1 Introduction

Both β-TCP and OCP have been proven to have promising osteoconductive characteristics. β-TCP has been applied in the form of granules and three-dimensional (3D) scaffolds (Hench and Polak 2002; Karageogiou and Kaplan 2005; Wang et al. 2015). The biocompatibility and degradability of conventional micrometer-sized β-TCP powder have been improved by the use of nanometer-sized β-TCP (Zhang et al. 2008; Kato et al. 2016). On the other hand, OCP is a metastable phase of HAp and tends to transform into HAp spontaneously (Brown et al. 1962). The intrinsic properties of OCP are hypothesized to be responsible for its excellent performance in vivo; moreover, implanted OCP granules have provided cores for nucleating multiple osteogenic sites (Suzuki et al. 1991, 2006). The combined usage of β-TCP with its better formability and OCP with its better osteoconductivity would boost the potential of both materials as a bone graft substitute. A practical way to achieve this is coating β-TCP scaffolds with OCP.

The purpose of the present study was to examine OCP formation on a β-TCP substrate in a simple calcium phosphate solution and to evaluate the effect of the particle size of the β-TCP substrate on the overgrowth of OCP under physiological conditions. To achieve this, micrometer- and nanometer-sized β-TCP particles were used to form β-TCP substrate.

2 Materials and Methods

Reagent-grade β-TCP powder (Wako, Ltd.) and atomized powder were molded into substrate composed with micrometer-sized particles (0.5–3 μm) (micro-TCP substrate, Fig. 28.1a) and with nanometer-sized particles (<100 nm) (nano-TCP substrate, Fig. 28.1b).

Fig. 28.1
figure 1

SEM images of the surfaces (a1, b1) and cross sections (a2, b2) of (a) micro-TCP substrate and (b) nano-TCP substrate before immersion

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Each substrate section was immersed in a calcifying solution (5 mM CaCl2, 5 mM K2HPO4+KH2PO4, 50 mM CH3COONa, pH 6.2, 37 ± 0.5 °C) for required periods. After the reaction terminated, the substrate was rinsed in Mill-Q water and in 99.5% ethanol and dried in air. The crystals in the overgrown layer and inside the substrate were characterized using powder XRD, microbeam XRD, thin-film in-plane XRD, FE-SEM, and TEM.

3 Results and Discussion

3.1 Early Stage of Overgrowth

On the micro-TCP substrate (Fig. 28.2), crystal growth started with the formation of sparse island-like aggregates of string-like precipitates, which gradually grew into small flakes (1–10 min) and subsequently increased in size (−30 min). Plate-like OCP crystals grew directly on the β-TCP particles and completely covered the substrate surface after 40 min. Thin-film in-plane XRD of the 40 min overgrown layer exhibits a shoulder peak at 4.7° (2θ), which corresponds to the (100) peak of OCP. It became strong after 60 min. On the nano-TCP substrate (Fig. 28.2), particles with a size of 10–20 nm precipitated after 1 min. These particles fused into strings (3 min), which subsequently formed root-like structure (5 min). The top part of the root-like deposits grew into small flakes (10 min), which completely covered the substrate surface. Thirty minutes overgrown layer exhibits a shoulder peak at 4.7° (2θ). Subsequently, plate-like OCP crystals grew on the thin layer of the flakes (60 min).

Fig. 28.2
figure 2

Time-resolved SEM observations and thin-film in-plane XRD measurements of TCP substrate after 1–60 min immersion. Characteristic XRD peaks of OCP (JCPDF 26-1056) and β-TCP (JCPDF 09-0169) are labeled. An arrow indicates shoulder peak of (100)OCP at 4.7° (2θ)

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3.2 Later Stage of Overgrowth

Figure 28.3a, b, respectively, show powder XRD and thin-film in-plane XRD profiles of the micro-TCP substrate after 20 h. Both XRD profiles of the nano-TCP substrate are almost the same. In the powder XRD profile, the (002) and (004) peaks are strong, while the other peaks are very weak. In the thin-film in-plane XRD profile, the peaks in the a-axis direction, (100), (200), and (010), are strong, while the peak in the c-axis direction, (002), is weak. This is due to the c-axial orientation of the OCP crystals on the substrate. The overgrown crystals are identified as OCP by combined analysis of both XRDs.

Fig. 28.3
figure 3

(a) Powder XRD, (b) thin-film in-plane XRD profiles of micro-TCP substrate and SEM images of (c) micro-TCP substrate and (d) nano-TCP substrate after 20 h immersion. (c1, d1) top view and (c2, d2) cross-sectional view. (c3) and (d3) are higher magnifications of (c2) and (d2)

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On both substrates, OCP crystals grew in the same manner. As the immersion period increased, the length of OCP crystals in the c-axis direction increased: 4–6 μm after 60 min, 10 μm after 3 h, 15–20 μm after 5 h, and 66–70 μm after 20 h. In the case of the nano-TCP substrate, plate-like OCP crystals (PL) grew on the layer of small flake-like crystals (F-layer), under which the layer of string-like precipitates (S-layer) had formed, and as a result, a three-layer structure was observed (Fig. 28.3d3). On the contrary, in the case of the micro-TCP substrate, OCP crystals grew directly on β-TCP particles, and no such structure was observed (Fig. 28.3c3).

Inside of the substrates, tiny OCP crystals (Fig. 28.3c3) and small amounts of HAp-nanofibers precipitated in the micro-TCP substrate (Fig. 28.4a, c), whereas only HAp-nanofibers were formed in the nano-TCP substrate (Fig. 28.4b). HAp-nanofibers were formed much more in the nano-TCP substrate than in the micro-TCP substrate, and they were localized around particles less than 500 nm in the micro-TCP substrate (Iijima and Onuma 2017). Thus, it was concluded that nano-TCP particles induced the formation of HAp-nanofibers (Onuma and Iijima 2017).

Fig. 28.4
figure 4

2θ-intensity converted microbeam XRD profile of (a) micro-TCP substrate and (b) nano-TCP substrate after 20 h immersion. Characteristic XRD peaks of OCP (JCPDF 26-1056) and ΗΑp (JCPDF 09-0432) are labeled, respectively, in (a, b). (c) HR-TEM image of a HAp-nanofiber formed inside of the micro-TCP substrate and its FFT

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Thus, varying the particle size of the β-TCP had great effect on the early stage of overgrowth and precipitates inside the substrates. Various types of OCP-coating layers were formed on β-TCP substrate. There is a general consensus that the physical properties of the coating layer, i.e., its thickness and topography, affect the in vivo performance of the coated material (Curtis and Wilkinson 1997; Anselme and Bigerelle 2011). These findings will facilitate the structural design of OCP-coating layers on a β-TCP scaffold.