Strong hydrogels that respond to force, heat and light

Posted: Jun 03, 2017
Strong hydrogels that respond to force, heat and light (Nanowerk News) Researchers have developed novel hydrogels not only exhibited remarkable optical response from pale yellow to purple color and from green to red fluorescence under external stimuli of force, heat, and UV light, but also simply reversed its color back to the original one by white light.
Their results have been published in Advanced Materials ("A Novel Design of Multi-Mechanoresponsive and Mechanically Strong Hydrogels").

Synthesis procedure of mechanoresponsive poly(AM-co-MA/SP) hydrogel via micellar copolymerization of acrylamide and methyl acrylate/spiropyran. The hydrogel exhibits color change from yellow to purple upon force, UV light, and heat stimuli and color reversion from purple to yellow upon white light. Both color change and reversion are triggered by a reversible structural transformation between spiropyran (SP, a ring-close, yellow) and merocyanine (MC, a ring-open, purple) states. (© Wiley-VCH Verlag) (click on image to enlarge)
Unlike other mechanoresponsive materials such as benzocyclobutene and 1,2-dioxetane, spiropyran (SP) is a multi-stimuli-responsive mechanophore, which can change its color and fluorescence in response to force, heat, and light.
Under these external stimuli, SP undergoes a reversible structural transformation between spiropyran (a ring-close) and merocyanine (MC, a ring-open) states, leading to the reversible optical property change.
However, the poor solubility of SP in aqueous solution makes it very challenging to directly incorporate SP into highly hydrophilic hydrogels (50%–90% water content), while still retaining SP mechanophore function.
More importantly, no SP-based tough hydrogels have been reported to date, thus little is known about the SP-induced mechanotransduction and toughening mechanisms in the hydrogels.
To overcome these challenges, here the resaerchers developed a new micellar-copolymerization method to incorporate highly hydrophobic SP into highly hydrated polymer network, resulting in SP-crosslinked poly(AM-co-MA/SP) (polyacrylamide-co-methylacrylate/spiropyran) hydrogels that exhibit SP-induced multimechano-responsive/recovery and mechanically strong properties.

Multi-stimuli-responsive poly(AM-co-MA/SP) hydrogel exhibits reversible color changes between purple color under the stimuli of a) heat, b) UV light (365 nm), c) stretching, d) compression and pale yellow after white light exposure. e) The hydrogel also shows the fluorescent responses to compression, as indicated by black “X” versus green background using the GFP filter and red “X” versus black background using the Cy5 filter. f) SEM image of poly(AM-co-MA/SP) hydrogel with microsphere structures (scale bar = 10 um), and g) in situ confocal image of poly(AM-co-MA/SP) hydrogel with strong color contrast between green fluorescence of microspheres and almost invisible background of PAM chains (scale bar = 50 µm). (© Wiley-VCH Verlag) (click on image to enlarge)
In their paper, the authors present proof-of-concept experiments to demonstrate that their hydrogels can achieve multi-stimuli-responsive property (color and fluorescence changes under the stimuli of external force, UV light, and heat), light-induced self-recovery capacity (∼75% toughness recovery and ∼74% stiffness recovery after the first loading), and strong mechanical properties (tensile stress of 1.45 MPa, tensile strain of 570%, fracture energy of 7300 J m-2) at optimal conditions.

By Michael Berger – Michael is author of three books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology, Nanotechnology: The Future is Tiny, and Nanoengineering: The Skills and Tools Making Technology Invisible Copyright © Nanowerk LLC

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