Photodegradable polyacrylamide tanglemers enable spatiotemporal control over chain lengthening in high-strength and low-hysteresis hydrogels

Abstract

Covalent hydrogel networks suffer from a stiffness-toughness conflict, where increased crosslinking density enhances the modulus of the material but also leads to embrittlement and diminished extensibility. Recently, strategies have been developed to form highly entangled hydrogels, colloquially referred to as tanglemers, by optimizing polymerization conditions to maximize the density and length of polymer chains and minimize the crosslinker concentration. It is challenging to assess entanglements in crosslinked networks beyond approximating their theoretical contribution to mechanical properties; thus, in this work, we synthesize and characterize polyacrylamide tanglemers using a photolabile crosslinker, which allows for direct assessment of covalent trapping of entanglements under tension. Further, this chemistry allows tuning of the modulus in situ by crosslink photocleavage (from tensile modulus (E_T) = 100 kPa to <25 kPa). Beyond cleavage of crosslinks, we demonstrate that even non-degradable tanglemer formulations can be photo-softened and completely degraded through Fe³⁺-mediated oxidation of the polyacrylamide backbone. While both photodegradation methods are useful for spatial patterning and result in softer gels with reduced fracture strength, only crosslink photocleavage improves gel extensibility via light-induced chain lengthening (ε_F = 700% to >1500%). Crosslink photocleavage in tanglemers also affords significant control over localized swelling and diffusivity. In sum, we introduce a simple and user-directed approach for probing entanglements and asserting spatiotemporal control over stress–strain responses and small molecule diffusivity in polyacrylamide tanglemers, suggesting a multitude of potential soft matter applications including controlled release and tunable bioadhesive interfaces.