Fabrication and characterization of a biodegradable Mg–2Zn–0.5Ca/1β-TCP composite

An overview of translational research in bone graft biomaterials

Natural bone healing is often inadequate to treat fractures with critical size bone defects and massive bone loss. Immediate surgical interventions through bone grafts have been found to be essential on such occasions. Naturally harvested bone grafts, although are the preferred choice of the surgeons; they suffer from serious clinical limitations, including disease transmission, donor site morbidity, limited supply of graft etc. Synthetic bone grafts, on the other hand, offer a more clinically appealing approach to decode the pathways of bone repair through use of tissue engineered biomaterials. This article critically retrospects the translational research on various engineered biomaterials towards bringing transformative changes in orthopaedic healthcare. The first section of the article discusses about composition and ultrastructure of bone along with the global perspectives on statistical escalation of bone fracture surgeries requiring use of bone grafts. The next section reviews the types, benefits and challenges of various natural and synthetic bone grafts. An overview of clinically relevant biomaterials from traditionally used metallic, bioceramic, and biopolymeric biomaterials to new generation composites have been summarised. Finally, this narrative review concludes with the discussion on the emerging trends and future perspectives of the promising bone grafts.

Current strategies in biomaterial-based periosteum scaffolds to promote bone regeneration: A review

The role of periosteum rich in a variety of bone cells and growth factors in the treatment of bone defects has gradually been discovered. However, due to the limited number of healthy transplantable periosteum, there are still major challenges in the clinical treatment of critical-size bone defects. Various techniques for preparing biomimetic periosteal scaffolds that are similar in composition and structure to natural periosteal scaffold have gradually emerged. This article reviews the current preparation methods of biomimetic periosteal scaffolds based on various biomaterials, which are mainly divided into natural periosteal materials and various polymer biomaterials. Several preparation methods of biomimetic periosteal scaffolds with different principles are listed, their strengths and weaknesses are also discussed. It aims to provide a more systematic perspective for the preparation of biomimetic periosteal scaffolds in the future.

A review of current challenges and prospects of magnesium and its alloy for bone implant applications

Medical application materials must meet multiple requirements, and the designed implant must mimic the bone structure in shape and support the formation of bone tissue (osteogenesis). Magnesium (Mg) alloys, as a "smart" biodegradable material and as "the green engineering material in the twenty-first century", have become an outstanding bone implant material due to their natural degradability, smart biocompatibility, and desirable mechanical properties. Magnesium is recognised as the next generation of orthopaedic appliances and bioresorbable scaffolds. At the same time, improving the mechanical properties and corrosion resistance of magnesium alloys is an urgent challenge to promote the application of magnesium alloys. Nevertheless, the excessively quick deterioration rate generally results in premature mechanical integrity disintegration and local hydrogen build-up, resulting in restricted clinical bone restoration applicability. The condition of Mg bone implants is thoroughly examined in this study. The relevant approaches to boost the corrosion resistance, including purification, alloying treatment, surface coating, and Mg-based metal matrix composite, are comprehensively revealed. These characteristics are reviewed to assess the progress of contemporary Mg-based biocomposites and alloys for biomedical applications. The fabricating techniques for Mg bone implants also are thoroughly investigated. Notably, laser-based additive manufacturing fabricates customised forms and complicated porous structures based on its distinctive additive manufacturing conception. Because of its high laser energy density and strong controllability, it is capable of fast heating and cooling, allowing it to modify the microstructure and performance. This review paper aims to provide more insight on the present challenges and continued research on Mg bone implants, highlighting some of the most important characteristics, challenges, and strategies for improving Mg bone implants.

Fabrication and characterization of biodegradable zinc matrix composites reinforced by uniformly dispersed beta-tricalcium phosphate via graphene oxide-assisted hetero-agglomeration

The development of biodegradable Zn matrix composites has been considered a promising approach to achieving enhanced mechanical properties, controllable degradation rate, good biocompatibility, and good osseointegration as orthopedic implants. However, scant literature regarding Zn matrix composites has been reported because of the great difficulty in dispersing the nano-sized bioactive reinforcements uniformly within the Zn matrix. In the present study, a novel and effective method were employed to obtain Zn matrix composites reinforced by uniformly dispersed beta-tricalcium phosphate (β-TCP) via graphene oxide (GO)-assisted hetero-agglomeration and subsequent spark plasma sintering process. A very low-content (0.04 vol%) few-layered GO was used as a coupling reagent to connect the Zn matrix and nano-sized TCP particles. In an appropriate polarity solvent, the negatively charged GO sheets could combine with both the positively charged Zn powder and TCP particles by electrostatic attraction and charge neutralization. Due to the nature of hetero-agglomeration, the flexible GO sheet could adhere to the large Zn powder and attracted a certain amount of TCP particles to form a Zn/GO/TCP sandwich structure by charge neutralization thereby forming a uniform dispersion of TCP particles within Zn matrix. After the spark plasma sintering (SPS) process, the TCP particles incorporated with very thin ZnO layers (thickness of a few dozen nanometers) formed a homogeneous and unique 3D network-like distribution in as-sintered TCP/Zn composites. A unique "snap pea"-like structure was confirmed at the grain boundary of α-Zn grains, which consisted of the TCP particles as "pea" and thin ZnO layer as "pod". Due to the uniform dispersion of bioactive TCP particles and unique structure of the TCP incorporating grain boundary, as-sintered 3TCP/Zn matrix composites possessed yield strength (YS) of 140.8 ± 7.7 MPa, failure strain of 36.0 ± 2.8%, the moderate degradation rate of 19.1 ± 3.3 μm·y^-1 and good cytocompatibility to MC3T3-E1 cells. Moreover, osteogenic differentiation activity evaluation revealed that the addition of TCP could significantly improve the expressions of the osteogenic differentiation-related gene (ALP) in MC3T3-E1 cells, thereby resulting in improved osteogenic capability. Therefore, biodegradable 3TCP/Zn matrix composites fabricated by GO-assisted hetero-agglomeration and subsequent SPS process could be a promising material as orthopedic implants.

Effects of Incorporating Β-Tricalcium Phosphate with Reaction Sintering into Mg-Based Composites on Degradation and Mechanical Integrity

For applications of biodegradable load-bearing implants, we incorporated 10 or 20 vol% β-tricalcium phosphate (β-TCP) into Mg-based composites through reaction sintering in the spark plasma sintering process. We previously reported that the evolved microstructure enhanced mechanical properties before degradation and modified in vitro degradation behaviors. In this study, immersion tests in physiological saline and subsequent compression tests in the air were conducted to investigate the effects of degradation on mechanical integrity. In the immersion tests, Mg/β-TCP composites showed no visible disintegration of sintered particles due to interfacial strength enhanced by reaction sintering. Local corrosion was observed in the Mg matrix adjacent to the reaction products. In addition, Mg/10% β-TCP showed dense degradation products of Mg(OH)2 compared with Mg and Mg/20% β-TCP. Those degradation behaviors resulted in reducing the effective load transfer from the Mg matrix to the reaction products as reinforcement. The yield strength decreased by 18.1% for Mg/10% β-TCP and 70.9% for Mg/20% β-TCP after six days of immersion. These results can give a broad view of designing spark plasma sintered Mg/bioceramic composites with the consideration of mechanical integrity.

Recent Developments in Magnesium Metal-Matrix Composites for Biomedical Applications: A Review

Recently, there is a growing interest in developing magnesium (Mg) based degradable biomaterial. Although corrosion is a concern for Mg, other physical properties, such as low density and Young's modulus, combined with good biocompatibility, lead to significant research and development in this area. To address the issues of corrosion and low yield strength of pure Mg, several approaches have been adopted, such as, composite preparation with suitable bioactive reinforcements, alloying, or surface modifications. This review specifically focuses on recent developments in Mg-based metal matrix composites (MMCs) for biomedical applications. Much effort has gone into finding suitable bioactive, bioresorbable reinforcements and processing techniques that can improve upon existing materials. In summary, this review provides a comprehensive overview of existing Mg-based composite preparation and their mechanical and corrosion properties and biological responses and future perspectives on the development of Mg-based composite biomaterials.

Microstructure, mechanical properties, and degradation of Mg-Ag alloy after equal-channel angular pressing

Enhancing the strength of Mg-based biodegradable alloys without decreasing their corrosion resistance is a major engineering challenge. In addition, the growing demand for effective reduction of infections and inflammation after implant placement motivates the design of alloys with appropriate compositions or coatings. One promising alloying element is silver, whose antibacterial effect has long been known. Therefore, a Mg-4% Ag alloy was selected for this study. The alloy was investigated under three conditions: as-cast, after T4 treatment, and after T4 treatment with subsequent equal-channel angular pressing (ECAP) using a newly developed double-ECAP die, which offers an equivalent strain per pass of 1.6. The first pass through the double-ECAP die was conducted at 370 °C and the second at 330 °C using route B_C. The microstructure of the as-cast Mg-4% Ag consisted of large grains (several hundred microns) and a dendritic structure with micron-sized Mg₅₄Ag₁₇ precipitates. T4 heat treatment caused dissolution of the dendrites and formation of a solid solution without changing the grain size. Consequently, the ultimate compressive strength (UCS) was increased by approximately 30%, and the compressive strain at fracture reached approximately 23%. The compressive yield strength (CYS) remained nearly constant at approximately 30 MPa. Subsequent ECAP led to strong grain refinement (from 350 μm to 38 μm after one pass and 15 μm after two passes) and further increases in the CYS and UCS, to 45 and 300 MPa after the first pass and 62 and 325 MPa after the second pass, respectively. The as-cast alloy exhibited a very high degradation rate in a simulated body fluid at approximately 36 °C. The degradation rate of the alloy after T4 treatment was much lower. Subsequent ECAP had no significant effect on the degradation properties. Thus, it can be concluded that grain refinement has little effect on the degradation rate.

Microstructure, mechanical, corrosion properties and cytotoxicity of beta‑calcium polyphosphate reinforced ZK61 magnesium alloy composite by spark plasma sintering

Magnesium alloy (ZK61) and beta-tricalcium phosphate (β-TCP) composite ZK61/xβ-TCP (x = 0, 5, 10, 15 wt%) are fabricated using spark plasma sintering (SPS). In this study, the microstructure, mechanical properties, degradation behavior in simulated body fluid and cytotoxicity tests of composite were investigated. The results show that when the content of β-TCP was 5 wt%, which could be evenly distributed on the boundary of ZK61 particles. But agglomeration phenomenon appeared when the content of β-TCP reached 15 wt%. The hardness and the compressive strength increase with increasing of β-TCP content, and ZK61/15β-TCP achieves a maximum Vickers hardness of 94.81 HV_0.1 and compressive strength of 402 ± 9 MPa. The immersion tests indicate that corrosion resistance of the composites are better than that of ZK61 matrix, especially ZK61/5β-TCP. Corrosion products of the composite surface are mainly Mg(OH)₂, HA and Ca₃(PO₄)₂. The cytotoxicity tests indicate that composite extracts have no toxicity to L-929 cells. These results suggest that ZK61/xβ-TCP composites are promising candidate for degradable implant materials.

A Magnesium-based Nanobiocomposite Processed by a Novel Technique Combining High Shear Solidification and Hot Extrusion

BACKGROUND

Most of the currently available Mg-based biomaterials corrode too fast in the physiological environment, causing many problems including hydrogen bubble release and premature mechanical failure. It is commonly recognized that high biodegradation rate is the major factor limiting their clinical applications.

OBJECTIVE

The present research aims to develop a new magnesium (Mg)-based biomaterial with a controlled biodegradation rate.

METHODS

A magnesium-hydroxyapatite (Mg-1.61Zn-0.18Mn-0.5Ca/1HA) nanocomposite was developed by a novel technique which combines high shear solidification and hot extrusion, followed by heat treatment. The microstructure and biodegradation rate of the nanocomposite in HBSS Hanks' Balanced Salt Solution were assessed. Biodegradation behaviour was studied using electrochemical corrosion and immersion test. Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) were used to characterize the surface microstructure, biodegradation morphology and to analyse the biodegradation products. Few patents were also cited in the article.

RESULTS

Under the optimized procedure of high shear solidification, extrusion and heat treatment at 400°C, the Mg-1.61Zn-0.18Mn-0.5Ca/1HA exhibited a satisfactory biodegradation rate of 0.12±0.04 mm/year.

CONCLUSION

This technology shows a potential of breakthrough innovation in the manufacturing of Mg-based biomaterials with a decreased biodegradation rate.

Assessment of magnesium-based biomaterials: from bench to clinic

This review presents the operation procedures of commonly used standard methods for assessment of Mg-based biomaterials from bench to clinic.

Next-Generation Biomaterials for Bone-Tissue Regeneration: Mg-Alloys on the Move

Disorders related to the bone health are becoming a significant concern due to subsequent rise in ageing human population. It is estimated that more than two million bone-surgeries are performed worldwide with an annual cost of $2.5 billion. In order to replace damaged bone-tissues and restore their function, biomaterials consisting of stainless steels, cobalt-chromium and titanium alloys are implanted. However, these permanent (non-biodegradable) implants often lead to stress-shielding effects and ions release as they interact with the cells and fluids in the body. It is required to overcome these issues by improving the quality of implant materials and increasing their service life. Recently, research in biodegradable materials, consisting of magnesium alloys in particular, has received global attention owning to their biocompatibility and closer mechanical properties to the natural bone. However, due to their rapid corrosion rate in the body fluids, clinical applications of Mg-alloys as viable bone-implants have been restricted. A number of Mg-alloys have been tested since (both in vivo and in vitro) to optimize their biodegradation rare and corrosion properties. The present review summarizes the most recent developments in Mg-alloys designed with biodegradation tailored to the bone-cells growth and highlights the most successful ways to optimize their surface properties for optimum cell/material interaction.

Mechanism for corrosion protection of β-TCP reinforced ZK60 via laser rapid solidification

It remains the primary issue to enhance the corrosion resistance of Mg alloys for their clinical applications. In this study, β-tricalcium phosphate (β-TCP) was composited with Mg-6Zn-1Zr (ZK60) using laser rapid solidification to improve the degradation behavior. Results revealed rapid solidification effectively restrained the aggregation of β-TCP, which thus homogenously distributed along grain boundaries of α-Mg. Significantly, the uniformly distributed β-TCP in the matrix promoted the formation of apatite layer on the surface, which contributed to the formation of a compact corrosion product layer, hence retarding the further degradation. Furthermore, ZK60/8β-TCP (wt. %) composite showed improved mechanical strength, as well as improved cytocompatibility. It was suggested that laser rapidly solidified ZK60/8β-TCP composite might be a potential materials for tissue engineering.

Substantial enhancement of corrosion resistance and bioactivity of magnesium by incorporating calcium silicate particles

We discovered that calcium silicate is an effective reinforcement phase to improve the corrosion resistance, mechanical strength and biological performance of Mg or Mg-based alloys to overcome their major drawbacks for orthopedic implant applications.