Masked Deposition of Gallium-Indium Alloys for Liquid- Embedded Elastomer Conductors

Home , Galinstan

www.afm-journal.de www.MaterialsViews.com Masked Deposition of Gallium-Indium Alloys for Liquid- Embedded Elastomer Conductors

Rebecca K. Kramer , * Carmel Majidi , and Robert J. Wood FULL PAPER

stretchable radio frequency (RF) radiation A fabrication method is introduced that utilizes masked deposition and selec- sensor,[ 6 ] wearable tactile interfaces,[ 7,8 ] and tive wetting to produce hyperelastic electronic circuits that are composed of a joint angle sensor that matches the nat- [9] a thin elastomer fi lm embedded with microchannels of liquid-phase gallium- ural elasticity of human skin. As with so- called “wavy” circuits and other stretchable indium (Ga-In) alloy. This method exploits the low melting-point and control- electronics,[ 10,11 ] elastomers embedded lable wetting dynamics of Ga-In alloys, as well as the ability for Ga-In alloys with microchannels of liquid-phase Ga-In to form irregularly-shaped, free-standing, micrometer-scale structures via alloy will continue to have a central role in gallium surface oxidation. Masked deposition eliminates the need for manual assistive wearable technologies that match injection fi lling, which enables certain geometries that cannot be produced the elasticity of human tissue and preserve natural ranges of motion. by injection and allows for the automated, high-volume production of Ga-In Liquid-embedded elastomer electronics based “liquid-embedded elastomer electronics” (LE3). With this approach, LE3 (LE3) are currently produced by casting a circuits can be produced with isolated features that have planar dimensions of low-modulus silicone elastomer (elastic less than 200 μ m and edge-to-edge feature separations as small as 25 μ m. modulus, E ≈ 0.1–1 MPa) in a micro- machined mold, sealing the molded features with an additional layer of elastomer, and then fi lling the embedded channels with Ga-In alloy using a needle and syringe. While reliable, this manual method of fabri- 1. Introduction cation is labor intensive, geometry limiting, and not readily scalable to automated high-volume manufacturing. In this letter, we Liquid-phase gallium-indium (Ga-In) alloys, such as galinstan introduce an alternative fabrication process (seen in Figure 1 ) (68.5% Ga, 21.5% In, 10% Tn, by weight) are non-toxic, low vis- − that eliminates needle injection and instead employs masked cosity (η = 2.4 × 10 3 Pa s) fl uids that are highly conductive (σ = patterning and deposition methods that are commonly used in 3.40 × 106 S/m) and have a thin oxide skin that allows them fl ex-circuit and soft lithography manufacturing. This fabrication to form stable free-standing structures.[ 1,2 ] Fluidic microchan- method exploits the thin oxide skin formed by droplets of Ga-In nels of Ga-In alloy embedded in an elastomer fi lm function alloy, a feature that allows the alloy to form stable, free-standing as highly stretchable (strain > 500%) electrical wiring or sen- structures that can be frozen in place prior to elastomer sealing. sors that change electrical conductivity in response to elastic In addition, this scalable approach to manufacturing exploits a deformation.[ 3–5 ] Recent applications include mechanically- variety of wetting behaviors between droplets of galinstan and tunable antennae for remote strain and pressure monitoring,[ 4 ] a thin metal fi lms. [ 12 ] This work complements several new fabrication techniques developed for Ga-In alloys, including vacuum induced patterning,[ 13,14 ] and contact printing.[ 15 ] Prof. R. K. Kramer School of Mechanical Engineering Purdue University West Lafayette, IN 47907, USA 2. Results E-mail: [email protected] Prof. C. Majidi Figure 2 displays a test pattern used to demonstrate the pos- Department of Mechanical Engineering sible trace widths and feature densities of the fabrication pro- Carnegie Mellon University cess. The test pattern included traces with widths of 1 mm, Pittsburgh, PA 15213, USA 500 μ m, 200 μ m, 100 μ m, 50 μ m, 25 μ m, and 10 μ m. Square Prof. R. J. Wood features were spaced with edge-to-edge separations of the same School of Engineering and Applied Sciences dimensions. Harvard University The minimum trace width that the fabrication process pro- Cambridge, MA 02138, USA duced is 200 μ m. Upon further inspection, it is apparent that Prof. R. J. Wood the trace designed to be 200 μ m in width is considerably less Wyss Institute for Biologically Inspired Engineering that in actuality ( Figure 3a), with the conductive liquid meas- Harvard University uring between approximately 100 μ m and 150 μ m in width Boston, MA 02115, USA along various points of the trace. This discrepancy between DOI: 10.1002/adfm.201203589 intended trace width and actual trace width is likely due to the

± ow lms . This ° Wetting uid ﬂ ] 130 12 ≈ [ les for both les for ). ° m. In addition, lms. wileyonlinelibrary.com μ www.afm-journal.de 125 ≈

square are approximately square 2 0.24% gallium, 22.69% ± 0.20% tin. The increased per- m, respectively. Discrepancies in m, respectively. ± μ square conductive liquid feature square conductive 2 lms in order to achieve selective wet- Figure 2 also demonstrates the ability to the ability to also demonstrates 2 Figure For completeness, we also tested contact completeness, For ] 1 [ 80 and 90 feature height may be caused by fl prior to encapsulation. pattern densely-packed features with the features pattern densely-packed pro- fabrication liquid-embedded-elastomer with a fea- features (squares cess. Isolated are shown of 1 mm in this case) ture length with an edge-to-edge to be reliably patterned as 25 separation as small b shows example profi 3 Figure liquid trace and a 1 mm wide conductive a 1 mm In a previous study we examined a variety of of wetting behaviors between droplets galinstan and thin metal fi 3. Discussion embedded in an elastomer sheet. The cross- embedded in an elastomer was found using section of these features microscope (Olympus, a 3D laser confocal is seen to be rounded LEXT OLS4000), and heights of the 1 mm in shape. The peak channel and 1 mm response was found to be tunable as a function of both composition and texture. Mainly, tin galinstan was found to wet reactively to 14511, pure), Aldrich, product nu. foil (Sigma but was repelled by sputtered (textured) fi of tin and indium (sputter targets from American Elements, 99.999% pure), among pro- other sputtered metals. The fabrication by cess given here exploits this phenomenon patterning areas of tin foil and sputter coated metal fi ve samples, with the average composition (per- ve samples, with It should be noted that wetting behavior of liquid-phase It should be noted that wetting behavior of liquid-phase Another important result of that study is the equilibrium is consistent with other reports of non-wetting between Ga-In is consistent with other reports of non-wetting between Ga-In alloys and PDMS. angle between the alloys and a PDMS substrate with recent liquids, this exposure to oxygen plasma. Unlike for water-based surface treatment did not notably the change the wettability of alloys (contact angle the substrate for Ga-In 0.09% indium, and 19.68% centage of tin demonstrates that the underlying tin foil is absorbed into the galinstan to create this new composition. This reactive process removes the solid metal seed layer, ting of galinstan on an elastomer surface. ting of galinstan on an elastomer surface. (PDMS) was also studied. alloys on a silicone elastomer Ga-In do not wet PDMS, but rather non- alloys found that Ga-In We uniformly stain the surface with a contact angle of state of galinstan and tin foil. Over a time period of several days, the materials react together to develop a stable alloy of new composition. The new alloy was analyzed using energy dispersive spectroscopy (EDS) in a scanning electron microscope over fi centage by weight) being 57.63%

lm would m in width. It m), this edge- μ μ 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim GmbH & Co. 2013 WILEY-VCH Verlag 500 © > m) this edge-effect dra- μ seed layer. nely patterned ooded with galinstan, which reactively wets the tin ooded with galinstan, which reactively wets 500 < , 5292–5296 ow for liquid-embedded elastomers fabricated by selective wet- ow for 23 , 2013

Process fl y thought that a more mild etchant may prevent ner features. features. ner Experiments with diluted ferric chloride (10–90% deionized water) were conducted to slow the etching process, as it was briefl over-etching and enable smaller traces. However, it was found and enable smaller traces. However, over-etching that diluting the etchant only slowed the etching process and therefore induce that achiev- did not change the results. We able trace width in this process is a function of the material and etchant combination, in addition to the material thickness. That is, scalability of the feature sizes is partially limited by the where a thinner seed fi aspect ratio of the seed layer, enable fi Adv. Funct. Mater. Funct. Adv. aggressive ferric chloride etch, which over-etched the edges of aggressive ferric chloride etch, which over-etched surrounding the the seed pattern, as seen by the dark outline larger features ( . For 2 features in Figure matically decreased the overall dimensions and completely removed the traces designed to be less than 200 is important to note that the seed layer for the gallium-indium alloy was altered prior to the liquid deposition on the surface, and therefore reduced feature sizes may be achievable using the selective wetting process on a more fi effect did not notably change the size of the resulting features. for the smaller traces ( However, foil. h,i) Excess galinstan is removed by wiping the surface with a thin-fi lm applicator. lm applicator. the surface with a thin-fi foil. h,i) Excess galinstan is removed by wiping using the PAA as a release the surface in water, j) The remaining sputter mask is removed from encapsulated by coating k) The liquid features are cooled in a freezer and subsequently layer. with an elastomer. ting of gallium-indium alloys. a,b) Tin foil is patterned on a PDMS surface with photo- ting of gallium-indium alloys. a,b) Tin foil is surface, and does c) A PAA release layer is spin-coated onto the elastomer lithography. d,e) The surface is sputtered with indium, not coat the super-hydrophobic photoresist. Lift-off is achieved by rinsing the sur- but the tin foil is protected by the photoresist. face with acetone. f,g) The surface is fl Figure 1 . www.MaterialsViews.com 5294 FULL PAPER Fgr 2. Figure enabled bythefabricationprocess.b,c)Theliquid-embeddedelastomerisfl wileyonlinelibrary.com metals withinelastomerfi nique yieldstheabilitytopattern highlyconductiveliquid LE3 circuitsusingmaskeddeposition. Thisfabricationtech- In closing,weintroduceaprocessforproducingGa-In based Conclusion 4. elastomer devices. metal, whichisidealforapplicationtoliquid-embedded- resulting inacompositestructureofonlyelastomerandliquid www.afm-journal.de seed layer. Over-etchingisevidencedbytheblackoutlinesurroundingfeatures.b)Exampleprofi les forbotha1mmchanne Fgr 3. Figure a) Zoomed-inimageofa200- A liquid-embedded-elastomerpatternedbyselectivewettingofGa-Inalloy. a)Atestpatterndisplayingpossiblefeaturesi

lms withoutthe needformolding, μ m widechannel.Theactualchannelwidthisreducedfromthedesignedduring patterning ofthe © 2013WILEY-VCHVerlag GmbH&Co. KGaA, Weinheim exible andstretchable. width andincreasefeaturedensities. For example,gaining rication techniquestocontinue toreducetheminimumtrace no inlet/outletlocationsarerequired forfi This processalsoyieldstheability tocreateisolatedfeatures,as and edge-to-edgeseparationsas smallas25 embedded elastomerdeviceswithfeaturessmallerthan200 phase changeofaconductiveliquidmetal.Ultimately, liquid- photolithography, andexploitsthewettingproperties and incorporates techniquesfromfl bonding, ormanualstepssuchassyringe-fi Future investigationswillincludefurther experimentalfab- ex-circuit manufacturing and Adv. Funct. lling. www.MaterialsViews.com

Mater. lling. Thisprocess l anda1mm μ m areproduced.

2013 zes anddensities , 23 , 5292–5296 2 square. μ m FULL PAPER 5295 ex-circuit (as wileyonlinelibrary.com www.afm-journal.de C for several minutes, to C for several minutes, to Revised: February 22, 2013 Revised: February ° s high surface tension. The The s high surface tension. Received: December 4, 2012 ’ ect the views of the National Published online: May 16, 2013 Published online: May 16, 2013 m. m. μ In this particular example, we used sputtered fi lms lms used sputtered fi In this particular example, we ] 12 [ C), which causes the galinstan to harden and hold its form. This C), which causes ° Next, the sample is rinsed with acetone to lift off the photoresist the sample is rinsed with acetone to lift Next, environment, When the sample is removed from the nitrogen to the sample is placed on a hotplate for several minutes Finally, the liquid-embedded-elastomer device may then be released Finally, 19 − sample is then baked on a hotplate at 85 sample is then baked the sample is sputter- PAA solution. Finally, evaporate solvents in the liquid-phase Ga-In previously found that We coated with indium. sputtered surfaces of various compositions, alloys are repelled by lms was The non-wetting property of the fi including tin and indium. of the surface texture, rather than the material found to be a function composition. PTFE/photoresist surface, which is hydrophobic. Hence, a solution a solution is hydrophobic. Hence, surface, which PTFE/photoresist 00627) diluted in nu. Cat acid) (PAA) (Polysciences, of poly(acrylic surface, is applied to the entire (10% PAA by volume) deionized water PAA solution The the elastomer. solution selectively wets wherein the lm on the fi s to yield a thin liquid at 800 RPM for 30 is spin-coated be be noted that the PAA solution must elastomer surface. It should that is less than the seed tin layer thickness spin coated to a thickness in between the liquid metal and the seed layer to ensure good contact to the liquid metal subsequent steps, due of indium, where a sputtering time of 10 min corresponds to a mean time of 10 min corresponds to a mean of indium, where a sputtering of approximately 1.4 lm thickness fi exposing the underlying layer and corresponding sputtered indium, thus with liquid galinstan tin foil. The tin-patterned surface is then coated for at least 24 h, while and kept in a nitrogen environment chamber substrate. The nitrogen the galinstan reactively wets to the tin foil oxidizing, while the environment prevents the galinstan from heavily a mask, preventing non- patterned sputter-coated indium behaves as uniform wetting of the elastomer surface. removed using a thin- galinstan that is not wetted to the surface is Film Applicator). The film applicator (Elcometer 4340 Automatic the liquid height, thus thin-film applicator allows for definition of of the liquid-embedded enabling some control over the cross-section by placing the entire pattern. Any residual liquid metal is striped poly(acrylic acid) on sample in a deionized water bath, where the results in lift-off of the the surface of the elastomer dissolves and liquid metal from the corresponding sputtered layer and undesired surface. into a freezer with an evaporate remaining solvents, and then placed of galinstan operating temperature below the melting temperature ( liquid pattern to remain phase-change enables the cross-section of the step. The liquid patterns intact during the subsequent encapsulation nal layer of coating a fi are embedded into an elastomeric matrix by spin followed by a cure step. elastomer over the entire surface of the device, from the freezer and It should be noted that the time between removal should not warm above spin-coating is critical, as the patterned features the melting temperature prior to encapsulation. from the glass slide substrate. Wiring may be incorporated by ]) or interfacing the device with a polyimide-based fl either inserting external copper wires at appropriate locations (as in 18 ref. [ in ref. [ 9 ]). Acknowledgements This work was partially supported by the National Science Foundation (grant number NSF DMR-0820484) and the Wyss Institute for and ndings, Biologically Inspired Engineering. Any opinions, fi conclusions or recommendations expressed in this material are those of the authors and do not necessarily refl Science Foundation. m can μ m thick μ 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim GmbH & Co. 2013 WILEY-VCH Verlag at substrate. In this at substrate. In © m-thick fi lm of tin foil m-thick fi C for 5 min to evaporate C for 5 min to evaporate ° μ m in height). We expect We m in height). 70 mm, Electron Microscopy

× μ Photoresist is spun onto the tin Photoresist is spun onto the tin ] 17 [ Experiments were completed using Experiments were completed using ]

16 ] [ 19 [ exible photoresist, thus it is particularly well- C for 10 min. The photoresist is then exposed C for 10 min. The photoresist is then exposed , 5292–5296 ° 23 , uorooctyl)silane (Sigma Aldrich) for at least 3 h. 2013

. The sample is then developed in an aqueous solution . The sample is then developed in an aqueous solution 2 The foil is fully cleaned with acetone, IPA, and deionized water, and deionized water, The foil is fully cleaned with acetone, IPA, After curing the elastomer layer, a 20- After curing the elastomer layer, 20 mTorr) with an open vessel containing a few drops of trichloro(1H, with an open vessel containing a few drops of trichloro(1H, 20 mTorr) ≈ Adv. Funct. Mater. Funct. Adv. foil at 800 rpm for 20 s. After spin coating, the sample is soft-baked foil at 800 rpm for 20 s. After spin coating, the sample is soft-baked on a hotplate at 85 under a mask with a collimated UV source with an exposure energy of 2140 mJ/cm of (0.2 M) sodium hydroxide at room temperature. Post-development, of (0.2 M) sodium hydroxide at room temperature. Post-development, exposed tin is chemically etched in a heated ferric chloride bath, where the photoresist masks the desired tin pattern from removal. Residual ferric chloride is removed by rinsing the sample in deionized and the remaining tin foil pattern acts as a seed layer for water, liquid metal deposition in later steps. The sample surface is then exposed to oxygen plasma (65 W for 30 s). Silicon-based elastomers, such as PDMS, that have recently been exposed to oxygen plasma are rendered hydrophilic by the formation of hydroxyl groups on the plasma exposure has no notable effect on the the surface. Yet, liquids. A superhydrophobic photosensitive nanocomposite fi lm is lm is nanocomposite fi liquids. A superhydrophobic photosensitive photoresist then spin-coated onto the surface. The superhydrophobic Polysciences) incorporates PTFE nanoparticles (Microdispers-200, by weight, well-mixed into a positive photoresist matrix (1:15 ratio with a centrifugal mixer). and subsequently baked on a hotplate at 85 with the features between 80–90 with the features between to of the process will yield the ability that further optimization and densely packed features, and will be create much smaller devices, such as soft resistors, inductors, used to create novel (gated) electronic components, with appli- capacitors, and active skins, robotics, medical devices, soft cations in soft sensing electronics. orthotics, and stretchable work, we use clean glass slides (50 mm an evacuated chamber Sciences). The glass slides are placed in ( 1H, 2H, 2H-perfl silicone-based elastomer is spin-coated in a layer of Subsequently, lm with prescribed fi liquid form onto the glass slide, yielding a thin Sylgard 184, Dow thickness. Here, we use polydimethylsiloxane (PDMS; 10:1 mass ratio of elastomer base to curing agent), however, Corning; c on the specifi subsequent steps in the process are not dependent elastomer may be properties of PDMS and any thus any silicone-based used. over the elastomer. (Sigma Aldrich, product nu. 14511, pure) is rolled good adhesion between The inherent tackiness of PDMS allows for tacky elastomer is used, the elastomer and foil. In cases where a less using an amino-epoxy adhesion between the layers may be achieved chemical bonding method. 5. Experimental Section 5. Experimental begins with vapor of liquid-embedded-elastomer devices Fabrication a fl deposition of a hydrophobic monolayer on more control over the trace and feature heights may inherently feature heights may over the trace and more control approach separation. A different edge-to-edge allow for smaller result in of the seed layer may and patterning to deposition control over the cross-sectional widths. Increased smaller trace (the elas- for thinner devices features may allow area of liquid is approximately 300- 2 Figure tomer sheet shown in Photoposit SP24 (Shipley), which is a positive working resist generally working resist generally Photoposit SP24 (Shipley), which is a positive Photoposit SP24 is used within the printed circuit board industry. also known to be a fl suited for compliant substrates. In addition, we chose this resist due ne line resolution (1:1 resist thickness to resist to its associated fi geometries can be achieved, and etched dimensions of 25 c sensitivity chart was available from be produced). As no specifi we adapted the i-line compatible exposure the resist manufacturer, et al. process initiated by Corbett www.MaterialsViews.com 5296 FULL PAPER wileyonlinelibrary.com Wu , Z. Cheng, S. Wood , [6] J. R. Kramer , K. R. Majidi, C. [5] Weitz , A. D. Weiss , A. E. Larsen, J. R. Chiechi, C. R. Dickey , D. M. [1] [1 J A Rgr , . oea Y Hag Huang, Y. Someya, T. Rogers, A. Wood , J. J. [ 10] R. Sahai, R. Majidi, C. Kramer , Santos, K. J. V. R. Posner , [9] D. J. Wong , P. D. R. Wood , [8] J. R. Majidi, C. Kramer , K. R. [7] Y-. ak C Mjd , . rmr P Brr , . . od Wood , J. R. Berard, P. Kramer , R. Majidi, C. Wood , Park, Y.-L. J. Whitesides, R. [4] M. Chen, B.-R. G. Park, Dickey , Y.-L. D. [3] M. Weiss , A. E. Chiechi, C. R. [2] [1 R K Kae , . Soe J C Wae , . . od unpublished. Wood , J. R. Weaver , C. J. Stone, A H. Kramer , K. Rogers, R. A. J. [ 12] Kim , H. D. [ 11] www.afm-journal.de 105017–105023. Whitesides, M. G. 1926. on IntelligentRobotsandSystemsinSanFrancisco, CA, 179 Automation inShanghai, Microeng. Chem. 1607. , 62–69. ,

2008

2010 , , 120 , , 20 , 148–150. , Lab Chip , 125029–125034. , Adv. Funct. Mater. Adv. Mater. China,

2010

, , 2011 10 IEEE Sens.

, 3227–3234. , 2008

, 1103–1107. , IEEE Int.Conf. onRoboticsand 2008 Smart Mater. Struct. Science , , , , 20 18 Sens. Actuators,A

, 4887–4892. , 2012 © , 1097–1104. , 2013WILEY-VCHVerlag GmbH&Co. KGaA, Weinheim

IEEE/RSJ Int.Conf. 2010 , , 12 , 2711–2718. , , , J. Micromech.

2011 327

2011 , 1603– , , 1919– , Angew.

2012 , , 20 , , [1 B L Cmy G J Hys M D Dce , . . utc , . . Tabor , E. C. Justice, S. R. Dickey , D. M. Hayes, J. G. Cumby , L. B. [ 13] [1 Ceia bnig ehd Frt te lsoe srae s exposed is surface elastomer the First, method: bonding Chemical Wood , [ 19] J. R. Kramer , K. R. Lu, Paik, W. K. Swenson, J. J. [ 18] E. Johnston, K. Strole, J. Zhigang, Corbett , W. S. Kim, Sundqvist, [ 17] Pan, T. K. J. Hong, D. Hjort, L. Kim, K. [ 16] S. Hagman, D. A. Hyun, Jeong, K. H. J. Ahn, S. C. [ 15] Li, M. Wang , S. Park, J. [ 14] . . ekned Heikenfeld, C. J. 60 with lightpressure.Placingthecontactedsurfacesinanovenat oven at60 deionized water, andtheﬁ lms areplacedonahotplateorinan least 20min,excesssilaneisdiscarded,thesurfacesarerinsedwith (3-glycidyloxypropyl)trimethoxysilane (98%SigmaAldrich).Afterat The foillayerisalsotreatedwithplasmaandcoatedin5%(v/v) 5% (v/v)(3-aminopropyl)triethoxysilane(99%SigmaAldrich). to oxygenplasma(65Wfor30s)andimmediatelycoatedin Robots andSystems, Electron. Packag.Manufact. Chip Jeon, S. Huang, Y. Rogers, A. J. ° C forseveralhoursﬁ nalizes thechemicalbond.

2012 , , 12 ° C todry. Thesurfacesmaythenbebondedtogether , 4657–4664. , J. Microelectromech.Syst. Appl. Phys.Lett.. San Francisco, CA,

2005 Nat. Commun. , ,

2012 Adv. Funct. 28 IEEE/RSJ Int.Conf. onIntelligent , 312–321. , , , 2011 101

2010 www.MaterialsViews.com

, 174102–. , Mater. , 414–420. ,

2012 , , 19

2013 , 246–253. , , , 3 , , 916-923. , 23 IEEE Trans. , 5292–5296 Lab