Actuators are used in a wide variety of electromechanical systems and in robotics, in applications such as steerable catheters, aircraft wings that adapt to changing conditions, and wind turbines that reduce drag.
Engineers have discovered a simple, economical way to make a nano-sized actuator that weighs 1.6 milligrams and can lift 265 milligrams hundreds of times in a row. Its strength comes from a process of inserting and removing ions between very thin sheets of molybdenum disulfide (MoS2), an inorganic crystalline mineral compound. The device works like a muscle, and converts electrical energy to mechanical energy.
The actuator is an inverted-series-connected (ISC) biomorph actuation device. By applying a small amount of voltage, the device can lift something that’s far heavier than itself. The simple restacking of atomically thin sheets of metallic MoS2 leads to actuators that can withstand stresses and strains comparable to or greater than other actuator materials.
Molybdenum disulfide — a naturally occurring mineral — is commonly used as a solid-state lubricant in engines. It is a layered material like graphite, with strong chemical bonding within thin layers, but weak bonding between the layers. Thus, individual layers of MoS2 can be easily separated into individual thin sheets via chemistry.
The extremely thin sheets, also called nanosheets, remain suspended in solvents such as water. The nanosheets can be assembled into stacks by putting the solution onto a flexible material and allowing the solvent to evaporate. The restacked sheets can then be used as electrodes — similar to those in batteries — with high electrical conductivity to insert and remove ions. Inserting and removing ions leads to the expansion and contraction of nanosheets, resulting in force on the surface. This force triggers the movement — or actuation — of the flexible material.
The MoS2-based electrochemical device has mechanical properties such as stress, strain, and work capacity that are extraordinary considering the electrodes are made by simply stacking weakly interacting nanosheets.