(Nanowerk Spotlight) Scientists have struggled to create synthetic materials that can sense and respond to their environment with the precision needed for advanced robotics and smart devices. A key challenge has been developing materials that can efficiently convert multiple types of environmental changes – like variations in light or moisture – into controlled mechanical movements. While responsive materials exist, they typically react slowly, work with only one type of stimulus, or lose effectiveness over repeated use.
Existing approaches using cellulose nanofibers can respond to moisture but do not inherently absorb light. Previous attempts to make them light-responsive involved adding materials like graphene and MXene, but these combinations struggled with structural limitations that slowed response times. A new composite material overcomes these challenges by integrating a specially designed covalent organic framework, which not only enhances light absorption but also creates ordered channels that accelerate water transport, significantly improving both speed and efficiency.
Recent advances in molecular engineering have enabled the creation of precisely ordered crystalline structures at the nanoscale level. These developments have opened new possibilities for materials that can efficiently absorb light and channel water molecules through their internal structure.
Researchers from the Harbin Institute of Technology have now developed a composite material that demonstrates significant advances in addressing these challenges. Their work, published in Advanced Functional Materials (“Multi-Responsive COF-Enhanced Smart Actuator”), describes a material that can quickly bend and move in response to both near-infrared light and humidity changes, while also generating electrical energy from these movements.
The team’s innovation centers on a new type of crystalline polymer called a covalent organic framework (COF). They created this framework by combining two chemical compounds: tetra (p-amino-phenyl) porphyrin and squaric acid (dubbed COF-TASA by the researchers). The resulting material can convert 79.9% of absorbed near-infrared light into heat – the highest efficiency reported for this class of materials. They combined this framework with cellulose nanofibers and a specialized polymer called polyvinylidene fluoride (PVDF) that can generate electricity when bent or deformed.
Schematic of the reaction system for COF-TASA. (Image: Reprinted with permission by Wiley-VCH Verlag)
The resulting composite shows remarkable improvements in both speed and reliability. When exposed to near-infrared light, it can bend to a specific shape in just 7 seconds, significantly outperforming previous materials. When humidity levels change, the material bends in the opposite direction. Importantly, it maintains these capabilities over multiple cycles without degradation – a crucial requirement for practical applications.
The material’s structure explains its enhanced performance. The new framework creates organized channels within the composite that allow water molecules to move efficiently through the material. Computer simulations revealed that these channels significantly improve how water diffuses through the structure, explaining the material’s faster response times compared to previous attempts.
A key aspect of the team’s work was creating a bilayer membrane that could translate environmental changes into physical movement. The researchers constructed this by combining two distinct layers: one made of their COF-enhanced cellulose nanofiber composite, and another of polyvinylidene fluoride (PVDF). When exposed to near-infrared light, the COF layer heats up and loses moisture, causing it to shrink, while the PVDF layer expands due to heat. This asymmetric deformation causes the entire membrane to bend. Similarly, when exposed to humidity, only the COF layer absorbs moisture and expands, while the PVDF layer remains unchanged, causing the membrane to bend in the opposite direction. This dual-response mechanism, combined with PVDF’s ability to generate electrical charges when deformed, allows the material to both move and generate electricity in response to environmental changes.
Schematic fabrication process of the COC/PVDF bilayer membrane. (Image: Reprinted with permission by Wiley-VCH Verlag)
The researchers demonstrated several practical applications. They created a humidity control system where the material acts as a switch, automatically opening and closing electrical circuits based on moisture levels. They also developed simple robots that could lift and transport small objects by bending in response to either light or humidity changes.
Perhaps most significantly, the material can convert its mechanical movements into electrical energy through its PVDF component. When the material bends in response to environmental changes, the PVDF generates small electrical voltages. This capability suggests applications in self-powered sensors and energy harvesting devices that could collect and use energy from environmental changes.
The team’s comprehensive understanding of the underlying mechanisms, supported by both experimental data and computer simulations, provides a foundation for future improvements. Their analysis showed that the ordered internal structure created by the framework not only improves water molecule movement but also enhances the material’s ability to absorb and convert light energy.
This advancement brings us closer to creating truly adaptive materials for applications ranging from soft robotics to environmental sensing. The combination of fast response times, multi-stimulus sensitivity, and energy harvesting capability offers new possibilities for self-powered devices that can automatically respond to environmental changes. Future applications could include adaptive building materials that respond to weather conditions, smart textiles that react to body moisture, and autonomous systems that can operate without external power sources.
The research demonstrates that precisely engineered molecular structures can create materials with multiple advanced capabilities, setting a new direction for the development of smart, responsive materials that can both sense and adapt to their environment while generating useful energy.
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