Thermoresponsive Double Network Hydrogels with Exceptional Compressive Mechanical Properties.

The utility of thermoresponsive hydrogels, such as those based on poly(N-isopropylacrylamide) (PNIPAAm), is severely limited by their deficient mechanical properties. In particular, the simultaneous achievement of high strength and stiffness remains unreported. In this work, a thermoresponsive hydrogel is prepared having the unique combination of ultrahigh compressive strength (≈23 MPa) and excellent compressive modulus (≈1.5 MPa). This is accomplished by employing a double network (DN) design comprised of a tightly crosslinked, highly negatively charged 1st network based on poly(2-acrylamido-2-methylpropane sulfonic acid (PAMPS) and a loosely crosslinked, zwitterionic 2nd network based on a copolymer of thermoresponsive NIPAAm and zwitterionic 2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (MEDSAH). Comparison to other DN designs reveals that this PAMPS/P(NIPAAm-co-MEDSAH) DN hydrogel's remarkable properties stem from the intra- and internetwork ionic interactions of the two networks. Finally, this mechanically robust hydrogel retains the desirable thermosensitivity of PNIPAAm hydrogels, exhibiting a volume phase transition temperature of ≈35 °C.