Coupled effects of molecular weight and grafting density on protein adsorption in phosphorylcholine-based zwitterionic brushes.

Zwitterionic polymer brushes are widely used to suppress nonspecific protein adsorption, yet their performance depends jointly on molecular weight and grafting density. Rational design remains difficult because these parameters are often examined separately or represented by a single conformational descriptor. Here, bovine serum albumin adsorption was systematically investigated across 22 poly(2-methacryloyloxyethyl phosphorylcholine) brush states prepared using a polydopamine-mediated grafting-to method that enabled systematic variation of molecular weight and grafting density. Quartz crystal microbalance with dissipation monitoring was used to quantify apparent adsorbed mass and track the evolution of the interfacial response. We demonstrate that protein adsorption on phosphorylcholine-based zwitterionic brushes exhibits multiple sequential interfacial responses. Three characteristic response modes were distinguished based on their viscoelastic signatures across the design space. The initial low-dissipation mode showed the strongest architecture dependence and dominated adsorption at low molecular weight. The intermediate, more dissipative mode contributed comparatively little adsorption. The later low-dissipation mode was the major contribution for several brush states, mainly in the high-molecular-weight region. Total adsorption varied nonmonotonically across the design space, and the lowest adsorption, approximately 96 ng cm-2, was observed at intermediate molecular weight and grafting density, where both initial and later adsorption were limited. These findings suggest that the antifouling performance of poly(2-methacryloyloxyethyl phosphorylcholine) brushes is determined by the combined contributions of multiple adsorption modes rather than by the suppression of any single adsorption response.