Adhesives are essential in various industries, including the aerospace, biomedical, electronics, construction, marine, and automotive fields. As these industries continue to advance, the demand for high-performance, multifunctional adhesives is also growing.

Such widespread use, however, has also accentuated their environmental issues. Conventional adhesives, while effective and economical, release petroleum-based chemicals that are harmful to soil and water quality. Their production also contributes to environmental pollution, and the adhesives themselves are difficult to recycle or dispose of. Developing reusable and more eco-friendly alternatives is therefore becoming increasingly important.

Concurrently, there is a growing demand for advanced adhesives with diverse functionalities. For example, stimuli-responsive adhesives that can vary their adhesive strength based on external stimuli are attracting significant attention. Among them, photo-responsive adhesives are especially attractive, as they allow localized wireless control of adhesion through light while also possessing non-invasive properties.

To meet both performance and sustainability goals, a research team led by Professor Kwang-Un Jeong, along with Ph.D. student Mintaek Oh from the Department of Nano Convergence Engineering at Jeonbuk National University, South Korea, has developed an eco-friendly, photo-switchable smart adhesive. The research study was made available online this past summer, and published in the Chemical Engineering Journal in the fall.

“We synthesized a tetrahydrogeraniol-based monomer, a derivative of rose oil, and successfully fabricated an eco-friendly adhesive containing 95% of it,” Jeong explained. “The final adhesive, incorporating a small amount of a functional monomer that responds to light and adheres strongly to various substrates, exhibits high-responsive adhesion, allowing quantitative control of its bonding strength. It is eco-friendly, cost-effective, versatile, and reusable.”

To fabricate the adhesive, the researchers first synthesized two key components: an acid azobenzene-based methacrylate monomer (AAMM) and a biomass-based tetrahydrogeraniol methacrylate monomer (TGMM). AAMM consists of azobenzene, carboxylic acid, and methacrylate. Azobenzene derivatives are well known for their reversible photo-switchable behaviour, making them ideal for light-responsive systems. The carboxylic groups can form strong hydrogen bonds with a variety of substrates, facilitating strong adhesion. 

TGMM, made from rose-oil-derived tetrahydrogeraniol, contributes to biodegradability and sustainability. It also helps maintain balance between flexibility and mechanical stability in the adhesive. By copolymerizing AAMM and TGMM through their methacrylate groups, the researchers successfully created a new eco-friendly and photo-switchable adhesive, termed the T/A adhesive. 

During testing, the optimized T/A adhesive demonstrated excellent photo-switchable adhesion, with strong adhesion to a wide range of substrates, including metals, plastics, rubber, glass, cork, and paper. When exposed to ultravioletlight, the adhesive becomes more liquid-like, and its adhesion strength decreases.  After subsequent exposure to visible light, it returns to its original adhesion strength and becomes more solid-like again, thus demonstrating fully reversible, light-controlled adhesion. 

In addition to light, adhesion strength can also be switched using heat and chemical treatments. Specifically, increasing temperature beyond 500C significantly reduces adhesion strength, which can then be restored by cooling. Similarly, the adhesive can be dissolved by solvents like chloroform and recovered when the solvent evaporates. The researchers also found that the reused T/A adhesives in all three processes were able to retain more than 90% of their original adhesive strength across repeated usage cycles. 

The researchers further demonstrated a smart UV sensor built using the new adhesive and a spring-based mechanism that can serve as a UV-light sensitive switch in electrical circuits. In this device, the T/A adhesive holds the spring to maintain an open circuit. When exposed to UV-light, its adhesion strength reduces, causing the spring to close the circuit. 

“Our smart, reusable adhesive is promising for a wide range of practical applications,” Oh stated. “It can play a valuable  role in environmental monitoring, smart electronics, and adaptive assembly systems, where controllable and reusable adhesion is critical. It could also enable reusable light-responsive smart packaging, wearable devices with detachable sensors, and reconfigurable robotic components.”

Overall, by replacing petroleum-based adhesives with biomass-derived alternatives, this research paves the way for more sustainable and smart adhesive technologies.

Reference
Title of original paper: Eco-Friendly and Photo-Switchable Smart Adhesives from Biomass-Based Copolymers with Acid Azobenzene Functions