By Ed Korczynski, Sr. Technical Editor
Medical and health/wellness monitoring devices provide critical information to improve quality-of-life and/or human life-extension. To meet the anticipated product needs of wearable comfort and relative affordability, sensors and signal-processing circuits generally need to be flexible. The SEMICON West 2016 Flexible Electronics Forum provided two days of excellent presentations by industry experts on these topics, and the second day focused on the medical applications of flexible circuits.
Flexible ultra-thin silicon
While thin-film flexible circuits made with printed thin-film transistors (TFT) have been developed, they are inherently large and slow compared to silicon ICs. Beyond dozens or hundreds of transistors it is far more efficient to use traditional silicon wafer manufacturing technology…if the wafers can be repeatedly thinned down below 50 microns without damage.
Richard Chaney, general manager of American Semiconductor, presented on a “FleX Silicon-on-Polymer” approach that provides a replacement polymer substrate below <1 micron thin silicon to allow for handling and assembly. Processed silicon-on-insulator (SOI) wafers are front-side temporarily bonded to a “handle-wafer”, then back-side grinded to the buried oxide layer, then oxide chemically removed, and then an application-specific polymer is applied to the backside. After removing the FleX wafer from the handle-wafer, the polymer provides physical support for dicing and the rest of assembly.
For the last few years, the company has been doing R&D and limited pilot production by shipping lots of wafers through partner applications labs, but in the second-half of 2015 acquired a new manufacturing facility in Boise, ID. Process tools are being installed, and the first product dice are “FleX-OPA” operational amplifiers. Initial work was supported by the Air Force Research Laboratory (AFRL), but in the last 12-18 months the company has seen a major increase in sample requests and capability discussions from commercial companies.
Printed possibilities
Bob Street of Xerox’s Palo Alto Research Center (PARC) presented on “Printed hybrid arrays for health monitoring.” There are of course fundamentally different sensor needs for different applications, and PARC is working on many thin-film transducers and circuits:
Gas sensing – outer environment or human breath,
Optical sensing – monitoring body signals such as blood oxygen,
Electrochemical sensing – detect specific enzymes, and
Pressure/Accelerometers – extreme physical conditions such as head concussions
“There are many and various ways that you can do health monitoring,” explained Street. “There will be sensors, and local electronics with amplifiers and logic and switches. One of the prime features of printing is that it is a versatile system for depositing different materials.”
PARC has built an amazing printing system for R&D that includes different functional dispense heads for ink-jet, aerosol, and extrusion so that a wide varieties of viscosities can be handled. The system also include integrated UV-cure capability. Printing tends to have the right spatial resolution on the scale of 50-100 microns for the target applications spaces.
PARC worked on an early system to monitor for head concussions and store event information. They used printed PVDF material to print accelerometers and pressure sensors, as well as ferroelectric analog memory. Various commercially available materials are used to print organic thin-film transistors (OTFT) for digital logic. For complementary digital logic, different metals would conventionally be needed for contacts to the n-type and p-type TFTs, but PARC found an additive layer that could be applied to one type such that a single metal could be used for both.
A gas sensor prototype that can can detect 100-1000ppm of carbon-monoxide was printed using carbon nano-tubes (CNT) as load resistors. They printed a 4-stage complementary inverter to provide gain, using 7 different materials. “This is a case where a very simple device uses many layers,” explained Street. “Four drops of one materials does it, so you wouldn’t look at using a subtractive process for this.”
Rigid/flex integration
Dr. Azar Alizadeh, GE Global Rsearch, presented on “Manufacturing of wearable sensors for human health & performance monitoring.” Wearables in healthcare applications include medical, high exertion, occupational, and wellness/fitness. The Figure shows a flexible blood pressure-sensor that measures from a finger-tip. Future flexible devices are expected to provide more nuanced biometric information to enable personalized medicine, but any commercially viable disposable device will have to cost <$10 to drive widespread adoption. Costs must be limited because just in the US alone the annual amount spent to serve ~50M patients in hospitals is >$880B.
Finger-tip optical blood-pressure sensor created with printed photodetector by GE Corp.