A Closer Look at Medical Sensor Solutions

By Carolyn Mathas

Contributed By Electronic Products

With advancing technology and an aging population, it should come as no surprise that the medical sensor market is exploding. A recent research report by MarketsandMarkets, for example, suggests that the total market for healthcare sensors is expected to reach $13.11 billion by 2017.

At the same time, there may be somewhat less awareness of one essential reason underlying this prolific growth: Medical electronics, including sensors, have become more sensitive and accurate. As a result, sensors are playing an important role in such applications as diagnostics, monitoring, drug delivery, and in-home treatment. Sensors are currently used in dialysis machines, insulin pumps, heart-rate-monitoring devices, blood-pressure-monitoring devices, medical imaging equipment, and blood glucose monitors, among many other applications.

MEMS, CCD, and CMOS technology for miniature sensors and micro-fluidic chips have revolutionized this segment by shrinking device size and providing greater accuracy, advanced control and low power consumption.

Various types of sensors can be found in medical devices, including pressure, magnetic, inertial, touch, optical, temperature, voltage, and current sensors. Applications include pressure sensors for blood pressure, respiratory, or kidney dialysis monitoring; accelerometers for pacemakers; silicon microphones, microactuators, and microelectrodes for hearing aids; microelectrodes for cochlear implants; micro-pumps for infusion drug delivery; IR ear thermometers; atomizers for nasal or pulmonary drug delivery; ultrasound sensors for medical imaging; accelerometers for human fall detection, and more. There is also a high demand for implantable biosensors in such medical areas as cardiac care and pharmaceutical delivery. Examples of sensing in implantable applications include detecting heart rhythm irregularities or sensing the amount of glucose in the bloodstream.

The accuracy that sensors provide dramatically adds to the safety, reliability, and intelligence of healthcare equipment and procedures. Especially critical in a variety of monitoring uses, they provide early detection of illness and of quality-of-life challenges.

Let’s now take a look at some specific examples.

The sensing and stimulation sections of a medical IC can be referred to collectively as the analog front end (AFE). In the area of heart-rate monitors, Analog Devices’ AD8232 is a single-lead AFE, an integrated signal-conditioning block for ECG measurement applications. It extracts, amplifies, and filters small biopotential signals when the environment is noisy, such as that created by motion or remote electrode placement. This design allows for an ultralow power analog-to-digital converter (ADC) or an embedded microcontroller to acquire the output signal easily.

As illustrated in Figure 1, the AD8232 consists of a specialized instrumentation amplifier (IA), an operational amplifier (A1), a right-leg drive amplifier (A2), and a mid-supply reference buffer (A3). In addition, the AD8232 includes leads-off detection circuitry and an automatic fast restore circuit that brings back the signal shortly after leads are reconnected.

Analog Devices AD8232 block diagram
Figure 1: Block diagram of the AD8232 heart-rate monitor. 

Features include a fully integrated single-lead ECG front end, a low supply current of 170 μA (typical), a common-mode rejection ratio of 80 dB (dc to 60 Hz), two or three electrode configurations, high signal gain (G = 100) with DC blocking capabilities, and a two-pole adjustable high-pass filter. A fast restore feature improves filter settling. The device has a three-pole adjustable low-pass filter with adjustable gain, a single-supply operation — 2.0 to 3.5 V — and an internal RFI filter.

The AD8232 is available in a 4 × 4 mm 20-lead LFCSP package. Performance of the device is specified from 0° to 70°C and is operational from −40° to +85°C.

In anesthesia delivery systems, airflow sensors are designed to measure the flow of air, oxygen, and nitrous oxide, mixed with an accurate concentration of anesthetic vapor (such as isoflurane) and to ensure that the unit delivers this mixture to the patient at a desired pressure and flow.

Mass airflow sensors such as the AWM90000 series from Honeywell Sensing and Control can play a key role here. The AWM90000 Series gives designers reliability, repeatable flow sensing and the ability to customize the sensor functions to meet their specific application. The AWM90000 micro bridge mass airflow sensors are available in two versions, Mass Flow and Differential Pressure.

Honeywell’s AWM 92100V (Figure 2), for example, has a flow range of ±200 sccm (standard cubic centimeters per minute) with a pressure drop of only 0.49 mBar, typically, while the AWM 92200V is a differential pressure version that has a range of ±2 in. H2O. The series sensors feature 1 ms response time, operate with a supply voltage from 8.0 to 15.0 Vdc, and consume only 50 mW of power. The compact plastic package will withstand a maximum overpressure of 25 psi without compromising performance.

AWM92100V mass airflow sensor from Honeywell Sensing and Control
Figure 2: The AWM92100V mass airflow sensor from Honeywell Sensing and Control. 

The sensors provide bidirectional sensing capability, a highly stable null and full-scale, an extremely low pressure drop, very low hysteresis and repeatability errors (less than 0.35 percent of reading), combined with fast response and low power consumption.

The silicon chip design is created from a thin-film, thermally isolated bridge structure, containing both heater and temperature sensing elements. This provides rapid response to the air or gas flow amount and direction, delivering a proportional output voltage. Amplified versions provide an enhanced output signal and less external circuit options. A variety of port styles provides greater application flexibility.

Disposable devices based on sensors are a rapidly growing medical device segment, which is forecast to grow to $6.2 billion by 2018. Made up of biosensors, image sensors, and accelerometers, they are used mostly in the form of strips — which held a 49.6 percent share of disposables in 2013. There are several types of disposable sensors where the sensor is located externally from the body although body fluids come in contact with it. One example of this is the disposable blood pressure sensor (DPS). These sensors are used in surgical procedures and ICUs (intensive-care units) to continuously monitor the blood pressure of the patient. The informational profile of the patient is then logged in by plugging the disposable blood pressure sensor into a monitor. These sensors are required to be replaced every 24 hours in order to avoid contamination.

Aimed at the disposables market, Freescale Semiconductor has developed a high-volume, miniature pressure sensor package as a sub-module component or disposable unit. The chip carrier package uses Freescale’s unique sensor die with piezoresistive technology, combined with on-chip thin film temperature compensation and calibration. The MPX2300DTI Pressure Sense NED 4SIP Chip Pak (Figure 3) features integrated temperature compensation and calibration, is ratiometric to supply voltage, is constructed of a white, medical-grade polysulfone case material (ISO 10993), that Freescale reports has passed extensive biological testing and is delivered in easy-to-use tape and reel.

Freescale MPX2300
Figure 3: The MPX2300, 2300DTI Pressure Sense NED 4SIP Chip Pak from Freescale. 

The Freescale Pressure Sensors have been designed for medical usage by combining the performance of the company’s shear stress pressure sensor design and the use of biomedically approved materials to provide a sensor that can be used in applications such as invasive blood pressure monitoring. It can be sterilized using ethylene oxide. The portions of the pressure sensor that are required to be biomedically approved are the rigid housing and the gel coating.

A silicone dielectric gel covers the silicon piezoresistive sensing element. The gel is said to be a nontoxic, nonallergenic elastomer system which meets all USP XX Biological Testing Class V requirements. The properties of the gel allow it to transmit pressure uniformly to the diaphragm surface, while isolating the internal electrical connections from the corrosive effects of fluids such as saline solution. According to Freescale, the gel provides electrical isolation sufficient to withstand defibrillation testing as specified in the Association for the Advancement of Medical Instrumentation (AAMI) Standard for blood pressure transducers. A biomedically approved opaque filler in the gel prevents bright operating room lights from affecting the performance of the sensor.

The device is used for medical diagnostics, infusion pumps, blood pressure monitors, pressure catheter applications, and patient monitoring.

While this article has presented just a few examples, it should be noted that sensors targeting health care are an important aspect of the dual trends toward (1) remote monitoring of patients and (2) emphasizing, where possible, at-home vs. hospital care. Highly accurate, reliable, and cost effective, the number of applications for health-related sensors continues to grow, as do revenues in the segment.

For more information on the parts discussed in this article, use the links provided to access product information pages on the Digi-Key website.

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