Spider-Silk-Inspired Nanocomposite Polymers for Durable Daytime Radiative Cooling.

Passive daytime radiative cooling (PDRC) materials, that strongly reflect sunlight and emit thermal radiation to outer space, demonstrate great potential in energy-saving for sustainable development. Particularly, polymer-based PDRC materials, with advantages of easy-processing, low cost, and outstanding cooling performance, have attracted intense attention. However, just like other polymer devices (for example polymer solar cells) working under sunlight, the issue of durability related to mechanical and UV properties needs to be addressed for large-scale practical applications. Here, a spider-silk-inspired design of nanocomposite polymers with potassium titanate (K2 Ti6 O13 ) nanofiber dopants is proposed for enhancing the durability without compromising their cooling performance. The formed tough interface of nanofiber/polymer effectively disperses stress, enhancing the mechanical properties of the polymer matrix; while the K2 Ti6 O13 can absorb high-energy UV photons and transform them into less harmful heat, thereby improving the UV stabilities. Taking poly(ethylene oxide) radiative cooler as an example for demonstration, its Young's modulus and UV resistance increase by 7 and 12 times, respectively. Consequently, the solar reflectance of nanocomposite poly(ethylene oxide) is maintained as constant in a continuous aging test for 720 h under outdoor sunlight. The work provides a general strategy to simultaneously enhance both the mechanical stability and the UV durability of polymer-based PDRC materials toward large-scale applications.

Highly Sensitive Ultrastable Electrochemical Sensor Enabled by Proton-Coupled Electron Transfer.

Electrochemical sensors are critical to artificial intelligence by virtue of capability of mimicking human skin to report sensing signals. But their practical applications are restricted by low sensitivity and limited cycling stability, which result from piezoionic mechanism with insufficient sensing response. Here, we report a highly sensitive ultrastable sensor based on proton-coupled electron transfer, which is different from piezoionic mechanism. The sensor gives a high sensing signal output of 117 mV, which is 16 times higher than that of counterpart device (7 mV). It delivers excellent working stability with performance retention as high as 99.13% over 10 000 bending cycles in air, exceeding that of the best-known sensors reported previously. The flexible sensor displays high sensitivity in detecting real-time signals of human activities with large and subtle deformations, including wrist bending, moving speed, pulse wave and voice vibration. Smart functions, such as braille language and handwriting recognitions, are demonstrated for artificial intelligence.

Curvature-controlled delamination patterns of thin films on spherical substrates.

Periodic delamination patterns in multilayer structures have exhibited extensive applications in microelectronics and optics devices. However, delamination behaviors of a closed thin shell on spherical substrates are still elusive. Herein, a unique instability mechanism of buckle delamination in a closed thin film weakly bonded to spherical substrates is studied by experiments, simulations, and theoretical analyses. The system of an Al film depositing on polystyrene spheres subjected to thermal mismatch strain is used for demonstration. Unlike traditional phenomena of wrinkling and wrinkle-induced delamination under increasing misfit strain, the weak adhesion between the core and shell results in a periodic pattern of delaminated hexagonal dimples that emerges directly from the smooth sphere configuration, before which no wrinkling occurs. Both substrate curvature and interfacial adhesion are revealed to control the dimple size and delamination width. These findings open a new venue for manifesting new controllable features for surface microfabrication.

MnO2 Synergized with N/S Codoped Graphene as a Flexible Cathode Efficient Electrocatalyst for Advanced Honeycomb-Shaped Stretchable Aluminum-Air Batteries.

Aluminum-air batteries possess high theoretical specific capacities and energy densities. However, the desired application performance in the field of flexible electronics is limited by the rigid battery structure and slow kinetics of the oxygen reduction reaction (ORR). To address these issues, flexible, stretchable, and customizable aluminum-air batteries with a reference to honeycomb shape are composed of multilayer single battery units to achieve large scalability and start-stop control. The single aluminum-air battery combines MnO2 with N/S codoped graphene to improve the electrocatalytic activity. Benefiting from an efficient electrocatalyst and reasonable structural design, the single aluminum-air battery exhibits excellent electrochemical characteristics under deformation conditions with a high specific capacity and energy density (1203.2 mAh g-1 Al and 1630.1 mWh g-1 Al). Furthermore, the obtained honeycomb-shaped stretchable aluminum-air batteries maintain a stable output voltage over the 2500% stretching. More interestingly, the stretchable honeycomb structure not only can solve the start-stop control problem but also has the potential to reduce the self-corrosion in disposable metal-air batteries. In addition, owing to the customizable shapes and sizes, the honeycomb-shaped stretchable aluminum-air batteries facilitate the integrated application of flexible batteries in wearables.

Surface buckling delamination patterns of film on soft spherical substrates.

This study aims to provide a fundamental understanding of the morphological transition of film buckling-delamination in an elastomeric bilayer spherical shell system. We developed an experimental system in which surface delamination buckles emerge because of biaxial compression of the elastomeric bilayer spherical shell driven by an air-pressured (pneumatic) device. A flat PDMS plate was first isotropically expanded and shaped into a hemisphere by air pressure. Subsequently, the hemisphere substrate was covered with a thin PDMS film. By releasing the air pressure, the substrate contracts and the outer film surface were subjected to biaxial compression; this resulted in various surface patterns of film buckling-delamination. It was found that the surface morphology transitions from initial delamination sites and that the buckles propagate on the entire surface of the sphere. This pattern formation is dependent on the surface strain distribution, i.e., radial strain and circumferential strain. In order to control the surface pattern, we systematically changed the material and system parameters such as film thickness, Young's modulus, and interfacial adhesion condition. In addition, finite element (FEM) computation was carried out to simulate the surface pattern and to elucidate the mechanism of buckling-delamination morphological transition.

A Nano-shield Design for Separators to Resist Dendrite Formation in Lithium-Metal Batteries.

Lithium dendrite growth during repeated charge and discharge cycles of lithium-metal anodes often leads to short-circuiting by puncturing the porous separator. Here, a morphological design, the nano-shield, for separators to resist dendrites is presented. Through both mechanical analysis and experiment, it is revealed that the separator protected by the nano-shield can effectively inhibit the penetration of lithium dendrites owing to the reduced stress intensity generated and therefore mitigate the short circuit of Li metal batteries. More than 110 h of lithium plating life is achieved in cell tests, which is among the longest cycle life of lithium metal anode and five times longer than that of blank separators. This new aspect of morphological and mechanical design not only provides an alternative pathway for extending lifetime of lithium metal anodes, but also sheds light on the role of separator engineering for various electrochemical energy storage devices.

Nacre-Inspired Composite Electrolytes for Load-Bearing Solid-State Lithium-Metal Batteries.

Solid-state lithium-metal batteries with solid electrolytes are promising for next-generation energy-storage devices. However, it remains challenging to develop solid electrolytes that are both mechanically robust and strong against external mechanical load, due to the brittleness of ceramic electrolytes and the softness of polymer electrolytes. Herein, a nacre-inspired design of ceramic/polymer solid composite electrolytes with a "brick-and-mortar" microstructure is proposed. The nacre-like ceramic/polymer electrolyte (NCPE) simultaneously possesses a much higher fracture strain (1.1%) than pure ceramic electrolytes (0.13%) and a much larger ultimate flexural modulus (7.8 GPa) than pure polymer electrolytes (20 MPa). The electrochemical performance of NCPE is also much better than pure ceramic or polymer electrolytes, especially under mechanical load. A 5 × 5 cm2 pouch cell with LAGP/poly(ether-acrylate) NCPE exhibits stable cycling with a capacity retention of 95.6% over 100 cycles at room temperature, even undergoes a large point load of 10 N. In contrast, cells based on pure ceramic and pure polymer electrolyte show poor cycle life. The NCPE provides a new design for solid composite electrolyte and opens up new possibilities for future solid-state lithium-metal batteries and structural energy storage.

Interaction between mechanosensitive channels embedded in lipid membrane.

The study of the gating mechanism of mechanosensitive channels opens a window to the exploration of how different mechanical stimuli induce adaptive cellular behaviors of both the protein and the lipid, across different time and length scales. In this work, through a molecular dynamics-decorated finite element method (MDeFEM), the gating behavior of mechanosensitive channels of small conductance (MscS) in Escherichia coli (E. coli) is studied upon membrane stretch or global bending. The local membrane curvature around MscS is incorporated, as well as multiple MscL (mechanosensitive channels of large conductance) molecules in proximity to MscS. The local membrane curvature is found to delay MscS opening and diminishes moderately upon membrane stretching. Mimicking the insertion of lysophosphatidylcholine (LPC) molecules into the lipid, both downward and upward bending can active MscS, as long as the global membrane curvature radius reaches 34 nm. Based on the different MscS pore evolutions observed with the presence of one or more MscLs nearby, we propose that when coreconstituted, multiple MscL molecules tend to be located at the local membrane curvature zone around MscS. In another word, as MscL "swims around" in the lipid bilayer, it can be trapped by the membrane's local curvature. Collectively, the current study provides valuable insights into the interplay between mechanosensitive channels and lipid membrane at structural and physical levels, and specific predictions are proposed for further experimental investigations.

Strain-Guided Oxidative Nanoperforation on Graphene.

Increased applications of nanoporous graphene in nanoelectronics and membrane separations require ordered and precise perforation of graphene, whose scalablility and time/cost effectiveness represent a significant challenge in existing nanoperforation methods, such as catalytical etching and lithography. A strain-guided perforation of graphene through oxidative etching is reported, where nanopores nucleate selectively at the bulges induced by the prepatterned nanoprotrusions underneath. Using reactive molecular dynamics and theoretical models, the perforation mechanisms are uncovered through the relationship between bulge-induced strain and enhanced etching reactivity. Parallel experiments of chemical vapor deposition (CVD) of graphene on SiO2 NPs/SiO2 substrates verify the feasibility of such strain-guided perforation and evolution of pore size by exposure of varied durations to oxygen plasma. This scalable method can be feasibly applied to a broad variety of 2D materials (e.g., graphene and h-boron nitride) and nanoprotrusions (e.g., SiO2 and C60 nanoparticles), allowing rational fabrication of 2D material-based devices.

Buckling morphology of an elastic ring confined in an annular channel.

This paper studies the buckling morphology transition of an elastic ring confined in an annular channel. Under uniform axial strain, the ring would first form one inward blister and then transit to an "S" shape, but does not induce more blisters due to an energy barrier caused by the annular shape of the channel. In order to overcome the energy barrier, external perturbation is employed and a stable morphology with multiple blisters may be obtained. A theoretical framework is then established to calculate the bifurcation points of the shape transition, which agrees well with finite-element (FEM) simulation results. The diagrams of the stable buckling morphologies with respect to the geometrics of the elastic rings are presented, which may provide useful insights for practical applications, for example, the design of a peristaltic pump.

Mechanical modeling of pimple growth.

Pimple is one of the most common skin diseases for humans, whose growth cause pain yet the corresponding mechanical analysis is lacking. A finite element model is developed to quantify the deformation field with the expansion of follicle, and then the mechanical stimulus is related to the sensation of pain during the development of pimple. Parametric studies show the dependence of mechanical stimulus and pain level on the pimple-surrounded structures, follicle depth and mechanical properties of the epidermis. The findings in this paper may provide useful insights on prevention or pain mitigation of pimples, as well as those related to other tissue growth and respective cosmetic concerns.

Rapid Programmable Nanodroplet Motion on a Strain-Gradient Surface.

When a nanodroplet is placed on a lattice surface, an inhomogeneous surface strain field perturbs the balance of van der Waals force between the nanodroplet and surface, thus providing a net driving force for nanodroplet motion. Using molecular dynamics and theoretical analysis, we study the effect of strain gradient on modulating the movement of a nanodroplet. Both modeling and simulation show that the driving force is opposite to the direction of strain gradient, with a magnitude that is proportional to the strain gradient as well as nanodroplet size. Two representative surfaces, graphene and copper (111) plane, are exemplified to demonstrate the controllable motion of the nanodroplet. When the substrate undergoes various types of reversible deformations, multiple motion modes of nanodroplets can be feasibly achieved, including acceleration, deceleration, and turning, becoming a facile strategy to manipulate nanodroplets along a designed two-dimensional pathway.

Gating and inactivation of mechanosensitive channels of small conductance: A continuum mechanics study.

Mechanosensitive channels of small conductance (MscS) in Escherichia coli (E. coli) serve as a paradigm for understanding the gating behaviors of the MscS family of ion channels. In this work, we develop a continuum mechanics framework to explore the conformational states of MscS during the gating transition. A complete gating transition trajectory from the closed to the open state along with partially open intermediates is obtained, and the open structure is close to the available structural model from crystallographic studies. The computational efficiency of the modeling framework makes it possible to explore the roles of various structural elements (e.g., loops that connect transmembrane helices) and specific interactions in the gating transition. It is observed that removing either the Asp62-Arg131 salt bridge or the Phe68-Leu111 non-polar interaction leads to essentially non-conducting structures even with a membrane tension close to the lysis limit. The loop connecting TM2 (the second transmembrane helix) and TM3 is found to be essential for force transmission during gating, while the loop connecting TM1 and TM2 does not make any major contribution. Based on the different structural evolutions observed when the TM3 kink is treated as a loop or a helical segment, we propose that the helical propensity of the kink plays a central role in inactivation; i.e., under prolonged sub-threshold membrane tension, transition of the initially flexible loop to a helical segment in TM3 may lead to MscS inactivation. Finally, the gating transition of MscS under different transmembrane voltages is explored and found to be essentially voltage independent. Collectively, results from the current continuum mechanics analysis provide further insights into the gating transition of MscS at structural and physical levels, and specific predictions are proposed for further experimental investigations.

Three-dimensional auxetic properties in group V-VI binary monolayer crystals X3M2 (X = S, Se; M = N, P, As).

The mechanical behaviors of a series of two-dimensional (2-D) crystals X3M2 (X = S, Se; M = N, P, As) are explored through density functional theory (DFT) calculations. Two atomic structures exist for 2-D X3M2, namely α-heart and ß-heart, which exhibit different out-of-plane Negative Poisson's Ratio (NPR) phenomena. For α-heart structures, a general out-of-plane NPR exists under in-plane tension in the y direction, while for ß-heart type structures, a general NPR exists under in-plane tension in the x direction. Through DFT calculations, we revealed that a NPR is a general phenomenon for 2-D X3M2 with heart-shaped atomic structures, and the absolute value of the out-of-plane NPR for 2-D X3M2 is always one order of magnitude larger than other 2-D crystals reported before. At the same time, the in-plane NPR at certain angles around two in-plane principal axes also exists for 2-D X3M2 except for S3P2 due to the special heart-shaped structure. The coexistence of out-of-plane and in-plane NPRs for 2-D crystals has not been reported before and these special three-dimensional (3-D) auxetic properties in 2-D crystals may enormously enrich research studies on novel mechanical, electrical and chemical behaviors of 2-D crystals. This discovery will extend the family of 2-D crystals with a NPR and also provide new insights for designing novel negative Poisson's ratio materials.

Humidity effect on ion behaviors of moisture-driven CO2 sorbents.

Ion hydration is a fundamental process in many natural phenomena. This paper presents a quantitative analysis, based on atomistic modeling, of the behavior of ions and the impact of hydration in a novel CO2 sorbent. We explore moisture-driven CO2 sorbents focusing on diffusion of ions and the structure of ion hydration complexes forming inside water-laden resin structures. We show that the stability of the carbonate ion is reduced as the water content of the resin is lowered. As the hydration cloud of the carbonate ion shrinks, it becomes energetically favorable to split a remaining water molecule and form a bicarbonate ion plus a hydroxide ion. These two ions bind less water than a single, doubly charged carbonate ion. As a result, under relatively dry conditions, more OH- ions are available to capture CO2 than in the presence of high humidity. Local concentrations of dissolved inorganic carbon and water determine chemical equilibria. Reaction kinetics is then driven to a large extent by diffusion rates that allow water and anions to move through the resin structure. Understanding the basic mechanics of chemical equilibria and transport may help us to rationally design next-generation efficient moisture-driven CO2 sorbents.

PVDF/Palygorskite Nanowire Composite Electrolyte for 4 V Rechargeable Lithium Batteries with High Energy Density.

Solid electrolytes are crucial for the development of solid state batteries. Among different types of solid electrolytes, poly(ethylene oxide) (PEO)-based polymer electrolytes have attracted extensive attention owing to their excellent flexibility and easiness for processing. However, their relatively low ionic conductivities and electrochemical instability above 4 V limit their applications in batteries with high energy density. Herein, we prepared poly(vinylidene fluoride) (PVDF) polymer electrolytes with an organic plasticizer, which possesses compatibility with 4 V cathode and high ionic conductivity (1.2 × 10-4 S/cm) at room temperature. We also revealed the importance of plasticizer content to the ionic conductivity. To address weak mechanical strength of the PVDF electrolyte with plasticizer, we introduced palygorskite ((Mg,Al)2Si4O10(OH)) nanowires as a new ceramic filler to form composite solid electrolytes (CPE), which greatly enhances both stiffness and toughness of PVDF-based polymer electrolyte. With 5 wt % of palygorskite nanowires, not only does the elastic modulus of PVDF CPE increase from 9.0 to 96 MPa but also its yield stress is enhanced by 200%. Moreover, numerical modeling uncovers that the strong nanowire-polymer interaction and cross-linking network of nanowires are responsible for such significant enhancement in mechanically robustness. The addition of 5% palygorskite nanowires also enhances transference number of Li+ from 0.21 to 0.54 due to interaction between palygorskite and ClO4- ions. We further demonstrate full cells based on Li(Ni1/3Mn1/3Co1/3)O2 (NMC111) cathode, PVDF/palygorskite CPE, and lithium anode, which can be cycled over 200 times at 0.3 C, with 97% capacity retention. Moreover, the PVDF matrix is much less flammable than PEO electrolytes. Our work illustrates that the PVDF/palygorskite CPE is a promising electrolyte for solid state batteries.

Oxidation-induced negative Poisson's ratio of phosphorene.

Mechanical properties of phosphorene oxides are investigated by using density functional theory calculations. Intriguingly, as the oxygen coverage approaches 50%, negative Poisson's ratios are found in the in-plane principal directions, originating from the asymmetric atomic structure upon oxygen adsorption. It is demonstrated that the structure of phosphorene oxide is largely weakened compared with that of pristine phosphorene. Our work indicates that certain unconventional mechanical properties, such as the negative Poisson's ratio, can be achieved by tuning the atomic structure through simple processes, which offers a new avenue to design specific properties at the nanoscale.

Bioinspired, Spine-Like, Flexible, Rechargeable Lithium-Ion Batteries with High Energy Density.

The rapid development of flexible and wearable electronics proposes the persistent requirements of high-performance flexible batteries. Much progress has been achieved recently, but how to obtain remarkable flexibility and high energy density simultaneously remains a great challenge. Here, a facile and scalable approach to fabricate spine-like flexible lithium-ion batteries is reported. A thick, rigid segment to store energy through winding the electrodes corresponds to the vertebra of animals, while a thin, unwound, and flexible part acts as marrow to interconnect all vertebra-like stacks together, providing excellent flexibility for the whole battery. As the volume of the rigid electrode part is significantly larger than the flexible interconnection, the energy density of such a flexible battery can be over 85% of that in conventional packing. A nonoptimized flexible cell with an energy density of 242 Wh L-1 is demonstrated with packaging considered, which is 86.1% of a standard prismatic cell using the same components. The cell also successfully survives a harsh dynamic mechanical load test due to this rational bioinspired design. Mechanical simulation results uncover the underlying mechanism: the maximum strain in the reported design (≈0.08%) is markedly smaller than traditional stacked cells (≈1.1%). This new approach offers great promise for applications in flexible devices.

Closed-edged bilayer phosphorene nanoribbons producing from collapsing armchair phosphorene nanotubes.

A new phosphorous allotrope, closed-edged bilayer phosphorene nanoribbon, is proposed via radially deforming armchair phosphorene nanotubes. Using molecular dynamics simulations, the transformation pathway from round PNTs falls into two types of collapsed structures: arc-like and sigmoidal bilayer nanoribbons, dependent on the number of phosphorene unit cells. The fabricated nanoribbions are energetically more stable than their parent nanotubes. It is also found via ab initio calculations that the band structure along tube axis substantially changes with the structural transformation. The direct-to-indirect transition of band gap is highlighted when collapsing into the arc-like nanoribbons but not the sigmoidal ones. Furthermore, the band gaps of these two types of nanoribbons show significant size-dependence of the nanoribbon width, indicative of wider tunability of their electrical properties.