Conducting Polymers: SFU Engineers patent a new kind of silicon rubber that can conduct electricity

Ajit Khosla and Bonnie Gray stretch their new electrically-conducting polymer for flexible microchip applications.



January 14, 2009

It was a cloudy afternoon in November 2007 when Ajit Khosla walked into his PhD thesis supervisor’s office and handed her a five centimeter disk of thin dark rubbery material. “I was sitting there playing with it, stretching it and twisting it while he showed me experimental results—voltage/resistance plots,” says Engineering Science professor Bonnie Gray. The data were remarkable. The material could conduct electricity like some metals but it worked even if you tied it in knots. “I think we can publish this,” said Gray. But Khosla said, “Can we actually patent it?”

Gray, an expert in microfluidics, had never patented anything before. Nor had Khosla, but he remembered how his former MSc supervisor had patented many inventions.

Khosla came to SFU to continue researching lab-on-a-chip systems, a way to do chemistry and biology right on microchips with tiny amounts of liquid. Conventionally, these systems reside on microscopic bits of rigid glass or silicon but Gray’s group had been working on thin flexible polymer substrates made of a kind of silicone rubber. This elasticity is needed for real world applications like wearable electronics and biomedical implants. Conventional metal electrical connections on these stretchy substrates can snap when twisted or bent. Gray had already developed methods for connecting microscopic fluid-carrying tubes, but her tiny electrical connections were breaking. She asked Khosla to come up with something better.

He decided to try adding particles of metal to polymer ingredients in the hope that the combination would conduct electricity. He tested at least 25 different substances, many of which had been published or patented by other researchers. “I even bought graphite at Canadian Tire and tried that,” says Khosla. He experimented with known conducting plastics but they were too fragile. Other polymers could not be patterned easily into small electronic lines. Eventually Khosla discovered that a blend of 35% nano-silver and several polymer chemicals provided a workable combination of conductivity, high flexibility, and micro-mouldability. “The nano-silver particles are so small that the total surface area is huge, which is probably one reason why these materials conduct so well,” says Khosla. He adds that other nanoparticles, including carbon nanotubes, also give good results.

The two SFU researchers received a provisional patent for their material in December 2008. They are excited because the polymer is very easy to mould into microscopic shapes using conventional lithography processes. “Lots of people are working on conductive polymers, including polymers containing nanoparticles, but our material conducts electricity better than any other flexible conducting polymer we have seen published or patented,” says Gray. She adds that the main reason they obtained the patent was to make sure nobody else got it first, preventing them from using the technology. “Although I am excited about the material, for me, as a microfluidics and micromechanical systems researcher, I am even more excited about using it”. She envisions using it for on-going collaborative projects, including contact lenses with embedded microfluidics and microelectronics, and a comfortable bra with flexible sensors to screen for breast cancer.  Another idea is implanted electronically active pressure sensors for studying joints such as the knee. “We’re going to eventually use this for everything,” says Gray.

Khosla will spend two or three more years completing his doctorate, characterizing and improving the quality of the new conducting polymer. “Right now I’m really excited about using silver nano-rods,” says Khosla who believes this might improve conductivity even more.