Coating Techniques for Functional Enhancement of Metal Implants for Bone Replacement: A Review

Abstract

To facilitate patient healing in injuries and bone fractures, metallic implants have been in use for a long time. As metallic biomaterials have offered desirable mechanical strength higher than the stiffness of human bone, they have maintained their place. However, in many case studies, it has been observed that these metallic biomaterials undergo a series of corrosion reactions in human body fluid. The products of these reactions are released metallic ions, which are toxic in high dosages. On the other hand, as these metallic implants have different material structures and compositions than that of human bone, the process of healing takes a longer time and bone/implant interface forms slower. To resolve this issue, researchers have proposed depositing coatings, such as hydroxyapatite (HA), polycaprolactone (PCL), metallic oxides (e.g., TiO₂, Al₂O₃), etc., on implant substrates in order to enhance bone/implant interaction while covering the substrate from corrosion. Due to many useful HA characteristics, the outcome of various studies has proved that after coating with HA, the implants enjoy enhanced corrosion resistance and less metallic ion release while the bone ingrowth has been increased. As a result, a significant reduction in patient healing time with less loss of mechanical strength of implants has been achieved. Some of the most reliable coating processes for biomaterials, to date, capable of depositing HA on implant substrate are known as sol-gel, high-velocity oxy-fuel-based deposition, plasma spraying, and electrochemical coatings. In this article, all these coating methods are categorized and investigated, and a comparative study of these techniques is presented.

Keywords: biocompatible metals; coating techniques; hydroxyapatite; surface modification.

Figure 1

Flowchart of sol-gel coating process coating steps, in brief.

Figure 2

SEM micrographs of composite coated samples heated to 600 °C and 1000 °C after 21 days of exposure to simulated body fluids (SBF) with different compositions [62].

Figure 3

Schematic illustration of a high-velocity suspension flame-spray (HVSFS) experimental setup for nano-oxide ceramic coatings [73].

Figure 4

Polished cross-section of atmospheric plasma spray (APC) (a) and high-velocity oxy-fuel plasma coating (HVOF) (b) TiO₂ coatings and respective fracture sections (c,d) [80].

Figure 5

Schematic plasma spray coating technique and the targeted substrate [89].

Figure 6

Morphology of HA-Zirconia coating stainless steel 316L substrate at different magnifications (a) as-sprayed (b) heat treated [99].

Figure 7

Schematic view of simple electrophoretic deposition process [108].

Figure 8

SEM micrographs of HA-coated samples by electrodeposition at (a) 1 V, (b) 2 V, (c) 3 V, (d,e) 5 mA/cm² and (f) 10 mA/cm² [122].

Figure 9

Surface of the coated samples and the detached coatings after adhesion strength tests [136].

Figure 10

Degraded Mg-1.2Zn-0.5Ca alloy coupons aged at different durations after in vitro immersion in the SBF solution for different times of 1, 7, and 28 days. (a) Degradation of Mg-1.2Zn-0.5Ca (b) degradation of PEO coated Mg-1.2Zn-0.5Ca samples, (c) degradation of PEO-coated Mg-1.2Zn-0.5Ca samples sealed with PCL, and (d) degradation of PEO/PCL-coated Mg-1.2Zn-0.5Ca samples dipped in PDAM. In the non-coated samples, a severe cracking scheme is visible which is a result of dehydration of Mg(OH)₂ as the main byproduct of corrosion. PEO-treated samples represent no severe sign of corrosion after 28 days. The PEO/PCL-treated samples provide higher corrosion resistance with a slight degradation of PCL layers on the PEO-deposited layer. Finally, the PEO/PCL/PDAM samples provide high corrosion resistance with no signs of pitting or cracking on the surface while sites of HA formation can be seen obviously on the outer layer which is resulted by addition of PDAM [149].

Figure 11

SEM micrographs of MAO-treated Ti CP before and after immersion in α-MEM without cells: (a) MAO-90s, (b) MAO-600s, (c) MAO-90s 5-day immersion, (d) MAO-600s 5-day immersion, (e) MAO-90s 14-day immersion, (f) MAO-600s 14-day immersion [158].