Microplasma spraying of hydroxyapatite coatings on additive manufacturing titanium implants with trabecular structures

Abstract

This paper presents new results of microplasma spraying (MPS) of laboratory-synthesized hydroxyapatite (HA) powder coatings onto trabecular substrates obtained by selective laser melting (SLM) of a certified titanium medical alloy powder. The aim of the study was to establish the possibility of combining the technologies of MPS and additive manufacturing (AM) for the possible production of custom-designed implants with increased surface biocompatibility, as well as to establish the MPS parameters that ensure chemical purity of the HA coating and satisfactory adhesion of the coatings to the substrate. The structural-phase compositions of the initial HA powder and the plasma-sprayed HA coating were studied by X-ray diffraction analysis and transmission electron microscopy, and the adhesion strength of the coating was tested according to the F1147 standard of the American Society for Testing and Materials (ASTM). The main results of the study are the following: the application of the MPS technology for HA coating with an average thickness of 150±50 μm on trabecular substrates obtained by the SLM method has been shown. The parameters of MPS of HA coatings onto titanium implants with a trabecular surface have been established. It is also proved that using the appropriate MPS parameters, it is possible to obtain a HA coating with a 95% level of HA phases, 93% level of crystallinity, and the adhesion strength to the trabecular substrate of 24.7±5.7 MPa, which complies with the requirements of the international medical standard (International Organization for Standardization [ISO] 13779-2:2018). These results are of significance for a wide range of researchers developing plasma spray technologies for the manufacture of biocompatible coatings.

A Brief Review of Current Trends in the Additive Manufacturing of Orthopedic Implants with Thermal Plasma-Sprayed Coatings to Improve the Implant Surface Biocompatibility

“…Adaptive implant is a term referring to the use of enveloped design algorithms, such as scaling up the basic design of a medical device to adapt it to a patient's unique anatomy. Thus, AM allows the design and manufacture of customized implants with controlled volumetric porosity, which is most valuable for orthopedics, and complex geometries such as porous structures, honeycombs, and implants with a trabecular (lattice) surface in one production step [19]. Figure 1 shows 3D-printed titanium alloy products of complex geometry obtained in one SLM operating cycle, such as a detail of the endoprosthesis of the intervertebral disk (Figure 1a) and a honeycomb structure (Figure 1b), and also illustrates their excellent correspondence to stereolithographic models according to computed tomography data (Figure 1c,d).…”

“…Figure 1 shows 3D-printed titanium alloy products of complex geometry obtained in one SLM operating cycle, such as a detail of the endoprosthesis of the intervertebral disk (Figure 1a) and a honeycomb structure (Figure 1b), and also illustrates their excellent correspondence to stereolithographic models according to computed tomography data (Figure 1c,d). from their stereolithographic (stl) models [19].…”

“…This is a variable that depends on the complexity and size of the model [7,13,21]. According to the results of computed tomography analysis, the ratio of the volume of defects to the volume of material for the honeycomb structure was 0.41% and for the endoprosthesis part was 2.28% [19].…”

“…In addition, the development and use of 3D CAD models of implants for 3D printing provide a good opportunity to combine AM processes with subsequent robotic processing of the implant surface [19]. Surface treatment of the implant, as noted in the reviews [9,10], is necessary to improve the antibacterial properties as well as the properties of biocompatibility and bioactivity of the implant surface.…”

“…It should also be noted that in a review article [24], poor surface roughness is noted as a disadvantage of SLM technology; however, there are currently no systematic studies on how SLM or SLS (Selective Laser Sintering) parameters affect the surface roughness of scaffolds or the relationship between roughness and biocompatibility of scaffolds [26]. Thus, it can be assumed that the high roughness values achieved in the SLM process (for example, the average roughness Ra = 26.6 ± 3.4 µm of 3D printed tensile specimens of the titanium alloy Ti6Al4V mentioned in [19]) may increase the biocompatibility of 3D printed titanium implants. However, more research is needed to establish the nature of the dependence of biocompatibility on roughness, and researchers should focus on certain materials.…”