Medical sensors are a particular challenge to developers because human physiology more often calls for measuring small values and doing so outside the body. Small size and low power consumption are other critical characteristics. In this vein we examine three notable and recent medical sensors: One for media-isolated pressure, one for mass airflow, and one for blood oxygen levels.
A media isolated pressure sensor comes from All Sensors Corporation. The first, SPM 401Series, is said to provide excellent performance in various applications, especially for low-pressure and smaller solutions. These media-isolated sensors are said to operate in hostile environments and yet, like a silicon sensor, have excellent sensitivity, linearity, and hysteresis. The pressure sensor is compatible with 316L stainless steel for increased corrosion resistance and improved resistance to pitting from chloride ion solutions. The steel also provides increased strength at high temperatures. A piezo-resistive sensor chip, housed in a fluid-filled cylindrical cavity, is isolated from the measured media by a stainless steel diaphragm and body.
A mass airflow sensor, from Honeywell, works on the transfer of heat. Airflow is directed across the surface of the sensing elements. Output voltage varies in proportion to the mass air or other gas flowing through the inlet and outlet ports of the sensor package. Dual Wheatstone bridges control airflow measurement – one provides closed-loop heater control, the other contains the dual sensing elements. The heater circuit minimizes shift due to ambient temperature changes by providing an output proportional to mass flow. The circuit keeps the heater temperature at a constant differential (160°C) above ambient air temperature, which is sensed by a heat-sunk resistor on the chip. The voltage output of the device corresponds to the differential voltage across the Wheatstone bridge circuit. Adequate air or gas filtering in most applications is possible with a disposable five-micron filter in series on the upstream side of the airflow device.
Advanced skin-surface medical sensors are close at hand, says GE researcher Anil Duggal. The slim, wireless devices, which the company is developing with the support of the Nano-Bio Manufacturing Consortium and the U.S. Air Force Research Laboratory, stick to the wrist like Band-Aids. The sensors analyze sweat, check vital signs, and even keep track of patients’ medical progress after treatment. “This will improve patient experience and get doctors better data about patients,” Duggal says, noting that the sensors will also be able to track heart rate, blood pressure and blood-oxygen saturation levels – and could potentially allow for wireless ECGs, allowing a doctor check heart activity while the patient is at work or play.
To power the sensors, Duggal and team resurrected organic light-emitting diodes. Once the next big thing in lighting, OLEDs glow when electricity flows through specialized organic polymers. They could be embedded in printed rolls of flexible sheets. The sensors are being tested in clinical trials to monitor hydration levels of people during intense exercise. The research team is working to expand this testing to measure stress as well.
