| Dec 09, 2025 |
Electrically controlled hydrogels switch metal friction from grip to glideElectrically tunable ionic hydrogels use small voltages to switch friction at metal contacts between high-friction and ultra-low-friction states, enabling voltage-programmed grip, release and motion in soft-robotic and mechanical systems. |
| (Nanowerk Spotlight) Friction in mechanical contacts consumes a large share of the energy supplied to machines. Tribology, the study of friction, wear and lubrication, attributes about 23 % of global energy use to sliding and rolling interfaces in engines, vehicles, manufacturing lines and power systems. Most designs treat friction as fixed. Engineers choose a lubricant, coating or surface texture, obtain a nearly constant coefficient of friction and then build the rest of the system around that choice. |
| A different approach is to treat friction as a controllable quantity. The technical goal is a solid interface whose coefficient of friction, the ratio between frictional force and normal load, can be shifted reversibly over a wide range using modest voltages, across practical contact areas and without free liquid lubricant. Work on superlubricity has shown that some systems can reach ultra-low friction states with coefficients at or below 0.03. |
| Other studies have used electric fields to change friction by moving charges or ions at an interface, but these usually rely on liquid lubricants enriched with special molecules or particles, confined in sealed, small-scale gaps, and they often offer only limited tunability. |
| Ionic hydrogels offer another option. They are soft, water-rich polymer networks that contain dissolved salts and conduct ions. These materials deform easily, respond to electric fields and already serve as active components in soft actuators, sensors and ionic conductors. Their combination of mechanical compliance and ionic mobility suggests that they could act as solid friction layers whose properties are set electrically rather than fixed by composition alone. |
| A study in the journal Advanced Materials ("Electrically Tunable Friction: From Sticky to Slippery with Ionic Hydrogels") develops this idea by turning an ionic hydrogel into an electrically tunable friction interface that spans dry-like and ultra-low friction states. |
| The work uses a polyvinyl alcohol based ionic hydrogel loaded with the salt lithium bis(trifluoromethane)sulfonimide, LiTFSI. By applying direct voltages from −30 V to +30 V between this hydrogel and a metal surface, the researchers tune the coefficient of friction more than fifty-fold, from values near dry contact, around 1–2, down to ≈0.03, an ultra-low regime where sliding requires only a small fraction of the normal load. |
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| Schematic illustration of potential applications and design principles of electrotunable friction: a) Schematic of a crawling robot inspired by worm-like peristalticmotion and electrotunable friction; b) Structural diagram of the PVA/LiTFSI via freeze-thaw cycling, driven by hydrogen bonding and Li─O interactions; c) Illustration of friction modulation between an ionic hydrogel and a metal ball under different electric fields. Proposed mechanism: when the ionic hydrogel is connected as the anode and the metal ball as the cathode, a removable boundary film forms on the metal surface. In contrast, connecting the metal ball as the anode leads to the formation of an oxide film; d) Time-dependent friction force between the PVA/LiTFSI versus a metal ball with a diameter of 2 mm at various voltages. Experiments were conducted under a normal load of 20 mN and a sliding velocity of 2 mm s−1. (Image: Reproduced from DOI:10.1002/adma.202518350, CC BY) (click on image to enlarge) |
| The material starts from polyvinyl alcohol, or PVA, a hydrophilic polymer widely used in hydrogels and biomedical coatings. PVA forms strong hydrogen bonds that provide mechanical strength but can yield relatively high friction, especially as the material dries. In this study, an aqueous PVA solution is mixed with LiTFSI and processed through a freeze-thaw cycle to form a clear ionic hydrogel, labeled PVA/LiTFSIₓ, where x denotes salt content. Structural analysis shows that LiTFSI disperses uniformly at the molecular level. The salt disrupts some PVA crystallites and increases the fraction of softer, amorphous segments, while Li⁺ cations act as temporary crosslinks between chains. |
| The resulting network combines robustness, high ionic conductivity and freeze resistance and serves as both structural support and internal ion reservoir at the sliding interface. |
| To quantify friction control, the team uses a ball-on-disk configuration in which metal balls made of steel or titanium alloy slide over flat samples of the ionic hydrogel. The ball and the hydrogel also act as electrodes in a direct-current circuit, with the hydrogel typically connected as anode and the metal as cathode. During sliding, the researchers apply voltages between −30 V and +30 V, measure the friction force and calculate the coefficient of friction under controlled normal loads and speeds. |
| At 0 V, the PVA/LiTFSI hydrogel against steel exhibits a high initial coefficient of friction of about 2 that gradually decreases toward about 1 as the hydrogel loses some water during the test. When a voltage of −30 V is applied, with the metal ball as negative electrode, the behavior changes sharply. The coefficient of friction drops to ≈0.03. In this ultra-low state, the lateral force during sliding is only about three hundredths of the normal load. Tests with titanium alloy balls show similar voltage-dependent trends, indicating that the effect does not depend strongly on the metal. |
| Systematic measurements map friction as a function of voltage. As the potential shifts from 0 V down to −30 V, the average coefficient of friction falls from dry-like values to ≈0.03. When the voltage is stepped up and down, the friction follows in a reversible way. Negative voltages drive the system into the slippery regime. At 0 V and at positive voltages, the coefficient of friction returns to higher values characteristic of dry contact. Time-dependent tests, in which the voltage alternates between −30 V, 0 V and +30 V, show that the interface can switch repeatedly between sticky and slippery states. |
| To isolate the role of ions, the study compares this system with a PVA hydrogel doped with ammonium chloride, NH₄Cl, and with pure PVA hydrogels without added salt. The ammonium based ionic hydrogel also displays clear voltage-dependent friction, with lower friction at negative bias and higher friction at positive bias. Pure PVA responds only weakly, with little change under an applied field. This comparison points to the motion of mobile ions under the electric field as the key factor in the electrotunable friction. |
| The proposed mechanism focuses on how ions and salt-rich regions reorganize inside the hydrogel when voltage is applied. In the resting state, Li⁺ and TFSI⁻ ions are distributed throughout the PVA network. Under an electric field, TFSI⁻ anions migrate toward the positively charged side and Li⁺ cations migrate toward the negatively charged side. |
| At negative voltages, with the metal ball as cathode, this ion motion draws a LiTFSI-rich liquid layer to the interface. The study describes this as electroosmotic extraction, where the electric field pulls ion-containing water toward the contact. That process builds a thin, liquid-like lubricating film between the hydrogel and the metal. Because this film is smoother and easier to shear than the solid network, it lowers the resistance to sliding and drives the coefficient of friction into the ultra-low regime. |
| In simpler terms, a negative voltage pumps out a very thin internal lubricant from within the hydrogel and positions it exactly where the two surfaces touch. The hydrogel still holds its shape, but the immediate contact zone behaves more like a lightly lubricated junction than a dry solid-on-solid pair. |
| At positive voltages, the situation reverses. Ion migration enriches Li⁺ near the hydrogel surface, and the positively biased metal becomes more prone to electrochemical reactions with water and oxygen in the environment. Oxidation and related reactions can roughen the metal or modify its surface chemistry. A rougher, more chemically active metal surface tends to stick more strongly to the hydrogel. Under +30 V, the coefficient of friction climbs back into the range of about 1–2 and the sliding pair behaves more like a dry contact. |
| Durability and energy cost are critical for applications. Under −30 V, wear on both the hydrogel and metal surfaces remains negligible over the reported test durations. A thin, colored film appears on the metal but can be removed by rinsing with water or ethanol. After cleaning, wear marks on both hydrogel and metal are difficult to detect, which suggests that the lubricating layer does not cause permanent damage under these conditions. The electrical power needed to maintain control is low. At −30 V, the power consumption is on the order of 9 μW, while the reduction in frictional force is substantial over different loads and speeds. |
| The study also embeds the ionic hydrogel in simple mechanical systems to illustrate electrotunable friction in operation. A robotic gripper with PVA/LiTFSI-coated fingers can release a bottle not only by reducing grip force but also by lowering friction electrically at about 32 V while the mechanical grip remains fixed. A two-block crawling robot, with each block resting on ionic hydrogel layers and linked by a telescopic actuator, uses a direct voltage of about 18 V applied alternately at each end to switch friction states. |
| During extension, the low-friction head moves forward while the high-friction tail anchors. During contraction, the friction pattern reverses and the tail advances. Each cycle, lasting about 2.7 s, produces net motion driven by controlled changes in friction. |
| Within friction control research, this Advanced Materials study stands out because it combines a wide electrical tuning range, dry operation and macroscopic contact areas in a single material system. Many earlier electrically responsive interfaces rely on liquid lubricants in sealed geometries and shift friction only from moderate values down to slightly lower ones. Here the ionic hydrogel functions as both a solid structural layer and an internal source of liquid lubricant, so it can move from dry-like friction to ultra-low friction without an external fluid. |
| By demonstrating that a soft ionic hydrogel can serve as an electrically switchable friction interface, the work supports a design view in which friction becomes a parameter that can be programmed rather than a fixed constraint. That prospect is relevant for soft robots, adaptive grippers, haptic interfaces and wearable systems that must alternate between grip and glide while remaining safe and efficient. |
| Open questions include long-term cycling stability, sensitivity to drying and contamination and integration with compact control electronics. Even with these challenges, wide-range electrical control of friction at modest voltages and low power adds a useful element to the toolbox for responsive mechanical and tribological systems. |
By
Michael
Berger
– Michael is author of four books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology (2009),
Nanotechnology: The Future is Tiny (2016),
Nanoengineering: The Skills and Tools Making Technology Invisible (2019), and
Waste not! How Nanotechnologies Can Increase Efficiencies Throughout Society (2025)
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