Calcium-binding phospholipids as a coating material for implant osteointegration

Abstract

Among the many biomolecules involved in the bone mineralization processes, anionic phospholipids play an important role because of their ability to bind calcium. In particular, phosphatidylserine is a natural component of the plasmalemma and of the matrix vesicles generated from the osteoblast membrane to create nucleation centres for calcium phosphate crystal precipitation. In the present work, we demonstrate that calcium-binding phospholipids can be used as biomimetic coating materials for improving the osteointegration of metal implants. Relatively thick phosphatidylserine-based coatings were deposited on titanium coupons by dip-coating. Upon dehydration in a simulated body fluid phospholipids were quickly crosslinked by calcium and re-arranged into a three-dimensional matrix able to induce rapid formation of a calcium phosphate mineral phase. The rate of mineralization was shown to be dependent on the adopted coating formulation. In the attempt to closely mimic the cell membrane composition, heterogeneous formulations based on the mixing of anionic phospholipids (either phosphatidylserine or phosphatidylinositol) with phosphatidylcholine and cholesterol were synthesized. However, surface plasmon resonance studies as well as scanning electron microscopy and elemental analysis demonstrated that the homogeneous phosphatidylserine coating was a more effective calcification environment than the heterogeneous formulations.

Figure 1

Cryo-SEM analysis of PC : PS : C coating after 1 h incubation in Vogel's SBF. (a) Complete SBF (×5000), (b) complete SBF after 1-h treatment of the sample in human serum (×5000) and (c) calcium-depleted SBF (×15 000), circle highlights a single liposome.

Figure 2

PC : PS : C coating after 7 days incubation in Vogel's SBF. (a) Photograph of a partially coated titanium disk, (b) Cryo-SEM of the phospholipid exposed surface (×5000), (c) Cryo-SEM of the phospholipid exposed surface of a serum-treated sample (×5000) and (d) EDX of the mineral phase.

Figure 3

Atomic Force micrographs of pure PS coated disks in Vogel's SBF for (a) 15 min and (b) 30 min.

Figure 4

Cryo-SEM/EDX analysis of pure PS coated disks after 7-days incubation in Vogel's SBF. (a) Phospholipid matrix exposed surface, (b) phospholipid matrix fracture surface and (c) EDX of the mineral phase.