Multimaterial four-dimensional printing of complex polyethylene glycol-thiol-ene hydrogel structures using digital light processing

Three-dimensional (3D) printing of hydrogels has advanced rapidly across numerous disciplines, including tissue engineering, medical devices, and biotechnology, enabling applications including cell scaffolds, drug delivery systems, and biosensors. However, the rapid fabrication of multi-layered complex hydrogel structures remains a significant challenge when employing traditional 3D printing methods. In this study, we demonstrated a four-dimensional printing (4D) approach that leveraged the shape-morphing properties of multi-layered hydrogels to efficiently create complex and fine featured structures using a digital projection stereolithographic printer. We optimized various poly(ethylene glycol) (PEG)-thiol-ene resin formulations and printing parameters to develop seven 3D printable resins that exhibited a wide range of volumetric swelling ratios, from 1.21 to 10.75, and a corresponding decrease in Young's modulus, from 98.10 kPa to 0.35 kPa. By varying the combinations of hydrogel layers with distinct swelling ratios and Young's moduli in the printed bilayer constructs, we could create curved structures with controllable bending angles ranging from 139° to 479° upon immersion in phosphate-buffered saline (PBS). We further demonstrated that the bending angles of these bilayer structures could be predicted using Timoshenko beam equation for lower-to-moderate swelling mismatch systems, while higher-swelling mismatch systems exhibited larger prediction deviations. By spatially patterning these resins within flat, multi-layered prints, we achieved programmed actuation into complex, doubly curved geometries such as domes and saddles. Furthermore, our approach enabled the fabrication of complex, nature-inspired curvilinear structures such as flowers, octopuses, and butterflies. This shape-morphing hydrogel printing method significantly reduces fabrication time, eliminates the need for structural supports, and maintains high precision and reproducibility. Overall, our technique offers a rapid and versatile strategy for producing small-scale, complex, multi-layered hydrogel structures, reducing print time from over an hour to just minutes. This approach shows potential utility in future bioengineering and soft robotics applications.