Full length articleAtomic layer deposited ZrO2 nanofilm on Mg-Sr alloy for enhanced corrosion resistance and biocompatibility
Graphical abstract
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.
Section snippets
Material preparation
A merchant Mg-1Sr alloy with the dimensions of 10 mm × 10 mm × 5 mm were grounded with SiC paper of different grit sizes (600, 800, 1200, 2400 and 4000) successively and were subsequently ultrasonically cleaned with acetone and ethanol for 15 min, respectively. At last, the samples were dried by flowing cool air.
Atomic layer deposition
The deposition of a ZrO2 layer was performed in a commercial ALD reactor (F-100-41, MNT Micro and Nanotech Co., LTD, Wuxi, China) through successive cyclic reactions between Tetrakis
Surface characterization
The surface morphologies of the modified samples are shown in Fig. 1. As demonstrated in Fig. 1a, nanoparticles are uniformly distributed on the surface of the ZrO2-100 sample. With the increase of deposition circles, the size of the nanoparticles is reduced slightly, as shown in Fig. 1b (200 cycles), Fig. 1c (300 cycles), and Fig. 1d (400 cycles). This is because more deposition circles result in thicker and denser nanoparticles. EDS (Fig. 1e and f) proves that the Zr-containing materials have
The formation mechanism of ZrO2 coating
ZrO2 coating was deposited on Mg-Sr alloy substrate through ALD with different cycles. Fig. 11 shows the formation mechanism of ZrO2 coating. In this work, the ALD process requires two exclusive precursors, i.e. TDMAZ and water, as illustrated in Fig. 11. This process contains two half-reactions, indicated as follows:
where, hydroxy (OH) group is originated from the natural oxidation of Mg substrate, and the octothorpe (#) indicates the connection of these groups with the substrate. And the
Conclusion
A ZrO2 ceramic coating has been successfully formed on the surface of Mg-Sr alloys by using the ALD method. By regulating the deposition cycle of ALD, the thickness of ZrO2 thin film can be controlled precisely; consequently, the corrosion resistance of Mg-Sr alloys can be regulated precisely. In vitro studies reveal that a ZrO2 thin film not only enhances the corrosion resistance of Mg-Sr alloy but also is beneficial for the growth of cells and tissues in light of the great biocompatibility of
Acknowledgements
This work is jointly supported by the National Natural Science Foundation of China, Nos. 51422102, and 51671081, and the National Key Research and Development Program of China No. 2016YFC1100600 (sub-project 2016YFC1100604), as well as Shenzhen Peacock Program (1108110035863317).
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The two authors contribute to this work equally.