A gel with multiple triggers
Scientists at the University of Birmingham have developed a new material that changes from a gel to a liquid-like state under ultraviolet light and can be rebuilt using heat or dismantled by acid. The material, described in the Journal of the American Chemical Society, is the first multi-responsive gel reported to be built from 'foldamers' – synthetic molecules that fold into defined shapes and can be assembled, disassembled and reassembled on demand.
The gel can be switched from a solid-like state into a flowing, liquid-like state using UV light. Heating reverses the process and reforms the gel, while acid provides a separate mechanism for breaking down the molecular network. This combination of responses could be relevant to pharmaceutical applications where control over when and where a therapeutic molecule is released is important.
The material consists of helical foldamer molecules connected by palladium ions, which act as four-way molecular connectors to create an extended network that traps liquid. UV light changes the shape of light-sensitive components within the foldamers, and this change is amplified throughout the network, causing the gel to lose its solid-like structure. Heating allows the network to reform, while acid acts through a different mechanism by disrupting the interactions between the foldamers and palladium ions.
Researchers from Birmingham's School of Chemistry also converted the material into a water-containing hydrogel without disrupting the molecular connections that hold it together. The work brought together expertise in designing new gels, led by Dr. Sarah Pike; supramolecular chemistry, led by Dr. Chiara Arno; and atomic-level structure characterization, led by Dr. Dominik Kubicki.
Understanding the structure
Understanding how the gel is assembled at the atomic scale required a technique called dynamic nuclear polarization-enhanced solid-state nuclear magnetic resonance spectroscopy (DNP NMR). A tiny quantity of gel was packed into a ceramic rotor and spun at nearly one million revolutions per minute inside a powerful magnet. The measurements revealed how palladium atoms connect the foldamer molecules to form the gel's molecular backbone.
Dr. Dominik Kubicki, Associate Professor in Materials Characterisation at the University of Birmingham, said that DNP NMR gave an atomic-level picture in a material that is otherwise exceptionally difficult to study, allowing the team to solve a major challenge in gel science. Using DNP NMR reduced an experiment estimated to take around seven years with conventional NMR to just 12 hours.
Potential applications
The researchers say the platform could eventually help inform the development of drug delivery systems capable of releasing therapeutic molecules in response to specific conditions. Future applications could include targeted drug delivery, controlled release of therapeutic molecules, biomedical materials, smart sensing, and catalysis.
Dr. Maria Chiara Arno, Associate Professor in Polymeric Biomaterials at the University of Birmingham, said: "Supramolecular materials are assembled using reversible interactions rather than permanent chemical bonds. That gives us an opportunity to create materials that are robust under normal conditions but can be reorganised or dismantled when we apply the right signal."
She added that the work represents a significant advance in building materials that behave more like biological systems by responding intelligently to their surroundings, for example releasing drugs only when exposed to a specific trigger such as changes in acidity within diseased tissue.
Dr. Sarah Pike, Associate Professor of Organic Chemistry at the University of Birmingham, said: "A very small change in molecular shape translates into a visible change in the whole material—demonstrating how carefully designed molecular components can give us control over the behavior of a bulk gel."
She noted that the research is at a fundamental stage, but the ability to program more than one response into the same material could ultimately inform the design of smart sensors, switchable catalysts, and materials that capture and release selected molecules on demand. The researchers stress that the work remains at a fundamental stage.