They use pure electrostatic attraction between flexible electrodes of opposing polarities, augmented with liquid dielectric.
The base unit of the actuator is almost a hinge.
Taking the central cross in the photo (above right) as an example, the upper (bare) strip is attached to the lower (red) strip permanently at the centre, creating two of these ‘hinges’, one to the left of the centre and one to the right (for reference, there are eight hinges in total in the photo).
When opposite potentials are applied to the strips, force develops between them – greatest at the very narrow gap within the permanent join and immediately adjacent to it.
With enough voltage, the parted strips next to the join will be pulled together, then further increasing the voltage pulls the next part together – continuing until the two strips are in contact along their whole lengths.
Bristol-electrostatic-actuator-front-closed-650In this way, with enough voltage, the four-strip eight-hinge assembly above pulls completely flat – photo left.
It is the vertical contraction that leads to the 99.8% length contraction claim.
The electrostatic pressure that tends to attract the strips together is proportional to the permittivity of the dielectric between the strips (and the square of applied potential difference), so immersing the whole assembly in an insulating liquid would mean more force would develop compared with the in-air case.
By putting a drip of silicone oil where the strips touch, the Bristol team achieved almost the same effect – increasing permittivity locally in the narrow gap where almost all of the attractive force is developed.
As the hinge closes, the drip is squashed into the next narrowest part, and so on, so it is effective in all states of closure.
According to ‘Electro-ribbon actuators and electro-origami robots‘, the paper in Science Robotics which describes this work, the liquid drop provided 92% of the force developed by the hinge when submerged.
Also according to the paper, dielectrophoretic forces draw the drop back into the junction of the hinge as voltage is removed.
A secondary effect is that the oil has a higher breakdown voltage than air, opening the door to the use of higher voltages.
The silicone used has ~2.7x the permittivity and 6.7x the breakdown strength of air, according to the paper, which could increase closing force by ~120x if no other insulating layer was involved.
The specific arrangement above has been dubbed an ‘electro-ribbon actuator’. The paper also includes other physical arrangements, which together are being called ‘electro-origami’.
“With electro-origami, we can replace electromagnetic motors with light, scalable, silent alternatives,” said co-inventor Dr Majid Taghavi. “Because electrostatic devices do not require high currents, they produce much less heat and can be much more efficient than electric motors.”
The team has funding to pursue commercial applications.
You can also see a video clip of the electro-origami fold here.