Injectable Hydrogels from a Thermoresponsive Triblock Terpolymer: Structural Investigation Using Small-Angle Neutron...

Aqueous solutions from block copolymers with blocks featuring lower critical solution temperature behavior have attracted great interest for 3D bioprinting, as they are in the liquid state at room temperature but become a soft and tissue-like solid at body temperature by forming a high-water-content hydrogel. Here, we use small-angle neutron scattering (SANS) to resolve the temperature-dependent self-assembled structures of a triblock terpolymer featuring a hydrophobic and two thermoresponsive blocks with strongly different cloud point temperatures Tcp in D2O. Solutions having polymer concentrations of 1 and 15 wt % were studied around the transitions that occur at the cloud point Tcp, the gelation temperature Tgel (which is observed for the concentrated solution), and the phase separation temperature Tps. In dilute solution, the crew-cut spherical micelles formed at low temperatures convert to cylindrical micelles upon approaching Tcp. We attribute this shape change to the formation of hydrophobic channels when one of the thermoresponsive blocks collapses. Upon heating towards Tps, more isotropic aggregates from micelles are observed, and above Tps, the micelles are densely packed in large aggregates, which we attribute to the dehydration of the other thermoresponsive block. In concentrated solution, the same mechanism results in a sphere-to-cylinder transition as the system crosses from the cloudy liquid into the gel state, followed by the formation of an extended network with the cylindrical micelles acting as hydrophobic channels and physical cross-links. Further heating across Tps induces the collapse of the network and the reorganization into large, compact aggregates, consistent with the previously observed macroscopic phase separation. Thus, these results establish the relation between the dehydration of the thermoresponsive blocks, the size, shape, inner structure, and correlations of the self-assembled micelles and the mechanisms leading to hydrogel formation and phase separation, which helps to design tailor-made injectable thermoresponsive hydrogels for 3D bioprinting and biomedical applications.