The quest to develop a wireless micro-robot for biomedical applications requires a small-scale “motor” that can be wirelessly powered through biological media. While magnetic fields can be used to power small robots wirelessly, they do not provide selectivity since all actuators (the components controlling motion) under the same magnetic field just follow the same motion. To address this intrinsic limitation of magnetic actuation, a team of German researchers has developed a way to use microbubbles to provide the specificity needed to power micro-robots for biomedical applications.
This week in Applied Physics Letters, from AIP Publishing, the team describes this new approach that offers multiple advantages over previous techniques.
“First, by applying ultrasound at different frequencies, multiple actuators can be individually addressed; second, the actuators require no on-board electronics which make them smaller, lighter and safer; and third, the approach is scalable to the sub-millimeter size,” said Tian Qiu, a researcher at the Max Planck Institute for Intelligent Systems in Germany.
The research team encountered some surprises along the way. Normally a special material, like a magnetic or piezoelectric material, is required for an actuator. In this case, they used a standard commercial polymer that simply traps air bubbles, and then used the air-liquid interface of the trapped bubbles to convert the ultrasound power into mechanical motion.
“We found that a thin surface (30-120 micrometers effective thickness) with appropriate topological patterning can provide propulsion force using ultrasound, and thousands of these bubbles together can push a device at millimeter scale,” Qiu said. “The simplicity of the structure and material to accomplish this task was a pleasant surprise.”
The team is already looking forward to developing their actuator further.
“The next steps are to increase the propulsive force of the functional surface, to integrate the actuator into a useful biomedical device, and then to test it in a real biological environment, including in vivo,” Qiu said.
The adoption of micro-structured surfaces as wireless actuators opens promising new possibilities in the development of miniaturized devices and tools for fluidic environments accessible by low intensity ultrasound fields. These functional surfaces could serve as ready-to-attach wireless actuators, powering miniaturized biomedical devices for applications such as active endoscopes.
The chemical element gallium could be used as a new reversible adhesive that allows its adhesive effect to be switched on and off with ease
Some adhesives may soon have a metallic sheen and be particularly easy to unstick. Researchers at the Max Planck Institute for Intelligent Systems in Stuttgart are suggesting gallium as just such a reversible adhesive. By inducing slight changes in temperature, they can control whether a layer of gallium sticks or not. This is based on the fact that gallium transitions from a solid state to a liquid state at around 30 degrees Celsius. A reversible adhesive of this kind could have applications everywhere that temporary adhesion is required, such as industrial pick-and-place processes, transfer printing, temporary wafer bonding, or for moving sensitive biological samples such as tissues and organs. Switchable adhesion could also be suitable for use on the feet of climbing robots.
The principle is actually quite simple: Above 30 degrees Celsius, gallium metal is liquid, and below 30 degrees it is solid. So if a drop of liquid gallium is introduced between two objects and then cooled to less than 30 degrees, the gallium layer solidifies and sticks the two objects together. When it is time to separate the objects, the temperature is raised to transition the gallium layer to its liquid state and they can be pulled apart with a small amount of unloading force. As an adhesive, gallium works in a similar fashion to hot glue, widely used in DIY applications. The difference is that far less heating and cooling are sufficient in the case of gallium, it lifts much more easily and cleanly from the surface, it is highly repeatable, and it is electrically conductive.
A soft actuator using electrically controllable membranes could pave the way for machines that are no danger to humans
In interacting with humans, robots must first and foremost be safe. If a household robot, for example, encounters a human, it should not continue its movements regardless, but rather give way in case of doubt. Researchers at the Max Planck Institute for Intelligent Systems in Stuttgart are now presenting a motion system – a so-called elastic actuator – that is compliant and can be integrated in robots thanks to its space-saving design.
The actuator works with hyperelastic membranes that surround air-filled chambers. The volume of the chambers can be controlled by means of an electric field at the membrane. To date, elastic actuators that exert a force by stretching air-filled chambers have always required connection to pumps and compressors to work. A soft actuator such as the one developed by the Stuttgart-based team means that such bulky payloads or tethers may now be superfluous.
Many robots have become indispensable, and it is accepted that they may be dangerous to humans in their workspace. In the automotive industry, for example, they assemble cars with speed and reliability, but are well shielded from direct contact with humans. These robots go through their motions precisely and relentlessly, and anyone who gets in the way could be seriously injured.
Robots with soft actuators that cannot harm humans, on the other hand, are tethered by pneumatic hoses and so their radius of motion is restricted. This may be about to change. “We have developed an actuator that makes large changes in form possible without an external supply of compressed air”, says Metin Sitti, Director at the Max Planck Institute for Intelligent Systems.