Impact of the physical properties of contact lens materials on the discomfort: role of the coefficient of friction

Contact Lens Discomfort (CLD) is the main cause in contact lens (CLs) discontinuation, referred in literature as drop-out phenomenon. Despite such evidence was reported in several clinical studies, a relationship between physico-chemical properties of CLs and CLD is not still totally understood. In this regard, the friction of CLs surfaces seems to be related to discomfort feeling events, probably due to an alteration of the lubricate function of the tear film after the CL placement inside the ocular environment. In the last years, many studies have been finalized to the friction measurements of CLs surface, finding conflicting data due to a lack in standardized protocol. The aim of this review is primarily to show evident relationships between CLs surface properties (i.e. wettability, tear evaporation, tear film quality, etc.) and the coefficient of friction (CoF), resulting therefore the most relevant physical quantity in the CLs characterization. In addition, we reported the most recent studies in CLs tribology, which highlight that the introduction of a standard protocol in CoF measurements is necessary to obtain reproducible results, considering the aim to evaluate in a more precise way the relationship between this material surface property and comfort in CLs users.