Dual-responsive and super absorbing thermally cross-linked hydrogel based on methacrylate substituted polyphosphazene

Stimuli-responsive hydrogels have played a crucial role in biomaterials being extensively studied for applications such as tissue engineering and drug delivery. Here, we report on the synthesis and characterization of a new hydrogel based on methacrylate substituted polyphosphazene (PMAPhos). The hydrogel was obtained spontaneously after the polyphosphazene substitution reaction and its mechanical properties were improved after annealing. By annealing the hydrogel in the presence of a 6% w/w methacrylic acid solution, we could obtain a poly(methacrylic acid) (PMAA) branched and slightly cross-linked polyphosphazene network, which showed ultra-high absorbency being able to absorb distilled water, at maximum, as much as 870 times of its own weight. Moreover, PMAPhos showed dual-responsive behaviour responding to both pH and temperature changes. Unlike many other similar systems, the hydrogel structure collapsed not only at lower but also at higher pHs. The gel volume phase transition as a function of temperature was determined by both differential scanning calorimetry (DSC) and by swelling experiments. However, after the hydrogel annealing, the thermal response was suppressed and overridden by the pH-response behaviour of the PMAA branches. Hydrogel morphology and swelling behaviour were also analyzed directly by Cryo-transmission electron microscopy, which revealed the fibrous polymer network. Remarkable features of this new hydrogel are: the biodegradation offered by the polyphosphazene backbone and simplicity of preparation where no co-polymerizations were used to reach the dual-responsive behaviour. Besides, the particular pH-responsive behaviour found for this hydrogel is a useful characteristic for applications aiming for controlled delivery of substances in specific sites of the human body.