A solvent-free adhesive bonds in seconds under visible light and is fully recyclable using microwave energy, offering strong performance across substrates without heat, UV light, or solvents.
(Nanowerk Spotlight) Adhesives are essential to the design and assembly of products in electronics, transportation, construction, and packaging. Many of these systems rely on thermoset polymers, which form crosslinked chemical networks that cannot be reshaped or removed once cured. These materials offer mechanical strength, chemical resistance, and thermal stability, but their permanence makes them difficult to recycle. This limitation poses challenges for sustainable manufacturing, particularly in applications where repairability or material recovery is important.
Efforts to replace these materials with recyclable alternatives have focused on covalent adaptable networks. These polymer systems form reversible chemical bonds that respond to specific stimuli, such as heat, light, or chemical agents. Some rely on disulfide exchange, ester transesterification, or photochemical reactions to enable reversible bonding.
However, these systems often require harsh conditions to work. Many need ultraviolet light in a narrow spectral range, added solvents, or high temperatures above one hundred and fifty degrees Celsius. These limitations restrict their compatibility with sensitive materials and slow down their adoption.
Lipoic acid, a naturally occurring molecule with a cyclic disulfide ring, has emerged as a building block for dynamic adhesives. It offers the potential for reversible polymerization through disulfide bond exchange. Earlier lipoic acid-based adhesives have demonstrated recyclability, but only when used with catalysts, solvents, or thermal input. Systems that use visible light for curing have been developed, but often require additional reagents or perform poorly when applied to diverse surfaces.
A study published in Advanced Materials (“Untying the Knot: A Fully Recyclable, Solvent‐Free, Wide‐Spectral Photocurable Thermoset Adhesive”), describes a new approach that addresses these constraints. The research team reports the design of a recyclable adhesive that cures in under thirty seconds under visible light and can be depolymerized in a household microwave without added solvents or elevated heat.
Monomer synthesis and bulk characterization. A) Schematic illustrations of the monomer synthesis (I) and the curing process (II). B) A photograph represents the curing process of TetraALA. On the left, the monomer in its liquid form flows to the bottom of the vial. After irradiation for 30 seconds (when the vial’s black lid is on top), the monomer solidifies and remains on the bottom of the vial. C) Tensile tests of the cured monomer after 30 s at 405 nm. (Image: Reprinted from DOI:10.1002/adma.202502040, CC BY)
The core of the system is a molecule called TetraALA. It is synthesized by esterifying lipoic acid with pentaerythritol in a single reaction step using commercially available reagents. The synthesis avoids purification and uses only volatile byproducts that can be removed easily. The resulting monomer contains four reactive disulfide units, which allow it to form a crosslinked network when exposed to light in the presence of a photoinitiator.
When irradiated at 405 nanometers, the liquid monomer polymerizes into a brittle solid with more than 90 percent conversion after thirty seconds. The material has a glass transition temperature of 37 degrees Celsius and a tensile strength of more than five megapascals. This strength exceeds previously reported values for lipoic acid-based adhesives and places it within the range of commercial thermoset adhesives.
The cured adhesive, named TetraALA, was tested on a range of materials including glass, polycarbonate, aluminum, and printed circuit board substrates. Adhesion strength was consistent across all cases, with failure modes indicating cohesive rather than adhesive failure. Bonds to glass were especially strong, often resulting in breakage of the glass itself during testing. Adhesion values were comparable to polyurethane systems and stronger than other recyclable adhesives based on dynamic covalent networks.
To confirm its adaptability to different curing conditions, the researchers evaluated the adhesive under four wavelengths of visible light: 405, 470, 530, and 630 nanometers. Different photoinitiators were used depending on the wavelength, enabling flexible deployment of the material. The adhesive performed well across all wavelengths, with only slight differences in strength that were attributed to interactions between metal ions and the lipoic acid groups.
The adhesive’s most distinctive feature is its recyclability using microwave irradiation. Instead of applying heat or solvents, the researchers placed the cured material in a household microwave operating at low power. Infrared spectroscopy showed that over 93 percent of the cured polymer reverted to its original monomer state after thirty seconds of exposure. Thermal imaging showed that the temperature of the material remained below 50 degrees Celsius during the process, ruling out thermal decomposition as the main mechanism.
The authors propose that microwave-induced molecular vibrations disrupt the disulfide bonds, triggering a ring-closing reaction that returns the polymer to its monomer form. The depolymerized adhesive can then be reused by reapplying light in the same conditions. The team demonstrated this process through four full cure-recycle cycles. Performance remained consistent across cycles in terms of mechanical strength, bond quality, and chemical composition. After the fourth cycle, fresh photoinitiators were needed due to degradation of the original ones, but the base material remained fully functional.
Some minor changes were observed after multiple recycles. Swelling tests showed a small increase in polymer flexibility, likely due to rearrangements in the crosslinked network. Creep compliance and stress relaxation measurements also indicated a slight reduction in crosslink density. However, these effects did not significantly affect performance. The adhesive retained its bonding strength and mechanical integrity throughout the cycles.
The researchers also assessed how the adhesive performs in wet environments. Glass substrates were bonded underwater using tri-distilled and tap water. In both cases, the adhesive maintained its strength. In tap water, performance improved over time, with a 60 percent increase in adhesion after 48 hours. Spectroscopic analysis showed that calcium and other metal ions in tap water displaced the original counterions in the adhesive network, forming stronger coordination bonds. This result suggests potential for use in underwater or humid environments.
To evaluate its potential in biomedical settings, the adhesive was applied to chicken skin, a standard model for wet biological tissue. Despite the presence of oils and partial light absorption, the adhesive cured under red light and achieved bond strength comparable to medical-grade cyanoacrylate glues. The study did not assess biocompatibility, which would be essential for clinical applications, but the result shows that the adhesive can function under biologically relevant conditions.
Because the cured material is transparent and has a high refractive index, the team explored optical uses as well. A laser was passed through a stack of twenty-two glass slides bonded with thin layers of the adhesive. The light passed through without scattering, and the adhesive layers reduced reflection losses compared to air gaps. In another test, the adhesive was used to bond two silicone prisms into a functional beam splitter, showing its suitability for use in optical components where transparency and low distortion are critical.
This adhesive combines mechanical strength, fast curing, visible-light activation, and solvent-free microwave recycling in one system. Its performance across different substrates and conditions, along with its compatibility with water and optical applications, makes it notable among reversible adhesives. The synthesis is simple, the materials are accessible, and the process requires no specialized infrastructure.
The authors point to several areas for future research. The exact mechanism by which microwave energy induces depolymerization remains open to study. Computational modeling could help clarify how vibrational energy interacts with bond strengths in disulfide networks. The scalability of microwave recycling for complex assemblies, especially those with non-flat geometries, also presents an engineering challenge.
This study demonstrates a practical adhesive system that overcomes several long-standing limitations in materials design. By integrating visible-light curing, solvent-free formulation, strong adhesion to diverse substrates, underwater stability, and closed-loop recyclability using low-power microwave energy, the researchers present a cohesive solution to technical and environmental challenges that have restricted progress in reversible adhesives.
The material performs reliably across repeated cycles of use and recovery without requiring harsh conditions or complex infrastructure. Its compatibility with both structural and optical applications expands its potential scope.
While further investigation is needed to understand the underlying mechanisms of microwave-induced depolymerization and to adapt the system for large-scale or non-planar configurations, the work provides a clear framework for designing recyclable adhesive systems that meet industrial performance standards without compromising sustainability.
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Shlomo Magdassi (The Hebrew University of Jerusalem)
, 0000-0002-6794-0553 corresponding author
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