Concept of biodegradable shape memory polymers

Structural concepts for tissue-compatible and biodegradable polymers, thermoplastic elastomers, and thermosets with shape memory capabilities will be introduced. Their thermal and mechanical properties and degradation behaviour will be explained. An important precondition for the shape memory effect of polymers is elasticity. An elastic polymeric material consists of flexible segments, so-called network chains, which are connected via netpoints or junctions. The permanent shape of such a polymer is determined by the netpoints. The network chains take a coil-like conformation in unloaded condition. If the polymer is stretched, the network chains become extended 

2 General schematic representation of the shape memory effect. 

An elastic polymer gains a shape memory functionality if the deformation of the material can be stabilized in the temperature range which is assigned for the specific application. This can be realized by using the network chains as a kind of molecular switch. For this purpose it should be possible to vary the segment flexibility as a function of temperature. Ideally, this process should be reversible. One way to obtain a switch functionality is given by the introduction of a thermal transition of the network chains at a temperature Above the segments are flexible, while below the transition 

For biomedical applications, a thermal transition of the segment chains in the range between room and body temperature is of great interest. Suitable segments for degradable shape memory polymers can be found by regarding 

Appropriate macrodiols are produced via ring opening polymerization of cyclic diesters or lactones initialized by low molecular weight diols (see Fig. 16.4). The sequence structure of cooligomers can be influenced by application of a transesterification catalyst. The molecular weight of the oligomers can be controlled by the monomer/initiator ratio. Typically, the molecular masses Mn being obtained are between 500 g mol and 10 000 g mol T The net 

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