Click chemistry stereolithography for soft robots that self-heal†
Although soft robotics promises a new generation of robust, versatile machines capable of complex functions and seamless integration with biology, the fabrication of such soft, three dimensional (3D) hierarchical structures remains a significant challenge. Stereolithography (SLA) is an additive manufacturing technique that can rapidly fabricate the complex device architectures required for the next generation of these systems. Current SLA materials and processes are prohibitively expensive, display little elastic deformation at room temperature, or exhibit Young's moduli exceeding most natural tissues, all of which limit use in soft robotics. Herein, we report a low-cost build window substrate that enables the rapid fabrication of high resolution (∼50 μm) silicone (polydimethylsiloxane) based elastomeric devices using an open source SLA printer. Our thiol–ene click chemistry permits photopolymerization using low energy (He < 20 mJ cm−2) optical wavelengths (405 nm < λ < 1 mm) available on many low-cost SLA machines. This chemistry is easily tuned to achieve storage moduli, 6 < E < 283 kPa at engineering strains, γ = 0.02; similarly, a large range of ultimate strains, 0.5 < γult < 4 is achievable through appropriate selection of the two primary chemical constituents (mercaptosiloxane, M.S., and vinylsiloxane, V.S.). Using this chemo-mechanical system, we directly fabricated compliant machines, including an antagonistic pair of fluidic elastomer actuators (a primary component in most soft robots). During printing, we retained unreacted pockets of M.S. and V.S. that permit autonomic self-healing, via sunlight, upon puncture of the elastomeric membranes of the soft actuators.