the mantle of squids mainly comprises two groups of muscle fibers: the circumferential fibers that constitute the bulk of the mantle wall, and the radial fibers that are distributed in the mantle wall as to partition the circumferential ones. [2] Squids expand their mantle radially, filling the mantle cavity with water. Then the circumferential muscles contract, pushing the water out of the mantle cavity through the funnel. Repetition of these movements results in a pulsed jet. Inspired by these natural systems, there are many efforts devoted to developing soft active materials with anisotropic structures by hybridizing liquid crystals or nanoparticles. Anisotropic materials fulfill specific functions and enable various applications including mass transport, [3] structural colors, [4] actuations, [5,6] and soft robotics. [1a,b,7] In addition, programmable, complex ordered materials display local response under stimuli and show intriguing 3D configurations, imitating actuation or locomotion of living organisms. [6a,e,7c] Although photo-and surface-mediated molecular alignments are versatile to fabricate liquid crystalline elastomer/network films with distributed ordered structures. [6b,8] The thickness and dimensions of the films are usually limited in micrometer scale, and the strategy cannot be extended to other systems like hydrogels with sophisticated alignments. So far, preparation of anisotropic Living organisms use musculatures with spatially distributed anisotropic structures to actuate deformations and locomotion with fascinating functions. Replicating such structural features in artificial materials is of great significance yet remains a big challenge. Here, a facile strategy is reported to fabricate hydrogels with elaborate ordered structures of nanosheets (NSs) oriented under a distributed electric field. Multiple electrodes are distributed with various arrangements in the precursor solution containing NSs and gold nanoparticles. A complex electric field induces sophisticated orientations of the NSs that are permanently inscribed by subsequent photo-polymerization. The resultant anisotropic nanocomposite poly(N-isopropylacrylamide) hydrogels exhibit rapid deformation upon heating or photoirradiation, owing to the fast switching of permittivity of the media and electric repulsion between the NSs. The complex alignments of NSs and anisotropic shape change of discrete regions result in programmed deformation of the hydrogels into various configurations. Furthermore, locomotion is realized by a spatiotemporal light stimulation that locally triggers time-variant shape change of the composite hydrogel with complex anisotropic structures. Such a strategy on the basis of the distributed electric-field-generated ordered structures should be applicable to gels, elastomers, and thermosets loaded with other anisotropic particles or liquid crystals, for the design of biomimetic/ bioinspired materials with specific functionalities.
Anisotropic structures are ubiquitous in nature, affording fascinating morphing behaviors. Biomimetic morphing materials can be developed by spatially controlling the orientations of molecules or nanofillers that produce anisotropic responses and internal stresses under external stimuli. However, it remains a serious challenge to fabricate materials with sophisticated anisotropic architectures. Here, a facile strategy to fabricate morphing hydrogels with elaborately ordered structures of nanosheets, which are oriented under distributed electric field and immobilized by polymerization to form a poly(N-isopropylacrylamide) matrix, is proposed. Diverse sophisticated anisotropic structures are obtained by engineering the electric field through the patterns and relative locations of the electrodes. Upon heating, the monolithic hydrogels with through-thickness and/or in-plane gradients in orientation of the nanosheets deform into various three-dimensional configurations. After incorporating gold nanoparticles, the hydrogels become photoresponsive and capable of programmable motions, for example, dynamic twisting and flipping under spatiotemporal stimuli. Such a strategy of using patterned electrodes to generate distributed electric field should be applicable to systems of liquid crystals or charged particles/molecules to direct orientation or electrophoresis and form functional structures. The biomimetically architectured hydrogels would be ideal materials to develop artificial muscles, soft actuators/robots, and biomedical devices with versatile applications.
Shape memory hydrogels have drawn increasing attention in recent years. Practical applications require these hydrogels to have good mechanical properties as well as contactless stimulations to trigger the shape deformations. Here we report a stiff and tough shape memory hydrogel that can transform to various configurations sequentially by phototriggered site-specific deformations. Response of the shape memory hydrogel to near-infrared (NIR) light irradiation was achieved by incorporating gold nanorods (AuNRs) into the glassy gel matrix of poly(methacrylic acid-co-methacrylamide) without compromising the excellent mechanical properties. Owing to the photothermal effect of the AuNRs, the localized temperature rise led to a dramatic decrease in Young’s modulus (from 200 to 2 MPa) of the prestretched hydrogel and bending deformation with a programmable direction and amplitude. More complex three-dimensional configurations can be obtained by multidirectional prestretching and shape memorizing the individual parts of the nanocomposite hydrogel. Furthermore, the AuNRs embedded in the gel were aligned along the prestretching direction, leading to anisotropic plasmon resonance. These photomediated programmable deformations of tough shape memory hydrogels should find applications in the biomedical and engineering fields.
Reported here is a multi-response anisotropic poly(N-isopropylacrylamide) hydrogel developed by using a rotating magnetic field to align magnetic double stacks (MDSs) that are fixed by polymerization. The magnetoorientation of MDSs originates from the unique structure with γ-Fe 2 O 3 nanoparticles sandwiched by two silicate nanosheets. The resultant gels not only exhibit anisotropic optical and mechanical properties but also show anisotropic responses to temperature and light. Gels with complex ordered structures of MDSs are further devised by multi-step magnetic orientation and photolithographic polymerization. These gels show varied birefringence patterns with potentials as information materials, and can deform into specific configurations upon stimulations. Multi-gait motions are further realized in the patterned gel through dynamic deformation under spatiotemporal light and friction regulation by imposed magnetic force. The magneto-orientation assisted fabrication of hydrogels with anisotropic structures and additional functions should bring opportunities for gel materials in biomedical devices, soft actuators/robots, etc.
Gradient materials exist widely in natural living organisms, affording fascinating biological and mechanical properties. However, the synthetic gradient hydrogels are usually mechanically weak or only have relatively simple gradient structures. Here, we report on tough nanocomposite hydrogels with designable gradient network structure and mechanical properties by a facile post-photoregulation strategy. Poly(1-vinylimidazole-co-methacrylic acid) hydrogels containing gold nanorods (AuNRs) are in a glassy state and show typical yielding and forced elastic deformation at room temperature. The gel slightly contracts its volume when the temperature is above the glass-transition temperature that results in a collapse of the chain segments and formation of denser intra- and interchain hydrogen bonds. Consequently, the mechanical properties of the gels are enhanced, when the temperature returns to room temperature. The mechanical performances of hydrogels can also be locally tuned by near-infrared light irradiation due to the photothermal effect of AuNRs. Hydrogels with arbitrary two-dimensional gradients can be facilely developed by site-specific photoirradiation. The treated and untreated regions with different stiffness and yielding stress possess construct behaviors in stretching or twisting deformations. A locally reinforced hydrogel with the kirigami structure becomes notch-insensitive and exhibits improved strength and stretchability because the treated regions ahead the cuts have better resistance to crack advancement. These tough hydrogels with programmable gradient structure and mechanics should find applications as structural elements, biological devices, etc.
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