Exothermic nonreversing process in the phase transition of poly( N- isopropylacrylamide) studied with stochastic temperature-modulated DSC

Nanoclay-Based Self-Supporting Responsive Nanocomposite Hydrogels for Printing Applications

Stimuli-responsive hydrogels and/or composite hydrogels have been of great interest for various printing applications including four-dimensional printing. Although various responsive hydrogels and/or composite hydrogels have been found to respond to given stimuli and change shapes as designed, the fabrication of three-dimensional (3D) structures from such responsive hydrogels is still a challenge due to their poor 3D printability, and most of the responsive material-based patterns are two-dimensional (2D) in nature. In this study, Laponite nanoclay is studied as an effective additive to improve the self-supporting printability of N-isopropylacrylamide (NIPAAm), a thermoresponsive hydrogel precursor while keeping the responsive functionality of NIPAAm. Graphene oxide (GO) is further added as a nanoscale heater, responding to near-infrared radiation. Due to the different shrinking ratios and mechanical properties of the poly( N-isopropylacrylamide) (pNIPAAm)-Laponite and pNIPAAm-Laponite-GO nanocomposite hydrogels, printed 2D patterns deform in a predictable way. In addition, 3D microfluidic valves are directly printed and cured in air, which can effectively control the flow directions in response to different stimuli as validated in a microfluidic system. Because Laponite nanoclay can be mixed with various responsive hydrogel precursors to improve their 3D printability, the proposed Laponite nanoclay-based nanocomposite hydrogels can be further expanded to prepare various 3D printable responsive nanocomposite hydrogels.