Water-in-Water Emulsions, Ultralow Interfacial Tension, and Biolubrication

A water-in-water (W/W) emulsion consists of droplets formed by the spontaneous liquid–liquid separation of two immiscible aqueous phases. The inherent properties of the W/W interfaces, low or ultralow interfacial tension (γW/W = 1–1000 μN/m) and a large thickness of several nanometers, beget the poor inherent stability of emulsions. Herein, we report a nanofibril emulsifier having Schiff base reactivity to generate a W/W emulsion. The W/W emulsion has superior stability (stable > 6 months) because collagen nanofibrils, acting as a stabilizer of W/W emulsions, can simultaneously satisfy the requirements of size and overall coverage ratio of the phase interfaces. W/W emulsions having γW/W ∼10 μN/m were used as synthetic synovial fluids, showing superior lubrication performance with a coefficient of friction in the range of 0.003–0.011, which has been demonstrated to be suitable for joint lubrication. An intraarticular injection assessment further confirmed this protective effect on articular cartilage in vivo. Our study reveals the mechanism of emulsion stabilization and opens up the possibility of osteoarthritis (OA) treatment using the biolubrication effects of W/W emulsions for lubricated artificial implant surfaces.