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Smart Polymers and Their Applications International Journal of Engineering Technology, Management and Applied Sciences www.ijetmas.com September 2014, Volume 2 Issue 4, ISSN 2349-4476 Smart Polymers and Their Applications Rushi Ghizal Gazala Roohi Fatima Seema Srivastava Physics Department. Physics Department. Physics Department. Integral University, Kursi Road. Integral University, Kursi Road, Integral University, Kursi Road Lucknow-226066, U.P, India. Lucknow-226066, U.P, India. Lucknow-226066, U.P, India. ABSTRACT Smart polymers are materials that respond to small external stimuli. These are also referred as “stimuli responsive” materials or “intelligent” materials. The stimuli include salt, UV irradiation, temperature, pH, magnetic or electric field, ionic factors etc. Smart polymers are very promising applicants in drug delivery, tissue engineering, cell culture, gene carriers, textile engineering, oil recovery, radioactive wastage and protein purification. The study is focused on the entire features of smart polymers and their most recent and relevant applications. Keywords: Smart polymers, Stimuli responsive materials, drug delivery, tissue engineering. INTRODUCTION The term “smart polymers” encompasses a wide spectrum of different compounds with unique potential for various applications. The characteristic features that actually make these polymers “smart”, is their ability to respond to very slight changes in the surrounding environment. The uniqueness of these materials lies not only in the fast microscopic changes occurring in their structure but also these transitions being reversible, i.e, these systems are able to recover their initial state when the sign or stimuli ends [1]. Smart polymers are biocompatible, strong, resilient, flexible, easy to sharpen and color. They keep the drug’s stability and are easy to manufacture, good nutrient carriers to the cells, easily charged using adhesion ligands and is possible to inject them in vitro as liquid to create a gel with the body temperature [2]. The responses are manifested as changes in one or more of the following- shape, surface characteristic, solubility, formation of an intricate molecular assembly, a sol-gel transition and others. The environmental trigger behind these transitions can be either change in temperature [3-8], pH shift [3,9,10], increase in ionic strength, presence of certain metabolic chemicals, addition of an oppositely charged polymer and polycation-polyanion complex formation, changes in electric [11] and magnetic field [12], light [13-14] or radiation forces. Smart polymers are becoming increasingly more prevalent as scientist learn about the chemistry and triggers that induce conformational changes in polymer structures and devise ways to take advantage of and control them. New polymeric materials are being chemically formulated that sense specific environmental changes in biological systems. 1. CLASSIFICATION OF SMART POLYMERS Smart polymers can be classified according to their physical features or to the stimuli they’re responding. Regarding the physical shape, they can be classified as free linear chain solutions, reversible gels covalently cross linked and polymer chain grafted on a surface [15]. The signs or stimuli that trigger the structural changes on smart polymers can be classified in three groups, 1. Physical stimuli(temperature, ultrasounds, light, mechanical stress), 2. Chemical stimuli(pH and ionic strength) and, 3. Biological stimuli(enzymes and biomolecules). Table1, presents Smart Polymers according to the stimuli they’re responding. 104 Rushi Ghizal, Gazala Roohi International Journal of Engineering Technology, Management and Applied Sciences www.ijetmas.com September 2014, Volume 2 Issue 4, ISSN 2349-4476 Table1. Stimuli-Responsive Smart Polymeric Materials Type of Stimulus Responsive Polymer Material Reference(s) pH *dendrimers [16-19] *poly(L-lysine)ester [20] *poly(hydroxyproline) [21] *Lactose-PEG grafted poly(L-lysine) nanoparticle [22] *poly(L-lysine)-g-poly(histidine) [22] *poly(propyl acrylic acid) [23] *poly(ethacrylic acid) [23] *polysilamine [24] *Eudragit S-100 [25] *Eudragit L-100 [26] *Chitosan [27] *PMAA-PEG copolymer [28] Ions *alginate (Ca2+ ) [29] *chitosan (Mg2+ ) [30] Organic solvent Eudragit S-100 [31] Temperature PNIPAAm [32] Magnetic field PNIPAAm hydrogels containing ferromagnetic [33 - 34] material PNIPAAm-co-acrylamide. Ru2+→Ru3+ (redox reaction) PNIPAAm hydrogels containing Tris (2,2-bipyridyl) [35] ruthenium (II). Temperature (sol-gel transition) *poloxamers [36-38] * chitosan-glycerol phosphate-water [39] * prolastin [40] * hybrid hydrogels of polymer and protein domains [41-42] Electric potential polythiophen gel [43] IR radiation poly(N-vinyl carbazole) composite [44] UV radiation Polyacrylamide crosslinked with 4-(methacryloylamino) [45-46] azobenzene Polyacrylamide-triphenylmethane leuco derivatives. Ultrasound dodecyl isocyanate-modified PEG-grafted poly(HEMA). [47] 2. DISCUSSION ON SOME TYPES OF SMART POLYMERS 2.1. pH sensitive smart polymers The pH sensitive polymers are able to accept or release protons in response to pH changes. These polymers contain in their structure acidic groups (carboxylic or sulphonic) or basic groups (amino salts) [48]. In other words pH sensitive polymers are polyelectrolytes that have in their structure acid or basic groups that can accept or release protons in response to pH changes in the surrounding environment. In the human body we can see remarkable changes of pH that can be used to direct therapeutic agents to a specific body area, tissue or cell compartment (Table 2). These conditions make the pH sensitive polymers the ideal pharmaceutical systems to the specific delivery of therapeutic agents. 2.1.1. Polymers with functional acid groups Polyacids or polyanions are pH sensitive polymers that have great number of ionizable acid groups in their structure (like carboxylic acid or sulphonic acid). The carboxylic groups accept protons at low pH values and release protons at high pH values [50]. Thus when the pH increases the polymer swells due to the electrostatic repulsion of the negatively charged groups. The pH in which acids become ionized depends on the polymer’s pKa (depends on polymers composition and molecular weight). 105 Rushi Ghizal, Gazala Roohi International Journal of Engineering Technology, Management and Applied Sciences www.ijetmas.com September 2014, Volume 2 Issue 4, ISSN 2349-4476 Table2. pH values from several tissues and cells compartments [49]. Tissue/Cell compartment pH Blood 7.4-7.5 Stomach 1.0-3.0 Duodenum 4.8-8.2 Colon 7.0-7.5 Lysosome 4.5-5.0 Golgi complex 6.4 Tumor-Extracellulare medium 6.2-7.2 Examples of polyanions are poly(acrylic acid)(PAA) or poly(methacrycic acid) (PMAA). Thus in oral drug delivery system, the poly(acrylic acid) polymer retains the drug on the presence of acid pH (stomach), delivering it in alkaline pH (small intestine). The drug delivery occurs due to the ionization of pendant groups of carbolic acid, forcing the polymer to swell. 2.1.2. Polymers with functional basic groups Polybases or polycations are protonated at high pH values and positively ionized at neutral or low pH values, i.e they go through a phase transition at pH 5 due to deprotonation of the pyridine groups. Example are poly(4-vinylpyridine)(PVP), poly(2-vinylpyridine) (PVAm), poly(2-diethylaminoethyl methacrlate) (PDEAEMA), with amino groups in their structure which in acid environments gain proton and in basic environment releases the protons. 2.2. Thermo-responsive polymers These smart polymers are sensitive to temperature and change their microstructural features in response to change in temperature. These are the most studied, most used and most safe polymers in drug administration systems and biomaterials. Thermo-responsive polymers present in their structure a very sensitive balance between the hydrophobic and the hydrophilic groups and a small change in the temperature can create new adjustments [51]. This type of system exhibit a critical solution temperature at which the phase of polymer and solution is changed in accordance with their composition. Those systems exhibiting one phase above certain temperature and phase separation below it possess an upper critical solution temperature (USTC). On the other hand, polymer solutions that appear as monophasic below a specific temperature and biphasic above it, generally exhibit the so called lower critical solution temperature (LCST). These represent the type of polymers with most number of applications. If the polymeric solution has a phase below the critical temperature, it will become insoluble after heating, i.e, it has one lower critical solution temperature (LCST). Above the critical solution temperature (LCST), the interaction strengths (hydrogen linkages) between the water molecules and the polymer become unfavorable, it dehydrates and a predominance of the hydrophobic interaction occurs causing the polymer swelling [52]. The LCST can be defined as the critical temperature in which the polymeric solution shows a phase separation, going from one phase (isotropic state) to two phases (anisotropic phases). The polymers with a lower critical solution temperature (LCST) are mostly used in drug delivery systems. The therapeutic agents as drugs, cells or proteins can be mixed with the polymer when this is on its ligand state (temperature below the transition temperature) being able to be injected in the human body on the subcutaneous layer or in the damaged area and forming a gel deposit on the area where it was injected
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