Micro-robots open up factory-on-a-desk possibilities

Tiny, submersible robots may suggest a single-cell retrieval system, desktop biofactories, or even tools for minimally invasive surgery, according to a 30 June report in the international journal, Science.

At 670 micrometers tall and 170 to 240 micrometers wide, the new microrobots are shorter than a hyphen and no wider than a period. Unlike many previous designs, the Swedish-made microrobots can function in salty broths, blood, urine, cell-culture medium and other liquids, suggesting a host of biotechnology uses.

‘Being able to manipulate many individual cells at the same time is becoming increasingly important for genomics, proteomics, and metabolic research,’ says Edwin Jager, a graduate student at Sweden’s Linkopings Universitet and lead author of the Science paper. ‘We think that these microrobots would be helpful for fundamental studies, or for manufacturing other small devices, especially if we set up arrays of them.’

The robot’s miniature hand might someday pluck single cells, bacteria, multi-cellular organisms and other biological entities from a sample, then transfer them to an analysis station. Coupled with a multisensor area, the microrobots also may suggest lab-on-a-chip designs, or ‘factory-on-a-desk’ tools, programmed to assemble various microstructures.

How do the microrobots work? Imagine a human hand, opening and closing. Similarly, conducting polymers such as polypyrrole can be forced to shrink and swell on command. The researchers combine layers of gold and polypyrrole, then use electricity to manipulate contractions of the polymer.

To make the microrobot grab a glass bead, for instance, researchers plump up the polymer by drawing positively charged ions, called cations, away from an electrolyte solution and into the material. Shrinking the polymer is as easy as applying a positive charge to the gold, which oxidizes the polypyrrole and causes cations to flee.

Previous microrobots have included electronic devices featuring rods and levers, artificial flying insects made of polysilicon, and a walking silicon microrobot. But, ‘none of these operate in water, and would not be suitable as microactuators for the manipulation of cells,’ Jager notes. Whenever pure silicon is exposed, he explains, it oxidizes and stops working. In the new design, silicon provides a skeletal framework, which is encased and protected by gold and polypyrrole ‘micromuscles.’

The Swedish team’s invention was etched into part of a four-inch silicon wafer, which allowed them to create separate gold-and-polypyrrole electrodes for each joint in a robotic arm. From one-fourth of a single wafer, they created 140 microrobots with an elbow, a wrist, a hand, and two to four fingers.

The team used photolithography to pattern the titanium-coated silicon. Next, using a patterned chromium layer as a kind of glue, they added a thin layer of gold to the template. More etching defined individual electrodes on the silicon. This step was followed by a rigid material called BCB (Benzocyclobutene, or Cyclotene), which provided a framework for the electrodes. Finally, the polypyrrole was deposited over the gold. After a final etching step, the robots were released and ready for action.

Submerged in an electrolyte solution, several robots were wired to an electrical source and videotaped as they as hoisted a glass bead. Much like puppeteers pulling one string or another, the researchers stimulated the microrobots’ fingers, wrists and elbows by applying a charge to specific joints. To open and close the microrobots’ hand, for example, they successively applied positive and negative potentials.

The microrobots successfully moved a bead up to 250 micrometers. They also hoisted beads 270 micrometers, from one miniature conveyor belt or ‘track’ to another, proving their potential as tiny factory workers.