Biomedical Engineering Reference
In-Depth Information
Strengthening polymers
While polymers do not have a grain structure in the same way that metals
and ceramics do, they can be strengthened in a number of different ways. As
previously mentioned, increasing the crystallinity and molecular weight of
a polymer will strengthen it. in the same way that grain boundaries impede
dislocation motion, anything in a polymer's structure that will impede the
slippage of segments of molecular chains will strengthen it. Crosslinking,
mentioned briefly before, involves forming strong bonds between molecules
previously only linked by weak Van der Waals forces. these strong bonds
then stop chains from moving. Drawing, as mentioned before, strengthens
semi-crystalline polymers in the drawing direction by aligning the molecular
chains in the structure through uniaxial force.
5.4 Physiological effects
the way a material behaves in a standard room temperature atmosphere may
vary substantially from its performance
in vivo
, where it must withstand a
corrosive saline environment at 37°C, while negotiating interactions with
other surrounding body fluids, tissues, biomolecules and cells. Some of
the microstructural properties associated with
in vivo
behaviour have been
described in the previous section, such as the effect of porosity on surface
area and the consequential number of reaction sites with which body fluids
may interact and the various structural properties of polymers. this section
will discuss the properties that influence
in vivo
reactions such as corrosion,
dissolution, degradation, inflammation and cell interaction with surfaces.
5.4.1 Metallic corrosion
the tendency to corrode is the primary problem facing metal implant
use in the body. Corrosion degrades structural integrity and can create
by-products that adversely affect biological functions. the ions, organic
substances and dissolved oxygen contained in the body's environment make
it electrochemically reactive to metals, leading to corrosion. Metals vary in
their tendency to corrode, depending on electrode potential. this potential
can be measured by AC, DC or impedance electrochemistry, as described by
Lemons
et al.
(1999). Some metals, such as Au and Pt, are inert, whereas
metals like Cr, Co, Al, Zn and ti are quite reactive and easily corrode. Why,
then, are the three main alloys in total joint replacement ti alloys, Co alloys
and stainless steel?
While aqueous corrosion in the body can be a hindrance, a similar
electrochemical reaction with the oxygen in air, called dry corrosion, can
be an advantage. Upon exposure to air, some metals immediately form an
