A study on the improvement of corrosion property on biodegradable magnesium via laser surface treatment

Abstract

Magnesium alloys have received extensive recognition as orthopedic materials. They possess mechanical qualities identical to those of bones that no other biomaterial has, and they resolve within the body by promoting bone production. However, the corrosion rate of magnesium alloys is accelerated when they are present in bodily fluids with high concentrations of chloride compounds. Moreover, the rapid change in hydrogen gas and pH that occurs at the beginning of corrosion can cause excessive inflammation and fibrosis in the surrounding tissue, which limits its use as a human body implant. As a result, surface treatment and coating of the material are critical for controlling early stage corrosion reactions.

Laser surface engineering is emerging as a useful tool for the surface modification of materials owing to its simplicity and effectiveness. Surface modification engineering using lasers is possible for non-contact machining and selective processes depending on the conditions of the laser, such as the laser beam size, wavelength, and power. Different effects can be expected; therefore, it is used in a variety of fields. In this study, femtosecond (fs) and nanosecond (ns) laser surface modification or coating processing of Mg and Mg alloys was performed in three different ways to improve the bio-corrosion properties.

In the first part, galvanic corrosion was reduced by controlling the laser power and repetition rate, resulting in a significant decrease in the corrosion rate of the Mg alloy. The initial corrosion rate decreased by 65% compared to the non-treated samples due to Mg ion elution and the amount of hydrogen gas analysis under hanks balanced salt solution (HBSS). Therefore, we observed surface and phase changes with scanning electron microscopy (SEM) and X-ray diffraction (XRD). By fs laser surface modification, we can control the corrosion rate more precisely, which will allow biomaterials to be used in various fields through optimization of laser treatment on diverse metals.

In the second part, the Mg surface was modified to achieve super-hydrophilic wettability with a micro–nanotextured surface so that the adhesion ability of the biodegradable coating layer was improved to control the corrosion rate. The laser treatment and non-laser treatment groups were compared after coating with the biodegradable polymer PLGA (poly lactic-co-glycolic acid). In the immersion test, the Mg ion assay showed that coatings on laser-treated Mg had fewer Mg ions than the control group. The cross-section of the laser-treated group showed coating layers that infiltrated deeper into the surface. These data indicate that the adhesion strength was improved physically owing to the embedded coating layer between the pits and grooves.

In the third part, hydroxyapatite (HAp) was coated on Mg using nanosecond laser coating, combining the advantages of chemical and physical treatments. The photothermal heat generated in the liquid precursor by the laser improved the adhesion of the coating through the precipitation and growth of HAp at the localized nanosecond laser focal area and increased the corrosion resistance and cell adhesion of Mg. Physical, crystallographic, and chemical bonding were analyzed to explore the mechanism through which the surface adhesion between Mg and the HAp coating layer increased. The applicability of the coating to Mg screws used in clinical devices and the improvement of its corrosion properties were confirmed. The liquid-environment-based laser surface coating technique offers a simple and quick process that does not require any chemical ligands; therefore, it overcomes a potential obstacle in its clinical use.

Laser surface engineering is emerging as a useful tool for surface modification of materials due to its simple and effective process. Surface modification engineering by laser is possible for non-contact machining and selective process depending on the conditions of the laser such as laser beam size, wavelength, power and so on. It is possible to expect different effects, therefore it is used in variety of fields. In this study, femtosecond (fs) and nanosecond (ns) laser surface modification or coating processing to Mg and Mg alloy in three different ways to improve bio-corrosion properties.

In first part, by controlling the laser power and repetition rate, the galvanic corrosion was reduced, resulting in significantly decreasing the corrosion rate of Mg alloy. Initial corrosion rate was decreased as 65% compared to non-treated samples by Mg ion elution and amount of hydrogen gas analysis under HBSS (Hanks solution). Therefore, we observed the surface and phase change with SEM (Scanning Electron Microscope) and XRD (X-ray diffraction). By fs laser surface modification, we can control corrosion rate more precisely, which will allow biomaterials to be used in various fields through optimization of laser treatment on diversity of metal.

In second part, Mg surface was modified to super hydrophilic wettability with micro- nano textured surface so that adhesion ability of biodegradable coating layer was improved for controlling corrosion rate. Laser treatment group and non-laser treatment group were compared after coating with biodegradable polymer PLGA (poly lactic-co-glycolic acid). In immersion test, Mg ion assay showed that coatings on laser treated Mg have less Mg ion than the control group. The cross-section of laser treated group showed coating layers infiltrate deeper onto the surface. From these data, adhesion strength was improved physically due to imbedded coating layer between pits and grooves.

In third part, hydroxyapatite (HAp) was coated on Mg using ns laser coating, combining the advantages of chemical and physical treatments. Photothermal heat generated in the liquid precursor by the laser improved the adhesion of the coating through the precipitation and growth of HAp at the localized ns laser focal area and increased the corrosion resistance and cell adhesion of Mg. The physical, crystallographic, and chemical bonding were analyzed to explore the mechanism through which the surface adhesion between Mg and the HAp coating layer increased. The applicability of the coating to Mg screws used for clinical devices and improvement in its corrosion property were confirmed. The liquid environment-based laser surface coating technique offers a simple and quick process that does not require any chemical ligands, and therefore, overcomes a potential obstacle in its clinical use.

마그네슘 합금은 기존 생체재료에 비해 골과 유사한 물성을 가지고 이식 후 골 형성 촉진과 동시에 인체 내에서 분해되기 때문에 새로운 정형외과용 재료로써 주목받고 있다. 그러나 마그네슘 합금의 부식속도는 염화 화합물이 풍부한 체액에서 가속화되고, 특히 부식 초기에 발생하는 급격한 수소기체 및 pH 변화는 과도한 염증 반응 및 주변조직의 섬유화를 유발할 가능성이 있어, 마그네슘 합금을 인체이식용 소재로 사용하기에 제약이 되고 있다. 마그네슘 합금의 초기 부식속도를 제어하기 위하여 다양한 코팅기술에 관한 연구가 활발히 진행되고 있으나 코팅층의 안정성 등 코팅을 이용한 방법도 해결해야 할 문제가 있다.

레이저는 일반 공정 분위기에서 비접촉식 가공이 가능하고 국소 부위에 따른 선택적인 공정이 가능할 뿐만 아니라, 레이저의 펄스폭이나 파장대에 따른 다양한 공정효과를 기대할 수 있기 때문에 많은 활용분야에서 사용되고 있다. 따라서 본 연구에서는, 코팅 기법의 단점 보완 및 성능향상을 위해 레이저를 활용한 표면 처리 및 발전된 코팅기법을 통해, 마그네슘 및 합금의 초기 부식속도 제어와 생체적합성을 향상시키고자 하였다.

첫 번째로, 펨토초 레이저 조건을 다양하게 적용하여 부식을 억제하는 최적 조건을 찾았으며 초기 부식속도를 알기 위해, 각각의 조건에 따라 마그네슘 이온 용출량을 조사한 후, 생체 모사액인 Hanksbalanced salt solution(HBSS)에서 부식평가를 진행하고 주사전자현미경(SEM), X-ray 회절분석법(XRD)를 이용하여 표면과 상 변화를 관찰하였다. 단상이 아닌 금속합금에서 대표적인 부식 메카니즘인 갈바닉 부식을 레이저를 이용한 제 2상의 고용을 통해 부식 저항성을 향상시켰고, 레이저 영향부의 빠른 냉각속도로 인해 결정립이 미세화되어 부식층이 균일하게 형성되므로, 국부 부식을 억제하는 효과를 보였다.

두 번째로, 펨토초 레이저 텍스처링 레이저 공법의 효과를 이용하여 마그네슘 표면을 초친수성의 성질을 갖도록 개질하고, 이를 통해 분해속도 조절을 위한 고분자 코팅능력의 향상을 유도하였다. 레이저 표면 처리하지 않은 마그네슘 그룹과 표면 처리한 마그네슘 그룹에 동일한 조건으로 생분해성 고분자 화합물인 PLGA (poly-lactic-co-glycolic acid)를 동일한 조건으로 스핀코팅하여 일련의 실험들을 진행하였다. 미디어 용액에서 침지부식 후에 마그네슘 용출량을 확인해본 결과, 처리 전에 비해 표면 개질 후 코팅을 진행한 그룹에서 마그네슘 이온양의 감소를 확인할 수 있었다. 이는 부식 후의 단면 결과 역시 일치된 반응을 보여주었다. 특히, 코팅층의 접착력을 측정하여 레이저 처리에 의해 코팅 효과가 향상된 그룹의 코팅이, 그렇지 않은 그룹의 코팅에 비해 더 효과적으로 작용한다는 것을 증명할 수 있었다.

세 번째로, 뼈의 무기질 성분인 하이드록시 아파타이트 (HAp)의 성분이 함유된 전구체 용액 내 레이저 처리를 통해, 코팅재 합성과 코팅이 동시에 가능하게 하였다. 공정상의 이점뿐만 아니라 기존 침지 코팅 방식보다 향상된 코팅능을 다양한 방법으로 확인하였고, 실제 임플란트 스크류로 사용되는 제품에 이 코팅 방식을 적용해봄으로써 산업 현장에서의 적용 가능성을 확인할 수 있었다. 적용된 코팅법에 의해 형성된 레이저 유도 코팅층은 스크래치 하중에도 견디는 성능으로, 장기간 부식 실험에서 향상된 부식저항성 효과를 보였다.

향후, 위의 연구결과를 토대로, 생분해성 제품군에 따라 (예. 스크류, 플레이트, 스캐폴드, 스텐트 등등) 원하는 대로 부식 속도를 제어하여 생분해성 합금의 활용 범위가 더욱 넓어질 뿐만 아니라, 레이저를 이용한 표면처리 및 용액내 코팅 방법을 활용하여 코팅이 적용되는 다양한 분야에도 효과적으로 적용될 것이라 예상된다.