Impact of manufacturing technology and material composition on the surface characteristics of hydrogel contact lenses

Background: Our aim was to investigate the impact of manufacturing method and material composition on the surface characteristics of hydrogel contact lenses. Methods: Five lens types were examined; three polyhydroxyethyl methacrylate (pHEMA) lenses, each manufactured by a different technique, namely, lathing, spin-casting and cast-moulding, a HEMA/methacrylic acid cast-moulded lens and a HEMA/glycerol methacrylate cast-moulded lens. Six lenses of each type were examined (front and back) using scanning electron microscopy (SEM). Additionally, both surfaces of three lenses from each of the pHEMA lens groups were examined, partially hydrated, using an atomic force microscope (AFM). Qualitative data were gathered for both SEM and AFM studies in addition to root-mean-square (RMS) roughness values for the lenses investigated with AFM. Results: The surfaces of the lathed lenses were covered in lathing/polishing marks. RMS roughness values for the anterior surface (10.9 ± 4.3 nm) were significantly greater (p = 0.02) than those of the posterior surface (9.3 ± 0.8 nm). The two surfaces of the spun-cast lens appeared similar by SEM but AFM RMS roughness values were greater (p = 0.02) for the anterior (12.3 ± 1.8 nm) than the posterior (5.8 ± 1.9 nm) surface. Both SEM and AFM showed similar topographic appearances for the surfaces of the cast-moulded pHEMA lens, although RMS roughness values were greater (p = 0.02) for the anterior (5.8 ± 0.9 nm) than the posterior (3.9 ± 0.3 nm) surface. All three cast-moulded lenses had more processing debris than the lathed and spun-cast pHEMA lenses. Overall, the surfaces of the lathed lens were 'rougher' than those of the cast-moulded lens (p = 0.01). Conclusion: The surface topographies of the hydrogel contact lenses are dependent on the method of manufacture. Cast-moulded lenses are associated with apparently 'stickier' surfaces, which may be indicative of surface degradation during the manufacturing process.