Stretchable elastomer paints form conductive and insulating layers directly on curved surfaces, enabling multilayer soft electronics for sensing and energy harvesting without transfers or specialized fabrication steps.
(Nanowerk Spotlight) Soft electronic devices need to work on objects that are not flat or rigid. These objects may stretch, bend, or have uneven textures, yet most electronic materials are still designed for surfaces that stay still and smooth. This gap matters because several emerging technologies, such as wearable health monitors and soft robotic systems, require electrical components that can match body movement or irregular 3D shapes.
Methods that build devices on flat films and then attach them to the final surface are often unreliable. They can lead to fractures, weak contact, or detachment, especially when the surface curves sharply or moves during operation.
Multiscale molecular architecture and preparation process of ionic conductive elastomer paint (ICEP) and ionic conductive elastomer coating (ICEC). a) Schematics illustrating the challenges of realizing conformal soft ionotronics on complex surfaces through a conventional method combining ex situ fabrication and subsequent transfer, which is prone to cause interfacial gaps and wrinkles of devices. b) Schematics illustrating the synthesis of u-TPU, i.e., the polymer backbone of ICEP and ICEC. c) Top panel: The direct conformal manufacturing process of multilayer ionotronic devices on geometrically complex surfaces. Bottom panel: Zoomed-in views depicting the hierarchical structure of the ICEC and the multiscale competition mechanism underpinning the largely tunable properties of ICEC: At the chain scale, phase separation-mediated strengthening competes with LiTFSI-induced softening; At the molecular scale, the reinforcement provided by hydrogen bonding arrays and 𝜋–𝜋 stacking interactions counterbalances the softening tendency arising from LiTFSI’s plasticizing behavior. d–f) Photographs of highly conformal ICEC formed on complex curved surfaces (e.g., a Möbius strip), irregular surfaces, and stretchable surfaces. (Image: Reproduced from DOI:10.1002/adfm.202519415, CC BY) (click on image to enlarge)
The authors introduce two liquid elastomer paints that can be applied with common tools and then dried into solid layers. One paint becomes a stretchable electrical conductor that moves charge using ions. The other becomes a flexible insulator. These layers can be built up in sequence on complex objects, creating devices such as sensors and generators without requiring film transfer, special curing conditions, or surface pretreatment.
The ionic conductive elastomer paint is based on a polyurethane that has been modified to dissolve in a solvent along with a lithium salt called lithium bis(trifluoromethanesulfonyl)imide. This creates a viscous liquid that can be brushed, dipped, or printed onto a surface. After the solvent evaporates, the coating forms a solid elastomer that conducts electric charge through mobile ions rather than electrons.
Because the polymer is not chemically cross linked, dried coatings can be dissolved back into the same solvent and recovered for reuse. The second paint, also an elastomer, forms a dielectric layer that blocks electrical charge. Together, the two paints enable stacked structures to be formed directly in place.
The mechanical behavior of the conductive coating is one of its defining advantages. By adjusting the amount of lithium salt in the paint, the final elastomer can be made either strong and firm or highly stretchable. At 15 percent salt by weight, the coating reaches a tensile strength of over 35 megapascals and a modulus of 4.35 megapascals. These values describe how much force the material can withstand and how much it resists stretching.
At 45 percent salt, the tensile strength is lower and the modulus drops to 0.71 megapascals, but the stretch at break increases to 1280 percent. This means a sample can be pulled to more than twelve times its length before breaking. At this higher salt level, the hysteresis ratio is 12 percent at 500 percent strain, meaning the coating returns close to its original size after being stretched and released. This low energy loss is useful for devices that need repeatable performance under motion.
The study attributes this tunability to two opposing mechanisms in the polymer. Urea groups in the polyurethane form temporary bonds that increase strength and stiffness. The lithium salt, along with small amounts of absorbed water, disrupts these interactions and increases chain mobility at higher concentrations. This balance allows one formulation to span a wide range of mechanical properties.
Adhesion to the target surface is another core feature. When applied as a liquid, the paint fills microscopic pits and textures. Once dried, the coating bonds physically to the substrate. Adhesion energy reaches about 1000 joules per square meter on acrylic, and between about 35 and 200 joules per square meter on metal and glass, depending on salt level. These values are higher than those measured when prefabricated elastomer films are laminated onto the same substrates, which supports the benefit of in situ formation.
The ionic conductivity of the dried coating increases with salt content, from 1.08 × 10−3 S m−1 at the lowest tested level to 53.23 × 10−3 S m−1 at the highest.
The coatings are also transparent, transmitting more than 80 percent of visible light for typical thicknesses. They maintain conductivity under repeated stretching and do not crack or delaminate when the underlying surface changes shape.
The paper demonstrates several devices made with the paints. In one example, a single conductive layer is brushed onto a brick to create a moisture induced electricity generator. The top surface of the coating absorbs more water vapor from the air than the side touching the brick. This difference produces a gradient in ion concentration that drives charge movement, generating about 200 millivolts and 0.25 microamperes at fixed humidity and temperature.
A resistive strain sensor is formed by coating the inner wall of a pneumatic robotic arm. As the arm bends, the coating stretches, changing its electrical resistance in line with the motion. Because the high salt formulation has low hysteresis, the readings remain stable when the arm flexes repeatedly.
The two paints are also used to make multilayer devices. A triboelectric nanogenerator is created by dipping a rod first into the conductive paint and then into the dielectric paint. When tapped against a metal plate, the device outputs about 2 volts and 15 microamperes. Similar generators made by brushing onto curved surfaces produce about 0.6 volts and 30 microamperes during finger tapping.
A capacitive pressure sensor is built by placing the dielectric layer between two conductive layers on a curved robotic surface. As pressure increases, the capacitance rises. The device monitors how tightly a robotic arm grips a baseball and remains intact under repeated bending.
Because the coatings can be removed with the same solvent used in their preparation, they support repair and material recovery without damaging the surface beneath. The paints also cure under ambient conditions without requiring ultraviolet light, vacuum, or controlled atmosphere.
By combining stretchable ion-based conductivity, direct adhesion, multilayer compatibility, and material recovery, the elastomer paints described in the paper show a way to make soft electronic systems that work on complex surfaces without relying on film transfer or specialized fabrication environments.
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