Piezoionic Elastomers by Phase and Interface Engineering for High-Performance Energy-Harvesting Ionotronics

Bioinspired learning and memory in ionogels through fast response and slow relaxation dynamics of ions

Mimicking biological systems' sensing, learning, and memory capabilities in synthetic soft materials remains challenging. While significant progress has been made in sensory functions in ionogels, their learning and memory capabilities still lag behind biological systems. Here, we introduce cation-π interactions and a self-adaptable ionic-double-layer interface in bilayer ionogels to control ion transport. Fast ion response enables sensing and learning, while slow ion relaxation supports long-term memory. The ionogels achieve bioinspired functions, including sensitization, habituation, classical conditioning, and multimodal memory, with low energy consumption (0.06 pJ per spike). Additionally, the ionogels exhibit mechanical adaptability, such as stretchability, self-healing, and reconfigurability, making them ideal for soft robotics. Notably, the ionogels enable a robotic arm to mimic the selective capture behavior of a Venus flytrap. This work bridges the gap between biological intelligence and artificial systems, offering promising applications in bioinspired, energy-efficient sensing, learning, and memory.

Recent Advances in Polyvinylidene Fluoride with Multifunctional Properties in Nanogenerators

Amid the global energy crisis and rising emphasis on sustainability, efficient energy harvesting has become a research priority. Nanogenerators excel in converting abundant mechanical and thermal energy into electricity, offering a promising path for sustainable solutions. Among various nanogenerator's materials, Polyvinylidene fluoride (PVDF), with its distinctive molecular structure, exhibits multifunctional electrical properties including dielectric, piezoelectric and pyroelectric characteristics. These properties combined with its excellent flexibility make PVDF a prime candidate material for nanogenerators. In nanogenerators, this material is capable of efficiently collecting and converting energy. This paper discusses how PVDF's properties are manifested in three types of nanogenerators and compares the performance of these nanogenerators. In addition, strategies to improve the output performance of nanogenerators are demonstrated, including physical and chemical modification of materials, as well as structural optimization strategies such as hybrid structures and external circuits. It also introduces the application of this material in natural and human energy harvesting, as well as its promising prospects in medical technologies and smart home systems. The aim is to promote the use of PVDF in self-powered sensing, energy harvesting and smart monitoring, thereby providing valuable insights for designing more efficient and versatile nanogenerators.

Mechano-Driven Neuromimetic Logic Gates Established by Geometrically Asymmetric Hydrogel Iontronics

The human brain's neural network demonstrates exceptional efficiency in information processing and recognition, driving advancements in neuromimetic devices that emulate neuronal functions such as signal integration and parallel transmission. A key challenge remains in replicating these functions while minimizing energy consumption. Here, inspired by neuronal signal integration and axonal bidirectional transmission, mechano-driven hydrogel logic gates leveraging the piezoionic effect is presented, offering a novel bionic approach with significantly reduced power consumption. By exerting external force on the thick and thin sides of the geometrically asymmetric hydrogel, spike signals of differing amplitudes and opposite polarities can be generated, corresponding to '1' and '0', respectively. The differential mobility of anions and cations plays a crucial role in the piezoionic effect. This geometric asymmetry amplifies ion convection, improving force-to-electricity conversion efficiency, while the inclusion of salts with varying ion size can further enhance this disparity, even reversing the signal direction. Arranging asymmetric hydrogel iontronics in series-parallel configurations enables the emulation of complex neuronal logic operations, facilitating ionic spike signal addition and subtraction. This hydrogel-based logic control has been directly applied in human-machine interaction to control robot arms and offers significant potential for the advancement of artificial intelligence, robotics, and wearable technologies.

Exploration of deep operator networks for predicting the piezoionic effect

The piezoionic effect holds significant promise for revolutionizing biomedical electronics and ionic skins. However, modeling this multiphysics phenomenon remains challenging due to its high complexity and computational limitations. To address this problem, this study pioneers the application of deep operator networks to effectively model the time-dependent piezoionic effect. By leveraging a data-driven approach, our model significantly reduces computational time compared to traditional finite element analysis (FEA). In particular, we trained a DeepONet using a comprehensive dataset generated through FEA calibrated to experimental data. Through rigorous testing with step responses, slow-changing forces, and dynamic-changing forces, we show that the model captures the intricate temporal dynamics of the piezoionic effect in both the horizontal and vertical planes. This capability offers a powerful tool for real-time analysis of piezoionic phenomena, contributing to simplifying the design of tactile interfaces and potentially complementing existing tactile imaging technologies.

A Biomimetic Passive Mechanotransduction Mechanism Based on Interfacial Regulation of Ionic p-n Junctions

Natural skin receptors use ions as signal carriers, while most of the developed artificial tactile sensors utilize electrons as information carriers. To imitate the biological ionic sensing behavior, here, we present a kind of biomimetic, ionic, and fully passive mechanotransduction mechanism leveraging mechanical modulation of interfacial ionic p-n junction (IPNJ) through microchannels. Sensors based on this mechanism do not rely on an external power supply and can encode external tactile stimuli into highly analogous signal outputs to those of natural skin receptors, in terms of both signal type (i.e., ionic potential difference) and signal intensity (≈120 mV). More importantly, the instant interfacial IPNJ regulation characteristic endows the sensors with superior performance when compared to the state-of-the-art piezoionic sensors, including a low detection limit of 0.01 N, fast response/recovery speeds (16 ms/16 ms), ultralow power consumption (pW level), excellent reproducibility (over 100,000 cycles), and good capabilities to resolve both static and dynamic mechanical stimulations. As demonstrations, machine-learning-assisted high accuracy (over 99%) surface texture recognition and object classification are successfully demonstrated with the sensors integrated on robotic hands. This work enriches the family of mechanical sensing mechanisms and provides a path to mimicking natural tactile sensory systems for smart skins, artificial prostheses, and intelligent robots.

Piezoelectric-Augmented Thermoelectric Ionogels for Self-Powered Multimodal Medical Sensors

A paradigm ionogel consisting of ionic liquid (IL) and PVDF-HFP composites is made, which inherently possesses dual-function ionic thermoelectric (iTE) and piezoelectric (PE) attributes. This study investigates an innovative "PE-enhanced iTEs" effect, wherein the ionic thermopower exhibits a 58% enhancement while the ionic conductivity arises more than 2× within a PE-induced internal electric field. By harnessing these multifaceted features, fully self-powered, multimodal sensors demonstrate their superior energy conversion capabilities, which possessed minimum sensitivities of 0.13 mV kPa-1 and 0.96 mV K-1 in pressure and temperature alterations, respectively. The PE augmentation of iTEs is maximized by ≈3× under rising water pressure. Their swift and sophisticated responses to various in vivo vital signs simultaneously in a hemorrhagic shock scenario, indicative of good prospects in the clinical medicine field are showcased.

Intercellular Ion-Gradient Piezoheterogated Biphasic Gel for Ultrahigh Iontronic Generation

Piezoionic materials have attracted considerable attention for their ability to generate iontronic signals or power in response to stress stimuli. However, the limited intrinsic transport distinction between cations and anions within most ionic materials results in weakened iontronic power conversion efficiencies under stress fields. Here, we report a piezoheterogated biphasic gel for ultrahigh iontronic generation, characterized by high-internal microphase heterointerfaces that facilitate the distinct transport of various ion species. Due to the ion confinement effect of cell-like multicompartments, a stable intercellular ion gradient within biphasic gel systems can be established in situ, constructing the chemical potential to further enhance ionic transmission efficiency and obtain a high-density net ion flux in the piezoionic process. Consequently, as a reliable piezo cell, a record maximum power of 150 W/m3 over 24 h can be realized. Meanwhile, we develop piezoionic devices that can interface with paralyzed vagus nerves and successfully regulate the blood pressure of rodents through their neuromodulation. By matching the ion species with heterointerface gating effects to regulate the ionic transmission efficiency, the piezo signal neuromodulation process can be further governed. We anticipate that the bioinspired heterointerface engineering strategy can provide new insights into designing high-performance piezoionic systems for promising abiotic-biotic applications.

All-polymer piezo-ionic-electric electronics

Piezoelectric electronics possess great potential in flexible sensing and energy harvesting applications. However, they suffer from low electromechanical performance in all-organic piezoelectric systems due to the disordered and weakly-polarized interfaces. Here, we demonstrated an all-polymer piezo-ionic-electric electronics with PVDF/Nafion/PVDF (polyvinylidene difluoride) sandwich structure and regularized ion-electron interfaces. The piezoelectric effect and piezoionic effect mutually couple based on such ion-electron interfaces, endowing this electronics with the unique piezo-ionic-electric working mechanism. Further, owing to the massive interfacial accumulation of ion and electron charges, the electronics obtains a remarkable force-electric coupling enhancement. Experiments show that the electronics presents a high d33 of ~80.70 pC N-1, a pressure sensitivity of 51.50 mV kPa-1 and a maximum peak power of 34.66 mW m-2. It is applicable to be a transducer to light LEDs, and a sensor to detect weak physiological signals or mechanical vibration. This work shows the piezo-ionic-electric electronics as a paradigm of highly-optimized all-polymer piezo-generators.

A universal framework for determining the effect of operating parameters on piezoionic voltage generation

The piezoionic effect, the generation of a transient voltage in a polymer matrix infused with ion embedded solvent upon the application of a mechanical stimulus, has demonstrated potential applications in ionic sensing, actuation, interfaces, and energy harvesting. Considerable progress has been made to increase voltage output based on optimizing the morphology and composition of materials. However, regardless of the materials used, in order to design and operate piezoionic devices efficiently, the effect of operating parameters, for example, the strength, speed, and location of the mechanical stimulus, as well as the collection of the piezoionic signal using electrodes are of equal importance. Yet, there has not been any systematic exploration of such operating parameters, leading to the present ad hoc approaches to the design, operation, and performance evaluation of piezoionic systems. In this work, we systematically show the effect of operating parameters on piezoionic voltage generation and provide a universal framework to describe new observations. To elucidate the relationship between the piezoionic response and the underlying mechanism, we propose a novel spatial-temporal strategy for characterizing the piezoionic effect. To ensure generality, newfound insights are modeled and cross-validated over a wide range of experimental data. New observations and new theoretical attributions resulting from this work provide the first systematic method towards optimizing the structure, geometry, and test of piezoionic devices.

Highly Stretchable, Transparent, Solvent-Resistant Multifunctional Ionogel with Underwater Self-Healing and Adhesion for Wearable Strain Sensors and Barrier-Free Information Transfer

Ionogels with excellent deformability, high ionic conductivity, and a sensitive stimulus response have been widely used and rapidly developed in flexible wearable systems. However, previously reported ionogels are mainly limited to atmospheric environments applications and have difficulty meeting the requirements of solvent-resistant, self-healing, and adhesion properties in underwater environments. Herein, a multifunctional ionogel capable of underwater applications is prepared by one-step photoinitiated polymerization of a fluorine-containing monomer (2,2,3,4,4,4-hexafluorobutyl acrylate, HFBA) and acrylic acid (AA) in a hydrophobic ionic liquid ([EMIM][TFSI]). The dynamic physical interactions of hydrogen bonds and ionic dipoles endow the ionogel with remarkable transparency, tunable mechanical properties, and underwater self-healing properties. Moreover, the fluoropolymer matrix offers high resistance to water and various solvents and exhibits strong underwater adhesion on different substrates. Thus, the sensor based on the ionogel exhibits excellent sensing properties, including high sensitivity, fast response, and superior durability. In particular, the ionogel can be used as a wearable underwater sensor to perform barrier-free information transfer. This study provides a design idea for the development of underwater flexible strain sensors.

Unveiling Gating Behavior in Piezoionic Effect: toward Neuromimetic Tactile Sensing

The human perception system's information processing is intricately linked to the nonlinear response and gating effect of neurons. While piezoionics holds potential in emulating the pressure sensing capability of biological skin, the incorporation of information processing functions seems neglected. Here, ionic gating behavior in piezoionic hydrogels is uncovered as a notable extension beyond the previously observed linear responses. The hydrogel can generate remarkably high voltages (700 mV) and currents (7 mA) when indentation forces surpass the threshold. Through a comprehensive analysis involving simulations and experimental investigations, it is proposed that the gating behavior emerges due to significant diffusion differences between cations and anions. To showcase the practical implications of this breakthrough, the piezoionic hydrogels are successfully integrated with prostheses and robot hands, demonstrating that the gating effect enables accurate discrimination between gentle and harsh touch. The advancement in neuromimetic tactile sensing has significant potential for emerging applications such as humanoid robotics and biomedical engineering, offering valuable opportunities for further development of embodied neuromorphic intelligence.

Self-Adhesive, Stretchable, and Thermosensitive Iontronic Hydrogels for Highly Sensitive Neuromorphic Sensing-Synaptic Systems

Artificial sensory afferent nerves that emulate receptor nanochannel perception and synaptic ionic information processing in chemical environments are highly desirable for bioelectronics. However, challenges persist in achieving life-like nanoscale conformal contact, agile multimodal sensing response, and synaptic feedback with ions. Here, a precisely tuned phase transition poly(N-isopropylacrylamide) (PNIPAM) hydrogel is introduced through the water molecule reservoir strategy. The resulting hydrogel with strongly cross-linked networks exhibits excellent mechanical performance (∼2000% elongation) and robust adhesive strength. Importantly, the hydrogel's enhanced ionic conductance and heterogeneous structure of the temperature-sensitive component enable highly sensitive strain information perception (GFmax = 7.94, response time ∼ 87 ms), temperature information perception (TCRmax = -1.974%/°C, response time ∼ 270 ms), and low energy consumption synaptic plasticity (42.2 fJ/spike). As a demonstration, a neuromorphic sensing-synaptic system is constructed integrating iontronic strain/temperature sensors with fiber synapses for real-time information sensing, discrimination, and feedback. This work holds enormous potential in bioinspired robotics and bioelectronics.

Giant Iontronic Flexoelectricity in Soft Hydrogels Induced by Tunable Biomimetic Ion Polarization

Flexoelectricity features the strain gradient-induced mechanoelectric conversion using materials not limited by their crystalline symmetry, but state-of-the-art flexoelectric materials exhibit very small flexoelectric coefficients and are too brittle to withstand large deformations. Here, inspired by the ion polarization in living organisms, this paper reports the giant iontronic flexoelectricity of soft hydrogels where the ion polarization is attributed to the different transfer rates of cations and anions under bending deformations. The flexoelectricity is found to be easily regulated by the types of anion-cation pairs and polymer networks in the hydrogel. A polyacrylamide hydrogel with 1 m NaCl achieves a record-high flexoelectric coefficient of ≈1160 µC m-1, which can even be improved to ≈2340 µC m-1 by synergizing with the effects of ion pairs and extra polycation chains. Furthermore, the hydrogel as flexoelectric materials can withstand larger bending deformations to obtain higher polarization charges owing to its intrinsic low modulus and high elasticity. A soft flexoelectric sensor is then demonstrated for object recognition by robotic hands. The findings greatly broaden the flexoelectricity to soft, biomimetic, and biocompatible materials and applications.

Nacre-Mimetic Structure Multifunctional Ion-Conductive Hydrogel Strain Sensors with Ultrastretchability, High Sensitivity, and Excellent Adhesive Properties

Recently, conductive hydrogels have emerged as promising materials for smart, wearable devices. However, limited mechanical properties and low sensitivity greatly restrict their lifespan. Based on the design of biomimetic-layered structure, the conductive hydrogels with nacre-mimetic structure were prepared by using layered acrylic bentonite (AABT) and phytic acid (PA) as multifunctional "brick" and "mortar" units. Among them, the unique rigid cyclic multihydroxyl structure of the "organic mortar" PA preserves both ultrastretchability (4050.02%) and high stress (563.20 kPa) of the hydrogel, which far exceeds most of the reported articles. Because of the synergistic effect of AABT and PA, the hydrogel exhibits an excellent adhesive strength (87.74 kPa). The role of AABT in the adhesive properties of hydrogels is proposed for the first time, and a general strategy for improving the adhesive properties of hydrogels by using AABT is demonstrated. Furthermore, AABT provides ion channels and PA ionizes abundant H+, conferring a high gauge factor (GF = 14.95) and excellent antimicrobial properties to the hydrogel. Also, inspired by fruit batteries, simple self-powered flexible sensors were developed. Consequently, this study provides knowledge for functional bentonite filler modified hydrogel, and the prepared multifunctional ionic conductive hydrogel shows great application potential in the field of intelligent wearable devices.