Page 367 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

344 BIOMATERIALS

orientation and volume fraction. This allows high-fiber-volume fractions and three-dimensional ori-
entation not achievable in isotropic short-fiber composites.
Table 14.1 lists some fibers common in biomedical composites. There are many naturally occurring
fibers, such as cotton, flax, collagen, jute, wood, hemp, hair, wool, silk, etc., but these have extremely
varying properties and present many processing challenges. Among these, collagen fibers have been
successfully utilized in tissue engineering of skin and ligament. Borosilicate glass fiber is ubiquitous
in the composites industry but not common in biomedical composites, where, instead, adsorbable
bioglass fibers made from calcium phosphate have found some applications. Carbon fiber is as
strong as glass fiber but is several times stiffer owing to its fine structure of axially aligned graphite
crystallites and is also lighter than glass. It is used extensively to make high-strength lightweight
composites in prosthetic structural components, where the fatigue resistance of carbon-fiber
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composites is also an advantage. Carbon fibers tend to be brittle and are anisotropic, particularly in
their thermal properties. They also add electrical conductivity to a composite, which can have
corrosive effects next to metallic implants. Among polymers, highly oriented aramid fibers such as
Kevlar are used in orthopedic applications because of their high resistance to impact fracture.
However, Kevlar has very poor compressive properties, making it unsuitable for bending applications,
and it is difficult to process due to its strong cut-through resistance. Teflon and polyester (Dacron)
fibers are used to make vascular prostheses that are flexible. Polylactide and polyglycolide and their
copolymers are used to make fiber composites in which adsorbability is more important than
mechanical properties.

14.3.3 Particles
Particles can be added to a matrix to improve mechanical properties such as toughness and hardness.
Other properties, such as dimensional stability, electrical insulation, and thermal conductivity, can
also be controlled effectively by particles, especially when added to polymer matrices. Particulate
reinforcement is randomly distributed in a matrix, resulting in isotropic composites. Particles can
either strengthen or weaken a matrix, depending on its shape, stiffness, and bonding strength with
the matrix. Spherical particles are less effective than platelet- or flakelike particles in adding stiff-
ness. Hard particles in a low-modulus polymer increase stiffness, whereas compliant particles such
as silicone rubber, when added to a stiff polymer matrix, result in a softer composite. Fillers are
nonreinforcing particles such as carbon black and glass microspheres that are added more for
economic and not performance purposes.
Particulate reinforcement in biomedical composites is used widely for ceramic matrices in
dental and bone-analogue applications. The most common such particle form is hydroxyapatite, a
natural component of bone where it exists in a composite structure with collagen. Hydroxyapatite
particles have very poor mechanical properties and may serve more as a bioactive than reinforcement
component.

14.3.4 Interface

The transfer and distribution of stresses from the matrix to the fibers or particles occur through the
interface separating them. The area at the interface and the strength of the interfacial bond greatly
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affect the final composite properties and long-term property retention. A low interfacial area denotes
poor wetting of the fiber with the matrix material. Wetting can be enhanced by processing methods
in which there is greater pressure (metal matrices) or lower-viscosity flow (polymer matrices). When
mechanical coupling is not sufficient, coupling agents are often used to coat fibers to improve chemical
compatibility with the matrix.
Interfacial shear strength determines the fiber-matrix debonding process and thus the sequence
and relative magnitude of the different failure mechanisms in a composite. Strong interfaces com-
mon in polymer matrix composites make ductile matrices very stiff but also lower the fracture tough-
ness. Weak interfaces in ceramic matrix composites make brittle matrices tough by promoting matrix
crack but also lower strength and stiffness. 5