Effect of covalently bound RGD-peptide on PEEK on human osteoblasts


Background: Due to its excellent properties, the high-performance biomaterial PEEK (polyetheretherketone) might represent a viable alternative to titanium as implant material. However, due to its bioinertness PEEK has to be surface-modified to achieve a sufficient osseointegration when being used as material for dental implants. Therefore, a coating of the covalently bound peptide Arg-Gly-Asp-Ser (RGD) on the PEEK surface might represent a possible solution to enhance cell adhesion onto PEEK.
Aim/Hypothesis: The Aim of the study was to evaluate the effects of a RGD-peptide coating on human osteoblasts in vitro compared to titanium and pure PEEK assuming that the covalently bound RGD-peptide enhances the cell adhesion and cell differentiation.
Material and Methods: Twenty Disc-shaped specimens were fabricated from PEEK (Vestakeep® i2 R, Evonik Industries, Essen, Germany) and titanium (n=10). The samples were polished up to 4000 grit. Afterwards, ten of the PEEK samples were coated by RGD-peptide (RGD-PEEK) by means of Schiff base formation using ethylene diamine and following crosslinker-mediated fixation of the RGD-peptide, whereas ten samples were left untreated (PEEK).
Each sample was incubated at 37 °C with 1 ml cell suspension containing 30,000 human osteoblasts. To evaluate the cell adhesion, cell differentiation and cell viability, the cell-covered area (CCA), the alkaline phosphatase (ALP) and the metabolization of alamar blue (AB) were determined after 1 and 21 days.
Results: After 1 and 21 days, the PEEK-samples showed the highest CCA (21.4 ± 5.8 % and 36.8 ± 15.6 %), followed by RGD-PEEK (14.8 ± 1.6 % and 33.0 ± 3.9 %) and titanium (7.7 ±4.7 % and 31.0 ± 18.4 %), whereas the increase in the CCA was significant for the RGD-PEEK samples. The differences in the CCA between the three groups were not significantly different.
After 1 day, the PEEK-samples showed the highest ALP extinction value (0.2 ± 0.01), followed by RGD-PEEK (0.19 ± 0.04) and titanium (0.07 ± 0.02). The ALP values increased after 21 days, whereas RGD-PEEK showed the highest extinction value (0.51 ± 0.16) which was significant different to day 1, followed by PEEK (0.31 ± 0.11) and titanium (0.29 ± 0.15). No significant difference was found between the groups 21 days.
The extinction values of AB increased from day 1 to day 21 for all groups (PEEK: from 0.89 ± 0.06 to 1.23 ± 0.06; RGD-PEEK: from 0.87 ± 0.06 to 1.18 ± 0.02; titanium: from 0.95 ± 0.2 to 1.15 ± 0.06). 
Conclusion and Clinical implications: Regarding the parameter ALP, which reflects the cell differentiation of the osteoblasts, and the CCA, representing the cell adhesion ability of the distinct surface, the covalently bound RGD-peptide as PEEK coating seems to represent one option to achieve a sufficient osseointegration of a PEEK dental implant. This has to be evaluated in vivo.