Surface-attached orthogonal gradient hydrogels

Gradient materials play a significant role in the creation of artificial implants due to their potential to reduce stress concentration when two or more structures with different mechanical properties are joined together, $e.g.,$ tendon, a fibrous protein that connects the soft and hard muscle tissues in our body. We employ free radical polymerization to synthesize random copolymers containing 90{\%} of N-isopropyl acrylamide (NIPAAm), 5{\%} photo-active methacrylyloxybenzophenone (MABP) and 5{\%} thermally-active styrenesulfonylazide (SSAz) crosslinkers. The presence of MABP and SSAz facilitates adjusting gel density on a flat support in two orthogonal directions by spatially and independently controlling UV dosage and temperature. The swelling behavior ($\alpha )$ of the gels in water and methanol is examined using a spectroscopic ellipsometry and the degree of swelling depends on the extent of crosslinking that ranges from $\alpha $ $=$ 1-1.2 (highly crosslinked gels) to $\alpha \quad =$ 4-5 (loosely crosslinked gels). We compare the network properties surface-attached gels and un-attached identical counterparts and confirm that the linear swelling ratio of surface-attached networks is higher than that of the corresponding un-attached gels.