Piezoionics: Mechanical‐to‐ionic transduction for sensing, biointerface, and energy harvesting

Piezoionic materials consisting of a polymer matrix and mobile ions can produce an electrical output upon an applied pressure inducing an ion concentration gradient. Distinct from charges generated by the piezoelectric or triboelectric effects, the use of generated mobile ions to carry a signal closely resembles many ionic biological processes. Due to this similarity to biology, the piezoionic effect has great potential to enable seamless integration with biological systems, which accelerates the advancement of medical devices and personalized medicine. In this review, a comprehensive description of the piezoionic mechanism, methods, and applications are presented, with the aim to facilitate a dialogue among relevant scientific communities. First, the piezoionic effect is briefly introduced, then the development of mechanistic understanding over time is surveyed. Next, different types of piezoionic materials are reviewed and methods to enhance the piezoionic output via materials properties, electrode interfaces, and device architectures are detailed. Finally, applications, challenges, and outlooks are provided. With its novel properties, piezoionics is expected to play a key role in the overcoming of grand challenges in the areas of sensing, biointerfaces, and energy harvesting.


INTRODUCTION
Intercellular communication in biological systems exploits the use of a variety of ions and small molecules. [1]Ions, with their diverse charge, size, and mobility interact with biomolecular structures to perform critical functions within living organisms, making ion transport a crucial for biological functions. [2]This is especially evident in regulating processes such as signal transduction, action potential, volume regulation, and the cell cycle. [3]Ionic transport is prevalent in many biological processes, particularly in the peripheral sensory system where it serves as the foundation for converting various external stimuli into biological potential signals by utilizing ions as information carriers.For example, as the largest organ in humans, skin can generate sensory signals by employing ionic channels, in the cell membrane of mechanoreceptors, that open when subjected to mechanical deformation. [4]Therefore, in a variety of ways, ions are as significant to biology as electrons are to artificial intelligent systems. [5]8] Among them, piezoionics, a new frontier that utilizes ions as charge and energy carriers, has garnered increasing attention.Piezoionics involves converting a mechanical stimulus into an electrical signal based on ionic transport, exhibiting numerous applications such as wearable electronics, medical devices, human-machine interfaces, and industrial control.Conventionally, piezoionics has been associated with two mechanical-to-ionic transduction mechanisms.The first one is based on the electric double layer (EDL) located at an electrolyte-conductor interface that has a larger unit area capacitance (UAC).[11][12] The other one is based on an ion gradient that is generated using a microstructural gradient. [13]n fact, the piezoionic effect can be fundamentally defined as the mechanical-to-ionic conversion process by which an applied mechanical stress induces a transient separation of anions and cations in a material, such as a solventinfused polymer, resulting in the generation of a voltage or current. [14]Piezoionic materials have advantages such as self-powering capability, flexibility, and biocompatibility, and are suitable for a variety of medical applications, such as neural interfaces and artificial skin.Most importantly, piezoionic ion transduction produces a direct output of ions, ensuring direct integration with ionic biological processes.Piezoionics has several potential advantages such as having highly controllable properties via the rational design of multiple parameters such as ion types, sizes, and charges.The capability to generate energy through mechanical conduction is especially attractive, enabling self-powered sensing devices or energy harvesters.Piezoionics, albeit with its numerous native advantages, is nonetheless a novel field that lacks prior publications systematically elaborating various critical aspects in detail.
This review provides an in-depth overview of the field of piezoionics, covering various important aspects such as fundamental mechanism, materials methodology, application, future outlook, and current debate.The paper begins with a description of the gemination and evolution of the field from a historical perspective, then moves on to surveying material design and fabrication, which is primarily focused on hydrogels containing free or fixed ions.Then, various applications in sensing, biointerfaces, and energy harvesting are detailed with performance analyses of prototypes.Finally, future outlooks are provided especially to shed light on possible pathways to address current technical challenges.

WHAT IS PIEZOIONICS?
Piezoionics, as its name suggests, is the process by which ion motion is produced in response to applied pressure.Predecessors of the piezoionic concept began to emerge back in the late 1990s, when P. G. de Gennes explained the actuation and sensing process in electroactive conducting polymer composites. [15]However, the term piezoionics was first introduced in 2015. [16]Initially, the piezoionic effect, with its generated voltage and current, was attributed to the Donnan potential arising from inhomogeneous ionic distribution, which dates back to a model by Spinks based on deformation induced ion flux. [17]However, Sawahata et al. attributed piezoionics to Nernst-Donnan potential generation induced by a heterogeneous mechanical deformation of polyelectrolyte hydrogel. [18]Nevertheless, there remains controversy over whether this effect arises from Donnan potentials at the electronic conducting polymerelectrolyte interface or stress-gradient induced ion movement.Recent research showed that the electrical response increased linearly with stress gradient, regardless of porous or non-porous electrodes, which suggests that the primary mechanism was stress gradients rather than Donnan potential. [19]n 2022, Madden et al. have demonstrated an artificial mechanoreceptor mimicing the somatosensory network in human skin, where the generated voltage magnitude and duration are attributed to the difference in mobility between cations and anions. [14]Based on this notion, therefore, the piezoionic effect can be defined as follows.In response to mechanical stress in a solvent-infused polymeric material, the separation of anionic and cationic species occurs, which can be measured as a voltage or a current.The key components involved in piezoionics are an applied stress, ions, a solvent, and a polymer network.This means the char-F I G U R E 1 System with which the piezoionic effect can be defined.The system consists of components such as (i) an applied stress, (ii) ions dissolved in a (iii) solvent, which is infused in a (iv) polymer network, also referred to as the network, backbone, or matrix.acteristics of solvent-infused polymeric materials, that is, ionic size, ionic charge, solvent type, and their chemical and physical nature are crucial to both the fundamental understanding and potential applications of piezoionics (Figure 1).A major categorial difference, compared to, for example, piezoresistive [20][21][22] and piezocapacitive [23] mechanisms, is that the piezoionic effect is a signal/energy generation mechanism.This means that, rather than redirecting or modulating another energy source, the piezoionic process produces an output.In the context of sensing and energy generation, the piezoionic effect enables self-power sensors and energy harvesters, respectively, highlighting its potential for innovative applications.

MATERIAL FOR PIEZOIONICS
Owing to the fact that piezoionics relies heavily on ion mobility, a liquid phase should be a component of the material system, which after the withdrawal of the applied pressure can recover to its original shape.Hence, materials for piezoionics are a class of flexible smart materials made up of a polymeric skeleton containing a fluid with mobile ions, such as room-temperature ionic liquids or dissolved salts in aqueous solutions. [24]It is well known that the gel state is generally a three-dimensional (3D) network with a liquid phase, [25] making the gel a good material choice for piezoionics.Among various kinds of gels, hydrogels, which are homogeneous materials in most cases, are cross-linked water-infused polymer networks formed through physical, ionic, or covalent interaction. [26]In contrast to hydrogels, ionogels have the dispersion medium being an ionic liquid rather than water.Ionogels also offer key advantages such as high ionic conductivity, non-volatility, and excellent thermal stability.To date, piezoionics studies have mainly been based on hydrogels and ionogel. [27,28]ypically, these piezoionic materials are composed of a soft polymer either impregnated with electrolytes containing mobile ions or polyelectrolytes containing immobilized ions and mobile counterions.Hence, piezoionic hydrogels and ionogels can be categorized into three main types: one component, double component, and multicomponent (Figure 2).

One-component piezoionic material
[31][32][33] For instance, polyacrylic acid (PAA) is a popular anionic electrolyte chosen as a decent candidate for mechanical-electrical conversion.Pan et al. utilized PAA hydrogels, which ionize H + in water because of their −COOH group, as a model system. [34]The fabricated PAA hydrogels exhibited the piezoionic effect with 6 mV voltage output and 2 μA current at a pressure of 5 N, which can be used as a self-powered sensor serving as a voice recognizer.
Apart from such polyelectrolytes which can ionize H + in water like PAA, a large number of polyelectrolytes directly produced by polymerization of ionic monomers have also been developed as models for the piezoionics study.Odent et al. selected anionic and cationic hydrogels, which were fabricated by 3-sulfopropyl acrylate potassium salt (SPA) and [2-(acryloyloxy) ethyl] trimethylammonium chloride (AETA) containing mobile potassium (K + ) or chloride (Cl − ) counterions, respectively, to investigate their piezoionic effects, and their piezoionic behaviors are different owing to different ion type and charge density. [35]

Double-component piezoionic material
In piezoionics, ion generation occurs not only from the polymer itself, like the polymer carrying charges, but also from the addition of ions like ionic liquids or solvated salts.Double-component material, consisting of a neutral polymer substrate and solvated salt or ionic liquid (Figure 2), shows excellent ionic conductivity and good stability, [36] and is regarded as a good piezoionic material.For polymer substrates, poly(vinyl alcohol) (PVA) is one of the most representative polymer backbones, which can form dense hydrogen bonds in its crystalline domains, thus providing a large number of physical cross-linking sites. [37]For mobile ionic components, ionic liquids, which are non-volatile in ambient, that is, do not suffer from water loss, are a good choice. [38]Recently, a piezoionic ionogel based on PVA and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (EMITFSI, ionic liquid) has achieved a voltage output of 2.11 mV at 1.5% external strain (Figure 3A-C). [39]Solvated salt is another alternative to be used to fabricate double-component material for piezoionics research.In a double-component material system, it is worth exploiting the Hofmeister effect or ion-specific effect between the polymeric skeleton and salts. [40]This is due to the effects of salts on the water structure, indirectly disturbing the properties of the co-solute, rather than direct interactions between salts and co-solute. [41]For example, the mechanical properties of Hofmeister series materials are significantly affected by the interaction modes between different salt solutions and the PVA polymer chains. [42]

Multicomponent piezoionic material
To enrich the function of hydrogels/ionogels for piezoionics, many researchers pay attention to fabricating hydrogel/ionogel composite.The most representative example is the double network (DN) hydrogel, which consists of two semi-penetrating or interpenetrating polymer networks that exhibit superior mechanical strength and toughness compared to either network alone. [43]Taking polyacrylamide (PAAm) and PVA as an example, polar polymers PAAm facilitate the movement of ions due to their favorable electrical properties, [44] while non-polar PVA possesses excellent mechanical properties. [45]Thus, the combination of such two different polymers has great potential to address the challenges of polymer electrolyte materials that require good performance both electrically and mechanically.Recently, multicomponent ionogels based on graphene-PAAm/PVA electrolytes have been developed to enhance the mechanical property of hydrogels and offer ion channels for ion mobility (Figure 3D). [46]Each component plays a different role in the above-mentioned ionogels.Specially, the PAAm network has a strong affinity for ions, the PVA network can enhance the mechanical properties of hydrogels, while the graphene nanosheets not only improve mechanical performance but also form channels to facilitate ion transport.

METHODS TO AMPLIFY OUTPUT SIGNALS
As defined previously, the piezoionic effect is the separation of anionic and cationic species in a (solvent-infused polymeric) material in response to an applied mechanical stress, thereby generating a voltage or current.Therefore, one of the most important aspects of the piezoionic effects lies in maximizing the output. [16]Researchers continuously strive to enhance the output signal or power of piezoionics  [38] (D) Synthesis process scheme of graphene-polyacrylamide/PVA ion gel electrolyte. [45] different methods, including ion gradient enlargement, electrode interface optimization, and superposition, which are detailed below.

Ion gradient enlargement
One way to produce a large piezoinic output is to exploit the difference in cationic and anionic mobilities to produce a large ion gradient.Dynamic structures and heterogeneously induced ion redistribution are good options to enlarge this gradient. [47]For example, Li and co-workers fabricated a knot-stretchable piezoionic yarn consisting only of a single roll of carbon nanotube yarn wrapped in a gel electrolyte.When asymmetric stretching begins, structural deformation (such as loop separation) starts at the moving end and then spreads along the yarn to the other end.This structural inhomogeneity suggests that absorbed ions are dynamically "squeezed" by the yarn, creating a charge density gradient along the yarn, which is referred to as the asymmetric ion squeezing method.The utilization of the large charge density gradient enables the generation of a large voltage. [47]nspired by the electric eel discharge, Schroeder et al. induced the formation of ion concentration gradients in PAAm hydrogels over compartments surrounded by repeating arrays of cation-and anion-selective hydrogel membranes, resulting in a ∼110 V open circuit voltage. [48]Meanwhile, designing 3D-printed stacked ionic assemblies with various ion types, charge densities, and cross-linking densities can also enlarge the gradient between cation and anion.As illustrated in Figure 4A, Odent et al. designed such two-compartment hydrogel systems, with compartments having different ion concentrations, forming large ion concentration gradients, where compressive strains of up to 50% generated a voltage of up to 70 mV. [35]

Electrode interface optimization
Developing appropriate surface electrodes and electrical contacts is critical for collecting and transferring the generated electrical charge. [49]For piezoionic output signal collection, noble metal electrodes are widely chosen due to their excellent conductivity.Electrodes are typically constructed using thin metal layers, which are a few microns thick and directly attached to the substrate. [50]Other methods for producing electrodes include sputtering and evaporation of thin metallic films. [51,52]However, establishing strong metal-polymer interfaces with adequate electrical and mechanical properties has been a major challenge. [53,54]Attributed to the modulus mismatch between metals and polymers at material interfaces, these electrodes experience interfacial slip upon the application of strain and stress. [55,56]Lu et al. developed a bio-inspired interface based on an in situ growth strategy, where metal ion-containing precursors are first seeded into the polyelectrolyte soil and then a tree-root-like interface is formed by a chemical reduction process (Figure 4B). [57]n this analogy, the tree and soil represent the noble metal electrodes and polyelectrolytes, respectively, while the root represents the interface.The tree-root-like material interface design not only facilitates charge transport but also enhances resistance to external strain, leading to a voltage signal of 26 mV generation, which is better than that of conventional devices based on a surface contact-based interface.

The technique of superposition
Another promising approach to enhancing the output signal is to utilize the superposition principle, that is, combining different voltage generation mechanisms to build up a large aggregated output.Piezoelectric materials, as one of the most F I G U R E 4 (A) Illustration of designing 3D-printed stacked ionic assemblies during the 3D printing process. [34](B) Bio-inspired interface design for strain sensors and experimental method for interfacial in situ growth. [56]ll-known electromechanical conversion materials, are a good alternative to achieve superposition performance due to ease of integration. [58]Villa et al. presented a polymeric nanocomposite simultaneously equipped with piezoelectric and piezoionic properties.This nanocomposite, composed of an ionic liquid combined with physically embedded barium titanate (BaTiO 3 ) nanoparticles with piezoelectric properties, can generate output voltages as high as 8 mV over a small pressure range (<10 kPa). [59]Additionally, triboelectric nanogenerators (TENGs) as another electromechanical conversion material, which can convert mechanical energy into electrical energy by utilizing the coupling effect of contact electrification (CE) and electrostatic induction, [60] can participate in the superposition process together with piezoionics to enlarge the output signal.Kim et al. demonstrated an ion-doped gelatin hydrogel (IGH), which has both triboelectric and piezoionic properties, as well as controllable ion transport properties, can achieve its voltage output from 0.522 V (0.2 M LiCl) to 0.755 V (0.4 M KCl), an increase of approximately 44.6%. [61]

ELECTRODE MATERIALS SELECTION
Electrode selection plays a critical role in piezoionic research, but obtaining stable electrode-gel interfaces and creating electrodes that can sustain repeated deformations without damage or loss of performance remains a significant challenge. [62]Common electrode materials utilized in piezoionics include noble metals, conductive polymers, and carbon nanomaterials.

Noble metals
Noble metals have garnered extensive research attention due to their good conductivity and electrochemical stability. [63]ommon metal electrode materials usually include platinum, silver, and gold. [64]At present, only gold among noble metals has participated in the research of piezoionic electrode materials.This is mainly due to its high conductivity, stability, and flexibility.When gold is incorporated into the electrode, the specific capacity of the metal-embedded electrode can be increased.Au nanoparticles have been shown to improve electrode conductivity and structural stability. [65]Villa et al. proposed a piezoionic system consisting of flexible clusterassembled gold electrodes fabricated by supersonic cluster beam deposition. [59,66]The introduction of such electrodes resulted in a 10-fold increase in the charge accumulation efficiency without affecting the mechanical properties of the nanocomposite. [59]However, due to the high cost and low availability of precious metals, combining them with other readily available and inexpensive materials may be one of the most attractive approaches to optimize their properties.Hence, the incorporation of Au with other materials, like polymers, metal oxides, and activated carbon, might be a good choice in future investigations.

Conductive polymers
Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is a conductive polymer with mixed ionicelectronic properties, specific capacitance, and high ionic conductivity. [67]It has been identified as a strong candidate for electroactive electrodes due to its high electrical conductivity, easy processability, optical transparency, as well as environmental stability. [68]Furthermore, due to its ability to compensate for ions in the electrolyte, the density of the holes in PEDOT:PSS can be reversibly tuned over a wide range, making it an ideal electrode for ion-based electronics, such as supercapacitors, [69] electrochemical transistors, [70] and sensors. [71]Additionally, the commercially available colloidal aqueous dispersion of PEDOT:PSS allows its processability to be accessible via different casting methods.Plesse et al. investigated how solvent annealing and additive concentration (ionic liquid (IL), Aquivion ® ) affect the electrical and mechanical performances of modified PEDOT:PSS electrodes. [68]It can be found that the electrical conductivity of PEDOT:PSS is increased by 10 2 -10 3 times by adding solvents (volatile or non-volatile), while the plasticizing effect (IL) or morphological change caused by polar solvents reduces the mechanical property of electrodes.As described in Figure 5A, the flexible free-standing PEDOT:PSS is used as an electrode to research the piezoionic performance of ionic polymer-polymer composites.Under optimal conditions, its output electrical signal was as high as ∆E = 2.9 mV (∆ε = ± 2%).

Composite nanomaterials
Noble metals have been shown to exhibit poor mechanical properties for electrodes, which has been a significant impediment. [72]In addition, the doping/dedoping process of conductive polymers induces volume change, leading to a decline in operational stability. [19]These challenges have driven the adoption of electrode materials like graphene and carbon nanotubes.Benefiting from electrical conductivity, structural stability, and porous structure, [73,74] these materials have shown promise in piezoionic research and have been investigated as potential replacements for traditional electrode materials. [49,75]When graphene composite is used as an electrode to replace Au electrodes, it has demonstrated significant improvements in voltage generation for piezoionic sensors, potentially because of its large surface area contacting ionic liquid. [76]The use of two-dimensional MXene (Ti 3 C 2 T x , T = OH, O, and F) as an electrode material has garnered significant attention due to its excellent mechanical properties, superior electrochemical performance, and high electrical conductivity. [77,78]When ultrathin MXene nanosheets, prepared by etching and exfoliation (Figure 5B), have been used as electrode materials for piezoionic studies, 219 mV voltage output could be observed.This can be attributed to the fact that the ultrathin structure is favorable for the formation of ultrafast ion channels, thus providing ample space for ion movement. [79]

APPLICATION
The multi-responsiveness of the piezoionic mechanism, generating voltage or current outputs, can enable applications such as sensing, biointerfaces, and energy harvesting.

Pressure sensors
[82] A pressure sensor is one that converts applied compressive stress into a detectable signal, most commonly an electrical signal.Thus far, pressure sensors of different mechanisms have been discovered, including triboelectric, [83] transmissive, [84] capacitive, [85] piezoelectric, [86] and piezoresistive. [87]Recently, piezoionic materials, as a brand new class of ionic composite materials, can be used as pressure sensors, which produce an electrical output through the ion gradient generated by the difference in the movement of anions and cations under the stimulation of pressure. [88]It is most different from the piezoelectric or triboelectric effect in that piezoionic effect produces mobile ions that resemble the signal generation and transmission in biological systems, making this material an ideal alternative for wearable biomimetic sensors.Andrew et al. introduced an all-fabric piezoionic pressure sensor named "PressION", which can be used to extract important physiological signals from different positions of the body, such as heartbeat, pulse, joint motion, phonation, and step data (Figure 6A). [89]

Biointerfaces
The peripheral sensory system relies heavily on ionic transport in various biological processes. [90]When a force is imposed on skin, the mechanical deformation of the skin induces the opening of ion channels in the mechanoreceptor cell walls, resulting in a sensory signal generated within the cell, which is subsequently conveyed to the brain.Piezoionic signals have similar characteristics to those produced by the mechanoreceptors regarding charge, voltage, and time response, making them suitable for use in creating artificial organs that are highly compatible in terms of signaling, F I G U R E 5 (A) Chemical structure of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) and variation of electrical response (ΔE) with stress frequency (∆ε = ± 2%) of modified structure. [67](B) Synthesis procedures of ultrathin MXene nanosheets. [79]I G U R E 6 (A) Monitoring of various physiological signals. [89](B) Illustration of neuromodulation in a mice model. [14](C) Illustrations of electrochemiluminescence skin device used for stress-sensitive, localized mechanical stimuli visualization. [93]s they already "speak the same language" as the nervous system.Piezoionics can serve as a sensory transduction or neural interfacing mechanism in either a bio-bio interface or a bio-machine interface.Researchers have reported the use of piezoionic devices to mimic transmembrane potential, which is maintained at a resting state of approximately −70 mV by the sodium-potassium pump.The minimum potential change required for generating an action potential has been reported to be around 15 mV.[91] The high charge density demonstrated by piezoionics makes it especially promising in terms of the neural world, where the injection of a certain minimum amount of charge is required, while the signal voltage must be kept low for safety reasons.Based on these principles, Madden selected PAAm hydrogel with piezoionic properties as a research tool, by which they stimulate nerves in mice to prove that piezoionic self-powered properties can, albeit with the aid of an external voltage amplifier, achieve nerves regulation.As shown in Figure 6B, a sequence of compressions was exerted on the piezoionic sensor that was linked to the peripheral nerves.Despite the absence of any visible hindlimb contraction upon utilizing stainless-steel electrodes, a distinct electromyography (EMG) signal could be detected, indicating muscle activation.[14]

Energy harvesting
Due to the ability of the piezoionic effect to generate an ionic voltage or current, mechanical energy harvesting is an intrinsic capability, especially harvesting at low frequency.As reported by Madden, [14] piezoionics can generate a transient signal output with a wide temporal range and a much higher charge density (4-6 orders of magnitude higher for a given voltage) than conventional piezoelectric and triboelectric mechanisms.For example, the energy harvesting capability of piezoionics holds significant potential for application in electroluminescence, providing a fascinating means of converting mechanical stimuli into visual signals while simultaneously enabling energy harvesting.Certain designs use the flux of intracellular Ca 2+ generated by a shear force acting on the cell, activating an electrochemical reaction, thereby producing light. [92]As Lee et al. reported, an electrochemiluminescence (ECL) activity piezoionic composites (a mixture of luminophores and electrolytes) can achieve spatial analysis and visualization of stresses applied locally to the ECL skin (Figure 6C). [93]

FUTURE PERSPECTIVES AND CONCLUSION
Further investigations of piezoionics with regard to materials, methodology, and applications are warranted.For materials, most research on piezoionics is currently based on hydrogels, leaving other material types largely unexplored.Elastomers with low glass transition temperature and dynamic structure might be a good alternative.On the one hand, segment mobility of such elastomers can enable ion movement.On the other hand, such elastomers can overcome limitations inherent in hydrogels, such as instability owing to rapid water evaporation in ambient conditions and poor adhesion to other materials.For example, dynamic supramolecu-lar ionic conductive elastomers have been developed with high ionic conductivity, self-healing capability, and mechanical compatibility via a phase-locked strategy. [94]Regarding methodology, improving the output performance of piezoionics is critical.A rational design methodology could be inspired by the studying of ionic conductors as a close analogue.For example, the reversible ion-dipole bond triggered ion-pumping process, by introducing abundant reversible disulfide bonds, might be beneficial to achieving a large output. [95]For microstructure modification, one can draw on design examples such as using a Calathea zebrina leaf as the template for preparing an ionic gel, [96] or utilizing reversible H-bond triggered ion pumping to emulate the structural and functional features of biological multicellular structures. [97]Furthermore, instrument accuracy and environmental factors should be considered to eliminate interference with the output.Concerning applications, ion carriers support the transmission and processing of information in piezoionics could mimic many biological functions, which is still to date largely unexplored.Applications that piezoionics are especially suitable include prostheses that restore normal biological function in cases of illness or trauma, as well as diagnostic and therapeutic devices that monitor physiology and intervene on demand.Opportunities to address critical grand challenges of in personalized healthcare and sustainable energy using piezoionic are widely available.
In this review, recent progress in the study of piezoionics has been systematically analyzed encompassing the development of mechanism, material, methodology, and applications in sensing, biointerfaces, and energy harvesting.Piezoionics, as an emerging mechanical-to-ionic transduction approach, have obtained increasing attention due to their unique properties enabled by ions as signal and energy carriers to produce an ionic or mixed ionic-electronic output, and at an unprecedented high current density with respect to the output voltage.Moreover, given that there are many types of ions as opposed to only one type of electron, analogous to color versus blackand-white photography, piezoionics has a large design space that provides a new level of material choices and controllability.The use of ions provides a whole new world to propel the smart personalized medicine and sustainable energy revolutions, connecting people, things, data, and processes with an unprecedented level of interconnectedness.

F I G U R E 2
Illustration of one-component, double-component, and multicomponent hydrogel/ionogel for piezoionics.