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 (γ_W/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 γ_W/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.

水包水 (W/W) 乳液由两个不混溶的水相自发液-液分离形成的液滴组成。W/W 界面的固有特性、低或超低的界面张力 (γ _W/W  = 1–1000 μN/m) 和几纳米的大厚度，导致乳液固有的稳定性差。在此，我们报告了一种具有席夫碱反应性以生成 W/W 乳液的纳米原纤维乳化剂。W/W乳液具有优异的稳定性（稳定>6个月），因为作为W/W乳液稳定剂的胶原纳米原纤维可以同时满足相界面的尺寸和整体覆盖率的要求。具有 γ _{W/W 的}W/W 乳液约 10 μN/m 用作合成滑液，显示出优异的润滑性能，摩擦系数范围为 0.003-0.011，已被证明适用于关节润滑。关节内注射评估进一步证实了这种对体内关节软骨的保护作用。我们的研究揭示了乳液稳定的机制，并开辟了使用 W/W 乳液对润滑人工植入物表面的生物润滑作用治疗骨关节炎 (OA) 的可能性。