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Preparation of bone-like composite coating using a modified simulated body fluid with high Ca and P concentrations

https://doi.org/10.1016/j.surfcoat.2006.02.036Get rights and content

Abstract

A carbonate hydroxyapatite (HA)–collagen (Col) composite coating on NiTi shape memory alloy (SMA) was synthesized by biomimetic growth in modified simulated body fluid (MSBF), which containing high Ca and P ions concentrations. The morphology of composite coating was uniform and porous. The major phase of coating, identified by Infrared Spectroscopy (IR) and X-ray diffraction (XRD), was HA with B-type carbonate substitution. It was also proved by IR results that the collagen was present in the coating. Transmission Electron Microscopy (TEM) and high resolution TEM analysis showed that the c-axis of HA crystals aligned parallel to the longitudinal direction of the collagen fibrils. The relative orientation maintained in the composite coating may benefit the NiTi SMA to achieve better properties in hard tissue replacement.

Introduction

Bone tissue consists mainly of hydroxyapatite and collagen; since the organic and inorganic phases organize themselves in vivo in a so-called multi-level hierarchical structure [1]. Bone is considered a chemically bonded composite between apatite and type I collagen [2]. This definition inspired earlier studies of hydroxyapatite–collagen composite [3], [4], [5], [6]. However with the evolution of the researches and experiments, it became obvious that only mimicking the composition of bone is not sufficient to obtain composites with properties similar to natural bone [7]. Some researchers have employed the method of co-precipitation to form bulk calcium phosphate–collagen composites by self-assembly of collagen and the precipitation of calcium phosphate [8], [9], [10], [11], [12], [13], [14]. In vivo evaluation did proved that the composite favorably enhanced new bone growth [15], [16]. However, the composite was still limited in practical application because of its poor mechanical properties. Glutaraldehyde cross-linked hydroxyapatite–collagen composite showed improved mechanical behavior even though the microstructural characteristics of hydroxyapatite and collagen disappeared [17].

Applying calcium phosphate–collagen composite coating to the surface of metallic implant is without doubt a novel method to achieve an excellent combination of the advantages of the coating and substrate. The metallic substrate provides strong mechanical support to the implant, whereas the bioactive coating promotes growth and healing of new bone tissue. Some studies of calcium phosphate–collagen composite coating indicated a problem with the homogeneity of collagen absorption [18], [19]. However, electrolytic deposition of a calcium phosphate–collagen coating yielded a uniform distribution of composite coating on the working electrode [20]. To the best of our knowledge, the structural relationship between hydroxyapatite and collagen during the composite coating formation on the substrate has not been reported yet.

The aim of this work is to produce a bone-like apatite–collagen composite coating on NiTi shape memory alloy by the method of biomimetic growth. The biomimetic process consists of soaking implants under moderate conditions of pH and temperature into simulated body fluid (SBF) solutions that have a similar inorganic content as human blood plasma [21]. To shorten the immersion periods while maintaining the proper concentrations and proportions of apatite and collagen, we modified the SBF by increasing the Ca and P ion concentrations. We will demonstrate that the biomimetic growth technique in modified simulated body fluid (MSBF) results in formation of carbonate hydroxyapatite–collagen composite coatings on NiTi alloy substrate, and that the characteristic orientation between hydroxyapatite and collagen is being maintained in the composite coating.

Section snippets

Materials and methods

NiTi shape memory alloy blocks (50.8 at.% Ni; Shenyang Tianhe New Materials Development Co., Ltd.) of 10 × 3 × 3 mm dimensions were used as substrates. All chemical regents were purchased from Sigma-Aldrich. Water-soluble type I collagen powder was purchased from Mingrang Bio-tech Co., Ltd.

The NiTi substrates were abraded with no. 360, 600, 800 sand papers in succession, and then cleaned ultrasonically in deionized water for 3 min. The cleaned samples were dipped in 30% HNO3 aqueous solution at 60 °C

ESEM

Fig. 1 shows the morphology of the surface of NiTi SMA after biomimetic growth in MSBF for 3 days. The coating formed on the surface of the substrate was uniform (Fig. 1a). It could be seen that the coating was composed by many small spherical particles, forming a loose arrangement. The average diameter of the single particle was about 1 μm. There were colloid-like connections among the small particles (Fig. 1b).

The stability of the MSBF containing high calcium and phosphate concentration is due

Conclusion

A carbonate hydroxyapatite–collagen composite coating was obtained on NiTi SMA by biomimetic growth in MSBF containing high Ca and P ions concentrations. The morphology of the composite coating was uniform. The main phase of the coating was HA with B-type carbonate substitution. In the formation process of the composite coating, the c-axis of HA crystals aligned themselves parallel to the longitudinal direction of the collagen fibrils. This meant that the composite coating contained the

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Project No. 50471048), Specialized Research Fund for the Doctoral Program of Higher Education (Project No. 20040056016), and Scientific and Technological Project of Tianjin (Project No. 043186311).

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