By Joyce P. Brayboy, U.S. Army Research Laboratory January 3, 2013
ADELPHI, Md. (Jan. 3, 2013) -- Origami artists can fold a sheet of paper into a crane, a flower or even an Army tank.
Likewise, an Army electrical engineer and a visiting research student from Johns Hopkins University, or JHU, wanted to design and model thin, micrometer scale materials to form useful three dimensional structures.
Self-assembling structures that seem to mimic the age old traditional Japanese art form of paper folding have potential uses in defense of the nation.
Christopher Morris, Ph.D., who works with micro-materials and devices at the U.S. Army Research Laboratory, or ARL, U.S. Army Research, Development and Engineering Command , known as RDECOM, envisioned transforming lithographically-patterned fabricated sheets into "microgrippers" similar to ones he had seen demonstrated by JHU at a conference for potential use in micro-scale surgery.
Their recent paper, "Laser triggered sequential folding of microstructures," was published in Applied Physics Letters and highlighted in Nature Photonics. As a result, that demonstrated how a small team of researchers looked at the same problem from different angles and conjured up a way to control folding pathways, and enable sequential folding on a millimeter scale using a low-intensity laser beam.
It all began when Morris reached out to the JHU professor, David Gracias, Ph.D., to discuss the possibilities of his findings for defense applications.
"Self-actuation for defense uses seemed like a natural extension of what we were exploring at JHU with chemical acuation triggers for medical procedures. Sequentially folding complex devices didn't come easily, but the results were worth the wait," said Kate Laflin, a research student who has studied self-assembly, microactuators, and their applications in chemical and biological sensing since 2008.
Laflin said she left behind her aspirations in medical research for the defense assignment.
"People have been very supportive at ARL and at JHU. The lab is a collaborative atmosphere, in that we're working toward the same goal -- protecting the warfighter."
As the two began looking into tagging applications, Morris suggested trying lasers as a trigger, Laflin said. "What was so exciting is that it worked at such a low intensity."
Once Laflin and Morris tested the use of lasers, they wanted to push the boundaries of selective response on a micro-level, she said.
"It's a simple concept, but to be able to design and model a structure at the micrometer, and ultimately, nanometer scale, it took steps that didn't seem intuitive at first," Morris said. "After a good theoretical understanding, design, modeling and testing -- it came together for us."
It takes about 20 hours in ARL's clean room to fabricate a sheet of millimeter-sized structures that essentially are battery-free wireless actuators that fold when exposed to a laser with an intensity of just 680 mW/cm2, which can be eye-safe at infrared wavelengths. A common household light source would not be strong enough to cause folding, but the metallic structures may respond to high-powered light-emitting diode, or LED, lighting.
So far, this lighting could be directed at the structures from up to three feet away using a handheld laser, and depending on the wavelength and intensity of the laser irradiation, the folding will occur within 67 milliseconds to 21 seconds. It takes minutes at best for the folding to occur with a larger structure, Morris said.
At the millimeter scale the structures could attach, jump, apply friction, and perform as mechanical switches to serve a number of defense functions.
"There is a lot of cool science using the underlying technique of absorbing optical energy to release the energy stored in pre-stressed metallic bilayers," Morris said.
The micro-device folding that the team has demonstrated can be applied to; the remote initiation of energetic materials, micro thrusters for robotics, the attachment of transponder tags to fabric surfaces, and could possibly be integrated with logic/memory circuits, sensors, transponder tags, and optical modules such as light emitting diodes.
"Imagine making something so small you can't even see it," Laflin said.
The metallic structures act as mechanical switches. The benefit researchers have found from using a mechanical as opposed to electrical switch is, there is no electric current leakage.
Now that the team has figured out a way to direct and control actuation in an uncontrolled environment, reversible actuation is on the horizon.
"Reversible actuation is good for mechanical switches and also for displays," Morris said.
"Our hope is that new uses will spur from this basic scientific exploration of novel fabrication and self-assembly of materials, and will help future Soldiers in ways they may not even see," Morris continued. "Imagine a kind of pixie dust; the structures will be so small that a cluster could be present without appearing to the natural eye.
"We are enabling true microsystems, where all of the energy and functions are self-contained in a millimeter- or smaller-sized package," Morris said.