We report a bilayer of sodium alginate/polyvinylidene fluoride (SA/PVDF) that is chemically bonded through a series of interfacial coupling reactions. The SA layer is hydrophilic in structure and is capable of strong interaction with water molecules, thus presenting high sensitivity to humidity, whereas the PVDF layer is hydrophobic, inert to humidity. This structural feature results in the bilayer having asymmetric humidity-responsive performances that can thus make its shape change with directionality, which cannot be achieved in an SA single layer. The responsive process to humidity can be adjusted by exposure of the bilayer to sunlight by means of a photothermal effect that accelerates dehydration of the bilayer to cause more rapid shape deformations. When the sunlight is removed, the bilayer adsorbs humidity again and returns to its original shape, indicating good reversibility. To exactly regulate the shape deformations of the bilayer with external stimuli, we employ Ca-treated filter paper to customize crosslinking reactions in the SA layer as desired patterns which are capable of causing different mechanical tensors and swellabilities in the bilayer so as to regulate and control the actuations for self-folding, curling, twisting, and coiling in response to sunlight and humidity.On the other hand, the chemically bonded bilayer has stronger interfacial toughness and is capable of reaching 300 J m, which is around 12 times the interfacial toughness of the physically combined bilayer; as a result, the chemically bonded bilayer is capable of sustaining continuous shape deformations without interfacial failure. The directionally mechanical actuations can be utilized in designing an indicator to roughly indicate the range of intensity of sunlight by coupling the chemically bonded bilayer into a typical electric circuit, in which the range of intensity of sunlight can be easily estimated by visual observation of the light-emitting diodes.
The well-known Marangoni effect perfectly supports the dynamic mechanism of organic solvent-swollen gels on water. On this basis, we report a series of energy conversion processes of concentrated droplets of polyvinylidene fluoride/dimethyl formamide (PVDF/DMF) that can transfer chemical-free energy to kinetic energy to rapidly rotate itself on water. This droplet (22.2 mg) is capable to offer kinetic energy of 0.099 μJ to propel an artificial paper rocket of 31.8 mg to move over 560 cm on water at an initial velocity of 7.9 cm s. As the droplet increases to 35.0 mg, a paper goldfish of 10.6 mg can be driven to swim longer at a higher initial velocity of 20 cm s. The kinetic energy of the droplet can be further converted to electrical energy through an electromagnetic generator, in which as a 0.5 MΩ resistor is loaded, the peak output reaches 6.5 mV that corresponds to the power density of 0.293 μW kg. We believe that this report would open up a promising avenue to exploit energies for applications in miniature robotics.
Soft robots, sensors, and energy harvesters require materials that are capable of converting external stimuli to visible deformations, especially when shape-programmable deformations are desired. Herein, we develop a polymer film that can reversibly respond to humidity, heating, and acetone vapors with the generation of shape-programmable large deformations. Poly(vinylidene fluoride) film, capable of providing acetone responsiveness, is designed with microchannel patterns created on its one side by using templates, and the microchannels-patterned side is then treated with hygroscopic 3-aminopropyltriethoxysilane (APTES) to give humidity/heating-responsive elements. The APTES-modified microchannels lead to anisotropic flexural modulus and hygroscopicity in the film, resulting in the shape-programmed kinematics depending on the orientations of surface microchannels. As the microchannels align at oblique/right angles with respect to the long axis of the film strips, the coiling/curling motions can be generated in response to the stimuli, and the better motion performances are found in humidity- and heating-driven systems. This material utilized in self-adaptive soft robots exhibits prominent toughness, powerful strength, and long endurance for converting humidity and heat to mechanical works including transportation of lightweight objects, automatic sensing cap, and mimicking crawling in nature. We thus believe that this material with shape-programmable multisensing capability might be suitable for soft machines and robotics.
Shape-programmed deformation of soft polymer films is essential for applications in robotics, self-adaptive devices, and sensors. In comparison to bilayer polymer actuators, the challenge remains to manipulate single-layered soft actuators for rapid, reversible, and shape-programmed deformations in response to external stimuli owing to their homogeneous composite structures. Herein, this work reports a soft polymer film actuator that has a single-layered structure, yet demonstrates the shape-programmed motility. The actuator is composed of polyvinylidene fluoride film as a matrix and patterned by photocrosslinking of acrylamide and N′,N′-methylenebisacrylamide, which generates soft–hard alternating segments in the structure. As it is exposed to acetone vapors, the soft–hard structures lead to an unequal response that results in the shape-programmed deformation. The actuator is elastic (strain: 160%) and tough (stress: 40 MPa) and can maintain its rapid, reversible, and shape-programmed motions for a few hours, even longer. The soft–hard structure enables the film actuator (3.5 mg) to give a contracting stress of 4 MPa that is used in an automatic device able to lift a cargo of 5.09 g, ∼1453 times heavier than the film itself. The power output reaches 474 J kg–1, ∼100 times higher than the reported soft actuators. This simple application indicates a potential for the soft actuator used in acetone vapor sensing devices.
The combination of physical pre-patterning and chemical post-crosslinking strategies enables a humidity-sensing actuator with differential mechanical tensors for controlled interfacial sensitivity.
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