Highly Conductive, Photolithographically Patternable Ionogels for Flexible and Stretchable Electrochemical Devices

Evolution of the Microrobots: Stimuli-Responsive Materials and Additive Manufacturing Technologies Turn Small Structures into Microscale Robots

The development of functional microsystems and microrobots that have characterized the last decade is the result of a synergistic and effective interaction between the progress of fabrication techniques and the increased availability of smart and responsive materials to be employed in the latter. Functional structures on the microscale have been relevant for a vast plethora of technologies that find application in different sectors including automotive, sensing devices, and consumer electronics, but are now also entering medical clinics. Working on or inside the human body requires increasing complexity and functionality on an ever-smaller scale, which is becoming possible as a result of emerging technology and smart materials over the past decades. In recent years, additive manufacturing has risen to the forefront of this evolution as the most prominent method to fabricate complex 3D structures. In this review, we discuss the rapid 3D manufacturing techniques that have emerged and how they have enabled a great leap in microrobotic applications. The arrival of smart materials with inherent functionalities has propelled microrobots to great complexity and complex applications. We focus on which materials are important for actuation and what the possibilities are for supplying the required energy. Furthermore, we provide an updated view of a new generation of microrobots in terms of both materials and fabrication technology. While two-photon lithography may be the state-of-the-art technology at the moment, in terms of resolution and design freedom, new methods such as two-step are on the horizon. In the more distant future, innovations like molecular motors could make microscale robots redundant and bring about nanofabrication.

Vitrimer ionogels towards sustainable solid-state electrolytes

The growing demand for flexible, stretchable, and wearable devices has boosted the development of ionogels used as polymer electrolytes. Developing healable ionogels based on vitrimer chemistry is a promising approach to improve their lifetimes as these materials are usually subjected to repeated deformation during functioning and are susceptible to damage. In this work, we reported in the first place the preparation of polythioether vitrimer networks based on the not extensively studied associative S-transalkylation exchange reaction using thiol-ene Michael addition. Thanks to the exchange reaction of sulfonium salts with thioether nucleophiles, these materials demonstrated vitrimer properties such as healing and stress relaxation. The fabrication of dynamic polythioether ionogels was then demonstrated by loading 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide or 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM triflate) within the polymer network. The resulting ionogels exhibited Young's modulus of 0.9 MPa and ionic conductivities in the order of 10^-4 S cm^-1 at room temperature. It has been found that adding ionic liquids (ILs) changes the dynamic properties of the systems, most likely due to a dilution effect of the dynamic functions by the IL but also due to a screening effect of the alkyl sulfonium OBrs-couple by the ions of the IL itself. To the best of our knowledge, these are the first vitrimer ionogels based on an S-transalkylation exchange reaction. While the addition of ILs resulted in less efficient dynamic healing at a given temperature, these ionogels can provide materials with more dimensional stability at application temperatures and can potentially pave the way for the development of tunable dynamic ionogels for flexible electronics with a longer lifespan.

Materials development in stretchable iontronics

Stretchable iontronics have recently been developed as an ideal interface to promote the interaction between humans and devices. Since the materials that use ions as charge carriers are typically transparent and stretchable, they have been used to fabricate devices with diverse functions with intrinsic transparency and stretchability. With the development of device design, material design has also been investigated to mitigate the issues associated with ionic materials, such as their weak mechanical properties, poor electrical properties, or poor environmental stabilities. In this review, we describe the recent progress on the design of materials in stretchable iontronics. By classifying stretchable ionic materials into three types of components (ionic conductors, ionic semiconductors, and ionic insulators), the issues each component has and the strategies to solve them are introduced, specifically in terms of molecular interactions. We then discuss the existing hurdles and challenges to be handled and shine light on the possibilities and opportunities from the insight of molecular interactions.

Muscle-Mimetic Highly Tough, Conductive, and Stretchable Poly(ionic liquid) Liquid Crystalline Ionogels with Ultrafast Self-Healing, Super Adhesive, and Remarkable Shape Memory Properties

Here, we report a simple method for preparing muscle-mimetic highly tough, conductive, and stretchable liquid crystalline ionogels which contains only one poly(ionic liquid) (PIL) in an ionic liquid via in situ free radical photohomopolymerization by using nitrogen gas instead of air atmosphere. Due to eliminating the inhibition caused by dissolved oxygen, the polymerization under nitrogen gas has much higher molecular weight, lower critical sol-gel concentration, and stronger mechanical properties. More importantly, benefiting from the unique loofah-like microstructures along with the strong internal ionic interactions, entanglements of long PIL chains and liquid crystalline domains, the ionogels show special optical anisotropic, superstretchability (>8000%), high fracture strength (up to 16.52 MPa), high toughness (up to 39.22 MJ/m³), and have ultrafast self-healing, ultrastrong adhesive, and excellent shape memory properties. Due to its excellent stretchability and good conductive-strain responsiveness, the as-prepared ionogel can be easily applied for high-performance flexible and wearable sensors for motion detecting. Therefore, this paper provides an effective route and developed method to generate highly stretchable conductive liquid crystalline ionogels/elastomers that can be used in widespread flexible and wearable electronics.

Photopolymerizable Ionogel with Healable Properties Based on Dioxaborolane Vitrimer Chemistry

Ionogels are solid polymer gel networks loaded with ionic liquid (IL) percolating throughout each other, giving rise to ionically conducting solid electrolytes. They combine the mechanical properties of polymer networks with the ionic conductivity, non-volatility, and non-flammability of ILs. In the frame of their applications in electrochemical-based flexible electronics, ionogels are usually subjected to repeated deformation, making them susceptible to damage. It appears critical to devise a simple and effective strategy to improve their durability and lifespan by imparting them with healing ability through vitrimer chemistry. In this work, we report the original in situ synthesis of polythioether (PTE)-based vitrimer ionogels using fast photopolymerization through thiol-acrylate Michael addition. PTE-based vitrimer was prepared with a constant amount of the trithiol crosslinker and varied proportions of static dithiol spacers and dynamic chain extender BDB containing dynamic exchangeable boronic ester groups. The dynamic ionogels were prepared using 50 wt% of either 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide or 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate, both of which were selected for their high ionic conductivity. They are completely amorphous (T_g below -30 °C), suggesting they can be used at low temperatures. They are stretchable with an elongation at break around 60%, soft with Young's modulus between 0.4 and 0.6 MPa, and they have high ionic conductivities for solid state electrolytes in the order of 10^-4 S·cm^-1 at room temperature. They display dynamic properties typical of the vitrimer network, such as stress relaxation and healing, retained despite the large quantity of IL. The design concept illustrated in this work further enlarges the library of vitrimer ionogels and could potentially open a new path for the development of more sustainable, flexible electrochemical-based electronics with extended service life through repair or reprocessing.

Ionic Liquid-Based Gels for Applications in Electrochemical Energy Storage and Conversion Devices: A Review of Recent Progress and Future Prospects

Ionic liquids (ILs) are molten salts that are entirely composed of ions and have melting temperatures below 100 °C. When immobilized in polymeric matrices by sol-gel or chemical polymerization, they generate gels known as ion gels, ionogels, ionic gels, and so on, which may be used for a variety of electrochemical applications. One of the most significant research domains for IL-based gels is the energy industry, notably for energy storage and conversion devices, due to rising demand for clean, sustainable, and greener energy. Due to characteristics such as nonvolatility, high thermal stability, and strong ionic conductivity, IL-based gels appear to meet the stringent demands/criteria of these diverse application domains. This article focuses on the synthesis pathways of IL-based gel polymer electrolytes/organic gel electrolytes and their applications in batteries (Li-ion and beyond), fuel cells, and supercapacitors. Furthermore, the limitations and future possibilities of IL-based gels in the aforementioned application domains are discussed to support the speedy evolution of these materials in the appropriate applicable sectors.

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.

Development of a Borane-(Meth)acrylate Photo-Click Reaction

In the development of 3D printing fuels, there is a need for new photoinitiating systems working under mild conditions and/or leading to polymers with new and/or enhanced properties. In this context, we introduce herein N-heterocyclic carbene-borane complexes as reagents for a new type of photo-click reaction, the borane-(meth)acrylate click reaction. Remarkably, the higher bond number of boranes relative to thiols induced an increase of the network density associated with faster polymerization kinetics. Solid-state NMR evidenced the strong participation of the boron centers on the network properties, while DMA and AFM showed that the materials exhibit improved mechanical properties, as well as reduced solvent swelling.

Highly Transparent, Stretchable, and Conductive Supramolecular Ionogels Integrated with Three-Dimensional Printable, Adhesive, Healable, and Recyclable Character

In this work, we report the easy fabrication of highly transparent (optical transmittance above 93%), stretchable (1500-2500% elongation at break), and conductive (up to 2.25 S m^-1 at 25 °C) supramolecular ionogels that simultaneously integrate with three-dimensional (3D) printable, healable, adhesive, and recyclable character. The supramolecular ionogel is designed using a linear amphiphilic poly(urethane-urea) (PUU) copolymer and ionic liquid (IL) as the elastic scaffold and electrolyte, respectively, via a simple cosolvent method. Intriguingly, the 3D-printed highly conductive (2.25 S m^-1 at 25 °C) supramolecular ionogel structure shows record-high mechanical performance with a breaking tensile strain and stress of 945% and 1.51 MPa, respectively, and is able to lift 3400× or bear 10000× its weight without fracture. Furthermore, both the solution casting and 3D-printed ionogel films show high sensitivity and reliability for sensing a wide range of strains, including various human motions. The results present some new insights into the structural, mechanical, and functional design of novel multifunctional ionogels with distinguished mechanical performance and tractable processability, which will extend them to a wide range of flexible electronic applications, including artificial intelligence, wearable/conformable electronics, human/machine interactions, soft robotics, etc.

3D Printable, Highly Stretchable, Superior Stable Ionogels Based on Poly(ionic liquid) with Hyperbranched Polymers as Macro-cross-linkers for High-Performance Strain Sensors

Stretchable ionogels have recently emerged as promising soft and safe ionic conductive materials for use in wearable and stretchable electrochemical devices. However, the complex preparation process and insufficient thermomechanical stability greatly limit the precise rapid fabrication and application of stretchable ionogels. Here, we report an in situ 3D printing method for fabricating high-performance single network chemical ionogels as advanced strain sensors. The ionogels consist of a special cross-linking network constructed by poly(ionic liquid) and hyperbranched polymer (macro-cross-linkers) that exhibits high stretchability (>1000%), superior room-temperature ionic conductivity (up to 5.8 mS/cm), and excellent thermomechanical stability (-75 to 250 °C). The strain sensors based on ionogels have a low response time (200 ms), high sensitivity with temperature independence, long-term durability (2000 cycles), and excellent temperature tolerance (-60 to 250 °C) and can be used as human motion sensors. This work provides a new strategy to design highly stretchable and superior stable electronic devices.

Work-hardening Photopolymer from Renewable Photoactive 3,3'-(2,5-Furandiyl)bisacrylic Acid

The design of a photopolymer around a renewable furan-derived chromophore is presented herein. An optimised semi-continuous oxidation method using MnO2 affords 2,5-diformylfuran from 5-(hydroxymethyl)furfural in gram quantities, allowing the subsequent synthesis of 3,3'-(2,5-furandiyl)bisacrylic acid in good yield and excellent stereoselectivity. The photoactivity of the diester of this monomer is confirmed by reaction under UV irradiation, and the proposed [2+2] cycloaddition mechanism supported further by TD-DFT calculations. Oligoesters of the photoreactive furan diacid with various aliphatic diols are prepared via chemo- and enzyme-catalysed polycondensation. The latter enzyme-catalysed (Candida antarctica lipase B) method results in the highest Mn (3.6 kDa), suggesting milder conditions employed with this protocol minimised unwanted side reactions, including untimely [2+2] cycloadditions, whilst preserving the monomer's photoactivity and stereoisomerism. The photoreactive polyester is solvent cast into a film where subsequent initiator-free UV curing leads to an impressive increase in the material stiffness, with work-hardening characteristics observed during tensile strength testing.

Microfluidic chip with pillar arrays for controlled production and observation of lipid membrane nanotubes

Lipid membrane nanotubes (NTs) are a widespread template for in vitro studies of cellular processes happening at high membrane curvature. Traditionally NTs are manufactured one by one, using sophisticated membrane micromanipulations, while simplified methods for controlled batch production of NTs are in growing demand. Here we propose a lab-on-a-chip (LOC) approach to the simultaneous formation of multiple NTs with length and radius controlled by the chip design. The NTs form upon rolling silica microbeads covered by lipid lamellas over the pillars of a polymer micropillar array. The array's design and surface chemistry set the geometry of the resulting free-standing NTs. The integration of the array inside a microfluidic chamber further enables fast and turbulence-free addition of components, such as proteins, to multiple preformed NTs. This LOC approach to NT production is compatible with the use of high power objectives of a fluorescence microscope, making real-time quantification of the different modes of the protein activity in a single experiment possible.

3D Printing Microactuators for Soft Microrobots

Current additive manufacturing, including three-dimensional (3D) and so-called four-dimensional printing, of soft robotic devices is limited to millimeter sizes. In this study, we present additive manufacturing of soft microactuators and microrobots to fabricate even smaller structures in the micrometer domain. Using a custom-built extrusion 3D printer, microactuators are scaled down to a size of 300 × 1000 μm², with minimum thickness of 20 μm. Microactuators combined with printed body and electroactive polymers to drive the actuators are fabricated from computer-aided design model of the device structure. To demonstrate the ease and versatility of 3D printing process, microactuators with varying lengths ranging from 1000 to 5000 μm are fabricated and operated. Likewise, microrobotic devices consisting of a rigid body and individually controlled free-moving arms or legs are 3D printed to explore the microfabrication of soft grippers, manipulators, or microrobots through simple additive manufacturing technique.

High-performance double-network ionogels enabled by electrostatic interaction

Production of highly conductive and mechanically robust ionogels is urgently needed for the development of diverse flexible electrical devices, but it remains challenging. Herein, we report a facile strategy to prepare high-performance ionogels (ionic conductivity of 1.9 S m-1, fracture strain of 170%) via electrostatic interaction between mechanically robust charged gel double networks and conductive ionic liquids. Ionogels based on charged polymer networks (with electrostatic interaction) exhibit obvious higher optical transmittance, ionic conductivity, and better mechanical properties compared with those based on neutral polymer networks (without electrostatic interaction). Ionic conductivity and mechanical properties of the ionogels can also be regulated by the double-network structure of the gels. We further develop an ionic skin sensor with the high-performance ionogels used as ionic conductors, which can exhibit excellent sensing performance even under harsh conditions. We envision that this new class of high-performance ionogels would be an attractive alternative to traditional hydrogels, and would extend the applications of ionic conductors to extreme environments.

A chemically bonded supercapacitor using a highly stretchable and adhesive gel polymer electrolyte based on an ionic liquid and epoxy-triblock diamine network

Despite significant advances in the development of flexible gel polymer electrolytes (GPEs), there are still problems to be addressed to apply them to flexible electric double layer capacitors (EDLCs), including interfacial interactions between the electrolyte and electrode under deformation. Previously reported EDLCs using GPEs have laminated structures with weak interfacial interactions between the electrode and electrolyte, leading to fragility upon elongation and low power density due to lower utilization of the surface area of the carbon material in the electrode. To overcome these problems, we present a new strategy for constructing an epoxy-based GPE that can provide strong adhesion between electrode and electrolyte. The GPE is synthesized by polymerization of epoxy and an ionic liquid. This GPE shows high flexibility up to 509% and excellent adhesive properties that enable strong chemical bonding between the electrode and electrolyte. Moreover, the GPE is stable at high voltage and high temperature with high ionic conductivity of ∼10⁻³ S cm⁻¹. EDLCs based on the developed GPE exhibit good compatibility between the electrode and electrolyte and work properly when deformed. The EDLCs also show a high specific capacitance of 99 F g⁻¹, energy density of 113 W h kg⁻¹, and power density of 4.5 kW g⁻¹. The excellent performance of the GPE gives it tremendous potential for use in next generation electronic devices such as wearable devices.

A chemically bonded supercapacitor using a stretchable and adhesive gel polymer electrolyte based on ionic liquid and epoxy for flexible devices.

Conjugated Polymer Actuators and Devices: Progress and Opportunities

Conjugated polymers (CPs), as exemplified by polypyrrole, are intrinsically conducting polymers with potential for development as soft actuators or "artificial muscles" for numerous applications. Significant progress has been made in the understanding of these materials and the actuation mechanisms, aided by the development of physical and electrochemical models. Current research is focused on developing applications utilizing the advantages that CP actuators have (e.g., low driving potential and easy to miniaturize) over other actuating materials and on developing ways of overcoming their inherent limitations. CP actuators are available as films, filaments/yarns, and textiles, operating in liquids as well as in air, ready for use by engineers. Here, the milestones made in understanding these unique materials and their development as actuators are highlighted. The primary focus is on the recent progress, developments, applications, and future opportunities for improvement and exploitation of these materials, which possess a wealth of multifunctional properties.

Novel Chemical Cross-Linked Ionogel Based on Acrylate Terminated Hyperbranched Polymer with Superior Ionic Conductivity for High Performance Lithium-Ion Batteries

A new family of chemical cross-linked ionogel is successfully synthesized by photopolymerization of hyperbranched aliphatic polyester with acrylate terminal groups in an ionic liquid of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄). The microstructure, viscoelastic behavior, mechanical property thermal stability, and ionic conductivities of the ionogels are investigated systematically. The ionogels exhibit high mechanical strength (up to 1.6 MPa) and high mechanical stability even at temperatures up to 200 °C. It is found to be thermally stable up to 371.3 °C and electrochemically stable above 4.3 V. The obtained ionogels show superior ionic conductivity over a wide temperature range (from 1.2 × 10^-3 S cm^-1 at 20 °C up to 5.0 × 10^-2 S cm^-1 at 120 °C). Moreover, the Li/LiFePO₄ batteries based on ionogel electrolyte with LiBF₄ show a higher specific capacity of 153.1 mAhg^-1 and retain 98.1% after 100 cycles, exhibiting very stable charge/discharge behavior with good cycle performance. This work provides a new method for fabrication of novel advanced gel polymer electrolytes for applications in lithium-ion batteries.

Facile Fabrication of a Self-Healing Temperature-Sensitive Sensor Based on Ionogels and Its Application in Detection Human Breath

The biocompatible strechable ionogels were prepared by a facile solution-processed method. The ionogels showed outstanding stretchable and self-healing properties. The electrical property could revert to its original state after 4 s. The repaired ionogels could still bear stretching about 150%. Moreover, the ionogels exhibited high sensitivity and wide-detection range to temperature. The temperature-sensitive sensor could detect the human breath frequency and intensity, showing potential application in detecting disease.

Chemically crosslinked liquid crystalline poly(ionic liquid)s/halloysite nanotubes nanocomposite ionogels with superior ionic conductivity, high anisotropic conductivity and a high modulus

A novel type of chemically crosslinked liquid crystalline nanocomposite ionogel electrolyte based on poly(ionic liquid) (PIL) with superior ionic conductivity and high anisotropic conductivity was designed and synthesized using the in situ photopolymerization of sheared soft ionogels containing charged halloysite nanotubes (HNTs) and ionic liquid monomers. The oriented structure was investigated using scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS). The chemically crosslinked backbone of the PIL and the addition of HNTs endowed the flexible ionogels with a combined very high modulus (up to 26.7 MPa) and mechanical strength (up to 4.4 MPa). Crucially, the obtained ionogels exhibited high mechanical stability even at temperatures up to 200 °C. Remarkably, in terms of the conductivities, the resulting pre-sheared ionogels displayed superior room temperature ionic conductivity (up to 6 mS cm-1) and a very high conductivity anisotropy ratio (up to 1600), owing to the alignment of the HNTs with oppositely charged surfaces and the high ionic conductivity of the polyelectrolyte PILs. Furthermore, flexible solid-state supercapacitor devices based on the high ion-conductive nanocomposite ionogels were fabricated, which demonstrated high and temperature-dependent specific capacitance, and remarkable cycling stability and flexible performance.

Novel fabrication of soft microactuators with morphological computing using soft lithography

A simple and cost-effective method for the patterning and fabrication of soft polymer microactuators integrated with morphological computation is presented. The microactuators combine conducting polymers to provide the actuation, with spatially designed structures for a morphologically controlled, user-defined actuation. Soft lithography is employed to pattern and fabricate polydimethylsiloxane layers with geometrical pattern, for use as a construction element in the microactuators. These microactuators could obtain multiple bending motions from a single fabrication process depending on the morphological pattern defined in the final step. Instead of fabricating via conventional photolithography route, which involves multiple steps with different chromium photomasks, this new method uses only one single design template to produce geometrically patterned layers, which are then specifically cut to obtain multiple device designs. The desired design of the actuator is decided in the final step of fabrication. The resulting microactuators generate motions such as a spiral, screw, and tube, using a single design template.