(648h) Zwitterionic Contribution to the Hydration Lubrication Dynamics of Poly(2-methacryloyloxyethyl phosphorylcholine)

Poly(2-methacryloyloxyethyl phosphorylcholine), or pMPC, is a biocompatible polymer shown to exhibit remarkable tribological properties and has received extensive experimental study as a possible artificial synovial fluid [1], especially as a lubricant for orthopedic implants [2]. These studies have attributed the lubricity of pMPC to a hydration lubrication [3] mechanism, but insight related to the underpinnings of this mechanism at the atomic scale is still lacking. In previous work [4,5], we showed that the polymer’s zwitterionic character, specifically the charged phosphoryl and choline groups, are integral to the hydration layers that form. This is due to a combination of the polymer’s natural affinity for hydration (i.e. hydrophilicity), and more subtly, the inter-monomer phosphoryl-choline interactions which aid in “unfurling” the polymer structure, allowing for increased hydration of the phosphoryl groups. Although our previous efforts have been instrumental in determining the atomic contributions to the hydration lubrication mechanism, our understanding of the intermonomeric effects that assist this mechanism (monomer-water hydrogen bonding, monomer-monomer hydrogen bonding, etc) is still lacking.

In this work we explore the hydration structure and dynamics of pMPC polymers of varying chain lengths, pMPCn, in solution using molecular dynamics simulations. System setup and parameterization was aided by the open source Molecular Simulation and Design Framework6 (MoSDeF), developed at Vanderbilt. We compare bulk solvated pMPCn to poly(2-methacryloyloxyethyl phosphate)n (pMPn) to quantify the effect that the choline group has on hydrogen bonding between the pMPC polymer and water. When the choline group is removed, total hydrogen bonding between the pMP polymer and water is found to be reduced by approximately a factor of 2. The choline group plays an essential, but unintuitive role in the stabilization and increased hydration of the polymer through inter-monomer hydrogen bonding of the choline-phosphoryl groups. Although these interactions between the phosphoryl and choline groups limit possible water-phosphoryl hydrogen bonding, the “unfurling” effect these interactions have on the polymer exposes more hydration sites overall, leading to the pronounced hydration lubrication mechanism observed.

References

[1] Chen, M., Briscoe, W. H., Armes, S. P., & Klein, J. (2009). Lubrication at Physiological Pressures by Polyzwitterionic Brushes. Science, 323(5922), 1698–1701. https://doi.org/10.1126/science.1169399

[2] Kyomoto, M., Moro, T., Saiga, K., Hashimoto, M., Ito, H., Kawaguchi, H., … Ishihara, K. (2012). Biomimetic hydration lubrication with various polyelectrolyte layers on cross-linked polyethylene orthopedic bearing materials. Biomaterials, 33(18), 4451–4459. https://doi.org/10.1016/j.biomaterials.2012.03.028

[3] Klein, J. (2013). Hydration lubrication. Friction, 1(1), 1–23. https://doi.org/10.1007/s40544-013-0001-7

[4] Klein, C., Iacovella, C. R., McCabe, C., & Cummings, P. T. (2015). Tunable transition from hydration to monomer-supported lubrication in zwitterionic monolayers revealed by molecular dynamics simulation. Soft Matter, 11(17), 3340–3346. https://doi.org/10.1039/C4SM02883J

[5] W.L. Roussell, C. Klein, C.R. Iacovella, P.T. Cummings, C. McCabe, Molecular Origins of the Ultra-Low Friction Exhibited by Biocompatible Zwitterionic Polymer Brushes Young Scientist, May 2015

[6] “MoSDeF” [Online]. Available: https://github.com/mosdef-­hub.