A universal packaging substrate for mechanically stable assembly of stretchable electronics.

Stretchable electronics commonly assemble multiple material modules with varied bulk moduli and surface chemistry on one packaging substrate. Preventing the strain-induced delamination between the module and the substrate has been a critical challenge. Here we develop a packaging substrate that delivers mechanically stable module/substrate interfaces for a broad range of stiff and stretchable modules with varied surface chemistries.

Creep-free polyelectrolyte elastomer for drift-free iontronic sensing.

Artificial pressure sensors often use soft materials to achieve skin-like softness, but the viscoelastic creep of soft materials and the ion leakage, specifically for ionic conductors, cause signal drift and inaccurate measurement. Here we report drift-free iontronic sensing by designing and copolymerizing a leakage-free and creep-free polyelectrolyte elastomer containing two types of segments: charged segments having fixed cations to prevent ion leakage and neutral slippery segments with a high crosslink density for low creep. We show that an iontronic sensor using the polyelectrolyte elastomer barely drifts under an ultrahigh static pressure of 500 kPa (close to its Young's modulus), exhibits a drift rate two to three orders of magnitude lower than that of the sensors adopting conventional ionic conductors and enables steady and accurate control for robotic manipulation.

Microstructured Polyelectrolyte Elastomer-Based Ionotronic Sensors with High Sensitivities and Excellent Stability for Artificial Skins.

High-performance flexible pressure sensors are highly demanded for artificial tactile sensing. Using ionic conductors as the dielectric layer has enabled ionotronic pressure sensors with high sensitivities owing to giant capacitance of the electric double layer (EDL) formed at the ionic conductor/electronic conductor interface. However, conventional ionotronic sensors suffer from leakage, which greatly hinders long-term stability and practical applications.

Experimental characterization of elastocapillary and osmocapillary effects on multi-scale gel surface topography.

Surface topography significantly affects various surface properties of polymer gels. Unlike conventional materials where surface topography is largely a geometric property, the surface topography of a polymer gel is governed by the competition between capillary, elastic, and osmotic effects, which leads to complex stimuli-responsive effects. Elastocapillary deformation and osmocapillary phase separation are two phenomena that are known to flatten gel surface topography.

Polyelectrolyte elastomer-based ionotronic sensors with multi-mode sensing capabilities via multi-material 3D printing.

Stretchable ionotronics have drawn increasing attention during the past decade, enabling myriad applications in engineering and biomedicine. However, existing ionotronic sensors suffer from limited sensing capabilities due to simple device structures and poor stability due to the leakage of ingredients. In this study, we rationally design and fabricate a plethora of architected leakage-free ionotronic sensors with multi-mode sensing capabilities, using DLP-based 3D printing and a polyelectrolyte elastomer.

Progress of Hydrophobic Ionogels: A Review.

The emergence of hydrophobic ionogels composed of hydrophobic polymer matrices and hydrophobic ionic liquids has drastically broadened the applications of ionic devices, especially for underwater explorations. Compared with traditional ionogels, hydrophobic ones are capable of achieving long-term stability in ambient and aqueous environments. In this review, the latest research developments of intrinsically hydrophobic ionogels are summarized, with particular emphases placed on the materials, mechanisms and applications.

Encapsulating eutectogels for stretchable humidity-resistant strain sensors.

Eutectogels are stretchable ionic conductors extensively developed in recent years, owing to their distinct advantages of low cost, non-volatility, non-toxicity, and outstanding biocompatibility. However, the susceptibility to humidity caused by the exchange of water molecules between the interiors of eutectogels and the external environment greatly restricts their practical applications. Here, a dip-coating strategy is proposed to fabricate a P(MEA--IBA) elastomer-coated P(AAC--AAM) eutectogel to achieve satisfactory humidity-resistant capability.

Stretchable Heterogeneous Polymer Networks of High Adhesion and Low Hysteresis.

Adhesives are ubiquitous, but the mutual exclusion between hyperelasticity and adhesiveness impedes their uses in emerging techniques such as flexible/stretchable electronics. Herein, we propose a strategy to synthesize hyperelastic adhesives (HEAs), by designating hyperelasticity and adhesiveness to the bulk and the surface of a polymer network, respectively. The bulk is hyperelastic but nonadhesive, and the surface is viscoelastic but adhesive, while the HEA is hyperelastic and adhesive.

Strong Interfaces Enable Efficient Load Transfer for Strong, Tough, and Impact-Resistant Hydrogel Composites.

Many biological hydrogels are mechanically robust to bear quasi-static and impact loads. In contrast, the mechanical properties of synthetic hydrogels against impact loads remain substantially unexplored, albeit their mechanical robustness under quasi-static loads has been extensively developed. Here, we report on the design and synthesis of strong, tough, and impact-resistant hydrogel composites by reinforcing Ca-alginate/polyacrylamide hydrogels with glass fabrics and conferring strong interfaces between the hydrogel matrix and the fibers.

Harnessing osmotic swelling stress for robust hydrogel actuators.

The volumetric expansion of hydrogels driven by osmotic swelling stress has enabled hydrogel actuators for myriad applications. However, most existing studies disregard optimizing the osmotic swelling stress for powerful actuation and simply utilize the osmotic swelling stress to trigger certain modes of actuation. In this work, we probe the osmotic swelling stress of hydrogels using polyacrylamide as a model system.

Highly stable flexible pressure sensors with a quasi-homogeneous composition and interlinked interfaces.

Electronic skins (e-skins) are devices that can respond to mechanical stimuli and enable robots to perceive their surroundings. A great challenge for existing e-skins is that they may easily fail under extreme mechanical conditions due to their multilayered architecture with mechanical mismatch and weak adhesion between the interlayers. Here we report a flexible pressure sensor with tough interfaces enabled by two strategies: quasi-homogeneous composition that ensures mechanical match of interlayers, and interlinked microconed interface that results in a high interfacial toughness of 390 J·m.

Simulation of the peel of hydrogels with stiff backing.

In this work, the peel of hydrogels under a stiff backing constraint was studied using a finite element method. The finite element method was first validated by comparing the simulation results to theoretical predictions and experimental measurements. Then, the method was used to investigate the effects of adhesion thickness, adhesion length and backing thickness on the peel behaviors, as well as the stress distribution within the adhesion layer.

Tough porous nanocomposite hydrogel for water treatment.

Developing a cost-effective, stable, and recyclable adsorbent with high adsorption capacity and rapid adsorption kinetics is highly demanded for water treatment but has been proven challenging. Herein, we report a one-step strategy to synthesize tough porous nanocomposite hydrogel, by introducing biochar nanoparticles and interconnected pores into a polyacrylamide hydrogel matrix as an exemplary system. The polyacrylamide hydrogel provides the overall mechanical strength to carry loads and facilitate recycling, the biochar provides adsorptive locus for high adsorption capacity, and the interconnected pores expedite solvent transport for rapid adsorption kinetics.

Fatigue Damage-Resistant Physical Hydrogel Adhesion.

Strong adhesion between hydrogels and various engineering surfaces has been achieved; yet, achieving fatigue-resistant hydrogel adhesion remains challenging. Here, we examine the fatigue of a specific type of hydrogel adhesion enabled by hydrogen bonds and wrinkling and show that the physical interactions-based hydrogel adhesion can resist fatigue damage. We synthesize polyacrylamide hydrogel as the adherend and poly(acrylic acid--acrylamide) hydrogel as the adhesive.

Direct-ink-write printing of hydrogels using dilute inks.

Direct-ink-write (DIW) printing has been used in myriad applications. Existing DIW printing relies on inks of specific rheology to compromise with printing process, imposing restrictions on the choice of printable materials. Reported ink viscosity ranges from 10 to 10 Pa·s.

Ionotronic Luminescent Fibers, Fabrics, and Other Configurations.

A family of recently developed devices, hydrogel ionotronics, uses hydrogels as ionic conductors, and uses hydrophobic elastomers as dielectrics. This development has posed a challenge: integrate hydrogels and hydrophobic elastomers-in various manufacturing processes-with strong, stretchable, and transparent adhesion. Here, a multistep dip-coating process is described to enable hydrogel ionotronics of diverse configurations.

Hydrogel Paint.

For a hydrogel coating on a substrate to be stable, covalent bonds polymerize monomer units into polymer chains, crosslink the polymer chains into a polymer network, and interlink the polymer network to the substrate. The three processes-polymerization, crosslinking, and interlinking-usually concur. This concurrency hinders widespread applications of hydrogel coatings.

Design Molecular Topology for Wet-Dry Adhesion.

Recent innovations highlight the integration of diverse materials with synthetic and biological hydrogels. Examples include brain-machine interfaces, tissue regeneration, and soft ionic devices. Existing methods of strong adhesion mostly focus on the chemistry of bonds and the mechanics of dissipation but largely overlook the molecular topology of connection.

Fatigue Fracture of Self-Recovery Hydrogels.

Hydrogels of superior mechanical behavior are under intense development for many applications. Some of these hydrogels can recover their stress-stretch curves after many loading cycles. These hydrogels are called self-recovery hydrogels or even fatigue-free hydrogels.

Bonding dissimilar polymer networks in various manufacturing processes.

Recently developed devices mimic neuromuscular and neurosensory systems by integrating hydrogels and hydrophobic elastomers. While different methods are developed to bond hydrogels with hydrophobic elastomers, it remains a challenge to coat and print various hydrogels and elastomers of arbitrary shapes, in arbitrary sequences, with strong adhesion. Here we report an approach to meet this challenge.

3D Printing of Transparent and Conductive Heterogeneous Hydrogel-Elastomer Systems.

A hydrogel-dielectric-elastomer system, polyacrylamide and poly(dimethylsiloxane) (PDMS), is adapted for extrusion printing for integrated device fabrication. A lithium-chloride-containing hydrogel printing ink is developed and printed onto treated PDMS with no visible signs of delamination and geometrically scaling resistance under moderate uniaxial tension and fatigue. A variety of designs are demonstrated, including a resistive strain gauge and an ionic cable.