CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No. 10-2020-0068227, filed on Jun. 5, 2020, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

1. Field

The present invention relates to a method of forming an apatite coating using a laser, and more particularly, to a method of forming an apatite coating, the method including immersing a substrate in a precursor solution containing Ca2+ions and PO43+ions and emitting a laser beam to the substrate.

2. Description of the Related Art

Titanium-based alloys that are the most widely used as metallic biomaterials for medical purposes are reported as superior materials to conventional biometals due to low modulus of elasticity, excellent biocompatibility, and high corrosion resistance. However, bioinert titanium-based alloys cannot directly induce osteogenesis and require a long treatment time to bond to adjacent bones, and a spontaneously generated titanium oxide coating is too thin and rapidly disappears, failing to induce regeneration of adjacent bone tissue.

Thus, bioactivity is imparted to an implant by surface treatment to solve problems as described above such as direct bonding failure between the implant and bones and relaxation for reducing an implant-bond integration time. The bioactivity of titanium, used as a main material for implants, is further improved by physical or chemical surface treatment, thereby reducing a healing time after an implant is introduced into a human body, and research has been conducted into more effective surface treatment.

In this regard, hydroxyapatite has been used as a material applied to the surface of titanium for the surface treatment. Hydroxyapatite is a basic component constituting hard tissue of the human body and has been used as a bone graft material or a bone regeneration material. Hydroxyapatite with a chemical structure of Ca10(PO4)6(OH)2 is distributed in dental enamel of the human body mainly in the outermost enamel layer having a thickness of 1 to 2 mm. Hydroxyapatite is known to have a remineralization effect by directly filling up micropores of demineralized enamel.

Various methods such as anodizing, sol-gel method, plasma spraying, chemical vapor deposition (CVD), and plasma electrolytic oxidation (PEO) have been used to form a hydroxyapatite coating on the surface of a substrate such as titanium by surface treatment.

First, the anodizing is a method of forming a relatively thick layer of an oxide and a metal salt on the surface of a metal using an external power source. A metal, an oxide layer of which is to be formed, is installed at an anode, and another insoluble metal is brought into contact with a cathode to allow a current to flow in an electrolyte. By flowing a current for anodizing, a thin film of an hydroxide of the metal is formed at a very low voltage, and a metal oxide layer is formed at a voltage of about 10 V. However, once the oxide layer is formed, resistance increases causing concentration of an internal stress in the metal oxide layer, and the oxide layer is destroyed at 70 V. When the voltage is increased again, a second porous oxide layer is formed and sparks may occur during this process. Since the oxide layer is formed by forcibly applying electricity thereto, electrical efficiency may decrease and a local area where the sparks occur receives thermal stress to deteriorate physical properties of titanium and decrease adhesion, thereby deteriorating final physical properties thereof.

The sol-gel method is a method of preparing a solution that is converted into a gel by hydrolysis or polymerization using alcohol, water, acid, and the like to form a coating film. A homogenized solution is applied to a substrate in a state with a relatively low viscosity and converted into a gel to form a film on the substrate. A wet coating method such as dip-coating, which is an application of the sol-gel method, is a low temperature process and has advantages of forming a coating layer regardless of an area and controlling a thickness or a microstructure of the coating layer. However, there may be disadvantages of requirement of additional post-heat treatment for crystallization, limited formation of a flat coating, and requirement of an adhesive inserted into an intermediate layer to obtain a sufficient binding force between the coating and the substrate.

The plasma spraying, a thermal spraying method, is a process of depositing a metallic material and a nonmetallic material, such as ceramic, having a high melting point on a substrate in a molten or semi-molten state. Although plasma spraying is advantageous in that the material and the size of the substrate are not limited without causing deformation in the substrate, this method is applicable in the field, a thick coating may be formed, the thickness of a coating is easily controlled, and various types of coating materials may be used, it is difficult to apply plasma spraying to implants since a coating layer has a porosity of 0.6 to 15%, a ceramic coating layer formed on titanium by mechanical bonding instead of metallic bonding is weak against impact, and adhesion between the coating layer and the substrate is weak.

The plasma electrolytic oxidation (PEO) is a surface treatment process of forming a dense coating layer with excellent mechanical stability by inducing microdischarge on the surface of a metallic material immersed in an electrolyte. Properties of the coating layer formed by the PEO are controlled by various process parameters including the electrolyte. Particularly, electrolyte conditions and current density are the most important factors affecting formation and physical properties of the coating layer in titanium and titanium alloys. The electrolyte generally used in this case is potassium phosphate, sodium phosphate, glycerol phosphate, and tartaric acid salts. Although such additives generally facilitate the plasma electrolytic oxidation process by increasing electrical conductivity and the pH, the additives may react with hydroxyapatite to lower purity and form another compound. Therefore, there are problems of a low crystallinity of hydroxyapatite on the surface of an implant and a low hydroxyapatite content in the coating layer.

RELATED ART DOCUMENT

Patent Document

(Patent Document 1) Japan Patent Laid-open Publication No. 2012-030993

SUMMARY

The present invention has been proposed to solve the above problems, and an object of the present invention is to provide a method of forming an apatite coating by emitting a laser beam onto a surface of a substrate immersed in a precursor solution.

However, these problems are exemplary, and the scope of the present invention is not limited thereby.

Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments.

According to an aspect of the present invention, provided is a method of forming an apatite coating including: (a) immersing a substrate in an apatite-forming precursor solution including Ca2+ions and PO43−ions; (b) emitting a laser beam onto a surface of the substrate immersed in the precursor solution; and (c) forming an apatite coating in a region exposed to the laser beam, wherein an output power of the laser beam is set within a range enabling the surface of the substrate to be melted.

In an embodiment of the present invention, the method may further include (d) removing the precursor solution and then removing a part of the apatite coating by emitting a laser beam to the region where the apatite coating is formed after step (c).

In an embodiment of the present invention, the substrate may include at least one material selected from the group consisting of a metal, a ceramic, a polymer, and a combination thereof.

In an embodiment of the present invention, the metal may include one selected from the group consisting of titanium (Ti), a Ti alloy, magnesium (Mg), a Mg alloy, stainless steel, and a cobalt (Co)-chromium (Cr) alloy.

In an embodiment of the present invention, the polymer may include one selected from the group consisting of polylactic acids (PLA), polyglycolic acid (PGA), poly D-lactic acid (PLDA), polyetheretherketone (PEEK), polycaprolactone (PCL), and ultra-high molecular weight polyethylene (UHMWPE).

In an embodiment of the present invention, the substrate is in a form selected from the group consisting of a plate form, a granular form, and a powder form.

In an embodiment of the present invention, the region exposed to the laser beam may be melted to form the apatite coating inside the substrate in step (c).

In an embodiment of the present invention, the precursor solution may be selected from the group consisting of Dulbecco's Modified Eagle's Medium (DMEM), Human blood plasma (HBP), and simulated body fluid (SBF).

In an embodiment of the present invention, the precursor solution may be used by increasing concentrations of Ca2+ions and PO43−ions by 1 to 400 times.

In an embodiment of the present invention, the emitting of a laser beam may be performed by repeating a scanning process with the laser beam in one direction by a predetermined distance more than once.

In an embodiment of the present invention, the emitting of a laser beam may be performed by repeating a scanning process with the laser beam in a zigzag direction by a predetermined distance more than once.

In an embodiment of the present invention, an output power of the laser beam may be 100 W/mm2or more.

According to another aspect of the present invention, provided is an apparatus for forming an apatite coating including a precursor solution container configured to contain a precursor solution for forming apatite and providing an environment allowing the precursor solution to be in direct contact with a substrate, and a laser generator disposed to emit a laser beam onto a surface of the substrate through the precursor solution contained in the precursor solution container in a state where the precursor solution is in direct contact with the substrate.

In an embodiment of the present invention, the apparatus may further include a substrate receiving part on which the substrate is placed, wherein the precursor solution container has an opening at one or more positions allowing the precursor solution contained therein to be in direct contact with the substrate.

In an embodiment of the present invention, the opening of the precursor solution container may have a structure sealed by the substrate.

In an embodiment of the present invention, the apparatus may further include a substrate receiving part on which the substrate is placed, wherein the substrate receiving part is formed inside the precursor solution container.

