Introduction: Porous iron was proposed for biodegradable metal-based bone scaffolds due to its strength, degradability and essential roles in human body metabolism[1],[2]. The foam structure greatly enhanced its degradation rate, but its excessive degradation could be harmful to the wound healing, especially at the early stage of operation[3]. Coating can therefore be developed to modulate the degradation rate of the iron foam structure. In this work, the degradation of pure iron foam was controlled by calcium phosphate (CaP) coating, which also has the potential to increase surface bioactivity toward bone cell proliferation.
Methods: Open-porous pure iron foam (purity 99.9%, pore size = 800 um, porosity = 88%) was used as the substrate material. Before the coating process, the substrate was pre-treated in the 2% HNO3 solution to remove the oxide on the surface. CaP coating was deposited by chemical conversion method. The electrolyte was prepared by dissolving 2 mol/L Ca(NO3)2 in 10 ml/L H3PO4 at pH 2.8 and 60°C. After deposition, the specimens were rinsed in distilled water and then were dried. Coating characterization was done by using SEM, EDS and XPS. Compression test was conducted using 1 kN load cell at 0.001 mm/s strain rate on specimens with dimension 15x12.5x2.6 mm3. Corrosion resistance of the CaP coated and uncoated iron foam was evaluated by electrochemical tests, including open circuit potential (OCP), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in modified Hank’s solution at 37°C.
Results and Discussion: Figs. 1a-1d show typical surface morphologies of the CaP coated and uncoated iron foam. It can be observed that the iron foam is completely covered with the random distributed flakes of high roughness CaP coating. The EDS result (Fig. 1e) shows that both of the Ca and P contents are around 13at%, while the Fe content is 1.3at%. The Ca/P atom ratio is 1.1, slightly higher than the stoichiometric Ca/P ratio of dibasic calcium phosphate dihydrate (DCPD). The XPS survey scanning spectrum (Fig. 1f) indicates that chemical composition of all coatings consists of O, P, Ca and Fe, except the high concentration of carbon, which is common in XPS analysis due to extraneous hydrocarbons from the environment. The high-resolution spectra of the Ca 2p peak (Fig. 1g) splits into two peaks as a result of spin orbit splitting. The binding energy of the Ca 2p3/2 peak of the CaP coating can be fitted to DCPD (347.6 eV).
Fig. 1: The typical surface morphologies of: (a,b) uncoated and (c,d) CaP coated iron foam, (e) EDS and (f-g) XPS results.
The compression test results (Fig. 2) show an improvement on the stiffness of the coated iron compared to the uncoated samples. However, the coating reduces yield stress, compression strength, and toughness. This could be related to the change of the foam behavior in the plateau region.
Fig. 2: Compression stress vs strain curves of the CaP coated and uncoated iron foam.
The electrochemical test results (Fig. 3) show a lower degradation rate of the coated iron compared to the uncoated samples by 7.4 times from the polarization test results (Fig. 3b). From the EIS curves (Fig. 3c), it could be observed that the coating resistance is more than 25 times higher than that of the pure iron substrate. The lower corrosion rate for the coated foam is helpful to decrease the harmful degradation products of pure iron implants at the early stage of operation.
Fig. 3: The electrochemical tests results of the CaP coated and uncoated iron foam: (a) OCP, (b) potentiodynamic polarization and (c) EIS.
Conclusion: The conversion coating process was able to deposit CaP coating on the surface of 3D structure porous iron. The CaP coating changes the elastic and plastic behavior of the iron foam. The CaP coated iron has lower degradation rate compared to the uncoated samples by 7.4 times from the polarization test results.
This work was supported by the NSERC-Canada, CFI-Canada, the Research Center of CHU de Quebec, Division of Regenerative Medicine, and the China Scholarship Council (CSC). SY was awarded of a mobility grant from CSC.
References:
[1] P. P. Mueller, et-al, Biomaterials 2006;27:2193-2200.
[2] N. M. Daud, et-al, Journal of Orthopaedic Translation 2014;2:177-184.
[3] M. Peuster, et-al, Biomaterials 2006;27:4955-4962.