Page 376 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOMEDICAL COMPOSITES 353

to have short processing time and near-net shape. Particulate composites used to repair cavities have
a polymer resin matrix filled with stiff inorganic or organic inclusions. The resin monomer is typi-
cally a methacrylate or a urethane dimethacrylate ester derivative such as bis-GMA cured on site by
cross-linkers, ultraviolet light (UV), or light-emitting diodes. The stiff filler particles that increase
strength and impart wear resistance can be glass ceramics, calcium silicate, calcium fluoride, crys-
talline quartz, and silicon nitride whiskers. These are usually silane treated or fused with silica
particles to improve retention in the matrix, especially in stress-bearing restorations and to reduce
water absorption. The filler can be from 50 to 80 percent by volume of the composite and have vary-
ing sizes from 20 nm to 50 mm. In a microfilled dental resin, fused silica particles 20 to 40 nm in
size can be incorporated to modest volume fractions up to 40 percent to produce a composite that is
translucent and can be polished to a high gloss but not mechanically strong enough for posterior
teeth and difficult to handle because of low viscosity. Hybrid dental resins have particle sizes of dif-
ferent orders of magnitude from around 0.1 to around 10 mm, allowing for higher filler volume, up
to 80 percent and higher viscosity for easier handling, as well as lower water absorption compared
with microfilled resins. Commercial dental composite resins have polymerization shrinkages vary-
ing from 1.6 to 2.5 percent, 18 followed by water absorption of up to 1.5 percent, 19 causing dimen-
20
sional changes. They also have poor adhesion to dentin, making it important to use bonding agents
to prevent fitting and leakage problems.
All-ceramic dental composites are used for stress-bearing restorations of dental crowns and
bridges. Fracture toughness is a very important concern and is addressed by designing crack deflec-
tion and bridging mechanisms into the composite. A common type is the alumina-glass composite
known as In-Ceram, in which a skeleton of alumina particles is slip cast and sintered, followed by
melt infiltration of glass into the porous core. The composition in one example is 75 percent by
volume a-alumina particles of average 3 mm size and 25 percent glass. 21 Thermal expansion
mismatch between the alumina and glass was shown not to affect fracture toughness significantly.
Recent development of aqueous tape-casting of the alumina core has made it easier to conform the
22
composite to a tooth model. The composite can be many times harder than the enamel of the opposing
teeth and can thus wear them out. They are coated with either alkali aluminosilicate dental porce-
lains or calcium phosphate composites to reduce the surface hardness. 23
24
Another application of dental composites is orthodontic archwires. One example is a unidirec-
tional pultruded S2-glass-reinforced dimethacrylate thermoset resin. 25 Depending on the yarn of
glass fiber used, the fiber volume fraction varied from 32 to 74 percent. The strength and modulus
were comparable with those of titanium wires. Orthodontic brackets were also made from composites
26
with a polyethylene matrix reinforced with ceramic hydroxyapatite particles, resulting in isotropic
properties and good adhesion to enamel.

14.8.3 External Prosthetics and Orthotics
Traditional prosthetic and orthotic materials such as wood, aluminum, and leather have been largely
replaced by high-performance composites and thermoplastics. 27 The requirements of low weight,
durability, size reduction, safety, and energy conservation have made fiber-reinforced plastics very
attractive in this area. This is one application where traditional composites design and manufacturing,
particularly using thermosets, are common because the products are structural and external to the
body. For transtibial (TT) and transfemoral (TF) prostheses, composites have been used for the socket-
frame component that interfaces with the residual limb and transmits the load and for the shank
component that supports the load over the ground. TT and TF prostheses have a target weight limit
of 1 and 2 kg, respectively, which makes carbon-fiber-reinforced (CFR) composites ideal. Socket
frames in the ISNY system have been made from carbon fiber tape and acrylic resin produced by
laminate layup. The matrix used is a blend of rigid and flexible methyl methacrylate resins for
tailoring the stiffness of the socket. CFR epoxy tubing has been used to replace stainless steel in arti-
ficial arms. Satin-weave carbon-fiber cloth in epoxy prepreg has been used to make the shank of the
Endolite TF prosthesis. Hybrid composites for the shank with carbon and nylon fibers in polyester
resins had better impact resistance than just carbon- or nylon-only composites. Using thermoplastics,
CFR Nylon 6,6 made easily by injection molding has excellent vibration-damping characteristics