APPLICATIONS

Hydrogels and superabsorbent polymers have applications in biomedical fields, pharmaceutical fields, and as personal hygiene products because of their high water content and the consequent biocompatibility. Soft contact lenses made of hydrogels are popular because of their comfort, disposability, and relatively low cost. Tissue engineering, molecular imprinting, wound dressing materials, immune isolation, drug delivery are among many medical applications of hydrogels. Immuno-isolation is the process of protecting implanted material such as biopolymers, cells, or drug release carriers from an immune reaction. Microencapsulation of these implanted cells with hydrogel-based capsules is common to protect them from the immune system. Alginate, chitosan, agarose, and polyethylene glycol have been used for encapsulation. These hydrogels are non-interfering with cellular functions of the inside cells, encapsulation can be done under physiological conditions, and provide microenvironment for the trapped cell survival for longer time.^31-33

BIOCATALYST

Recently, smart biocatalyst where enzymes are conjugated to stimuli-response polymer have gained significant attention. Based on presence or absence of external stimuli the polymer attached to enzymes changes its conformation to protect the enzyme from external environment and regulate enzyme activity, thus acting as molecular switch.

Smart polymers can be used to design reversible soluble or insoluble biocatalyst. Reversible biocatalysts catalyze on enzyme reaction in their soluble state and thus can be used in reactions with insoluble or poorly soluble substrates. Reversible soluble biocatalysts are formed by the phase separation of smart polymers in aqueous solutions following a small chance in the external conditions, when the enzyme molecule is bound covalently to polymer. As the reaction is complete, the conditions are changed to cause the catalyst to precipitate so that it can be separated from the product and be reused. Stimuli that are used to reuse include pH, temperature, ionic strength, and addition of chemical species like calcium.

Thermo or temperature-responsive biocatalysts are the conjugated product of temperature response polymer and the enzyme. Thermoresponsive polymer shows reversible phase transition at a particular temperature.This temperature is known as LCST, below this temperature polymers exist in a solution state and start precipitating or becoming turbid above this temperature, also known as cloud point.

For example, trypsin immobilized on a pH-responsive copolymer of methylmethacrylate and methacrylic acid is used for repeated hydrolysis of casein. Similarly simplex cells are immobilized inside beads of the thennoresponsive polymer gel as a biocatalyst. A biocatalyst sensitive to magnetic field is produced by immobilizing invertase and y-Fe,0₃ in poly(N-isopropylacrylamide-co- acrylamide) gel. The heat generated by exposure of y-Fe,0, to a magnetic field causes the gel to collapse, which is followed by a sharp decrease in the rate of sucrose hydrolysis.

Industrial biocatalyst polymers such as poly-N-isopropyl aciyl amide (PNiAAm), polyvinylcaprolactum (PNVC1), poly(7V,7V-diethyl acrylamide) (PDEAM), poly(iV-ethylmethacrylamide) (PNEMAM), poly(methyl vinyl ether) (PMVE), and poly(2-ethoxyethyl vinyl ether (PEOVE) are typical examples of thermoresponsive polymers. Among them, PNiPAAm has been extensively used in the development of smart industrial biocatalysts. Structurally, the iV-isopropyl moiety in the polymer is responsible for its smart behavior in response to the temperature changes, which interacts with the water molecules at temperatures below the LCST, but dissociates above the LCST. The LCST of PNiPAAm is approximately 32.5°C.

SENSORS

Gel has ability to change volume in response to a specific stimulus and is utilized in many sensing designs that are currently under development. A variety of gels with various chemical structures have been proposed, which can collectively achieve response to a broad range of stimuli. Temperature sensors are in high demand for industrial applications and to identify sites of infection, inflammation, or other pathology in healthcare. Many polymer systems were therefore investigated for applications in gel-based temperature sensors. Most of these systems contain polymers that exhibit LCST, that is, shrink when heated, or UCST, that is, swell when heated.

4.4.2.1 USED Л5 GLUCOSE SENSORS

In designing glucose biosensors, polymers with an extended pi-electron system such as thiophene, pyrrole, and thiazole are widely being explored. This is because of their conductivity, good stability, biocompatibility and reproducibility.

One of the most popular applications of pH-sensitive polymers is the fabrication of insulin delivery systems for the treatment of diabetic patients. Delivering insulin is different from delivering other drugs, since insulin has to be delivered in an exact amount at the exact time of need. Many devices have been developed for this purpose and all of them have a glucose sensor built into the system. In a glucose-rich environment, such as the bloodstream after a meal, the oxidation of glucose to gluconic acid catalyzed by glucose oxidase (GluOx) can lower the pH to approximately 5.8. Glucose oxidase is mostly used in glucose sensing.

BIOMIMETIC ACTUATORS

There have been attempt to mimic the efficient conversion of chemical energy into mechanical energy in living organisms. A cross-linked gel of Poly(vinyl alcohol) chains entangled with the polyacrylic acid chains has good mechanical properties and shows rapid electric field association bending deformations: a gel rod of 1 mm diameter bends semi-circularly within 1 s on the application of electric field. Polymer gels capable of mechanical response to electric field have also been developed using the cooperative binding of the positively charged surfactant molecule to the polyanionic polymer poly (2 acrylamido-2-methyl-l-propane sulfonic acid). Copolymer gels consist of N-Isopropylacrylamide and acrylic acid would be useful for constructing biochemomechanical systems. A pH-induced change in the-COOH ionization of acrylic acid alters the repulsive forces, the attractive force is produced by hydrophobic interactions arising from the dehydration of N-isopropylacrylamide moieties. The biomimetic actuators could be used in future soft machines.

BIOSEPARATIONS

Conjugate systems have been used in physical affinity separation and immunoassays. In affinity precipitation of biomolecule, the bioconjugate is synthesized by coupling a ligand to a water-soluble smart polymer. The ligand polymer conjugate selectively binds the target protein from the crude extract and the protein-polymer complex is precipitated from the solution by the changes in the environment like pH, temperature, ionic strength, or addition of some reagents. Finally the desired protein is dissociated from the polymer and the later can be recovered from the reuse for another cycle. Various ligands like protease inhibitors, antibiotics, nucleotides, metal chelates, and carbohydrates have been used in affinity precipitation.

REDUCES RADIOACTIVE WASTE

Polymer-bound smart catalysts are useful in waste minimization, catalyst recovery, and catalyst reuse. Polymeric smart coatings have been developed that are capable of both detecting and removing hazardous nuclear contamination. Such applications of smart materials involve catalyst chemistry and sensor chemistry.

Recently, scientists in Germany and India are reporting the development of a new polymer that reduces the amount of radioactive waste produced during routine operation of nuclear reactor. In the study the researchers created an absorbent material that unlike unconventional ion exchange resins has the unique ability of disregarding iron bases ions.⁴²

BIOTECHNOLOGY AND MEDICINE

Smart polymers may be physically mixed with or chemically conjugated to biomolecules to yield a large family of polymer biomolecule to yield a large family of polymer-biomolecule system that can respond to biological as well as to physical and chemical stimuli. Biomolecules that can be polymer conjugated include proteins and oligopeptides, sugars, polysaccharides, single- and double- stranded oligonucleotides, DNA plasmids, simple lipids, phospholipids, and synthetic diug molecule. These polymer-biomolecule complexes are referred as affinity smart biomaterials or intelligent bioconjugates. Also such polymers have been used in developing smart surfaces and smart hydrogels that can respond to external stimuli. Such polymeric biomaterials have shown a range of different applications in the field of biotechnology and medicine. The researchers have used these polymers for biomedical applications to downstream processing and biocatalyst. The latest thrilling breakthrough was achieved by the group of Stayton and Hoffman, at the University ofWashington, USA. The researchers developed a clever way to use smart polymers that provide size selective switches to turn proteins on and off.

SMART POLYMER IN PROTEIN PURIFICATION

Smart polymers are used for the concentration of protein solutions and for the isolation as well as purification of biomolecules. Recombinant thermostable lactate dehydrogenase from the thermophile Bacillusstearo thermophilus was purified by affinity partitioning in an aqueous two-phase polymer system formed by dextran and a copolymer of N-vinyl caprolactam and 1-vinyl imidazole. The enzyme partitioned preferentially into the copolymer phase in presence of Cu ions. The enzyme lactate dehydrogenase from porcine muscle has better access to the ligands and binds to the column. With the decrease in temperature the polymer molecules undergo transition to a more expanded coil conformation. Finally, the bound enzyme is replaced by the expanded polymer chains. This system was used for lactate dehydrogenase purification.³⁶

PROTEIN FOLDING

In order to attain the native structure and function of proteins, the refolding process is a major challenge in currently ongoing biochemical research. Using smart polymer reduces the hydrophobicity of surfactant which facilitates or hinders the conformational transition of unfolded protein, depending upon the magnitude of unfolded protein. Refolding of bovine carbonic anhydrase was examined in presence of PPO-Ph-PEG at various temperatures. The refolding yield of carbonic anhydrase was strongly enhanced and aggregate formation of PPO-Ph-PEG at specific temperature of 50-55°C. Eudragit S-100, a pH-sensitive smart polymer is supposed to increase the rate of refolding and refolding percentage of denatured protein. This was found to assist refolding of a-chymotrypsin, which is known to bind to the polymerrather than non-specifically.^33-35

AUTONOMOUS FLOW CONTROL IN MICROFLUIDICS

The concept of “lab in a chip” has evolved out of efforts to miniaturize analytical instruments. By using photolithography on a chip, one can create microchannels and work with very small volumes. Smart materials show considerable promise in designing microactuators for autonomous flow control inside these microfluidic channels. Saitoh et al.⁴² have explored the use of glass capillaries coated with Poly-N-Isopropylacrylamide for creating an on/off valve for the liquid flow. Below LCST the PNIPAm coated capillary allowed the flow of water, above LCST the flow was blocked as the coating was now hydrophobic. Beebe et al., on the other hand, used a pH-sensitive methacrylate to control the flow inside the microchannels. The hydrogel-based micro fluid valve opened or closed depending upon the pH of the solution. The design has the potential of being self-regulating/ autonomous since the valve can be controlled by feedback of H^- produced or consumed in the reaction. Undoubtedly we will see many other innovative designs for such applications in coming years.