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Sensing Changes in Medical Electronics

Wed, 02/13/2008 - 11:31am
Industries currently using acoustic wave sensors are finding them to be useful in an assortment of application areas. The medical device community is taking note and looking at this technology for potential monitoring in specific medical devices. This article reviews these sensors and describes the application areas in which they would be useful.

By Kerem Durdag

Generic monolithic crystal filter operating principles

Acoustic wave devices, which are fundamentally based on mature technology, have recently been successfully employed for sensing applications, using unique design and packaging approaches. Current applications include industrial markets such as temperature monitoring, tire pressure monitoring, and oil condition monitoring sensors. The inroads made in these markets indicate that acoustic wave sensors have a distinct potential of addressing medical sensor needs in the future.

Overview of the Wave Sensor

Competitively priced due to mature manufacturing methodologies and inherently rugged due to advanced packaging techniques, acoustic wave sensors are a robust option. They can conduct measurements instantaneously and are sensitive with a wide operating range resulting from material characterization and selection. Complemented by additional functionalities such as a small size, low power requirements, and being passively and wirelessly interrogated (no sensor power source required), the sensors can be applicable in wide-scale and diverse applications.Acoustic wave sensors function by generating an acoustic wave on a piezoelectric material when a bias is applied. As the acoustic wave propagates through or on the surface of the material, any changes to the characteristics of the propagation path affect the velocity and/or amplitude of the wave, which are monitored and correlated to the corresponding physical quantity being measured.

Solid-state two port MCF viscosity sensor

There are many types of acoustic wave sensors which possibly may offer solutions in the medical device market, each with a specific construction and operating mechanism. For example, the two-port monolithic crystal filter (MCF) device combines the best properties of both the BAW (bulk acoustic wave) and SAW (surface acoustic wave) devices (Figure 1). It employs separate input and output transducers in order to allow differential signal measurements, like the SAW structures, but also allows the sensor crystal to be employed as a physical barrier between the electronics and the sensing medium. In doing so, it overcomes packaging challenges that are present in regular quartz crystal microbalance devices.

MCF Device

The MCF device uses input and output IDTs (interdigital transducers; the comb-like pattern of metal on the device that converts the electric field energy to mechanical wave energy and then back to an electric field) to launch and receive the acoustic wave. The continuous exchange of energy between the top and bottom surfaces of the plate allows the signal between the IDTs to be influenced by changes on the opposite surface.Implementing appropriate material selection and signal conditioning together with optimized IDT design, the MCF sensor is a viscosity sensor for embedded real time, in-line fluid monitoring applications (Figure 2) in a screw-on bolt packaging for easy installation. The hermetically packaged chip with an abrasion resistant proprietary hard-coat surface can be fully embedded in contact with fluids with temperatures ranging from -25C to 125C, and is immune to the effects of vibration and flow condition due to the absence of moving parts. The principle of operation is elegant; the fluids' viscosity determines the thickness of the fluid hydro-dynamically coupled to the surface of the sensor. As the acoustic wave penetrates the fluid, viscosity is calculated by measuring the power loss between the input and output transducers. The sensor requires no calibration, can be manufactured reliably, and offers flexibility of integration in industrial end-user instrumentation and control systems.

Further, these characteristics are complemented by another outstanding property of SAW devices which is particularly pertinent to medical devices; namely, their ability to operate with no wire connection or battery. They are connected only by a radio frequency link to a transceiver or reader unit. This is due to the SAW devices operating at very low input signal levels and high electrical efficiency.

A high-frequency electromagnetic wave is emitted from an RF transceiver and is received by the antenna of the SAW sensor that is fabricated onto the sensor surface using standard semiconductor techniques (Figure 3). The antenna is connected to the IDTs leading to the conversion of the received signal into an acoustic wave, which propagates along the sensor similarly to the previous description. Depending on the construction of the device, the IDTs can retransmit to the receiver. The received signal is amplified, converted to a baseband frequency in the RF module, and then analyzed by a signal processor for appropriate translation and display. Because the operating frequencies are in the GHz range, SAW sensors are well protected from electromagnetic interference that often occurs in the vicinity of industrial equipment, such as motors and high-voltage lines.

Representation of a wireless temperature sensor

Temperature Sensors

Commercial SAW temperature sensors are available (Figure 4) as a 433.78 MHz one-port SAW resonator structure specifically designed to have a linear frequency versus temperature characteristic. With a temperature coefficient frequency of 16.2 ppm/C (~7028 Hz/C), it is operable from 0 to 120C. When combined with an antennae and interrogation unit, this SAW sensor chip makes a great solution for numerous wireless temperature sensing applications.Given the small size of these temperature sensors and their capability for wireless operation, they have the potential to be the ideal solution for remote health diagnostics market demand. Using such sensors to remotely transfer body temperature and other vitals to a secure information distribution hub for analysis by physicians is no longer categorized as simply wishful thinking, but rather, a real possibility that would allow more cost-effective and patient empowered monitoring and dialogue.

Commercially available wire temperature sensor

Viscosity Sensors

Though much work remains to be done, the viscosity sensor has the potential of being used in medical applications such as the determination of blood and plasma viscosity (haematological characteristics that affect blood flow). There is research that indicates that changes in blood viscosity, considered a vital health marker, are related to onset of cardiovascular disease. Also, the monitoring of blood coagulation, important to observe during surgical procedures, would provide another tool to the medical community for point-of-care applications. Finally, measurement of blood viscosity after application of rheogesic drugs can show if medication is effective for post-operative analysis. If the analysis time can be decreased from several hours to several minutes, it provides a significant decision making and economic benefit to the community at large.It is important to note that viscosity is but one element of monitoring blood characteristics; the other one is elasticity. Viscosity is related to the energy dissipated during flow primarily due to sliding and deformation of red blood cells and red blood cell aggregates. The elasticity is related to the energy stored during flow due to orientation and deformation of red blood cells and in itself, is an indication of sickling disorders that may also contribute to vascular complications.

Conclusion

The key advantages of such an in-situ health monitoring sensor would be its instantaneous notification for diagnostic action, its ability to analyze very small samples, and its ability to be manufactured in scale for integration to instrumentation, akin to the present glucose meters. Since there would be no moving parts, with presence of the abrasion and scratch proof surface, a robust sensor can be produced for the applications in question.Leading-edge research and development efforts like those described here provide a path of realizing capabilities that can not only provide efficiencies to the current healthcare system, but empower the patient to become proactive in the monitoring of their own well-being.

Kerem Durdag is the director of business development for SenGenuity at Vectron International. In this role, he is responsible for business and sales channel development activities regarding solid-state acoustic wave (fluid, physical, and gas) sensors to a variety of markets and applications. Durdag can be reached at 207-856-6977, x106 or kdurdag@sengenuity.com.
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