IEEE Spectrum on ‘The Internet of Disposable Things’
The December 2018 edition of IEEE Spectrum features a report by Alissa M. Fitzgerald on the rise of the “Internet of Disposable Things” — wireless sensors made of biodegradable paper and plastic. Moving away from silicon and quartz means these sensors can find their way into food products, or even the human body.
Mark G. Allen, scientific director of the Singh Center for Nanotechnology and Alfred Fitler Moore Professor of Electrical and Systems Engineering and Mechanical Engineering and Applied Mechanics, is a pioneer in this field. As an expert in MEMS, or microelectromechanical systems, and the cofounder of a company that implantable heart monitors, he and his research group are figuring out new ways to get these sensors to be safer, less expensive and more flexible.
Fitzgerald, the founder of a MEMS design firm herself, described Allen’s role in this transformation:
You can see the move toward flexibility and biodegradability already. One of the first implantable MEMS pressure sensors to be approved by the U.S. Food and Drug Administration, created by CardioMEMS (now part of St. Jude Medical), was a rigid sensor made of quartz, designed to be implanted adjacent to a stent, in order to monitor the efficacy with which the stent does its job of holding an artery open. The sensor includes an antenna buried within the quartz and a thin membrane of quartz that covers a cavity, thus forming a capacitor. When the blood’s pressure squeezes down on the membrane, it changes the capacitance and therefore the resonant frequency of the sensor’s circuit. The change in frequency can be passively measured by an external reader, which interrogates the sensor with an RF signal of known frequency and then compares it with the altered frequency of the return signal.
The problem with a quartz sensor — and with those made of silicon as well — is that they are relatively rigid and quite brittle. It would be better to use more flexible materials, which can conform to the body’s contours and narrow spaces such as those inside blood vessels. Even better would be biodegradable materials, which could dissolve harmlessly in the body after a sensor has finished its work, avoiding any need for surgical removal.
One of CardioMEMS’s cofounders, the University of Pennsylvania’s Mark Allen, subsequently shifted away from rigid silicon and quartz sensors to making flexible and biodegradable sensors. Graduate students in Allen’s group are now engineering sensors and even batteries from polymers such as poly(lactic co-glycolic) acid (PLGA), polyvinyl alcohol (PVA), and polycaprolactone (PCL), the same materials used in dissolvable surgical sutures. Interconnects and antennas are formed from biodegrading metals such as magnesium or zinc. Using MEMS microfabrication techniques such as photolithography and electroplating, the students have demonstrated biodegradable, millimeter-size pressure sensors with the same wireless data-transmission capability of their quartz ancestors.
Continue reading at IEEE Spectrum.