MEDICAL DEVICES BASED ON BIOINSPIRED CERAMICS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 379

chemical reactions has been attempted, the so-called bioactive bonding. The
implant surface is often coated with bioactive ceramics, which is designed to
elicit a specifi c biological response at the interface of the material, resulting
in the formation of a bond between tissues and the material [Hench, 1993;
Yamamuro, 1990]. This concept includes a large number of bioactive materials
including hydroxyapatite [Darimont, 2002], glass ceramics [Andersson, 1990] and
glasses [West, 1990], which have been successfully applied as coatings in metal
prosthesis.
11.4.1 Hydroxyapatite and Substituted Apatites
Hydroxyapatite (HA), Ca 10 (PO 4 ) 6 (OH) 2 , is the calcium phosphate biomaterial
most frequently used due to its high biocompatibility and bond integration, being
the material most similar to the mineral component of the bones. HA fi nds broad
clinical applications in the repair of bone defects, the bone augmentation and in a
variety of other areas in the muscoskeletal and dental fi elds not involving high
loads. Its poor mechanical properties are the greatest impediment to its broader
use in load-bearing applications in the human body. However, as HA is so similar
to the receptor tissue, HA coatings have been widely applied in metallic implants
in load-bearing orthopaedic applications [Hallab, 2004], improving dramatically
the adhesion of these implants to the living bone tissue.
Despite HA being the main mineral component of the bone, its mineral phase
cannot be described only as hydroxyapatite but also as a multi-substituted calcium
phosphate, and, in fact, there has been an increasing interest in the production of
enhanced HA materials, such as the silicon substituted hydroxyapatite (Si-HA)
[Balas, 2003; Marques, 2001]. As a matter of fact, the role of Si during the miner-
alization that takes place at early stages of bone development is well known [Car-
lisle, 1970] and investigations on Si defi ciency in the diet showed that it stands
behind diseases like osteoporosis [Pérez-Granados, 2002].
The HA, being only osteoconductive, does not satisfy the criteria of an
ideal bioactive material because HA resorbs very slowly and the dissolution
products do not stimulate the genes in the osteogenic cells. Silicon-substituted
HA coatings or scaffolds release small concentrations of silicon and calcium
ions, which have been found to stimulate seven families of genes in osteoblasts,
increasing proliferation and bone extracellular matrix production [Jones,
libro 2005]. Genes are activated by small concentrations (less than 20 parts per
million) of hydrated silicon (Si(OH) 4 ). Therefore, small amounts of silica (SiO 2 )
can be substituted for calcia (CaO) in synthetic hydroxyapatite. There is also
enough experimental evidence of the improved bioactivity of the Si-HA when
compared to the ordinary HA. In vivo experiments have shown that bone
ingrowth in silica-substituted HA granules was signifi cantly greater than that in
phase-pure HA granules, the former being classifi ed as an osteoproductive or
regenerative material [Jones, 2005; Porter, 2003]. Therefore, it is clear that the
production of coatings from this new material can be of interest to the medical
industry.