Periodic Reporting for period 2 - E-MOTION (Molecular materials for a new generation of artificial muscles)
Reporting period: 2023-04-01 to 2024-09-30
Actuator devices converting energy into motion are a fundamental part of everyday life. However, there is currently an unmet need in actuation technologies to provide soft, smooth, noiseless movement that can mimic human motion and dexterity. The development of such “artificial muscles” is burgeoning in both interest and importance as it would enable significant advances in areas as important as robotics, medicine and aeronautics. To enable a step-change in this field, E·MOTION proposes to develop unprecedented macroscopic-scale soft materials based on switchable spin crossover (SCO) molecules with remarkable actuating performances. Using an innovative combination of material engineering methods these materials will be endowed with electrical actuation, self-sensing and energy harvesting properties, which will be a major breakthrough. More fundamentally, E·MOTION aims at understanding in-depth structure vs. mechanical property relationships in these switchable materials, which is essential for processing and optimizing their function. A multiscale experimental and theoretical approach will be used to assess how the molecular deformation and change in molecular connectivity under external stimuli affect macroscopic mechanical properties as well as the cycle life. Finally, E·MOTION will take a major shift on the side of actuator design by the development of original fibre-braided actuators as well as 3D-printed, microfluidic actuator devices made of these materials. These advanced actuator designs will then be thoroughly analysed for their ability to mimic complex muscular schemes.
The main achievements of the project during the mid-term of the project are as follows:
- We synthesized and characterized size- and shape-controlled nanoparticles of various SCO complexes. The particles were then dispersed in different commercial polymer matrices to obtain composite films, which serve us to construct actuator devices. Notably, we developed a novel family of complexes, Fe(NH2trz)3(BF4)2−x(SiF6)x/2, which appear quasi-ideal for actuation purposes with adjustable, near room temperature operation, large strains and needle shaped particle morphology. Beyond simple particle dispersion, we synthesized also chemically-coupled SCO-polymer composites as well as self-healing composites.
- We have synthesized a series of unprecedented compounds, which combine SCO-active and electroactive properties. These comprise a novel family of Hofmann clathrates of general formula {Fe(R-pbpy+)2[μ2-M(CN)4]2}, which couple the SCO with redox-active ligands, providing an innovative strategy for the development of SCO switches, which can be actuated via a reversible electrochemical reaction (instead of temperature change).
- We fabricated various spin crossover based composite materials using different electroactive polymer matrices, including piezopolymers and conducting polymers, and investigated the electromechanical couplings between the constituents. We also prepared multilayer structures comprising a piezoresistive organic semiconductor layer and an SCO-polymer composite layer. When inserted into a field effect transistor, the strain coupling between the two layers afforded for electrically sensing of the molecular spin state changes with high sensitivity.
- We analyzed structure - mechanical property relationships in bulk and nanoscale spin crossoover materials using variable temperature and pressure x-ray diffraction, DMA and nanoindentation techniques, combined with Molecular Dynamics simulations - providing us the necessary inputs to start working towards effective actuator designs. Remarkably, we discovered important anelastic effects at the spin transition, manifested by elastic softening and associated mechanical damping phenomena.
- We conducted a deep experimental (DMA) and theoretical analysis of the mechanical properties of the SCO-polymer composites. Thanks to these efforts, we are now able to predict material properties and design materials with enhanced performance.
- We fabricated a series of electrothermally actuated bending actuators, which were characterized using a custom-built bench, including open-loop identification and closed-loop PID control.
- We have embarked in the design (by means of finite element analysis) and fabrication (by means of 3D printing) of more complex actuator concepts. Notably, we developed shape-morphing actuators, which can realize various pre-programmed movements.
- We synthesized and characterized size- and shape-controlled nanoparticles of various SCO complexes. The particles were then dispersed in different commercial polymer matrices to obtain composite films, which serve us to construct actuator devices. Notably, we developed a novel family of complexes, Fe(NH2trz)3(BF4)2−x(SiF6)x/2, which appear quasi-ideal for actuation purposes with adjustable, near room temperature operation, large strains and needle shaped particle morphology. Beyond simple particle dispersion, we synthesized also chemically-coupled SCO-polymer composites as well as self-healing composites.
- We have synthesized a series of unprecedented compounds, which combine SCO-active and electroactive properties. These comprise a novel family of Hofmann clathrates of general formula {Fe(R-pbpy+)2[μ2-M(CN)4]2}, which couple the SCO with redox-active ligands, providing an innovative strategy for the development of SCO switches, which can be actuated via a reversible electrochemical reaction (instead of temperature change).
- We fabricated various spin crossover based composite materials using different electroactive polymer matrices, including piezopolymers and conducting polymers, and investigated the electromechanical couplings between the constituents. We also prepared multilayer structures comprising a piezoresistive organic semiconductor layer and an SCO-polymer composite layer. When inserted into a field effect transistor, the strain coupling between the two layers afforded for electrically sensing of the molecular spin state changes with high sensitivity.
- We analyzed structure - mechanical property relationships in bulk and nanoscale spin crossoover materials using variable temperature and pressure x-ray diffraction, DMA and nanoindentation techniques, combined with Molecular Dynamics simulations - providing us the necessary inputs to start working towards effective actuator designs. Remarkably, we discovered important anelastic effects at the spin transition, manifested by elastic softening and associated mechanical damping phenomena.
- We conducted a deep experimental (DMA) and theoretical analysis of the mechanical properties of the SCO-polymer composites. Thanks to these efforts, we are now able to predict material properties and design materials with enhanced performance.
- We fabricated a series of electrothermally actuated bending actuators, which were characterized using a custom-built bench, including open-loop identification and closed-loop PID control.
- We have embarked in the design (by means of finite element analysis) and fabrication (by means of 3D printing) of more complex actuator concepts. Notably, we developed shape-morphing actuators, which can realize various pre-programmed movements.
The main progress beyond the state of the art comprises:
- We synthesized a new family of SCO compounds incorporating redox-active ligands. We show that these compounds are able to undergo a quasi-reversible electrochemical redox process as well as SCO. These findings open the door to a new generation of multiswitchable spin crossover molecules, which are adressable not only by thermal, but also by electrical stimuli.
- We developed an original Molecular Dynamics method for the theoretical investigation of nano-mechanical actuator devices based on SCO molecules. The method relies on the experimental crystal structure of compounds and is able to simulate the SCO behavior, including the couplings between the electronic and structural degrees of freedom. This approach removes bottlenecks associated with continuum mechanics simulations, which ignore local, microscopic effects and require preliminary mechanical characterizations, and overcomes also current limitations of atomistic 'spring-ball' modeling methods, which neglect structural details and cannot provide a reliable description of the real time evolution of the system.
- We revealed unexpected mechanical properties of spin crossover - polymer composite materials, including significant strain amplification and mechanical damping at the spin transition and identified the intra- and inter-molecular processes behind these phenomena. Furthermore, we developed a micromechanical homogenization approach allowing for a quantitative prediction of the mechanical actuating properties of such composite materials, which goes significantly beyond the state of the art.
- We constructed an organic field-effect transistor (OFET), which allowed us for the electrical sensing of spin state switching with unprecedented sensitivity at the micrometer scale. This breakthrough allows us to anticipate the development of lightweight, flexible devices combining the mechanical actuation capability of spin crossover composites with the strain sensitivity of organic electronic materials, providing scope for self-sensing of the actuator.
As for the expected results, we anticipate:
- Fabrication of 3D printed actuator devices with optimized material and structural design, allowing for the transformation of weak forces of individual molecules into a much greater muscle-like contraction force.
- Endowing the molecule-based actuators with numerous advanced functionalities, such as closed-loop control, camouflage, bistability, chemical actuation, self-sensing and self-regulation.
- Develop solutions for demanding applications in lightweight robots, human assist devices, microfluidic systems and smart textiles.
- We synthesized a new family of SCO compounds incorporating redox-active ligands. We show that these compounds are able to undergo a quasi-reversible electrochemical redox process as well as SCO. These findings open the door to a new generation of multiswitchable spin crossover molecules, which are adressable not only by thermal, but also by electrical stimuli.
- We developed an original Molecular Dynamics method for the theoretical investigation of nano-mechanical actuator devices based on SCO molecules. The method relies on the experimental crystal structure of compounds and is able to simulate the SCO behavior, including the couplings between the electronic and structural degrees of freedom. This approach removes bottlenecks associated with continuum mechanics simulations, which ignore local, microscopic effects and require preliminary mechanical characterizations, and overcomes also current limitations of atomistic 'spring-ball' modeling methods, which neglect structural details and cannot provide a reliable description of the real time evolution of the system.
- We revealed unexpected mechanical properties of spin crossover - polymer composite materials, including significant strain amplification and mechanical damping at the spin transition and identified the intra- and inter-molecular processes behind these phenomena. Furthermore, we developed a micromechanical homogenization approach allowing for a quantitative prediction of the mechanical actuating properties of such composite materials, which goes significantly beyond the state of the art.
- We constructed an organic field-effect transistor (OFET), which allowed us for the electrical sensing of spin state switching with unprecedented sensitivity at the micrometer scale. This breakthrough allows us to anticipate the development of lightweight, flexible devices combining the mechanical actuation capability of spin crossover composites with the strain sensitivity of organic electronic materials, providing scope for self-sensing of the actuator.
As for the expected results, we anticipate:
- Fabrication of 3D printed actuator devices with optimized material and structural design, allowing for the transformation of weak forces of individual molecules into a much greater muscle-like contraction force.
- Endowing the molecule-based actuators with numerous advanced functionalities, such as closed-loop control, camouflage, bistability, chemical actuation, self-sensing and self-regulation.
- Develop solutions for demanding applications in lightweight robots, human assist devices, microfluidic systems and smart textiles.