Gelation of degalactosylated xyloglucan from nano to macroscopic scale

Partially degalactosylated xyloglucans are able to form thermo-reversible gels in aqueous solution at physiological temperature. This property makes them very promising materials for tissue engineering. Moreover, due to a highly branched, hydroxyl group-rich molecular structure, xyloglucans are particularly interesting for manufacturing of micro/nanoparticles to be used as nano-scalar delivery devices of active ingredients in cosmetic and pharmaceutical formulations. Here we present results on the temperature-induced self-assembly of degalactosylated xyloglucan at varying polymer concentration from dilute to semi-dilute regime. Our aim is to investigate the gelation mechanism for gaining information valuable in tailoring structural and mechanical properties of either nanogel particles or macrogel depots. We used static and dynamic light scattering, rheology, and scanning electron microscopy to follow the gelation kinetics and characterize the system in its final state. Results obtained at low polymer concentration show that, on increasing temperature, the polymer chains and pre-existing clusters form larger structures having higher density and homogeneity over the length scale of a few nanometers. Despite the system polydispersity, signatures of concomitant coil-globule and LCST transition can be envisaged. Both demixing and coil-to globule transition are induced by the worsening of the solvent quality with increasing temperature. At increasing polymer concentration, gelation on macroscopic scale occurs quickly, but continuous adjustments are observed over times comparable to those required for nanogel formation. The microstructure of the gel evolves from a disordered arrangement of partially folded sheets, forming large and heterogeneous cavities, to a layered microstructure with regularly stacked sheets connected by filaments and transverse walls. The morphological evolution of the gel suggests the formation of several crosslinking lines suturing the polymeric membranes formed by ribbon-like structures self-assembly.