Ocular bio-lubricating materials: from lubrication mechanism to dry eye syndrome treatment

The human eye, a mechanically dynamic and physiologically vital organ, sustains continuous mechanical activity through repetitive blinking-averaging approximately 20 000 cycles daily, while exhibiting exceptional lubrication performance characterized by an ultralow coefficient of friction (<0.01). This remarkable lubricating functionality is mediated by the tear film, a multifunctional biological lubricant combining boundary lubrication mechanisms (via adsorbed mucins and lipids) and fluid film lubrication mechanisms to minimize friction and wear, and preserve ocular surface integrity. Failure of such ocular lubrication can cause tear film instability or ocular surface damage, leading to discomfort, visual dysfunction and dry eye syndrome. Ocular lubrication involves multiple structures and lubricants with highly complex biomolecular interactions. Insights into the structure of eyes, lubricant composition and causes of functional impairments are essential for addressing friction-related diseases in biological systems. This review examines ocular lubrication by first exploring the biological structure of the eyes and typical lubrication modes. Then, the characterization tools, such as tribometer, atomic force microscope and surface force balance in the field of ocular lubrication, are introduced, followed by a comparison of their working principles, applicable conditions and application fields. Finally, the specific causes of dry eye syndrome are outlined, along with current bio-lubricants, contact lenses and other ocular-inspired bio-lubricating materials.