This stretchy gel repairs itself and signals stress with color

Sep 12, 2025

A stretchable, self-healing gel visibly changes color when pulled or heated, combining strength and built-in sensing in a single material designed for wearable devices and soft robotics.

(Nanowerk Spotlight) Soft wearable electronics, artificial skin, and flexible biomedical devices all depend on materials that can stretch, recover from damage, and adapt to physical stress. But combining those properties in a single material has proven difficult. Many soft polymers can bend and flex but break under repeated strain. Others are tough enough to last but lose performance after damage or lack the ability to provide useful feedback. Materials that can both endure stress and signal it in real time, while also repairing themselves, are rare. Where strength and responsiveness are needed together, design trade-offs often limit performance. To address this, researchers are exploring ways to build polymers that do more than passively deform. One approach is to incorporate molecular systems that convert mechanical force into controlled, reversible structural changes. Among these are mechanically interlocked molecules, which consist of ring-shaped structures threaded onto linear components. These molecules can slide under tension without breaking any bonds. That motion, while invisible to the eye, can be engineered to produce measurable changes in properties such as fluorescence. In principle, this allows stress at the macroscale to be translated into molecular motion, and then into an optical signal. The challenge has been in embedding these sliding molecular systems into soft, stretchable materials while maintaining both their mobility and their function. In a new study published in Advanced Functional Materials (“Effective Sliding Motions of Vibration‐Induced Emission Stoppers in Mechanically Interlocked Molecules as Artificial Muscle Tougheners and In Situ Molecular Shuttling Sensors for Self‐Healable Mechano‐Fluorescent Polyurethane Organogels”), researchers in Taiwan report a polyurethane-based gel that overcomes this problem. By chemically integrating sliding rotaxane and daisy chain molecules into the polymer network, they created a material that is tough, self-healing, and able to change its fluorescence in response to stretching. These interlocked molecules act both as internal reinforcements and as sensors of strain, enabling the material to deform and recover while providing a visual signal of mechanical stress. The goal of the study was to determine whether mechanically interlocked molecular units could be covalently incorporated into a self-healing polymer gel without restricting their motion, and whether that motion could then be used to enhance mechanical properties and enable real-time stress sensing. The researchers designed two types of interlocked systems: a [2]rotaxane, which features a single ring sliding along a linear axle, and a [c2] daisy chain, which consists of two intertwined rotaxanes capable of more complex movements. Each molecule contained a DPAC unit, a fluorophore known to exhibit vibration-induced emission. When unconstrained, DPAC emits orange light. When constrained or bent, the emission shifts to blue. The team integrated these molecules into a polyurethane network crosslinked with cellulose nanocrystals. The nanocrystals provide physical reinforcement and facilitate self-healing by forming hydrogen bonds throughout the network. By attaching the interlocked molecules covalently to the polymer, the researchers ensured that their mechanical movement would be coupled to the deformation of the bulk material. When stretched, the sliding motion of the macrocycles restricts the conformational freedom of the DPAC units, causing the emission color to shift from orange to blue. When the material relaxes, the molecules return to their original configuration and the orange emission is restored. Chemical structures and cartoon depictions of DPAC-based [2]rotaxanes and [c2] daisy chain molecules, i.e., a) RDPAC/1 and b) RDPAC/2 (before and after shuttling, respectively) along with c) DDPAC/C and d) DDPAC/E (contracted and expanded forms, respectively). PL spectra of e) [2]rotaxane RDPAC/1 and f) [c2] daisy chain DDPAC/C (orange lines before shuttling) in anisole/DMSO (75/25, v/v) solutions were neutralized by adding base (DBU) to become respective solutions of [2]rotaxane RDPAC/2 and [c2] daisy chain DDPAC/E (blue lines after shuttling), where the inset figures show their PL emission colors and acid-base responses (Concentration: 0.2 mg mL−1, 𝜆ex = 365 nm). (Image: Reprinted from DOI:10.1002/adfm.202519737, CC BY) (click on image to enlarge) Tests showed that a small amount of the daisy chain component—only 1.5 percent by weight—was sufficient to significantly enhance the mechanical performance of the gel. The most effective formulation reached a toughness of 142 megajoules per cubic meter and a strain capacity over 4600 percent. This represents a 2.6-fold increase in toughness compared to the same gel without interlocked molecules. The rotaxane-based gel also improved mechanical performance but to a lesser degree, likely due to differences in sliding range and structural constraints. Fluorescence measurements confirmed that the materials acted as ratiometric stress sensors. As the gel was stretched, the emission color gradually shifted from orange to blue, with the ratio of intensities at 603 nanometers and 451 nanometers changing in proportion to the applied strain. This shift was visible under ultraviolet light and could be recorded across different points on the gel surface. Local variations in color revealed how stress was distributed across the material during deformation, providing a potential method for real-time strain mapping in soft devices. The fluorescence response was reversible and repeatable. After stretching, the emission returned to its original state over several hours at room temperature or within minutes under mild heating. The materials also exhibited temperature-sensitive behavior. As temperature increased, the emission shifted toward orange, corresponding to increased mobility of the DPAC units. This temperature dependence was linear and consistent across cycles, allowing the gel to act as a dual-mode sensor for both strain and heat. Time-resolved fluorescence studies confirmed that the changes in emission were due to physical restriction of molecular motion rather than chemical transformation. The fluorescence lifetime of the blue emission increased under strain, while the lifetime of the orange emission decreased, consistent with the expected changes in DPAC conformation. Rheological analysis showed that the gels retained their mechanical integrity under repeated deformation. Storage and loss moduli remained stable across cycles of low and high strain. The gels recovered their structure after large deformations, and stress relaxation measurements confirmed that the network could dissipate energy effectively. Temperature sweeps showed that the internal hydrogen bonding network softened with heat, in line with the observed optical and mechanical changes. The most effective formulation used a modified daisy chain in which the macrocycle, rather than the axle, served as the point of attachment to the polymer. This design reduced interference with the DPAC unit and allowed greater freedom of motion during sliding. Compared to systems where the interlocked molecules were physically mixed into the gel without covalent bonding, the chemically integrated designs showed higher toughness and more distinct emission changes. This highlights the importance of structural integration for translating molecular-scale motion into macroscopic function. The work demonstrates a material that can stretch, recover, and respond to stress in a controlled and visible way. It addresses a central limitation in the design of soft materials, showing that mechanical interlocked molecules can be used not only as structural elements but as active components that change behavior under force. By linking molecular motion to both optical and mechanical outcomes, the researchers present a platform that may support future developments in soft sensing, adaptive materials, and damage-tolerant electronics.

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)
Copyright ©




Nanowerk LLC