**3. Evaluation of osteoconductivity**

The evaluation methods for the bioactivity of the implants are classified into *in vitro* and *in vivo* methods. In this review, the *in vivo* evaluation method is described. In *in vivo* evaluation, many types of animals at different ages were used in various studies, and differentresearchers used a different implanted part of the animals. Moreover, a unified quantification criterion has not yet been established, and the criteria used in various studies are not compatible with one another. Therefore, we use the bone–implant contact ratio, *R*B–I, as an osteoconductive index based on the observation of body tissue on the implants. Bone–implant contact was determined by linear measurement of direct bone contact with the implant surface. The sum of the length of the bone formation on the implant surface was measured and expressed as a percentage of the total implant length (bone–implant contact ratio) in the cancellous bone and the cortical bone parts [17, 18, 25, 26, 33].

**Figure 6.** Bone-implant contact ratio, RB-I, for the various surface coated samples (rat's tibia, 14 days). \*p < 0.05, (1) cortical bone part, (2) cancellous bone part. (A) needle-like, (B) plate-like, (C) net-like, (D) spherical-like, (E) as-polished Ti. Hap CO3Ap (A-1, B-1, C-1) or HAp/gel. composite (A-2, B-2) CO3Ap/CaCO3 composite (A-1, B-1, C-1, D-1) or HAp/col. composite (C-2)

Figure 6 shows the bone–implant contact ratios, *R*B–I, of the samples coated under the various conditions mentioned above and classified based on the following four surface morphologies: (A) needle‐like, (B) plate‐like, (C) net‐like, and (D) sphere‐like. The samples are then compared with the control implant ((E) noncoated Ti). In Fig. 6, the samples are distinguished according to color based on whether or not the coating contained CaCO3 and collagen or gelatin (white: HAp; gray: CO3–Ap or HAp/gelatin; black: CO3–Ap/CaCO3 or HAp/collagen). The *R*B–I value of HAp‐coated samples (white bar) is the same as or higher than that of the as‐polished one (E). In particular, *R*B–I in the cancellous bone part is highest in the sample coated with the needle‐like HAp (A‐1). The influence of the different surface morphologies on *R*B–I is apparent [17, 18]. A small amount of CO3 included in CO3–Ap does not influence osteoconductivity, and an increased amount of CO3 (>15 mass%CO3), including that in CO3–Ap/CaCO3, has a negative effect on (black bar in (A‐1), (B‐1), (C‐1), and (D‐1)) [25, 26]. The *R*B–I value of HAp/gelatin‐ coated samples is the same as that of HAp (gray and white bars in (A‐2) and (B‐2)), and we did not find a positive effect of the addition to HAp on the osteoconductivity, or any negative effects within the limit of gelatin content used. In the HAp/collagen films (C‐2), osteoconduc‐ tivity was improved, and maximum *R*B–I was obtained when the collagen content was the same as that in natural bone. The addition of too much collagen, exceeding that amount of collagen content in natural bone, inhibited the improvement of the osteoconductivity [33].

#### **4. Conclusion**

**3. Evaluation of osteoconductivity**

Biomedical Engineering

294

bone parts [17, 18, 25, 26, 33].

\* \*

\* \* 6060

00

 **contact ratio,**

**‐Implant contact ratio,**

**(2)**

00

HAp/col. composite (C-2)

2020

4040 **Bone‐Implant**

2020

4040

*R***B‐I (%)**

*R***B‐I (%)**

8080

6060

00 1010 2020 00 1010 2020

**Total CO3 content in the coating,** *C***CO3 / mass%**

00 1010 2020

00 1010 2020 3030

③④ ② ① ⑤ ⑧③ ⑥ ② ① ⑦

**Figure 6.** Bone-implant contact ratio, RB-I, for the various surface coated samples (rat's tibia, 14 days). \*p < 0.05, (1) cortical bone part, (2) cancellous bone part. (A) needle-like, (B) plate-like, (C) net-like, (D) spherical-like, (E) as-polished Ti. Hap CO3Ap (A-1, B-1, C-1) or HAp/gel. composite (A-2, B-2) CO3Ap/CaCO3 composite (A-1, B-1, C-1, D-1) or

\*

\* \* \*

\* \*

6 5 6 6 4 5 65 46 4 6 6

**(A-1) (B-1) (C-1) (D-1)**

B‐I

**(1)**

The evaluation methods for the bioactivity of the implants are classified into *in vitro* and *in vivo* methods. In this review, the *in vivo* evaluation method is described. In *in vivo* evaluation, many types of animals at different ages were used in various studies, and differentresearchers used a different implanted part of the animals. Moreover, a unified quantification criterion has not yet been established, and the criteria used in various studies are not compatible with one another. Therefore, we use the bone–implant contact ratio, *R*B–I, as an osteoconductive index based on the observation of body tissue on the implants. Bone–implant contact was determined by linear measurement of direct bone contact with the implant surface. The sum of the length of the bone formation on the implant surface was measured and expressed as a percentage of the total implant length (bone–implant contact ratio) in the cancellous bone and the cortical

Micro-Nano Mechatronics — New Trends in Material, Measurement, Control, Manufacturing and Their Applications in

sum of the length of the part of bone formation on the implant surface total implant length *R* (%) 100 (4)

4

*n* = 6 6 5 6 4 4 66 5 4 6 5

6

\* \* \*

**(E) (A-2) (B-2) (C-2)**

\*

\*

**Collagen or gelatin content in the coating,** *C***c,gF/mass%** 00 2020 4040 00 2020 4040 00 2020 4040

in natural bone

\* \*

The inside of the human body is equivalent to a water environment atroom temperature, since the water content in the body is about 60%. It is thought that hydroformed HAp has greater osteoconductivity than HAp synthesized using pyroprocessing, because synthesized HAp in the aqueous solution at neutral pH and room temperature is similarto that formed in the body. In addition, titanium dioxide, TiO2, which does not exist in the human body, is a remarkable compound with respect to its osteoconductivity. It is important to research and improve the osteoconductivity of substances such as HAp, TiO2, and CaTiO3. However, we need to pay attention to the properties of their compounds, such as surface roughness [35], crystallinity, and corrosivity, all of which influence osteoconductivity. Furthermore,the evaluation criterion for osteoconductivity has not been adequately established.

The development of implants with high functionality is an important problem that urgently needs to be solved, instead of merely making progress in medical technology. It is thought that nothing can compete with such implants in the progress and development of the individual technology. We hope that these important problems can be solved using the combination of the discovery of new bioactive compounds (organic and inorganic) and their coating techni‐ ques, alloy designs for the implants, and/or the growth of related surrounding techniques for them.

> [11] Nie X, Leyland A, Matthews A. Deposition of layered bioceramic hydroxyapatite/ TiO2 coatings on titanium alloys using a hybrid technique of micro‐arc oxidation and

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