Building a Nanomanipulator
MIT’s Martin Culpepper shows how to build simple machines that move with nanometer precision.
Nanotechnologists promise a lot: electronics forged from individual molecules, superstrong, lightweight materials, ultrafine capsules that carry drugs to specific organs or cells in the body. But to tinker with materials on that scale, researchers need tools to probe and nudge their invisibly small specimens. And manufacturers will need equipment to mass-produce these future marvels. Such instruments don’t come cheap. The going price for a nanomanipulator – a machine so named not because it is tiny itself but because it can move things around with nanometer precision – is tens of thousands of dollars. MIT mechanical-engineering professor and TR100 honoree Martin Culpepper believes he can produce better instruments for less than $3,000 apiece using a different approach to machine design. Existing nanomanipulators, he points out, “have got a bunch of different joints and linkages to assemble together.” Because the gaps between pieces can be many nanometers wide, this “old paradigm,” as he calls it, is impractical for nanoscale motion. Instead, Culpepper’s machine is built around one piece that bends and flexes ever so slightly. He shows TR’s Dan Cho how to provide supersmall movement without an astronomical price tag.
1. Like Butter. The nanomanipulator begins in a machine shop, where two of Culpepper’s graduate students, Soohyung Kim and Nathan Landsiedel, cut pieces out of metal. They place a plate of titanium on the bed of a water-jet cutter and feed instructions from a computer disk into the adjoining console. With a moving nozzle that shoots a millimeter-wide stream of water laced with particles of garnet, the machine can cut complicated shapes in a matter of minutes.
2. Flex Time. Culpepper holds out one of the newly cut pieces. This is the heart of his machine: three flat strips branching out symmetrically from a common center and surrounded by a wiry frame to form a vague triangle. There is purpose in this curious geometry. The center, or “stage,” of the triangle is where a probe would be fixed in a complete instrument. “You hold these three points,” says Culpepper, gesturing toward three washer-shaped fixtures suspended on the bent arms between the corners of the triangle, then “you push each one of these tabs to the side or up and down.” He indicates the ends of the flat strips and demonstrates how pushing two tabs toward each other moves the center away from them both. Press down on all three and the center moves upwards. By pushing different combinations of tabs, he can cause the stage to slide or twist in any possible direction. This is what engineers call six-axis motion, something existing nanomanipulators struggle to achieve.
3. Getting Attached. Culpepper bolts the flexible piece onto the aluminum base of his machine. A twist of one of the long knobs protruding from the sides moves the adjacent washer-shaped fixture by a few micrometers. With these knobs, Culpepper can adjust the triangle’s shape, tuning it to different tasks. “We can make it have a larger or smaller range of motion, or finer resolution,” he explains, so that “people won’t have to spend several thousand dollars just to do one task.”
4-5. Prime Movers. Culpepper next attaches the base to three actuators, the components that push and pull the flexible tabs on command. Each actuator consists of aluminum cylinders wrapped in hand-wound copper wire. Inside are long rod-shaped pieces capped on the ends with strong magnets. When current is passed through the wires, it creates a magnetic field within the actuator, pushing the magnets and mechanism to one side or another, or up and down.
6. Mind the Gap. On top of the device, Culpepper fixes a bulky piece strewn with wires. This tricornered aluminum crown contains six cylindrical capacitance sensors that precisely monitor the movement of the stage. Culpepper is contemplating a more compact, laser-based measurement system for future versions. “It can move without the sensors,” he adds, “but at the levels of precision we’re aiming for, it’s critical to measure.”
7. Silent Running. With the nanomanipulator assembled and wired up, Culpepper steps over to the computer to test it. It’s not much for the naked eye to see, but as he types commands into the keyboard, the stage of the manipulator performs a nanoscale contortion routine. Culpepper keeps an eye on the numbers, watching for unexpected disturbances. Air vibrations stirred by ordinary voice conversations can throw the stage’s position off, though careful structural design has minimized this effect. Lab instruments that incorporate the manipulator – which Culpepper will begin designing this fall – will be fully protected from these tremors. Coupon-clipping nanotechnologists may soon have cause to celebrate.