Journal of Materials Science & Technology, 2020, 46(0): 136-138 DOI: 10.1016/j.jmst.2019.12.014

Prospective

Opportunities and challenges of biodegradable Zn-based alloys

H.F. Li,*, Z.Z. Shi,, L.N. Wang,*

Beijing Advanced Innovation Center for Materials Genome Engineering, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China

Corresponding authors: * E-mail addresses:huafangli@ustb.edu.cn(H.F. Li),ryansterne@163.com(Z.Z. Shi),luning.wang@ustb.edu.cn(L.N. Wang).

Received: 2019-12-10   Accepted: 2019-12-16   Online: 2020-06-1

Abstract

Following the footsteps of biodegradable Mg-based and Fe-based alloys, biodegradable Zn-based alloy is a newcomer and rising star in the family of biodegradable metals and alloys. The combined superior mechanical properties, appropriate degradation rates, excellent biocompatibility of biodegradable Zn-based alloys have brought worldwide research interest on the design, development and clinical translation of Zn-based alloys. The present perspective has summarized opportunities and challenges in the development of biodegradable Zn-based alloys.

Keywords: Biodegradable Zn-based alloys ; Clinical applications ; Mechanical properties ; Degradation behavior ; Biocompatibility ; Corrosion fatigue ; Bio-tribocorrosion

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Cite this article

H.F. Li, Z.Z. Shi, L.N. Wang. Opportunities and challenges of biodegradable Zn-based alloys. Journal of Materials Science & Technology[J], 2020, 46(0): 136-138 DOI:10.1016/j.jmst.2019.12.014

1. Introduction

Biodegradable metals and alloys have been research hotspot recently considering they can dissolve completely upon fulfilling the mission to support or assist with tissue and organ healing, without needing the costly secondary surgical operation to remove it [1]. Besides the earlier developed biodegradable Mg-based and Fe-based alloys, biodegradable Zn-based alloys have aroused researcher’s extensive interest and attention as they have overcome the major concerns and drawbacks that have found in Mg-based and Fe-based biodegradable materials: too fast degradation rate of Mg-based alloys [2] and too slow degradation rate of Fe-based alloys [3]. In our present perspective, the advantages and opportunities as well as the challenges of biodegradable Zn-based alloys have been summarized.

2. Opportunities of biodegradable Zn-based alloys

2.1. Selectable fabrication and processing methods

As for biomaterials, fabrication and processing methods are of great importance as they have a profound impact on the biomedical materials’ various properties and clinical performances, such as microstructures, mechanical properties, degradation behavior, blood compatibility and biocompatibility, etc. Numerous fabrication and processing methods have been employed in developing biodegradable Zn-based alloys, including the conventional methods, such as casting, rolling, extrusion, drawing and forging [[4], [5], [6]]. On the other hand, several advanced and novel fabrication and processing methods: severe plastic deformation [7], additive manufacturing (AM) [8,9], have been used to fabricate and develop biodegradable Zn-based alloys. These selectable fabrication and processing methods make it easy and feasible to develop biodegradable Zn-based alloys with combined properties suitable for various biomedical applications.

2.2. Tunable mechanical properties

A key requirement of biomedical materials and devices is that they must have adjustable and suitable mechanical properties for specific clinical applications. For instance, biomaterials for orthopaedic application are required to encounter low Young’s Modulus in order to avoid the “stress shield effect”, which would cause bone loss and failure of device implantation [10,11]. As for the cardiovascular applications, sufficient radial strength and flexibility are prerequisite for the successful stent implantation and further vascular stenosis treatment [12,13]. The mechanical properties of biodegradable Zn-based alloys can be adjusted via adding various alloying elements and employing different fabrication and processing methods. The tunable mechanical properties are critical for the clinical translation of Zn-based alloys.

2.3. Suitable degradation behavior

Degradation behavior is one of key factors that determine the performance of the biodegradable alloys. Biodegradable Mg-based alloys are encountered to have high degradation rates which cause extensive H2 formation and accumulation, mechanical loss and disintegration [2]. On the contrary, the biodegradable Fe-based alloys have lower degradation rates than the clinical requirement, which would cause tissue inflammation and bring the concern of biosafety [3]. Compared with Mg-based alloys and Fe-based alloys, biodegradable Zn-based alloys have relatively suitable degradation behavior with ideal degradation rate [14,15]. The ideal corrosion rate of Zn-based alloys would guarantee the structure and mechanical integrate at the initial period of implantation, and then dissolve completely after tissue recovery, without causing gas pocket or inflammation which frequently occur in Mg-based alloys and Fe-based alloys.

2.4. Excellent biocompatibility of Zn

Zinc is an indispensable essential nutrient element in human body. It is a catalyst for more than 50 different enzymes and regulate gene expressions [16]. It plays an important role in growth and stimulates bone formation. Zinc is a potent inhibitor of osteoclastic bone resorption in vitro [17]. Zinc has an important role in states of cardiovascular disease and shows protective effects in coronary artery disease and cardiomyopathy [18]. Biodegradable Zn and its alloys demonstrate excellent hemocompatibility [19] and cytocompatibility [20]. Pure zinc stent implanted into rabbit abdominal aorta did not show severe inflammation, platelet aggregation, thrombosis formation, or obvious intimal hyperplasia up to one year time period [21]. Zn-1X (X = Mg, Ca, Sr) alloys implanted into mice’s femoral shaft of the distal femur showed enhanced ability of osteogenesis and promoted more new bone formation at the periosteum [20]. Zn-Cu stent implanted into porcine coronary arteries exhibited no inflammation responses or thrombosis formation up to 24 months [22]. The satisfactory biocompatibility of biodegradable Zn-based alloys has reassured their biosafety and is undoubtedly a strong impetus to promote the further research, development and clinical translation of biodegradable Zn-based alloys.

2.5. Extensive application prospects

Considering their ease to fabrication and processing, excellent mechanical properties, suitable degradation behavior, satisfactory biocompatibility, biodegradable Zn-based alloys have extensive application prospects ranging from bone fracture fixation pins, screws, plates, etc. to antibacterial biomedical devices and cardiovascular stents, cerebrovascular stents, carotid artery stents, urethral stents, esophagus, gastrointestinal, and biliary stents. Besides, biodegradable Zn-based alloys can be applied as responsive biosensors, considering their sensitivity to a stimulus (i.e. pH, protein content, variation of stresses, etc.).]. An example of responsive biosensors based on biodegradable Zn-based alloys is designing wireless implants with radiofrequency-driven resistor-inductor-capacitor (RLC) resonators [23].

3. Challenges of biodegradable Zn-based alloys

Although biodegradable Zn-based alloys have great potential to be new generation of biodegradable devices, there are several challenges to overcome for perspectively further successful design, develop and clinical translation of biodegradable Zn-based alloys.

3.1. Understanding natural aging and creep behavior of biodegradable Zn-based alloys

The low melting temperature of zinc would cause natural aging and creep at room temperature [24]. Tensile strength and yield strength would decrease with natural aging [25]. It has been reported that biodegradable Zn-0.1Li alloy exhibits appreciable creep deformation at human body temperature (37 °C) [26]. As for the biomedical materials and devices, one important requirement is the stability of their structure and mechanical properties, deformation or mechanical loss would happen when the materials are under natural aging and creeping effect. In order to guarantee the clinical performance and low implantation and repair failure rate, how the natural aging and creeping behavior effect the microstructure and mechanical properties of biomedical Zn-based alloy have to be systematically investigated and clarified in the future development and research on biodegradable Zn-based alloys.

3.2. Investigating corrosion fatigue (CF) and stress corrosion cracking (SCC) properties of biodegradable Zn-based alloys

It is well known that human body environment is a combination of corrosive solutions (such as blood, spinal fluid, gastric fluid, synovial fluid, urine, saliva, bile, etc.) and stress loading (such as vasorelaxation, spinal motion, joint bearing, etc.). Considering the harsh environment of human body, another indispensable requirement for biomedical materials and medical devices is to have ability to resist the combined corrosive and mechanical attack and invasion, i.e. corrosion fatigue (CF) and stress corrosion cracking (SCC). Investigating corrosion fatigue (CF) and stress corrosion cracking (SCC) properties of biodegradable Zn-based alloys in human body environment is imperative for further development and successful clinical applications of biodegradable Zn-based alloys.

3.3. Exploring bio-tribocorrosion of biodegradable Zn-based alloys

Besides corrosion fatigue (CF) and stress corrosion cracking (SCC), another inevitable issue of biodegradable Zn-based alloys is the bio-tribocorrosion behavior in human body environment. Similar to the aforementioned CF and SCC, bio-tribocorrosion is an another nonnegligible concern of a material as a result of synergistic physicochemical and mechanical stimulation [27]. A thorough exploring and understanding of the bio-tribology and bio-tribocorrosion mechanisms is therefore critical and crucial in developing reliable and credible biodegradable Zn-based implants and medical devices.

4. Concluding remark

As an up-and-coming new rising star, biodegradable Zn-based alloys have great potential for clinical applications as novel biomedical materials and devices as they have various selectable fabrication and processing methods, tunable mechanical properties, suitable degradation behavior, acceptable biocompatibility. Further studies should focus on the understanding natural aging and creep behavior, investigating corrosion fatigue (CF) and stress corrosion cracking (SCC) properties, as well as exploring bio-tribocorrosion of biodegradable Zn-based alloys.

Acknowledgements

This work was supported financially by the National Natural Science Foundation of China (Nos. 31700819 and 51871020), the Young Elite Scientists Sponsorship Program by CAST (YESS, No. 2018QNRC001) and the Fundamental Research Funds for the Central Universities (No. 06500098).

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