Biomedical Engineering Reference
In-Depth Information
in this chapter, the most used polymers in orthopaedic applications will
be described. especial emphasis will be given to their physical and chemical
properties.
8.2 Ultra high molecular weight polyethylene
(UHMWPE)
Ultra-high molecular weight polyethylene (UHMWPe) is a unique polymer
with outstanding physical and mechanical properties, such as its chemical
inertness, lubricity, impact resistance and abrasion resistance. it is the gold
standard in articulating bearing surfaces of joint replacements.
Polyethylene is a polymer formed from ethylene (c
2
H
4
), which is a gas
which has a molecular weight of 28. the generic chemical formula for
polyethylene is -(c
2
H
4
)
n
-, where
n
is the degree of polymerization.
1
A
schematic of the chemical structures of ethylene and polyethylene is shown
in Fig. 8.1.
there are several kinds of commercial polyethylene (low-density
polyethylene: LdPe; linear low-density polyethylene: LLdPe; high-density
polyethylene: HdPe; ultra high molecular weight polyethylene: UHMWPe)
which are synthesized with different molecular weights and chain architectures.
LdPe and LLdPe refer to low-density polyethylene and linear low-density
polyethylene, respectively. the molecular weight of these polyethylenes is
typically lower than 50 000 g mol
-1
. the only difference between them is the
architecture, LdPe has a branch architecture whereas LLdPe has a linear
architecture. HdPe is a linear polymer with a molecular weight of up to
200 000 g mol
-1
. UHMWPe, in comparison, has a molecular weight of up
to 6 million g mol
-1
with as many as 200 000 ethylene units. the physical
and mechanical properties of UHMWPe and HdPe are summarized in table
8.1.
the better packing of linear chains with the resulting increased crystallinity
in the UHMWPe form provides the improved mechanical properties required
for orthopaedic use even though there is a decrease in both ductility and
fracture toughness.
H
H
H
H
C
C
C
C
n
H
H
H
Polyethylene
H
Ethylene
8.1
Chemical structure of ethylene and polyethylene.
