Polyacrylamide Hydrogels with Reversibly Photocontrolled Stiffness for 2D Mechanobiology

We report the development of polyacrylamide hydrogels with photoswitchable stiffness using solely visible light and their application to cell culture. We have previously shown that azobenzenes can control the binding constants of dynamic covalent boronic ester bonds (<i>Chem. Sci</i>. <b>2018</b>, <i>9</i>, 5987; <i>J. Am. Chem. Soc</i>. <b>2020</b>, <i>142</i>, 19969). Here we show that these photoswitchable dynamic bonds can be incorporated into polyacrylamide hydrogels that are stable for at least 10 days in buffer without changes in stiffness or photoresponse. Reversible stiffening and softening are achieved with green and blue irradiation, respectively. We prepared soft (877 ± 79 Pa) and stiff (8.4 ± 0.3 kPa) hydrogels that undergo rapid, photoreversible changes in modulus over at least 3 light irradiation cycles. <i>In vitro</i> studies show that the hydrogels are nontoxic to HepG2 cells. The cells undergo the expected changes in morphology, actin stress fiber formation, and Yes-associated protein (YAP) subcellular localization upon stiffening and softening the hydrogel substrate with visible light. These results validate the suitability of our visible-light-controlled hydrogel as a versatile platform for cellular mechanotransduction studies.