1. Introduction
For several years, there has been an increased demand for bone replacement or hard tissue damaged from various factors such as osteoarthritis, osteoporosis, dentistry, war-related injuries, and traffic accidents [
1]. Currently in Indonesia, bone implants are imported, but with increasing numbers of patients requiring bone replacements there is a need for biomaterials that can be used to regenerate skeletal tissue [
1,
2,
3].
The treatment for bone damage is to use metals such as 316L stainless steel (316L SS) to replace damaged bones. However, this treatment creates problems due to the low level of biocompatibility of the metal, which causes pain and bruising in the surrounding tissue [
4]. The 316L SS devices can also result in problems from galvanic corrosion, crevices, and the release of dangerous Cl and Fe ions into tissues. Therefore, because these surfaces are not bioactive, they must be modified using osteoconductive materials such as bioceramics, including hydroxyapatite (HA). These new alternatives are used because they can effectively reconstruct human bone tissue [
5,
6,
7].
HA (Ca
10(PO
4)
6(OH)
2) is an alternative material used in biomedical applications [
2]. HA has the lattice parameters of
a = 9.433Å and
c = 6.875Å, and a variable Ca/P mol ratio of 1.67 [
8,
9]. HA’s advantages are its bioactivity, biocompatibility, and non-corrosiveness. Currently, HA is deposited as a coating on metal coating materials to help suppress the release of harmful metallic ions [
10]. Accordingly, HA works as a bioceramic, which has excellent biological characteristics that facilitate bone repair and reconstruction [
11].
A recent study was conducted to develop minerals similar to natural bone in terms of crystallinity, chemical composition, and microstructure. Natural bone consists of the mineral carbonate (CO
3)
2−, which varies based on age, approximately 2%–8% [
11] by total weight, in addition to other bone-forming elements. The combination of HA and carbonate minerals from external sources and natural bone is called carbonated hydroxyapatite (CHA). CHA shows better biological properties due to its low crystallinity and increased surface area; consequently, it indicates better bioactivity and is applicable in biomedical applications [
12].
CHA (Ca
10−x(PO
4)
6−x(CO
3)
x(OH)
2−x with 0 ≤
x ≤ 2), as a bioceramics candidate for bone implants, consist of three types: B-type (the carbonate ion substitutes the phosphate ion), A-type (the carbonate ion substitutes the hydroxyl ion), and AB-type (the carbonate ion covers the phosphate and hydroxyl ions simultaneously). B-type CHA is the most widely used in biomedical applications [
13]. Generally, B-type CHA can reabsorb osteoclasts and is highly soluble in apatite lattices in clinical tests [
14]. The combination of carbonate ions and tetrahedral phosphates in B-type CHA causes alterations in the lattice parameters of the crystal structure of CHA [
15].
Synthetic CHA can be produced through the reaction of synthetic compounds and the reaction of natural compounds. Natural compounds such as biogenic materials can be obtained from some shells and bones [
8]. In this work, the CHA was fabricated using abalone mussel shells (
Haliotis asinina) as the natural ingredient and source of calcium from Indonesia because of the higher content of calcium carbonate (CaCO
3), which is 90%–95% [
16].
CHA can be synthesized using several techniques, including co-precipitation [
12,
17,
18], nano emulsion [
19,
20], sol-gel [
21], mechanical alloying [
22], and mechanochemical-hydrothermal methods [
23,
24]. In this work, the co-precipitation method was selected based on specific considerations: several CHA synthesis approaches do not require any organic solvent, thereby making the process cost-effective; the co-precipitation method is simple as well as cost-effective with a high throughput (87%), which makes it suitable for large-scale production [
9].
Biopolymers are increasingly used in biomedical applications due to their chemical similarities with the extracellular matrix of many tissues and their beneficial biological performance [
25]. To refine the mechanical and biological behavior of inorganic composite materials, a great deal of interest has focused on organic polymer coatings [
1]. As a biopolymer, HCB is interesting for its fully interconnected pores of uniform size and high mechanical strength in the direction of the pores [
26]. The HCB architecture is identified by orderly unidirectional macropore or channel that penetrate the materials. Moreover, as a natural polymeric porogen and non-toxic pore-forming agent in a scaffold, HCB structures offer great strength with low weight and less material [
27].
Presently, the biomaterials used for dental and orthopedic applications are Ti alloys, stainless steel, magnesium-based alloys, and cobalt-chromium alloys [
1,
28,
29]. Ti and its alloys have become the most popular biomaterials for orthopedic and dental implants [
30]. Ti-alloys have proven to be relevant for their excellent corrosion resistance, suitable mechanical properties, attractive biocompatibility, non-toxicity, perfect antibacterial character, average elastic modulus, good strength-to-weight ratio, and superior photocatalysis [
1,
11,
29]. Various methods for the coating process on metal surfaces have also been developed, including dip coating [
29], electrophoretic deposition (EPD) [
1,
3,
30,
31], sol-gel coating [
6,
32], and plasma spraying [
10]. Among such methods, dip coating and EPD are the easiest; however, the EPD method has a greater chance of cracking due to manual withdrawal of the coating. The presence of cracking in the layer greatly affects the characteristics when implanted in the body [
3]. In addition, the coating process using the dip-coating method requires more time [
29]. Therefore, it is necessary to combine the two methods in order to have an impact in controlling surface morphology, layer thickness, free cracking, and the formation of a homogeneous layer relatively quickly. The combination of the two methods results in an electrophoretic deposition dip coating (EP2D). This tool is a combination of a series of dip coater devices and EP2D tools that were integrated on the computer. This tool is used for the coating process of CHA and the scaffold CHA/HCB to Ti alloy.
In this work, CHA was fabricated via co-precipitation using calcium carbonate (CaCO
3) from abalone mussel shells with stirring time variations. The characteristics of CHA were characterized, including its effect on crystallographic properties, Ca/P molar ratio, its thermal properties, and the functional groups of CHA samples. The best synthesized CHA was mixed with HCB porogen at a concentration of 40 wt.% for the scaffold fabrication treatment. The physicochemical properties of the scaffold CHA/HCB were analyzed using scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDS), X-ray diffractometer (XRD), and Fourier transform infrared spectroscopy (FTIR). This study has added the HCB from Indonesia as a biopolymer and porous agent scaffold to CHA. The CHA/HCB confirms the non-toxicity scaffold as shown in previous research [
33]. Therefore, the authors tried to coat the Titanium (Ti) alloy with the scaffold CHA/HCB to create CHA/HCB/Ti coatings. CHA and the scaffold CHA/HCB 40 wt.% were used to coat the Ti alloy using the EP2D method developed by the authors with immersion times of 10, 20, and 30 min. The physicochemical properties of CHA/Ti and CHA/HCB/Ti coatings were characterized using SEM and XRD. Evaluation based on compressive strength parameters of CHA/Ti and CHA/HCB/Ti used a universal testing machine (UTM).
5. Conclusions
This work presents a successful fabrication of CHA based on abalone mussel shells with stirring times of 15, 30, and 45 min and the scaffold CHA/HCB 40 wt.%. The morphology of samples showed a high aggregation and more regular shape with sizes below 1 μm. The synthesized CHA formed the same phase as B-type CHA. CHA with the stirring time of 45 min could have lower transmittance values and smaller crystallite sizes. The scaffold CHA/HCB 40 wt.% exhibited the potential scaffold for bone growth and lower crystallinity. Therefore, CHA and the scaffold CHA/HCB 40 wt.% have the potential for coating on the Ti alloy.
This study also presents a successful CHA/Ti and CHA/HCB/Ti coating process using the EP2D method with immersion time variations of 10, 20, and 30 min. The longer the immersion time, the thicker is the bioceramic layer deposited on the substrate surface so that the compressive strength tended to increase, as shown in the compressive strength for CHA/HCB/Ti. However, the compressive strength of the CHA/Ti coating for the immersion time of 20 min tended to decrease. The XRD pattern showed the CHA, HA, and Ti phases for the CHA/Ti coating, but the CHA/HCB/Ti coating only had the CHA and Ti phases. As shown in the SEM analysis, the thickness value of CHA/Ti for an immersion time of 20 min decreased because in the first period of EPD at a constant voltage, thickness increased with time. Still, over a more extended period, this effect was not observed. However, the immersion time could result in an increase in coatings’ thickness, even after a short immersion time of 10 min, as shown in the CHA/HCB/Ti coating. Based on these preliminary results, CHA and scaffold CHA/Ti can potentially be applied to coatings with the Ti alloy.
The development of the EP2D is expected to impact controlling surface morphology, layer thickness, free cracking, and the formation of a homogeneous layer relatively quickly. These results can be seen on layer thickness for CHA/HCB/Ti coatings with immersion times of 10, 20, and 30 min at 71–88 μm. Additionally, the compressive strength for all immersion time variations for CHA/HCB/Ti coatings was about 54–83 MPa, which was within the average of human cancellous bone (0.2–80 MPa). The layer thickness and compressive strength that meet the requirements for bone implant applications are obtained quickly. Therefore, it can attain the goal of developing an electrophoretic deposition dip coating (EP2D) tool integrated into the computer and the use of HCB for CHA/HCB/Ti coating.