Shape and stiffness memory ionogels with programmable pressure-resistance response

Review of ionic liquid and ionogel-based biomaterials for advanced drug delivery

Ionic liquids (ILs) play a crucial role in the design of novel materials. The ionic nature of ILs provides numerous advantages in drug delivery, acting as a green solvent or active ingredient to enhance the solubility, permeability, and binding efficiency of drugs. They could also function as a structuring agent in the development of nano/micro particles for drug delivery, including micelles, vesicles, gels, emulsion, and more. This review summarize the ILs and IL-based gel structures with their advanced drug delivery applications. The first part of review focuses on the role of ILs in drug formulation and the applications of ILs in drug delivery. The second part of review offers a comprehensive overview of recent drug delivery applications of IL-based gel. It aims to offer new perspectives and attract more attention to open up new avenues in the biomedical applications of ILs and IL-based gels.

Supramolecular Ionogels for Use in Locating Damage to Underwater Infrastructure

The capacity to self-detect and locate damage to underwater infrastructure in emergencies is vital, as materials and technologies that securely facilitate energy and information transmission are crucial in several fields. Herein, the development of a multifunctional supramolecular ionogel (SIG) and SIG-based devices for use in detecting and locating damage to underwater infrastructure is reported. The SIG is fabricated via the single-step photoinitiated copolymerization of hydroxy and fluorinated monomers in a fluorinated ionic liquid. Hydrogen-bond/ion-dipole-interaction synergy ensures that the SIG is highly ionically conductive and extremely mechanically strong, with underwater self-healing and adhesion properties. It can be used as an underwater ionic cable to provide reporting signals via changes in strain; furthermore, SIG-based devices can be fixed to underwater infrastructure to locate damage via resistance monitoring. The SIG can also be attached to the human body for use in underwater communication, thereby safeguarding maintenance personnel while repairing underwater infrastructure. This study provides a novel pathway for developing supramolecular materials and devices.

Ultrasensitive Ionic Conductors with Tunable Resistance Switching Temperature Enabled by Phase Transformation of Polymer Cocrystals

Phase-transformable ionic conductors (PTICs) show significant prospects for functional applications due to their reversible resistance switching property. However, the representative design principle of PTICs is utilizing the melt-crystallization transition of ionic liquids, and the resistance switching temperatures of such PTICs cannot be tuned as desired. Herein, a new strategy is proposed to design PTICs with on-demand resistance switching temperatures by using the melt-crystallization transition of polymer cocrystal phase, whose melting temperature shows a linear relationship with the polymer compositions. Owing to the melt of polymer cocrystal domains and the tunable migration of ions in the resistance switching region, the obtained PTICs display ultrahigh temperature sensitivity with a superior temperature coefficient of resistance of -8.50% °C-1 around human body temperature, as compared to various ionic conductors previously reported. Therefore, the PTICs can detect tiny temperature variation, allowing for the intelligent applications for overheating warning and heat dissipation. It is believed that this work may inspire future researches on the development of advanced soft electrical devices.

Bicontinuous vitrimer heterogels with wide-span switchable stiffness-gated iontronic coordination

Currently, it remains challenging to balance intrinsic stiffness with programmability in most vitrimers. Simultaneously, coordinating materials with gel-like iontronic properties for intrinsic ion transmission while maintaining vitrimer programmable features remains underexplored. Here, we introduce a phase-engineering strategy to fabricate bicontinuous vitrimer heterogel (VHG) materials. Such VHGs exhibited high mechanical strength, with an elastic modulus of up to 116 MPa, a high strain performance exceeding 1000%, and a switchable stiffness ratio surpassing 5 × 103. Moreover, highly programmable reprocessing and shape memory morphing were realized owing to the ion liquid-enhanced VHG network reconfiguration. Derived from the ion transmission pathway in the ILgel, which responded to the wide-span switchable mechanics, the VHG iontronics had a unique bidirectional stiffness-gated piezoresistivity, coordinating both positive and negative piezoresistive properties. Our findings indicate that the VHG system can act as a foundational material in various promising applications, including smart sensors, soft machines, and bioelectronics.

Spider-Silk-Inspired Tough, Self-Healing, and Melt-Spinnable Ionogels

As stretchable conductive materials, ionogels have gained increasing attention. However, it still remains crucial to integrate multiple functions including mechanically robust, room temperature self-healing capacity, facile processing, and recyclability into an ionogel-based device with high potential for applications such as soft robots, electronic skins, and wearable electronics. Herein, inspired by the structure of spider silk, a multilevel hydrogen bonding strategy to effectively produce multi-functional ionogels is proposed with a combination of the desirable properties. The ionogels are synthesized based on N-isopropylacrylamide (NIPAM), N, N-dimethylacrylamide (DMA), and ionic liquids (ILs) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]). The synergistic hydrogen bonding interactions between PNIPAM chains, PDMA chains, and ILs endow the ionogels with improved mechanical strength along with fast self-healing ability at ambient conditions. Furthermore, the synthesized ionogels show great capability for the continuous fabrication of the ionogel-based fibers using the melt-spinning process. The ionogel fibers exhibit spider-silk-like features with hysteresis behavior, indicating their excellent energy dissipation performance. Moreover, an interwoven network of ionogel fibers with strain and thermal sensing performance can accurately sense the location of objects. In addition, the ionogels show great recyclability and processability into different shapes using 3D printing. This work provides a new strategy to design superior ionogels for diverse applications.

Ionogels for flexible conductive substrates and their application in biosensing

Ionogels are highly conductive gels made from ionic liquids dispersed in a matrix made of organic or inorganic materials. Ionogels are known for high ionic conductivity, flexibility, high thermal and electrochemical stability. These characteristics make them suitable for sensing and biosensing applications. This review discusses about the two main constituents, ionic liquids and matrix, used to make ionogels and effect of these materials on the characteristics of ionogels. Here, the material properties like mechanical, electrochemical and stability are discussed for both polymer matrix and ionic liquid. We have briefly described about the fabrication methods like 3D printing, sol-gel, blade coating, spin coating, aerosol jet printing etc., used to make films or coating of these ionogels. The advantages and disadvantages of each method are also briefly summarized. Finally, the last section provides a few examples of application of flexible ionogels in areas like wearables, human-machine interface, electronic skin and detection of biological molecules.

MXene/PPy@PDMS sponge-based flexible pressure sensor for human posture recognition with the assistance of a convolutional neural network in deep learning

The combination of flexible sensors and deep learning has attracted much attention as an efficient method for the recognition of human postures. In this paper, an in situ polymerized MXene/polypyrrole (PPy) composite is dip-coated on a polydimethylsiloxane (PDMS) sponge to fabricate an MXene/PPy@PDMS (MPP) piezoresistive sensor. The sponge sensor achieves ultrahigh sensitivity (6.8925 kPa-1) at 0-15 kPa, a short response/recovery time (100/110 ms), excellent stability (5000 cycles) and wash resistance. The synergistic effect of PPy and MXene improves the performance of the composite materials and facilitates the transfer of electrons, making the MPP sponge at least five times more sensitive than sponges based on each of the individual single materials. The large-area conductive network allows the MPP sensor to maintain excellent electrical performance over a large-scale pressure range. The MPP sensor can detect a variety of human body activity signals, such as radial artery pulse and different joint movements. The detection and analysis of human motion data, which is assisted by convolutional neural network (CNN) deep learning algorithms, enable the recognition and judgment of 16 types of human postures. The MXene/PPy flexible pressure sensor based on a PDMS sponge has broad application prospects in human motion detection, intelligent sensing and wearable devices.

Biomimetic Spun Silk Ionotronic Fibers for Intelligent Discrimination of Motions and Tactile Stimuli

Innovation in the ionotronics field has significantly accelerated the development of ultraflexible devices and machines. However, it is still challenging to develop efficient ionotronic-based fibers with necessary stretchability, resilience, and conductivity due to inherent conflict in producing spinning dopes with both high polymer and ion concentrations and low viscosities. Inspired by the liquid crystalline spinning of animal silk, this study circumvents the inherent tradeoff in other spinning methods by dry spinning a nematic silk microfibril dope solution. The liquid crystalline texture allows the spinning dope to flow through the spinneret and form free-standing fibers under minimal external forces. The resultant silk-sourced ionotronic fibers (SSIFs) are highly stretchable, tough, resilient, and fatigue-resistant. These mechanical advantages ensure a rapid and recoverable electromechanical response of SSIFs to kinematic deformations. Further, the incorporation of SSIFs into core-shell triboelectric nanogenerator fibers provides outstanding stable and sensitive triboelectric response to precisely and sensitively perceive small pressures. Moreover, by implementing a combination of machine learning and Internet of Things techniques, the SSIFs can sort objects made of different materials. With these structural, processing, performance, and functional merits, the SSIFs prepared herein are expected to be applied in human-machine interfaces.

An Ultrastretchable Gradient Ionogel Induced by a Self-Floating Strategy for Strain Sensing

The fabrication of gradient ionogels for flexible strain sensors remains challenging because of the complex preparation procedures, and it is still difficult to prepare highly stretchable ionogels (strain > 10000%). In this study, a strategy is proposed to successfully fabricate gradient ionogels and apply them to flexible strain sensors by utilizing the self-floating character of the polysiloxane cross-linker. A gradient ionogel with ultrahigh stretchability (>14000%) is prepared via a one-step in situ photopolymerization process of the precursor with long-chain poly(dimethylsiloxane) bis(2-methyl acrylate) (PDMSMA). PDMSMA, which has a self-floating ability and excellent flexibility, induces a gradient composition distribution in the ionogel, thereby endowing the ionogel with superior stretchability and gradient changes in conductivity and adhesivity from the top to the bottom layer. Because of multiple molecular interactions, the bottom surface of the ionogel possesses good resilience and self-adhesion, whereas the top surface, which has a high PDMSMA content, shows a nonsticky performance. As a result, a singular gradient ionogel having both a sticky bottom surface and a nonsticky top surface is achieved. Furthermore, the flexible strain sensor that is created based on these gradient ionogels exhibits high sensitivity (its gauge factor reaching 5.08), a wide detection range (1-1500%), fast response times, and good linearity. Notably, the detection signal remains repeatable over 1000 uninterrupted strain cycles. The fabricated strain sensor was further utilized to monitor joint movements and physiological signals. This work provides a facile strategy for fabricating gradient ionogels and shows their application potential in the field of flexible electronics.

Multifunctional Ionic Conductive Anisotropic Elastomers with Self-Wrinkling Microstructures by In Situ Phase Separation

Multifunctional flexible sensors are the development trend of wearable electronic devices in the future. As the core of flexible sensors, the key is to construct a stable multifunctional integrated conductive elastomer. Here, ionic conductive elastomers (ICEs) with self-wrinkling microstructures are designed and prepared by in situ phase separation induced by a one-step polymerization reaction. The ICEs are composed of ionic liquids as ionic conductors doped into liquid crystal elastomers. The doped ionic liquids cluster into small droplets and in situ induce the formation of wrinkle structures on the upper surface of the films. The prepared ICEs exhibit mechanochromism, conductivity, large tensile strain, low hysteresis, high cycle stability, and sensitivity during the tension-release process, which achieve dual-mode outputs of optical and electrical signals for information transmission and sensors.

Recent Advancements in Physiological, Biochemical, and Multimodal Sensors Based on Flexible Substrates: Strategies, Technologies, and Integrations

Flexible wearable devices have been widely used in biomedical applications, the Internet of Things, and other fields, attracting the attention of many researchers. The physiological and biochemical information on the human body reflects various health states, providing essential data for human health examination and personalized medical treatment. Meanwhile, physiological and biochemical information reveals the moving state and position of the human body, and it is the data basis for realizing human-computer interactions. Flexible wearable physiological and biochemical sensors provide real-time, human-friendly monitoring because of their light weight, wearability, and high flexibility. This paper reviews the latest advancements, strategies, and technologies of flexibly wearable physiological and biochemical sensors (pressure, strain, humidity, saliva, sweat, and tears). Next, we systematically summarize the integration principles of flexible physiological and biochemical sensors with the current research progress. Finally, important directions and challenges of physiological, biochemical, and multimodal sensors are proposed to realize their potential applications for human movement, health monitoring, and personalized medicine.

Non-equilibrium-Growing Aesthetic Ionic Skin for Fingertip-Like Strain-Undisturbed Tactile Sensation and Texture Recognition

Humans use periodically ridged fingertips to precisely perceive the characteristics of objects via ion-based fast- and slow-adaptive mechanotransduction. However, designing artificial ionic skins with fingertip-like tactile capabilities remains challenging because of the contradiction between structural compliance and pressure sensing accuracy (e.g., anti-interference from stretch and texture recognition). Inspired by the formation and modulus-contrast hierarchical structure of fingertips, an aesthetic ionic skin grown from a non-equilibrium Liesegang patterning process is introduced. This ionic skin with periodic stiff ridges embedded in a soft hydrogel matrix enables strain-undisturbed triboelectric dynamic pressure sensing as well as vibrotactile texture recognition. By coupling with another piezoresistive ionogel, an artificial tactile sensory system is further fabricated as a soft robotic skin to mimic the simultaneous fast- and slow-adaptive multimodal sensations of fingers in grasping actions. This approach may inspire the future design of high-performance ionic tactile sensors for intelligent applications in soft robotics and prosthetics.

Technology Roadmap for Flexible Sensors

Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Ionogels: recent advances in design, material properties and emerging biomedical applications

Ionic liquid (IL)-based gels (ionogels) have received considerable attention due to their unique advantages in ionic conductivity and their biphasic liquid-solid phase property. In ionogels, the negligibly volatile ionic liquid is retained in the interconnected 3D pore structure. On the basis of these physical features as well as the chemical properties of well-chosen ILs, there is emerging interest in the anti-bacterial and biocompatibility aspects. In this review, the recent achievements of ionogels for biomedical applications are summarized and discussed. Following a brief introduction of the various types of ILs and their key physicochemical and biological properties, the design strategies and fabrication methods of ionogels are presented by means of different confining networks. These sophisticated ionogels with diverse functions, aimed at biomedical applications, are further classified into several active domains, including wearable strain sensors, therapeutic delivery systems, wound healing and biochemical detections. Finally, the challenges and possible strategies for the design of future ionogels by integrating materials science with a biological interface are proposed.

Variable-Sensitivity Force Sensor Based on Structural Modification

Force sensors are used in a wide variety of fields. They require different measurement ranges and sensitivities depending on the operating environment because there is generally a trade-off between measurement range and sensitivity. In this study, we developed a variable-sensitivity, variable-measurement-range force sensor that utilizes structural modification, namely changes in the distance between the force application point and the detection area, and changes in the cross-sectional area. The use of shape-memory materials allows the sensor structure to be easily changed and fixed by controlling the temperature. First, we describe the theory of the proposed sensor. Then, we present prototypes and the experimental methods used to verify the performance of the sensor. We fabricated the prototypes by attaching two strain gauges to two sides of a shape-memory alloy and shape-memory polymer plates. Experiments on the prototypes show that the relationship between the applied force and the detected strain can be changed by bending the plate. This allows the sensitivity and measurement range of the sensor to be changed.

Starch-based shape memory sponge for rapid hemostasis in penetrating wounds

Death caused by excessive blood loss has always been a global concern. Timely control of bleeding in incompressible penetrated wounds remains a great challenge. Here, we developed a shape memory sponge (SQG) based on modified starch and gelatin (Gel) to control the hemorrhage of penetrating wounds. The porous structure of SQG greatly enhanced the absorption of blood, and the adhesion of erythrocytes and platelets. The water absorption rate of SQG reached 1178.72 ± 12.18% in 10 s. SQG quickly recovered its shape in water (∼3 s) and exhibited high mechanical strength (∼38 kPa), acting as a physically packed barrier to facilitate hemostasis. Furthermore, the positively charged sponges were conducive to activating platelets and promoting the release of coagulation factors. SQG sponges possessed the lowest blood coagulation index (BCI) of 21.32 ± 0.19%, and presented good biocompatibility and obvious inhibitory effect on Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Moreover, SQG sponges controlled complete bleeding in 69 ± 20 s and a bleeding loss of 334 ± 138 mg was observed, nearly 50% lower than that of gelatin sponge in rabbit liver penetrating wounds. Overall, SQG possesses a combination of potent shape recovery, rapid hemostasis, and excellent antibacterial and degradation ability, enabling promising applications for hemostasis in non-compressible penetrating wounds.

Protein Crystallization-Mediated Self-Strengthening of High-Performance Printable Conducting Organohydrogels

Conductive polymers have many advanced applications, but there is still an important target in developing a general and straightforward strategy for printable, mechanically stable, and durable organohydrogels with typical conducting polymers of, for example, polypyrrole, polyaniline, or poly(3,4-ethylenedioxythiophene). Here we report a protein crystallization-mediated self-strengthening strategy to fabricate printable conducting organohydrogels with the combination of rational photochemistry design. Such organohydrogels are one-step prepared via rapidly and orthogonally controllable photopolymerizations of pyrroles and gelatin protein in tens of seconds. As-prepared conducting organohydrogels are patterned and printed to complicated structures via shadow-mask lithography and 3D extrusion technology. The mild photocatalytic system gives the transition metal carbide/nitride (MXene) component high stability during the oxidative preparation process and storage. Controlling water evaporation promotes gelatin crystallization in the as-prepared organohydrogels that significantly self-strengthens their mechanical property and stability in a broad temperature range and durability against continuous friction treatment without introducing guest functional materials. Also, these organohydrogels have commercially electromagnetic shielding, thermal conducting properties, and temperature- and light-responsibility. To further demonstrate the merits of this simple strategy and as-prepared organohydrogels, prism arrays, as proofs-of-concept, are printed and applied to make wearable triboelectric nanogenerators. This self-strengthening process and 3D-printability can greatly improve their voltage, charge, and current output performances compared to the undried and flat samples.

The Soft-Strain Effect Enabled High-Performance Flexible Pressure Sensor and Its Application in Monitoring Pulse Waves

Flexible and wearable pressure sensors attached to human skin are effective and convenient in accurate and real-time tracking of various physiological signals for disease diagnosis and health assessment. Conventional flexible pressure sensors are constructed using compressible dielectric or conductive layers, which are electrically sensitive to external mechanical stimulation. However, saturated deformation under large compression significantly restrains the detection range and sensitivity of such sensors. Here, we report a novel type of flexible pressure sensor to overcome the compression saturation of the sensing layer by soft-strain effect, enabling an ultra-high sensitivity of ~636 kPa −1 and a wide detection range from 0.1 kPa to 56 kPa. In addition, the cyclic loading-unloading test reveals the excellent stability of the sensor, which maintains its signal detection after 10,000 cycles of 10 kPa compression. The sensor is capable of monitoring arterial pulse waves from both deep tissue and distal parts, such as digital arteries and dorsal pedal arteries, which can be used for blood pressure estimation by pulse transit time at the same artery branch.

Emerging Iontronic Sensing: Materials, Mechanisms, and Applications

Iontronic sensors represent a novel class of soft electronics which not only replicate the biomimetic structures and perception functions of human skin but also simulate the mechanical sensing mechanism. Relying on the similar mechanism with skin perception, the iontronic sensors can achieve ion migration/redistribution in response to external stimuli, promising iontronic sensing to establish more intelligent sensing interface for human-robotic interaction. Here, a comprehensive review on advanced technologies and diversified applications for the exploitation of iontronic sensors toward ionic skins and artificial intelligence is provided. By virtue of the excellent stretchability, high transparency, ultrahigh sensitivity, and mechanical conformality, numerous attempts have been made to explore various novel ionic materials to fabricate iontronic sensors with skin-like perceptive properties, such as self-healing and multimodal sensing. Moreover, to achieve multifunctional artificial skins and intelligent devices, various mechanisms based on iontronics have been investigated to satisfy multiple functions and human interactive experiences. Benefiting from the unique material property, diverse sensing mechanisms, and elaborate device structure, iontronic sensors have demonstrated a variety of applications toward ionic skins and artificial intelligence.