Collapse of Thermoresponsive Brushes and the Tuning of Protein Adsorption

Protein adsorption onto brush displaying surfaces is strongly affected by collapse, an effect utilized in protein chromatography and in harvesting cell sheets for tissue engineering applications. For relatively small particles, the free energy penalty incurred upon insertion into the brush F-ins is related to the work expended against the osmotic pressure of the unperturbed brush. Within the self-consistent field (SCF) theory of brushes, the scale of F-ins decreases with the brush thickness < z > because the value of the osmotic pressure at the grafting surface Pi(0) similar to < z > irrespective of the interaction free energy. Brush collapse thus favors adsorption because it reduces F-ins via two effects: (i) lowering of the osmotic pressure and (ii) a possible decrease of the inserted volume of the particle. These general results are supplemented by numerical solutions of SCF equations for the collapse of thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) brushes as described by the empirical free energy of Afroze et al. (J. Mol. Struct. 2000, 554, 55). These yield the monomer concentration profiles c(z) and the corresponding osmotic pressure profiles Pi(z) as functions of the altitude z and the temperature T. c(z) and Pi(z) are then used to characterize F-ins for collapsed and swollen brushes as well as the adsorption isotherms for three adsorption mechanisms of relevance to ex vivo biotechnology applications involving PNIPAM brushes: (a) primary adsorption at the wall, (b) adsorption onto a ligand embedded within the brush, and (c) ternary adsorption within the brush due to weak attraction between the polymer and the adsorbing particle. Our results rationalize existing experimental results concerning the interactions between proteins and PNIPAM brushes and predict the effects of tuning parameters such as grafting density, polymerization degree and protein dimensions.