Medical Device Link.

DESIGN

Old problems demand new solutions

In Germany alone, more than 10 million people suffer from hypertonia. Ten per cent of those are candidates for long term surveillance. Catheters are currently available for short term blood pressure monitoring during surgery and there are extracorporeal systems such as blood pressure cuffs for long term monitoring with individual measurements. In these cases, the catheters are restricted to clinical evaluation use because of the required permanent access to the body, and the extracorporeal systems hinder patients in daily life and do not allow continuous measurements.

The wireless pressure monitoring system is aimed at overcoming these limitations by providing up to 30 measurements per second of temperature and blood pressure with a fully telemetrically controlled implant that works without a battery. Additional precise information is gained on transient heart rhythm and possible anomalies such as syncopes. Furthermore, the wireless transmission of transient pressure conditions with a miniaturised system offers new diagnostics and therapies in fields such as cranial and gastrointestinal pressure monitoring.

Technological background

Figure 1: Capacitive CMOS pressure sensor with integrated ASIC circuitry.

Experts from companies and institutions in the fields of microsensors, medical encapsulation and catheter technology have worked together to develop a fully implantable sensor unit (see end for full list of partners).1 The heart of the system is a capacitive complementary metal oxide semiconductor (CMOS) sensor with application specific integrated circuitry (ASIC) (Figure 1). To ease a mechanically robust and thus reliable assembly, the sensor chip is inserted into a microinjection moulded polymer carrier. To meet the challenging demands of miniaturisation, a highly precise geometry without distortion (wall thicknesses of less than 100 µm over a length of up to 9 mm) is indispensable to allow a reliable assembly (Figure 2). The sensor and sensor carrier are inserted into a laser cut tube of stainless steel with an outer diameter of 1.05 mm, a wall thickness of 0.05 mm and length of 16 mm.

Figure 2: Microinjection moulded polymer carrier with wall thickness of less than 100 µm and a length of up to 9 mm.

Encapsulation of pressure sensors with soft polymers such as silicones is most common. When applying the sensors in a wet environment, a major problem is drift of the sensor because of water uptake of the polymer. Currently available systems try to overcome this drawback by appropriate preconditioning and offset compensation of the sensor. In the case of the telemetrically controlled sensor, extensive work and expertise were invested to meet these challenges and to achieve long term stable sensor signals.

Figure 3 shows the encapsulated sensor tip. This sensor tip is connected via a cable to a telemetric unit, which enables transmission of energy and data through inductive coupling. Figure 4 illustrates the entire implant. The shape is designed to minimise clotting and disturbance of the blood flow profile. An extracorporeal wearable telemetric reader unit allows for the storage and evaluation of the collected data. At a signal rate of 30 Hz, the achieved accuracy in the measurement range of 900–1400 mbar (675 –1050 mmHg) within a temperature range from +30 °C to +45 °C is ±3.3 mbar (±2.5 mmHg).

In vivo evaluation

Figure 3: The encapsulated sensor tip.

For device evaluation, acute and chronic settings in sheep are being performed to examine medical and technical aspects. From the medical point of view, safe access of the sensor tip into the femoral artery, sealing of the vessel and the subcutaneous fixation of the transponder unit are currently under evaluation. For this purpose a collagen based insertion instrument has been developed and tested. From the technical perspective, the behaviour of the sensor assembly in the wet and corrosive environment inside the body has been the focus of intensive testing. Furthermore, dependable transmission of energy and measurement data through the skin is crucial for the reliable function of the implant.

Figure 4 : Microinjection moulded polymer carrier with wall thickness of less than 100 µm and a length of up to 9 mm. (click image to enlarge)

Figure 5 shows a transient pressure curve, recorded in a vascular model with a distance of up to 15 cm between the encapsulated transponder unit and the extracorporeal reader unit. This distance is realistic assuming a subcutaneous placement of the transponder unit and attachment of the transmission coil of the wearable reader unit to the body of a patient. In the course of this experiment, the system successfully and reliably recorded pressures from 5–110 mmHg. First experiments in sheep were performed by inserting the sensor tip into the femoral artery. Results of a series of short term and long term in vivo tests will be published soon.

Other applications

Figure 5 : The transient pressure curve in a vascular model. (click image to enlarge)

Blood pressure monitoring is only one of a number of possible applications of an implantable pressure sensor unit. The reiterative recording of the blood pressure at 30 Hz allows conclusions to be made on the transient blood ejection rate of the heart; so far these have not been attainable with the wearable systems currently available for long term recording of blood pressure.

The achieved degree of miniaturisation has been possible because of the wireless transmission of energy and thus the ability of the implant to work without a battery. Digital data transfer allows wireless transmission and recording of a variety of body pressures without the interference and noise known with analogous data transmission.

Other embodiments of the sensor tips of even smaller dimensions and increased length of cable are under development for implantation in deeper and smaller anatomical structures. Because of the galvanic isolation of the sensor from any electrical source, the sensor tip described in this article represents an interesting alternative to catheters and endoscopes.

Acknowledgements

The presented results were achieved in the course of the HYPER-IMS project, coordinated by Dr Osypka GmbH Medizintechnik (www.osypka.de) and financially supported by the German Federal Ministry of Education and Research (BMBF, grant number 16SV2133). Partners in the consortium are MHM Harzbecher Medizintechnik GmbH (www.mhm-harzbecher.de); AME, RWTH Aachen, Germany (www.ame.hia.rwth-aachen.de); IWE1, RWTH Aachen, Germany (www.iwe1.rwth-aachen.de); Universitätsklinikum Heidelberg, Germany (www.klinikum.uni-heidelberg.de); Fraunhofer IMS, Duisburg, Germany (www.ims.fraunhofer.de); Bytec Medizintechnik GmbH, Stolberg, Germany (www.bytec-gmbh.de).