AAWSAP DIRD, Biomaterials, January 2010

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one free from concerns of immunogenicity. These polymers can be optically clear,
exhibit good flexibility, and have strength comparable to that of many plastics.
BIODEGRADATION ADVANTAGES
In the human body, biodegradable polymers have good compatibility but also
decompose to harmless materials and over time dissolve altogether. Biodegradable
polymers undergo a chemical hydrolysis in the salty and wet environment of tissues by
way of a labile chemical backbone of the polymer. The degradation starts immediately
upon water exposure and occurs in two steps.
In the first step, the material thoroughly hydrates, and the water attacks the polymer
chains, converting long chains into shorter, water-soluble fragments. The desirable
aspect of this process is a reduction in molecular weight without a loss in physical
properties, since the device matrix is still held together, even with the shorter chains.
In the second step, the shorter polymer chains are attacked by enzymes that are
naturally present in tissues. Basically, a metabolization of the fragments by the body
tissues results in a rapid loss of polymer mass, what is referred to as bulk erosion. All
the commercially available synthetic devices and sutures degrade by bulk erosion.
DEGRADABLE BIOMATERIALS
Different biodegradable polymers have different lifetimes in tissues, ranging from a few
days to years. Combining two different biopolymers-for example, short-lived (days)
PLA (polylactide) and longer lived (months) PGA (polyglycolide)-reveals that polymers
can be produced with intermediate decomposition times. Thus, their decay times can be
custom determined through their formulation.
Biodegradable polymers fulfill a physician's desire to have an implanted device that will
not require a second surgical intervention for removal, which is desirable in many
applications. In orthopedic applications, for example, a fractured bone that has been
fixated with a rigid, nonbiodegradable stainless implant has a tendency for refracture
upon removal of the implant, making removal undesirable. This refracturing results
from the offloading of the stress on the bone by the stainless steel support because the
bone has not carried a sufficient load during the healing process. However, a fixation
system prepared from a biodegradable polymer can be engineered to degrade at a rate
that will slowly transfer the load to the healing bone, thereby avoiding the risk of
refracture and eliminating the need to remove the implant.
POLYLACTIC ACID AND POLYGLYCOLIC ACID
Polylactic acid (PLA), polyglycolic acid (PGA), and their copolymers are the most widely
used of the biodegradable polymers. These materials, when exposed to the sun and
weather, will degrade into water and carbon dioxide and essentially vanish, given
sufficient time.
In the human body, combinations of PLA and PGA are used to control the longevity of a
material by controlling its degradation rate when exposed to tissues. The degradation
products in the human body are also water and carbon dioxide.
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