**5. Conclusion**

Contemporary research in bioactive metals and ceramics for load-bearing application is focused on bridging the gap between mechanical properties and biocompatibility. The fabrication techniques detailed in this chapter have demonstrated that great strides have been made and in doing so, can potentially be applied to improve on existing orthopaedic implants. The chapter also presented materials that are yet to be used in load-bearing application, such as zinc and niobium, but have great potential in doing so. With regards to metals, mitigating the toxicity of their respective ions is the major focus. This can be achieved through alloying with elements that are less toxic, or improving the coating on the implant to ensure ion release is minimised. Emerging biodegradable metals, such as magnesium and iron, are highly promising as they can reduce the overall healthcare cost. These metals degrade in the body through corrosion, and, as they are naturally found in the body, they can be excreted. The message from bioactive metals is that if the metal is naturally found in the body, then it is

<sup>6</sup> Wear from the container for example in which the powders are mixed can form into the mixture.

<sup>7</sup> Intriguingly, this is a clear example of how an attempt to address an issue results in more questions and possibilities in biomaterial engineering.

corroded and thus resorbed. If not, then if binding should occur, improved binding is achieved if a ceramic coating is formed, for example, TiO2 on titanium-based implant. This is interesting considering that metals such as titanium and tantalum are extracted in their oxide form and are followed by arduous processing to achieve high levels of purity, for only osseous tissues to show preference to their oxide form. Perhaps if certain steps necessary for achieving high purity can be avoided, then this could lessen the costs associated with the manufacturing steps, and thus implant fabrication.

Other forms of ceramic-metal composites can be attained through the powder blending route. Metallic powders of iron and magnesium can be homogenously mixed with CaP powders, followed by consolidation and densification. This can be an ideal method if the strategy is to impart the desired qualities (e.g. ductility) throughout a porous structure rather than purely on the surface. Considerations such as mixing time and sintering temperatures need to be

CaP, respectively. In order to avoid the latter issue, Choy et al. [69] used microwave synthesis of a Ti-CaP composite to avoid using high temperatures, where materials absorb electromag‐ netic energy that are produced by the microwave and subsequently convert it into heat energy. Additional benefits of the technique include fast reaction rate and efficient energy transfor‐ mation. A Ti-CaP composite with excellent mechanical properties comparable to cortical bone was fabricated by mixing and reacting Ti with two precursors of HA (calcium carbonate and dicalcium phosphate dihydrate in this case). Interestingly, it was discovered that the in situ synthesis method chosen resulted in the presence of Ti, HA, TTCP, and CaTiO3, indicating that the calcium precursors were able to react indiscriminately with the Ti. Incidentally, CaTiO3

A titanium-magnesium porous composite is one example of two metals combined, and can be achieved in a number of ways to form a semi-biodegradable metallic implant—including powder blending, melt infiltration casting, or as a layered structure. Porosity and compressive strength suitable for bearing loads are attainable, but this depends on the amount of magne‐

Contemporary research in bioactive metals and ceramics for load-bearing application is focused on bridging the gap between mechanical properties and biocompatibility. The fabrication techniques detailed in this chapter have demonstrated that great strides have been made and in doing so, can potentially be applied to improve on existing orthopaedic implants. The chapter also presented materials that are yet to be used in load-bearing application, such as zinc and niobium, but have great potential in doing so. With regards to metals, mitigating the toxicity of their respective ions is the major focus. This can be achieved through alloying with elements that are less toxic, or improving the coating on the implant to ensure ion release is minimised. Emerging biodegradable metals, such as magnesium and iron, are highly promising as they can reduce the overall healthcare cost. These metals degrade in the body through corrosion, and, as they are naturally found in the body, they can be excreted. The message from bioactive metals is that if the metal is naturally found in the body, then it is

or decomposition of

determined without resulting in contaminations by milling apparatus6

sium, which is significantly altered in situ as corrosion of Mg takes place.

Wear from the container for example in which the powders are mixed can form into the mixture.

<sup>7</sup> Intriguingly, this is a clear example of how an attempt to address an issue results in more questions and possibilities in

was claimed to facilitate apatite formation in vitro.7

**4.2. Same material composites**

212 Advanced Techniques in Bone Regeneration

**5. Conclusion**

6

biomaterial engineering.

Many ceramics and glasses display excellent bioactivity and are toxic-free. This is to be expected considering that they have been synthesised based on the composition of natural bone. Forming ceramics and glass into complex shapes is difficult irrespective of the applica‐ tion due to their inherent properties; however, progress is being made to eliminate such factors. The chapter on ceramics and glasses focused predominantly on fabrication routes with "ideal" porous structure. Such techniques have elucidated to how compressive strength for loadbearing application is attainable in porous CaP if excellent control over the physical properties can be achieved. However, the bone exhibits multiple stress states which will all need to be addressed before clinical application is considered. To achieve desirable flexural (bending) strength and fracture toughness, the fabrication method could use a CaP reinforced with ceramics that possess high toughness and flexural strength. Reinforcing dense β-TCP compo‐ site with varying amounts of the bioactive TiO2 is known to increase the fracture toughness and flexural strength similar to that of cortical bone. Recent progress showed that an eightfold increase in compressive strength in HA reinforced with TiO2 above the required amount, with respect to TiO2-free HA. Alternatively, hydroxyapatite rod-like particulates, also known as whiskers, can be incorporated into the CaP matrix. Although a form of CaP, the whiskers are able to preserve their morphology during sintering, forming a distinct phase from the surrounding CaP. When dispersed throughout the microstructure, whiskers improve the fracture resistance by deflecting microcrack propagation, as well as absorbing the energy generated by the microcrack. Factors such as the aspect ratio, whisker orientation, and content volume influence their effectiveness. Incidentally, natural bone exhibits crack deflection behaviour. Therefore, in theory, fabricating CaP using freeze casting or extrusion with reinforced whiskers or TiO2 can enhance flexural strength and/or fracture toughness, and thus CaP implants can be made suitable for multiple stress states.

The final section of the chapter presented examples of how ceramics and materials can be combined to produce synthetic implants with excellent bioactivity and mechanical properties. A CaP coating can be applied to titanium or magnesium to impart bioactivity or improve corrosion resistance, respectively. However, the extra process required additional costs.

Titanium-based materials still remain as the exemplary implant for load-bearing application. The fatigue resistance and analysis of multiple stress responses of biodegradable materials have not been concluded, and indeed, there are concerns with their ability to reach the benchmark set by titanium-based materials. However, if they can be engineered to withstand the initial load and allow for natural bone to remodel, then what synthetic material currently available (and possibly for the very long foreseeable future) is able to outperform the natural bone? Furthermore, the great diversity in bone morphology means that there is no ideal scaffold and thus each application requires a bespoke graft with matching mechanical properties, which will need to be addressed.

It is evident from this chapter that material engineers are exhausting their resources and developing ingenious methods in the process to improving bioactive implants. There are issues regarding the rate of degradation of bioresorbable implants; however, different materials (e.g. magnesium and zinc) degrade at varying rates. It will be interesting to see if technology can allow for a multi-layered bioresorbable implant can be engineered (the different layers corresponding to different bioresorbable materials). Investigations into finding ways of degrading titanium could be interesting. The conjecturing of future directions in the field is limitless and thus it can be said with confidence that the future of the field is very encouraging.
