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Nature communications(IF12.353),published in August 2018,reported a paper called“In-built thermo-mechanical cooperative feedback mechanism for self-propelled multimodal locomotion and electricity generation”written by Ghim Wei Ho's research group from National University of Singapore.

Utilization of ubiquitous low-grade waste heat constitutes a possible avenue towards soft matter actuation and energy recovery opportunities.While most soft materials are not all that smart relying on power input of some kind for continuous response,they conceptualize a self-locked thermo-mechano feedback for autonomous motility and energy generation functions. Here, the low-grade heat usually dismissed as ‘not useful’is used to fuel a soft thermo-mechano-electrical system to perform perpetual and untethered multimodal locomotions.The innately resilient locomotion synchronizes self-governed and auto-sustained temperature fluctuations and mechanical mobility without external stimulus change,enabling simultaneous harvesting of thermo-mechanical energy at the pyro/piezoelectric mechanistic intersection.The untethered soft material showcases deterministic motions (translational oscillation,directional rolling,and clockwise/anticlockwise rotation),rapid transitions and dynamic responses without needing power input, on the contrary extracting power from ambient.This work may open opportunities for thermo-mechano-electrical transduction,multigait soft energy robotics and waste heat harvesting technologies.

In this work, they devise a self-governing thermo-mechano-electrical system (TMES) to exploit low-grade ambient heat for a diverse adaptive mechanical actuation,coupled with thermo-mechanical transducing behavior.The structure design and system concept is presented in Fig.1a.A thermo-mechanical deformation is first actuated on a hot surface based on the bimorph principle, which requires a bilayer structure with opposite coefficient of thermal expansion (CTE).To fulfill the perpetual motility and thermo-mechanical energy harvesting,materials selection and microstructure design are the essential elements, conferring TMES with mechanical robustness and functional ductility.The TMES mainly consists of a three-dimensionally (3D) aligned ferroelectric polyvinylidene fluoride (PVDF) and polydopamine modified reduced graphene oxide-carbon nanotube layer (PDG-CNT) with nacre-like brick-and-mortar microstructures.A TMES film up to 100cm2 was made (Fig.1b),which allows easily obtaining of TMES strips at different alignment angles depending on the cutting directions.The PDG-CNT synthesized in our work possesses nacre-like brick-and-mortar microstructures (Fig.1c),and π-π bonding and strong adhesion between PDG and CNT allows PDG-CNT water ink being layer-by-layer deposited on hydrophilic treated PVDF surface without delamination,hence ensuring mechanical robustness of TMES to bear large and various deformations.

The structure design and system concept of TMES

Their results demonstrate the use of TMES that opens up a promising avenue of smart soft material system that operates on self-regulated thermo-mechano feedback—based on structural instability that readily transit between various steady states.The monolithic TMES is capable of processing autonomous extraction of thermal energy from a constant low-grade heat environment and converting it to diverse locomotions and energy generation functions.Kinematic tracking,mechanical analysis,and dynamic thermal imaging disclose the essential principles that activate and maintain the ceaseless thermo-mechanical synchronized mechano-thermal feedback loop,consequently generating adaptive locomotions and cooperative pyro/piezoelectricity.

Such soft material system has shown to effectively interface with low-grade ambient heat and adapt to unstructured environment,while not limited by the dependence on electrical or pneumatic tethers.Ultimately,the self-locked thermo-mechanical feedback mechanism described here may offer new possibilities for autonomous multigait soft energy robotics and waste heat harvesting technologies with versatile thermo-mechano-electrical transduction.

The multi walled carbon nanotubes used in the PDG-CNT come from XFNANO.It is a great honor for XFNANO to provide high quality carbon nanotubes for researchers.