Gemini Interfaces in Aqueous Lubrication with Hydrogels

The sliding interfaces found in the body-within the eyes, the digestive system, and the articulating joints, for example-are soft and permeable yet extremely robust, possessing low friction. The common elements among these systems are hydrophilic biopolymer networks that provide physical surfaces, elasticity, and fluid permeability. Stiff, impermeable probes are traditionally used to assess the frictional properties of most surfaces, including soft, permeable materials. However, both sides of physiological articulating interfaces are soft and hydrated. Measuring the friction response on just one-half of the cornea-eyelid interface or the cartilage-cartilage interface using a stiff, impermeable probe may not reproduce physiological lubrication. Here, we present lubricity measurements of the interface between two soft, hydrated, and permeable hydrogels. We explore the distinctions between the self-mated "Gemini" hydrogel interface and hydrogels sliding against hard impermeable countersurfaces. A rigid impermeable probe sliding against a soft permeable hydrogel exhibits strong frictional dependence on sliding speed, and a hydrogel probe sliding against flat glass shows a strong friction dependence on time in contact. The twin Gemini interface shows very low friction mu < 0.06, with little dependence on sliding speed or time in contact. This consistently low-friction Gemini interface emulates the physiological condition of two like permeable surfaces in contact, providing excellent lubricity.