borrows the structure of nature. So far, researchers have developed various artificial muscles (also called drivers) that can reversibly contract, bend or rotate when subjected to external stimulation.
In recent years, fibrous artificial muscles have received widespread attention. After being stimulated by the external environment, it can contract/elongate and deform like a muscle, which has more advantages than the traditional motor and heater driving form.
However, there are currently some difficulties in the field of artificial muscle fibers. One of them is how to accurately control the driving volume, and its control accuracy is very low at present. In addition, if the artificial muscles remain in a state of contraction for a long time, they need to continuously inject external energy into them, which is not conducive to later application and energy saving.
Figure | Related Papers (Source: ACS Nano)
Recently, the team of Suzhou Institute of Nano Technology and Nanobionics, Chinese Academy of Sciences (hereinafter referred to as " Suzhou Institute of Nano Institute of Nano, Chinese Academy of Sciences ") published a paper on ACS Nano , entitled "Stepwise Artificial Yarn Muscles with Energy-Free Catch States Driven by Aluminum-Ion Insertion)[1]. Di Jiangtao, a researcher at the Suzhou Institute of Nanometer, Chinese Academy of Sciences, and Deputy Director Li Qingwen are the corresponding authors of the paper , and the first author of the paper is doctoral student Ren Ming.
Figure | Schematic diagram of aluminum ion embedding causing artificial muscle fibers to contract (Source: ACS Nano)
Inspired by the fact that the muscles of bivalve molluscs can maintain contraction behavior for a long time under low energy consumption, the team developed a new driving method for ion embedding so that muscle fibers do not need to consume extra energy when maintaining a specific driving state. In addition, muscle fibers have also significantly improved the accuracy of step-drive.
In this study, the above challenges were solved by reversible Faraday embedding and detachment between collapsed carbon nanotubes . This new driving mechanism enables artificial muscle fibers to achieve energy-free high tension contraction retention, and programmable step-by-step drive.
When energy is not provided, artificial muscle fibers can almost completely maintain the achieved contraction stroke even under loads of up to nearly 100,000 times the weight of muscle. The drive mechanism allows programmatic control of stroke steps as low as 1% during the reversible drive process. In addition, the isometric contraction stress generated by artificial muscles is about 40 times that of skeletal muscle , and can achieve precise control of step size.
At the same time, artificial muscles have high energy storage capabilities. When fully charged, the energy stored by the muscles is as high as 102mAh/g, allowing artificial muscles to serve as batteries to power secondary muscles or other devices. This is of great significance to the development of self-energized artificial muscles and the integrated drive system with multiple muscle groups and multiple actions. The basic principle of artificial muscle fiber driving of
artificial muscle fibers mainly involves the intercalation process of aluminum ions, which is similar to the working mechanism of aluminum ion battery . Di Jiangtao said: "A long time ago, I wanted to introduce the ion intercalation mechanism of the battery into artificial muscles to solve the precise driving problem of artificial muscles."
Specifically, aluminum ions can be embedded in muscle fiber materials during charging, so that the fiber volume changes to a certain extent. The change in its volume can be amplified by the fiber helical structure, thereby causing significant changes in the length direction, that is, muscle-like contraction. When the reverse voltage is applied, the aluminum ions will de-embed from the inside of the fiber, and the fiber will return to its original length.
Figure | Zero energy consumption high tension capture state of artificial muscles and programmable step-driven (Source: ACS Nano)
The contraction of artificial muscle fibers utilizes a special structure-intercalation compound formed by aluminum ions and carbon.The working mechanism of intercalation compounds is different from the ion adsorption-dominated electric double layer capacitors, or the pseudocapacitance mechanism. The formation of intercalation compounds can increase the interaction between the host and guest materials, so the muscles can still maintain a driving state under conditions of external stress and power outage.
Since the formation of this compound can be regulated by an electric field, driving control will be more convenient. For example, the driving amount can be controlled by controlling the amount of charge of the reaction, thereby achieving step-driving.
This research took more than two years from the birth of ideas to the presentation of papers. Among them, the biggest challenge is how to enable ions to be embedded into the fiber. Di Jiangtao said: "For the cylindrical carbon nanotube structure, aluminum ions tend to adsorption on the surface, which mainly reflects the adsorption of electric double layer, and the adsorption amount will also be limited to certain limits."
They made many attempts in the early stage, which ended in failure. Later, by controlling the growth of carbon nanotubes at the source, carbon nanotubes with relatively large tube diameters were prepared. After the collapse of the carbon nanotubes with large diameters, a layered structure similar to graphite will be formed, thus providing a migration path and storage space for the intercalation of aluminum ions. In contrast, commonly used cylindrical carbon nanotubes do not have ionic embedding sites.
As a new driver, artificial muscles are still in the basic research stage. Due to its advantages such as flexibility and high strength, artificial muscles will have certain application prospects on software robot . At the same time, the driving voltage required is very low, basically within 5V. From the perspective of energy consumption, it also makes it more advantageous and has no obvious thermal effect.
Figure | Examples of energy storage and application of artificial muscles (Source: ACS Nano)
Previous muscle fibers require mechanical stretching to restore to the initial muscle length before starting. The artificial muscle fibers designed by this research can use stored energy to recover muscle. When high-capacity muscles are charged, the elbows bend and do work. High-capacity muscles can power low-capacity muscles, causing them to contract, thus returning the elbow to its initial position.
In addition, the energy stored in muscle fibers can power a toy car. During charging, artificial muscle fibers contract and pull the toy car forward. The charged artificial muscle fibers are used to power an mini motor to drag the car back and forth.
In addition, the artificial muscle fibers designed in this study use a large amount of ionic liquid as the basis of supporting electrolyte, so it will be more adaptable to the environment and can achieve stable driving under high temperature and high pressure conditions. However, this structure is still relatively sensitive to humidity, which requires further improvement.
Next, the team will improve the driving volume and response speed of artificial muscles by optimizing the electrode structure, with the current driving volume of about 20%. Their ultimate goal is to make high-performance muscle fibers as the basic drive unit to achieve diversified applications.
It is understood that at the Suzhou Institute of Nanotechnology, Chinese Academy of Sciences, the functional nanocarbon material team led by Researcher Li Qingwen is mainly committed to the control preparation and application research of nanocarbon materials, and has made important achievements in the separation of high-purity semiconductor carbon nanotubes, the continuous preparation of carbon nanotube fibers and thin film gas phases, and the industrial application of carbon nanotube materials.
Reference materials:
1.Ren, M., Xu, P., Zhou, Y., Wang, Y., Dong, L., Zhou, T., Chang, J., He, J., Wei, X., Wu, Y., et al. (2022). Stepwise Artificial Yarn Muscles with Energy-Free Catch States Driven by Aluminum-Ion Insertion. ACS Nano. https://doi.org/10.1021/acsnano.2c05586.
