ABSTRACT
Nature requires selectively designed molecular assemblies and interfaces to maintain biological function and sustain life, which provides a specific structure, particular function, and environment. Environmental responsive nanocarriers are nanostructures which are capable of bringing chemical and physical modifications for receiving internal or external stimuli. These changes are due to variations in physical and chemical polymer properties. The stimuli is obtained from changes in the environment of the materials, sucha as changes in pH, temperature, enzyme concentration, light or electric pulses, magnetic field, ultrasound intensity, etc. (Stuart et al. 2010). This environmental sensitivity is helpful in controlling the biodistribution of drugs and targeting diseased areas in the body. This is achieved through the development of environmental responsive nanocarriers which need the utilization of biocompatible materials that are capable of responding to particular stimuli, or which are able to undergo a hydrolytic cleavage, specific protonation, or change in the molecular conformational in response to a specific stimulus (Mura et al. 2013). Polysaccharides are one of the most popular polymeric materials for stimuli-responsive drug delivery systems (Cheng et al. 2013). The structure and property diversity of polysaccharides is due to their unique features such as their wide molecular weight range, reactive groups, and variability of chemical composition. Biochemically and chemically, polysaccharides can be modified easily due to the variable derivable groups present on their molecular chains, which lead to formation of various types of derivatives. Polysaccharides are safe, stable, non-toxic, and biodegradable (Concheiro 2008). Polysaccharides like alginate, chitosan, dextran, hyaluronic acid, and carboxymethyl cellulose are environmental responsive polymers. Various factors like pH, temperature, and ionic strength of the medium affect conformation of polysaccharides chains. By the action of physical or chemical stimuli, trigger phase transitions of crosslinked and isolated chains can be exploited for sensitive conformation (Alvarez-Lorenzo et al. 2013). Polysaccharide can be affected by stimuli like the application of heat to increase the temperature or electric fields which alter charge distribution.(Brulé et al. 2011). The polysaccharides become attractive components of smart drug delivery systems (DDSs) because of their response to the variety of stimuli. Because of these smart responsive DDSs, the drug can be specifically released in the affected cells or tissues (Alvarez-Lorenzo and Concheiro 2008). In addition, most of the natural polysaccharides help with bioadhesion via non-covalent bonds which can form with biological tissues, mainly mucous membranes and epithelia. These non-covalent bonds form due to the presence of hydrophilic groups like carboxyl, hydroxyl, and amino groups. Both chitosan and alginate are good bioadhesive materials. These can prolong the time of residence and thus increase the absorption of loaded drugs (Wang et al. 2011). The recognition and binding to desirable surfaces can be facilitated by the polysaccharides through mimicking the surface of the bacteria, viruses, and eukaryotic cells (Concheiro 2008). Thus, the novel environment-responsive, biocompatible, and even targetable DDSs can be obtained with the integration of the polysaccharides features (Hamidi et al. 2008). This review discusses the most significant progress in developing environment-responsive polysaccharide nanocarrier drug delivery systems in response to specific stimuli both endogenous and exogenous.
