High-Porosity Foam-Based Iontronic Pressure Sensor with Superhigh Sensitivity of 9280 kPa^(-1)

Flexible pressure sensors with high sensitivity are desired in the fields of electronic skins,human-machine interfaces,and health monitoring.Employing ionic soft materials with microstructured architectures in the functional layer is an effective way that can enhance the amplitude of capacitance signal due to generated electron double layer and thus improve the sensitivity of capacitive-type pressure sensors.However,the requirement of specific apparatus and the complex fabrication process to build such microstructures lead to high cost and low productivity.Here,we report a simple strategy that uses open-cell polyurethane foams with high porosity as a continuous three-dimensional network skeleton to load with ionic liquid in a one-step soak process,serving as the ionic layer in iontronic pressure sensors.The high porosity(95.4%) of PU-IL composite foam shows a pretty low Young's modulus of 3.4 kPa and good compressibility.A superhigh maximum sensitivity of 9,280 kPa^(-1) in the pressure regime and a high pressure resolution of 0.125% are observed in this foam-based pressure sensor.The device also exhibits remarkable mechanical stability over 5,000 compression-release or bending-release cycles.Such high porosity of composite structure provides a simple,cost-effective and scalable way to fabricate super sensitive pressure sensor,which has prominent capability in applications of water wave detection,underwater vibration sensing,and mechanical fault monitoring.

Silver nanowires for anti-counterfeiting

Silver nanowire(AgNW)films have been widely used as flexible transparent electrodes due to the high electrical conductance and high transmittance in the visible range.The infrared(IR)properties of AgNW films,however,are seldom discussed.Here,we show that ultrathin AgNWs present high transmittance in the visible range(~95%)and high reflectance(60-70%)in the IR range of 8e14 mm.Such a significant difference makes AgNW films invisible to naked eyes but visible under an IR camera,being an ideal selection for anti-counterfeit technologies,while no excitation is required.We have demonstrated that AgNW films can be spray-coated on either flat or microstructured surfaces with high conformability and high scratch-resistance,and thus these anti-counterfeiting materials can be applied to a variety of applications.The AgNW-based anti-counterfeiting may be helpful for combating counterfeiting crimes in the fields of medicines,artwork,banknotes,brand luxuries,industrial parts,and so on.

Metallic nanostructures for light trapping in energy-harvesting devices

Solar energy is abundant and environmentally friendly.Light trapping in solar-energy-harvesting devices or structures is of critical importance.This article reviews light trapping with metallic nanostructures for thin film solar cells and selective solar absorbers.The metallic nanostructures can either be used in reducing material thickness and device cost or in improving light absorbance and thereby improving conversion efficiency.The metallic nanostructures can contribute to light trapping by scattering and increasing the path length of light,by generating strong electromagnetic field in the active layer,or by multiple reflections/absorptions.We have also discussed the adverse effect of metallic nanostructures and how to solve these problems and take full advantage of the light-trapping effect.

Iontronic pressure sensor with high sensitivity and linear response over a wide pressure range based on soft micropillared electrodes

Electronic skins and flexible pressure sensors are important devices for advanced healthcare and intelligent robotics.Sensitivity is a key parameter of flexible pressure sensors.Whereas introducing surface microstructures in a capacitive-type sensor can significantly improve its sensitivity,the signal becomes nonlinear and the pressure response range gets much narrower,significantly limiting the applications of flexible pressure sensors.Here,we designed a pressure sensor that utilizes a nanoscale iontronic interface of an ionic gel layer and a micropillared electrode,for highly linear capacitance-to-pressure response and high sensitivity over a wide pressure range.The micropillars undergo three stages of deformation upon loading:initial contact(0-6 k Pa)and structure buckling(6-12 k Pa)that exhibit a low and nonlinear response,as well as a post-buckling stage that has a high signal linearity with high sensitivity(33.16 k Pa-1)over a broad pressure range of 12-176 k Pa.The high linearity lies in the subtle balance between the structure compression and mechanical matching of the two materials at the gel-electrode interface.Our sensor has been applied in pulse detection,plantar pressure mapping,and grasp task of an artificial limb.This work provides a physical insight in achieving linear response through the design of appropriate microstructures and selection of materials with suitable modulus in flexible pressure sensors,which are potentially useful in intelligent robots and health monitoring.

A stretchable and adhesive ionic conductor based on polyacrylic acid and deep eutectic solvents

Hydrogels are a widely used ionic conductor in on-skin electronic and iontronic devices.However,hydrogels dehydrate in the open air and freeze at low temperatures,limiting their real applications when they are attached on skin or exposed to low temperatures.Polymer-ionic liquid gels can overcome these two obstacles,but synthetic ionic liquids are expensive and toxic.In this work,we present an ionic conductor based on polyacrylic acid(PAAc)and deep eutectic solvents(DESs)that well addresses the aforementioned challenges.We polymerize acrylic acid in DESs to get the PAAc–DES gel,which exhibits excellent stretchability(>1000%),high electrical conductivity(1.26 mS cm^(−1)),high adhesion to the skin(~100 Nm^(−1)),as well as good anti-drying and anti-freezing properties.We also demonstrate that the PAAc-DES gel can be used as an on-skin electrode to record the surface electromyographic signal with high signal quality,or as a transparent stretchable electrode in iontronic devices that can work at–20℃.We believe that the PAAc–DES gels are an ideal candidate as epidermal electrodes or transparent stretchable electrodes.

Sugar transfer of nanomaterials and flexible electrodes

Nanomaterials with various dimensionalities(e.g.,nanowires,nanofilms,two-dimensional materials,and three-dimensional nanostructures)have shown great potential in the recent development of flexible electronics.Conventionally,organic solvents are inevitable while integrating nanomaterials onto flexible substrates,where polymer mediator-assisted transfer techniques are involved.This often damages the flexible substrate and thus hamper the large-scale application of nanomaterials.Here we report a method using watersoluble sugar as a mediator to facilely transfer nanomaterials onto rigid or flexible substrates.This method requires no organic solvent during transfer.More importantly,the morphology and properties of transferred nanomaterials,such as shape,microstructure,resistivity,and transmittance are well preserved on the target substrate.We believe that this universal and rapid transfer method can greatly advance the applications of nanomaterials in the field of flexible devices and beyond.

Integration of Soft Electronics and Biotissues

The collection of physiological signals as well as the electrical stimulation to the biotissues are significant but challenging.There is a huge gap between the living systems and electronics.Biotissues are wet and soft,while electronics are dry and relatively stiff;biotissues conduct ions,while electronic materials often conduct electrons.As a result,forming a stable interface for bidirectional electrical communications between electronics and the living systems is difficult.In this perspective,we review recent landmark progresses made in this field,and propose a few future directions that scientists may further work on.

Atomic origin of the traps in memristive interface

In recent years, trap-related interfacial transport phenomena have received great attention owing to their potential applications in resistive switching devices and photo detectors. Not long ago, one new type of memristive interface that is composed of F-doped SnO2 and Bi2S3 nano-network layers has demonstrated a bivariate-continuous-tunable resistance with a swift response comparable to the one in neuron synapses and with a brain-like memorizing capability. However, the resistive mechanism is still not clearly understood because of lack of evidence, and the limited improvement in the development of the interfacial device. By combining I-V characterization, electron energy-loss spectroscopy, and first- principle calculation, we studied in detail the macro/micro features of the memristive interface using experimental and theoretical methods, and confirmed that its atomic origin is attributed to the traps induced by O-doping. This implies that impurity-doping might be an effective strategy for improving switching features and building new interfacial memristors.