Biomedical Engineering Reference
In-Depth Information
composite. If fibres are long and aligned, mechanical properties should be
improved along the longitudinal direction. Unaligned long fibres or short
fibres still offer a great deal of surface area with which to interact with the
matrix while retaining isotropic properties. Fibres can improve toughness
by bridging cracks or by fibre pull-out, wherein energy is absorbed by the
frictional force required to pull the fibre out of place when fractured. In either
case, the main goal of a fibre is to either stop crack propagation altogether or
to elongate the path the crack has to take to propagate across the material.
If we return, briefly, to the original ideal goal of mimicking bone to
achieve its properties, let us consider another scenario. Again, hA, as it is
very similar to the inorganic component of bone, seems a good choice of
matrix material. the main second phase of bone is collagen, in the form of
bundled fibrils running longitudinally up and down the bone. If we remember
that reinforcement size is an important factor, we know we need something
on the scale of nanometres in diameter to simulate the reinforcing effect of
the fibrils. Carbon nanotubes, with their nano-scale dimensions, high aspect
ratio and excellent strength seem like a good possibility. this composite idea
is discussed in more detail in the literature (White
et al.
, 2007).
A final consideration in forming composites is, of course, how they will
react in the body and how the body will interact with them. it is important
to consider the biological response to all constituent materials. in the case
of carbon nanotube-reinforced hA, there is still a good deal of research
that needs to be done to evaluate the biological impact of carbon nanotubes
before the idea would be feasible
in vivo
.
5.6 Summary
this chapter broadly categorises material properties into three areas: mechanical
properties, molecular and microstructural properties and physiological effects.
Mechanical properties include, among others, strength, stiffness, toughness
and fatigue. Molecular and microstructural properties contain density, porosity,
surface area, grain size, composition, atomic structure, and polymer molecular
weight and thermal transition temperatures. Properties such as wear, surface
characteristics and tendencies toward corrosion, dissolution and degradation
comprise the final category of physiological effects. Explanations of these
properties and general characterisation advice are provided. The final section
of the chapter seeks to compare material classes with respect to each of these
property areas and suggests composites as a way to 'compromise' on properties
and combine the best attributes of at least two separate materials.
One should take away two main points from this chapter. First, that material
properties are controllable. At the same time, however, one must consider
the second point, that material properties are all interrelated. Alterations in
one property change another and, ultimately, all properties come together
