Surface Activation and Pretreatments for Biocompatible Metals and Alloys Used in Biomedical Applications

Abstract

To improve the biocompatibility of medical implants, a chemical composition of bone-like material (e.g., hydroxyapatite) can be deposited on the surface of various substrates. When hydroxyapatite is deposited on surfaces of orthopedic implants, several parameters must be addressed including the need of rapid bone ingrowth, high mechanical stability, corrosion resistance, biocompatibility, and osseointegration induction. However, the deposition process can fail due to poor adhesion of the hydroxyapatite coating to the metallic substrate. Increasing adhesion by enhancing chemical bonding and minimizing biocoating degradation can be achieved through surface activation and pretreatment techniques. Surface activation can increase the adhesion of the biocoating to implants, providing protection in the biological environment and restricting the leaching of metal ions in vivo. This review covers the main surface activation and pretreatment techniques for substrates such as titanium and its alloys, stainless steel, magnesium alloys, and CoCrMo alloys. Alkaline, acidic, and anodizing techniques and their effects on bioapatite deposition are discussed for each of the substrates. Other chemical treatment and combination techniques are covered when used for certain materials. For titanium, the surface pretreatments improve the thickness of the TiO₂ passive layer, improving adhesion and bonding of the hydroxyapatite coating. To reduce corrosion and wear rates on the surface of stainless steel, different surface modifications enhance the bonding between the bioapatite coatings and the substrate. The use of surface modifications also improves the morphology of hydroxyapatite coatings on magnesium surfaces and limits the concentration of magnesium ions released into the body. Surface treatment of CoCrMo alloys also decreased the concentration of harmful ions released in vivo. The literature covered in this review is for pretreated surfaces which then undergo deposition of hydroxyapatite using electrodeposition or other wet deposition techniques and mainly limited to the years 2000-2019.

Figure 1

The approximate number of published articles of HAp deposition on different types of biomedical substrates from 2000 to 2019.

Figure 2

A schematic of apatite formation on the surface of alkali and heat-treated porous Ti based alloy scaffold soaking in SBF [26].

Figure 3

SEM images of chemically treated Ti in (a) NaOH, (b) H₃PO₄ + H₂O₂ solution, and (c) electrochemically treated in NH₄F + glycerol + water electrolyte (20V for 2 h) [38].

Figure 4

SEM images of titanium oxides that are anodically prepared under different anodizing conditions: (a) chemical etch in 0.5 wt% HF for 30s, (b) aqueous 0.3 wt% HF + 1 M H₃PO₄ at 20 V, (c) aqueous 0.5 wt% HF + 1 M H₃PO₄ at 20 V, (d) aqueous 0.5 wt% HF + 1 M H₃PO₄ at 10 V, and (e) aqueous 0.5 wt% HF + 1 M H₃PO₄ at 150 V [42].

Figure 5

Variations of a metal surface at the impact point of a grain during sandblasting process. R: radius grain; r_k: radius grain spike; r_s: radius melting zone; r_t: radius texture disturbance [43].

Figure 6

SEM images of three types of inclusions after initiation and propagation of pitting corrosion in X70 steel: (a) Type A (particles of (Al, Ca)O and (Mn,Ca)S); (b) Type B ((Al,Ca)O), and (c) Type C ((Mn, Ca)S). Steel was immersed in 0.1 mol/L NaCl and 0.5 mol/L NaHCO₃ solutions at 25°C for times indicated in figure [50].

Figure 7

Schematic diagram illustrating the corrosion failure and species present for surface modified magnesium and its alloys.

Figure 8

SEM images of HAp coatings electrochemically deposited onto CoCrMo alloy with (a) 0 mM aspartic acid and (b) 10 mM aspartic acid. (Courtesy I. Coskun and T.D. Golden, 2018).