A new polymer-based material that can heal itself when exposed to ultraviolet light for less than a minute has been developed by an international team of researchers. They envision that the rehealable material could be used in automotive paints, varnishes for floors and furniture, and many other applications. The team, led by Stuart J. Rowan from Case Western Reserve University, also includes Christoph Weder from the Adolphe Merkle Institute of the University of Fribourg in Switzerland and Rick Beyer from the Army Research Laboratory at Aberdeen Proving Ground in Maryland. Although the polymers are not ready for commercial use, the researchers said they can prove that the concept works. Schematic representation of the optical healing of the new metallo-supramolecular polymers developed by the CWRU/AMI/ARL team. Where irradiated with ultraviolet light, the originally solid material is liquefied and can quickly fill up cracks. After the light is switched off, the material solidifies, and the original properties are restored. (Image: Schematic courtesy of Marc Pauchard for Adolphe Merkle Institute, Case Western Reserve University, US Army Research Laboratory) "These polymers have a Napoleon Complex: In reality they're pretty small but are designed to behave like they're big by taking advantage of specific weak molecular interactions," said Rowan. "Their molecular design allows the materials to change their properties in response to a high dose of ultraviolet light," added Weder. The new materials were created by a mechanism known as supramolecular assembly. Unlike conventional polymers, which consist of long, chain-like molecules with thousands of atoms, these materials are composed of smaller molecules, which are assembled into longer, polymer-like chains using metal ions as "molecular glue." The result: The new materials, which the scientists call "metallo-supramolecular polymers," behave in many ways like normal polymers. But when irradiated with intense ultraviolet light, the assembled structures are temporarily unglued. This transforms the originally solid material into a liquid that flows easily. When the light is switched off, the material reassembles and solidifies again: The original properties are restored. Artist’s illustration of the concept of healable polymers. The new polymers developed by the CWRU/AMI/ARL team mimic the unique feature of biological tissues of being repairable. Unlike human skin, the new polymers don’t need stitches, though. The researchers have shown that the combination of supramolecular polymers with a light-heat conversion scheme is a particularly effective approach to healable materials. (Image: Dominique Bersier and Gina Fiore for Adolphe Merkle Institute, Case Western Reserve University, US Army Research Laboratory) Using lamps such as those dentists use to cure fillings, the researchers repaired scratches in their polymers. Wherever they waved the light beam, the scratches filled up and disappeared, much like a cut that heals and leaves no trace on skin. Tests showed that the researchers could repeatedly scratch and heal their materials in the same location. "We can simply use heat to heal these materials," said Mark Burnworth, a graduate student at Case Western Reserve. "But by using light, we have more control, as it allows us to target only the defect and leave the rest of the material untouched." The researchers systematically investigated several new polymers to find an optimal combination of mechanical properties and healing ability. They found that metal ions driving the assembly process via weaker chemical interactions serve best as the light-switchable molecular glue. Schematic representation of the molecular design of the new metallo-supramolecular polymers developed by the CWRU/AMI/ARL team. The new materials were created by a mechanism known as supramolecular assembly: comparably short, string-like molecules (green) with sticky end groups (blue) are assembled into much longer chains using metal ions (gray) as "molecular glue." In this state, the materials behave in many ways like normal polymers. Upon irradiation with ultraviolet light, the structures are temporarily disassembled. This transforms the originally solid material into a liquid that flows easily. When the light is switched off, the structures reassemble, and the original properties are restored. (Image: Gina Fiore for Adolphe Merkle Institute, Case Western Reserve University, US Army Research Laboratory) They also found that the materials that assembled in the most orderly microstructures had the best mechanical properties. But, healing efficiency improved as structural order decreased. "Understanding these relationships is critical for allowing us to improve the lifetime of coatings tailored to specific applications, like windows in abrasive environments," Beyer said. According to Rowan, "One of our next steps is to use the concepts we have shown here to design a coating that would be more applicable in an industrial setting." Their findings are published in the April 21 issue of Nature. For more information, visit: www.case.edu