Biodegradable polymers are defined as polymers that can be degraded by the micro-organism within a suitable period. Biodegradable polymers are degraded by enzymatic hydrolysis and oxidation to non-toxic small molecules, which can be metabolized by or excreted from the body. Biodegradable polymers and their degraded products do not cause any serious effects on the environment. Biodegradable polymers are also called smart biodegradable polymers because of their ability to respond to very slight changes in the surrounding environment. Smart biodegradable polymers have immense potential in drug delivery systems since they are able to release, at the appropriate time and site of action, entrapped drugs in response to specific physiological triggers. Smart polymeric materials respond with a considerable change in their properties to small changes in their environment: environmental stimuli like temperature, pH, chemicals, and light. In this chapter, synthesis, characterization, and various applications of smart nanoparticles are summarized.
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Over the last few decades, smart polymeric Nanoparticles shave been used in various applications such as biomedical and biochemical sciences in many ways. Recent decades have witnessed the appearance of synthetic functional polymers that respond in some desired way to a change in temperature, pH, electric or magnetic field, or some other parameter. These polymers were originally called stimulus responsive’ but the name ‘smart’ polymers was coined based on their similarity to biopolymers. (Bridges et al, 2000)
Smart polymers undergo fast, reversible changes in microstructure from a hydrophilic to a hydrophobic state. These changes are triggered by small changes in the environment but are apparent at the macroscopic level as precipitate formation from a solution or order-of-magnitude changes in the size and water content of polymers. These macroscopic changes are also reversible, the system returning to its initial state when the trigger is removed. The driving force behind these transitions varies, with common stimuli including the neutralization of charged groups by either a pH shift or the addition of an oppositely charged polymer, changes in the efficiency of hydrogen bonding with an increase in temperature or ionic strength, and collapse of polymers and interpenetrating polymer networks. An appropriate balance of hydrophobicity and hydrophobicity in the molecular structure of the polymer is believed to be required for the phase transition to occur. Such highly nonlinear responses by smart polymer systems have mainly been observed in water and, occasionally, in organic solvents or polymer blends. (Jeong et al,1997). As the stimulus-responsive behavior occurs in aqueous solutions, these polymers are becoming increasingly attractive for biotechnology and medicine (Figure 1).
There has been considerable interest in developing and generating biodegradable Nanoparticles with unique and effective potential for applications. The present review aims to bring together the exciting design of these polymeric Nanoparticles and expanding their uses by focusing on polymers as key examples of this technology. Nanoparticles generally vary in size from 10 to 1000 nm. They are made of a macromolecular material which can be of synthetic or natural origin. Depending on process used for their preparation, two different types of Nanoparticles can be obtained, namely nanospheres and nanocapsules. Nanocapsules have a matrix-type structure in which a drug is dispersed, whereas nanocapsules exhibit a membrane-wall structure with an oily core containing the drug. Because these systems have very high surface areas, drugs may also be adsorbed on their surface. (Tanget al,1996).
Figure 1. Uses of smart polymers in biotechnology and medicine
A majority of these reviews have dealt with the NPs of poly (D,L-lactide), poly(lactic acid) PLA, poly(D,L-glycolide) PLG, poly(lactide-co-glycolide), PLGA, poly(N-isopropylacrylamide) NIPAm and poly-(cyanoacrylate) PCA. The present review details the latest developments on the above mentioned polymers as well as NPs based on Chitosan, gelatin, sodium alginate and other hydrophilic / biodegradable polymers (Godbey et al,1999). The PLA, PLG and PLGA polymers being tissue-compatible have been used earlier as CR formulations in parentral and implantation drug delivery applications. In addition, poly(e-caprolactone), PCL, which was first reported by Pitt et al. for the CR of steroids and narcotic antagonists as well as to deliver ophthalmic drugs and poly(alkylcyanoacrylate), PACA, arenow being developed as NPs. In addition, frequently used polymers like poly (methylidene malonate), gelatin, chitosan and sodium alginate will also be included in this review.
In addition to the above reported reversible physical cross-linking approach for preparing thermo-sensitive biodegradable polymers, an approach based on chemical cross-linking for preparing biodegradable and thermo-sensitive polymers appears to be highly desirable. These bio-conjugates have been prepared by random polymer conjugation to lysine amino groups on the protein surface, and also by site-specific conjugation of the polymer to specific amino acid sites, such as cystiene sulfydryl groups, that are genetically engineered into the known amino acid sequence of the protein. We have conjugated several different smart polymers to streptavidin, including temperature, pH, and light sensitive polymers. (Godbey & Mikos,2001)