Biologically enzyme-responsive polymers

Enzyme-responsive polymers stand apart from other stimuli-responsive polymers in their ability to respond to a biological molecule to regulate the function of natural materials. Ihis is both a strength and a limitation while enzyme-responsive polymers are uniquely suited to perform tasks in their niche area (i.e., in a biological surrounding), they cannot readily be used in other applications that do not preserve the activity of enzymes. 

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 predisposition of ERMs to be used as biomaterials comes with some limitations. As the material relies on enzymes as stimuli, the operating conditions must preserve the function of the enzyme. This typically restricts the use of enzyme-responsive polymers to applications that respond under these circumstances. Although several enzymes are known that operate outside these conditions (e.g., at elevated temperatures or in organic solvents), no stimuli-responsive materials have yet been developed that respond to these enzymes. Moreover, since the application of ERMs is focused on biological systems, the polymers used have to be biocompatible and be able to withstand the effects of a complex biological environment in which they have to maintain their function. 

Figure 5.20 Schematic representation of a responsive polymer brush combined with biological molecules. A change in pH, temperature, or salt concentration leads to a change between a protective state in which an enzyme and a receptor molecule are hidden away deep inside of a brush layer, and an active state in which they are exposed to the solution.

Macroscopic transitions can also be triggered by biology-to-material interactions in the so-called biointer active polymers. These materials incorporate receptors for biomolecules which, when stimulated, cause localized or bulk modifications in the material properties. Those polymers that respond to selective enzyme catalysis are known as enzyme responsive... 

Polymer hydrogels consist of a cross-finked network of hydrophilic polymers with a very large water content (up to 99%) (Wichterle and Lim, 1960).They are structurally similar to the ECM and have therefore frequently been used as ECM mimics (Fedorovich et al., 2007 Shoichet, 2010). Other biological applications include the use as injectable scaffolds, (temporary) cell culture supports and drug delivery matrices (Mano, 2008). For all these applications, the introduction of enzyme-responsive functionalities into the polymer hydrogel is attractive because it would either more closely mimic the ECM (which is itself enzyme responsive) or allow drug delivery in response to the presence of a specific enzyme. 

Characterisation of the enzyme-responsive material prepared through the methods outlined above is essential not only to test the enzyme responsiveness of the polymer but also to characterise the material s overall performance under the conditions in which it will be used in its ultimate application. This section will provide a brief overview of standard and specialised techniques that have been employed for this purpose and covers mechanical, chemical, physical and biological properties as well as enzyme responsiveness. While we will discuss the reason for choosing particular techniques and explain their advantages and Umitations in the context of the analysis of enzyme-responsive materials, explanations of the working principles of the techniques will not be provided and the reader is referred to other specialised textbooks on this topic instead. 

Over the past 5 years, a number of researchers have started to explore and mimic these approaches in the laboratory. Enzyme-assisted formation of supramolecular polymers has several unique features. These include selectivity, confinement and catalytic amplification, which allow for superior control as observed in biological systems. These systems are finding applications in areas where supramolecular function is directly dictated by molecular order, for example in designed biomaterials for 3D cell culture, templating, drug delivery, biosensing, and intracellular polymerisations to control cell fate. Overall, biocatalytic production of supramolecular polymers provides a powerful new paradigm in stimuli-responsive nanomaterials. 

Phase transition in gels in response to biochemical reactions [27,28]. Polymer gels were synthesized in which an enzyme (urease) or a biologically active protein (lectin) was immobilized. The volume phase transitions were observed in such gels when biochemical reactions took place. Such mechano-biochemical gels will be used in devices such as, sensors, selective absorbers, and biochemically controlled drug release. 

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