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Microscopic Optical Sensor Circuits Demonstrated

OWIC sensors

Optical wireless integrated circuits (Alejandro Cortese, Cornell University)

23 Apr. 2020. Researchers exhibited solar-powered integrated circuit sensors with LED lights, smaller than can be seen with the naked eye, for mass production at low cost. A team of physicists and engineers from Cornell University in Ithaca, New York describe their circuits in the 17 April issue of Proceedings of the National Academy of Sciences (paid subscription required).

Technologies in many fields need progressively smaller sensors to keep track of physical phenomena, as smaller and unobtrusive systems, such as wearable devices become increasingly popular. Not only do sensors need to be smaller than before, they also need to run independently, without physical connections to larger systems or central hubs. The lab of Cornell physicist Paul McEuen studies materials and systems at the smallest of scales, and devised a sensor circuit meeting these requirements measuring 100 microns or millionths of a meter in size.

A team from McEuen’s group, led by postdoctoral researcher Alejandro Cortese, designed their optical wireless integrated circuits, or OWICs, specifically for ultra-small applications, such as medical sensing. In addition, their devices need to be easily fabricated, low in cost, and able to be produced in large quantities. McEuen says in Cornell statement that the team, “pushed it another order of magnitude down in size and made it mass fabricate-able,” and adds, “we constrained ourselves and said we’re not going to do it unless we can make them by the million.”

Just because their OWICs are small does not mean they were simple to design. Cortese, the paper’s first author, and colleagues report needing 100 different steps using 30 different materials to fabricate an OWIC. The circuit is built on a gallium arsenide surface, a semiconductor material that operates at higher frequencies than silicon, and is more resistant to radiation and heat. In addition, gallium arsenide circuits emit light more effectively than silicon. This light-emitting property is important in OWICs, since the circuits both use solar energy for power and are built with integrated light-emitting diodes or LEDs. In the end, OWICs require 15 layers of photolithography, fabricated under microscopes.

Once designed, however, the circuits can be fabricated in mass quantities, each about the size of a single-cell organism, with 30,000 of the devices fitting on a U.S. penny, and each device costing less than one cent to make. In the paper, the Cornell team demonstrates OWICs for measuring voltage, temperature, pressure, and conductivity under a range of conditions. For a practical biomedical application, the researchers worked with neuro-technology engineering professor Chris Xu, to embed a temperature sensor in brain tissue and transmit readings with blinking coded signals from the LEDs.

The researchers envision OWICs as building blocks in a larger technology platform, where thousands of the circuits can be packed into a single device to perform a number of tasks. “And that means,” notes Cortese, “you can increase the range of things the device can sense, how the device communicates out, or it’s ability to complete more complex tasks. We really developed this as a platform so that a lot of people have space to develop new devices, new applications.”

The university filed for a provisional patent on the technology. In addition, Cortese, McEuen, and co-author Alyosha Molnar founded the start-up company OWiC Technologies in Ithaca commercializing the microscopic sensor circuit technology. The company, formed in November 2019, is first-place winner of a virtual campus pitch competition held last week. OWiC Technologies expects its initial applications will be embedded sensors to authenticate products and reduce counterfeiting.

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