Plasma-Sprayed Bioactive Ceramic Coatings with High Resorption Resistance Based on Transition Metal-Substituted Calcium Hexaorthophosphates

• Main Author: Robert B. Heimann
• Format: Article
• Language: English
• Published: MDPI AG 2019-06-01
• Series: Materials
• Subjects: NaSiCons; NZP structure; osseoconductive coatings; plasma spraying; solubility; ionic conductivity; cell proliferation; <i>in vivo</i> testing
• Online Access: https://www.mdpi.com/1996-1944/12/13/2059

Calcium (titanium, zirconium) hexaorthophosphates with a [NZP] (sodium zirconium phosphate) structure belonging to the NaSiCon (<b>Na</b> <b>S</b>uper<b>i</b>onic <b>Con</b>ductor) family were deposited by atmospheric plasma spraying onto the surfaces of Ti6Al4V substrates. (NaSiCon <i>sensu strictu</i> refers to solids with the chemical formula Na_1+xZr₂Si_xP_3&#8722;xO₁₂, 0 &lt; x &lt; 3. In a broader sense, it is also used for similar compounds where Na, Zr and/or Si are replaced by isovalent elements). The microstructure of the coatings revealed the incongruent melting of the precursor material as ascertained by electron probe microanalysis (EPMA). The adhesion of the coatings to the substrate surface was within the limits specified for biomedical coatings. The solubility of the coatings was tested by immersion in 0.2 molar tris&#8722;hydroxymethyl&#8722;amino&#8722;methane&#8722;HCl (TRIS&#8722;HCl) buffer and found to be at least one order of magnitude lower than that of conventional hydroxylapatite coatings deposited under comparable conditions. <i>In vitro</i> biocompatibility tests with primary rat bone marrow cells (BMCs) showed a substantial cell proliferation in the presence of fetal bovine serum. Animal tests confirmed that coatings based on calcium (titanium, zirconium) hexaorthophosphates applied to Ti6Al4V rods implanted in the femoral medulla of sheep led to the strong neoformation of dense bone at a stable interface implant-bioceramic coating without coating delamination. Hence, based on their multifarious advantageous properties in the biomedical context, CaTi_4-xZr_x(PO₄)₆ ceramics may be considered the &#8216;Sleeping Beauty&#8217; of osseoconductive coatings for the stem of hip endoprostheses and dental root implants, osteosynthetic fixation devices, and bioelectric devices including bone growth stimulators.