Sung-Youn Cho

Biodegradability engineering of biodegradable Mg alloys: Tailoring the electrochemical properties and microstructure of constituent phases

Crystalline Mg-based alloys with a distinct reduction in hydrogen evolution were prepared through both electrochemical and microstructural engineering of the constituent phases. The addition of Zn to Mg-Ca alloy modified the corrosion potentials of two constituent phases (Mg + Mg2Ca), which prevented the formation of a galvanic circuit and achieved a comparable corrosion rate to high purity Mg. Furthermore, effective grain refinement induced by the extrusion allowed the achievement of much lower corrosion rate than high purity Mg. Animal studies confirmed the large reduction in hydrogen evolution and revealed good tissue compatibility with increased bone deposition around the newly developed Mg alloy implants. Thus, high strength Mg-Ca-Zn alloys with medically acceptable corrosion rate were developed and showed great potential for use in a new generation of biodegradable implants.

Long-term clinical study and multiscale analysis of in vivo biodegradation mechanism of Mg alloy

Significance In the past decade, countless studies have been performed to control the mechanical and corrosion property of magnesium-based alloy, which degrades in the physiological environment, to overcome the flaws of the inert implant materials and shift the paradigm of conventional bone fixation devices. Controlled degradation of Mg-5wt%Ca-1wt%Zn alloy results in the formation of biomimicking calcification matrix at the degrading interface to initiate the bone formation process. This process facilitates early bone healing and allows the complete replacement of biodegradable Mg implant by the new bone within 1 y of implantation, as demonstrated in 53 cases of successful long-term clinical study. There has been a tremendous amount of research in the past decade to optimize the mechanical properties and degradation behavior of the biodegradable Mg alloy for orthopedic implant. Despite the feasibility of degrading implant, the lack of fundamental understanding about biocompatibility and underlying bone formation mechanism is currently limiting the use in clinical applications. Herein, we report the result of long-term clinical study and systematic investigation of bone formation mechanism of the biodegradable Mg-5wt%Ca-1wt%Zn alloy implant through simultaneous observation of changes in element composition and crystallinity within degrading interface at hierarchical levels. Controlled degradation of Mg-5wt%Ca-1wt%Zn alloy results in the formation of biomimicking calcification matrix at the degrading interface to initiate the bone formation process. This process facilitates early bone healing and allows the complete replacement of biodegradable Mg implant by the new bone within 1 y of implantation, as demonstrated in 53 cases of successful long-term clinical study.

In vivo corrosion mechanism by elemental interdiffusion of biodegradable Mg–Ca alloy†

We elucidated the in vivo corrosion mechanism of the biodegradable alloy Mg-10 wt % Ca in rat femoral condyle through transmission electron microscope observations assisted by focused ion beam technique. The alloy consists of a primary Mg phase and a three-dimensional lamellar network of Mg and Mg(2)Ca. We found that the Mg(2)Ca is rapidly corroded by interdiffusion of Ca and O, leading to a structural change from lamellar network to nanocrystalline MgO. In contrast to the fast corrosion rate of the lamellar structure, the primary Mg phase slowly changes into nanocrystalline MgO through surface corrosion by O supplied along the lamellar networks. The rapid interdiffusion induces an inhomogeneous Ca distribution and interestingly leads to the formation of a transient CaO phase, which acts as a selective leaching path for Ca. In addition, the outgoing Ca with P from body fluids forms needle-type calcium phosphates similar to hydroxyl apatite at interior and surface of the implant, providing an active biological environment for bone mineralization.

Bone formation within the vicinity of biodegradable magnesium alloy implant in a rat femur model

The purposes of this preliminary study were to investigate the effect of increased Ca contents (5–10 wt% Ca) in Mg-Ca alloy on the mechanical properties and osseous healing rate in a standard rat defect model. Mechanical tests were performed using a compression system followed by qualitative histological analysis using the hemotoxylin and eosin (H&E) staining method and quantitative reverse transcriptase polymerase chain reaction (reverse transcriptase PCR). Mg-Ca alloy degraded fast in vivo while displaying a high level of the bone formation markersOC and ALP. Favorablemechanical strength properties were displayed as Ca content increased from 5 wt% to 10 wt% to show its potential to be considered as a load bearing implant material. The resultfrom this study suggests that the developed Mg-Ca alloy has the potential to serve as a biocompatible load bearing implant material that is degradable and possibly osteoconductive.