In-situ thermoreversible gelation of block and star copolymers of poly(ethylene glycol) and poly(N-isopropylacrylamide) of varying architectures

We report the development of a new gelation mechanism and a new family of p olymers that self-assembles to form gels in a thermoreversible fashion. The polymers are block or star copolymers with a central hydrophilic poly(ethy lene glycol) (PEG) segment (A) and temperature responsive poly(N-isopropyla crylamide) (PNIPAAm) terminal segments (B). Copolymers of various architect ures, AB, A(B)(2), A(B)(4), and A(B)(8), were synthesized to investigate th e structures and properties relationship. At 5 degreesC, the viscosities of 20 wt % solutions were between 700 and 950 cP, and they could be easily in jected through a 25 G needle. Upon warming to body temperature, A(B)(2), A( B)(4), and A(B)(8) formed a strong associative network gel with aggregates of PNIPAAm segments acting as physical cross-links, whereas AB formed a wea ker gel by micellar packing and entanglement. The values of elastic modulus , loss tangent, and yield strength were 1000-2500 Pa, 0.24-0.62, and 200-86 0 Pa, respectively. The gelation kinetic was fast; a typical gelation time for a solution of 5 mt in volume was less than a minute. No significant syn eresis was observed after 2 months at 37 degreesC. DSC results indicated th at the thermal behavior of material was completely reversible even after 30 heat-and-cool cycles. These materials are promising candidates for in-situ gelation applications such as injectable drug delivery, tissue engineering scaffolds, and anatomical barriers.