Soft materials that can actively deform and reconfigure in response to external stimuli have been extensively studied for their profound potential for autonomous, adaptive, and multifunctional material systems. Emerging pathway to create dynamicand adaptive architectures involves additive manufacturing (often referred to as 3D printing) of programmable soft matter. This approach has been recently termed 4D printing, with the 4th dimension being the time-dependent shape change after the printing.Here we demonstrate additive manufacturing of stimuli-responsive hydrogels and shape memory polymers 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 temperature dependent swelling of 3D printed Poly(N-isopropylacrylamide) (PNIPAAm) hydrogel. Swelling ratio and transition temperature of 3D printed PNIPAAm is tailored by PµSL process parameters. Based on the swelling characteristics of 3D printed PNIPAAm hydrogels, we demonstrate programmed responsive deformation in highly complex 3D PNIPAAm micro-structures. Secondly, we present 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). Mechanical testing results show dramatic change in mechanical response of the microlattice at different temperatures. Furthermore, shape memory effect of the SMP allows for full restitution of the original shape by heating even after substantial mechanical deformation. 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. Also presented are some of biomedical applications of 4D printing including a microneedle array with enhanced tissue adhesion and transformable culture tubes for rapid histological analysis of spheroids/organoids