Capacitive Pressure Sensors for Medical Applications

This content is excerpted from Sensor Technology Alert and Newsletter, a sensor intelligence service published by the Technical Insights unit of Frost & Sullivan.

Capacitive pressure sensors and switch fabrication using strained compensated silicon-germanium-boron (SiGeB) have been developed by a team of researchers led by Stavros Chatzandroulis at the Institute of Microelectronics, NCSR 'Demokritos', Greece. Researchers from the Department of Applied Sciences, National Technical University of Athens have also collaborated with Chatzandroulis in their quest to produce capacitive pressure sensors that exhibit good yields and high performance.

The capacitive pressure sensors and pressure switches were fabricated using a 2.4 micrometer thick strain compensated heavily boron doped SiGeB diaphragm. To seal the pressure sensor cavity and construct the device, the process relied heavily on the silicon fusion bonding between the two silicon wafers. Both rectangular and circular type pressure sensors and pressure switches were successfully fabricated using this process.

Talking to Sensor Technology Chatzandroulis emphasized, "Our capacitive sensors are particularly suitable for the medical field. In this field, miniaturization of both sensor and packaging size as well as minimization of the power requirements of the sensor are the major drivers. For example, blood pressure measuring catheters need to penetrate into small blood vessels putting stringent requirements on sensor size, which should be half the size of the vessel to be accessed. In small vessels, catheters usually have a diameter of [two millimeters]. Our capacitive sensors are well within these size requirements."

Drivers for the acceptance of this technology are applications such as abdominal aortas aneurysm and congestive heart failure. These are the major cardiovascular diseases affecting people, and an implantable pressure monitoring system, which would tailor treatment medication by measuring blood pressure, is a very attractive prospective. Implantable applications will require a miniature battery-less telemetric sensor, which will be able to communicate with an external handheld unit. In such applications, power consumption becomes an additional issue and these capacitive devices offer a clear advantage over piezoresistive ones. Other potential applications include intraocular pressure monitoring for the treatment of glaucoma (a serious disease characterized by an increased pressure in the eye that may result in blindness), intracranial pressure monitoring for the treatment of patients with head trauma, and monitoring of neurological patients.

The capacitive sensors developed in this research exhibit good yields and performance. They can be fabricated in extremely small sizes (currently 0.8 mm × 1.1 mm including a reference capacitance) while further miniaturization is easily achievable and is on the cards. The results of this capacitive type sensor operating in the medical pressure regime (0 to 300 mmHg) with a sensitivity to pressure of 1.5 fF/mmHg (10-15 or femto-farad/million of Hg) or 305 ppm/mmHg, and of a pressure switch operating in the 3 bar to 8 bar pressure range prove their versatility. They are thus suitable for a number of industrial and automotive applications.

Commercial applications include the potential applications mentioned above. A special interest exists in using the sensors in implantable pressure sensor monitors where their miniature size and inherent low power consumption give them additional advantages. Apart from the medical field, potential commercial applications of the sensors include remote wireless sensors where power minimization is of paramount importance. One such application is a tire pressure monitoring system for the monitoring of pressure in vehicle tires. Such systems will become mandatory in US light vehicles, with the phase-in of the TREAD (Transport Recall Enhancement, Accountability and Documentation) act during October 2005 to September 1, 2007.(Vehicles produced by final-stage manufacturers and alterers are to be equipped with a compliant TPMS (including a malfunction indicator) by September 1, 2008.)

The researchers are on a constant look-out for potential partners, which will help bring the technology to market. Partners are sought for in the medical and automotive field who will be willing to adopt the sensors into their products. A keen search is also on for potential funding partners with an interest in financing a spin-off to exploit the various areas of this technology.