A biocompatible stretchable material for brain implants and ‘electronic skin’

March 10, 2017

A printed electrode pattern of a new polymer being stretched to several times of its original length (top), and a transparent, highly stretchy “electronic skin” patch (bottom) from the same material, forming an intimate interface with the human skin to potentially measure various biomarkers (credit: Bao Lab)

Stanford chemical engineers have developed a soft, flexible plastic electrode that stretches like rubber but carries electricity like wires — ideal for brain interfaces and other implantable electronics, they report in an open-access March 10 paper in Science Advances.

Developed by Zhenan Bao, a professor of chemical engineering, and his team, the material is still a laboratory prototype, but the team hopes to develop it as part of their long-term focus on creating flexible materials that interface with the human body.

Flexible interface

“One thing about the human brain that a lot of people don’t know is that it changes volume throughout the day,” says postdoctoral research fellow Yue Wang, the first author on the paper. “It swells and de-swells.” The current generation of electronic implants can’t stretch and contract with the brain, making it complicated to maintain a good connection.

Illustration showing incorporation of ionic liquid-assisted stretchability and electrical conductivity (STEC) enhancers to convert conventional PEDOT:PSS film (top) to stretchable film (bottom). (credit: Wang et al., Sci. Adv.)

To create this flexible electrode, the researchers began with a plastic (PEDOT:PSS) with high electrical conductivity and biocompatibility (could be safely brought into contact with the human body), but was brittle. So they added a “STEC” (stretchability and electrical conductivity) molecule similar to the kind of additives used to thicken soups in industrial kitchens.

This additive transformed the plastic’s chunky and brittle molecular structure into a fishnet pattern with holes in the strands to allow the material to stretch and deform. The resulting plastic remained very conductive even when stretched 800 percent its original length.

Scientists at SLAC National Accelerator Laboratory, UCLA, the Materials Science Institute of Barcelona, and Samsung Advanced Institute of Technology were also involved in the research, which was funded by Samsung Electronics and the Air Force Office of Science Research.

Stanford University School of Engineering | Stretchable electrodes pave way for flexible electronics

Abstract of A highly stretchable, transparent, and conductive polymer

Previous breakthroughs in stretchable electronics stem from strain engineering and nanocomposite approaches. Routes toward intrinsically stretchablemolecularmaterials 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 dual functions to changemorphology andas conductivity-enhancingdopants inpoly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The polymer films exhibit conductivities comparable to the best reported values for PEDOT:PSS, with higher than 3100 S/cm under 0% strain and higher than 4100 S/cm under 100% strain—among the highest for reported stretchable conductors. It is highly durable under cyclic loading,with the conductivitymaintained 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 as high as 800%, which is superior to even the best silver nanowire– or carbon nanotube–based stretchable conductor films. The combination of excellent electrical andmechanical 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.