Tuning the thermal response of 3D-printed bilayer hydrogels via architectural control using binary ethanol–water solvent systems

Abstract

While stimuli-responsive materials can be prepared via many established procedures, digital light processing (DLP) 3D printing offers a simple and robust technique for the fabrication of hydrogels, including spatially-defined bilayer hydrogels. The use of synthesis solvent mixtures has recently gained attention as a facile alternative to more complicated chemical modifications to tune hydrogel morphology by exploiting solvent-monomer interactions and cononsolvency which, by extension, modulates stimuli-response time and magnitude. In this work, we utilized a binary solvent system consisting of ethanol and water to induce morphological changes within a thermally-responsive poly(N-isopropyl acrylamide) (pNIPAAm) hydrogel during polymerization. By varying the ratio of ethanol to water, we demonstrated that hydrogel properties, such as crosslink density, pore morphology, and thermal response, can be tuned and correlated. While mass expulsion was fastest in gels prepared in 100% ethanol, we found that gels prepared in 75%–25% ethanol–water and 50%–50% ethanol–water maintained mechanical integrity at high temperatures, allowing expulsion of water mass without large amounts of contraction. We utilized the experimental findings from the monolayer hydrogel studies and investigated the response of bilayer structures comprised of pNIPAAm hydrogel layer and a non-responsive poly(2-hydroxyethyl acrylate) (pHEA) hydrogel layer and applied a mathematical model to better understand the fundamental kinematics of these bilayer systems in response to temperature. We also demonstrated the utility of these bilayer hydrogels for use in soft robotics applications. Overall, this work highlights that modulation of binary solvent mixture ratios is a strategy that enables control of morphological and mechanical features of stimuli-responsive hydrogels via 3D printing.