Biomedical Engineering Reference
In-Depth Information
infrared- [24] or UV-light [25], application of electric fi eld [26, 27] or alternating
magnetic fi elds [28-31], or lowering of
T
switch
below ambient temperature by plas-
ticizers such as water [32]. The SME results from a combination of a suitable
molecular architecture and a programming procedure. Therefore, intrinsic mate-
rial properties such as thermal or mechanical properties can be adjusted to the
needs of specifi c applications by small variations of molecular parameters, such
as monomer ratio or main chain bonds. This approach of adjusting material
properties enables the design of polymer systems. Furthermore, this approach
enables the creation of multifunctional materials, which is an actual trend in
polymer science. Multifunctionality is the targeted combination of material func-
tions, which are not linked with each other [33]. Multifunctional SMP can be
realized as multimaterial systems, for example, by the incorporation of particles
in polymer matrices, in which each material contributes a certain function, or as
one component systems by the integration of suitable functional groups or build-
ing blocks [34]. Promising approaches can be the combination of biofunctionality,
hydrolytic degradability, and shape-memory functionality. Such multifunctional
SMPs have a high potential for applications in the biomedical fi eld such as MIS
(see Section 8.4) [35]. In contrast to metal implants or nondegradable polymers,
bioresorbable SMPs are advantageous as they do not require an additional surgery
for implant removal. In addition, bulky implants created from bioresorbable
SMPs and having a
T
switch
between room temperature and body temperature could
be inserted to the application site through a small incision in a compressed or
elongated temporary shape. As soon as the implant is placed in the body, it
assumes body temperature and changes into its bulky application- relevant shape.
Other promising biomedical applications include intelligent degradable suture
materials, which tighten a wound with a predefi ned stress, stimuli-sensitive matri-
ces for drug delivery applications, or active scaffolds for regenerative therapies.
The required bioresorbable SMPs can be realized by the introduction of hydro-
lyzable bonds as weak links in the polymer chain enabling the degradation of these
polymers in the presence of water, which may be supported by enzymes. Figure
8.1 shows hydrolysable bonds used in degradable polymers, in order of their
stability.
Biodegradable, synthetic polymers may have advantages compared to polymers
from natural sources. They can be tailored to meet the specifi c requirements of
certain applications, such as thermal and mechanical properties. In addition, the
processability of synthetic polymers, for example, by extrusion or injection molding
Figure 8.1
Relative stability of chemical bonds against hydrolysis occuring in common,
synthetic polymers.
