Biologically responsive polymer systems

In this section, the properties, characteristics and uses for pH-responsive polymer systems are described. The advancement in material science has led to the design of a variety of sensitive materials, which are used for development of new polymeric systems that can respond to biological stimuli. There is a highly promising role of pH-responsive polymer systems for drug and gene dehvery in the future, as it is described in the following sections of this chapter. 

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. 

Polyanionic polymers can enter into biological functions by distribution throughout the host and they behave similar to proteins, glycoproteins and polynucleotides which modulate a number of biological responses related to the host defense mechanism. These are enhanced immune responses, and activation of the reticuloendothelial system (RES) macrophages. 

The latter process has been detailed in numerous pharmacokinetic models relating the observations on absorption, distribution, and elimination of orally or parenterally administered conventional pharmaceutical agents. The former process is unique to the topic of sustained drug release and is related to the physical and chemical characteristics of the drug and its polymer delivery vehicle and the biological response elicited by the Implanted system in situ. 

Elastomers of silicone are widely used as biomaterials. In general, silicone elastomers have excellent biocompatibility, inducing only a limited inflammatory response following implantation. In fact, until very recently, it was assumed that silicones were almost completely inert in biological systems. It is now known, however, that certain silicone polymers can provoke inflammatory and immune responses. The biological response to implanted silicone, and the variability of that response among individuals, is the subject of considerable debate and interest. 

The labeling of nanoparticles with fluorescent dyes allows one to use them as markers in biomedical appUcations. One possibility is to immobilize the fluorescent dyes physically or chemically on the particle s surface (e.g. FITC-dextran [29]). However, either desorption can occur, or the surface is changed that much that the biological response (cell uptake, toxicity) is significantly modified or even totally hindered. Therefore, an incorporation of hydrophobic dyes into the polymeric nanoparticles leads to marker systems where only the polymer and the highly variable surface functionaUty are the relevant factors for particle-cell interactions. 

The examples mentioned above relied on hydrophobic collapse of one of the blocks of the block copolymer upon addition of a nonsolvent. There are numerous cases of linear block copolymer systems where one of the blocks is tunable, with switchable hydrophilicity in response to changes in external stimuli such as pH, temperature, light,or a combination of conditions. These stimuli-responsive polymers are sometimes referred to as smart polymers and possess immense potential in biomedical applications because of their ability to assemble/disassemble in response to artifidal or biologically triggered events. Although not discussed expli-dtly in this chapter, some of the materials described exhibit stimuli-responsive characteristics those and other polymers are... 

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