Injectable hydrogels based on alginates grafted with LCST side-chains of different chemistry.

A homologous series of thermoassociating copolymers was prepared by grafting onto alginates different amounts of three different temperature responsive polymers: poly(N-isopropylacrylamide), poly(di(ethylene glycol)methacrylate) and poly(ethylene oxide-co-propylene oxide). From a large set of analytical techniques combining rheology, calorimetry, NMR and SAXS, the relevant parameters controlling the sol/gel transition and the gel properties, mainly the degree of entanglement of macromolecules and the fraction of responsive stickers, were highlighted and interpreted objectively by considering the particularities of the phase diagrams of LCST polymers. Complementary analyses were implemented to investigate adhesiveness, injectabilty, gel swelling and molecular release in physiological environment of thermogelling formulations. In particular, it is shown that steady shear experiments allow to predict the injection forces by taking into account the characteristics of the system (syringe and needle), and that the rapid gelation of the formulations when they are heated at 37 °C delays the release of small molecules into the environment. The overall set of data is discussed in the framework of scaling relations in order to draw quantitative guidelines for the design of injectable thermoresponsive hydrogels.

Sol/gel transition of thermoresponsive Hyaluronan: From liquids to elastic and sticky materials.

Thermoassociating copolymers were prepared by grafting temperature responsive poly(N-isopropylacrylamide-stat-N-tert-butylacrylamide) telomers onto hyaluronan. By varying the composition of LCST side chains, from 50 to 100 wt% of NIPAM units, it is shown that the sol/gel transition of entangled solutions can be accurately controlled in the range of 10 to 35 °C with an abrupt transition and reversible properties. Complementary experiments, performed by DSC and NMR, demonstrate the close relationship between thermoassociation of LCST grafts, forming microdomains of low mobility, and macroscopic properties. Moreover, by performing tack experiments during heating we demonstrate that hyaluronan formulations abruptly switch from a weak adhesive viscous behavior to an elastic adhesive profile in the gel regime. As LCST side-chains form concentrated micro-domains of low mobility, physical gels can resist to dissociation above their sol/gel transition for relatively long periods when immersed in excess physiological medium. The thermoassociative behavior of these copolymers, whose properties can be finely tuned in order to form sticky gels at body temperature, clearly demonstrates their potential in biomedical applications such as injectable gels for drug delivery or tissue engineering.