Conducting Polymer Iongels Based on PEDOT and Guar Gum

Developing excellent plantar pressure sensors for monitoring human motions by using highly compressible and resilient PMMA conductive iongels

Based on real-time detection of plantar pressure, gait recognition could provide important health information for rehabilitation administration, fatigue prevention, and sports training assessment. So far, such researches are extremely limited due to lacking of reliable, stable and comfortable plantar pressure sensors. Herein, a strategy for preparing high compression strength and resilience conductive iongels has been proposed by implanting physically entangled polymer chains with covalently cross-linked networks. The resulting iongels have excellent mechanical properties including nice compliance (young's modulus < 300 kPa), high compression strength (>10 MPa at a strain of 90 %), and good resilience (self-recovery within seconds). And capacitive pressure sensor composed by them possesses excellent sensitivity, good linear response even under very small stress (∼kPa), and long-term durability (cycles > 100,000) under high-stress conditions (133 kPa). Then, capacitive pressure sensor arrays have been prepared for high-precision detection of plantar pressure spatial distribution, which also exhibit excellent sensing performances and long-term stability. Further, an extremely sensitive and fast response plantar pressure monitoring system has been designed for monitoring plantar pressure of foot at different postures including upright, forward and backward. The system achieves real-time tracking and monitoring of changes of plantar pressure during different static and dynamic posture processes. And the characteristics of plantar pressure information can be digitally and photography displayed. Finally, we propose an intelligent framework for real-time detection of plantar pressure by combining electronic insoles with data analysis system, which presents excellent applications in sport trainings and safety precautions.

A supramolecular approach for converting renewable biomass into functional materials

The rational transformation and utilization of biomass have attracted increasing attention because of its high importance in sustainable development and green economy. In this study, we used a supramolecular approach to convert biomass into functional materials. Six biomass raw materials with distinct chemical structures and physical properties were copolymerized with thioctic acid (TA) to afford poly[TA-biomass]s. The solvent-free copolymerization leads to the convenient and quantitative fabrication of biomass-based versatile materials. The non-covalent bonding and reversible solid-liquid transitions in poly[TA-biomass]s endow them with diversified features, including thermal processability, 3D printing, wet and dry adhesion, recyclability, impact resistance, and antimicrobial activity. Benefiting from their good biocompatibility and nontoxicity, these biomass-based materials are promising candidates for biological applications.

Schiff base and coordinate bonds cross-linked chitosan-based eutectogels with ultrafast self-healing, self-adhesive, and anti-freezing capabilities for motion detection

Ion conductors offer great potential for diverse electric applications. However, most of the ion conductors were fabricated from non - degradable petroleum-based polymers with non or low biodegradability, which inevitably leads to resource depletion and waste accumulation. Fabricating ion conductors based on renewable, and sustainable materials is highly desirable and valuable. Herein, a series of eutectogels were designed through dual-dynamic-bond cross-linking among ferric iron (Fe3+), protocatechualdehyde (PA), and chitosan (CS) in 1 - allyl-3 - methylimidazole chloride ionic liquid/urea (AmimCl/urea) eutectic-based ionic liquid. Due to the presence of AmimCl/urea eutectic-based ionic liquid, the obtained CS - PA@Fe eutectogels showed excellent ionic conductivity, superior anti-freezing properties that could maintain flexibility and high electrical properties at -20 °C. Dual-dynamic-bond cross-linking of catechol-Fe coordinate and dynamic Schiff base bonds equip CS - PA@Fe eutectogels with excellent injectable, and self-healing abilities. Additionally, due to the presence of phenolic hydroxyl groups of PA, the obtained CS - PA@Fe eutectogels present good adhesiveness. Based on the CS - PA@Fe eutectogels, multifunctional flexible strain sensors with high sensitivity, stability, as well as rapid response speed at wide operating temperature ranges were successfully fabricated. Thus, this study offers a promising strategy for fabricating naturally occurring biopolymers based eutectogels, which show great potential as high-performance flexible strain sensors for next-generation wearable electronic devices.

Organic Mixed Ionic-Electronic Conductors for Bioelectronic Sensors: Materials and Operation Mechanisms

The field of organic mixed ionic-electronic conductors (OMIECs) has gained significant attention due to their ability to transport both electrons and ions, making them promising candidates for various applications. Initially focused on inorganic materials, the exploration of mixed conduction has expanded to organic materials, especially polymers, owing to their advantages such as solution processability, flexibility, and property tunability. OMIECs, particularly in the form of polymers, possess both electronic and ionic transport functionalities. This review provides an overview of OMIECs in various aspects covering mechanisms of charge transport including electronic transport, ionic transport, and ionic-electronic coupling, as well as conducting/semiconducting conjugated polymers and their applications in organic bioelectronics, including (multi)sensors, neuromorphic devices, and electrochromic devices. OMIECs show promise in organic bioelectronics due to their compatibility with biological systems and the ability to modulate electronic conduction and ionic transport, resembling the principles of biological systems. Organic electrochemical transistors (OECTs) based on OMIECs offer significant potential for bioelectronic applications, responding to external stimuli through modulation of ionic transport. An in-depth review of recent research achievements in organic bioelectronic applications using OMIECs, categorized based on physical and chemical stimuli as well as neuromorphic devices and circuit applications, is presented.

Gelatin and Tannic Acid Based Iongels for Muscle Activity Recording and Stimulation Electrodes

Iongels are soft ionic conducting materials, usually composed of polymer networks swollen with ionic liquids (ILs), which are being investigated for applications ranging from energy to bioelectronics. The employment of iongels in bioelectronic devices such as bioelectrodes or body sensors has been limited by the lack of biocompatibility of the ILs and/or polymer matrices. In this work, we present iongels prepared from solely biocompatible materials: (i) a biobased polymer network containing tannic acid as a cross-linker in a gelatin matrix and (ii) three different biocompatible cholinium carboxylate ionic liquids. The resulting iongels are flexible and elastic with Young's modulus between 11.3 and 28.9 kPa. The morphology of the iongels is based on a dual polymer network system formed by both chemical bonding due to the reaction of the gelatin's amines with the polyphenol units and physical interactions between the tannic acid and the gelatin. These biocompatible iongels presented high ionic conductivity values, from 0.003 and up to 0.015 S·cm^-1 at room temperature. Furthermore, they showed excellent performance as a conducting gel in electrodes for electromyography and electrocardiogram recording as well as muscle stimulation.

Emerging iongel materials towards applications in energy and bioelectronics

In the past two decades, ionic liquids (ILs) have blossomed as versatile task-specific materials with a unique combination of properties, which can be beneficial for a plethora of different applications. The additional need of incorporating ILs into solid devices led to the development of a new class of ionic soft-solid materials, named here iongels. Nowadays, iongels cover a wide range of materials mostly composed of an IL component immobilized within different matrices such as polymers, inorganic networks, biopolymers or inorganic nanoparticles. This review aims at presenting an integrated perspective on the recent progress and advances in this emerging type of material. We provide an analysis of the main families of iongels and highlight the emerging types of these ionic soft materials offering additional properties, such as thermoresponsiveness, self-healing, mixed ionic/electronic properties, and (photo)luminescence, among others. Next, recent trends in additive manufacturing (3D printing) of iongels are presented. Finally, their new applications in the areas of energy, gas separation and (bio)electronics are detailed and discussed in terms of performance, underpinning it to the structural features and processing of iongel materials.

Additive Manufacturing of Conducting Polymers: Recent Advances, Challenges, and Opportunities

Conducting polymers (CPs) have been attracting great attention in the development of (bio)electronic devices. Most of the current devices are rigid two-dimensional systems and possess uncontrollable geometries and architectures that lead to poor mechanical properties presenting ion/electronic diffusion limitations. The goal of the article is to provide an overview about the additive manufacturing (AM) of conducting polymers, which is of paramount importance for the design of future wearable three-dimensional (3D) (bio)electronic devices. Among different 3D printing AM techniques, inkjet, extrusion, electrohydrodynamic, and light-based printing have been mainly used. This review article collects examples of 3D printing of conducting polymers such as poly(3,4-ethylene-dioxythiophene), polypyrrole, and polyaniline. It also shows examples of AM of these polymers combined with other polymers and/or conducting fillers such as carbon nanotubes, graphene, and silver nanowires. Afterward, the foremost applications of CPs processed by 3D printing techniques in the biomedical and energy fields, that is, wearable electronics, sensors, soft robotics for human motion, or health monitoring devices, among others, will be discussed.

Orthogonal photochemistry-assisted printing of 3D tough and stretchable conductive hydrogels

3D-printing tough conductive hydrogels (TCHs) with complex structures is still a challenging task in related fields due to their inherent contrasting multinetworks, uncontrollable and slow polymerization of conductive components. Here we report an orthogonal photochemistry-assisted printing (OPAP) strategy to make 3D TCHs in one-pot via the combination of rational visible-light-chemistry design and reliable extrusion printing technique. This orthogonal chemistry is rapid, controllable, and simultaneously achieve the photopolymerization of EDOT and phenol-coupling reaction, leading to the construction of tough hydrogels in a short time (tgel ~30 s). As-prepared TCHs are tough, conductive, stretchable, and anti-freezing. This template-free 3D printing can process TCHs to arbitrary structures during the fabrication process. To further demonstrate the merits of this simple OPAP strategy and TCHs, 3D-printed TCHs hydrogel arrays and helical lines, as proofs-of-concept, are made to assemble high-performance pressure sensors and a temperature-responsive actuator. It is anticipated that this one-pot rapid, controllable OPAP strategy opens new horizons to tough hydrogels.

Elastic and Thermoreversible Iongels by Supramolecular PVA/Phenol Interactions

Iongels have attracted much attention over the years as ion-conducting soft materials for applications in several technologies including stimuli-responsive drug release and flexible (bio)electronics. Nowadays, iongels with additional functionalities such as electronic conductivity, self-healing, thermo-responsiveness, or biocompatibility are actively being searched for high demanding applications. In this work, a simple and rapid synthetic pathway to prepare elastic and thermoreversible iongels is presented. These iongels are prepared by supramolecular crosslinking between polyphenols biomolecules with a hydroxyl-rich biocompatible polymer such as poly(vinyl alcohol) (PVA) in the presence of ionic liquids. Using this strategy, a variety of iongels are obtained by combining different plant-derived polyphenol compounds (PhC) such as gallic acid, pyrogallol, and tannic acid with imidazolium-based ionic liquids, namely 1-ethyl-3-methylimidazolium dicyanamide and 1-ethyl-3-methylimidazolium bromide. A suite of characterization tools is used to study the structural, morphological, mechanical, rheological, and thermal properties of the supramolecular iongels. These iongels can withstand large deformations (40% under compression) with full recovery, revealing reversible transitions from solid to liquid state between 87 and 125 °C. Finally, the polyphenol-based thermoreversible iongels show appropriated properties for their potential application as printable electrolytes for bioelectronics.

A conductive PEDOT/alginate porous scaffold as a platform to modulate the biological behaviors of brown adipose-derived stem cells

The development of three-dimensional conductive scaffolds is vital to support the adhesion, proliferation and myocardial differentiation of stem cells in cardiac tissue engineering. Herein, we describe a facile approach for preparing a poly(3,4-ethylenedioxythiophene)/alginate (PEDOT/Alg) porous scaffold with a wide range of desirable properties. In the PEDOT/Alg scaffold, chemically crosslinked alginate networks are formed using adipic acid hydrazide as the crosslinker, and PEDOT is synthesized in situ in the alginate matrix simultaneously. PEDOT exists in the alginate matrix as particles and its morphology can be modulated by adjusting the ratio of PEDOT/alginate. The results also show that the swelling properties, degradation behaviors, mechanical strength and conductivity of the PEDOT/Alg scaffold can be controlled via adjusting the PEDOT/alginate ratio. The introduction of PEDOT can overcome the brittle nature of the pure alginate scaffold. Moreover, the PEDOT/Alg scaffold exhibits excellent conductivity (as high as 6 × 10^-2 S cm^-1). The introduction of PEDOT improves the protein absorption capacity of the alginate scaffold. To explore its potential application in cardiac tissue engineering, brown adipose-derived stem cells (BADSCs) are seeded in the prepared PEDOT/Alg porous scaffold. The results suggest that the PEDOT/Alg porous scaffold can support the attachment and proliferation of BADSCs. Moreover, it is beneficial for the cardiomyogenic differentiation of BADSCs, especially under electrical stimulation. Overall, we conclude that the PEDOT/Alg porous scaffold may represent an ideal platform to modulate the biological behaviors of BADSCs.

All-Polymer Conducting Fibers and 3D Prints via Melt Processing and Templated Polymerization

Because of their attractive mechanical properties, conducting polymers are widely perceived as materials of choice for wearable electronics and electronic textiles. However, most state-of-the-art conducting polymers contain harmful dopants and are only processable from solution but not in bulk, restricting the design possibilities for applications that require conducting micro-to-millimeter scale structures, such as textile fibers or thermoelectric modules. In this work, we present a strategy based on melt processing that enables the fabrication of nonhazardous, all-polymer conducting bulk structures composed of poly(3,4-ethylenedioxythiophene) (PEDOT) polymerized within a Nafion template. Importantly, we employ classical polymer processing techniques including melt extrusion followed by fiber spinning or fused filament 3D printing, which cannot be implemented with the majority of doped polymers. To demonstrate the versatility of our approach, we fabricated melt-spun PEDOT:Nafion fibers, which are highly flexible, retain their conductivity of about 3 S cm-1 upon stretching to 100% elongation, and can be used to construct organic electrochemical transistors (OECTs). Furthermore, we demonstrate the precise 3D printing of complex conducting structures from OECTs to centimeter-sized PEDOT:Nafion figurines and millimeter-thick 100-leg thermoelectric modules on textile substrates. Thus, our strategy opens up new possibilities for the design of conducting, all-polymer bulk structures and the development of wearable electronics and electronic textiles.

Bio-Based Polymer Electrolytes for Electrochemical Devices: Insight into the Ionic Conductivity Performance

With the continuing efforts to explore alternatives to petrochemical-based polymers and the escalating demand to minimize environmental impact, bio-based polymers have gained a massive amount of attention over the last few decades. The potential uses of these bio-based polymers are varied, from household goods to high end and advanced applications. To some extent, they can solve the depletion and sustainability issues of conventional polymers. As such, this article reviews the trends and developments of bio-based polymers for the preparation of polymer electrolytes that are intended for use in electrochemical device applications. A range of bio-based polymers are presented by focusing on the source, the general method of preparation, and the properties of the polymer electrolyte system, specifically with reference to the ionic conductivity. Some major applications of bio-based polymer electrolytes are discussed. This review examines the past studies and future prospects of these materials in the polymer electrolyte field.

Highly Conductive, Stretchable, and Cell-Adhesive Hydrogel by Nanoclay Doping

Electrically conductive materials that mimic physical and biological properties of tissues are urgently required for seamless brain-machine interfaces. Here, a multinetwork hydrogel combining electrical conductivity of 26 S m^-1 , stretchability of 800%, and tissue-like elastic modulus of 15 kPa with mimicry of the extracellular matrix is reported. Engineering this unique set of properties is enabled by a novel in-scaffold polymerization approach. Colloidal hydrogels of the nanoclay Laponite are employed as supports for the assembly of secondary polymer networks. Laponite dramatically increases the conductivity of in-scaffold polymerized poly(ethylene-3,4-diethoxy thiophene) in the absence of other dopants, while preserving excellent stretchability. The scaffold is coated with a layer containing adhesive peptide and polysaccharide dextran sulfate supporting the attachment, proliferation, and neuronal differentiation of human induced pluripotent stem cells directly on the surface of conductive hydrogels. Due to its compatibility with simple extrusion printing, this material promises to enable tissue-mimetic neurostimulating electrodes.

Conducting Polymer Scaffolds Based on Poly(3,4-ethylenedioxythiophene) and Xanthan Gum for Live-Cell Monitoring

Conducting polymer scaffolds can promote cell growth by electrical stimulation, which is advantageous for some specific type of cells such as neurons, muscle, or cardiac cells. As an additional feature, the measure of their impedance has been demonstrated as a tool to monitor cell growth within the scaffold. In this work, we present innovative conducting polymer porous scaffolds based on poly(3,4-ethylenedioxythiophene) (PEDOT):xanthan gum instead of the well-known PEDOT:polystyrene sulfonate scaffolds. These novel scaffolds combine the conductivity of PEDOT and the mechanical support and biocompatibility provided by a polysaccharide, xanthan gum. For this purpose, first, the oxidative chemical polymerization of 3,4-ethylenedioxythiophene was carried out in the presence of polysaccharides leading to stable PEDOT:xanthan gum aqueous dispersions. Then, by a simple freeze-drying process, porous scaffolds were prepared from these dispersions. Our results indicated that the porosity of the scaffolds and mechanical properties are tuned by the solid content and formulation of the initial PEDOT:polysaccharide dispersion. Scaffolds showed interconnected pore structure with tunable sizes ranging between 10 and 150 μm and Young's moduli between 10 and 45 kPa. These scaffolds successfully support three-dimensional cell cultures of MDCK II eGFP and MDCK II LifeAct epithelial cells, achieving good cell attachment with very high degree of pore coverage. Interestingly, by measuring the impedance of the synthesized PEDOT scaffolds, the growth of the cells could be monitored.

Facile Soaking Strategy Toward Simultaneously Enhanced Conductivity and Toughness of Self-Healing Composite Hydrogels Through Constructing Multiple Noncovalent Interactions

Tough and stretchable conductive hydrogels are desirable for the emerging field of wearable and implanted electronics. Unfortunately, most existing conductive hydrogels have low mechanical strength. Current strategies to enhance mechanical properties include employing tough host gel matrices or introducing specific interaction between conductive polymer and host gel matrices. However, these strategies often involve additional complicated processes. Here, a simple yet effective soaking treatment is employed to concurrently enhance mechanical and conductive properties, both of which can be facilely tailored by controlling the soaking duration. The significant improvements are correlated with co-occurring mechanism of deswelling and multiple noncovalent interactions. The resulting optimal sample exhibits attractive combination of high water content (75 wt %), high tensile stress (∼2.5 MPa), large elongation (>600%), reasonable conductivity (∼25 mS/cm), and fast self-healing property with the aid of hot water. The potential application of gel as a strain sensor is demonstrated. The applicability of this method is not limited to conductive hydrogels alone but can also be extended to strengthen other functional hydrogels with weak mechanical properties.

Bacterial Cellulose Ionogels as Chemosensory Supports

To fully leverage the advantages of ionic liquids for many applications, it is necessary to immobilize or encapsulate the fluids within an inert, robust, quasi-solid-state format that does not disrupt their many desirable, inherent features. The formation of ionogels represents a promising approach; however, many earlier approaches suffer from solvent/matrix incompatibility, optical opacity, embrittlement, matrix-limited thermal stability, and/or inadequate ionic liquid loading. We offer a solution to these limitations by demonstrating a straightforward and effective strategy toward flexible and durable ionogels comprising bacterial cellulose supports hosting in excess of 99% ionic liquid by total weight. Termed bacterial cellulose ionogels (BCIGs), these gels are prepared using a facile solvent-exchange process equally amenable to water-miscible and water-immiscible ionic liquids. A suite of characterization tools were used to study the preliminary (thermo)physical and structural properties of BCIGs, including no-deuterium nuclear magnetic resonance, differential scanning calorimetry, thermogravimetric analysis, scanning electron microscopy, and X-ray diffraction. Our analyses reveal that the weblike structure and high crystallinity of the host bacterial cellulose microfibrils are retained within the BCIG. Notably, not only can BCIGs be tailored in terms of shape, thickness, and choice of ionic liquid, they can also be designed to host virtually any desired active, functional species, including fluorescent probes, nanoparticles (e.g., quantum dots, carbon nanotubes), and gas-capture reagents. In this paper, we also present results for fluorescent designer BCIG chemosensor films responsive to ammonia or hydrogen sulfide vapors on the basis of incorporating selective fluorogenic probes within the ionogels. Additionally, a thermometric BCIG hosting the excimer-forming fluorophore 1,3-bis(1-pyrenyl)propane was devised which exhibited a ratiometric (two-color) fluorescence output that responded precisely to changes in local temperature. The ionogel approach introduced here is simple and has broad generality, offering intriguing potential in (bio)analytical sensing, catalysis, membrane separations, electrochemistry, energy storage devices, and flexible electronics and displays.

Poly(3,4-ethylenedioxythiophene) (PEDOT) Derivatives: Innovative Conductive Polymers for Bioelectronics

Poly(3,4-ethylenedioxythiophene)s are the conducting polymers (CP) with the biggest prospects in the field of bioelectronics due to their combination of characteristics (conductivity, stability, transparency and biocompatibility). The gold standard material is the commercially available poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). However, in order to well connect the two fields of biology and electronics, PEDOT:PSS presents some limitations associated with its low (bio)functionality. In this review, we provide an insight into the synthesis and applications of innovative poly(ethylenedioxythiophene)-type materials for bioelectronics. First, we present a detailed analysis of the different synthetic routes to (bio)functional dioxythiophene monomer/polymer derivatives. Second, we focus on the preparation of PEDOT dispersions using different biopolymers and biomolecules as dopants and stabilizers. To finish, we review the applications of innovative PEDOT-type materials such as biocompatible conducting polymer layers, conducting hydrogels, biosensors, selective detachment of cells, scaffolds for tissue engineering, electrodes for electrophysiology, implantable electrodes, stimulation of neuronal cells or pan-bio electronics.