Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Human skin is a remarkable organ. It consists of an integrated, stretchable network of sensors that relay information about tactile and thermal stimuli to the brain, allowing us to maneuver within our environment safely and effectively. Interest in large-area networks of electronic devices inspired by human skin is motivated by the promise of creating autonomous intelligent robots and biomimetic prosthetics, among other applications. The development of electronic networks comprised of flexible, stretchable, and robust devices that are compatible with large-area implementation and integrated with multiple functionalities is a testament to the progress in developing an electronic skin (e-skin) akin to human skin. E-skins are already capable of providing augmented performance over their organic counterpart, both in superior spatial resolution and thermal sensitivity. They could be further improved through the incorporation of additional functionalities (e.g., chemical and biological sensing) and desired properties (e.g., biodegradability and self-powering). Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.

25th Anniversary Article: The Evolution of Electronic Skin (E-Skin): A Brief History, Design Considerations, and Recent Progress

Organic solar cells are promising for technological applications, as they are lightweight and mechanically robust. This study presents flexible organic solar cells that are less than 2 μm thick, have very low specific weight and maintain their photovoltaic performance under repeated mechanical deformation.

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Ultrathin and lightweight organic solar cells with high flexibility

IO N Materials that are both conductive and stretchable could enable a spectrum of applications such as stretchable displays, [ 1 ] stretchable radiofrequency antennas, [ 2 ] artifi cial muscles [ 3 ] and conformal skin sensors. [ 4–7 ] A variety of such materials have been recently developed, such as wavy thin metals, [ 8 , 9 ] metal-coated net-shaped plastic fi lm, [ 10 ] graphene fi lms [ 11 ] and carbon nanotube (CNT)-based composites. [ 12–19 ] But several limitations typically exist in these materials including low conductivity, [ 13 , 15 , 16 , 19 ]

Highly Conductive and Stretchable Silver Nanowire Conductors

Hydrogels for biomedical applications.

Tough hydrogels, polymeric network structures with excellent mechanical properties (such as high stretchability and toughness), are emerging soft materials. Despite their remarkably mechanical features, tough hydrogels exhibit two flaws (freezing around the icing temperatures of water and drying under arid conditions). Inspired by cryoprotectants (CPAs) used in the inhibition of the icing of water in biological samples, a versatile and straightforward method is reported to fabricate extreme anti-freezing, non-drying CPA-based organohydrogels with long-term stability by partially displacing water molecules within the pre-fabricated hydrogels. CPA-based Ca-alginate/polyacrylamide (PAAm) tough hydrogels were successfully fabricated with glycerol, glycol, and sorbitol. The CPA-based organohydrogels remain unfrozen and mechanically flexible even up to -70 °C and are stable under ambient conditions or even vacuum.

Rational Fabrication of Anti-Freezing, Non-Drying Tough Organohydrogels by One-Pot Solvent Displacement.

Stretchable conductors, including interconnects, fi lms, and circuits, are key and inevitable elements in the development of stretchable electronics, which have many potential applications in robotic skins, wearable displays, and solar cells, as well as medical implants for health monitoring and disease diagnostics, point of care treatment, and biological actuation. [ 1–5 ] One critical challenge in the realization of stretchable conductors is the simultaneous incorporation of excellent mechanical compliance (strain > > 1%) and stable metallic conductivity. Raised by the nature of materials, metals usually show high electronic performance and stability, but their mechanical stretchability is very poor. On the other hand, soft materials, such as polymers and elastomers, are typically mechanical soft yet exhibit poor conductivity. To advance this emerging fi eld, two major strategies, namely “materials that stretch” and “structures that stretch”, have been suggested to address the challenge. [ 1 ] In the former, conductive materials such as metals, carbon materials, and conducting polymers are fi lled into a rubbery matrix to form elastic conductive composites. [ 6–20 ] This strategy can readily yield highly elastic conductors with tensile strain larger than 100%, yet the conductivity is typically too low to be useful. Not until recently were single-walled-carbon-nanotube-based elastomers developed as promising stretchable conductors with conductivity as high as 5 × 10 3 S cm − 1 . [ 6 , 21 ] However, the conductivity decreased dramatically to 20 S cm − 1 when this elastic conductor was stretched to 140% tensile strain. In the latter strategy, thin fi lms or ribbons of highly conductive metal or alloy are confi gured into in-plane waves or out-of-plane buckles and then bonded onto elastomeric substrates. [ 22–37 ] When tensile force is applied on the substrate, the waves or buckles are straightened to achieve large strains, and they recover when the tensile is released. Therefore, the conductive path actually subjects the material to very little stress and maintains stable metallic conductance under reversible stretching and releasing. Currently, this strategy still requires

Stretchable Conductors with Ultrahigh Tensile Strain and Stable Metallic Conductance Enabled by Prestrained Polyelectrolyte Nanoplatforms

Liquid Metal‐Based Transient Circuits for Flexible and Recyclable Electronics

As one of the most outstanding materials, the analysis of the structure and function of hydrogels has been extensively carried out to tailor and adapt them to various fields of application. The high water content, which is beneficial for plenty of applications in the biomedical setting, prevents the adoption of hydrogels in flexible electronics and sensors in real life applications, because hydrogels lose their excellent properties, including conductivity, transparency, flexibility, etc., upon freezing at sub-zero temperatures. Therefore, depressing the liquid–solid phase transition temperature is a powerful means to expand the application scope of hydrogels, and will benefit the chemical engineering and materials science communities. This review summarizes the recent research progress of anti-freezing hydrogels. At first, approaches for the generation of anti-freezing (hydro)gels are introduced and their anti-freezing mechanisms and performances are briefly discussed. These approaches are either based on addition of salts, alcohols (cryoprotectants and organohydrogels), and ionic liquids (ionogels), modification of the polymer network or a combination of several techniques. Then, a concise overview of applications leveraged by the widened temperature resistance is provided and future research areas and developments are envisaged.

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Biomimetic anti-freezing polymeric hydrogels: keeping soft-wet materials active in cold environments

Owing to their structural advantages in comparison with bulk polydimethylsiloxane (PDMS), porous 3D substrates, which are also known as PDMS sponges, possess immense potential in energy generation, transport and storage, absorption, separation, photocatalysis, wearable electronics and life science applications. Extensive effort has therefore been devoted to the design and synthesis of porous PDMS sponges. The key to excellent performance in high-value applications is a deep knowledge of the beneficial aspects of their porosity, structure and morphology for a specific application. Several new routes for the fabrication of PDMS sponges, as well as novel applications, have been developed recently. Therefore, this review focuses on advances in the field of the fabrication and application of PDMS sponges since 2011. After a concise introduction, which includes remarks on the benefits of PDMS in comparison with other materials, strategies for the synthesis/fabrication of PDMS sponges are presented and their advantages and drawbacks are concisely discussed. A summary of the potential fabrication procedures will be beneficial for those entering the field, as it gives a quick overview of the feasible applications of a PDMS sponge fabricated via a specific method. Afterwards, different high-value applications of PDMS sponges are described in conjunction with specific and detailed examples of all the abovementioned applications. In this regard, suggestions are also made regarding which fabrication method is most beneficial for the application being discussed. Finally, recent advances in the fabrication and application of PDMS sponges are summarized, as well as issues, such as surface functionalization and pore size limitations, which are impeding reasonable future applications. We envision that this review will be helpful to those entering the field, as well as experienced researchers who need rapid access to recent literature, and will give new stimuli for the development of novel porous materials.

Xuechang Zhou

Papers

Rational Fabrication of Anti-Freezing, Non-Drying Tough Organohydrogels by One-Pot Solvent Displacement.

Stretchable Conductors with Ultrahigh Tensile Strain and Stable Metallic Conductance Enabled by Prestrained Polyelectrolyte Nanoplatforms

Liquid Metal‐Based Transient Circuits for Flexible and Recyclable Electronics

Biomimetic anti-freezing polymeric hydrogels: keeping soft-wet materials active in cold environments

Recent progress in fabrication and application of polydimethylsiloxane sponges