Researchers find that drying hydrogels briefly turns them into strong, reversible adhesives, opening new uses in robotics and material handling.
(Nanowerk Spotlight) A surprising shift in how hydrogels behave during drying may upend long-standing assumptions about these materials. In a study led by researchers at the University of Alberta and Tsinghua University, scientists observed that a polyacrylamide hydrogel—normally valued for its slipperiness—transforms into a highly adhesive and high-friction surface as it dries. This transition, which occurs between the fully hydrated and fully dry states, generates friction forces and adhesion strengths that far exceed those of most commercial adhesives.
The discovery introduces a new functional regime for hydrogels, with promising implications for areas such as soft robotics, dry adhesives, and temporary bonding interfaces.
Hydrogels are soft materials made from networks of polymer chains that hold large amounts of water. They are typically engineered for applications where low friction and hydration are key—such as tissue scaffolds, artificial cartilage, and contact lenses. Their lubricating behavior is well-documented: when wet, the friction coefficient of a hydrogel, which quantifies how much it resists sliding, can be as low as 0.01. Even in dry conditions, their friction coefficient generally stays under 0.2. These properties have reinforced the perception that hydrogels are unsuitable for tasks that require strong grip, firm attachment, or structural adhesion.
Attempts to improve the adhesive properties of hydrogels have typically involved modifying their molecular structure or adding microscopic interlocking features. While these strategies have led to some success, even the best-performing hydrogel adhesives rarely achieve adhesion strengths greater than 1.5 megapascals (MPa). By comparison, certain pressure-sensitive tapes and recently developed smart adhesives can reach values close to 3 MPa, but these often sacrifice reversibility or require complex activation steps such as heating or chemical treatment.
In contrast, the study published in Advanced Science (“Ultra‐High Friction and Adhesion in Hydrogel Layer Driven by Wet‐to‐Dry Transition Dynamics”) presents a passive, controllable approach that uses dehydration itself to unlock a new behavior in the hydrogel. The researchers began with a polyacrylamide hydrogel layer formed on a polydimethylsiloxane (PDMS) substrate—a common soft silicone used in flexible devices. When this hydrogel layer transitioned from a wet state to a partially dry one, its surface underwent structural collapse and contraction. This transition phase, though brief, was marked by a steep rise in friction and adhesion.
Reciprocating tribological behavior of the hydrogel layer under different conditions: a) COFs versus time when a ZrO2 ball is used as the upper friction pair and the hydrogel layer as the lower friction pair, as the hydrogel surface transitions from a wet state to a dry state and subsequently to an underwater lubricated state; b) A schematic illustration of the friction system corresponding to (a) along with microscopic images of the hydrogel layer in its various states; c) COFs versus time when a stainless steel ball serves as the upper friction pair, showing the transition of the hydrogel layer from a wet state to a dry state and then to an underwater lubricated state; d) COFs versus time when a nylon ball is used as the upper friction pair, with the hydrogel layer undergoing the same sequence of state changes. In this experiment, the water on the surface of the hydrogel layer was gently removed with a non-woven fabric at the start of the friction test to obtain a wet-state hydrogel. Over a period of 100–250 s, the hydrogel layer gradually dried; then, at 360 s, ≈5 μL of DI water was added to induce underwater lubrication. The upper friction pair balls had a diameter of 2 mm, a normal load of 5 mN, a reciprocating stroke of 1 mm, and sliding velocities of 8, 6, 4, 2, 1, or 0.5 mm s−1. (Image: Reprinted from DOI:10.1002/advs.202507827, CC BY) (click on image to enlarge)
Under a 2 millinewton load, the shear force required to slide a test probe across the surface increased to 115 millinewtons, resulting in a friction coefficient of 57.5. This value is not only far beyond typical hydrogel measurements but also higher than most dry materials designed for friction-based grip. Once the hydrogel became fully dry, the friction dropped again. Rehydrating the sample restored the original low-friction state, confirming that the high-friction phase occurs specifically during the intermediate wet-to-dry transition.
To understand this transformation, the team analyzed how the hydrogel’s internal structure changed during dehydration. As water evaporated, the three-dimensional polymer network began to collapse. Microscopic imaging showed that this shrinkage led to the formation of surface protrusions and wrinkles, which increased the real contact area between the hydrogel and any object in contact with it. These microstructures, combined with the closer packing of polymer chains, enhanced interactions like hydrogen bonding and van der Waals forces across the interface.
Importantly, this change was not just observed at small scales. The researchers performed adhesion tests across a range of materials—including glass, silicon wafers, stainless steel, and nonwoven fabric. On glass, the hydrogel layer achieved an adhesion strength of 3.48 MPa, while on silicon wafers it reached 3.64 MPa. These values not only surpass most engineered hydrogels but also exceed the performance of many commercial adhesives under similar testing conditions. On rougher or more porous surfaces, such as ceramic and wood, the adhesion strength was lower but still significant.
Atomic force microscopy provided further insight. When the hydrogel was fully dry, the surface layer remained compact, with limited deformation under pressure. During the transition, however, the outermost layer expanded and softened, allowing the AFM probe to penetrate deeper before encountering resistance. This indicated that the surface was more compliant and sticky during dehydration. Rheological measurements confirmed that both the storage modulus (which reflects stiffness) and the loss modulus (which reflects internal energy dissipation) increased during drying, further supporting the idea that drying reorganizes the hydrogel’s mechanical properties in a way that promotes adhesion.
Practical tests reinforced these results. A hydrogel-coated PDMS layer was used to grip a 2.64-kilogram weight between two glass plates. After the hydrogel was allowed to dry under compression, it held the load securely, with an estimated adhesion pressure of 1.04 MPa. When submerged in water, the sample quickly detached—demonstrating that the adhesion can be reversed cleanly without residue or damage by rehydrating the layer.
The underlying adhesion mechanism combines two main effects. First, dehydration causes the hydrogel to contract, forming micro-cavities or “negative pressure zones” that act like suction cups. Second, the chemical groups in the polymer—particularly the amide groups in polyacrylamide—form strong hydrogen bonds with common surface chemistries, such as the oxygen atoms in silicon oxide or the hydroxyl groups on glass. This dual contribution from mechanical interlocking and molecular bonding results in a temporary but strong attachment.
Tests across different environmental conditions showed that the hydrogel retained high adhesion even at elevated temperatures (75°C) and high humidity (90%). The effect was also observed in other hydrogel materials, such as polyacrylic acid and poly(N,N-dimethyl acrylamide), indicating that the behavior is not limited to a single chemical formulation. This broad compatibility suggests the potential for integrating similar hydrogel coatings into a range of applications, from reusable adhesives to robotic components that require adaptive grip.
Schematic diagram and observation of the hydrogel layer grasping and release process. a) Schematic diagram of the process of using a hydrogel layer to grasp and release an object; b) Microscopic morphologies of the hydrogel layer in a dry state, a water-absorbed swollen state, and in contact with a steel surface; In the third picture, the adhesion interface between the hydrogel and the target object was observed from the side of the PDMS substrate; c) Microscopic morphology of the contact interface between the hydrogel layer and the steel surface, and the changes during the water absorption process at its edge. The blue dashed curve in (c) indicates the boundary of the adhesive interface, while the black region represents areas where air is present between the hydrogel layer and the steel surface (non-adhesive region). (Image: Reprinted from DOI:10.1002/advs.202507827, CC BY) (click on image to enlarge)
One notable outcome of the experiments was that failure often occurred not at the hydrogel-object interface, but within the PDMS substrate itself. When researchers examined the detached surfaces, they found chemical signatures consistent with the substrate, not the hydrogel. This indicates that the interface between the hydrogel and the object was stronger than the internal cohesion of the PDMS, highlighting the robustness of the adhesion.
The findings recast the drying process—not as a loss of functionality for hydrogels, but as a means of activating a new functional state. This redefinition opens possibilities for soft robotic systems that rely on variable adhesion, temporary fixturing in microfabrication, and recyclable bonding methods that avoid chemical residues. Because the transition is reversible and controllable using water alone, it offers a straightforward alternative to adhesives that require mechanical latches, heating, or chemical activators.
By isolating and exploiting a narrow window in the material’s hydration cycle, the study introduces a previously unrecognized phase where hydrogels temporarily behave more like strong adhesives than soft lubricants. The work challenges assumptions about what hydrogels are capable of and proposes a mechanism that does not require new chemistry or complex processing—just a simple shift in moisture.
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