Fundamental Theory of Biodegradable Metals—Definition, Criteria, and Design

Abstract

Until now there has been no fundamental theory applicable for biodegradable metals (BMs). First, this paper optimizes the definition of BMs given in 2014. Second, the dual criteria of biodegradability and biocompatibility are proposed for BMs, and all metallic elements in the periodic table with accessible data are screened on the basis of these criteria. Regarding biodegradability, electrode potential, reactivity series, galvanic series, Pilling–Bedworth ratio, and Pourbaix diagrams are all adopted as parameters to classify the degradable and nondegradable nature of a material, especially in a physiological environment. Considering the biocompatibility at different levels, cellular biocompatibility, tissue biocompatibility, and human/clinical related biocompatibility parameters are put forward to comprehensively evaluate the biosafety of BMs. Third, for the material design of BMs, mechanical properties, chemical properties, physical properties and biological properties should be considered and balanced to guarantee that the degradation behavior of BMs match well with a tissue regeneration/repair procedure as the function of time and spatial location. Besides the selected metallic elements, some nonmetallic elements are selected as suitable alloying elements for BMs. Finally, five classification/research directions for future BMs are proposed: biodegradable pure metals, crystalline alloys, bulk metallic glasses, high entropy alloys, and metal matrix composites.

Alloying design of biodegradable zinc as promising bone implants for load-bearing applications

“…Alloying with Li, Mg, Cu, Ag, and Mn (≤2 wt%) resulted in a significant increase in strength and hardness. Among them, 199-350 43,59 Compressive yield strength (CYS), as extruded (MPa) 99-457 90-258 60,61 Elongation to failure, as extruded (%) 6-84 8-28 34,43 0.012-0.017 0.73-1.06 Dietary average daily intake (mg) 53 8.6 329 Recommended daily intake (mg) 53 12-15 280-350 Beneficial effects on bone 54 Necessary Li displayed the best strengthening effect on Zn followed by Mg.…”

“…Therefore, elements that play essential or beneficial roles in bone metabolism can be regarded as potential alloying elements. Metallic elements (K, Na, Ba, and Mo) and nonmetallic elements (O, P, S, Si, and Se) can be added into Zn to develop novel Zn alloy systems for better biocompatibility in the future [53][54][55] .…”

Recent research and progress of biodegradable zinc alloys and composites for biomedical applications: Biomechanical and biocorrosion perspectives

“…Engineered or natural materials that are used directly to supplement the functions of living tissue are known as biomaterials and they have been utilized as implant materials for a long time in the field of medical science [ [1] , [2] , [3] , [4] ]. Conventional non-degradable metallic biomaterials, such as stainless steels (SS), cobalt–chromium (Co–Cr) alloys, and titanium (Ti) and some of its alloys, are generally used as permanent or temporary implants to restore function by providing support to hard tissues.…”

Advances in biocermets for bone implant applications

“…They not only are the essential trace elements in the human body, but also can corrode gradually in physiological environment until to dissolve completely [4]. However, metal materials also present some limitations in bone implant applications, for instance, the incompatible mechanical properties and degradation rate, as well as poor bioactivity and possible cytotoxicity introduced by the high concentrations of metal ions [5][6][7].…”