Hydroxyapatite formation on metallurgical grade porous silicon nanosponge particles

Investigations into the development of potential bone substitutes have increased rapidly in the last decade. Titanium and cobalt chrome are currently the alloys of choice when it comes to the orthopedic medical device fields due to their excellent mechanical strength and corrosion-resistant properties. Yet these materials are unable to elicit a biologically functional bone-material interface without a bioactive surface coating or surface modification. Osteoconductivity is only achieved when suitable coatings are applied or their surface properties are suitably altered. The need for significant bony reconstruction implants as a result of prosthetic revision surgery also increases the need to produce longer lasting or permanent bone substitute materials. Hydroxyapatite is the principle constituent of bone and has been used as a mechanism to induce bone formation at particular biological sites in need of bone repair and growth. When applied as a surface coating, hydroxyapatite s chemical and physical properties allow osteointegration of medical devices and prostheses. The discovery of hydroxyapatite has resulted, not only in rapid advances and developments in the orthopedic and dental fields, but has also lead to a surge in investigations into further tailoring of the material to create new devices that meet clinical needs. Currently, the most commonly used method for assessing the potential bioactivity and bone-bonding ability of a material in-vitro involves using simulated body fluid. Previous research by Kokubo et al. has shown that in-vitro results obtained using these experiments correlate directly to in-vivo results and thus satisfies their use as potential bone-tissue substitutes. Porous silicon is a bioactive material and has been the subject of intense research since its original discovery at the Bell labs in 1956. Canham et al. was the first to suggest the possibility of creating biologically interfaced devices from porous silicon given its biostability, non-toxicity and ease of its topographical manipulation and optoelectronic properties. Porous silicon has been shown to induce the formation of a physiologically stable hydroxyapatite on its surface using in-vitro simulated body fluid experiments. Other studies today are also exploring the use of porous silicon as a promising potential bioactive therapeutic agent and drug delivery vehicle. Further research exploring the potential of using a silicon-substituted hydroxyapatite coating in in-vitro experiments have showed improved bioactivity and chemical stability under physiological conditions compared to normal hydroxyapatite. In 2010, Chadwick, Clarkin and Tanner showed that metallurgical grade porous silicon powder induced bone-like apatite formation on its surface in simulated body fluid inferring a bioactive nature and likely close bony apposition in-vivo. This chapter explores the use of porous silicon as a biomaterial and hydroxyapatite and porous silicon as a potential biomaterial for bone tissue engineering and other bioactive applications. It examines current research and future directions of such biomaterials.