A regular sewing machine is key to a new advance toward creating “soft” robotics, wearable electronics, and implantable medical systems made of elastic materials that are capable of extreme stretching.
New stretchable technologies could lead to innovations including robots that have humanlike sensory skin and synthetic muscles, as well as flexible garments that people might wear to interact with computers or for therapeutic purposes.
“However, you would need a low-cost, highly stretchable electrical conductor to interconnect sensors and other components in these applications,” says Babak Ziaie, professor in the School of Electrical and Computer Engineering and Weldon School of Biomedical Engineering at Purdue University.
Other researchers are pursuing approaches that require the use of complicated and expensive microfabrication techniques or exotic technologies such as liquid-metal-filled microchannels and ultrathin wires prone to breaking.
Now, a Purdue team has come up with a far simpler, less-costly alternative: Use a standard sewing machine to create ultra-stretchable interconnects out of conventional wire. The wire is sewn in a zigzag pattern and embedded in a rubbery, stretchable “elastomer” called Ecoflex, manufactured by Smooth-On Inc.
STRETCH AND REPEAT
The researchers have demonstrated that the interconnect is capable of stretching 500 percent of its length.
“This compares to only a few percent for an ordinary metal connection,” says Ziaie, who led the research with doctoral student Rahim Rahimi. “The structures are also highly robust, capable of withstanding thousands of repeated stretch-and-release cycles.”
Since it was not practical to sew the wire directly into the rubbery elastomer because of its squishy nature, the researchers developed a technique to first sew the wire into a sheet of polyethylene terephthalate (PET), the same material used to make transparencies for overheard projectors.
The zigzag wire pattern was stitched to the PET sheets with a water-soluble thread. Then the rubbery stretchable polymer was poured over the sheet, encasing the wire as it solidified. They used warm water to dissolve the thread, and the flexible polymer separated from the PET sheet with the wire embedded in it.
The researchers used their technique to demonstrate a stretchable “inductive strain sensor” for monitoring expansion of an inflatable urinary catheter balloon. Strain gauges measure how much a material stretches or deforms.
Because conventional strain gauges are made of rigid metal film, they can’t measure more than a small percentage of the deformation before breaking, whereas a soft strain gauge could continue stretching with the material.
Researchers will present a paper on the findings at the Solid-State Sensors, Actuators and Microsystems Workshop in Hilton Head Island, South Carolina. The National Science Foundation funded the research.
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