A new
design for photoresponsive shape memory hydrogels and their
possible applications are demonstrated in the present study. We show
that the photodissociable Fe3+-carboxylate coordination
can be utilized as a molecular switch to realize photocontrol of shape
memory on both macroscopic and microscopic scales and enable a number
of functions. Indeed, Fe3+-carboxylate coordination can
fix a large tensile strain (up to 680%) of the sodium alginate/polyacrylamide
hydrogel through cross-linking of sodium alginate chains, and subsequent
UV irradiation allows strain energy release in spatially selected
regions through reduction of Fe3+ to Fe2+. By
manipulating light irradiation, complex 3D structures are obtained
from 2D hydrogel sheets, and they exhibit complex solvent-driven actuation
behaviors due to a light-changeable modulus and cross-linking density
in the hydrogel. Based on the same approach, micropatterns can be
inscribed on the hydrogel surface using mask-assisted irradiation,
and they exhibit chain orientation-mediated anisotropic topography
change upon solvent exchange. Moreover, light-controlled strain energy
release also enables changing hydrogel surface wettability by solvent
replacement. The demonstrated mechanism for photoresponsive hydrogels
is highly efficient and applicable to many systems, which offers new
perspectives in developing hydrogels with multiple photoresponsive
functions.
Hydrogel actuators have gained considerable interest and experienced significant advancements in recent years. However, the programming of their actuating behaviors is still challenging. Herein, we report the development and regulation of gradient structures of hydrogels for programmable thermally responsive actuating behaviors. The hydrogel actuators are developed by controlling the photoreduction of Fe 3+ ions coordinated with carboxylate groups from the substrates and their limited diffusion into the precursor solutions to act as both initiators and crosslinkers. The developed hydrogels show welldefined external geometries and controllable thicknesses under spatiotemporal control of ultraviolet irradiation. The shapes and the actuation amplitudes of the hydrogel actuators can be independently regulated by controlling the formation and photodissociation of Fe 3+ −carboxylate coordination in the formed gradient networks. Some interesting applications such as the lifting of an object with a specific shape and directional walking are realized. The proposed method can be extended to other hydrogel actuators with different compositions and stimuli-responsive behaviors.
Endowing polymers with improved mechanical properties by functional fillers is a continuously pursued topic. Although composites with different fillerimproved properties have been reported, the inevitably raised costs often restrict the practical uses of these products. Herein, we report the utilization of the waste asphaltene sands (AS) produced from the coal-oil co-processing process as a functional filler to toughen the thermoplastic elastomers represented by SEBS. The FT-IR, XRD and elemental analysis showed that AS is a mixture consisting of organic compounds and inorganic minerals from the coal. After filling AS into SEBS, the inorganic minerals act as hard cores to strengthen the matrix while the surrounding aromatic and alkane compounds ensure a good compatibility of AS with SEBS. Composites with altered contents and sizes of AS particles were prepared by solvent processing and subsequent hot pressing.The optical microscopy images show a good dispersion of the AS particles in the SEBS substrate. As a result, the tensile strength of the composites are notably enhanced from 6.88 to 17.23 MPa after loading 20 wt% AS. Thus, the AS waste is very promising as the new and successful filler in developing polymers with improved mechanical properties and simultaneously reduced production cost.
Hydrogel shape memory and actuating functionalities are heavily pursued and have found great potential in various application fields. However, their combination for more flexible and complicated morphing behaviors is still challenging. Herein, it is reported that by controlling the light‐initiated polymerization of active hydrogel layers on shape memory hydrogel substrates, advanced morphing behaviors based on programmable hydrogel shapes and actuating trajectories are realized. The formation and photo‐reduction‐induced dissociation of Fe3+‐carboxylate coordination endow the hydrogel substrates with the shape memory functionality. The photo‐reduced Fe2+ ions can diffuse from the substrates into the monomer solutions to initiate the polymerization of the thermally responsive active layers, whose actuating temperatures and amplitudes can be facially tuned by controlling their thicknesses and compositions. One potential application, a shape‐programmable 3D hook that can lift an object with a specific shape, is also unveiled. The demonstrated strategy is extendable to other hydrogel systems to realize more versatile and complicated actuating behaviors.
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