Characterization of the surface of conventional hydrogel and silicone hydrogel contact lenses by time-of-flight secondary ion mass spectrometry

PURPOSE: To characterize the surfaces of unworn conventional hydrogel and silicone hydrogel contact lenses. METHODS: Near-identical formulations of poly(hydroxyethyl methacrylate) (pHEMA) were used to manufacture lathe-cut, spun-cast, and cast-molded contact lenses. The surfaces of two of each of these lens types and two of each of two commercially available silicone hydrogel lenses-balafilcon A (PureVision) and lotrafilcon A (Focus Night and Day)-were analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS). RESULTS: The ToF-SIMS spectra revealed the presence of the bulk polymer pHEMA at the surface of all three hydrogel lenses, along with other contaminants, such as poly(dimethyl siloxane), alkyl sulfates, alkyl-aryl sulfonates, dioctyl phthalate, Irgafos 168, sodium, chlorine, aluminum, potassium, calcium, copper, and fluorine, which are primarily derived from the various processing steps undertaken in lens manufacture, handling, and storage. The amount of bulk polymer detected at the surface of the PureVision lens was greater than that detected at the surface of the Night and Day lens. In addition, contaminants similar to those found on the surfaces of the conventional hydrogel lenses were detected. The Focus Night and Day lens appears to be coated with an organo-nitrogen material, which results from the plasma deposition of reactive precursors on the surface. CONCLUSIONS: We confirm that ToF-SIMS has the capacity to characterize the surface chemistry of contact lenses. The ongoing application of this technique can assist researchers and clinicians to understand the clinical performance of contact lenses.