Fabricating 3D Moisture- and NIR Light-Responsive Actuators by a One-Step Gradient Stress-Relaxation Process

Polymeric materials that can actuate under the stimulationof environmentalsignals have attracted considerable attention in fields includingartificial muscles, soft robotics, implantable devices, etc. To date,the improvement of shape-changing flexibility is mainly limited bytheir unchangeable shapes and structural and compositional distributions.In this work, we report a one-step treatment process to convert 2Dpoly(ethylene oxide)/sodium alginate/tannic acid thin films into 3D-shapedmoisture- and NIR light-responsive actuators. Spatial surface wettingof the film leads to the release of residual stress generated in filmformation in a gradient manner, which drives the wetted regions tobidirectionally bend. By controlling the position and bending amplitudeof the wetted regions, designated 3D shapes can be obtained. Moreover,Fe3+ ions in the aqueous solution used for surface wettingcan coordinate with carboxylate groups in sodium alginate chains toform a gradient cross-linking network. This gradient network can notonly stabilize the resulting 3D shape but also render the film withmoisture-responsive morphing behaviors. Fe3+ ions can alsoself-assemble with tannic acid molecules to form photothermal aggregates,making the film responsive to NIR light. We further show that filmswith versatile 3D shapes and different modes of deformation can befabricated by a one-step treatment process. This strategy is convenientand extendable to develop 3D-shaped polymer actuators with flexibleshape-changing behaviors.

Automated summary

A simplified treatment process is reported to convert 2D poly(ethylene oxide)/sodium alginate/tannic acid thin films into 3D-shaped moisture- and NIR light-responsive actuators. Wetting the film surface releases residual stress in a gradient manner, driving the wetted regions to bend bidirectionally. A gradient cross-linking network formed by Fe3+ ions and sodium alginate chains stabilizes the resulting 3D shape and provides moisture-responsive morphing behaviors. Fe3+ ions also enable the film to respond to NIR light by self-assembling with tannic acid molecules. Films with versatile 3D shapes and deformation modes can be conveniently fabricated using this one-step treatment process.