Carbon Nanotube Network Topology-Enhanced Iontronic Capacitive Pressure Sensor with High Linearity and Ultrahigh Sensitivity.

Flexible capacitive pressure sensors with high sensitivity over a wide pressure range are highly anticipated in the fields of tactile perception and physiological signal monitoring. However, despite the introduction of microstructures on the electrolyte layer, the deformability is still limited due to the size limitation of the microstructures, making it difficult to significantly improve the sensitivity of iontronic capacitive pressure sensors (ICPSs). Here, we propose an innovative strategy of combining carbon nanotubes (CNTs) topological networks and ionic hydrogel micropyramid array microstructures that can significantly enhance the sensitivity of flexible ICPSs for ultrasensitive pressure detection. Compared with other previously reported ICPSs, the sensor developed in this work exhibits an unprecedented sensitivity (Smin > 1050 kPa-1) and a high linear response (R2 > 0.99) in a wide pressure range (0.03-28 kPa) enabled by CNT percolation networks inside the microstructred electrolyte layer. This ultrasensitive and flexible ICPS also can effectively detect pressure from a variety of sources, including sound waves, lightweight objects, and tiny physiological signals, such as pulse rate and heartbeat. This work provides a general strategy to achieve an ICPS with both broader pressure-response range and higher sensitivity, which provides a stable and efficient way for a low-cost, scalable sensor for sensitive tactile sensing in human-computer interaction applications.

Fabrication and Characterization of a Novel Smart-Polymer Actuator with Nanodispersed CNT/Pd Composite Interfacial Electrodes.

As emerging smart polymers, ionic polymer-metal composites (IPMCs) are playing more and more important roles as promising candidates for next-generation actuators in terms of academic interest and industrial applications. It is reported that the actuation behaviors of IPMCs are dependent on the electrochemical kinetic process between metal/polymer interfaces to a great extent. Thus, the fabrication of tailored metal/polymer interface electrodes with large surface areas and superior interface characteristics is highly desirable in improving the actuation performance of IPMCs, which is still technologically critical for IPMCs. In this contribution, we developed a novel fabrication technology for carbon/metal composite electrodes with a superior interface structure and characteristics to optimize the actuation behaviors of IPMCs by exploiting the synergistic effect of combining a sulfonated multi-walled carbon nanotube (SCNT)/Nafion hybrid layer with nanodispersed Pd particles. The improved IPMCs showed significantly enhanced capacitance characteristics and highly facilitated charge-discharge processes. Moreover, their actuation behaviors were greatly improved as expected, including approximately 2.5 times larger displacement, 3 times faster deformation speed, 4 times greater output force, and 10 times higher volume work density compared to those of the IPMCs with traditional electrode structures. The advantages of the developed SCNT/Pd-IPMCs will greatly facilitate their applicability for artificial muscles.

Multiresponsive Ti3C2Tx MXene-Based Actuators Enabled by Dual-Mechanism Synergism for Soft Robotics.

Multiresponsive and high-performance flexible actuators with a simple configuration, high mechanical strength, and low-power consumption are highly desirable for soft robotics. Here, a novel mechanically robust and multiresponsive Ti3C2Tx MXene-based actuator with high actuation performance via dual-mechanism synergistic effect driven by the hygroexpansion of bacterial cellulose (BC) layer and the thermal expansion of biaxially oriented polypropylene (BOPP) layer is developed. The actuator is flexible and shows an ultrahigh tensile strength of 195 MPa. Unlike the conventional bimorph-structured actuators based on a single-mechanism, the actuator developed provides a favorable architecture for dual-mechanism synergism, resulting in exceptionally reversible actuation performance under electricity and near-infrared (NIR) light stimuli. Typically, the developed actuator can produce the largest bending angle (∼400°) at the lowest voltage (≤4 V) compared with that reported previously for single mechanism soft actuators. Furthermore, the actuator also can be driven by a NIR light at a 2 m distance, displaying an excellent long-distance photoresponsive property. Finally, various intriguing applications are demonstrated to show the great potential of the actuator for soft robotics.

Three-Fingered Soft Pneumatic Gripper Integrating Joint-Tuning Capability.

Compared with traditional rigid grippers, soft grippers are made of lightweight and soft materials and have the characteristics of flexible contact and strong adaptability, which are widely utilized to grasp fragile objects with complex contours and shapes. In this article, we design and fabricate a three-fingered stiffness-tunable soft gripper by integrating the joint-tuning capability. The soft fingers are composed of an internal bending actuator and an external fiber-jamming jacket, under an actuation of pneumatic pressure. Static and kinematic models are established to detect the bending angle and end trajectory of the internal bending actuator. Meanwhile, the bending angle and blocking force of bending actuator are experimentally measured and are comparably analyzed with the theoretical predictions. Jamming pressure is applied in the stiffness-tunable jacket to explore the variable stiffness and load-carrying capability of the soft finger. By incorporating the stiffness-tunable property, the grasping performance of various weights and types of goods, as well as the maximum grasping force of the soft gripper, is investigated. Finally, by patterning the stiffness-tunable jacket on the bending actuator, the variable curvature bending deformation and joint-tuning capability of the soft finger are achieved. This proposed soft gripper holds great potential applications in soft robotics community.

Enhancing of broadband sound absorption through soft matter.

This paper proposed a metamaterial design method that uses soft matter for constructing a unique soft acoustic boundary to effectively improve broadband sound absorption performance. Specifically, attaching a flexible polyvinyl chloride (PVC) gel layer with an elastic modulus as low as a few kilopascals and a thickness of a few millimeters to the inner wall of a cavity-type sound-absorbing metamaterial structure significantly improved the absorption performance of the composite structure in low-frequency broadband ranges. The sound absorption enhancement mechanism differed from those proposed in previous research. On the one hand, the soft PVC gel layer acted as a soft acoustic boundary, substantially reducing the sound speed and reflection and producing considerable elastic strain energy at the interface between two different media to improve the sound absorption performance. On the other hand, this PVC gel layer displayed extremely low stiffness and high damping, producing an abundance of plasmon-like resonance modes in low-frequency broadband ranges, achieving a resonance absorption effect. Since this sound absorption enhancement method did not require an increase in the external dimensions or a change in the structural parameters of the original absorber and achieved robust enhancement in a wide frequency band, it displayed potential application value in various engineering fields.

One-Step Synthesis of Microdome Patterns for Microstructured Pressure Sensors with Ultra-High Sensing Performance.

Pressure sensors usually suffer from a trade-off between sensitivity and the linear sensing range, which may be improved by manipulating the geometric microstructure of active sensing materials via the molding strategy, standard photolithography technique, and so on. However, these conventional microengineering techniques require specialized equipment, which are extremely complicated, high-cost, and time-consuming to manufacture. Herein, a mold-free, scalable, low-cost, and environment-friendly one-step thermofoaming strategy is proposed to fabricate surface morphology-tunable microdome-patterned composites (MPCs). The microstructured pressure sensor is then prepared by coating the MPCs with highly conductive graphene. Remarkably, the as-prepared pressure sensor presents a better overall sensing performance compared to the previous pressure sensors prepared using complicated microengineering methods. Moreover, an electromechanical response model and finite-element analysis are used to clarify the sensing mechanisms of the present microstructured pressure sensor. Furthermore, several successful application demonstrations are conducted under various pressure levels. Considering the advantages of the one-step fabrication strategy over conventional surface microengineering techniques and the high performance of the microstructured pressure sensor, the present pressure sensor has promising potential applications in health monitoring, tactile sensation, wearable devices, etc.

Direct Writing Corrugated PVC Gel Artificial Muscle via Multi-Material Printing Processes.

Electroactive PVC gel is a new artificial muscle material with good performance that can mimic the movement of biological muscle in an electric field. However, traditional manufacturing methods, such as casting, prevent the broad application of this promising material because they cannot achieve the integration of the PVC gel electrode and core layer, and at the same time, it is difficult to create complex and diverse structures. In this study, a multi-material, integrated direct writing method is proposed to fabricate corrugated PVC gel artificial muscle. Inks with suitable rheological properties were developed for printing four functional layers, including core layers, electrode layers, sacrificial layers, and insulating layers, with different characteristics. The curing conditions of the printed CNT/SMP inks under different applied conditions were also discussed. The structural parameters were optimized to improve the actuating performance of the PVC gel artificial muscle. The corrugated PVC gel with a span of 1.6 mm had the best actuating performance. Finally, we printed three layers of corrugated PVC gel artificial muscle with good actuating performance. The proposed method can help to solve the inherent shortcomings of traditional manufacturing methods of PVC gel actuators. The printed structures have potential applications in many fields, such as soft robotics and flexible electronic devices.

3D-printed construct from hybrid suspension as spatially and temporally controlled protein delivery system.

Protein delivery systems have been extensively applied in controlled releasing of protein or polypeptides for therapeutic treatment or tissue regeneration. While 3 D printing technology shows great promise in novel dosage form with tailoring dose size and drug release profile, 3 D printable protein delivery system has to face many difficult challenges. In this study, we developed a hybrid suspension combining Eudragit polyacrylate colloid as matrix material and Pluronic polyether hydrogel as diffusion channel for protein release. This hybrid suspension can be 3 D-printed into construct with complex shape and inner structures thanks to its pseudoplastic and thixotropic rheological properties. The protein can be incorporated in hybrid suspension either in its original or nanoparticle capsulated form. The experiment shows that the protein release from construct is a function of drying time, molecular weight (MW) of chitosan, as well as their own structural/diffusional properties. Also, the theoretical derivation suggests polyacrylate matrix tortuosity, chitosan erosion rate as well as hydrogel diffusion coefficient all contributed to the extended duration of release profile. In addition, cytotoxicity test through cell culture confirmed that the construct fabricated from hybrid suspension exhibit relative good bio-compatibility. Finally, heterogeneous constructs with zoned design were fabricated as protein delivery system, which demonstrated the capability of hybrid suspension technique for spatial and temporal release of macromolecular drugs to realize pharmaceutical effectiveness or guild cell organization.

Silicone-Ionic Liquid Elastomer Composite with Keratin as Reinforcing Agent Utilized as Pressure Sensor.

Development of a flexible pressure sensor is crucial for the future improvement of the wearable electronic devices designed to detect dynamic human motion. In this study, a novel pressure sensor with remarkably improved force sensing characteristics is obtained through combined usage of polydimethylsiloxane (PDMS) and ionic liquid (IL). Keratin is dispersed homogeneously in the PDMS matrix to serve as a reinforcing filler. High conductivity IL is employed as sensitivity-enhancing constituent in the elastomer, and the effect of the amount of IL on elastomers' pressure-sensing performance is investigated. The elastomer with 70 parts per hundred rubber (phr) IL shows excellent pressure-sensing performance. This novel pressure sensor demonstrates high linear sensitivity (0.037 kPa-1 ) in the large pressure region of 0-10 kPa. Response and recovery times are 8 and 11 ms, respectively, which are much shorter than previously reported. Moreover, the pressure sensor could distinguish different pressures via stable sensing signals in the pressure range of 0 to 50 kPa. The excellent performance of the novel pressure sensor has application potential in various fields, such as health monitoring and soft robotics.

Progress of low-frequency sound absorption research utilizing intelligent materials and acoustic metamaterials.

In recent years, increasing attention has been paid to the impacts of environmental noises on living creatures as well as the accuracy and stability of precise instruments. Due to inherent properties induced by large wavelength, the attenuation and manipulation of low-frequency sound waves is quite difficult to realize with traditional acoustic absorbers, yet particularly critical to modern designs. The advent of acoustic metamaterials and intelligent materials provides possibilities of energy dissipation mechanisms other than viscous dissipation and heat conduction in conventional porous sound absorbers, and therefore inspires new strategies on the design of subwavelength-scale structures. This short review aims to trace the current advancement on the low-frequency sound absorption research utilizing intelligent materials and metamaterials, including Helmholtz resonators and acoustic metamaterials based on micro-perforated plates, porous media, and decorated membrane, along with the tunable absorbing structures regulated with the function of electroactive polymers or magnetically sensitive materials. The effective principles and prospects were concluded and presented for future investigations of subwavelength-scale acoustic structures.

Rapid preparation of a Nafion/Ag NW composite film and its humidity sensing effect.

In this study, we present a novel humidity-responsive composite film based on the integration of two silver nanowire (Ag NW) layers deposited via the physical deposition process with an ionomer (Nafion) layer sandwiched between them. As a result of the ion migration inside the ionomer, the humidity to electrical transduction displays a fast response and high sensitivity in comparison with previous research. Meanwhile, the effects of the humidity gradient on the sensing response of the Nafion/Ag NWs composite film were investigated. Specifically, a higher humidity enables a faster response rate but a slower recovery rate, as compared to a lower humidity level. In addition, breathing tests are performed, which verified the opportunities for the design and fabrication of high-performance sensors for environment and biophysical monitoring. This research also revealed the potential applications in VOC (volatile organic compound) detection.

The Effects of Dimensions on the Deformation Sensing Performance of Ionic Polymer-Metal Composites.

As an excellent transducer, ionic polymer-metal composites (IPMCs) can act as both an actuator and a sensor. During its sensing process, many factors, such as the water content, the cation type, the surface electrode, and the dimensions of the IPMC sample, have a considerable impact on the IPMC sensing performance. In this paper, the effect of dimensions focused on the Pd-Au typed IPMC samples with various thicknesses, widths, and lengths that were fabricated and their deformation sensing performances were tested and estimated using a self-made electromechanical sensing platform. In our experiments, we employed a two-sensing mode (both current and voltage) to record the signals generated by the IPMC bending. By comparison, it was found that the response trend was closer to the applied deformation curve using the voltage-sensing mode. The following conclusions were obtained. As the thickness increased, IPMC exhibited a better deformation-sensing performance. The thickness of the sample changed from 50 µm to 500 µm and corresponded to a voltage response signal from 0.3 to 1.6 mV. On the contrary, as the length increased, the sensing performance of IPMC decreased when subjected to equal bending. The width displayed a weaker effect on the sensing response. In order to obtain a stronger sensing response, a thickness increase, together with a length reduction, of the IPMC sample is a feasible way. Also, a simplified static model was proposed to successfully explain the sensing properties of IPMC with various sizes.

A Compact Review of IPMC as Soft Actuator and Sensor: Current Trends, Challenges, and Potential Solutions From Our Recent Work.

Recently, attempts have been made to develop ionic polymer-metal composite (IPMC), which is garnering growing interest for ionic artificial muscle, as a soft actuator and sensor due to its inherent properties of low weight, flexibility, softness, and particularly, its efficient transformation of electrical energy into mechanical energy, with large bending strain response under a low activation voltage. In this paper, we focused on several current deficiencies of IPMC that restrict its application, such as non-standardized preparation steps, relaxation under DC voltage, solvent evaporation, and poor output force. Corresponding solutions to overcome the abovementioned problems have recently been proposed from our point of view and developed through our research. After a brief introduction to the working mechanism of IPMC, we here investigate the key factors that influence the actuating performance of IPMC. We also review the optimization strategies in IPMC actuation, including those for preparation steps, additive selection for a thick casting membrane, solvent substitutes, water content, encapsulation, etc. With consideration of the role of the interface electrode, its effects on the performance of IPMC are revealed based on our previous work. Finally, we also discuss IPMCs as potential sensors theoretically and experimentally. The elimination of the deficiencies of IPMC will promote its applications in soft robotics.

Printing Carbon Nanotube-Embedded Silicone Elastomers via Direct Writing.

Direct writing techniques for the printing of colloidal multiwalled carbon nanotubes (CNTs) embedded in polydimethylsiloxane (PDMS) were developed herein to fabricate complex structures including woodpiles, tetragonal scaffolds, and gradient mesh structures. The multiwalled CNTs served as a conductive filler and thickening agent for the printing ink. A suitable rheological behavior was obtained by mixing the CNTs with PDMS dissolved in an isopropyl alcohol solvent. A 7 wt % CNT loading in the PDMS was optimum for printing gap-spanning features at a nozzle moving speed of 20 mm/s. The printed structures, including a woodpile and gradient mesh structure, were capable of detecting changes in external mechanical pressure. Printed CNT/PDMS strips exhibit electrical actuation with good mechanical performance (strain of 8.9%) at a low actuation voltage (60 V). The performance characterization and application display demonstrated the possibility of developing custom complex CNT/PDMS structures for a broad range of applications, including soft robots and flexible electronic devices.

Tunable actuation behavior of ionic polymer metal composite utilizing carboxylated carbon nanotube-doped Nafion matrix.

In this study, we propose to neutralize the relaxation deformation of Nafion-ionic polymer metal composite (IPMC) by slow anode deformation of Flemion-IPMC caused by carboxyl groups (-COOH). Carboxylated carbon nanotubes (CCNT) as -COOH carriers were doped into a Nafion matrix. By adjusting the doping content from 0 wt% to 10 wt%, an IPMC with constant steady-state deformation has been achieved at a critical CCNT content of 2 wt%. Moreover, the increasing rate of the slow anode deformation with the CCNT content is tunable, which is found to be 2.26 mm s-1 %-1.

Influence of Ambient Humidity on the Voltage Response of Ionic Polymer-Metal Composite Sensor.

Electrical potential based on ion migration exists not only in natural systems but also in ionic polymer materials. In order to investigate the influence of ambient humidity on voltage response, classical Au-Nafion IPMC was chosen as the reference sample. Voltage response under a bending deformation was measured in two ways: first, continuous measurement of voltage response in the process of absorption and desorption of water to study the tendency of voltage variation at all water states; second, measurements at multiple fixed ambient humidity levels to characterize the process of voltage response quantitatively. Ambient humidity influences the voltage response mainly by varying water content in ionic polymer. Under a step bending, the amplitude of initial voltage peak first increases and then decreases as the ambient humidity and the inherent water content decrease. This tendency is explained semiquantitatively by mass storage capacity related to the stretchable state of the Nafion polymer network. Following the initial peak, the voltage shows a slow decay to a steady state, which is first characterized in this paper. The relative voltage decay during the steady state always decreases as the ambient humidity is lowered. It is ascribed to progressive increase of the ratio between the water molecules in the cation hydration shell to the free water. Under sinusoidal mechanical bending excitation in the range of 0.1-10 Hz, the voltage magnitude increases with frequency at high ambient humidity but decreases with frequency at low ambient humidity. The relationship is mainly controlled by the voltage decay effect and the response speed.

A spongy graphene based bimorph actuator with ultra-large displacement towards biomimetic application.

Bimorph actuators, consisting of two layers with asymmetric expansion and generating bending displacement, have been widely researched. Their actuation performances greatly rely on the difference of coefficients of thermal expansion (CTE) between the two material layers. Here, by introducing a spongy graphene (sG) paper with a large negative CTE as well as high electrical-to-thermal properties, an electromechanical sG/PDMS bimorph actuator is designed and fabricated, showing an ultra-large bending displacement output under low voltage stimulation (curvature of about 1.2 cm(-1) at 10 V for 3 s), a high displacement-to-length ratio (∼0.79), and vibration motion at AC voltage (up to 10 Hz), which is much larger and faster than that of the other electromechanical bimorph actuators. Based on the sG/PDMS bimorph serving as the "finger", a mechanical gripper is constructed to realize the fast manipulation of the objects under 0.1 Hz square wave voltage stimulation (0-8 V). The designed bimorph actuator coupled with ultra-large bending displacement, low driven voltage, and the ease of fabrication may open up substantial possibilities for the utilization of electromechanical actuators in practical biomimetic device applications.

Novel electromechanical actuation based on a spongy graphene paper.

A spongy graphene paper with an inside foldable corrugated structure is fabricated by electrothermal reduction of the oxygen-containing groups inside an RGO paper. Reversible macroscopical length contraction of the spongy graphene paper with a strain of 2.4% under 10 V voltage is exhibited, mostly attributed to the deformation of corrugated structures.