1208 A.Kumar et al.Prog.Polym.ScL.32(2007)1205-1237 different applications [33].This review focuses on LCST.whereas the poly-SPP block exhibits an the various potential applications of sps within the opriate above three defined categories.The main aim of the review is to highlight the recent developments polymers which stay in solution in the ful within last decade of SPs for applications 100C.Both oseparation,protein ing.micronu these polymers in water at d engin pli chemical valves and tissue at hig atu 2.Polymers as linear free chains in solution th poly-SPP block. and at low poly-SPP block forms colloidal polar ags gates In aqueous solution,the delicate balance between that are kept in solution by the PNiPAAm block.In hydrophobic-hydrophilic conditions controls phase this way.colloidal aggregates which switch rever transition of the polymer.As hydrophobic condi tions increas solub S conver pH-respon as Eu vhich the dragit (co-po ylmetha rylate with wate h chitosa (de.y vlated chitin)as th these polymers become increasingly protonated and Aqueous solutions of thermoresponsive polymers hydrophobic,and eventually precipitate and this are characterized by an inverse dissolution beha. transition can be sharp.For example Eudragi S-100 precipitates from aqueous solution on acid 21.The ons are homogenous at fica ar und pH erea dap a10 precipit at vely hig LCST emperature of the in soluti p nd it is ted that sp honthe phertion atures at play a role in the new direction like rotein folding also called These application areas are discussed here. demixtion,will be denoted "Ta"or critical point (CP). Poly-N-isopropylacrylamide (PNiPAAm) 2.1.Bioseparation gained its popularity mainly ecause of the sharp LCST of about 32 the eds 4A3 proces 144-461 or addition of surfactants [44.47.48]to the mprove the purity of the roduct.Use of SPma polymer solution.When heated above 32C.the contribute to the simple and cost-effective processes polymer becomes hydrophobic and precipitates out to separate target molecules.The separation of from solution and below LCST it becomes com- target substance can be performed in different ways pletely soluble because of hydrophilic state and using these polymers,like aqueous two-phas forms a clear solution. Water-soluble block co- polymer system (ATPS),affinity precipitation or mer ofr prepare polym nic atograpny. he thermor sponsive be ma pyD-N N-dimethyll at there. (SPP)by sequential free radical polymerization via the reversible addition-fragmentation chain transfer 2.1.1.Aqueous two-phase polymer system (RAFT)process.Such block co-polymers with two ATPS is an aqueous,liquid-liquid,biphasic hydrophilic blocks exhibit double thermoresponsive system which is obtained by mixing of aqueous behavior in water:the PNiPAAm block shows a solution of two polymers,or a polymer and a salt at different applications [33]. This review focuses on the various potential applications of SPs within the above three defined categories. The main aim of the review is to highlight the recent developments within the last decade of SPs for applications in areas like bioseparation, protein folding, microflui￾dics and actuators, chemical valves and tissue engineering applications. 2. Polymers as linear free chains in solution In aqueous solution, the delicate balance between hydrophobic–hydrophilic conditions controls phase transition of the polymer. As hydrophobic condi￾tions increase the polymer precipitates forming an altogether different phase. This conversion from soluble to insoluble form can be achieved by either reducing the number of hydrogen bonds which the polymer forms with water or by neutralizing the electric charges present on the polymeric network. Aqueous solutions of thermoresponsive polymers are characterized by an inverse dissolution beha￾vior, their isobaric phase diagrams presenting a LCST [39–42]. The solutions are homogenous at low temperature and a phase separation appears when the temperature exceeds a definite value. The LCST is the minimum of the phase diagram of the system, and in the practical cases to be treated in the following, the phase separation temperatures at which the phase transition occurs, also called demixtion, will be denoted ‘‘Td’’ or critical point (CP). Poly-N-isopropylacrylamide (PNiPAAm) gained its popularity mainly because of the sharp￾ness of its phase transition, LCST of about 32 1C which is close to the physiological temperature, and the easiness to vary its phase separation temperature by co-polymerization [42,43], addition of salts [44–46], or addition of surfactants [44,47,48] to the polymer solution. When heated above 32 1C, the polymer becomes hydrophobic and precipitates out from solution and below LCST it becomes com￾pletely soluble because of hydrophilic state and forms a clear solution. Water-soluble block co￾polymers were prepared from the non-ionic mono￾mer of N-isopropylacrylamide (NiPAAm) and the zwitterionic monomer 3-[N-(3-methacrylamidopro￾pyl)-N,N-dimethyl] ammonio-propane sulfonate (SPP) by sequential free radical polymerization via the reversible addition–fragmentation chain transfer (RAFT) process. Such block co-polymers with two hydrophilic blocks exhibit double thermoresponsive behavior in water: the PNiPAAm block shows a LCST, whereas the poly-SPP block exhibits an upper critical solution temperature. Appropriate design of the block lengths leads to block co￾polymers which stay in solution in the full temperature range between 0 and 100 1C. Both blocks of these polymers dissolve in water at intermediate temperatures, whereas at high tem￾peratures, the PNiPAAm block forms colloidal hydrophobic associates that are kept in solution by the poly-SPP block, and at low temperatures, the poly-SPP block forms colloidal polar aggregates that are kept in solution by the PNiPAAm block. In this way, colloidal aggregates which switch rever￾sibly can be prepared in water [49]. Another type of soluble SPs which respond to microchanges in pH are the ‘‘pH-responsive polymers’’—such as Eu￾dragit S-100 (co-polymer of methylmethacrylate and methacrylic acid) and the natural polymer, chitosan (deacetylated chitin). As the pH is lowered, these polymers become increasingly protonated and hydrophobic, and eventually precipitate and this transition can be sharp. For example Eudragit S-100 precipitates from aqueous solution on acid￾ification to around pH 5.5 whereas chitosan precipitates at a relatively higher pH of about 7. Such class of SP in solution phase has various applications, such as bioseparation of proteins, cells and bioparticles and it is also investigated that SP play a role in the new direction like protein folding. These application areas are discussed here. 2.1. Bioseparation The production of macromolecules and separa￾tion of biomolecules in purified form, through the process of bioseparation needs special efforts to bring down the overall cost of production and improve the purity of the product. Use of SP may contribute to the simple and cost-effective processes to separate target molecules. The separation of target substance can be performed in different ways using these polymers, like aqueous two-phase polymer system (ATPS), affinity precipitation or thermoresponsive chromatography. The thermore￾sponsive chromatography comes under smart sur￾faces and membranes section and will be discussed there. 2.1.1. Aqueous two-phase polymer system ATPS is an aqueous, liquid–liquid, biphasic system which is obtained by mixing of aqueous solution of two polymers, or a polymer and a salt at ARTICLE IN PRESS 1208 A. Kumar et al. / Prog. Polym. Sci. 32 (2007) 1205–1237