Responsive soft matter is a network of polymer chains that can exhibit large change in shape and property in response to environmental conditions, such as temperature, pH, light, and electric field. The unique attributes of this emerging class of materials have been extensively studied and applied in a broad range of areas, including soft robotics, microfluidics, and bio-medical engineering. However, manufacturing of responsive soft matter has been heavily relying on traditional manufacturing processes such as cutting, molding, and lithography, which are restricted inherently to two-dimensional (2D) space. In this work, we demonstrate a three-dimensional (3D) micro-fabrication of stimuli-responsive soft matter using projection micro-stereolithography (PµSL). PµSL is a high-resolution additive manufacturing technique that utilizes the state-of-the-art digital display as a dynamically reconfigurable photomask to rapidly build complex 3D micro-structures in a layer-by-layer fashion.
First, we present 3D printing of Poly(N-isopropylacrylamide) (PNIPAAm) and Poly(acrylic acid) (PAA), hydrogels that are responsive to temperature and electric field, respectively. Swelling of 3D printed hydrogels is characterized and tailored by controlling PµSL process parameters, including concentration of monomer and cross-linker, curing UV intensity, and dimensions. Based on these results, we demonstrate programmed responsive deformation in highly complex 3D hydrogel micro-structures that can shrink and grow, move an object, and even walk. Second part of the talk will be devoted to 3D printing of shape memory polymer (SMP) to achieve tunable and recoverable mechanical properties in three-dimensionally architected micro-structures. Temperature-responsive SMP exhibits significant change in elastic modulus around its glass transition temperature (Tg). Various types of 3D microlattices are fabricated using SMP. Not only does shape memory effect of the SMP allow for full recovery of the original shape upon heating even after substantial mechanical deformation, mechanical property and energy absorption of the printed microlattice can also be controlled by temperature. Lightweight, mechanically tunable, and geometrically recoverable microstructures have great potential for new smart structural systems that can effectively react and adapt to varying environments or unpredicted payloads.