Our Approach

Our artificial muscles project pursues a potential solution to this problem. It combines electrostatic actuation with microfluidics, liquid electrodes, and additive manufacturing (AM).

The basic idea is that a charged capacitor produces an attractive force between its two electrodes. Consequently, the material contracts like a muscle. That contraction pulls on the surrounding material, which acts like tendons. Normally, this force is small, for large capacitors. However, the force scales like the inverse square of the distance between the plates. So, if the scale of the device is reduced by a factor of 1,000, the force increases by a factor of 1,000,000. 

Hence, arrays of microcapacitors can be arranged in microfibers, all pulling in the same direction. The result is added elongation, so that while the individual microcapacitor shrinks by a microscopic distance, the overall fiber shrinks macroscopically, because those distances add, as in a system of springs connected in series. Next, individual fibers can be arranged in parallel in fiber bundles, where all fibers act in the same direction, so that the generated forces add together like springs connected in parallel. Hence, in a muscle fiber bundle, the longitudinal dimension adds elongation, while the two lateral dimensions add force. 

Our approach offers a pathway to a particularly energy-efficient actuation. Traditional step motors are electromagnetic motors in that the actuator force is a magnetic Lorentz force. That requires the generation of strong magnetic fields, which is usually done by running large currents in solenoids. However, the internal resistance of the solenoid wiring then generates significant Joule heating, which leads to energy loss and power inefficiency. Moreover, current needs to be run even to maintain the force without motion. In contrast, our approach is electrostatic – magnetic fields are unnecessary. Maintaining the force without motion requires no current because capacitors simply remain charged. Consequently, our artificial muscles would be a particularly energy-efficient actuation system. So, they could be powered by smaller batteries for longer operational times, allowing for greater miniaturization and wearables.

Calculations indicate that the proposed devices could generate up to 33MPa stress under the current extreme limits of manufacture and materials. COMSOL simulations of both individual devices and arrays offer strong evidence for the feasibility of the proposed techniques. 

Parameter sweeps of the simulations offer insights into the behavior of the proposed devices as well as suggest optimal values maximizing device performance. These results allow for efficient design to maximize the generated output force density. 

To realize our muscles, we use 3D printing, as this approach solves both the manufacturing and upscaling requirements. Microfluidic channels inbuilt inside the print are supposed to provide wiring and electrodes, by being filled with conductive material. However, a 3D-printer cannot print over empty space. Hence, 3D-printed prototype has microfluidic channels printed in sacrificial material inside cured resin. The resin serves as the dielectric for the microcapacitors and tendons for the muscles. This means the sacrificial material must be removed and replaced with conductive material. This is very challenging however, because traditional removal techniques are not usable in microfluidics due to the very high surface-to-volume ratio and very limited fluidic access from the outside world.

More recently, we developed an experimental protocol, which allows the successful removal of sacrificial material from the microchannels. This is a major technical step forward because it opens the door to producing the inbuilt wiring for the muscles. The next step would be to find the best conductive material, load it successfully into the microchannels, and ascertain its electrical conductivity.