Every year, more than 60 million tonnes of electronic waste (e-waste) are generated worldwide. Hidden inside nearly every discarded smartphone, laptop, and wearable device is an unexpected obstacle to recycling: adhesives.

Most of us have experienced this on a much smaller scale: peeling a sticker from a bottle or removing a screen protector without leaving sticky residue. What seems like a small everyday annoyance becomes a major engineering challenge when it affects billions of products.

Consider the smartphone in your pocket. Inside, the battery, display, camera module, and countless other components are held together with permanent adhesives that were never intended to come apart. When the device reaches the end of its life, these strong bonds make it extremely difficult to disassemble, repair, or recover valuable materials. Instead, many components are discarded long before they have reached the end of their useful life. Globally, e-waste now exceeds 60 million tonnes every year, yet much of its enormous material value remains locked inside these discarded devices. For this reason, recyclers often refer to e-waste as "urban mines"—rich sources of valuable materials that are incredibly difficult to access.

Adhesives are among the most overlooked materials in modern technology, yet they sit at the heart of this challenge. For decades, materials scientists have pursued a simple goal: an adhesive that bonds strongly when needed but can also be released on-demand without leaving residue, damaging the surface, or requiring harsh chemicals for removal. That challenge became the starting point for our research.

Our journey into switchable adhesives did not begin with adhesives at all. It began with liquid crystal elastomers (LCEs), a remarkable class of soft materials that combine the molecular order of liquid crystals with the elasticity of rubber. Like the liquid crystals used in display technologies, their molecules can switch between ordered and disordered states. Unlike the rigid materials found in screens, however, LCEs are soft, stretchable polymers whose stiffness, energy dissipation, and ultimately their adhesive behaviour change dramatically as this molecular order switches.

In 2019, while working with Dr. Takuya Ohzono in Prof. Eugene Terentjev's group at the Cavendish Laboratory, I kept noticing something curious. Every time I prepared an LCE film, it stuck firmly to its glass mould. The only reliable way to release it was to either heat it above the liquid-crystal transition or cool it below the glass transition. At first, this was simply an inconvenience during sample preparation. But the behaviour was too consistent to ignore.

When I mentioned the observation to Takuya and Eugene, we began to wonder whether the material itself was changing its adhesion. We quickly designed a simple experiment to measure the adhesive force in the two phases. The result surprised all of us: adhesion in the nematic phase was more than twice as strong as in the isotropic phase. A few months later, we published our first paper on switchable adhesion in Advanced Materials, marking the beginning of an entirely new direction for our research.

That simple experiment immediately raised a much bigger question. Pressure-sensitive adhesives work because they are soft enough to conform to a surface while dissipating the energy needed to resist peeling. Liquid crystal elastomers already possessed one of these essential ingredients: exceptionally high energy dissipation that could be switched on and off simply by changing phase (high in the liquid-crystal (nematic) phase and low in the isotropic phase). We realised that if we could also make LCEs soft enough to behave like true pressure-sensitive adhesives, they might combine the strong bonding of conventional adhesive tapes with something they cannot offer: clean, on-demand release.

That discovery opened an exciting new research direction, but it also revealed a major obstacle. Conventional LCEs are simply too stiff to behave like pressure-sensitive adhesives. Their tightly crosslinked polymer networks and rigid liquid-crystal building blocks prevent them from conforming to the surface. Simply pressing them against a substrate achieves very little. Instead, they typically need to be heated or annealed before they become soft enough to develop strong adhesion. That is the opposite of how a pressure-sensitive adhesive should behave: you press it onto a surface, and it sticks immediately.

For decades, adhesive scientists have known that pressure-sensitive adhesives only work within a very narrow thermomechanical window. This is captured by the Dahlquist criterion, developed by Carl Dahlquist at 3M in the 1960s. To behave like a true pressure-sensitive adhesive, a material must remain soft at room temperature and have a glass transition temperature below 0 °C. Conventional acrylic liquid crystal elastomers fall far outside this window, with glass transition temperatures ranging from about 10 °C to nearly 200 °C and storage moduli that are one to several orders of magnitude too high. In other words, they are simply too rigid to behave like conventional pressure-sensitive adhesives.

Rather than searching for an entirely new adhesive chemistry, we asked a simpler question: could we redesign the polymer network itself? Our solution was to introduce a small molecule known as a chain-transfer agent during polymerisation. Instead of allowing long polymer chains to grow uninterrupted, the chain-transfer agent repeatedly terminated growing chains and initiated new ones, producing a softer, more flexible network with fewer effective crosslinks. Importantly, this simple change preserved the unique liquid-crystal behaviour while transforming the material into one that could finally satisfy the requirements of a pressure-sensitive adhesive.

The results exceeded our expectations. For the first time, an acrylic liquid crystal elastomer behaved like a true pressure-sensitive adhesive: it bonded within seconds under light pressure yet released cleanly with gentle heating. Behind this simple behaviour lay a dramatic transformation in the material's mechanical properties, with its glass transition temperature falling from 193 °C to −11 °C and its stiffness dropping by four orders of magnitude.

 The new adhesive behaved exactly as we had hoped. It supported a 500 g weight, remained fully reusable over repeated cycles, and worked reliably on glass, metal, rubber, and 3D-printed plastics. Among its many potential applications, we are particularly excited by wearable medical sensors, where gentle heating could enable painless, residue-free removal while allowing the device to be reused.

One of the most exciting aspects of this work is its simplicity. Rather than relying on exotic chemistries or complex manufacturing methods, our approach stays within the well-established world of acrylic polymers, which already dominate the global pressure-sensitive adhesive market. That makes the strategy inherently scalable and compatible with existing manufacturing processes.

We hope this work provides a practical route towards adhesives that bond strongly when needed, release cleanly on demand, and can be used again. More broadly, it reminds us that transformative discoveries do not always require entirely new materials. Sometimes, simply rethinking the architecture of a familiar material is enough to unlock entirely new functionality. The full study is available in Communications Materials.