Biomedical Engineering Reference
In-Depth Information
simulations to predict the water diffusion as a function of the swelling degree of
the PLA matrix. The diffusion coefficients are then passed to the macroscale model.
In conclusion, the proposed multiscale analysis is capable to predict the evolution
with time of several properties of water/PLA mixtures, according to the change of
relevant indicators such as the extent of degradation and erosion of the PLA matrix.
11.1 Introduction
Biodegradable materials offer tremendous potential for the development of im-
plantable devices and systems for treating disease. Currently, biodegradable poly-
mers are used in diverse applications ranging from absorbable sutures [24], ortho-
pedic implants [37], drug delivery devices [23], scaffolds for tissue engineered con-
structs [1], medicated and biodegradable stents [42]. When applications involve ei-
ther negligible or well-known design requirements, the design of these classes of
implants are greatly facilitated. However, in situation where the requirements are
more complex, either in implants featuring complex geometries or in implants un-
der conditions that influence the course of degradation and erosion, the design pro-
cess is usually inhibited by the lack of rational models of biodegradable material
behaviour [32, 42]. In order to advance from prototype status to a reliable human-
implant devices, device designers must therefore rely on a combination of intuition
and trial-and-error approaches that often fail due to two major reasons: (i) the lack
of models able to describe the evolution of the material as it degrades and erodes,
and (ii) the difficulty to collect reliable experimental data quantifying and character-
izing this behaviour. Theoretical models to predict polymer degradation and erosion
would seem to be important tools for a number of different applications. If drug
elution is to be part of the therapy, drug delivery profiles should be programmable
at the design stage. For load bearing implants, mechanical properties and structural
integrity of the implant as well as their evolution should be accounted for. Because
the implant is ultimately absorbed, structural breakdown and loss of function must
be predicted and carefully designed for.
Polymer degradation is the deleterious change in properties of the material due
to irreversible changes in its chemical structure. A biodegradable polymer is a poly-
mer in which the degradation is mediated at least partially by a biological system
[35]. More precisely, polymer degradation is the chain scission process that breaks
polymer chains down to oligomers and finally to monomers, ultimately resulting
in a decrease of molecular weight. Polymers degrade by several different mecha-
nisms, depending on their inherent chemical structure and on the environment con-
ditions to which they are subjected. The prevailing mechanism of biological degrada-
tion for synthetic biodegradable aliphatic polyesters (the most commonly employed
biodegradable polymers in the medical such as polyglycolic acid and polylactic acid
[16]) is scission of the hydrolytically unstable backbone chain by passive hydrolysis.
By tailoring the polymer backbone with hydrolysable functional groups, the poly-
mer chains become labile to an aqueous environment and their ester linkages are
cleaved by absorbed water. There are two key factors that influence: (i) co-polymer
