Selective adsorption of phosphatidylcholine lipids improves lubrication

Phospholipids (PLs) lining on the outer layer of the cartilage surface, together with biomacromolecules hyaluronic acid (HA) and lubricin, are considered to be responsible for the extremely low friction coefficients at physiologically high pressure.¹ Hundreds of PLs bearing different headgroups, alkyl chain lengths, and degrees of unsaturation have been identified both in the synovial fluid and on the cartilage surface.^2,3 The major PL classes, phosphatidylcholine (PC), sphingomyelin (SPM), and phosphatidylethanolamine (PE), in forms of a single component or binary mixtures, are all capable of maintaining similar or superior lubrication ability to articular joints by means of hydration lubrication.^4,5 However, the PL mixture in synovial fluid is enriched in unsaturated PLs, which are not able to afford comparable lubrication on biomimicking HA-coated surfaces.⁶

In this work, we studied the adsorption of mixed saturated and unsaturated PC lipids, DSPC and POPC, on HA modified mica surface by fluorescent labelling and imaging, and measured the normal and shear forces between PC layers adsorbed on HA using a surface force balance (SFB). The results show that DSPC molecules can replace most of the already adsorbed POPC layer on HA, which largely reduces the friction coefficients by two orders of magnitude at elevated normal load even with mixtures containing 5 mol% DSPC-SUVs. Based on these preliminary results, we conclude that the gel-state PC lipids in the bulk dispersion selectively adsorb on biomimicking substrates and improves boundary lubrication. Taking into consideration that the PL composition in synovial fluid differs from that on the cartilage surface,^2,3 PLs in synovial fluid might selectively adsorb on the cartilage surface, in a more complicated way under biological conditions,⁷ which contributes to the remarkable biolubrication in articular joints.

References:

(1) J. Annu. Rev. Biomed. Eng. 2016, 18 (1), 235–258.

(2) J. Orthop. Res. 2001, 19, 671–676.

(3) J. Arthritis Rheum. 2013, 65 (9), 2323–2333.

(4) Biomaterials 2013, 34 (22), 5465–5475.

(5) Langmuir 2019, https://doi.org/10.1021/acs.langmuir.9b01660.

(6) Acta Biomater. 2017, 59, 283–292.

(7) Adv. Colloid Interface Sci. 2019, 274, 1–12.