George J. Dias

Calcium phosphate coatings on magnesium alloys for biomedical applications: A review

Magnesium has been suggested as a revolutionary biodegradable metal for use as an orthopaedic material. As a biocompatible and degradable metal, it has several advantages over the permanent metallic materials currently in use, including eliminating the effects of stress shielding, improving biocompatibility concerns in vivo and improving degradation properties, removing the requirement of a second surgery for implant removal. The rapid degradation of magnesium, however, is a double-edged sword as it is necessary to control the corrosion rates of the materials to match the rates of bone healing. In response, calcium phosphate coatings have been suggested as a means to control these corrosion rates. The potential calcium phosphate phases and their coating techniques on substrates are numerous and can provide several different properties for different applications. The reactivity and low melting point of magnesium, however, require specific parameters for calcium phosphate coatings to be successful. Within this review, an overview of the different calcium phosphate phases, their properties and their behaviour in vitro and in vivo has been provided, followed by the current coating techniques used for calcium phosphates that may be or may have been adapted for magnesium substrates.

Magnesium alloys: Predicting in vivo corrosion with in vitro immersion testing†

Magnesium (Mg) and its alloys have been proposed as degradable replacements to commonly used orthopedic biomaterials such as titanium alloys and stainless steel. However, the corrosion of Mg in a physiological environment remains a difficult characteristic to accurately assess with in vitro methods. The aim of this study was to identify a simple in vitro immersion test that could provide corrosion rates similar to those observed in vivo. Pure Mg and five alloys (AZ31, Mg-0.8Ca, Mg-1Zn, Mg-1Mn, Mg-1.34Ca-3Zn) were immersed in either Earles balanced salt solution (EBSS), minimum essential medium (MEM), or MEM-containing 40 g/L bovine serum albumin (MEMp) for 7, 14, or 21 days before removal and assessment of corrosion by weight loss. This in vitro data was compared to in vivo corrosion rates of the same materials implanted in a subcutaneous environment in Lewis rats for equivalent time points. The results suggested that, for the alloys investigated, the EBSS buffered with sodium bicarbonate provides a rate of degradation comparable to those observed in vivo. In contrast, the addition of components such as (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) (HEPES), vitamins, amino acids, and albumin significantly increased corrosion rates. Based on these findings, it is proposed that with this in vitro protocol, immersion of Mg alloys in EBSS can be used as a predictor of in vivo corrosion.

Magnesium biomaterials for orthopedic application: A review from a biological perspective

Magnesium (Mg) has a long history of investigation as a degradable biomaterial. Physicians first began using Mg for biomedical applications in the late 19th century. Experimentation continued with varying levels of success until the mid-20th century when interest in the metal waned. In recent years the field of Mg-based biomaterials has once again become popular, likely due to advancements in technology allowing improved control of corrosion. Although this has led to success in vascular applications, continued difficulties in predicting and controlling the corrosion rate of Mg in an intraosseous environment has impeded the development of Mg-based biomaterials for orthopedic applications. In this review, an initial summary of the basic properties and the physiological role of Mg are followed by a discussion of the physical characteristics of the metal which lend it to use as a degradable biomaterial. A description of the historical and modern applications for Mg in the medical field is followed by a discussion of the methods used to control and assess Mg corrosion, with an emphasis on alloying. The second part of this review concentrates on the methods used to assess the corrosion and biocompatibility of Mg-based orthopedic biomaterials. This review provides a summary of Mg as a biomaterial from a biological perspective.

Effect of Cross-Linking on Microstructure and Physical Performance of Casein Protein

The development of advanced materials from biorenewable protein biopolymers requires the generation of more exogenous bonds to maintain the microstructure and durability in the final products. Casein is the main protein of milk, representing about 80% of the total protein. In the present investigation the casein protein was solubilized and/or emulsified in aqueous alkaline solutions, and 2D films and 3D matrices were produced. The effects of silane (3-aminopropyl triethoxy silane), DL-glyceraldehyde and glutaraldehyde on tensile properties and water swelling/absorption of 2D casein films and also the microstructure of the freeze-dried 3D matrices were analyzed. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed that there were no significant changes in the molecular weight (19-23.9 kDa) of the casein proteins on exposure to alkaline solutions of sodium hydroxide and silane. The casein films produced without glycerol plasticizer and with heat treatment (130 °C for 18 h) were fragile. However, the fragile films were transformed into ductile and tough materials on exposure to moisture (i.e., conditioned for one week at 50 ± 2% relative humidity and 22 ± 2 °C) and showed a maximum average tensile strength of 49-52 MPa and modulus of 1107-1391 MPa. The chemical cross-linkers (i.e., DL-glyceraldehyde and glutaraldehyde) improved the microstructure of glycerol plasticized casein protein, when analyzed under scanning electron microscope (SEM). Furthermore, these chemical cross-linking agents enhanced the mechanical properties and water resistant properties of casein films.

Buffer-regulated biocorrosion of pure magnesium

Magnesium (Mg) alloys are being actively investigated as potential load-bearing orthopaedic implant materials due to their biodegradability in vivo. With Mg biomaterials at an early stage in their development, the screening of alloy compositions for their biodegradation rate, and hence biocompatibility, is reliant on cost-effective in vitro methods. The use of a buffer to control pH during in vitro biodegradation is recognised as critically important as this seeks to mimic pH control as it occurs naturally in vivo. The two different types of in vitro buffer system available are based on either (i) zwitterionic organic compounds or (ii) carbonate buffers within a partial-CO2 atmosphere. This study investigated the influence of the buffering system itself on the in vitro corrosion of Mg. It was found that the less realistic zwitterion-based buffer did not form the same corrosion layers as the carbonate buffer, and was potentially affecting the behaviour of the hydrated oxide layer that forms on Mg in all aqueous environments. Consequently it was recommended that Mg in vitro experiments use the more biorealistic carbonate buffering system when possible.

Substituted hydroxyapatites for bone regeneration: A review of current trends

At present hydroxyapatite (HA) is been extensively investigated for biomedical applications, largely as a result of its similarity in composition to the mineral portion of bone. Although HA undergoes osseointegration and is bioactive and osteoconductive, the inherent brittleness and low fracture toughness limits its use under load bearing conditions, also once implanted in the body, HA takes a long time to resorb. The crystal structure of HA is conducive to a variety of ionic substitutions. To accurately mimic the calcium deficient and carbonate-containing nature of HA in bone, both cationic and anionic substituents have been incorporated to synthetic HA. This article focuses on the incorporation of both the well established (Zn, Si, Sr, F, and carbonate) and latest ions (Ag, citrate, iron, niobate, and tantalates) into the HA structure and aims to highlight the key effects of these substitutions in terms of their chemical, physical, and biological properties. It can be shown that a minor substituent cannot only alter the microstructure, stability and crystallinity of the HA structure in an implant, but also have a significant effect on bone cells colonizing the implant, which in turn can influence the new bone formation and bone remodeling processes.

Osseous regeneration in the presence of oxidized cellulose and collagen.

Oxidized cellulose and collagen are two absorbable hemostatic scaffolding materials that are used widely in surgery. A histomorphological study was undertaken to determine the tissue response and extent of healing brought about by intraosseously implanting these two materials in the femur and tibia of sheep. There was no major difference in the rate of repair of the bone defects brought about by these two materials, with the bone defects being completely repaired by lamellar bone at 6–8 weeks. Therefore, our results suggest that, in most instances where collagen is presently used in surgical applications, it could be substituted by oxidized cellulose.

Keratin–hydroxyapatite composites: Biocompatibility, osseointegration, and physical properties in an ovine model

Reconstituted keratin has shown promise as an orthopaedic biomaterial. This in vivo study investigates the biological response of composite materials prepared from reconstituted keratin containing a high content of hydroxyapatite (HA) (40 wt % HA), implanted for up to 18 weeks in the long bones of sheep. Keratin-HA composites were compared with a commercially available polylactic acid (PLA) HA composite (BIO RCI HA®, Smith and Nephew). Porous keratin-HA materials displayed excellent biocompatibility and osseointegration, with full integration into bone by 12 weeks. Dense keratin-HA materials also showed excellent biocompatibility, with a more limited osseointegration, involving the penetration of new bone into the periphery of the implant after eight weeks. In contrast, the PLA-HA implant did not integrate with surrounding tissue. Microindentation showed that porous keratin-HA implants were initially soft, but became stiffer as new bone penetrated the implant from four weeks onwards. In contrast, although the initial rigidity of dense keratin-HA composites was maintained for at least two weeks, the implant material weakened after four weeks. The PLA-HA implant maintained its physical properties throughout the course of the trial. This study demonstrates the increased osseointegration/osteoconduction capacity of keratin-HA composites and provides further evidence supporting the suitability of keratin-based materials, such as bone graft substitutes and soft tissue fixation devices.

Monetite and brushite coated magnesium: in vivo and in vitro models for degradation analysis

The use of magnesium (Mg) as a biodegradable metallic replacement of permanent orthopaedic materials is a current topic of interest and investigation. The appropriate biocompatibility, elastic modulus and mechanical properties of Mg recommend its suitability for bone fracture fixation. However, the degradation rates of Mg can be rapid and unpredictable resulting in mass hydrogen production and potential loss of mechanical integrity. Thus the application of calcium phosphate coatings has been considered as a means of improving the degradation properties of Mg. Brushite and monetite are utilized and their degradation properties (alongside uncoated Mg controls) are assessed in an in vivo subcutaneous environment and the findings compared to their in vitro degradation behaviour in immersion tests. The current findings suggest monetite coatings have significant degradation protective effects compared to brushite coatings in vivo. Furthermore, it is postulated that an in vitro immersion test may be used as a tentative predictor of in vivo subcutaneous degradation behavior of calcium phosphate coated and uncoated Mg.

The in vitro and in vivo evaluation of the biocompatibility of Mg alloys

Magnesium (Mg) and its alloys are being widely investigated for their potential use as resorbable biomaterials for orthopaedic applications. However, the natural corrosion of the metals results in potentially harmful perturbations to the physiological environment, which requires a comprehensive understanding of their biocompatibility. Currently, most investigations proceed directly from in vitro biocompatibility studies to intraosseous implantation. However, this can result in the unnecessary elimination of appropriate materials due to over sensitive in vitro methods or the implantation of potentially harmful materials. This study involved the development of a relevant in vitro cell culture method, and an in vivo soft tissue implantation technique to provide an intermediate step between basic cell culture methods and large animal intraosseous investigations. A Live/Dead fluorescent assay was used to investigate the viability of both L929 and SaOS-2 cells exposed to Mg alloys, with the results compared to those seen with the intramuscular implantation of the same materials in Lewis rats. These methods were able to successfully provide data on the corrosion of Mg alloys, allowing the identification of slowly and safely corroding materials that may be used in future intraosseous investigations.