Ultra-minaturized Pressure Sensors for Intravascular Blood Pressure Measurements

Researches Edvard Källvesten Patrik Melvås Jessica Melin Thomas Frisk Göran Stemme Sponsors Radi Medical Systems VINNOVA

In a joint research project between S3/KTH and RADI Medical Systems, very small piezoresistive surface micromachined silicon pressure sensors have been designed, fabricated and commercialized for use in catheter based medical equipment for intravascular pressure measurements.

Figure 1: The ultra-miniaturaized pressure sensor chip (0.1 mm x 0.14 mm x 1.3 mm) on a "large" pressure sensor chip.

A new project was started in 1999 to investigate other sensing principles for catheter based measurements. One of the new sensors is based on an ultraminiaturized resonating beam located inside a reference vacuum cavity beneath the pressure sensitive diaphragm . The resonator gives a frequency output signal which is more immune to noise than traditional piezoresistive sensors and also has a higher pressure sensitivity.

Figure 2: A schematic drawing of the new encapsulated resonant beam pressure sensor.

Figure 3: SEM photograph showing the fabricated structure where the top diapgragm is shaped by the force transducing beam and the piezoresistor.

The same design is also used for non-resonant sensors where the diaphragm acts as both a pressure sensitive element and a lid which protects the piezoresistors. In these new designs the sensing beam acts as a lever arm to increase the sensitivity. Other sensor designs based on the resonance principle are also currently under development.

Figure 4: SEM photograph showing the fabricated pressure sensor with a leverage beam attached to the underside of the diaphragm.

In other designs the sensors diaphragm acts as both a pressure sensitive element and as a lid that protects the piezoresistors. In these new designs the sensing beam also acts as a lever arm to increase the sensitivity. To further increase the sensitivity a new H-shaped beam design was developed were the piezoresistor could be incorporated in the beam itself. A dual-beam pressure sensor, which improves the temperature compensation, has been tested. Figure 5: A schematic drawing of the encapsulated dual-beam strain-gauge pressure sensor. To decrease the over all production cost a new design was introduced where the number of wires to the pressure sensor were decreased from three to only two by introducing diodes in series with the sensing piezoresistors. By using the forward and reverse mode of operation of the diodes the temperature and pressure signals can be transmitted on the same pair of wires. Figure 6: A schematic drawing of the guide-wire with a pressure sensor that incorporates diodes to decrease the number of connecting wires.

Publications Melvås, P, et al. "Miniaturized pressure sensor using a free hanging strain-gauge with leverage effect for increased sensitivity", presented at Transducers'01 Melvås, P, et al. "Media protected surface micromachined leverage beam pressure sensor ", to be published in J. Micromech. Microeng, vol 11, 2001 Melvås, P, et al. "A surface micromachined resonant beam pressure sensor", presented at the IEEE 14th InternationalWorkshop on Micro Electro Mechanical Systems (MEMS'01) in Interlaken, Switzerland, January  2001. Kälvesten, E., et al. "The First Surface Micromachined Pressure Sensor for Cardiovascular Pressure Measurements." presented at the IEEE 11th International Workshop on Micro Electro Mechanical Systems (MEMS'98) in Heidelberg, Germany, January 25-29 1998