Biocompatible polymeric materials with switchable adhesion properties

We report on the design, investigation and comparison of the adhesive properties of poly-(N-isopropylacrylamide) and biocompatible polyethylene glycol-based thermoresponsive brushes. Specifically, a poly-(N-isopropylacrylamide), PNIPAM, poly-(oligo(ethylene glycol) methyl ether methacrylate-co-2-(2-methoxyethoxy) ethyl methacrylate), P(OEGMA-MEO2MA), and poly(oligo(ethylene glycol) methyl ether methacrylate-co-oligo(propylene glycol) methacrylate), P(OEGMA-OPGMA) brushes were synthesised on planar silicon wafers and silica particles via the surface-initiated atom transfer radical polymerisation. Switching of swelling and surface charge were investigated using spectroscopic ellipsometric and zeta-potential measurements, respectively. Adhesion properties were studied in situ in aqueous solutions at different temperatures using three kinds of AFM probes with well defined tips geometries and radii. We found that: (1) adhesion properties of all polymer brushes reversibly switch with temperature-polymer brushes are completely non-adhesive below the Low Critical Solution Temperature (LCST) and become sticky above the LCST; (2) adhesive force scales nonlinearly with the probe radius; (3) adhesion energy, obtained using Derjaguin/Muller/Toporov approaches, decreases with the increase of the tip radius that is attributed to different depth of penetration of the probes in the polymer layer; (4) adhesion decreases in the sequence PNIPAM - P(OEGMA-MEO2MA) - P(OEGMA-OPGMA). Based on dynamic light scattering and cryo-TEM experiments, the latter effect is attributed to the formation of a thin oligoethylene glycol shell around the hydrophobic polymer core in the case of P(OEGMA-OPGMA). We expect that the P(OEGMA-OPGMA) system, which demonstrates less sticky adhesion properties, could be particularly promising for cell adhesion experiments and tissue engineering.