Enhanced thermal response of 3D-printed bilayer hydrogels <i>via</i> nanoclay incorporation

There exist numerous opportunities to design stimuli-responsive bilayer hydrogels for enhanced actuation using simple and robust techniques. Specifically, digital light processing (DLP) 3D printing offers a robust technique for multi-layered hydrogel fabrication. However, nanocomposite hydrogels utilizing this technique have not yet been widely realized. Nanoclay incorporation has been shown to improve the actuation of poly(<i>N</i>-isopropyl acrylamide) (pNIPAAm) hydrogels; however, opportunities remain to study the relationship between clay morphology and thermal response, particularly in a 3D-printed bilayer system. In this work, we utilized an ethanol-water cosolvent, hydrogel precursor solution to incorporate montmorillonite (MMT) clay into 3D-printed pNIPAAm hydrogels. By varying the MMT loading, we demonstrated that a low loading of MMT (0.5 wt% relative to the mass of NIPAAm monomer) induced the greatest enhancement of the initial rate and final magnitude of actuation in the studied hydrogels. We utilized poly(2-hydroxyethyl acrylate) (pHEA) as a passive layer to form bilayers by sequentially printing pHEA before the pNIPAAm/MMT hydrogels, and used those hydrogels to demonstrate the accelerated actuation of 3D-printed pNIPAAm/MMT-pHEA bilayers compared to clay-free, pNIPAAm-pHEA bilayers. Through comparison to a mathematical framework and fabrication of an all-pNIPAAm bilayer, we suggested that the model has limitations for the prediction of bilayer curvature in these systems due to the inability of certain hydrogels to overcome the inertia of the passive layer. Overall, this work showcases the utility of MMT as a handle for tunability in 3D-printed pNIPAAm bilayer hydrogels.