Pandiyarajan Chinnayan Kannan1 Jan Genzer1

1, Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina, United States

Gradient materials play a vital role in the creation of artificial implants due to their tendency 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 demonstrate a versatile synthetic platform to mimic such gradient structures on flat supports using a random copolymer 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 allows complete control over the gel density and stiffness in orthogonal directions by spatially and orthogonally controlling UV dosage and temperature. We examine the swelling ratio (α) of these gradient gels in water using spectroscopic ellipsometry; α depends on the extent of crosslinking that ranges from α = 1-1.2 (highly crosslinked gels) to α = 4-5 (loosely crosslinked gels). We corroborate the gel density with swelling and find that densely crosslinked gels promote protein adsorption (or cell adhesion) while the loosely-bound gels repel proteins and cells. We attribute the latter phenomenon to entropic shielding and size exclusion factors.