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Encyclopedia of Physical Science and Technology EN002G-62 May 19, 2001 19:27

188 Biomaterials, Synthetic Synthesis, Fabrication, and Applications

higher levels of bioactivity for correspondingly lower TABLE VI Composites Used in Medical Devices
levels of calcium oxide within the glass due to porosity
Composite Medical device applications
generated during sample formation by this route which
provides additional sites for mineral nucleation and sites Glass, glass–ceramic or Bone cement
for bone cells. HAP with PMMA
Tricalcium phosphate/HAP Bone Substitutes
with PE
3. Toward Resorbable Implants Glass–ceramic, quartz Dental restorations
with BIS/GMA
Biomaterials which are used to repair the body need to
Drugs with various Drug delivery
last as long as the patient does. At present this is not the polymers/ceramics
case and some people may face several hip replacement Carbon or glass fiber with All of the above applications but with
operations, for example, each time there being less bone PMMA and other matrices increased strength and/or stiffness
material (or less healthy bone material) for incorporation
of devices. The current life expectancy of such replace-
ments is on the order of 10 years at present. This needs therapy. The chemical composition of the fibers can be
to be doubled on tripled in the future. None of the ma- important in establishing continuity between the metallic
terials described above is able to address the problem of component and the coating with titanium being an espe-
tissue alteration with age and disease. The skeletal system cially good candidate to achieve such an effect.
has the capacity to repair itself, this ability diminishing
with age and disease state of the material. The ideal solu-
1. Composites Based on HAP
tion to the problem is to use biomaterials to augment the
body’s own reparative process. Certain of the resorbable There are many applications for calcium phosphate bio-
implants such as tricalcium phosphate and some bioac- ceramics can be seen in Fig. 7. The form of calcium phos-
tive glasses are based on this concept. Problems which phate used in orthopedic clinical applications is usually
exist with the development of resorbable materials are (a) based on hydroxyapatite and β-tricalcium phosphate. The
the products of resorption must be compatible with cel- materials are widely used in composite formulations to-
lular metabolic processes and (b) the rate of resorption gether with:
must also be matched by the capacities of the body to pro-
cess and transport the products of this process. In addi- 1. Ceramics. Mixed calcium phosphates, calcium
tion, as the material is resorbed and new material formed, sulfates, zinc calcium phosphates, aluminium calcium
the properties of both phases will alter and compatibil- phosphates, metacalcium phosphates, sodium
ity must be maintained at all times. This is difficult to metacalcium phosphate, calcium carbonate,
achieve. magnesium calcium carbonate and magnesium
carbonate.
2. Biological derivatives. Bone derivatives (autografts,
E. Composites
allografts and xenografts), collagen, dura, fibrin,
Composite materials are used clinically in order to take amino acids, polyfunctional acids, inductive factors
advantage of the desirable properties of each of the con-
stituent materials while limiting the undesirable or dele- TABLE VII Bioceramic Composites
terious properties of the individual phases. Composites
Category Examples
cover a wide range of compositions and representative
materials are listed in Table VI. Most of what is dis- Inert Carbon fiber reinforced carbon
cussed in this section will relate to bioceramic compos- Carbon fiber polymeric matrix materials (polysulfone,
ites. Bioceramic composites are either bioinert, bioactive poly(aryl) ether ketone
or biodegradable. Examples of each of these classes of Carbon fiber reinforced bone cement
composite and their applications are given in Table VII. Bioactive A-W glass–ceramic ®
The ceramic phase can be the reinforcing material, the Stainless steel fiber reinforced Bioglass
matrix material or both. The incorporation of high strength Titanium fiber reinforced bioactive glass
fibers increases the mechanical strength of the composites Zirconia reinforced A-W glass–ceramic
while maintaining the bioactivity of the material. In the Calcium phosphate particle reinforced polyethylene
case of glass doped materials, the fracture toughness of Calcium phosphate fiber and particle reinforced
bone cement
the material increases dramatically and renders materi-
Resorbable Calcium phosphate fiber reinforced polylactic acid
als suitable for dental implantation and hip replacement