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MEMS gear up for medical mission

Posted: 03 Mar 2008 ?? ?Print Version ?Bookmark and Share

Keywords:microelectromechanical-systems? motion sensors? medical applications? accelerometer?

MEMS -based inertial and motion sensors are finding slots in medical applications, with demand fueled by implantable devices that must be highly reliable and very small, as well as by handheld devices for home monitoring and diagnostics.

The global MEMS market reached $7.78 billion in 2006 and is projected to hit $10.91 billion by 2011, with a CAGR of 6.7 percent, according to Bourne Research. In 2006, medical applications accounted for 12.7 percent of total MEMS revenues. In 2011; they're forecast to account for 11.3 percent of revenues. But the numbers don't tell the complete story.

'Last great frontier'
"The market share loss isn't due to declining sales but, rather, to much slower growth in comparison to other markets. There is a tremendous amount of development taking place in the medical sector. This is the last great frontier for MEMS, and its full impact is yet to be felt," said Marlene Bourne, president and principal analyst at Bourne Research.

Myriad medical applications are adopting MEMS inertial sensors, said Christophe Lemaire, customer marketing manager for the micromachined products division of Analog Devices Inc. "Many times, our existing products can be used as they are or tweaked to support some of these applications. For the most part, in the medical field, they are looking for very small size and very low power."

A growing market for MEMS is patient monitoring. "There is a great interest in trying to monitor the activity of people who are at medical risk or who are at risk of falling," said Lemaire. "Motion sensors are a part of this, and many times our existing sensors are well adapted to this kind of monitoring."

Suitable for oxygen concentrators, inhaled anesthetics and ventilators, Omron's D6F MEMS flow sensors can measure flow speed from 1mm/s to 40m/s.

Implantable medical devices such as pacemakers, meanwhile, demand extremely high reliability and a very small package size, for obvious reasons. Endevco Corp. developed its Model 40366 surface-mount accelerometer that is suitable for implantable medical devices such as rate-responsive heart pacemakers and defibrillators.

The Model 40366 is a variable-capacitance silicon MEMS device that is hermetically sealed with a shear bond strength of 1kg. The accelerometer delivers repeatability of less than 0.035pF in an SMT footprint that measures 2.05mm x 2.9mm, and is configured as a three-terminal half-bridge. Acceleration through the oblate axis causes one capacitor to increase in value as the other decreases, while air damping creates an inertial system that is overdamped, Endevco said.

For non-implantable medical devices, Omron Electronics launched a MEMS sensor that measures air/gas flow speed, aimed at oxygen concentrators, inhaled anesthetics and ventilator applications. The D6F series of MEMS flow sensors measures flow speed from 1mm/s to 40m/s. Omron said it is offering the first flow sensor to use thermopile technology, which converts thermal energy into electrical energy to measure air/gas flow speed more accurately and efficiently in medical applications. The thermopile method is said to deliver advantages such as low-cost implementation, low-power consumption, accurate measurement and high sensitivity.

Conventional flow sensors use a resistance measurement method, which tracks the electrical resistance change of a material due to changes in temperature. This method requires a labor-intensive adjustment of the resistance balance. The D6F, which measures 15mm x 20mm x 60mm, operates over a temperature range of -10C to 60C with a supply voltage of 12-24Vdc while consuming 15mA. It features an integral orifice with an analog output of 1-5Vdc.

ADI's two-axis accelerometer enables more accurate measurement by Omron's blood pressure wrist monitor.

Monitoring, diagnostics
Motion sensors are being used in monitoring and diagnostics medical applications. STMicroelectronics introduced ultracompact low-g linear accelerometers in 2007 for health monitoring. ST's LIS244AL accelerometer offers a full-scale output range of 2g. The part's compact, square footprint and low power consumption make it suitable for use in battery-powered portable devices.

It addresses a range of applications sensitive to package size and power consumption, including data-integrity protection, user interfaces, anti-theft systems, remote device control, intelligent motion-based power management, and sports and health-monitoring devices. The accelerometer incorporates a dual-axis MEMS sensor and a CMOS interface chip in a single package that provides two simultaneous, real-time analog outputs: one for side-to-side (lateral) and one for forward-backward (longitudinal) directions.

It's housed in an inexpensive 4mm x 4mm x 1.5mm plastic package and provides shock survivability up to 10,000g, according to ST.

The company also offers the LIS302 three-axis accelerometer family of motion sensors for managing power consumption of products that need to be powered on (or off) when they are touched or moved. In medical applications, the sensors can monitor motion or detect the position or activity level of patients.

Even older product lines such as ADI's ADXL330 low-g accelerometer are finding new purpose in unique medical applications. The ADI accelerometer is being used to measure the depth of compression during CPR in the shock pad used in portable defibrillators or as part of a standalone device, providing real-time audio feedback on the effectiveness of the motion. ADI's ADXL311 two-axis accelerometer is now being used in an Omron blood pressure monitor that fits around a user's wrist. It is said to enable more-accurate blood pressure measurement through cuff positioning sensing.

- Gina Roos

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