Stimuli-responsive “smart polymers

Materials that show a shape-memory effect can be deformed into a temporary shape and afterwards they can recover their original shape on exposnre to an external stimulus [65,66]. Shape-memory polymers (SMPs) are stimuli-responsive smart polymers that have dual shape, which responds to application of an external stimulus. SMP is conventionally processed to receive its permanent shape. Afterward, the polymer is deformed and the intended temporary shape is fixed [67,68]. This process is called programming. These polymers basically consist of two phases, fixed points or frozen... 

A novel approach to immobilization of enzymes via covalent attachment is the use of stimulus-responsive smart polymers, which undergo dramatic conformational changes in response to small alterations in the environment, such as temperature, pH, and ionic strength [401 03], The most prominent example is a thermo-responsive and biocompatible polymer (poly-iV-isopropyl-acrylamide), which exhibits a critical solution temperature around 32°C, below which it readily dissolves in water, while it precipitates at elevated temperatures due to the expulsion of water molecules from its polymeric matrix. Hence, the biolransformation is performed under conditions, where the enzyme is soluble. Raising the temperature leads to precipitation of the immobilized protein, which allows its recovery and reuse. In addition, runaway reactions are avoided because in case the reaction temperature exceeds the critical solution temperature, the catalyst precipitates and the reaction shuts down. 

In the last two decades, the development of poljoners which change their structures and properties in response to environmental stimuli such as pH, temperature, and light has attracted a great deal of attention (1-3). Such polymers have been called smart polymers, intelligent polymers, stimulus-sensitive pol3uners, or responsive polymers. They have been used in many applications, ranging from bioactive agent delivery to separation (4,5). Various delivery systems based on the smart polymers have been proposed because of their... 

Smart polymers Smart polymers include those polymers that act in response to very little change in the surrounding environment or to external stimulus, and thus they are also called environmentally sensitive or stimuli-responsive polymers or intelligent polymers. The unique character which makes these polymers intelligent and smart is their capability to respond even to minor changes in the nearby environment. These responses on one hand are fast and microscopic and on the other hand are reversible, which further enhance the imique character of these polymers. The response can be visualized or measured in terms of change in shape, solubility, sol-gel transition, surface characteristics and formation of complicated assembly of molecules, etc. [12,13]. This class of polymers will be discussed in detail later in the chapter. 

For the purposes of this section, smart polymers are defined as those which exhibit a nonlinear response such as a conformational change or a phase transition to an external stimulus. A very large number of responsive polymers have been reported in both open and patent literature, but the field continues to grow as new mechanisms of response and new types of polymer are emerging. 

By definition, ERMs are very well suited to bridge the gap between manmade materials and biological processes (see Table 6.1) (Ulijn, 2006 Zelzer et al., 2013). Responsiveness to enzymes provides not only a means of communication between the material and its biological environment, it also presents the possibility of designing materials that display reversible and dynamic responses to a stimulus. While reversibility is not an uncommon feature for smart polymers, a dynamic interaction wherein response of a material only persists in the presence of the stimulus or a cofactor is rare. Several reversible enzyme-responsive polymer systems have been prepared so far (Ku et al, 2011), but truly dynamic or fuelled polymer-based systems have yet to be developed. 

The conjugation of proteins to stimuli responsive polymers are used in the field of protein isolation and separation [29,30]. The ability of some polymers to undergo a reversible change in response to an external physical, chemical, or biochemical stimulus gave them the name smart polymers [31]. Different stimuli have been utilized, such as temperature, light, ionic strength, or electric field for physical factors, or pH and specific ions for chemical triggers or metabolites for biochemical reactions [32, 33]. 

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