4D Printing with Mechanically Robust, Thermally Actuating Hydrogels

摘要

A smart valve is created by 4D printing of hydrogels that are both mechanically robust and thermally actuating. The printed hydrogels are made up of an interpenetrating network of alginate and poly(N-isopropylacrylamide). 4D structures are created by printing the “dynamic” hydrogel ink alongside other static materials. 4D printing is an exciting emerging technology for creating dynamic devices that can change their shape and/or function on-demand and over time.1-3 4D printing combines smart actuating and sensing materials with additive manufacturing techniques to offer an innovative, versatile, and convenient method for crafting custom-designed sensors,4 robotics,5 and self-assembling structures.1 Stimuli-responsive volume-change materials incorporated into multimaterial structures can be harnessed to create movement in the same way that biological muscles achieve motion in animals and nastic movements are generated in plants.6, 7 Current 4D printed examples utilize water absorption1 or thermal shape memory2 to demonstrate impressive shape change, but are slow to respond, show limited reversibility, and restricted to bending type motions of flexible structures that generate little force. We here describe relatively fast and reversible, skeletal muscle-like linear actuation in 3D printed tough hydrogel materials and their incorporation into a smart valve that can control the flow of water. Hydrogels were utilized as the active material in the 4D printed structures since these materials have been processed with printers8 and some examples of hydrogels can drastically, and reversibly, alter their volume in response to changes within their environment.9, 10 We start with an ionic covalent entanglement (ICE) hydrogel11 that can be 3D printed12 and demonstrate high toughness.12 The latter is important since we require thin sections that respond quickly to external stimuli but also need to be mechanically robust to support the internal and external mechanical loads. ICE gels are a type of an interpenetrating polymer network hydrogel that is made up of an entanglement of one polymer network crosslinked with metal cations and a second polymer network crosslinked with covalent bonds.11, 13-15 This dual network structure is similar to that in double network hydrogels16, 17 and facilitates high toughness through large-scale crack-tip energy dissipation by unloading of the strands in the tight network,18 in this case due to dissociation of the ionic crosslinks.19 The loosely crosslinked covalent network serves to bridge the damage zones created by the loss of ionic bonds and prevents catastrophic crack propagation. Here, we use a thermally responsive covalent crosslinked network of poly(N-isopropylacrylamide) (PNIPAAm) to function as both the toughening agent and to provide actuation through reversible volume transitions. PNIPAAm is a widely studied temperature-sensitive hydrogel that exhibits a large reversible volume transition at a critical temperature, TC (≈32–35 °C).20, 21 The volume change is caused by a coil–globule transition of the polymer network strands22 and results in a large decrease in the water content when the temperature is increased above Tc.20 Alginate/PNIPAAm ICE gel inks with various concentrations of NIPAAm were prepared for extrusion printing. The inks had a constant alginate concentration of 4% (w/v) and NIPAAm concentrations of 10%, 15%, or 20% (w/v) with fixed amounts of covalent crosslinker and UV initiator. Printing and curing at subambient temperatures (10 °C) was necessary to prevent phase separation of the PNIPAAm (Supporting Information). Rheological characterization showed that the inks had appropriate flow12 and gelation behaviors at 10 °C for extrusion printing (Supporting Information) with an Envisiontec 3D-Bioplotter coupled with a UV-light source that enables in situ photopolymerization. The printed hydrogels were immersed in 0.1 m calcium chloride solution to crosslink the alginate polymer network an