Developed at the Georgia Institute of Technology, the robots are around 2mm in length, 1.8mm wide and 0.8mm thick, and weigh about five milligrams. They can be powered by vibrations from on-board piezoelectric generators, ultrasound sources or audio speakers. Their developers believe they may be able to work in swarms to sense environmental changes, move materials or even repair injuries inside the human body.
The technology arose from research led by Azadeh Ansari, from Georgia Tech’s School of Electrical and Computer Engineering. “We are working at the intersection of mechanics, electronics, biology and physics. It’s a very rich area and there’s a lot of room for multidisciplinary concepts,” she said.
Each robot consists of a piezoelectric actuator based on lead zirconate titanate (PZT), which vibrates when electric voltage is applied to it. This is glued onto a polymer body which is 3D printed using the process two-photon polymerisation lithography (TPP), which has springy legs that move up and down in response to the vibrations of the actuator: external vibrations from below the surface on which the robot is standing also generates the same effect. The size, diameter, design and overall geometry of the legs determines which frequency the robot will respond to, and the amplitude of vibration determines the speed at which they move. Ansari has created robots that can cover four times their own length in a second.
"As the micro-bristle-bots move up and down, the vertical motion is translated into a directional movement by optimising the design of the legs, which look like bristles," explained Ansari. "The legs of the micro-robot are designed with specific angles that allow them to bend and move in one direction in resonant response to the vibration."
A paper describing Ansari’s research has been accepted for publication in the Journal of Micromechanics and Microengineering. The paper describes the TPP process, which uses light to polymerise a liquid monomer substance, the remainder of which consume to be washed away from the printed structure. "It's writing rather than traditional lithography," Ansari explained. "You are left with the structure that you write with a laser on the resin material. The process now takes quite a while, so we are looking at ways to scale it up to make hundreds or thousands of micro-bots at a time." The first author on the allegation, graduate student DeaGyu Kim, has produced hundreds of the tiny structures, some with four legs and some with six, to determine the best configurations. TPP can produce smaller structures, but these tend to be subject to greater adhesion forces and are difficult to manipulate.
While the piezoelectric actuators can provide the vibration to move the robots, they can also be used to generate a voltage which the team believes could be used to power up on-board sensors when the robots are being driven by external vibration. In the current phase of research, Ansari and her team are attempting to join micro-bristle-bots made to respond to different vibrations together: these combination robots would be steerable by varying the frequency and amplitudes of vibration. "Once you have a fully steerable micro-robot, you can imagine doing a lot of interesting things," she said.
Ansari believes that these robots might be more useful than versions that use magnetic fields to produce movement, as it is not possible to single out individual robots within a swarm using this method. She is also interested in producing robots that can jump or swim.
"We can look at the collective behaviour of ants, for example, and apply what we learn from them to our little robots," Ansari added. "These micro-bristle-bots walk nicely in a laboratory environment, but there is a lot more we will have to do before they can go out into the outside world."