Page 49 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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

Biomaterials, Synthetic Synthesis, Fabrication, and Applications 189

(bone morphogenic protein), growth factors (bone, porous implant for the purpose of biological fixation in
epidermal tissue, cartilage, platelet, insulin). joint repair. UHMWPE is the polymer of choice for the
3. Therapeutic agents. Hormones, antibiotics, matrix material because of its abrasion-resistance, impact
chemotherapeutic drugs. resistance, corrosion resistance and zero moisture absorp-
4. Synthetic polymers. Polylactic acid (PLA), tion. The ashed bone provides the bioactive properties to
polyglycolic acid (PGA), polycaprolactone (PCA), prevent rejection.
polyamino acids, polyethylene (PE) and high
molecular weight derivatives, polysulfone,
polyhydroxybutyrate. V. THE WAY FORWARD,
5. Metals. Titanium-, cobalt-, and iron based alloys. TISSUE ENGINEERING

These materials can be developed in the form of partic- Although many materials, both synthetic and natural, are
ulates with a range of porosities, moldable forms, block used in medical devices and treatments it remains the goal
forms, scaffolds, fibers, and coatings. of scientists and clinicians to be able to replace diseased
or damaged tissues with living substitutes produced in
the laboratory. These substitutes would be available in
2. Bone Graft Materials
limitless quantities and be able to avoid rejection due to
Bone graft substitutes are available based on alumina the body’s own immune system. Tissue engineering is a
chemistry, silica, synthetic and natural calcium salts way forward. Tissue engineering encompasses the study
(phosphate, carbonate, sulfate, and hydroxide) and these of cellular responses to materials implants, manipulation
materials combined with natural polymers such as col- of the healing environment to control the structure of re-
lagen, and synthetic polymers such as PMMA, PHEMA, generated tissue, the production of cells and tissues for
and UHMWPE. Both sintered and nonsintered materials transplantation into the body, and the development of a
based on calcium phosphate are available with the nonsin- quantitative understanding of biological energetics. Engi-
tered versions showing greater biocompatibility (simply neers, chemists, life scientists, and clinicians all have im-
due to better resorption characteristics!). Materials can be portant roles to play in the furtherance of the discipline.
produced with a high degree of porosity thus mimicking Current areas of interest are the design of biocompatible
natural bone and allowing cells to permeate the implanted casings for cell transplants, the development of polymer
material over time. These can be produced from natural composites for patching wounds, the generation of scaf-
corals where the biomineralized skeleton of calcium folds that guide and encourage cells to form tissues, the
carbonate is replaced by calcium phosphate. Examples in- building of bioreactors for the large-scale production of
clude Interpre 200 and 500 with the materials being nearly therapeutic cells, and the establishment of experimental
nonresorbable. The same coral based materials, can be and mathematical models to predict cell behavior.
used in their calcium carbonate form without modification
with resorption and replacement by fibro-osseus bone tis-
A. Prevention of Unwanted Tissue Interactions
sue. Calcium sulfate is routinely used as a casting material
for fractures and is used for dental repairs together with Experimentation in this area began as early as 1933
porous hydroxyapatite granules (Hapset). The calcium with the use of synthetic nitrocellulose membranes to en-
sulphate is resorbed and can be replaced with the osseous compass cells and prevent an immune response. Current
tissue growing around the HA granules and holding them applications of the technology extend to the use of
in place. Another biomaterial which makes use of calcium laboratory-grown skin in the treatment of burns and ul-
hydroxide together with PMMA coated with PHEMA is cers, the treatment of cancer patients via an increase in
the hard tissue replacement polymer HTR. The implant their marrow cells with culture external to the body and
material consists of PMMA beads which are sintered in the detoxification of liver cells from patients with liver
together to give a porous mass which is then coated with failure.
PHEMA and calcium hydroxide. The PHEMA coating Some materials that are being investigated as im-
absorbs a lot of water and a gel is formed at the surface munoprotective coatings are, alginate–polylysine coacer-
containing calcium ions. This material is very bio- vates, polyacrylates, polyphosphazenes, materials based
compatible. on hyaluronic acid, and cellulose as well as hydro-
Otheralternativesforimplantsarebasedonnaturalbone gel membranes directly synthesized on cells. The ad-
rather than the synthetic derivatives. For example, ashed vantage of synthetic membranes as immunoprotective
bone can be used in combination with Ultra-High Molec- coatings is that they can be tailored for mechanical
ular Weight Polyethylene, UHMWPE for coating on a strength, biocompatibility, permeation characteristics, and