Catheter sensors

Introduction

Sensors inside our body are becoming more and more common. Advances in medical technology and, in particular, semiconductor manufacturing allow for ever smaller MEMS (MicroElectroMechanical Systems) pressure sensors made of silicon. At the same time, the demand from the medical technology sector for in-vivo pressure sensors, which can be used on human beings, is growing. These new, extremely miniaturized pressure sensors are introduced into the body with the help of catheters as part of a minimally invasive procedure and serve for medical diagnosis. They provide digitally available real-time data of intracranial or intraocular pressure, for controlling the aorta or kidney, for optimal positioning of the catheter for atherectomy, or for measuring the intracoronary fractional flow reserve in cases of arterial deposits.

In-vivo pressure measurement with external sensors

For many years, pressure has been measured through thin tubes filled with fluid and external sensors. However, this design has serious disadvantages compared to in-vivo measurement: Due to the large fluid column and the necessary elasticity of the tube, all pressure changes arrive "smoothed out" at the sensor. This is due on the one hand to the inertia of the fluid used as a pressure mediator. On the other hand, small pressure spikes leading to minimal changes in volume of the tube are simply absorbed. All in all, the thin fluid-filled tube acts as a low-pass filter. In the end, the pressure sensor used outside the body on the catheter can only measure the signal that reaches it: Its accuracy can be as good as it is, but potentially relevant pressure spikes are not correctly recorded, and a diagnosis is not possible in many cases. The only advantage is the connection to the environment, as it can directly measure the relative pressure. However, attention must be paid to its position relative to the height of the measurement point, influenced by ambient barometric pressure, to avoid distorting the measurement results.

Optical Pressure Measurement Technology

Compared to other methods, the optical in-vivo pressure measurement using a Fabry-Pérot interferometer is much more accurate. The miniaturization of this measurement principle was made possible only by fiber optic technology and has been available on the market for about 20 years. The structure of these sensors, also known as MicroOptoMechanical Systems (MOMS), is shown below.

Structure of a Fabry-Perot Pressure Sensor

The light emerging from the optical fiber is reflected by the flexible membrane and the barrel-shaped structure mounted firmly at the end. The cavity between them is evacuated. Both parts interfere with each other. If the bending of the membrane changes due to an external pressure change, the phase of the two interfering light waves changes, leading to constructive or destructive interference. Depending on the wavelength used, deformations down to the nanometer range can be reliably detected. A major advantage of MOMS is its immunity to electromagnetic interference, so these pressure sensors can be used in magnetic resonance tomographs (MRTs), in electro surgery, or even during patient stimulation by electrical shocks. Additionally, these sensors are significantly less sensitive to overpressure. Their disadvantage is the always forward-oriented membrane surface and the high cost of the overall system. Moreover, like the following in-vivo MEMS sensors, the absolute pressure can only be determined due to the lack of pressure connection to the environment. A further, but uncomplicated absolute pressure sensor in the evaluation electronics is needed to calculate the patient's air pressure-independent relative pressure.

Miniaturized MEMS Pressure Sensors

As a third alternative, the direct integration of a miniaturized MEMS pressure sensor into the catheter has established itself in recent years. Due to space constraints, these are usually not complete Wheatstone measurement bridges as used in ex-vivo pressure measuring cells, but only half bridges, which are completed by two additional external resistors. The decreased sensitivity can be compensated for by amplification, but this reduces the signal-to-noise ratio.

The MEM2000 has a special position as a full bridge sensor, but it is also significantly larger in its ceramic housing. However, it is also available as a pure silicon die with micro cables as a custom form. The table below provides a non-representative overview of currently available in-vivo measuring sensors on the market (excluding external sensors connected by tubing).

The IntraSense in-vivo sensor is also available with a coating to protect against light for endoscopes, and a pre-calibrated version is in the works. Since all MEMS sensors are sensitive to light, the respective manufacturer must take precautions in the event of intense illumination. Other sensors, like the P330 have a particularly high burst pressure, making them more robust against pressure fluctuations, while the model of the P41 is particularly compact.

The half-bridge sensors differ literally only by a hair's breadth: With a width of the die that is decisive for integration into a catheter of significantly less than 0.5 mm, these sensors are only as wide as a few human hairs. In addition, the silicon wafers and bonded glass substrates have been significantly minimized in thickness through grinding, lapping, and polishing. To process a membrane sensitive to low body pressures on such a minimally lateral surface, the use of the latest semiconductor manufacturing technologies such as Deep Reactive Ion Etching (DRIE) is required.

Conclusion

Due to an aging society and the resulting increase in demand for diagnosis of heart diseases, cataracts and kidney problems, the demand for in-vivo pressure measurement solutions will strongly increase in the future. While for specific applications under strong electromagnetic fields, the expensive optical pressure measurement is unavoidable, the majority of examinations will be performed using the latest generation of ultracompact MEMS pressure sensors.

References

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