Physical patterning of silicone hydrogel to alter surface wettability, tear lipid deposition and bacterial adhesion

Abstract

A hydrogel is defined as a three-dimensional network of polymer chains that can swell and retain a significant fraction of water inside its structure without dissolving in water. Outstanding properties of a hydrogel such as high biocompatibility, low toxicity and good tissue mechanical matching make it popular in the contact lens area. As a newly developed material, silicone hydrogel (SiHy) has significantly increased oxygen permeability and wearer comfort. However, three major problems still exist are tear film deposition, reduced surface wettability, and microbial contamination. Generally, chemical modifications can be applied to optimize hydrogel surface properties to improve the performance of hydrogels. In recent years, a new type of surface modification has been discovered to alter hydrogel properties by physically patterning the hydrogel surfaces with topographies. Therefore, we hypothesize that the SiHy surface properties can also be changed by adding surface topographies. In this project, patterns with different dimensions (diameters, heights and spacing distances) were applied to the SiHy surface, and we also evaluated their effects on hydrogel surface wettability, lipid deposition and microbial adhesion. Therefore, a variety of surface patterns were first fabricated on polypropylene (PP), by embossing with Silicon wafer, as molds with the mirror patterns for the subsequent SiHy fabrication. PP films from three different sources was used for patterning, and the roughness and patterning fidelity were investigated. Based on the characterization, PP pellets provided by our collaborator were chosen for mold fabrication as they created fewer defects. SiHy samples were subsequently fabricated by the collaborator using the PP mold. We used both a 3D laser confocal microscope and an atomic force microscope (AFM) to measure the actual surface structure dimensions of the PP molds and SiHy samples. Among different ways to examine the material surface wettability, in this study, the captive bubble method was chosen to determine SiHy wettability. The static and dynamic water contact angles were measured. Further studies were undertaken to test the lipid deposition and bacterial adhesion on surface patterned SiHy samples. Commercial hydrogel contact lenses were used in preliminary tests to evaluate the experimental setup. Based on the data that we obtained, pattern 11 showed outstanding performance in increasing surface wettability and reducing lipid deposition. However, it also generated higher microbial adhesion compared with other patterns. The principal component analysis (PCA) showed that the lipid deposition was more correlated to the contact angle hysteresis and static water contact angle, while the microbial adhesion was more correlated to the ratio of spacing and diameter. Therefore, surface topography did alter the surface properties of SiHy samples, but further studies are still necessary to figure out how performance on wettability, lipid deposition, and microbial adhesion can be improved synchronously.