Improvement of a pendulum-type apparatus for friction test of a contact lens to simulate the conditions of its actual usage

Friction between the contact lens (CL) and the corneal or conjunctival surfaces is considered one of key factors in triggering CL-associated adverse effects. However, the relationship between friction properties and these effects remains unclear. Traditional measurement methods often fail to replicate real-life conditions, thereby highlighting the need for more effective apparatus. In this study, the authors developed an optimised pendulum apparatus integrated with an inclinometer to enhance the measurement of CL friction coefficients, thereby improving its precision and relevance to clinical settings. This new design allows for faster and easier calculation of the friction coefficient based on the amplitude decay per libration cycle, surpassing the accuracy of previous video-based methods. The pendulum's hemisphere component was made from ethylene–propylene–diene monomer rubber (EPDM) 30, which has an elastic modulus similar to that of a human eyeball, creating a measurement environment that closely mimics real-world usage. The authors optimised the apparatus by evaluating the effects of hemisphere stiffness and saline volume on the friction coefficient. Measurements of multiple lenses recorded by the authors, particularly Lens A, made of narafilcon A, revealed significant consistency across different hemisphere materials with an optimal saline volume of 150 μL yielding a friction coefficient of 0.026 ± 0.003. No statistically significant differences in the friction coefficients were found across variations in the lens base curve, diameter, centre thickness, or power. This improved apparatus demonstrates the capability of effectively measuring friction coefficients under conditions that simulate clinical usage, providing rapid and reliable results. The findings validate the apparatus and suggest its potential for broader applications in assessing CL properties, thereby facilitating future research on the material characteristics and safety of various CLs, including decorative lenses.