Biomedical Engineering Reference
In-Depth Information
for use in numerous biomedical products and devices, such as degradable
sutures (dissolving stitches) and bone fixation plates. Dissolving stitches
are probably one of the most common uses of degradable materials as
medical devices.
PLA and PGA can be processed easily and their degradation rates,
physical and mechanical properties are adjustable over a wide range by
varying the molecular weight and copolymer used. Degradation is by
reaction with water [5]. Water is first taken up by the polymer, causing
swelling. Once water is taken up, it attacks the ester links in the polymer
chains (hydrolysis), causing chain scission, reducing the chain length.
PLA exists in three forms: poly(L-lactic acid) (PLLA), poly(D-lactic acid)
(PDLA) and poly(DL-lactic acid) (PDLLA), a mixture of D-PLA and
L-PLA. This is because the lactide monomer is chiral. PLLA can be highly
crystalline, which means that the polymer chains are highly organised
and tightly packed, preventing rapid water uptake. PDLLA contains a
random mixture of D-lactide and L-lactide units, making it impossible
to crystallise. PDLLA is therefore amorphous and the random orienta-
tion of the polymer chains allows space for water uptake. Therefore,
PDLLA degrades faster than PLLA (Figure 9.3). A PLLA suture may take
two years to degrade, whereas a PDLLA suture would dissolve in two
months. A wide range of physical properties and degradation times can
be achieved by varying the monomer ratios when using copolymers such
as lactide-glycolide copolymers or hydroxybutyrate-hydroxyhexanoate
copolymers. Playing with the chemistry and synthesis route can there-
fore give control over the degradation time. However, this does not
necessarily mean control over the degradation
rate
.
Although the polyesters can be manipulated to degrade after a certain
time, the degradation profile is not linear (Figure 9.3). After implan-
tation, degradation rate is initially slow, as water is adsorbed into the
polymer. The chemistry and structure of the polymer determines the rate
of water diffusion. Once there is high water content inside the polymer,
chain scission begins. Once the chain scission begins, the degradation
process is rapid, leading to sudden loss in mechanical properties. This is
because each time an ester bond is broken, a chain is cut, leaving car-
boxylic acid (
COOH) groups where the ester link used to be. Molecular
weight (chain length) is reduced - on average, it will be halved with each
scission. The chain scission reaction is accelerated when the pH goes
away from neutral, and therefore the presence of the acidic groups
accelerates degradation (autocatalysis). Once the chain length is below a
critical value, the chain will leave the device. Quite quickly there will be
many short chains leaving the device, which is called the 'whoosh effect'.
−
