Combining thermosensitive physical self-assembly and covalent cycloaddition chemistry as simultaneous dual cross-linking mechanisms for the preparation of injectable hydrogels with tuneable properties

Injectable hydrogels are promising materials for the sustained delivery of therapeutics, bioactives and cell therapies. Hydrogels that display suitable processing times and rapid gelling kinetics upon injection are particularly desirable as they can limit burst release of their payload. Therefore, we aimed to develop hydrogels that could be readily processed at ambient temperature, gelled rapidly at physiological temperature, and possessed tuneable properties (e.g., degradation and release profiles). To achieve this, a unique dual cross-linking mechanism was investigated, employing a combination of pentafulvene-maleimide Diels-Alder cycloaddition (DAC) chemistry and thermoresponsive physical cross-linking with a functionalised poly(ethylene glycol-b-propylene glycol-b-ethylene glycol) (PEG-PPG-PEG; Pluronic F-127) copolymer. A combination of temperature dependent nuclear magnetic resonance spectroscopy, dynamic light scattering, and rheological measurements were conducted to interrogate the gelation mechanism. Whereas the reaction between aqueous solutions of maleimide functionalised Pluronic and pentafulvene functionalised PEG star polymer resulted in hydrogels with gelation times (t_gel) between 30 and 55 mins at ambient temperature, significantly faster t_gel values (5–10 mins) were observed at physiological temperatures (37 °C). The relatively slow gelation at ambient temperature provided good working times and injectability through narrow gauge (26¹/₂ G) needles. While the initial Young's modulus (E) of the hydrogels was highly dependent on the gelation temperature, the E remained dynamic in response to further temperature changes post-gelation. The hydrogels displayed relatively low swelling (20–30% mass increases) and degradation times (t_d) up to 16 d, which provided for the sustained release of latex particles (cell mimic) over a period of 7–14 d. With their tuneable gelation kinetics and properties, the thermoresponsive DAC hydrogels have potential for the controlled release of therapeutics.