Owing to their water-rich structures, which are similar to those of biological tissues, hydrogels have long been regarded as promising scaffolds for artificial tissues and organs. However, in terms of the structural anisotropy, most synthetic hydrogels are substantially different from biological systems. Synthetic hydrogels are usually composed of randomly oriented three-dimensional polymer networks whereas biological systems adopt anisotropic structures with hierarchically integrated building units. Such anisotropic structures often play essential roles in biological systems to exhibit particular functions. In this context, anisotropic hydrogels provide an entry point for exploring biomimetic applications of hydrogels. Reflecting these aspects, an increasing number of studies on anisotropic hydrogels have been reported recently. This Minireview highlights the use and perspectives of these anisotropic hydrogels, particularly focusing on their preparation, structures, and applications.

Fluids that contain ordered nanostructures with periodic distances in the visible-wavelength range, anomalously exhibit structural colours that can be rapidly modulated by external stimuli. Indeed, some fish can dynamically change colour by modulating the periodic distance of crystalline guanine sheets cofacially oriented in their fluid cytoplasm. Here we report that a dilute aqueous colloidal dispersion of negatively charged titanate nanosheets exhibits structural colours. In this ‘photonic water', the nanosheets spontaneously adopt a cofacial geometry with an ultralong periodic distance of up to 675 nm due to a strong electrostatic repulsion. Consequently, the photonic water can even reflect near-infrared light up to 1,750 nm. The structural colour becomes more vivid in a magnetic flux that induces monodomain structural ordering of the colloidal dispersion. The reflective colour of the photonic water can be modulated over the entire visible region in response to appropriate physical or chemical stimuli.

The construction of hierarchical nanostructures with precise morphological and dimensional control has been one of the ultimate goals of contemporary materials science and chemistry, and the emulation of tailor-made nanoscale superstructures realized in the nature, using artificial building blocks, poses outstanding challenges. Herein we report a one-pot strategy to precisely synthesize hierarchical nanostructures through an in-situ initiation-growth process from a liquid crystalline block copolymer. The assembly process, analogous to living chain polymerization, can be triggered by small-molecule, macromolecule or even nanoobject initiators to produce various hierarchical superstructures with highly uniform morphologies and finely tunable dimensions. Because of the high degree of controllability and predictability, this assembly strategy opens the avenue to the design and construction of hierarchical structures with broad utility and accessibility.

The quasi-particle band structure, carrier mobility, and optical response of atomic-thin TiO2 nanosheets were accurately predicted with the many-body perturbation theory of G0W0+BSE calculations. The lepidocrocite-type TiO2 exhibits an unexpected direct band gap of 5.3 eV, different from the retaining indirect band gap character in anatase-type TiO2 nanosheet. Because of the dispersive valence band maxima from the strong overlap between O-2p orbitals, an extremely high hole mobility of 1069 cm2 V–1 s–1 was proposed for the lepidocrocite-type TiO2 nanosheet. Including the electron–hole exchange of excitonic effect, our simulations well reproduce the experimental absorption spectra implying a huge exciton binding energy. The strong anisotropic exciton originates from the crystal dependent effective mass in the lepidocrocite-type TiO2 nanosheet due to the asymmetric orbital overlaps. The strongly bound exciton still renders a sufficiently high potential for hydrogen production from aqueous solution, although it largely decreases the photonic excitation energy.

Although various biomimetic soft materials that display structural hierarchies and stimuli responsiveness have been developed from organic materials, the creation of their counterparts consisting entirely of inorganic materials presents an attractive challenge, as the properties of such materials generally differ from those of living organisms. Here, we have developed a hydrogel consisting of inorganic nanosheets (14 wt%) and water (86 wt%) that undergoes thermally induced reversible and abrupt changes in its internal structure and mechanical elasticity (23-fold). At room temperature, the nanosheets in water electrostatically repel one another and self-assemble into a long-periodic lamellar architecture with mutually restricted mobility, forming a physical hydrogel. Upon heating above 55 °C, the electrostatic repulsion is overcome by competing van der Waals attraction, and the nanosheets rearrange into an interconnected 3D network of another hydrogel. By doping the gel with a photothermal-conversion agent, the gel-to-gel transition becomes operable spatiotemporally on photoirradiation.

Crystalline Si-based nanosheets were successfully synthesized from CaSi2 by a simple soft chemical synthetic method in solution. By immersing CaSi2 powder or CaSi2/Si substrates in an inositol hexakisphosphate (IP6) solution, Ca atoms were extracted from the CaSi2 particles, then Si-based nanosheets were formed. The morphological, structural and optical properties of the Si-based nanosheets were investigated. It is noted that the thin Si-based nanosheets stacked with a void space formed bundle structures, and the nanosheets were easily exfoliated from the bundles to expose the surfaces corresponding to the Si{111} planes. Meanwhile, the surface of the Si nanosheets might be terminated by O, H, or OH bonds. The Si-based nanosheet bundles were then formed and directly rooted to the Si(111) substrates, and had a remarkably highly symmetrical morphology. This study demonstrated a simple method for preparing Si-based nanosheets, and electro- and photo-chemical applications would possibly be expected, such as in lithium ion batteries.

The transverse thermoelectric (Nernst) effect on pyrochlore molybdates is investigated experimentally. In Nd(2)Mo(2)O(7) and Sm(2)Mo(2)O(7) with the spin chirality, the Nernst signal, which mostly arises from the transverse heat current (or equivalently the transverse Peltier coefficient alpha(xy)), shows a low-temperature (20-30 K) positive extremum, whereas it is absent in (Gd(0.95)Ca(0.05))(2)Mo(2)O(7) with no single-spin anisotropy of the rare-earth ion and hence with no spin chirality. The correlation between the Hall conductivity sigma(xy) and alpha(xy) in Nd(2)Mo(2)O(7) also indicates the spin chirality plays a significant role in the spontaneous (anomalous) Nernst effect.

In our previous work, we have shown that "electrostatic forces", when generated anisotropically in aqueous media by 2D electrolytes upon cofacial orientation, enable the formation of a hydrogel with an anisotropic parameter, as defined by the ratio of elastic moduli E /E , of 3.0. Herein, we successfully developed the design strategy for a hydrogel with an anisotropic parameter of no less than 85. This value is not only 28 times greater than that of our previous anisotropic hydrogel but also 6 times larger than the current champion record in synthetic hydrogels (E /E ∼15). Firstly, we simply lowered ionic contaminants in the hydrogel and were able to enhance the anisotropic parameter from 3.0 to 18. Then, we chose a supporting polymer network allowing the hydrogel to carry a higher interior permittivity. Consequently, the anisotropic parameter was further enhanced from 18 to 85. Owing to the enhanced mechanical anisotropy, our new hydrogel displayed a superb ability of seismic isolation.