Photo-responsive bio-inspired adhesives: facile control of adhesion strength via a photocleavable crosslinker†
A photo-responsive bio-inspired adhesive consisting of a zwitterionic polymer, poly(sulfobetaine methacrylate) (pSBMA), DOPA (3,4-dihydroxyphenylalanine), and a photocleavable nitrobenzyloxycarbonyl containing crosslinker was synthesized by thermally-initiated free radical polymerization. The main component, pSBMA, is a highly hydrophilic zwitterionic polymer with a high potential for biomedical application. The DOPA functionality, an extensively studied major component of the adhesion properties in mussels, is known to universally enhance polymeric adhesion properties even in the presence of water. The third component, a nitrobenzyl-containing dimethacrylate crosslinker, is efficiently cleaved upon UV irradiation. Proton NMR analysis of the crosslinker, 2-nitro-1,3-benzenedimethanol dimethacrylate (NBDM), demonstrates that photocleavage of the o-nitrobenzyl ester occurs rapidly for 30 minutes, then gradually continues for another 3 hours. The same patterns of photocleavage were observed in a terpolymer adhesive containing the nitrobenzyl crosslinker, poly(N-methacryloyl-3,4-dihydroxyl-L-phenylalanine-co-sulfobetaine methacrylate-co-2-nitro-1,3-benzenedimethanol dimethacrylate), poly(MDOPA-co-SBMA-co-NBDM). The terpolymer adhesive demonstrated a reduction in adhesion properties over the course of irradiation corresponding to the cleavage rate observed by 1H NMR analysis of NBDM. After 30 minutes of UV irradiation the original adhesion strength, 341 kPa, was decreased to 223 kPa, corresponding to a 35% reduction. The adhesion strength continued to decrease to 150 kPa at the 3 hours mark, a 56% reduction compared to the original adhesion strength.