Engineers at Stanford University have used molecules similar to those used to thicken soup in industrial kitchens to chemically alter brittle plastics and make them stretchable like rubber bands and slightly enhancing their electrical conductivity, according to a report on the school website.
A printed electrode pattern of the new polymer being stretched to several times of its original length (top),
and a transparent, highly stretchy “electronic skin” patch forming an intimate interface with the human skin
to potentially measure various biomarkers (bottom). (Image credit: Bao Lab)
This breakthrough comes after more than a decade of research into flexible electronics by chemical engineer Zhenan Bao, who has been concerned with making electronics that are soft and flexible enough to be comfortable and effective for medical purposes. For instance, making an electronic implant that could stretch and contract as the brain swells and deswells during the day.
The researchers started with a plastic that was electrically conductive and biocompatible, but that was inflexible. A five-percent stretch of the plastic would cause it to break.
“The plastic in this experiment was actually made up of two different polymers that were tightly wound together,” the article explained. “One was the electrical conductor. The other polymer was essential to the process of making the plastic. When these two polymers combined they created a plastic that was like a string of brittle, sphere-like structures. It was conductive, but not flexible.”
Researchers hoped that a molecular additive could separate the polymers and prevent crystallization without affecting conductivity. They tried more than 20 additives before finding a molecule that changed the plastic’s structure into a fishnet pattern with holes that allowed the material to stretch.
It was also determined that when stretched to double its original length the plastic was slightly more conductive. It was conductive when stretched to as much as eight times its original length.
The work was recently published in Science Advances. The abstract read:
“Previous breakthroughs in stretchable electronics stem from strain engineering and nanocomposite approaches. Routes toward intrinsically stretchable molecular materials remain scarce but, if successful, will enable simpler fabrication processes, such as direct printing and coating, mechanically robust devices, and more intimate contact with objects.
“We report a highly stretchable conducting polymer, realized with a range of enhancers that serve a dual function: (i) they change morphology and (ii) they act as conductivity-enhancing dopants in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The polymer films exhibit conductivities comparable to the best reported values for PEDOT:PSS, with over 3100 S/cm under 0% strain and over 4100 S/cm under 100% strain—among the highest for reported stretchable conductors. It is highly durable under cyclic loading, with the conductivity maintained at 3600 S/cm even after 1000 cycles to 100% strain.
“The conductivity remained above 100 S/cm under 600% strain, with a fracture strain of 800%, which is superior to even the best silver nanowire– or carbon nanotube–based stretchable conductor films. The combination of excellent electrical and mechanical properties allowed it to serve as interconnects for field-effect transistor arrays with a device density that is five times higher than typical lithographically patterned wavy interconnects.”
See the new electrode stretched in the video below: