Biomedical Engineering Reference
In-Depth Information
such a device at the place of application. This has led to multifunctional materials
for biomedical applications, which combine biodegradability, controlled drug
release, and shape-memory capability. After implantation of a medical device, the
SME can affect a fi xation of the device at the site of implantation. Subsequently,
the controlled release of the loaded drug is used for treating infections, reducing
infl ammatory responses, or, potentially, supporting regeneration processes.
Finally, the degradation of the matrix can avoid a second surgery for removal of
the implant.
In multifunctional SMPs that involve controlled drug release, specifi c processes
in synthesis, processing, and programming have to be applied to enable each of
the functionalities. Key for a release of bioactive molecules is their incorporation
into the polymer matrix. Drug incorporation into the matrix can principally be
achieved by soaking of the synthesized matrix in a drug solution and a subsequent
drying step, or alternatively by mixing of defi ned amounts of drug with polymer
network precursors and subsequent crosslinking. The latter method often allows
higher drug loading, but is limited by the chemical stability of the drug under the
crosslinking conditions. Drug release from polymer matrices is most often ruled
either by diffusion or by degradation of the matrix. Diffusion-controlled release
from a degradable matrix can, for example, be achieved for small water-soluble
molecules from bulk-eroding materials [79-81]. Alternatively, the use of surface-
eroding materials such as polyanhydrides [82] or poly(orthoesters) [83] allows an
erosion-controlled drug release. So far, only a few systems representative for triple
functional materials combining biodegradability, SME, and controlled drug release
have been published. All of them belong to the bulk eroding polyesters with a
diffusion-controlled release. However, type and ratio of the monomers as well as
network architectures resulted in quite different capabilities of the networks.
First examples for triple functional SMPs were polymer networks from poly(
-
caprolactone -
co
-glycolide)dimethacrylate. In these semicrystalline networks,
hydrophobic and hydrophilic drugs could be incorporated either by swelling of the
fi nal networks in an organic solvent saturated with the respective drug or by
mixing defi ned amounts of the drug with the network precursors followed by
irradiation (
in situ
incorporation) [84]. A semicrystalline polymer network was
chosen so that the crystalline phases of the networks were used for the fi xation of
the temporary shape, while drug molecules should predominantly be incorporated
in the amorphous phases without having a too strong infl uence on the melting
point of the polymer crystallites or the shape fi xation and recovery [85]. Large
amounts of drug decreased the elongation at break of the networks, which reduced
their programmability. In materials with high crystallinity in the drug- free state,
no effect of drug loading on the thermomechanical properties and shape- memory
functionality was observed.
Drug -loaded networks, which were shown to have high shape- fi xity and shape-
recovery, were subjected to hydrolytic degradation compared with drug-free
samples. It was found that a diffusion-controlled release was realized before
erosion of the matrix would have led to changes in the rate of drug release. Fur-
thermore, independence of polymer functionalities could be demonstrated.
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