**2. Fabrication of bioactive metals by incorporation of apatite nuclei as precursors of apatite**

#### **2.1. Bioactive metals**

Metals have high mechanical strength and high fracture toughness. Among them, stainless used steel (SUS), cobalt-chromium (Co-Cr) alloys, titanium (Ti) and its alloys have been widely used as implant materials. However, most of them do not have bioactivity or have extremely poor bioactivity. Hence, these materials without any pretreatment cannot spontaneously form apatite coatings in living body in most cases. For this reason, effective methods for imparting high bioactivity to these metallic biomaterials are desired. As a representative surface modification of metals for bioactivity, hydroxyapatite coating by plasma spray method has been widely used in practical use as artificial hip joint [17]. However, this method required heating process at a temperature over 10,000°C and it is difficult to optimize the composition and crystallinity of hydroxyapatite for the bone conduction in living body. Kokubo et al. reported that NaOH and heat treatment are an effective way to impart bioactivity to the surface of Ti metal and its alloys [5, 18, 19]. In fact, the NaOH- and heat-treated Ti metal showed apatite formation within 1 day in SBF and attained high bioactivity. From these properties, the Kokubo's method has been already used in clinical use as a surface modification for artificial hip joint. However, this method cannot be applied to SUS and Co-Cr alloys [19]. As one of the most effective method for solving this problem, incorporation of apatite nuclei described in Section 1.4 can become an effective candidate to impart bioactivity to various kinds of bioinert metallic biomaterials. Recently, the author successfully imparted bioactivity to pure Ti metal [20–22], Ti-6Al-4 V alloy [23], Ti-15Mo-5Zr-3Al alloy [22, 24, 25], Ti-12Ta-9Nb-3 V-6Zr-O alloy [22], pure zirconium (Zr) metal [23], Co-Cr alloy [26] and SUS [27, 28] by incorporation of apatite nuclei on their surfaces based on the materials' design described in Section 1.4. Among them, the author introduces the details of bioactive SUS as a representative case of bioinert metallic biomaterials in this chapter.

average particle size. Then, the plates were treated by that with 3 μm (JIS #4000) of average particle size. **Figure 2** shows the SEM micrograph of the surface of the SUS plate after the sandblasting process. It can be seen that the SUS plate possessed the roughened surface with

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*2.3.2. Impartation of apatite-forming ability: incorporation of apatite nuclei as precursors of apatite*

**2.4. Apatite-forming ability of bioactive SUS: porous bone-like apatite coatings** 

**Figure 2.** SEM micrograph of the surface of the SUS plate after the sandblasting process.

Next, bioactivity of thus-obtained bioactive SUS was evaluated by soaking in SBF at pH 7.40, 36.5°C, which is corresponded to physiological environment. **Figure 4** shows the thin-film

After the micropores formation, the following process was conducted for apatite nuclei precipitation in the pores to impart bioactivity to the surface of the SUS plate. First, the pH value of SBF was increased to 8.40 by dissolving tris(hydroxymethyl)aminomethane at 25°C. Subsequently, the SUS plates were soaked in the SBF and the solution was pressed by cold isostatic pressing machine to make the solution penetrate into the pores. In order to precipitate apatite nuclei in the pores of the specimens, the solution was heated by using electromagnetic induction at 2.5 kW for 2 hours while soaking the specimens in the solution. Hereafter, the authors denote these treatments as 'alkaline SBF treatment'. **Figure 3** shows the SEM micrograph and the EDX profile of the surface of the SUS plate after the alkaline SBF treatment. It can be seen that the surface morphologies were slightly rounded off in comparison with just after the doubled sandblasting process shown in **Figure 2** and some types of coatings were formed on the plates. In the EDX profile, peaks of P and Ca were detected. It is considered that the SUS plate has been effectively heated in SBF by electromagnetic induction because the iron, which is a main chemical component of SUS, has high magnetic susceptibility. As a result, nucleation and growth of calcium phosphate were further promoted and the apatite nuclei formed under alkaline condition grew to some types of calcium phosphate coating on the surface of the SUS plate in the alkaline SBF treatment. Hereafter, the SUS plate after the alkaline SBF treatment is denoted as 'bioactive SUS'

fine pores formed by the sandblasting process.

**in biomimetic environment**

#### **2.2. SUS as an orthopedic material**

SUS is a typical biomaterial with high mechanical strength and high-corrosion resistance and has been already used as orthopedic implants such as artificial hip joint. However, SUS has no bioactivity. If an effective bioactivity treatment for SUS is established, range of its application is largely extended. Bioactivity treatment utilizing apatite nuclei described in Section 1.4 is one of the effective methods used for surface modification of SUS to impart bioactivity. As a novel micropores formation process, the authors established a formation process of roughened surface with fine pores on metals and organic polymers by doubled sandblasting process [29] by using the grinding particles with 14 μm of average particle size as first process, and then using the particles with 3 μm of average particle size as second process. The authors clarified that thus-formed fine pores contributed to the improvement of adhesion property of porous bonelike apatite layer formed on the bioactive materials in SBF in the process shown in **Figure 1** because of an improvement of mechanical interlocking effect. As described in this section, the authors formed micropores on the surface of SUS plates by the doubled sandblasting process. Then the authors precipitated apatite nuclei in the pores of SUS to impart bioactivity.

#### **2.3. Fabrication process of bioactive SUS**

#### *2.3.1. Micropores formation by the doubled sandblasting process*

First, in order to prepare micropores on the surface, the SUS plates (JIS SUS 316 L) were treated by a sandblasting process using alumina-grinding particles with 14 μm (JIS #800) of average particle size. Then, the plates were treated by that with 3 μm (JIS #4000) of average particle size. **Figure 2** shows the SEM micrograph of the surface of the SUS plate after the sandblasting process. It can be seen that the SUS plate possessed the roughened surface with fine pores formed by the sandblasting process.

been widely used as implant materials. However, most of them do not have bioactivity or have extremely poor bioactivity. Hence, these materials without any pretreatment cannot spontaneously form apatite coatings in living body in most cases. For this reason, effective methods for imparting high bioactivity to these metallic biomaterials are desired. As a representative surface modification of metals for bioactivity, hydroxyapatite coating by plasma spray method has been widely used in practical use as artificial hip joint [17]. However, this method required heating process at a temperature over 10,000°C and it is difficult to optimize the composition and crystallinity of hydroxyapatite for the bone conduction in living body. Kokubo et al. reported that NaOH and heat treatment are an effective way to impart bioactivity to the surface of Ti metal and its alloys [5, 18, 19]. In fact, the NaOH- and heat-treated Ti metal showed apatite formation within 1 day in SBF and attained high bioactivity. From these properties, the Kokubo's method has been already used in clinical use as a surface modification for artificial hip joint. However, this method cannot be applied to SUS and Co-Cr alloys [19]. As one of the most effective method for solving this problem, incorporation of apatite nuclei described in Section 1.4 can become an effective candidate to impart bioactivity to various kinds of bioinert metallic biomaterials. Recently, the author successfully imparted bioactivity to pure Ti metal [20–22], Ti-6Al-4 V alloy [23], Ti-15Mo-5Zr-3Al alloy [22, 24, 25], Ti-12Ta-9Nb-3 V-6Zr-O alloy [22], pure zirconium (Zr) metal [23], Co-Cr alloy [26] and SUS [27, 28] by incorporation of apatite nuclei on their surfaces based on the materials' design described in Section 1.4. Among them, the author introduces the details of bioactive SUS as a

SUS is a typical biomaterial with high mechanical strength and high-corrosion resistance and has been already used as orthopedic implants such as artificial hip joint. However, SUS has no bioactivity. If an effective bioactivity treatment for SUS is established, range of its application is largely extended. Bioactivity treatment utilizing apatite nuclei described in Section 1.4 is one of the effective methods used for surface modification of SUS to impart bioactivity. As a novel micropores formation process, the authors established a formation process of roughened surface with fine pores on metals and organic polymers by doubled sandblasting process [29] by using the grinding particles with 14 μm of average particle size as first process, and then using the particles with 3 μm of average particle size as second process. The authors clarified that thus-formed fine pores contributed to the improvement of adhesion property of porous bonelike apatite layer formed on the bioactive materials in SBF in the process shown in **Figure 1** because of an improvement of mechanical interlocking effect. As described in this section, the authors formed micropores on the surface of SUS plates by the doubled sandblasting process.

Then the authors precipitated apatite nuclei in the pores of SUS to impart bioactivity.

First, in order to prepare micropores on the surface, the SUS plates (JIS SUS 316 L) were treated by a sandblasting process using alumina-grinding particles with 14 μm (JIS #800) of

representative case of bioinert metallic biomaterials in this chapter.

**2.2. SUS as an orthopedic material**

14 Recent Advances in Porous Ceramics

**2.3. Fabrication process of bioactive SUS**

*2.3.1. Micropores formation by the doubled sandblasting process*

#### *2.3.2. Impartation of apatite-forming ability: incorporation of apatite nuclei as precursors of apatite*

After the micropores formation, the following process was conducted for apatite nuclei precipitation in the pores to impart bioactivity to the surface of the SUS plate. First, the pH value of SBF was increased to 8.40 by dissolving tris(hydroxymethyl)aminomethane at 25°C. Subsequently, the SUS plates were soaked in the SBF and the solution was pressed by cold isostatic pressing machine to make the solution penetrate into the pores. In order to precipitate apatite nuclei in the pores of the specimens, the solution was heated by using electromagnetic induction at 2.5 kW for 2 hours while soaking the specimens in the solution. Hereafter, the authors denote these treatments as 'alkaline SBF treatment'. **Figure 3** shows the SEM micrograph and the EDX profile of the surface of the SUS plate after the alkaline SBF treatment. It can be seen that the surface morphologies were slightly rounded off in comparison with just after the doubled sandblasting process shown in **Figure 2** and some types of coatings were formed on the plates. In the EDX profile, peaks of P and Ca were detected. It is considered that the SUS plate has been effectively heated in SBF by electromagnetic induction because the iron, which is a main chemical component of SUS, has high magnetic susceptibility. As a result, nucleation and growth of calcium phosphate were further promoted and the apatite nuclei formed under alkaline condition grew to some types of calcium phosphate coating on the surface of the SUS plate in the alkaline SBF treatment. Hereafter, the SUS plate after the alkaline SBF treatment is denoted as 'bioactive SUS'

#### **2.4. Apatite-forming ability of bioactive SUS: porous bone-like apatite coatings in biomimetic environment**

Next, bioactivity of thus-obtained bioactive SUS was evaluated by soaking in SBF at pH 7.40, 36.5°C, which is corresponded to physiological environment. **Figure 4** shows the thin-film

**Figure 2.** SEM micrograph of the surface of the SUS plate after the sandblasting process.

**Figure 3.** (a) SEM micrograph and (b) EDX profile of the surface of the SUS plate after the alkaline SBF treatment.

**2.5. Adhesion property of porous bone-like apatite coating formed on the bioactive SUS**

**Figure 5.** (a) SEM micrograph and (b) EDX profile of the surface of the bioactive SUS after the soaking in SBF for 1 day.

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ing was attained by the formation of micropores.

**2.6. Case of the other types of metals**

bioinert or poorly bioactive metals.

**nuclei as precursors of apatite**

**3.1. Bioactive organic polymers**

The adhesive strength between the bioactive SUS and the porous bone-like apatite coating formed by soaking in SBF for 14 days was measured by a modified ASTM C-633 method [30–33]. For the reference, the author prepared the specimens, which were applied same treatments without the doubled sandblasting treatment. The average adhesive strength between the formed apatite layer and the bioactive SUS with alkaline SBF treatment was 15.4 MPa, and for the SUS without the sandblasting treatment was 1.5 MPa. The SUS treated with doubled sandblasting treatment showed higher adhesive strength than untreated ones. This is because a mechanical interlocking effect between the SUS plate and the porous bone-like apatite coat-

As described in Section 2.1, the author reported that this bioactivity treatment was applicable not only to SUS but also to other kinds of metallic biomaterials such as Ti metal and its alloys, Zr metal and Co-Cr alloy by optimizing the condition of micropores formation and apatite nuclei precipitation according to the kinds of materials. Also in the case of these metals, apatite formation was induced within 1 day in SBF [20–26]. In addition, it is reported that most of the bioactive ceramics show apatite formation within 1 week in SBF [34]. Hence, it is suggested that the alkaline SBF treatment was an effective method to impart high bioactivity to

**3. Fabrication of bioactive organic polymers by incorporation of apatite** 

Organic polymers have various mechanical properties and are easily processed in various shapes such as sticks, plates, films, sponges and fibers. Because of these properties, organic polymers have been widely used as various implant materials such as artificial hip joint,

**Figure 4.** TF-XRD profile of the surface of the bioactive SUS after the soaking in SBF for 0 day, 1 day, 3 days and 7 days.

X-ray diffraction (TF-XRD) profiles of the surface of the bioactive SUS after soaking in SBF for 0 day (i.e., before soaking in SBF), 1 day, 3 days and 7 days. Before soaking in SBF, diffraction peaks of apatite were not detected. This result suggested that the calcium phosphate coating formed in the alkaline SBF treatment was consisted of amorphous calcium phosphate (ACP). After soaking in SBF for 1 day, 3 days and 7 days, diffraction peaks of apatite were clearly detected. **Figure 5** shows the SEM micrograph and the EDX profile of the surface of the bioactive SUS after soaking in SBF for 1 day. It was observed that the whole surface was covered with porous coating, which consisted of needle-like crystallites, which characterize bone-like apatite, in the SEM observation. In the EDX profile, the intensity of the peaks of P and Ca was relatively increased in comparison with those after the alkaline SBF treatment shown in **Figure 3**. From these results, it is indicated that apatite formation was induced within 1 day and high bioactivity was imparted to the SUS by the alkaline SBF treatment.

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**Figure 5.** (a) SEM micrograph and (b) EDX profile of the surface of the bioactive SUS after the soaking in SBF for 1 day.

#### **2.5. Adhesion property of porous bone-like apatite coating formed on the bioactive SUS**

The adhesive strength between the bioactive SUS and the porous bone-like apatite coating formed by soaking in SBF for 14 days was measured by a modified ASTM C-633 method [30–33]. For the reference, the author prepared the specimens, which were applied same treatments without the doubled sandblasting treatment. The average adhesive strength between the formed apatite layer and the bioactive SUS with alkaline SBF treatment was 15.4 MPa, and for the SUS without the sandblasting treatment was 1.5 MPa. The SUS treated with doubled sandblasting treatment showed higher adhesive strength than untreated ones. This is because a mechanical interlocking effect between the SUS plate and the porous bone-like apatite coating was attained by the formation of micropores.

#### **2.6. Case of the other types of metals**

As described in Section 2.1, the author reported that this bioactivity treatment was applicable not only to SUS but also to other kinds of metallic biomaterials such as Ti metal and its alloys, Zr metal and Co-Cr alloy by optimizing the condition of micropores formation and apatite nuclei precipitation according to the kinds of materials. Also in the case of these metals, apatite formation was induced within 1 day in SBF [20–26]. In addition, it is reported that most of the bioactive ceramics show apatite formation within 1 week in SBF [34]. Hence, it is suggested that the alkaline SBF treatment was an effective method to impart high bioactivity to bioinert or poorly bioactive metals.
