Biodegradable thermogelling polymers for biomedical applications
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Introduction The past decade has seen substantial developments in research, especially on biomaterials. A biomaterials class of great interest is stimuli-responsive biodegradable hydrogels, since they exhibit satisfactory performance in vivo in both tissue and blood-contacting environments. These hydrogels can be easily fabricated into different morphologies by altering the physical properties, for example, by changing the polymer concentration in an aqueous solution to obtain a soft or hard gel, for given applications. Thermogelling hydrogels, also known as thermogels, are a class of stimuli-responsive hydrogels that have gained much interest because they undergo macroscopic sol-to-gel transitions in response to temperature.1–5 They liquefy when cooled below room temperature and form a gel when heated to room or body temperature. This thermogelation feature runs contrary to typical natural phenomena, where water freezes or jelly solutions solidify at low temperatures. The process is reversible when the themogelling polymers are composed of physically cross-linked polymeric networks. This unique property of thermogels finds use in a wide range of biomedical applications, including injectable hydrogels to deliver drugs,6–8 tissue engineering scaffolds9–15 for cardiac, nerve, and cartilage tissues for regenerative medicine, and antiadhesion fillers.16
The mechanism behind the property of thermogelation lies in the aggregation of a micellar network structure, which is an interconnected lattice formed by the aggregation of selfassembled micelles (spherically shaped lipid molecules) in an aqueous solution.17 Thermogelling copolymers consist of hydrophilic and hydrophobic segments that can self-assemble into micelles in an aqueous solution. As the temperature increases, hydrophobic association of the core drives the individual micelles to aggregate into close-packed structures, leading to macroscopic gelation. The low critical solution temperature (LCST), below which the compounds in the mixture are miscible at all compositions, is also a factor in the micelle-forming thermogelation mechanism. For example, poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) have LCST ranges of 100–150°C and 10–30°C in water, respectively. Above the LCST, the polymer segments become hydrophobic and immiscible. When a PEG-PPG amphiphilic copolymer forms with a balanced hydrophilic–hydrophobic segment ratio, thermogelling behavior can be observed at or above the LCST of PPG (i.e., at room temperature or body temperature); the PPG segments become hydrophobic, which induces the formation of micelles. Scarpa et al. conducted the first study on thermoresponsive polymers in 1967 and described the reversible phasechange behavior of poly(N-isopropylacrylamide) (PNIPAAm).18
Sing Shy Liow, Institute of Materials Research and Engineering, A*STAR, Singapore; [email protected] Anis Abdul Karim, Institute of Materials Research and Engineering, A*STAR, Singapore; [email protected] Xian Jun Loh, A*STAR Personal Care Program; and Na
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