Freeze-dried hydrogel powders sprayed onto wet surfaces merge into functional coatings in under five seconds, enabling rapid soft robot fabrication.
(Nanowerk Spotlight) Soft robots can bend, stretch, and adapt to their surroundings, but flexibility alone is not enough for most applications. A soft surgical device might need magnetic particles to enable remote guidance through the body. A robotic gripper might benefit from thermochromic pigments that signal temperature changes. A wearable sensor requires materials that detect and transmit information about its environment. These responsive capabilities come from specialized functional materials that must be distributed evenly throughout a flexible matrix and applied as thin coatings, or functional skins, that conform to whatever shape the underlying robot takes.
Engineers have struggled to create such skins. Two main strategies have emerged, neither fully satisfactory. The first involves adhering pre-fabricated thin films directly onto target surfaces. This works reasonably well for flat or gently curved objects but fails when confronted with complex three-dimensional shapes.
The underlying problem is geometric: just as gift wrap cannot smoothly cover a sphere without folding or tearing, thin films cannot conform to surfaces with compound curvature. Viscoelastic materials that flow and mold more easily offer a partial solution, but their bulk tends to fill in and obscure fine surface textures rather than preserving them.
The second strategy deposits liquid precursors onto surfaces through dipping, spraying, painting, or printing, then cures them in place. Liquids can wet and conform to intricate geometries, avoiding the solid-contact problems of films. But liquid-based methods face their own trade-offs. Low-viscosity solutions spray easily but lack the cohesion needed for thick coatings and tend to pool unevenly under gravity. High-viscosity formulations maintain their shape better but demand complex printing equipment and custom recipes for each application.
Both approaches require curing steps (evaporation, heating, or ultraviolet exposure) that add time and constrain where coatings can be applied.
A research team based primarily at Zhejiang University, with collaborators from Fudan University, Chengdu University, and Universiti Teknologi Malaysia, has developed a method that sidesteps these limitations. Their approach, published in Advanced Functional Materials (“Spraying Conformal Hydrogel Skins as Functional Platform”), uses engineered powders that transform into continuous hydrogel skins upon contact with water.
The technique requires no elaborate equipment, no extended curing, and no specialized formulations for different substrate shapes. A simple spray of powder followed by a mist of water produces a functional coating in less than 5 seconds.
Design and demonstration of the powder-based spraying strategy for conformal functional hydrogel skins. a Functionally lyophilized hydrogel powders are sprayed onto a wet substrate surface, forming a conformal powder coating. b The powders undergo rehydration, transforming into swollen functional microgels. These hydrated microgels establish contact and subsequently coalesce into a continuous network via physical interactions. c The conformal functional hydrogel skin forms and adheres robustly to the substrate. d–f, A maple leaf, a scallop shell, and a conch coated with conformal magnetic (d), thermochromic (e), and fluorescent (f) hydrogel skins, respectively. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
The material at the heart of this system consists of two polymers: polyacrylic acid and polyethyleneimine. When dissolved separately and then mixed, these polymers gel almost instantaneously through electrostatic attraction between their charged molecular groups and hydrogen bonding between specific chemical sites.
Unlike conventional hydrogels that rely on permanent chemical cross-links, this network forms through reversible physical interactions. The researchers exploited this property by freeze-drying the hydrogel and grinding it into fine particles averaging 89.25 µm in diameter, small enough to settle into microscopic surface features.
The fabrication process requires only two steps. A powder gun deposits the particles onto a substrate that has been plasma-treated to improve adhesion and lightly moistened with water. The powders immediately absorb this surface moisture and stick loosely in place. A subsequent mist spray triggers full rehydration, causing each particle to swell into a soft microgel. These microgels then contact their neighbors and merge into a continuous skin through the same physical interactions that formed the original bulk material. The entire transformation completes within seconds without mechanical intervention.
To demonstrate that functional components could be incorporated, the researchers embedded neodymium iron boron magnetic microparticles into the hydrogel before freeze-drying. These particles, averaging about 5 µm in diameter, were coated with a thin silica shell to prevent corrosion in wet environments. The resulting magnetic powders dispersed uniformly within the polymer matrix, anchored by the pre-formed network structure that prevents the settling problems common in liquid precursor systems.
The magnetic hydrogel skin exhibited strong mechanical performance. At 33% magnetic particle loading by weight, the material had a Young’s modulus (a measure of stiffness) of approximately 140 kPa, similar to soft biological tissues. Fracture toughness exceeded 800 J/m². Thin strips could stretch to more than eight times their original length without rupturing. The skin remained intact when folded to a radius of curvature as tight as 200 µm and stayed firmly bonded to elastomer films during repeated stretching cycles without delamination.
The coating adhered strongly to a wide range of substrates, with interfacial toughness values ranging from 480 J/m² for silicone rubber to above 690 J/m² for glass. Notably, adhesion to biological tissues remained low at only 2 to 4 J/m², which would prevent the coating from causing tissue damage during biomedical applications.
The conformability of the powder-based approach sets it apart from alternatives. When applied to textured leather or silicon pyramid arrays with tip features as small as 60 µm, the hydrogel skin faithfully reproduced the underlying topography rather than bridging over it. Cross-sectional imaging revealed seamless interfaces without voids, and micro-computed tomography (an imaging technique that creates three-dimensional views of internal structure) confirmed a continuous architecture with uniformly distributed magnetic particles.
The team constructed several soft robots to demonstrate practical applications. A butterfly-shaped device, fabricated by coating both sides of an elastomer film with magnetic hydrogel skin approximately 180 µm thick, achieved wing flapping angles exceeding 90° under magnetic control. At an oscillating field of 20 mT and 10 Hz, the robot maintained consistent flapping at a frequency comparable to actual butterflies.
A natural maple leaf, selected for its finger-like lobes suitable for grasping, became a functional gripper after the researchers applied magnetic coatings approximately 160 µm thick on both sides. The conformal skin preserved the leaf’s original appearance and intricate vein texture while enabling magnetically controlled bending to angles of approximately 100°. The gripper could grasp objects with a force of roughly 0.08 N and transport them through rolling locomotion before releasing them when the magnetic field was removed.
Another demonstration addressed challenges in biomedical soft robotics. Liquid metals offer attractive properties for devices that must operate inside the body, but their native oxide films tend to stick to tissues, and residual metal remnants raise long-term safety concerns. The researchers encapsulated a liquid metal ball approximately 7.5 mm in diameter within a magnetic hydrogel skin roughly 1 mm thick.
The resulting capsule resisted compression to 60% strain without leaking. Guided by magnetic fields through an ex vivo pig stomach, it navigated gastric folds to reach a target site within 33 seconds. An alternating magnetic field then heated the capsule from 27.2 °C to 59.5 °C in one minute, reaching temperatures sufficient for thermal ablation therapy.
The platform extends beyond magnetic functionality. By substituting thermochromic microparticles or fluorescent quantum dots for the magnetic filler, the researchers created skins that change color with temperature or glow under ultraviolet light. A four-section crawling robot combined magnetic actuation on its underside with thermochromic sensing on top, transitioning from black to red within 10 seconds of entering a 60 °C zone. A jellyfish-like swimmer integrated fluorescent skin with magnetic propulsion, achieving speeds of approximately 110 mm/s while emitting yellow fluorescence in darkness. This optical visibility could aid retrieval in poorly lit environments.
The underlying polyacrylic acid/polyethyleneimine hydrogel system has established biocompatibility from prior studies in tissue repair. The material’s limited tendency to swell makes it suitable for humid or submerged environments, and spraying a glycerol-water mixture can prevent dehydration in dry conditions.
What distinguishes this work from earlier functional coating methods is its combination of speed, simplicity, and surface fidelity. The powder format provides storage stability and easy transport while delivering conformal coverage activated by nothing more than water mist. The researchers envision applications in soft robotics, flexible electronics, thermal management systems, and biomedical devices where functional surfaces must conform to complex geometries without sacrificing performance.
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