Digital Light Processing of Stiff yet Tough Single Network Hydrogels

Wide applicability of hydrogels towards tissue repair, drug delivery, or biomedical devices is often restricted due to weak mechanical properties and an inability to process them into structures that mimic tissue architecture. Conventional strategies to enhance hydrogel stiffness through increased number of crosslinks often result in embrittlement. To resolve the stiffness-toughness conflict, hydrogels with high degree of polymer chain entanglements have been developed, particularly with the use of low initiator concentrations and slow radical polymerization^[1]. Unfortunately, this approach is not directly amenable to processing methods such as Digital Light Processing (DLP), which relies on the rapid light-based crosslinking of liquid precursors to form 3D shapes in a bottom-up, layer-by-layer manner^[2,3]. To overcome this mechanical performance-processability conflict, we introduce one-step, Continuous curing after Light Exposure Aided by Redox initiation (CLEAR) printing approach towards formation of highly entangled networks. CLEAR printing is a dual-initiating approach that uses light to crosslink and set the shape of the part, while a complementary reaction (via redox initiators) slowly allows complete conversion of any unreacted monomer within the printed part under ambient conditions.<br/><br/>To demonstrate the advantage of CLEAR, we use acrylamide as a model monomer to form 3D printed, water-swollen highly entangled hydrogels with modulus (~250 kPa) and toughness (~600 kJ m^-3) values far higher than those (~100 kPa, ~150 kJ m^-3) attained with DLP alone. Despite their 85% water content, CLEAR printed hydrogels are also superior in compression and under shear than those printed with DLP alone. Interestingly, we also show that the CLEAR technique outperforms traditional approaches of improving green strength (e.g., light flood cure or thermal post-curing). We next leverage CLEAR to process highly entangled hydrogels into complex shapes with open, interconnected pores (e.g., trabecular structure), hollow channels, and knotted topologies with high print fidelity and resolution. These 3D printed structures can sustain cyclic (tensile or compressive) loading and recover their original shape with minimal hysteresis. Further, we show that CLEAR can be applied to other monomer systems through changing the type of monomer (e.g., N-isopropylacrylamide, acrylic acid) or the type of crosslinker (e.g., polyethylene glycol PEG diacrylate, gelatin methacrylamide) to obtain 3D printed, swollen single network hydrogels with modulus and toughness values as high as 300 kPa and 1.8 MJ m^-3. Additionally, we establish the applicability of CLEAR towards 3D printing of highly entangled elastomers with toughness of 30 MJ m^-3, nearly 10-fold higher than those printed with DLP alone.<br/><br/>As a potential biomedical application, we demonstrate the use of 3D printed, cytocompatible highly entangled hydrogels towards achieving robust adhesion onto wet tissues (e.g., <i>ex vivo</i> porcine heart, lung, stomach, intestine, tendon) with high interfacial toughness values ranging between 300 to 1000 J m^-2. We then process these hydrogels into 3D porous patches that can not only conform to curved tissue topology (e.g., heart) but are also compliant with the dynamic movement of native tissues. Finally, we show proof-of-concept hydrogel adhesives with 3D patterns to (i) form microfluidic connections directly onto tissues for delivery of drugs and (ii) metamaterial structures that enable directional, spatially selective adhesive strength.<br/><br/><b>Ref:</b><br/>[1] Kim+ <i>Science </i><b>2021</b>, <i>374</i>, 212.<br/>[2] Grigoryan+ <i>Science </i><b>2019</b>, <i>364</i>, 458.<br/>[3] Dhand+ <i>Adv. Mater.</i> <b>2022</b>, <i>34</i>, 2202261.