Surface-Grafted Hyperbranched Polyglycerol Coating: Varying Extents of Fouling Resistance across a Range of Proteins and Cells.

Abstract

Hyperbranched polyglycerol (HPG) was previously investigated as a nonfouling hydrophilic grafted layer on biomaterial surfaces, analogous to the well-known poly(ethylene oxide) (PEO), but the range of adsorbing cells and proteins tested was limited and at times the assays used were not the most sensitive. Thus, the questions arise whether HPG-grafted layers can indeed efficiently resist adsorption of a wider range of adsorbing biological entities, and how would different biological entities interact with such a coating. An HPG coating of 25 nm thickness was grafted onto a spin-coated and plasma-treated polystyrene (PS) layer on a silicon wafer substrate; this provided a well-suited system for surface analyses by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and atomic force microscopy (AFM), which verified the presence of a uniform, smooth grafted HPG layer. Adsorption of bovine serum albumin, lysozyme, fibrinogen, and endothelial cell growth medium 2 (EGM2) was reduced by >90%, with the adsorbed amounts close to the detection limit of XPS but still detectable by ToF-SIMS using principal component analysis. With human serum, however, the reduction in adsorption was slightly less pronounced. Smooth muscle cells (SMCs) and fibroblasts were virtually unable to attach onto the grafted HPG layer, with >99% reductions at 6 h compared with plasma-treated PS; the few attached cells remaining rounded and unable to spread. Their attachment might have resulted from coating defects. Testing with full blood showed that unlike for the control surface (plasma-treated PS), platelets did not adhere to the HPG surface, but there was attachment of some cells that stained CD11b positive and likely are neutrophils. Cells of the fungal organism Candida albicans were also able to attach onto the HPG surface to a limited extent, but in contrast to the control surface, the attached cells on HPG did not form hyphal extensions and thus seem to be compromised in their ability to invade and to form biofilms. Our data suggest that "low-fouling" is a better term than nonfouling for a grafted HPG layer as the resistance to adsorption is not uniform across a range of proteins and cells. It is also important in future work to study whether the cells that do attach can still exert their normal functions; our observation of the absence of hyphal extensions for C. albicans suggests that this may not be so. Hence, the potential utility of a grafted HPG layer may be not just a function of adsorbed amounts but also of the functionality of adsorbed proteins and cells.