Atomic layer deposited ZrO₂ nanofilm on Mg-Sr alloy for enhanced corrosion resistance and biocompatibility

Abstract

The biodegradability and good mechanical property of magnesium alloys make them potential biomedical materials. However, their rapid corrosion rate in the human body’s environment impairs these advantages and limits their clinical use. In this work, a compact zirconia (ZrO₂) nanofilm was fabricated on the surface of a magnesium-strontium (Mg-Sr) alloy by the atomic layer deposition (ALD) method, which can regulate the thickness of the film precisely and thus also control the corrosion rate. Corrosion tests reveal that the ZrO₂ film can effectively reduce the corrosion rate of Mg-Sr alloys that is closely related to the thickness of the film. The cell culture test shows that this kind of ZrO₂ film can also enhance the activity and adhesion of osteoblasts on the surfaces of Mg-Sr alloys.

Statement of Significance

The significance of the current work is to develop a zirconia nanofilm on biomedical MgSr alloy with controllable thickness precisely through atomic layer deposition technique. By adjusting the thickness of nanofilm, the corrosion rate of Mg-Sr alloy can be modulated, thereafter, the degradation rate of Mg-based alloys can be controlled precisely according to actual clinical requirement. In addition, this zirconia nanofilm modified Mg-Sr alloys show excellent biocompatibility than the bare samples. Hence, this work provides a new surface strategy to control the degradation rate while improving the biocompatibility of substrates.

Introduction

With the remarkable advantages of excellent mechanical property and biodegradability [1], [2], [3], which has a great potential to prevent a patient from having to undergo a second surgery after healing, the application of Mg-based alloy has getting more and more attention in the past decade. As an essential mineral, magnesium plays a significant role in human metabolism [4], [5], increasing bone cells’ growth [6] and the expression of osteogenic markers [7]. Furthermore, Mg promotes the formation of new bone, which is good for bone healing, and increases osteoconductivity [6], [8]. It is the fourth most abundant cation in the human body. Each adult human contains about 1 mol (24 g) Mg, and intracellular free Mg concentrations are about 0.5 mmoL/L [4]. In addition, the density range of Mg-based alloys, from 1.74 to 2.0 g/cm³, are very close to that of natural bone, from 1.8 to 2.1 g/cm³. The compressive strength of Mg alloys is closer to that of human bone when compared to other commonly used alloys as metallic implants [9]. Moreover, the Yong’s modulus of Mg alloys that is similar to that of human bone reduces the risk of stress shielding in orthopedic applications [10]. Strontium (Sr) is an important trace element. The body content of Sr in each adult is about 140 mg, and the average daily intake is about 2 mg [11]. As an element of natural bone, this element can accelerate tissue growth and reduce bone absorption [12], [13], [14]. Additionally, Sr is a part of the muscles and the heart. Sr behaves much like Ca in the human body, and the bone-seeking performance of Sr is preeminent [15]. Mg incorporated with Sr can enhance its mechanical capacity and corrosion resistance [13], [16]. With the series of advantages mentioned above, adding Sr to Mg substrate can further increase the possibility of the use of Mg for biomaterials.

However, as temporary implant biomaterials, Mg-based alloys, including Mg-Sr, suffer from rapid corrosion in the biological environment, which may significantly restrict their clinical use [6], [17], [18], [19], [20]. In the corrosion process, the amount of hydrogen gas produced may delay the healing of a wound and lead to the necrosis of tissues [21]. Also, this process might produce some toxic metallic elements [22], [23]. Therefore, the effective use of biodegradable Mg alloys is necessary for developing alloys and controlling or reducing the degradation rate to adjust the weightlessness rate and the rate of hydrogen evolution, thus decreasing the effect of biocompatibility.

So far, many efforts have been made to solve these problems. Besides the bulk design, such as the composition design [10], [24], [25] and the structure design [26], surface modification is an effective strategy not only for enhancing the corrosion resistance of metallic implants but also for endowing these materials with specific biological functions [27], [28], [29], [30], [31], [32], [33], [34], [35], [36].

Currently, atomic layer deposition (ALD) as a promising technology for the deposition of thin films has been developed rapidly due to the reproducibility, simplicity, and conformality of the acquired films [37], [38], [39]. In addition, the nanofilms deposited by ALD have precise thickness and high uniformity [39], [40]. In recent years, more and more researchers have been applying this technology to improve the corrosion resistance of materials [41], [42], [43].

With the advantages of excellent abrasion resistance, corrosion resistance, compression resistance, mechanical property, biocompatibility, high osseointegration, low toxicity, and firm stability in bone [44], [45], [46], [47], ZrO₂ is widely applied in biomedical materials, such as femoral heads and dental bridges [48]. Several techniques, such as cathodic arc deposition [49], plasma sputtering [50] and sol-gel [51], have been used in the formation of ZrO₂ due to its cytocompatibility, bioactivity, and antimicrobial quality. However, there are often some defects on these coatings during fabrication process and the process is often complicated. In this work, to explore the performance of ZrO₂, we prepared ZrO₂ ceramic coatings on Mg-Sr alloys deposited by ALD with different cycles for adjusting the thickness of films. The corrosion resistance and surface characterizations of these ceramic coatings were systematically studied. Moreover, the prepared ZrO₂ nanofilm exhibited good biocompatibility.