Biomedical Engineering Reference
In-Depth Information
phosphates, glass and glass-ceramics, and bone mineral.
Minerals in bone are numerous. In the past, bone has been
defatted, ground, and calcined or heated to yield a rela-
tively pure mix of the naturally occurring bone minerals. It
was recognized early that this mixture of natural bone
mineral was poorly defined and extremely variable.
Consequently, its use as an implant material was limited.
The calcium phosphate ceramic system has been the
most intensely studied ceramic system. Of particular in-
terest are the calcium phosphates having calcium-to-
phosphorus ratios of 1.5-1.67. TCP and HA form the
boundaries of this compositional range. At present,
these two materials are used clinically for dental and or-
thopedic applications. TCP has a nominal composition of
Ca 3 (PO 4 ) 2 . The common mineral name for this material is
whitlockite. It exists in two crystographic forms, a - and b-
whitlockite. In general, it has been used in the b-form.
The ceramic HA has received a great deal of attention.
HA is, of course, the major mineral component of
bone.
Zimmerman et al. (1991) and Lin (1986) introduced
an absorbable polymer composite reinforced with an
absorbable calcium phosphate glass fiber. This allowed
for the fabrication of a completely absorbable composite
implant material. Commercial glass fiber produced from
a lime-aluminum-borosilicate glass typically has a tensile
strength of about 3 GPa and a modulus of elasticity of
72 GPa. Lin (1986) estimates the absorbable glass fiber
to have a modulus of 48 GPa, comparing favorably with
the commercial fiber. The tensile strength, however, was
significantly lower, approximately 500 MPa.
Matrix systems
Ceramic matrix or metal matrix composites have im-
portant technological applications, but their use is re-
stricted to specific cases (e.g., cutting tools, power
generation equipment, process industries, aerospace),
with just a few examples for biomedical applications
(e.g., calcium phosphate bone cements).
Most biomedical composites have polymeric matrices,
mostly thermoplastic, bioabsorbable or not.
The most common matrices are synthetic nonab-
sorbable polymers. By far the largest literature exists for
the use of polysulfone, PEEK, UHMWPE, PTFE,
PMMA, and hydrogels. These matrices, reinforced with
carbon fibers, PE fibers, and ceramics, have been used as
prosthetic hip stems, fracture fixation devices, artificial
joint bearing surfaces, artificial tooth roots, and bone
cements. Also, epoxy composite materials have been
used. However, because of concerns about the toxicity of
monomers ( Morrison et al. , 1995 ) the research activity
on epoxy composite for implantable devices gradually
decreased.
Materials used and some examples of proposed ap-
plications are reported in Table 3.2.12-1 .
Not all the proposed systems underwent clinical trial
and only some of them are today regularly commercialized.
A review on biomedical applications of composites is
in Ramakrishna et al. (2001) .
Absorbable composite implants can be produced from
absorbable a-polyester materials such as polylactic and
polyglycolic polymers. Previous work has demonstrated
that for most applications, it is necessary to reinforce
these polymers to obtain adequate mechanical strength.
PGA was the first biodegradable polymer synthesized
( Frazza and Schmitt, 1971 ). It was followed by PLA and
copolymers of the two ( Gilding and Reed, 1979 ). These
a-polyesters have been investigated for use as sutures and
as implant materials for the repair of a variety of osseous
and soft tissues. Important biodegradable polymers in-
clude poly(ortho esters), synthesized by Heller and co-
workers ( Heller et al. , 1980 ), and a class of bioerodable
dimethyl-trimethylene
The
nominal
composition
of
this
material
is
Ca 10 (PO 4 ) 6 (OH) 2 .
TCPs and HA are commonly referred as bioceramics,
i.e., bioactive ceramics. The definition refers to their
ability to elicit a specific biological response that results
in the formation of bond between the tissues and mate-
rial (Hench et al., 1971). HA ceramic and TCPs are used
in orthopedics and dentistry alone or in combination with
other substances, or also as coating of metal implants.
The rationale behind the use of bioceramics in combi-
nation with polymeric matrix for composites is in their
ability to enhance the integration in bone, while im-
proving the device mechanical properties. An example
are the HA-PE composites developed by Bonfield
( Bonfield, 1988; Bonfield et al. , 1998 ), and today com-
mercialized with the name of HAPEX (Smith & Nephew
ENT, Memphis, TN).
Glasses
Glass fibers are used to reinforce plastic matrices to form
structural composites and molding compounds. Com-
mercial glass fiber plastic composite materials have the
following favorable characteristics: high strength-to-
weight ratio; good dimensional stability; good resistance
to heat, cold, moisture, and corrosion; good electrical
insulation properties; ease of fabrication; and relatively
low cost. De Santis et al. (2000) have stacked glass and
carbon/PEI laminae to manufacture a hip prosthesis with
constant tensile modulus but with bending modulus in-
creasing in the tip-head direction. An isoelastic intra-
medullary nail made of PEEK and chopped glass fibers has
been evaluated by Lin et al. (1997), and glass fibers
have been used to increase the mechanical properties of
acrylic resins for applications in dentistry ( Chen et al. ,
2001 ).
carbonates
(DMTMCs)
( Tang
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