Biofunctionalization of metallic implants by calcium phosphate coatings

Abstract

Metallic materials have been extensively applied in clinical practice due to their unique mechanical properties and durability. Recent years have witnessed broad interests and advances on surface functionalization of metallic implants for high-performance biofunctions. Calcium phosphates (CaPs) are the major inorganic component of bone tissues, and thus owning inherent biocompatibility and osseointegration properties. As such, they have been widely used in clinical orthopedics and dentistry. The new emergence of surface functionalization on metallic implants with CaP coatings shows promise for a combination of mechanical properties from metals and various biofunctions from CaPs. This review provides a brief summary of state-of-art of surface biofunctionalization on implantable metals by CaP coatings. We first glance over different types of CaPs with their coating methods and in vitro and in vivo performances, and then give insight into the representative biofunctions, i.e. osteointegration, corrosion resistance and biodegradation control, and antibacterial property, provided by CaP coatings for metallic implant materials.

Keywords: Biodegradation; Calcium phosphates; Metallic implant materials; Osteointegration; Surface biofunctionalization.

Graphical abstract

Fig. 1

Influence of surface properties of the implant material on the cell behaviors. (a) SEM images show different cell morphology and adhesion behaviors of human corneal epithelial cells cultured on (i) smooth and (ii–iii) groove patterned silicon oxide substrates, and human mesenchymal stem cells on poly(dimethylsiloxane) micropillar of different heights of (iv) 0.97 μm and (v) 12.9 μm. (b) Cell adhesion behaviors of NIH/3T3 cells on surfaces with wettability gradient. The cells displayed extended pseudopodia and adhered firmly on I and IV regions; while the cells showed much lower attachment on II, III and V regions. (Parts (i-iii) in (a) are reproduced with permission from Ref. [12], parts (iv-v) in (a) are reproduced with permission from Ref. [13], and (b) is reproduced with permission from Ref. [15].). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 2

Enhanced osteointegration of metallic implant materials by CaP coating. (a) Schematic representation of osseointegration induced by CaP coating [108]. (b) (i, ii) Fluoroscopic images and (iii, iv) HE stained pathological images of the cross-section of (i, iii) uncoated and (ii, iv) DCPD coated Mg implant after 4 weeks of implantation. (I: implant; N: newly formed osteoid tissue) (c) HE-stained pathological images of the Ti6A14V screw implant/bone interfaces: (i) uncoated, (ii) micron-HA-coated, (iii) nano-HA-coated, and (iv) polymeric bioabsorbable screw as control. (G: granulation tissue; T: tendons; MV: minimal vascularization). ((a) is reproduced with permission from Ref. [115] (b) is reproduced with permission from Ref. [144].).

Fig. 3

Schematic of the biodegradation control performance provided by the CaP coatings. (a) Extremely high degradation rate of the Mg-alloy implant in body fluid may possibly result in a gap between the implant and the new-formed bone. (b) The gap is shown by a typical tetracycline labeling 14 weeks post-operation in the femora of rabbits. (c) The CaP coatings can reduce the degradation rate to eliminate the interfacial gap and enhance the biocompatibility of the implants. Reproduced with permission from Ref. [114].

Fig. 4

Antibacterial performance of CaP and its composite coating on metallic implant materials. (a) Images and statistics of colony formation units of three kinds of different bacteria after cultured with uncoated, FHA, and HA coated pure Ti surfaces. (b) Schematic of the electrochemical deposition process and immobilization of BMP-2 on HA coatings. ((a) is reproduced with permission from Ref. [168] (b) is reproduced with permission from Ref. [173].).