WO2008139347A1

WO2008139347A1 - Sensor probe for measuring a physical property inside a bodily lumen

Info

Publication number: WO2008139347A1
Application number: PCT/IB2008/051637
Authority: WO
Inventors: Ronald Dekker; Jan F. Suijver; Jacob R. Haartsen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-05-09
Filing date: 2008-04-28
Publication date: 2008-11-20
Also published as: JP2010525913A

Abstract

A sensor probe (100) for measuring a physical propertyinside a bodily lumen. The sensor probe (100) comprises a catheter (1) having a hollow distalend (2) and a flexible monolithic integrated circuit foil (3), which comprises a sensor, wherein the flexible foil (3) is provided in and is extendable from the hollow distal end (2) ofthe catheter (1). This enables the measurement of, for example, a blood flow5 and/or pressureinside an artery as well as in a small and/ordif f icult to reach locations, such as an aneurysm (12).

Description

SENSOR PROBE FOR MEASURING A PHYSICAL PROPERTY INSIDE A BODILY LUMEN

FIELD OF THE INVENTION

The invention relates to a sensor probe for measuring a physical property inside a bodily lumen.

BACKGROUND OF THE INVENTION

Measuring a physical property inside a bodily lumen is beneficial in several applications. For example, measurement of a blood flow and pressure inside a coronary artery gives information on the cardiac output of a patient which is essential in managing critically ill patients with cardiac failure. In another example assessment of the blood flow results in important information on the formation and growth of aneurysms. Aneurysms are caused by weakening of the arterial wall that finally results in a bulge formation in different appearance shapes. When the wall of such an aneurysm weakens it can rupture, leading to hemorrhaging. However, a large fraction of the population has asymptomatic aneurysms, whereby the aneurysm has stabilized resulting in a low risk for the patient. In such cases, treatment of the aneurysm is not advised, as the treatment itself significantly increases the probability for risk to the patient, such as for example a stroke when the aneurysm is located in the brain. After an aneurysm formation has taken place, blood flow pattern within the aneurysm pouch is of a crucial importance to predict growth pattern and rupture occurrence. Assessment of several physical properties as predictors of the aneurysm formation and growth as well as the dynamic assessment of the physical properties inside the aneurysm pouch is crucial in order to understand and predict aneurysm behavior and to determine whether or not intervention is required. For this purpose, as an example, the blood flow velocity, the blood pressure, the oxygen level in the blood and the temperature inside the artery as well as in the aneurysm may be determined. From the comparison of these values, as well as their fluctuations over time, an assessment of the risk involved with the aneurysm can be made.

A blood flow sensor is known from US 5,373,850 which discloses a flow- velocity sensor probe comprising a thermistor for generating heat in a fluid and for detecting the temperature in the fluid. The flow- velocity sensor is fitted into a catheter by winding an electrically conducting wire around the catheter several tens to several hundreds of times. The thermistor for detecting the temperature is disposed so as to contact the wire as much as possible. The outer side of the wire is covered by a metal ring used as a means for radiating thermally conductive heat. In order that the catheter tube surface and the outer surface of the metal ring will be flush, the outer diameter of the catheter tube is reduced beforehand by the thickness of the wire and metal ring. Passing an electric current through the wire causes the wire to produce heat, which is transmitted to the outside via the metal ring. The extent of transmission depends upon the flow velocity of the exterior fluid. The flow velocity can be measured by measuring the temperature of the wire at such time with the thermistor. The known sensor has such a shape and size that it is not suitable for measuring the flow within small and difficult to reach locations inside the bodily lumen, such as, for example, an aneurysm pouch, which can have a diameter substantially below lcm.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a sensor probe for measuring a physical property inside an artery as well as in small and difficult to reach locations, such as an aneurysm. The invention is defined by the independent claims. Advantageous embodiments are defined by the dependent claims. This object is achieved by the sensor probe according to the invention for measuring a physical property inside a bodily lumen, which is characterized in that the sensor probe comprises a catheter having a hollow distal end and a flexible monolithic integrated circuit foil comprising a sensor, wherein the flexible monolithic integrated circuit foil is provided in and is extendable from the hollow distal end of the catheter. The flexible foil is provided inside the hollow end of the catheter and can be extended or retracted from that hollow distal end thereby exiting from the catheter. When the catheter is inside a part of the bodily lumen where the physical property needs to be measured, the sensor on the extended flexible foil enables the determination of that physical property. The flexibility of the foil enables a measurement of a physical property inside a location that is difficult to reach with the catheter by extending the flexible foil from the catheter and inserting or guiding it into a difficult to reach location, such as for example an aneurysm pouch in which case the catheter has to go around a sharp corner in order to reach that location.. The flexible monolithic integrated circuit foil allows for a miniaturization of the size of the foil and the sensor that is integrated on the flexible foil. This enables the measurement of physical properties inside the bodily lumen in locations that are too small for the catheter. Another advantage of the miniaturization of the measuring part of the sensor probe is a reduction of the influence of the sensor probe on the measured physical property. For example, in the case that a blood flow is measured, the blood flow will be influenced by the sensor probe. By applying the retracted flexible foil, having a smaller size than the catheter, the influence of the sensor probe on the blood flow will be reduced.

In an embodiment of the sensor probe according to the invention, the sensor comprises a heater and a temperature sensor for measuring a flow of a fluid inside the bodily lumen. When the sensor probe is, for example, inside an artery, the extended flexible foil will be in contact with blood inside the artery thereby enabling the heater and the temperature sensor of the flexible foil to determine the blood flow inside the artery. The flexibility of the foil allows for the measurement of the blood flow in difficult to reach locations inside the bodily lumen. Therefore, it is also possible to measure the flow inside an aneurysm pouch, which is difficult to reach for the catheter, by extending the flexible foil from the catheter into the aneurysm pouch. This enables the measurement of the blood flow in locations that are too small and/or difficult to reach for the catheter, such as an aneurysm pouch.

In a further embodiment of the sensor probe according to the invention, the flexible foil comprises a plurality of heaters and temperature sensors arranged adjacent to each other. This enables the simultaneous measurement of the fluid flow inside the bodily lumen at a number of locations, for example inside and outside of an aneurysm pouch.

In an embodiment of the sensor probe according to the invention, the temperature sensor comprises electrically connected parallel stripes forming a serpentine pattern. Preferably, the electrically connected parallel stripes comprise alternating n-type and p-type silicon. This results in a more sensitive temperature measurement, because silicon exhibits a relatively high Seebeck coefficient in the order of 200 micro V/K. By combining p- type silicon, which has a positive Seebeck coefficient, and n-type silicon, which has a negative Seebeck coefficient, a total Seebeck coefficient of approximately 400 micro V/K is obtained.

In an embodiment of the sensor probe according to the invention, the sensor comprises a pressure sensor. This enables the determination of the pressure, for example the blood pressure, inside the bodily lumen in difficult to reach locations. When the sensor also comprises the heater and the temperature sensor a more accurate characterization of the condition inside the bodily lumen is provided for. For example, an improved assessment of the risk involved with an aneurysm can be made by the determination of the pressure and flow of the blood inside the aneurysm pouch.

In an embodiment of the sensor probe according to the invention, the flexible foil further comprises an antenna. This advantageously provides for a wireless operation mode of the sensor probe, and/or for an efficient energy transfer to the flexible foil.

In an embodiment of the sensor probe according to the invention, the flexible foil has a first shape when it is inside the hollow distal end of the catheter and a second shape, which is different from the first shape, when it is outside the catheter. In this way the second shape of the flexible foil can be such that the flexible foil can be steered in a simpler way to a difficult to reach location inside the bodily lumen, while the first shape allows for the flexible foil to be inside the hollow distal end of the catheter, thereby allowing for the sensor probe to be directed smoothly to a required location while protecting the flexible foil and without the flexible foil hampering the movement of the sensor probe. For example, a physician is then able to actively steer the sensor probe to a location near the aneurysm and, after retracting the flexible foil from the hollow distal end of the catheter, subsequently actively steer the flexible foil inside of the aneurysm. In a further embodiment the flexible foil further comprises a shape memory alloy which provides for the second shape of the flexible foil after heating of the shape memory alloy. In this way the second shape is a specified shape of the flexible foil that is memorized by the shape memory alloy. This memorized, second shape can be induced by heating of the shape memory alloy when the flexible foil is outside the catheter. Preferably, the flexible foil further comprises a further heater for heating of the shape memory alloy. In this way the curvature or shape transformation of the shape memory alloy from the first shape to the memorized, second shape can be induced by a local heating of the flexible foil with the further heater. In an embodiment of the sensor probe according to the invention, the flexible foil further comprises a strain gauge for detecting of a stress in the flexible foil. The stress of the flexible foil obtained by the strain gauges gives information on the curvature and shape of the flexible foil. This is, for example, of aid to a physician in carefully positioning the flexible foil of the sensor probe to the inside of the aneurysm. In an embodiment of the sensor probe according to the invention, the flexible foil further comprises signal processing circuitry. In this way a further miniaturization of the sensor probe is achieved by integration of additional functionality on the flexible foil. For example, the signal processing may comprise amplification, AD conversion and/or data multiplexing. The object is also achieved with a medical apparatus comprising the sensor probe according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention will be further elucidated and described with reference to the drawings, in which:

Figs. Ia and Ib are a schematic cross-sectional view of an end part of a sensor probe inside an artery according to an embodiment of the invention;

Figs. 2a and 2b are schematic top views of an end part of a flexible foil which is comprised in a sensor probe according to an embodiment of the invention;

Fig. 3 is a schematic top view of a heater, a temperature sensor and electronic circuitry integrated on a flexible foil which is comprised in a sensor probe according to an embodiment of the invention;

Fig. 4a is a schematic cross-sectional view, showing the side of the flexible foil, of an end part of an embodiment of a sensor probe according to the invention;

Fig. 4b is a schematic cross-sectional view, rotated 90 degrees around the main axis of the catheter with respect to Fig. 4a and showing a top view of the flexible foil, of an end part of an embodiment of a sensor probe according to the invention; and

Fig. 4c is a schematic cross-sectional view, rotated 90 degrees around the main axis of the catheter with respect to Fig. 4a and showing a top view of the flexible foil in an extended position, of an end part of an embodiment of a sensor probe according to the invention.

The Figures are not drawn to scale. In general, identical components are denoted by the same reference numerals in the Figures.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. Ia shows a schematic cross-sectional view of an embodiment of an end part of a sensor probe 100 according to the invention for measuring a physical property, in this case a blood flow, inside a bodily lumen. A catheter 1 is positioned inside an artery 11 near an aneurysm 12. The catheter 1 comprises at its distal end a hollow part 2, in which a flexible foil 3 is located that has a first shape such that the flexible foil 3 fits in the hollow part 2. The flexible foil 3 comprises circuitry and a sensor for measuring, for example, the blood flow. The catheter 1 is used to position the flexible foil 3 into the body, and to position it in the proper vein. Additionally the catheter 1 is provided with wires for the electrical connections to the flexible foil 3. Fig. Ib is schematic cross-sectional view of the sensor probe 100 as shown in Fig. Ia, but now the flexible foil 3 is partially extending outside the hollow part 2 of the catheter 1. After retraction of the flexible foil 3 from the hollow part 2 the flexible foil 3 has, in this example, a second, bent shape that enables the flexible foil 3 to enter the aneurysm 12. In this way, for example, the blood flow inside the relatively small and difficult to reach aneurysm pouch 12 can be measured.

The flexible foil 3 is a monolithic integrated circuit foil, for example such as is disclosed in US 6,762,510 wherein the flexible foil 3 is manufactured with an IC (Integrated Circuit) process and subsequently transferred to a flexible carrier of, for example, polyimide. The IC process can be advantageously used to integrate different devices and circuitry on the flexible foil 3.

Fig. 2a schematically shows a top view of a part of the flexible foil 3 and Fig. 2b shows an enlargement of an end part of the flexible foil 3. The flexible foil 3 is in this case a small stripe with a width in the order of 300 micrometers and a length of several millimeters. On this stripe a linear array of small flow meters is integrated. This makes it possible to measure the flow at a number of points inside and outside of the aneurysm pouch 12 simultaneously. Each flow meter consists of a temperature sensor or thermopile 5 and a heater 4. A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of thermocouples that are, in this example, connected in series. Using a temperature (difference) sensor 5 that is based on thermocouples has the advantage that it is extremely sensitive and absolutely offset free. Traditionally a thermocouple is made from two pieces of metal with different Seebeck coefficients. The thermopower, or thermoelectric power, or Seebeck coefficient of a material is a measure of the magnitude of an induced thermoelectric voltage in response to a temperature difference across that material. Metals have relatively small Seebeck coefficients in the order of 20 micro V/K. Semiconductors exhibit much higher Seebeck coefficients in the order of 200 micro V/K. Furthermore, p-type silicon has a positive Seebeck coefficient while n-type silicon exhibits a negative Seebeck coefficient so that a total Seebeck coefficient in the order of 400 micro V/K is achieved by combining n-type silicon and p-type silicon. It is therefore advantageous to fabricate the legs 13, 14 of the thermopile 5 from n-type and p-type silicon or polysilicon layers 13, 14 that are available in the IC process.

As is shown in Fig. 3, the thermopile 5 has a serpentine pattern in which the n- type and p-type silicon or poly silicon legs 13, 14 are electrically connected with a relatively small n-type or p-type silicon or polysilicon connecting legs 15 such that the thermopile 5 forms one continuous resistor in a serpentine pattern. To prevent the formation of unwanted diode junctions the legs of the thermopile 5 are preferably connected using a metal contact. The heater 4 is, in this example, made from a metal layer. It should be noted that the flexible carrier made of polyimide provides for a material that has a very high thermal resistance allowing for sensitive temperature and flow measurements. The flexible foil 3 further comprises, in this example, additional circuitry 6. The additional circuitry 6 may comprise signal processing circuitry, such as amplification, or it may, for example, include analog to digital conversion circuitry and/or data multiplexing circuitry. Additionally in this example, the flexible foil 3 is provided with a magnetic loop receiver antenna 9 for enabling, for example, a wireless RF link. Furthermore, other sensors may be integrated on the flexible foil 3 for the measurement of other physical properties inside the bodily lumen. For example, the integration of a pressure sensor on the flexible foil 3 enables the measurement of the pressure inside the bodily lumen. Combining the pressure sensor with the flow sensor increases the accuracy of the characterization of the aneurysm behavior. Examples of other sensors that may be integrated on the flexible foil 3, whether in combination with each other or not, are a sensor for measuring the absolute temperature and a sensor for measuring the oxygen level of the blood.

The measurement of the blood flow with thermopile 5 and the heater 4 will now be explained. A small amount of energy is dissipated in the heater 4, for example by forcing a current through the heater 4. This energy dissipation will increase the temperature of the blood that flows down-stream a fraction of a degree compared to the temperature of the blood that flows up-stream. This difference in temperature of the blood down-stream and upstream is measured with the thermopile 5 and is a measure for the blood flow. Calculations show that temperature differences in the order of milli- Kelvin can easily be measured with the thermopile 5.

During positioning of the sensor probe 100 inside the bodily lumen, the flexible foil 3 remains retracted into the hollow end 2 of the catheter 1. Figs. 4a, 4b and 4c schematically depict the end of the catheter 1. Fig. 4a shows the probe sensor 100 with the flexible foil 3 from its side, and Figs. 4b and 4c show the flexible foil 3 from the top, which is in fact a 90 degrees rotated view around the main axis of the catheter 1 with respect to Fig. 4a. The catheter 1 itself is hollow and the hollow end 2 of the catheter 1 comprises the flexible foil 3 when it is retracted, as can be seen in Figs. 4a and 4b. The flexible foil 3 is in this embodiment connected to a thin wire 10, which slides in the hollow catheter 1. In this way the flexible foil 3 can be pushed out of the hollow end 2 of the catheter 1. The wire 10 can also be used to rotate the flexible foil 3 around its main axis for a more precise positioning, for example, inside the aneurysm pouch 12.

To position the flexible foil 3 inside the aneurysm pouch 12, it should be bent when it is shifted out of the catheter 1 into a shape which enables the flexible foil 3 to be steered into a difficult to reach location such as, for example, the aneurysm pouch 12. There are several ways to achieve this. First of all the shrinkage of the polyimide substrate during curing during the manufacturing of the flexible foil 3 can be used for this purpose. This shrinkage would cause a severe bending of flexible foil 3 were it not that directly after de lamination, both sides of the polyimide carrier are provided with a ceramic layer. On one side this is of course the circuit layer stack, on the other side it is a separator oxide layer. This oxide layer prevents an unwanted mixing of adhesive with polyimide. When the oxide layer is removed by wet etching, the foil bends naturally to a radius of less than a millimeter. Secondly an SMA (Shape Memory Alloy) can be used to induce the required curvature of the flexible foil 3. A property of SMA is that when it is annealed at 500 degrees Celcius, the shape of the SMA during the anneal is memorized. After cooling of the SMA it may be deformed, but on heating the SMA beyond a transition temperature the shape of the metal during anneal is restored. SMA furthermore exhibit super elasticity. An SMA like TiNi (51% - 49%) can be deposited by sputtering or evaporation and patterned by wet etching. In this way the SMA can be included in the flexible foil 3 during processing. After delamination the flexible foil 3 is bent to the desired curvature or shape and then annealed at 500 degrees Celcius. In this way the desired curvature or shape of the flexible foil 3 is memorized. The flexible foil 3 should be bent in a compressive mode during this procedure. When the temperature inside of the bodily lumen is higher than the transition temperature, the flexible foil 3 will resume the memorized bending automatically after shifting it out of the catheter 1. The transition temperature is determined by the exact composition of the SMA. When a transition temperature is selected above the temperature inside of the bodily lumen, the curvature can be induced by locally heating the SMA. This can be done by including miniature heater elements in the immediate vicinity of the SMA on the flexible foil 3. In this way the physician is able to actively steer the flexible foil 3 inside of the aneurysm 12. Other alternatives are also possible for enabling the transformation from the first shape to the second shape of the flexible foil 2. For example, piezo-electric or voice coil actuators may be integrated on the flexible foil 3, or polymers, such as for example polypyrrole, may be applied that swell under the influence of an electric current. The flexible foil 3 may furthermore be provided with a series of polysilicon strain gauges to detect a stress in the flexible foil 3. The information from the strain gauges may be of aid to the physician in carefully positioning the flexible foil 3 inside of the aneurysm pouch 12. Obviously the flexible foil 3 needs to be sealed with a biocompatible coating.

Parylene may be used for this purpose, which is a material that is widely accepted as a biocompatible coating in, for example, catheters and pacemakers. To prevent any problems in making electrical contacts to the flexible foil 3, the flexible foil 3 is preferably operated in a wireless mode. This eliminates the need for difficult and unreliable electrical contacts. In a convenient arrangement, the flexible foil 3 is on one side provided with a magnetic loop receiver antenna 9. This part of the flexible foil 3 fits into a widening of the wire 10 inside the catheter 1. This widening holds a transmitter loop antenna 8. In this way the receiver and transmit antennas 8, 9 are situated in a very close proximity to each other, allowing for an efficient energy transfer and RF link. This RF link may also be used for bi-directional communication.

The sensor probe 100 enables the measurement of physical properties in locations that are difficult to reach for the state-of-the art catheters. For example, the arteries inside the brain have a typical inner diameter of 2mm to 3mm and the aneurysm pouch 12 in that case has diameter in the order of 2mm to 5mm. In this case the laminar blood flow is not disturbed and/or influenced by the catheter when the catheter is smaller than approximately lmm. Nowadays catheters have dimensions larger than approximately lcm and these catheters can therefore not be applied to measure a physical property inside the arteries of the brain. However, the flexible foil 3 of the sensor probe 100 according to the invention provides for a bending radius in the order of lmm to 2mm and is therefore suitable for measuring physical properties inside the brain, for example inside the aneurysm pouch 12 in the brain.

In summary, the invention relates to a sensor probe for measuring a physical property inside a bodily lumen. The sensor probe comprises a catheter having a hollow distal end and a flexible monolithic integrated circuit foil, which comprises a sensor, wherein the flexible foil is provided in and is extendable from the hollow distal end of the catheter. This enables the measurement of, for example, a blood flow and/or pressure inside an artery as well as in a small and/or difficult to reach locations, such as an aneurysm.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of other elements or steps than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS:

1. A sensor probe (100) for measuring a physical property inside a bodily lumen, comprising a catheter (1) having a hollow distal end (2) and a flexible monolithic integrated circuit foil (3) comprising a sensor, wherein the flexible monolithic integrated circuit foil (3) is provided in and is extendable from the hollow distal end (2) of the catheter (1).

2. A sensor probe (100) as claimed in claim 1, in which the sensor comprises a heater (4) and a temperature sensor (5) for measuring a flow of a fluid inside the bodily lumen.

3. A sensor probe (100) as claimed in claim 2, in which the flexible foil (3) comprises a plurality of heaters (4) and temperature sensors (5) arranged adjacent to each other.

4. A sensor probe (100) as claimed in claim 2, in which the temperature sensor (5) comprises electrically connected parallel stripes (13,14) forming a serpentine pattern.

5. A sensor probe (100) as claimed in claim 4, in which the electrically connected parallel stripes (13,14) comprise alternating n-type and p-type silicon.

6. A sensor probe (100) as claimed in claim 1 or 2, in which the sensor comprises a pressure sensor.

7. A sensor probe (100) as claimed in claim 1, in which the flexible foil (3) further comprises an antenna (9).

8. A sensor probe (100) as claimed in claim 1, in which the flexible foil (3) has a first shape when it is inside the hollow distal end (2) of the catheter (1) and a second shape, which is different from the first shape, when it is outside the catheter (1).

9. A sensor probe (100) as claimed in claim 8, in which the flexible foil (3) further comprises a shape memory alloy which provides for the second shape of the flexible foil (3) after heating of the shape memory alloy.

10. A sensor probe (100) as claimed in claim 9, in which the flexible foil (3) further comprises a further heater for heating of the shape memory alloy.

11. A sensor probe (100) as claimed in claim 1, in which the flexible foil (3) further comprises a strain gauge for detection of a stress in the flexible foil (3).

12. A sensor probe (100) as claimed in claim 1, in which the flexible foil (3) further comprises signal processing circuitry (6).

13. Medical apparatus comprising the sensor probe (100) as claimed in claim 1.