Drawing inspiration from the mechanisms that lubricate the cartilage in our joints over a lifetime of wear, researchers designed extremely slippery hydrogels with self-renewing, lipid-based boundary layers, which result in a near 100-fold reduction in friction and wear over other hydrogels. The approach may provide a method for sustained hydrogel lubrication in a variety of biomedical applications that require low-friction low-wear materials like tissue engineering and biosensor development. For many biomedical applications, hydrogels must be able to slide against one another without generating excess friction or damaging wear. In many cases, hydrogel lubrication is achieved through fluids trapped in the hydrogel, forming a slippery interface at the surface. In contrast, the long-lasting and self-replenishing lubricity of the articular cartilage has been, in part, attributed to nonfluid boundary layers that expose phosphatidylcholine (PC) lipids. Inspired by the long-lasting lubrication of the complex biohydrogel in our joints, Weifang Lin and colleagues adapted the mechanism to lubricate synthetic hydrogels. Lin et al. synthesized several different hydrogels that incorporated trace amounts of PC lipids. The incorporated lipids migrate to the hydrogel's surface, creating a molecularly thin, slippery boundary layer. As the lipid-based boundary layer wears, more lipids are exuded, continually regenerating the lubricated surface. According to the results, the approach resulted in a significant reduction in both friction and wear compared to lipid-free hydrogels and over a wide range of conditions. What's more, the gels maintained their lubricity and self-lubricating capabilities after being dried and rehydrated. "Lin et al. have demonstrated a simple yet effective way of creating self-lubricating hydrogels through incorporation of lipids with minimal effect on bulk mechanical properties," writes Tannin Schmidt in a related Perspective. "Indeed, this discovery has potential applications and utility in numerous fields relevant to biology and medicine, and it will be of great interest to see where future research and applications lead," Schmidt writes.
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Science