization injection of water with at the temperature of 40–50  °C To investigate the response of the multiaxial force sen- for the fast removal (Fig.  3f ) after the PVA removal, sor, three-axial load cell (SM-50  N, CAS Korea) was the elastomer with empty microchannels is dried at implemented for the monitoring of force applied to the 50 °C for 2 h. Then, Galinstan is injected into the empty sensor in x, y, and z direction and the linear stage with a K im et al. Micro and Nano Syst Lett (2021) 9:2 Page 5 of 8 Fig. 4 Performance characterization. a Experimental setup. b Schematic view of the sensor. (x1–x4: labels for each LM microchannel, H: height of the dome, and D: diameter of the dome) c Response of the sensor with the dome height (H) of 0.9 mm to normal directional force. d Real-time response to various loadings e Response of the sensor when H is 2.2 mm. f Sensitivity to the displacement according to the height of the dome Kim et al. Micro and Nano Syst Lett (2021) 9:2 Page 6 of 8 loading tip for displacement-controlled loading (Fig. 4a) These characteristics result in the difference in sensi - Each LM microchannel was connected to 8-channel tivity the sensitivity of the multiaxial sensor regarding ADC reader (ADS1114, Texas Instruments) installed with respect to the dome height and diameter. In addi- on Arduino DUE (Arduino). LM microchannels were tion, the diameter of the dome would also affect the gap labeled as x1, x2, x3, and x4 as shown in the Fig.  4b. among the liquid metal channels, although not covered At the initial state (F = 0), the voltage is approximately in our study. 20  mV and its corresponding resistance is about 0.4 Ω The characteristics of the developed sensor were owing to the high electrical conductivity of liquid metal. investigated on the flat surfaces, where accurate control Thanks to the voltage dividing circuit and high resolu - of the direction and magnitude of the force is possible. tion ADC reader, the small change in voltage could be On the other hand, when the sensor is located on the monitored. curved or wavy surfaces, the trend or behaviour of the In the dome sensor, the positions of the inner LM sensor would be different because the microchannels microchannel pair and the outer pair are geometrically are deformed, requiring an additional calibration of the symmetric, respectively. (x2 and x3: the inner pair, x1 initial state. The characterization of the sensor on differ - and x4: the outer pair). Therefore, the sensor responses of ent surface geometry is beyond the scope of this study each pair under the applied normal force showed a simi- and it is left as the future work for the practical E-skin lar tendency. When a normal force of 1.2 N was applied, applications. the relative voltage changes (ΔV/V ) of the inner pair increased to 12.1 and 15.8, respectively. On the other Applications: finger loading hand, the responses of x1 and x4 of the outer pair were To demonstrate the feasibility of the proposed sensor, we only 0.4 and 0.8, respectively (Fig.  4c). It means that the applied various directional forces using a finger (Fig.  5a). developed multiaxial force sensors can distinguish the Various directional forces (F –F ) were consequently 1 4 direction of an applied force by utilizing the difference applied to the sensor; F : (−x, 0, −z), F : (+x, 0, −z), F : 1 2 3 position of LM microchannel by the change of dome (0, +y, −z), and F : (0, −y, −z) (Fig. 5b). To visualize the geometry. In addition, the responses of the sensors were direction and amplitude of loaded force, we multiplied recovered to their initial values after various loading the relative voltage changes of the top and the bottom cycles owing to the excellent stability of the liquid metal array and implemented 4 × 4 matrix array with 3D color- and the structure of the sensor (Fig.  4d). Meanwhile, map (MATLAB, Mathworks). The LM microchannels when the height of the dome (H) increased to 2.2  mm, can be divided into two groups: the top and the bottom the outer sensor pair shows slightly different responses: layer. There are four LM microchannels on the top layer; the response of x1 began to increase after 0.5  N, while x1, x2, x3, and x4, and four on the bottom; y1, y2, y3, and that of x4 increased after 1 N (Fig. 4e). This is due to the y4. Each component has its own response, or relative different gap between the LM microchannels along the voltage change (X = ΔV /V and Y = ΔV /V ) and i xi xi0 i yi yi0 dome. can be summarized as below. Force–displacement curves depending on the vari- ous height of the dome were measured to verify the X = (X , X , X , X ) sensitivity of the sensor. When H increases, less force 1 2 3 4 is applied with the same displacement, and the sensi- tivity of the sensor decreases (Fig.  4f ). For example, to Y = (Y , Y , Y , Y ) 1 2 3 4 apply 1  N of normal force, 200  µm of loading displace- ment is required for the dome height of H = 0.9  mm, S = Y ∗ X. while 900 µm of displacement is needed for H = 2.2 mm. When the force applied to the sensor without a dome To detect the direction of loading, the responses of LM structure (H = 0  mm), the response dramatically microchannels of the top and the bottom are multiplied increases with the loading displacement. As H increases, to build a 4 × 4 matrix (S). We were able to detect the larger displacement is required for the loading of force. loading position and their relative magnitude of the force Also, the dome with larger H provides larger radial dis- as shown in the Fig. 5c–f. placement among the adjacent channels, which leads to different response behavior of the sensor array. K im et al. Micro and Nano Syst Lett (2021) 9:2 Page 7 of 8 Fig. 5 Application: finger force detection. a Image of a finger applying the force. b Various loading condition. c–f Responses to the various loading forces Conclusion sensor using liquid metal microchannel array with a In this research, we introduce a multidirectional force dome structure. LM microchannels were fabricated using Kim et al. Micro and Nano Syst Lett (2021) 9:2 Page 8 of 8 organic solar cell. Nano Energy 74:104749. https ://doi.org/10.1016/j. 3D-printed molding. Since the microchannel arrays were nanoe n.2020.10474 9 located along the dome structure, which is in three- 7. James JW, Pestell N, Lepora NF (2018) Slip detection with a biomi- dimensional shape, various directional forces could be metic tactile sensor. IEEE Robot Autom Lett 3:3340–3346. https ://doi. org/10.1109/LRA.2018.28527 97 distinguished with high force sensitivity. The sensor 8. Wan Y, Qiu Z, Hong Y et al (2018) A highly sensitive flexible capacitive showed high signal recovery properties owing to intrin- tactile sensor with sparse and high-aspect-ratio microstructures. Adv sic electromechanical properties of liquid metal with Electron Mater 4:1–8. https ://doi.org/10.1002/aelm.20170 0586 9. Yamamoto Y, Yamamoto D, Takada M et al (2017) Efficient skin tempera- high flexibility. The various directional forces using force ture sensor and stable gel-less sticky ECG sensor for a wearable flexible were distinguished by the implication of sensor array and healthcare patch. Adv Healthc Mater 6:1–7. https ://doi.org/10.1002/ corresponding 3D color map. We expect this technology adhm.20170 0495 10. Kim K, Park J, Suh J, Hoon SJ et al (2017) 3D printing of multiaxial force could be used in various soft material-based applications sensors using carbon nanotube (CNT )/thermoplastic polyurethane including electronic skin, soft robotics, and wearable ( TPU) filaments. Sensors Actuators A Phys 263:493–500. https ://doi. devices. org/10.1016/j.sna.2017.07.020 11. Cho C, Ryuh Y (2016) Fabrication of flexible tactile force sensor using con- Acknowledgements ductive ink and silicon elastomer. Sensors Actuators A Phys 237:72–80. Not applicable. https ://doi.org/10.1016/j.sna.2015.10.051 12. Lee YR, Chung J, Oh Y, Cha Y (2019) Flexible shear and normal force sen- Authors’ contributions sor using only one layer of polyvinylidene fluoride film. Appl Sci. https :// KK designed the principle of the sensor, performed experiments, analyzed the doi.org/10.3390/app92 04339 data, and wrote the paper. JA performed the FEA simulation, JC fabricated the 13. Yin J, Santos VJ, Posner JD (2017) Bioinspired flexible microfluidic shear dome structure, YJ constructed the experimental system to characterize the force sensor skin. Sensors Actuators A Phys 264:289–297. https ://doi. multiaxial force sensor, and OG conducted the experiments. IP led the overall org/10.1016/j.sna.2017.08.001 direction of the project and wrote the paper. All authors have given their 14. Matich S, Hessinger M, Kupnik M et al (2017) Miniaturized multiaxial approval to the final version of the manuscript. All authors read and approved force/torque sensor with a rollable hexapod structure: Miniaturisierter the final manuscript. Kraft-Momenten-Sensor auf Basis einer gerollten Hexapod-Struktur. Tech Mess 84:S138–S142. https ://doi.org/10.1515/teme-2017-0046 Funding 15. Zhao Y, Zhao Y, Ge X (2018) The development of a triaxial cutting force This work was supported by a National Research Foundation of Korea (NRF) sensor based on a MEMS strain gauge. Micromachines. https ://doi. grant funded by the Korean government (MSIT ) (No. 2018R1A2B2004910). org/10.3390/mi901 0030 16. You X, He J, Nan N et al (2018) Stretchable capacitive fabric electronic skin Availability of data and materials woven by electrospun nanofiber coated yarns for detecting tactile and All data generated or analyzed during this study are included in this published. multimodal mechanical stimuli. J Mater Chem C 6:12981–12991. https :// doi.org/10.1039/C8TC0 3631D Competing interests 17. Gong S, Wang Y, Yap LW et al (2018) A location- and sharpness-specific The authors declare that they have no competing interests. tactile electronic skin based on staircase-like nanowire patches. Nanoscale Horizons 3:640–647. https ://doi.org/10.1039/c8nh0 0125a Author details 18. Sun X, Sun J, Li T et al (2019) Flexible tactile electronic skin sensor with Department of Mechanical Engineering, Korea Advanced Institute of Science 3D force detection based on porous CNTs/PDMS nanocomposites. Nano and Technology (KAIST ), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic Micro Lett 11:1–14. https ://doi.org/10.1007/s4082 0-019-0288-7 of Korea. Korea Institute of Machinery and Materials (KIMM), 156 Gajeong- 19. Kwon SN, Kim SW, Kim IG et al (2019) Direct 3D printing of graphene buk-Ro Yuseong, Gu Daejeon, Korea. nanoplatelet/silver nanoparticle-based nanocomposites for multiaxial piezoresistive sensor applications. Adv Mater Technol 4:1–9. https ://doi. Received: 2 August 2020 Accepted: 1 December 2020 org/10.1002/admt.20180 0500 20. Wang S, Li Q, Wang B et al (2019) Recognition of different rough surface based highly sensitive silver nanowire-graphene flexible hydrogel skin. Ind Eng Chem Res 58:21553–21561. https ://doi.org/10.1021/acs.iecr.9b049 47 21. Peng S, Wu S, Yu Y et al (2020) Multimodal capacitive and piezoresistive References sensor for simultaneous measurement of multiple forces. ACS Appl Mater 1. Dargahi J, Najarian S (2004) Human tactile perception as a standard for Interfaces 12:22179–22190. https ://doi.org/10.1021/acsam i.0c044 48 artificial tactile sensing-a review. Int J Med Robot Comput Assist Surg 22. Ji B, Zhou Q, Wu J et al (2020) Synergistic optimization toward the sensi- 01:23. https ://doi.org/10.1581/mrcas .2004.01010 9 tivity and linearity of flexible pressure sensor via double conductive layer 2. Hao J, Bonnet C, Amsalem M et al (2014) Transduction and encoding sen- and porous microdome array. ACS Appl Mater Interfaces. https ://doi. sory information by skin mechanoreceptors. Pflugers Arch Eur J Physiol org/10.1021/acsam i.0c089 10 467:109–119. https ://doi.org/10.1007/s0042 4-014-1651-7 23. Yang J, Tang D, Ao J et al (2020) Ultrasoft liquid metal elastomer foams 3. Kim K, Choi J, Jeong Y et al (2019) Highly sensitive and wearable liquid with positive and negative piezopermittivity for tactile sensing. Adv Funct metal-based pressure sensor for health monitoring applications: inte- Mater 2002611:1–10. https ://doi.org/10.1002/adfm.20200 2611 gration of a 3d-printed microbump array with the microchannel. Adv 24. Kim K, Choi J, Jeong Y, et al (2019) Strain-Insensitive Soft Pressure Sensor Healthc Mater 8:1–10. https ://doi.org/10.1002/adhm.20190 0978 for Health Monitoring Application Using 3D-Printed Microchannel Mold 4. Kim S, Amjadi M, Lee TI et al (2019) Wearable, ultrawide-range, and bend- and Liquid Metal. in 2019 20th International Conference on Solid-State ing-insensitive pressure sensor based on carbon nanotube network- Sensors, Actuators and Microsystems & Eurosensors XXXIII ( TRANSDUC- coated porous elastomer sponges for human interface and healthcare ERS & EUROSENSORS XXXIII). 2535–2538 devices. ACS Appl Mater Interfaces. https ://doi.org/10.1021/acsam i.9b076 Publisher’s Note 5. Choi J, Kwon D, Kim K et al (2020) Synergetic effect of porous elastomer Springer Nature remains neutral with regard to jurisdictional claims in pub- and percolation of carbon nanotube filler toward high performance lished maps and institutional affiliations. capacitive pressure sensors. ACS Appl Mater Interfaces 12:1698–1706. https ://doi.org/10.1021/acsam i.9b200 97 6. Choi J, Kwon D, Kim B et al (2020) Wearable self-powered pressure sen- sor by integration of piezo-transmittance microporous elastomer with http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Micro and Nano Systems Letters Springer Journals

All-soft multiaxial force sensor based on liquid metal for electronic skin

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References (3)

  • K Kim (2019)

    1

    Adv Healthc Mater, 8

  • J Dargahi (2004)

    23

    Int J Med Robot Comput Assist Surg, 01

  • J Hao (2014)

    109

    Pflugers Arch Eur J Physiol, 467

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Springer Journals
Copyright
Copyright © The Author(s) 2021
eISSN
2213-9621
DOI
10.1186/s40486-020-00126-9
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Abstract

Electronic skin (E-skin) capable of detecting various physical stimuli is required for monitoring external environments accurately. Here, we report an all-soft multiaxial force sensor based on liquid metal microchannel array for electronic skin applications. The proposed sensor is composed of stretchable elastomer and Galinstan, a eutectic gallium-indium alloy, providing a high mechanical flexibility and electro-mechanical durability. Liquid metal microchannel arrays are fabricated in multilayer and positioned along a dome structure to detect multi-directional forces, supported by numerical simulation results. By adjusting the height of the dome, we could control the response of the multiaxial sensor with respect to the deflection. As a demonstration of multiaxial force sensing, we were able to monitor the direction of multidirectional forces using a finger by the response of liquid metal microchannel arrays. This research could be applied to various fields including soft robotics, wearable devices, and smart prosthetics for artificial intel- ligent skin applications. Keywords: Electronic skin, Force sensor, Multiaxial sensor, Liquid metal, 3D printing understanding of the dynamics at the interfaces, the Introduction monitoring of multidirectional forces is essential. Human skin can detect and understand various mechani- Currently, various multiaxial force sensors have been cal stimuli using multilayered and array structures of reported for E-skin applications using MEMS-based mechanoreceptors [1, 2]. Many researchers have recently metal strain gauges [14, 15]. However, these sensors are reported various types of electronics skins (E-skin) mim- limited by their rigid silicon substrate and their adapt- icking versatile functions of skins such as pressure sens- ability on the soft human skin. Thus, there have been ing [3–6], tactile sensing [7, 8], and temperature sensing multiaxial force sensors using conductive yarns [16, 17], [9]. To understand the external environments or stimuli carbon nanotubes [18, 19] and metal nanowires [20, 21] in an accurate manner, E-skin should perceive various with flexible substrates like polydimethylsiloxane (PDMS) mechanical information emulating human perception and Ecoflex. However, solid-state electronics employing and natural touch. Previously many flexible tactile sen - metal film, carbon nanotubes, etc. have a limited stretch - sors that solely measure normal [10, 11] or shear forces ability and signal drifting in their long-term use. Also, [12, 13] have been reported. At the interface of the skin, the signals are easily affected by external environmental however, not only normal force but also tangential force conditions like temperature and humidity, limiting their is experienced, resulting multiaxial forces to the skin. applications in real-life. For the dexterous manipulation of objects and thorough In this research, we introduce a liquid metal (LM)- based soft multiaxial force sensor for E-skin applications. Detection of multiaxial force was achieved by the imple- *Correspondence: inkyu@kaist.ac.kr mentation of multiple liquid microchannels with 3D Department of Mechanical Engineering, Korea Advanced Institute dome structure (Fig.  1a). Four LM microchannels were of Science and Technology (KAIST ), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea fabricated on the top elastomer layer and the other four Full list of author information is available at the end of the article © The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. Kim et al. Micro and Nano Syst Lett (2021) 9:2 Page 2 of 8 Fig. 1 Liquid metal-based multiaxial force sensor with dome structure. a Schematic design of the proposed sensor. b Images of the proposed sensor channels were made on the bottom layer in perpendicu- matrix for spatial detection of applied forces. Each lar direction, along the 3D dome structure for the detec- microchannel has a width of 500 μm and a thickness of tion of multidirectional forces (Fig.  1b). The responses 200  μm. Furthermore, the multilayer LM microchan- to the normal force with respect to the sensor substrate nels are shaped into the 3D dome structure. When were investigated for the adjustment of sensitivity. As the morphology of the sensor is a plane or in 2D, it is a demonstration, various multidirectional forces were difficult to detect the tangential loadings and cause a applied with the finger and its corresponding responses severe shear stress inside. On the other hand, the 3D were monitored. dome structure can translate the tangential loading to change of cross-sectional area of LM microchan- Design and working principle nel along the dome. Unlike previous multiaxial force The LM-based multiaxial force sensor is composed sensors with dome structure [22], the dome structure of two parts: LM microchannel array with multilayer does not have to be rigid our proposed sensor. Pre- structure and the dome structure. Multi arrays of liq- vious multiaxial sensors used a rigid half-spherical uid metal microchannels are designed for the dis- structure to deliver external forces to the strain gauges crimination of loading location. It is not possible to or sensing structures beneath it by torque induced understand the loaded point or region with only one by the tangential force. When LM microchannels are LM microchannel. Thus, four straight microchannels located along the surface of the dome, however, they on the top elastomer layer and another four micro- are directly compressed by the external forces. This channels on the bottom layer were designed. The enables the sensor to become all-soft structure that number of microchannel in a unit area means the spa- can conformally adapted to arbitrary substrates. tial resolution of the sensor. It is possible to measure All-soft liquid metal-based multiaxial force sen- more accurate signals from various loading angles with sor detects the forces by the change of resistances of more microchannels. In the E-skin application in our multi LM microchannel array. Galinstan, which is research, the area of interest is within 5  mm × 5  mm an eutectic alloy of gallium, indium, and tin attracts for the measurement of finger force. Since the mini- increasing attentions for its excellent mechanical and mum feature size of the microchannel using FDM 3D electrical properties as a component of stretchable elec- printing process is limited as 500  µm, the maximum tronic applications [23, 24]. Due to its intrinsic prop- number of microchannel was determined as four. Since erties as a liquid metal, the microchannel filled with the top channels and the bottom channels are per- Galinstan can easily deformed into 3D dome shape with- pendicular to each other, they comprise four by four out any mechanical or electrical failure. When there is K im et al. Micro and Nano Syst Lett (2021) 9:2 Page 3 of 8 an external force applied to the LM microchannel, the of the sensor would shift from the center to the cross-sectional area of the microchannel decreases, side, which can be visualized as in Fig.  2b: 1(θ = 0° resulting the increase of the resistance. The responses of ) → 2(θ = 15°) → 3(θ = 30°). The deformations of the each LM microchannel differ with respect to the loading dome structure with respect to the loading angle θ condition. were simulated using finite element method (FEM) Liquid metal microchannels of the sensor on the (Fig . 2b). bottom layer were labeled as × 1, x2, x3, and x4 and those on the top layer were as y1, y2, y3, and Fabrication process y4 (Fig.  2a). In cross-sectional view (x–z plane), The LM-based multiaxial force sensor is fabricated by multidirectional load could be applied to the sensor the following procedures (Fig.  3). First, a master mold with the angle θ. When the normal force is loaded is designed for the liquid metal microchannel using (θ = 0°), more force would be delivered to the those 3D CAD (Fusion 360, Autodesk). Four microchannels LM microchannels at the center: x2, x3, y2, and y3 are sharing a single reservoir structure at the end. The than those on the side. As θ increases, the response reservoir provides a room for the liquid metal injection Fig. 2 Working principle of the proposed sensor. a Multi-layer liquid metal microchannels in x (x1, x2, x3, and x4) and y (y1, y2, y3, and y4) direction and different loading directions in x(y)-z plane. Different responses of the sensor array with respect to the loading direction (①, ②, and ③). b Finite element analysis (FEA) simulation results showing the corresponding deformations of the liquid metal microchannels Kim et al. Micro and Nano Syst Lett (2021) 9:2 Page 4 of 8 Fig. 3 Fabrication process of the proposed sensor. a 3D Printing of microchannel mold. b Placement of the 3D-printed PVA mold on the elastomer film. c Embedment of the mold by spin coating and curing. d Placement of the 2nd mold on the top. e Embedment of the mold. f Removal of the mold. g Filling of liquid metal inside microchannel using syringe injection or vacuum filling. h Fabrication of dome structure using the vacuum chamber. i Curing of additional elastomer for the fixation of dome structure. j Fabrication of the proposed sensor and stores remaining liquid metals after filling the microchannels using a syringe injection or vacuum fill - microchannel. The mold is 3D-printed with poly(vinyl ing method (Fig.  3g). 3D dome structure is fabricated alcohol) (PVA) filament which is a water-soluble mate - with a vacuum chamber. Vacuum was applied to the rial (Fig.  3a). The dimension of the microchannel mold crossing area of the liquid metal-filled plane sensor at is 500 μm width and 200 μm thickness which is a mini- the hole with a diameter of 5  mm (Fig.  3h). The height mum feature size of fused deposition modeling (FDM) of the dome was controlled by the amplitude of applied 3D printing. The 3D printed mold is positioned on the pressure. When the thickness of the elastomer was elastomer sheet (Fig.  3b). Due to their intrinsic stic- 0.8  mm, the height of the dome (H) was 0.9  mm with tion between the mold and the elastomer, the mold is Δp = −10  kPa and H = 2.2  mm with Δp = −30  kPa. To not move during the spin coating process. The mold fix the shape of the dome, an additional elastomer was is then embedded into the elastomer (Dragonskin10, filled and cured on the opposite side of the dome, fill - Smooth-on) by spin coating (300  rpm, 60  s), followed ing the concave hills (Fig. 3i). After the curing, the pres- by the additional mold embedment (Fig.  3c). The mold sure is released (Fig.  3j). Electric wires are then directly on the top layer is aligned perpendicular to that on the inserted into each reservoir of the channels for the inte- bottom so that the microchannels on the top and the gration with the circuit system. bottom can cross each other in a perpendicular direc- tion (Fig.  3d–e). Then, the PVA molds are dissolved by Performance characterization injection of water with at the temperature of 40–50  °C To investigate the response of the multiaxial force sen- for the fast removal (Fig.  3f ) after the PVA removal, sor, three-axial load cell (SM-50  N, CAS Korea) was the elastomer with empty microchannels is dried at implemented for the monitoring of force applied to the 50 °C for 2 h. Then, Galinstan is injected into the empty sensor in x, y, and z direction and the linear stage with a K im et al. Micro and Nano Syst Lett (2021) 9:2 Page 5 of 8 Fig. 4 Performance characterization. a Experimental setup. b Schematic view of the sensor. (x1–x4: labels for each LM microchannel, H: height of the dome, and D: diameter of the dome) c Response of the sensor with the dome height (H) of 0.9 mm to normal directional force. d Real-time response to various loadings e Response of the sensor when H is 2.2 mm. f Sensitivity to the displacement according to the height of the dome Kim et al. Micro and Nano Syst Lett (2021) 9:2 Page 6 of 8 loading tip for displacement-controlled loading (Fig. 4a) These characteristics result in the difference in sensi - Each LM microchannel was connected to 8-channel tivity the sensitivity of the multiaxial sensor regarding ADC reader (ADS1114, Texas Instruments) installed with respect to the dome height and diameter. In addi- on Arduino DUE (Arduino). LM microchannels were tion, the diameter of the dome would also affect the gap labeled as x1, x2, x3, and x4 as shown in the Fig.  4b. among the liquid metal channels, although not covered At the initial state (F = 0), the voltage is approximately in our study. 20  mV and its corresponding resistance is about 0.4 Ω The characteristics of the developed sensor were owing to the high electrical conductivity of liquid metal. investigated on the flat surfaces, where accurate control Thanks to the voltage dividing circuit and high resolu - of the direction and magnitude of the force is possible. tion ADC reader, the small change in voltage could be On the other hand, when the sensor is located on the monitored. curved or wavy surfaces, the trend or behaviour of the In the dome sensor, the positions of the inner LM sensor would be different because the microchannels microchannel pair and the outer pair are geometrically are deformed, requiring an additional calibration of the symmetric, respectively. (x2 and x3: the inner pair, x1 initial state. The characterization of the sensor on differ - and x4: the outer pair). Therefore, the sensor responses of ent surface geometry is beyond the scope of this study each pair under the applied normal force showed a simi- and it is left as the future work for the practical E-skin lar tendency. When a normal force of 1.2 N was applied, applications. the relative voltage changes (ΔV/V ) of the inner pair increased to 12.1 and 15.8, respectively. On the other Applications: finger loading hand, the responses of x1 and x4 of the outer pair were To demonstrate the feasibility of the proposed sensor, we only 0.4 and 0.8, respectively (Fig.  4c). It means that the applied various directional forces using a finger (Fig.  5a). developed multiaxial force sensors can distinguish the Various directional forces (F –F ) were consequently 1 4 direction of an applied force by utilizing the difference applied to the sensor; F : (−x, 0, −z), F : (+x, 0, −z), F : 1 2 3 position of LM microchannel by the change of dome (0, +y, −z), and F : (0, −y, −z) (Fig. 5b). To visualize the geometry. In addition, the responses of the sensors were direction and amplitude of loaded force, we multiplied recovered to their initial values after various loading the relative voltage changes of the top and the bottom cycles owing to the excellent stability of the liquid metal array and implemented 4 × 4 matrix array with 3D color- and the structure of the sensor (Fig.  4d). Meanwhile, map (MATLAB, Mathworks). The LM microchannels when the height of the dome (H) increased to 2.2  mm, can be divided into two groups: the top and the bottom the outer sensor pair shows slightly different responses: layer. There are four LM microchannels on the top layer; the response of x1 began to increase after 0.5  N, while x1, x2, x3, and x4, and four on the bottom; y1, y2, y3, and that of x4 increased after 1 N (Fig. 4e). This is due to the y4. Each component has its own response, or relative different gap between the LM microchannels along the voltage change (X = ΔV /V and Y = ΔV /V ) and i xi xi0 i yi yi0 dome. can be summarized as below. Force–displacement curves depending on the vari- ous height of the dome were measured to verify the X = (X , X , X , X ) sensitivity of the sensor. When H increases, less force 1 2 3 4 is applied with the same displacement, and the sensi- tivity of the sensor decreases (Fig.  4f ). For example, to Y = (Y , Y , Y , Y ) 1 2 3 4 apply 1  N of normal force, 200  µm of loading displace- ment is required for the dome height of H = 0.9  mm, S = Y ∗ X. while 900 µm of displacement is needed for H = 2.2 mm. When the force applied to the sensor without a dome To detect the direction of loading, the responses of LM structure (H = 0  mm), the response dramatically microchannels of the top and the bottom are multiplied increases with the loading displacement. As H increases, to build a 4 × 4 matrix (S). We were able to detect the larger displacement is required for the loading of force. loading position and their relative magnitude of the force Also, the dome with larger H provides larger radial dis- as shown in the Fig. 5c–f. placement among the adjacent channels, which leads to different response behavior of the sensor array. K im et al. Micro and Nano Syst Lett (2021) 9:2 Page 7 of 8 Fig. 5 Application: finger force detection. a Image of a finger applying the force. b Various loading condition. c–f Responses to the various loading forces Conclusion sensor using liquid metal microchannel array with a In this research, we introduce a multidirectional force dome structure. LM microchannels were fabricated using Kim et al. Micro and Nano Syst Lett (2021) 9:2 Page 8 of 8 organic solar cell. Nano Energy 74:104749. https ://doi.org/10.1016/j. 3D-printed molding. Since the microchannel arrays were nanoe n.2020.10474 9 located along the dome structure, which is in three- 7. James JW, Pestell N, Lepora NF (2018) Slip detection with a biomi- dimensional shape, various directional forces could be metic tactile sensor. IEEE Robot Autom Lett 3:3340–3346. https ://doi. org/10.1109/LRA.2018.28527 97 distinguished with high force sensitivity. The sensor 8. 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IP led the overall org/10.1016/j.sna.2017.08.001 direction of the project and wrote the paper. All authors have given their 14. Matich S, Hessinger M, Kupnik M et al (2017) Miniaturized multiaxial approval to the final version of the manuscript. All authors read and approved force/torque sensor with a rollable hexapod structure: Miniaturisierter the final manuscript. Kraft-Momenten-Sensor auf Basis einer gerollten Hexapod-Struktur. Tech Mess 84:S138–S142. https ://doi.org/10.1515/teme-2017-0046 Funding 15. Zhao Y, Zhao Y, Ge X (2018) The development of a triaxial cutting force This work was supported by a National Research Foundation of Korea (NRF) sensor based on a MEMS strain gauge. Micromachines. https ://doi. grant funded by the Korean government (MSIT ) (No. 2018R1A2B2004910). org/10.3390/mi901 0030 16. 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Published: Jan 4, 2021

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