text: A miniature robotic crab tinier than a flea has become the smallest-ever remote-controlled robot.
  Just a half-millimetre wide, it can walk, bend, twist, turn and even jump.
  The record-breaking micro machine was developed by engineers at Northwestern University and comes in the form of a tiny peekytoe crab.
  Although the research is exploratory at the moment, they believe the technology might bring the field closer to realising micro-sized robots that can perform practical tasks inside tightly confined spaces.
  Things like eliminating tumours or clearing blocked arteries inside the human body.
  The same team produced a winged microchip last year that was the smallest-ever human-made flying structure. They have also developed millimetre-sized robots resembling inchworms, crickets and beetles.
  ‘You might imagine micro-robots as agents to repair or assemble small structures or machines in industry or as surgical assistants to clear clogged arteries, to stop internal bleeding or to eliminate cancerous tumours — all in minimally invasive procedures,’ said Professor John Rogers, a Bioelectronics pioneer who led the experimental work.
  Smaller than a flea, the crab is not powered by complex hardware, hydraulics or electricity. Instead, its power lies within the elastic resilience of its body.
  ‘Our technology enables a variety of controlled motion modalities and can walk with an average speed of half its body length per second,’ said Professor Yonggang Huang, who led the theoretical work.
  ‘This is very challenging to achieve at such small scales for terrestrial robots,’
  To build the mini machine, the researchers used a shape-memory alloy material that transforms to its ‘remembered’ shape when heated.
  In this case, the researchers used a scanned laser beam to rapidly heat the robot at different targeted locations across its body. A thin coating of glass elastically returns that corresponding part of structure to its deformed shape upon cooling.
  Rogers said that as the robot changes from one phase to another — deformed to remembered shape and back again — it creates locomotion.
  He explained that not only does the laser remotely control the robot to activate it, the laser scanning direction also determines the robot’s walking direction.
  Scanning from left to right, for example, causes the robot to move from right to left.
  To manufacture such a tiny machine, the scientists turned to a technique they introduced eight years ago — a pop-up assembly method inspired by a child’s pop-up book.
  The team began by fabricated precursors to the walking crab structures in flat, planar geometries. Then, they bonded the precursors onto a slightly stretched rubber substrate.
  When the stretched substrate is relaxed, a controlled buckling process occurs that causes the crab to ‘pop up’ into precisely defined three-dimensional forms.
  The manufacturing method would allow the team to develop robots of various shapes and sizes, but the crab was down to the students.
  ‘With these assembly techniques and materials concepts, we can build walking robots with almost any sizes or 3D shapes,’ said Rogers.
  The findings were published in the journal Science Robotics.
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