Researchers at the University of Texas at Dallas have created powerful unipolar muscles from carbon nanotubes.
Researchers at the University of Texas at Dallas have created powerful unipolar electrochemical muscles that contract more actively when they move faster.
For more than 15 years, researchers at the University of Texas at Dallas and their colleagues in the United States, Australia, South Korea and China have been making artificial muscles by twisting and winding carbon nanotubes or polymer filaments. When temperatures change, these muscles work, contracting their length when heated and returning to their original length when cooled. However, they have their limitations.
Electrochemically controlled muscles made from carbon nanotubes (CNTs) are an alternative approach to creating fast, powerful, artificial muscles that can be used in robotics and other applications.
Electrochemically controlled muscles are especially promising because their energy conversion efficiency is not limited by the thermodynamic limit: they can contract more strongly, and they can also withstand heavy loads without consuming much energy.
But there are limitations to the electrochemical muscles of CNTs. First, muscle stimulation is bipolar, meaning that muscle movement, expansion or contraction changes direction during a potential scan. The potential at which a stroke changes direction is the zero charge potential, and the rate at which the potential changes over time is the potential scan rate.
Another question: this electrolyte is stable only in a certain voltage range. Outside this range, the electrolyte is destroyed.
To solve these problems, the researchers figured out that the inner surfaces of the spiral filaments of carbon nanotubes can be coated with a specific ion-conducting polymer that contains either positively or negatively charged chemical groups.
This polymer coating converts the bipolar excitation of the carbon nanotube filaments to unipolar excitation where the muscle acts in one direction over the entire range of electrolyte stability. The number of solvent molecules pumped into the muscle by each ion increases with the potential scan rate for some unipolar muscles, which increases the effective size of the ions.
Thus, muscle travel can increase by a factor of 3.8 with an increase in potential scan speed, while muscle travel from a carbon nanotube without polymer coating decreases by 4.2 times with the same changes in potential scan speed.