Recent advances in ocular lubrication

The ocular lubrication, where the eyelid constantly slides on the curved corneal surface, is considered as one of primary lubrication systems in bio-tribology. Under reliable lubrication conditions, sensitive ocular tissues remain intact from fatigue damage during spontaneous blink cycles. The tear film, evenly filled between cornea and conjunctiva, is a biological fluid with dynamic adjustment ability, which provides superior lubrication with the friction coefficient of below 0.01. However, the lubrication failure may result in a variety of uncomfortable symptoms such as inflammatory reactions, tissue damage and neurological abnormalities. Therefore, it is essential to clarify the fundamental mechanism of ocular lubrication, which helps to alleviate and even recover from various ocular symptoms. This review firstly demonstrates that the ocular components, containing lipids and mucins, contribute to maintaining the lubrication stability of tear film. Furthermore, the ocular lubrication state in various physiological environments and the physical effect on tear film dynamics are further discussed. As typical applications, the therapeutic agents of dry eye syndrome and contact lens with superior lubrication effects are introduced and their lubrication mechanisms are clarified. Finally, this review summarizes a series of the latest research inspired by ocular lubrication. Overall, this work will provide a valuable guidance on the theoretical research and extensive applications in the field of biological lubrication.