Surface Modification of Model Silicone Hydrogel Contact Lenses with Densely Grafted Phosphorylcholine Polymers

Abstract

When a biomaterial is inserted into the body, the interaction of the surface with the surrounding biological environment is crucial. Given the importance of the surface, the ability to alter the surface properties to support a compatible environment is therefore desirable. Silicone hydrogel contact lenses (CL) allow for improved oxygen permeability through the incorporation of siloxane functional groups. These groups however are extremely surface active and upon rotation, can impart hydrophobicity to the lens surface, decreasing lens wettability and increasing protein and lipid deposition. Lens biofouling may be problematic and therefore surface modification of these materials to increase compatibility is exceedingly recognized for importance in both industry and research. The current work focuses on the creation of a novel anti-fouling polymer surface by the incorporation of 2-methacryoyloxyethyl phosphorylcholine (MPC), well known for its biomimetic and anti-fouling properties. A controlled polymerization method was used to generate a unique double-grafted architecture to explore the effect of increasing surface density of polyMPC chains on corresponding anti-fouling properties. The novel free polymer was synthesized by a 3-step atom transfer radical polymerization (ATRP). First, poly(2-hydroxyethyl methacrylate) (polyHEMA) was polymerized by ATRP, where the hydroxyl (OH) groups of the polymer then underwent an esterification to create macroinitiating sites. From these sites, a second ATRP of poly(MPC) varying in length occurred, yielding the double-grafted polymer poly(2(2-bromoisobutyryloxy-ethyl methacrylate)-graft-poly(2-methacryloyloxyethyl phosphorylcholine (pBIBEM-g-pMPC). The polymer was designed for resistance to protein adsorption through a possible synergistic effect between the surface induced hydration layer by surrounding PC groups coupled with steric repulsion of the densely grafted chains. To test its potential as a surface modifier, the polymer was grafted from model silicone hydrogel CL through a 4-step surface initiated ATRP (SI-ATRP) in a similar manner to the free polymer. First, the ATRP initiator was immobilized from the HEMA OH groups of the unmodified CL, generating Intermedate-1. A polyHEMA brush was grafted from the initiating sites yielding pHEMA-50, followed by the generation of a second initiator layer (Intermediate- 2). A sequential ATRP of poly(MPC) then generated the target pMPC-50/pMPC-100 surfaces. For the free pBIBEM-g-pMPC polymer analysis, 1H-NMR and GPC determined polymers formed with a predictable MW and low polydispersity (PDI). For surface grafting, using a sacrificial initiator, 1H-NMR and GPC indicated that the pHEMA-50 and pMPC-50/pMPC-100 polymers were well-controlled, with a MW close to the theoretical and a low PDI. For surface chemical composition, ATR-FTIR showed the presence of the ATRP initiator (Intermediate-1 and 2) by the appearance of a C-Br peak and disappearance of the OH peak. XPS confirmed the chemical composition of the 4-step synthesis by a change in the fraction of expected surface elements. Both the surface wettability and EWC of the materials increased upon pMPC modification, further improving upon increasing pMPC chain length. The contact angle was as low as 16.04 ± 2.37º for pMPC-50 surfaces and complete wetting for pMPC-100. Finally, the single protein adsorption using lysozyme and bovine serum albumin (BSA) showed significantly decreased protein levels for pMPC-50/100 lenses, as much as 83% (p 0.00036) for lysozyme and 73% (p 0.0076) for BSA, with no significant difference upon chain length variation. The aforementioned data demonstrates that the novel polymer has potential in providing an anti-fouling and extremely wettable surface, specifically regarding silicone hydrogel CL surfaces.