Biocompatible Organic Coatings Based on Bisphosphonic Acid RGD-Derivatives for PEO-Modified Titanium Implants

Abstract

Currently, significant attention is attracted to the problem of the development of the specific architecture and composition of the surface layer in order to control the biocompatibility of implants made of titanium and its alloys. The titanium surface properties can be tuned both by creating an inorganic sublayer with the desired morphology and by organic top coating contributing to bioactivity. In this work, we developed a composite biologically active coatings based on hybrid molecules obtained by chemical cross-linking of amino acid bisphosphonates with a linear tripeptide RGD, in combination with inorganic porous sublayer created on titanium by plasma electrolytic oxidation (PEO). After the addition of organic molecules, the PEO coated surface gets nobler, but corrosion currents increase. In vitro studies on proliferation and viability of fibroblasts, mesenchymal stem cells and osteoblast-like cells showed the significant dependence of the molecule bioactivity on the structure of bisphosphonate anchor and the linker. Several RGD-modified bisphosphonates of β-alanine, γ-aminobutyric and ε-aminocaproic acids with BMPS or SMCC linkers can be recommended as promising candidates for further in vivo research.

Keywords: RGD peptide; bisphosphonic acid; fibroblasts; human osteosarcoma cells; in vitro tests; mesenchymal stem cells; plasma electrolytic oxidation; titanium implants.

Scheme 1

Reagents and conditions: (a) MeSO₃H, 4–5 h, 85–90 °C; (b) DMF, 3–16 h, 0→25 °C; (c) H₂O: acetone = 1:1, pH = 8–9, 1 h, 38–40 °C; (d) H₂O, pH = 7, 1–1.5 h, 38–40 °C.

Figure 1

SEM images of the PEO coating: (a) Top view; (b) Cross-section.

Figure 2

XRD pattern of the PEO coating with the labeled peaks and SemiQuant results.

Figure 3

Survey XPS spectra of the Ti-PEO coating.

Figure 4

Polarization curves in Ringer’s solution for the Ti samples, with PEO coating, and RGD modification.

Figure 5

Electrochemical properties in Ringer’s solution for the Ti samples, with PEO coating, and RGD modification: (a) Corrosion potential E_corr; (b) Corrosion current i_corr; (c) Polarization resistance R_p.

Figure 5

Electrochemical properties in Ringer’s solution for the Ti samples, with PEO coating, and RGD modification: (a) Corrosion potential E_corr; (b) Corrosion current i_corr; (c) Polarization resistance R_p.

Figure 6

Optical density showing viability and proliferation of fibroblasts (FLECH-104), human osteoblast-like cells (MG-63) and mesenchymal stem cells (MSC) cultured on the surface of Ti-PEO functionalized by RGD-derivatives (15–22) after 7 days (metal samples were kept for 1 h at room temperature in the solutions of compounds 15–22 with concentrations 1.3 × 10⁻³–1.8 × 10⁻³ M/L and then dried).