Supporting Information

Transcription

1 Supporting Information Silicones for Stretchable and Durable Soft Devices: Beyond Sylgard-184 Sungjune Park, Kunal Mondal, Robert M. Treadway, Vikash Kumar, Siyuan Ma, James Holbery, Michael D. Dickey* Department of Chemical and Biomolecular Engineering, North Carolina State University, 911 Partners Way, Raleigh, NC 27695, USA Applied Sciences Group, Microsoft Corporation, Redmond, WA, USA Corresponding Author * S-1

2 Mechanical properties of silicones. Figure S1 shows tensile-stress and strain profiles of two commercially available silicones. Despite the promising mechanical properties of these materials, we ruled them out for fabricating devices for reasons related to replica molding. As shown in Figure S1(a), Exsil TM 100 (Gelest) exhibits extremely high elongation at break (1864.5%), but has a very low Young s modulus (0.02 MPa). As a result, microchannels composed of Exxsil tend to collapse. In contrast, MED (Nu-sil) shows the highest tensile stress among all silicones we used, relatively high elongation (484 %), and Young s modulus (1.96 MPa) (Figure S2(b)). However, MED is processed from a solvent, which adds processing complications. Figure S1. (a) The stress-strain plot of highly soft elastomer (Exsil TM 100) and (b) elastomer showing the highest tensile stress (MED ). S-2

3 Resilience of silicones. To measure resilience of silicones as a function of strain, dog bone shaped molds of elastomers were prepared as indicated in ASTM D412 Type C standard. We characterized the resilience of the silicones by five cycles of stress-strain. We pre-stretched the materials once to just below the strain at break to minimize the Mullins effect and then additionally performed four more cycles. Resilience can be calculated by integrating the stressstrain curve from zero to the elastic limit (Figure S2). The calculated resilience of silicones is shown in Figure S3. Figure S2. Representative stress-strain curves for silicones during consecutive tensile testing: (a) Sylgard-184, (b) Sylgard-186, (c) Dragon Skin 10 slow and (d) Ecoflex Note in (a-d) that each strain % shows five cycles that generally overlap. S-3

4 Figure S3. Resilience as a function of strain for silicones calculated from cycle stress-strain curve in tension. S-4

5 Blending of silicones. To enhance mechanical properties of Sylgard-184, we blended it with silicones with excellent mechanical properties, such as Dragon Skin and Ecoflex. Once completely mixed, we cured these silicone blends at 60 C for 24 hrs. As shown in Figure S4, we calculated the viscosity of blended silicones as a function of weight percent of Sylgard-184. As weight percent of Sylgard-184 increases, the viscosity of blended silicones decrease. Figure S4. Viscosity of blended silicones as a function of weight percent of Sylgard-184. S-5

6 Table S1. Mechanical properties of blended silicones. Blended Silicones Tear strength (N/mm) Young s modulus (MPa) Dragon Skin 10 slow / Sylgard-184 (3:1) 3.44 ± ± 0.16 Dragon Skin 10 slow / Sylgard-184 (5:1) 5.71 ± ± 0.11 Dragon Skin 10 slow / Sylgard-184 (7:1) 8.54 ± ± 0.07 Ecoflex / Sylgard-184(1:1) 4.29 ± ± 0.21 Ecoflex / Sylgard-184(2:1) 3.31 ± ± 0.06 Ecoflex / Sylgard-184(5:1) 4.20 ± ± 0.01 Stretchable microfluidics filled with liquid metal. We identified highly stretchable and tear resistant silicones such as Sylgard-186, Elastosil-M4130 and M4630, and fabricated microfluidic devies using such materials. To create microchannels via replica molding, we cured silicones on a mold at 60 C for 4 hrs. However, Elastosil-M4630 was cured at room temperature for 12 h to prevent excessive interfacial bonding between the elastomer and Si mold, which would occur if curing occured at 60 C. As shown in Figure S5, microfluidic devices consisting of Sylgard-186, Elastosil-M4130 and M4630 filled with liquid metal can withstand high elongation, demonstrating stretchable microfluidic electronics. S-6

7 Figure S5. Photos of stretchable microfluidic device consisting of (a,b) Sylgard-186, (c,d) Elastosil-M4130, and (e,f) Elastosil-M4630 filled with liquid metal in the microchannels. (a,c,e) before- and (b,d,f) after stretching. The channel width of Sylgard-186, Elastosil-M4130 and M4630 devices is 400 μm, 600 μm and 800 μm, respectively. S-7