Biodegradable Shape - Memory Polymers - Biodegradable Polymers - page 201

mechanical deformation, which has been applied during the programming
process. R r quantifi es the ability of the polymer to memorize its permanent shape
and it is a measure of how far the applied strain during the programming

ε 1) is recovered during the SME. For that the strain that was applied

during the programming in the N th cycle,

m − ε p ( N −

ε m − ε p ( N − 1) is compared to the

change in strain during the SME p ( N ) (Eq. (8.2), Figure 8.5). The remaining

strain of the samples after two successively passed cycles in the stress-free state is
given by

ε m − ε

p ( N ). In the stress-controlled protocol, R f is represented by
the ratio of the tensile strain after unloading

ε p ( N − 1) and ε

ε u and the strain at σ m after cooling

of the N th cycle l ( N ) (Eq. (8.3), Figure 8.5). In such a protocol, R r quantifi es the

ability of the polymer to reverse the deformation that was applied in the program-
ming procedure

ε

1) during the following shape-memory transition. For
this purpose, the strain that was applied during the programming step in the N th
cycle

ε l − ε p ( N −

ε l ( N ) − ε p ( N − 1) is compared to the change of strain that occurs with the

SME ε l ( N ) − ε p ( N ) (Eq. (8.4), Figure 8.5).

8.3
Classes of Degradable SMP s

A strategy to functionalize SMP, so that they become biodegradable is the intro-
duction of hydrolytically cleavable bonds into such polymers (see Figure 8.1) [36].
In the design of these polymers, it has to be considered that the degradation
products should be either fully metabolized or excretable as fragments. This is of
exceptional importance when these SMP are intended for biomedical applications.
Furthermore, such degradable polymeric (bio)materials enable the application as
matrix materials for controlled drug release systems that requires the exact char-
acterization of the polymer's erosion behavior and the drug diffusion characteris-
tics. Degradable SMPs show two types of degradation mechanisms: surface- and
bulk erosion [37]. The degradation type depends on the diffusion of water into the
polymer and the reactivity of the polymer functional groups (see Figure 8.1).
Amorphous and crystalline segments, especially switching segments, display dif-
ferent degradation behavior. Amorphous segments degrade much faster due to
the easier water penetration in these areas. In contrast, the penetration of water
in crystalline segments is more inhibited by the dense packing of the crystalline
lamellae.
In this section, an overview about degradable materials that exhibit an SME is
given. SMPs can be divided into four types (see Table 8.1).
The requirements for an implant material are determined by the specifi c appli-
cation. The key properties of degradable biomaterials are their mechanical proper-
ties, their degradation rate and degradation behavior, as well as biocompatibility
and biofunctionality. Each application requires a specifi c combination of these
properties/functions.
In the following sections, four different types of degradable SMPs are described.