Due to the outstanding properties of natural materials
chemists and material scientists often follow the principles that nature has developed over millions of years for
the design of new synthetic materials (“bioinspired materials”). In this context, one of the most inspiring features is
the ability to change a specific property upon an external
trigger [1]. Hereby nature developed different mechanisms
resulting in stimuli-responsive materials, which feature
responses within seconds to minutes up to years. Having
developed perfectly designed structures that are able to
exploit these mechanisms, natural materials (and thereby
the living organisms) are able to adopt to the conditions
of their surrounding environment. The variety of the properties that can be changed upon application of an external
stimulus is enormous, as is the nature of the applied stimulus as well. Variation of mechanical properties is one
feature, which for example can be found in sea cucumbers. These animals are able to change the stiffness of their
skin by several orders of magnitude in case of danger (i.e.
an attack by a predator) [2]. Several animals (e.g., octopus,
squid, chameleon) can adopt their color, i.e. their appearance, according to their need (i.e. for camouflage, to warn
predators or conspecifics as well as to impress the other
sex). In contrast to these fast processes, the growth of plants
is influenced by external influences in the timeframe of
years (e.g., wind-bend trees or plants growing toward the
light) [3]. In addition to these macroscopic changes, natural
materials show a stimuli-responsiveness on the level of single molecules. For instance, proteins adopt different shapes
depending on the conditions used (e.g., temperature, pHvalue, salt concentration, etc.). These few examples already
demonstrate impressively the unique properties of these
materials developed by evolution.
Simplifying nature’s inspiring examples, many stimuliresponsive polymer systems have been developed by
researchers over the last decades [4–8]. The variety of
different stimuli, which can be applied, cover a wide
range (see Fig. 1). In response, the polymeric material will
feature dimensional changes. One specific kind of stimuliresponsive materials are shape-memory materials. These
materials respond with a change of their shape toward a
specific stimulus (mainly temperature), i.e. they will transform from a temporary shape to a permanent shape. The
interesting feature of this behavior is that the original shape
is “memorized” by the material and is reformed after deformation without additional mechanical efforts. Despite
the large variety of different stimuli-responsive natural
materials, the examples for natural shape memory