Abstract
Purpose :
An atomic force microscopy (AFM) indentation method was employed to measure the surface softness of a novel reusable biomimetic silicone hydrogel (lehfilcon A) contact lens with surface modification of a cross-linkable bioinspired 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer. The purpose of the study was to measure and compare the surface softness of the lehfilcon A contact lens, the cornea, and other reusable SiHy contact lenses including comfilcon A, senofilcon A, senofilcon C, and samfilcon A.
Methods :
An AFM nanoindentation method was developed to test corneas and contact lenses in fully hydrated conditions. The AFM probe used for the indentation experiments had a design (tip size, geometry and spring constant) especially suited to characterize soft materials and biological tissues, and it allowed precise contact-point determination as well as accounted for the fluid-load support effect. Indentation force-curves (FC) were generated for the cornea and contact lens samples at a fixed force, chosen to generate a contact pressure within the physiological range of the upper eyelid pressure on the human ocular surface. These FC were then analyzed to calculate the surface modulus values to conclude relative comparisons in the surface softness.
Results :
The improved sensitivity and accuracy of the indentation method allowed precise measurement of the extremely low surface softness for the cornea and lehfilcon A contact lens. The surface modulus of the lehfilcon A contact lens (46±7 kPa) was similar to that of the cornea sample (55±31 kPa), but significantly lower than those of the comfilcon A (231±51 kPa), senofilcon C (251±47 kPa), senofilcon A (257±36 kPa), and samfilcon A (266±39 kPa) contact lenses (p<0.05 for all).
Conclusions :
A new AFM indentation method with increased sensitivity and accuracy was developed to measure the surface modulus of a novel biomimetic MPC surface-modified SiHy contact lens as well as the cornea and other reusable SiHy contact lenses. The results indicated that the surface of the lehfilcon A contact lens was as soft as the cornea surface and was significantly softer than the comfilcon A, senofilcon A, senofilcon C, and samfilcon A contact lenses. The surface softness of a contact lens might affect the mechanical contact stress on ocular tissues thus its impact on eye comfort.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.