Biomedical Engineering Reference
In-Depth Information
order of solubility for some of the materials is tetracalcium phosphate
(Ca
4
P
2
O
9
)
>
amorphous calcium phosphate > alpha-tricalcium phosphate
(
α
-Ca
3
(PO
4
)
2
)
>
beta-tricalcium phosphate (
β
-Ca
3
(PO
4
)
2
)
>>
hydroxyapa-
tite (most stable form).
6.2.3 Polymers
Polymers are the most versatile class of biomaterials, being extensively applied in
medicine. The property of polymers depend on the monomers they are made of and
the how the monomers are organized. Compared with metals and ceramics, poly-
mers offer the advantage of cost-effective synthesis in desirable compositions with
a wide variety of chemical structures with appropriate physical, interfacial, and
biomimetic properties. Further, they are easy to work with. Traditionally, metals or
ceramics are chosen for hard-tissue applications, and polymers are selected for soft-
tissue applications. However, the very strength of a rigid metallic implant used in
bone fixation can lead to problems with stress shielding, whereas a bioabsorbable
polymer implant can increase ultimate bone strength by slowly transferring load to
the bone as it heals. Biopolymers (polymers used in biomedical application) are usu-
ally used for their flexibility and stability, but have also been used for low friction
articulating surfaces. Some of the common applications are listed in Table 6.1 and
Figure 6.4. For drug delivery, the specific properties of various degradable systems
can be tailored to achieve optimal release kinetics of the drug or active agent. The
basic design criteria for biopolymers call for compounds that are biocompatible,
have manufacturing compatibility, sterilizability, chemical resistance, rigidity, and
are capable of controlled stability or degradation in response to biological condi-
tions. Biopolymers can be classified according to the following criteria:
Natural, synthetic, or a combination of both (semisynthetic);
Degradable or nondegradable;
Structural or nonstructural.
Naturally derived polymers are abundant and usually biodegradable. For ex-
ample, polysaccharide chitin is the second most abundant natural polymer in the
world after cellulose. The principal disadvantage of natural polymers lies in the de-
velopment of reproducible production methods, because their structural complex-
ity often renders modification and purification difficult. Additionally, significant
batch-to-batch variations occur because of their preparation in living organisms.
Synthetic polymers are available in a wide variety of compositions with read-
ily adjusted properties. Most synthetic polymers are linear homopolymers and are
defined by monomer chemistry, stereochemistry, and a degree of polydispersity.
Copolymers (multimers) are comprised of more than one monomer, which can
be arranged in an alternating pattern, ordered in blocks, or randomly distributed.
Processing, copolymerization, and blending provide means of optimizing proper-
ty of a polymer to a required application. However, the primary difficulty is the
general lack of biocompatibility of the majority of synthetic materials, although
