Improved Biocompatible, Flexible Mesh Composites for Implant Applications via Hydroxyapatite Coating with Potential for 3-Dimensional Extracellular Matrix Network and Bone Regeneration

Abstract

Hydroxyapatite (HA)-coated metals are biocompatible composites, which have potential for various applications for bone replacement and regeneration in the human body. In this study, we proposed the design of biocompatible, flexible composite implants by using a metal mesh as substrate and HA coating as bone regenerative stimulant derived from a simple sol–gel method. Experiments were performed to understand the effect of coating method (dip-coating and drop casting), substrate material (titanium and stainless steel) and substrate mesh characteristics (mesh size, weave pattern) on implant’s performance. HA-coated samples were characterized by X-ray diffractometer, transmission electron microscope, field-emission scanning electron microscope, nanoindenter, polarization and electrochemical impedance spectroscopy, and biocompatibility test. Pure or biphasic nanorod HA coating was obtained on mesh substrates with thicknesses varying from 4.0 to 7.9 μm. Different coating procedures and number of layers did not affect crystal structure, shape, or most intense plane reflections of the HA coating. Moduli of elasticity below 18.5 GPa were reported for HA-coated samples, falling within the range of natural skull bone. Coated samples led to at least 90% cell viability and up to 99.5% extracellular matrix coverage into a 3-dimensional network (16.4% to 76.5% higher than bare substrates). Fluorescent imaging showed no antagonistic effect of the coatings on osteogenic differentiation. Finer mesh size enhanced coating coverage and adhesion, but a low number of HA layers was preferable to maintain open mesh areas promoting extracellular matrix formation. Finally, electrochemical behavior studies revealed that, although corrosion protection for HA-coated samples was generally higher than bare samples, galvanic corrosion occurred on some samples. Overall, the results indicated that while HA-coated titanium grade 1 showed the best performance as a potential implant, HA-coated stainless steel 316 with the finest mesh size constitutes an adequate, lower cost alternative.

Comparison of microstructure and tensile behavior of hydroxyapatite-coated PEEK meshes and cellulose-based fabrics and mats

“…Those results confirm similar nanostructure to the one obtained for HA powders dried at higher temperature (700°C). 35

Figure 3.Representative HR-TEM images for HA coating powders (scratched off from substrates’ surface) dried at various temperatures: (a, b) 90°C (red arrow in a indicates nanorod length), (c, d) 150°C (red arrow in c indicates nanorod length). Pure HA plane spaces of d = 0.81 nm (001), d = 0.34 nm (002), and d = 0.31 nm (102) are marked in panels b and d. Scale bar is 50 nm for (a, c) and 20 nm for (b, d).

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“…For instance, previous work on flexible, HA-coated metallic meshes showed that mesh size (i.e., open areas) had an effect on extracellular matrix development. 35 Therefore, PEEK meshes and cellulose mats should be considered for future biocompatibility studies.…”

“…The open area was 56% with 220 μm opening size and a mesh size of 86 × 86 (86 openings per 25.4 mm length), selected based on previous work on metal meshes, literature, and commercial availability. 14,35 Second, as cellulosic fabric, a commercial grade 32 thread count cotton cross-stitch fabric was selected. As previously mentioned, hybrid composites made from PMMA and flexible scaffolds showed promise for customizable implants.…”

“…These coating conditions were selected based on preliminary tests and previous research on HA-coated meshes. 35 The drying temperature for PEEK meshes and cotton-based fabrics was chosen considering preliminary tests and our previous research (in which it was 150°C). A drying temperature of 90°C was used for cellulose mat samples, based on preliminary drying tests to avoid visible shrinkage and burning.…”

“…It was shown to lead to thin coatings (< 2 μm) with good adhesion for substrates made from a range of materials (metals and polymers). [31][32][33][34][35] The main drawbacks of this process is that the coating coverage depends on sol viscosity and the thickness is not uniform. Moreover, the temperature during the drying phase can affect polymeric substrates and deteriorate their mechanical properties.…”