Neutral polyphosphocholine-modified liposomes as boundary superlubricants

TL;DR

This study demonstrates that neutral polyphosphocholine-modified liposomes act as boundary superlubricants, achieving ultralow friction by stabilizing water-soluble surface coatings capable of adsorbing on negatively charged surfaces.

Contribution

It introduces a novel neutral lipid-PMPC conjugate that stabilizes liposomes and achieves superlubricity, extending previous work on charged conjugates.

Findings

Neutral liposomes exhibit superlubricity with friction coefficients around 10^-3.

Negatively charged conjugates cannot adsorb on negatively charged surfaces.

The method enables stable liposome adsorption on negatively charged surfaces.

Abstract

Boundary lubrication is associated with two sliding molecularly thin lubricated film-coated surfaces, where the energy dissipation occurs at the slip-plane between lubricated films. The hydration lubrication paradigm, which accounts for ultralow friction in aqueous media, has been extended to various systems, with phosphatidylcholine (PC) lipids recognized as extremely efficient lubrication elements due to their high hydration level. In this work, we extend a previous study (Lin et al., Langmuir 35 (2019) 6048-6054), where a charged lipid-poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) conjugate was prepared, to the very different case of a neutral lipid-PMPC) conjugate. This neutral molecule stabilizes the liposomes by attaching highly water-soluble PMPC to the surface of liposomes with its lipid moieties incorporated in the lipid bilayers. Such neutral polyphosphocholinated liposomes provide a surface lubricity which is well within the superlubrication regime (coefficient of friction = ca. 10-3 or even lower). In contrast, negatively charged lipid/polyphosphocholine conjugates modified liposomes were unable to adsorb on negatively-charged (mica) surfaces. Our method provides stable liposomes that can adsorb on negatively charged surfaces and provide superlubricity.