NO344722B1

NO344722B1 - Medical catheter for temperature and pressure sensing and production method thereof

Info

Publication number: NO344722B1
Application number: NO20161579A
Authority: NO
Inventors: Rolf Kahrs Hansen; Magne Tvinnereim; Ronny Nesse
Original assignee: Spiro Medical As
Priority date: 2016-09-30
Filing date: 2016-09-30
Publication date: 2020-03-23
Also published as: NO20161579A1

Description

Medical catheter for temperature and pressure sensing and production method thereof

Field of invention

The present invention relates to measurement of pressure and temperature in the body (invasive, minimal invasive or non-invasive) of humans or animals with the use of a catheter with one or more sensors.

Diagnostics of humans or animals require knowledge about the status of the body which requires some form of state detection. This can be based either on visual or acoustic sensing by a clinician, but it normally requires some data from sensors in order to facilitate proper diagnostics.

The present invention covers the use of a catheter with sensors for temperature and/or pressure in order to establish data essential for a correct diagnosis. The use of the catheter can be

● Invasive – i.e. for blood, cardiac, intracranial, urinary use.

● Minimally invasive (embedded in ectodermal structures) – i.e. for rectal, oesophageal or gastrointestinal use.

● Non-invasive – i.e. for nose, pharynx use.

Related art

A number of catheters with micro sensors (micro machined silicone, MEMS) have been described in literature and patents. Catheter pressure sensors can typically be resistive type, capacitive type or photoelectric type. Catheter temperature sensor can be resistive type, thermistor type, thermocouple type or use of the bridge resistance of a resistive type pressure sensor as a means of detecting temperature.

Traditionally a cavity for the pressure sensor is moulded within the catheter and the sensor is isolated from mechanical stresses that might result in false pressure signals. A tubular metal casing is often used in order to encapsulate the sensor and a silicone or another soft compound is used to act as a pressure sensing window to the surroundings.

Connection to the sensor is normally with wires bonded by soldering, welding or by the use of conductive glue. As the sensors are extremely small (much less than one mm cross section), the traditional methods are therefore time consuming and very difficult and require time, skill and patience by operators. Hence they are also very expensive.

Clausen and Sveen /11/ describe and discuss the use of a pressure sensor for invasive use. The pressure sensor is mounted on a flexible printed circuit board (FPCB) which is placed in a silicone rubber catheter with the sensor inside the tip of the catheter and the flexible printed circuit board (FPCB) being integrated into the wall of the catheter. This catheter is rather complex which makes the manufacturing expensive and probably also unreliable.

EP1671669 /10/ describes a catheter with sensors mounted on a FPCB extending to the full length of the catheter where the FPCB is inserted in a tube and where the FPCB comprises a metal support plate.

US8025623 /9/ describes a pressure sensor module with a pressure side and a pressure reference side, a transmission medium coupled to the sensor and a carrier with a slot formed to expose the sensing side of the sensor. This modular approach is considered cost effective as it reduces manufacturing costs by allowing most of the delicate work associated with building a catheter pressure sensor transducer to be performed ahead of time on a separate module.

WO2007139479 /3/ describes various solutions incorporating folded flexible support members for sensors. The support member has one or more electrically conductive lines or patterns and the support member is at least partly formed into an elongated tube and the inside of the support member is at least partly sealed from the outside. This production approach is very cost efficient and elegant, but it is difficult to obtain robustness during use and it is difficult to obtain the required dielectric strength which is required for clinical use.

WO2011005165 /4/ describe a variant of /3/ with one or more external electrodes. CN102665813 /5/ describes another variant where the flexible conductor carrier includes at least one elongated inserted section intended to be arranged inside a lumen. WO2013074036 /6/ describes another variant which is a component made from a single piece of flexible material with a middle section formed from rolling into a tube and with flat end portions. These descriptions all include rolling of a flexible support material (typically a FPCB) into tubular catheters or sections of a catheter.

SE1400311 /7/ describes the arrangement of a sensor on a FPCB inside an outer casing where at least one cavity in the outer casing is filled with a rigid filling material that extends in the lengthwise direction of the tubular outer casing. This construction will stabilise the sensor so that it is not affected by false signals due to mechanical stress. The rigid material has to extend to both sides of the sensor and in practise it has to be protected by an additional rigid outer cylindrical casing. The sensor window itself must be exposed and sealed with a flexible compound. The complete sensor assembly thus consists of several materials and production processes and the production of such a device is cumbersome and requires substantial operator training and skill. The coupling between the different materials is also critical.

NO301210 /12/ describes a catheter using the temperature sensitive part of a resistive pressure sensor to detect temperature and respiration. This is a well-known method that has been used since the birth of resistive pressure sensors. Cranen and Boves in a paper from 1985 /13/ shows the use of pressure sensors in the pharynx and also discuss the temperature dependency of these pressure transducers.

US5113868 /14/ describes a capacitive pressure sensor for making low pressure measurements. The sensor may be mounted into a 0.5 mm OD catheter and employs a transducer which consists of a rectangular bulk silicone micro-diaphragm where the diaphragm may have encapsulating material upon it and where only the silicone diaphragm is exposed for measurements and is a separate section compared to the rest of the catheter.

The object of the present invention is to solve the problems related to the known art by providing a simple, flexible and robust catheter. These objects are obtained with a catheter as well as a method for producing the catheter as defined in the accompanying claims.

The present invention provides a catheter where the sensors are covered completely in a soft material surrounded by an outer tube, there is no “pressure sensor window” different from the rest of the catheter, and the production method is new as the entire catheter or catheter section is produced in very few operations including the sensor locations as well as the rest of the catheter or catheter section.

The invention will be described below with reference to the accompanying drawings, illustrating the invention by way of examples.

Figure 1 illustrates the cross and longitudinal section of the preferred embodiment of the invention.

Figure 2 illustrates a cross section of an alternative embodiment of the invention. Figure 3 illustrates a cross section of a second alternative embodiment of the invention.

Figure 4 illustrates a set of components or sensors positioned on a part of the circuit board according to the invention.

Figure 5 illustrates cross sections of several alternative embodiments of the invention.

Figure 6 illustrates the end part of the catheter according to the invention.

Figure 7 illustrates a strain relieving feature.

Figure 8 illustrates an additional feature to the invention.

Figure 9-11 illustrates the production method according to the invention.

Figure 12 illustrates the measurements according to the invention.

Description of the invention

Catheter construction

The present invention relates to a medical catheter especially for use by humans or animals. Referring to the figures, the key property is that the catheter is completely filled and consists of an outer tube (1) filled with the required components (3, 4, 9-11, 12, 15, 17) (wires, FPCB, sensors) and a filler material (2) that takes up the rest of the space.

In figure 1 the catheter is constituted by a tube (1) enclosing a circuit board (3) with a sensor (4) mounted on it. The circuit board extends along the tube and may include several sensors and other circuitry.

The pressure sensors (4) measure absolute pressure, hence no reference side has to be exposed to the environment. This also allows the use of a catheter tube that is filled with a soft compound (2) – i.e. no lumen or ventilation to the surroundings is necessary. The soft compound is a soft, gel like material (2), being sufficiently soft to transmit the pressure to the sensor, but not soft enough to equalize the pressure in the catheter along its length. This way the local pressure is measured at each sensor being independent of the pressure measured at a distance from the sensor. As will be discussed below this material may be UV curable silicone or similar.

The components in the catheter thus includes at least one micro-sensor (4, 17) for measurement of temperature and/or pressure. Sensors can be analogue pressure sensors (4) and temperature sensors (17) or they can be digital sensors (4, 17) that can deliver a calibrated digital signal over a digital electrical bus. Typical analogue sensors include thermistors, thermocouples and absolute pressure sensors based on resistive type halfbridge or full bridge pressure sensors. Due to size constraints, these sensors are preferably bare dies mounted on a FPCB. The latter type is also – with the proper signal conditioning electronics - capable of providing a temperature signal as well as a pressure signal as the resistance over the full bridge is dependent on temperature.

Typical digital sensors include Microelectromechanical Systems (MEMS, also called Micro-machined Silicone sensors) from various suppliers. This includes both temperature and pressure sensors.

Mounting of the sensor dies to the FPCB is done by bonding, as illustrated in Figure 2, where the bond wires (5) are protected with glob-tops (glue) (6). Encapsulated sensors and components like thermistors and resistors are mounted using surface mount techniques (SMC) (7,18) as illustrated in figure 3.

The pressure sensors should be stabilized mechanically as mechanical stress may lead to forces on the pressure-sensing diaphragm, especially on bonded components. This is done by fixing a stiff plate (15) to the underside of the FPCB as shown in figure 8, preferably using a hard glue.

In order to optimize the pressure sensor sensitivity and to optimize the time response of temperature sensors, the distance to the outer catheter surface is important. This distance is a trade-off between medical safety (the dielectric strength) and the sensitivity and time response. In order to ensure optimal distance, a drop of glue (16) of a defined size is placed on the backside of the FPCB (3) or the before mentioned stiffener (15) in figure 8. Similarly, drops of glue can be placed on the component side of the FPCB also. In the case that the glob-top (6, figure 2) fulfils this function on the component side or in the case that the outer tube alone fulfils the electrical safety requirements, the additional distance drop on the component side is not required.

The FPCB can be used along the entire length of the catheter, or there can be one piece of FPCB for one or more sensors so that the rest of the catheter is made up by wires (12) that are connected to the FPCB by soldering. Side by side soldering can in many cases be very difficult because catheters are typically thin, therefore the soldering points can be distributed along the length of the catheter so that the solder pads can be made large enough to accommodate easy, manual soldering (figure 4).

Some applications will require a rather stiff catheter and some will require a soft catheter. In some cases the flexibility should be different in different longitudinal sections – for example a catheter for use through the nose into the oesophagus. In this case, the distal part of the catheter could be stiffer than the middle section in order to ease insertion and at the same time have a soft material in the pharynx and nasopharynx as these parts are very sensitive. In some cases flexibility should be different in different directions. These properties are solved by the present invention by use of an internal fibre or “spine” so that the catheter is easier to bend in one direction than in the perpendicular direction. The thickness and material of the fibre will control the flexibility and the cross section of the fibre can be rectangular or oval in order to obtain direction-dependent flexibility.

In figure 5 this is illustrated as cross sections of the tube where the supporting wire (9,10,11) has three alternative shapes as flat (9), oval (10) and circular (11). The drawings also show the cross sections with either a circular wire (12) including necessary conductor leads or the circuit board (3). The wire and circuit boards a connected in a series along the tube length.

In order to facilitate insertion and placement during use, the present invention uses a spherical end (13,14) on the catheter as shown in figure 6, being formed typically with UV curing biomedical glue. The sphere diameter depends on the application. In the case of blood vessels the sphere diameter will typically be the same as the catheter diameter. In case of an oesophagus catheter, the sphere size will typically be 3 mm diameter so that insertion is smoothly carried out and such that the swallow reflex and the peristaltic activity tends to hold the catheter in place in the oesophagus.

In some cases it is necessary to mark a certain position along the catheter. In the present invention, this is solved by using two or more different colours on the outer tube. If the proximal colour is for example blue and the distal colour is for example yellow, it is easy to see for example during insertion via nose to the oesophagus when the blue/yellow division is behind the uvula – hence the colour division can be used to control the position in the patient. It is essential the outer tubes ends are sealed off in order to avoid ingress of gas or liquid. Sealing may be obtained with a biomedical glue.

In some cases direct mechanical pressure on a pressure sensor should be avoided – for example when measuring hydrostatic pressure (for example in blood vessels or in the breathing channel). According to a possible embodiment of the invention illustrated in figure 8 this is solved by using “collars”(19) next to the pressure sensor so that skin do not touch the material over the sensor diaphragm directly. The collars can be of the same material as the end sphere (the tip of the catheter).

In some cases, pressure sensors are sensitive to longitudinal strain and thereby produce false signals. In this case, wires or FPCB can be bent slightly as to function as strain relieving springs, as shown in figure 7 (the bend angles are approximate). One such feature on each side of a pressure sensor will strain relieve the pressure sensor and make it less sensitive to longitudinal strain.

The present invention describes a catheter where only two materials are exposed to the human or animal – namely the outer tube and the glue that is used for the end sphere, the seals and the collars. The results is a smooth catheter, easy to clean and disinfect. It has a defined flexibility depending on the application. The flexibility can be different for different sections of the catheter, and it can be different in different directions.

Calibration and signal readout

In the case of digital sensors, the signal readout is carried out digitally via a computer bus. Most conveniently a serial bus like I2C. This will limit the number of conductors and several sensors can “talk” on the same bus provided they have different addresses. In the case of analogue sensors, more wires are needed and the signals have to be amplified and digitized. Typically, pressure sensors are temperature dependent and this has to be compensated individually because two sensors will have different characteristics. The present invention solves this by using electronic boards with processors and amplifiers where each board has a unique I2C address. Thereby, several boards can be connected on the same I2C bus and inserted in the same pressure and temperature controlled chamber. The catheters can then be calibrated individually under computer control and the calibration parameters saved in flash memory.

The catheter pressure sensors will be influenced by the environment as material characteristics will be affected by for example humidity. This is an inherent property of most solid materials – materials with shear forces. These will act directly on the pressure sensor diaphragm as this is covered by a soft compound. Especially the offset (the output at a certain pressure) is affected. It is part of this invention to calibrate the catheters in an environment which is representative to the environment it will be used in. Intended use in the pharynx or oesophagus means that calibration shall be done in 100 % relative humidity and at a temperature between 30°C and 38°C.

In case one wants to limit the use of the catheter – as no catheter will last forever – the present invention includes facility to set the usage limit in flash memory so that when the limit is exceeded, the catheter does not work anymore, or a warning can be issued. The present invention counts one use when the catheter has been running for N minutes after start up. N being for example 60 minutes. This makes it possible to test the catheter “dry” (i.e. in the office) without increasing the usage count.

Electrical connections and wiring

FPCBs are produced either as full-length boards or in sections (figure 4) with solder pads for connection of wires. Dies are connected as shown in figure 2 with wires (6) and glob-top (5) and surface mount components as shown in figure 3.

Non-metal stiffeners (15, figure 8) are mounted under components where this is necessary. This is the case for die pressure sensors that are bonded.

If the distance to the outer surface is important (sensitivity for pressure sensors and time response for temperature sensors), a distance drop (16, Figure 9) is placed at the back of the sensor or the stiffener plate.

Tube production, method I

One possible production method is illustrated in figure 9.

In order to produce the tube it is necessary to fill the inner space with a soft compound (2). In order to do this, a soft tube (1) and peel-able heat shrink tube (20) are pulled over the electronic part (FPCB with components or wires/FPCB and components). It is essential that the soft tube (1) has the following properties:

● The inner diameter is larger than the final diameter of the soft filler (2).

● The shrink temperature of the heat shrink tube (20) is high enough to melt the soft tube (1).

● The soft tube has a colour determined by the marking requirements mentioned above.

Soft material (2) is then injected into the soft tube from one end using a filler pump (22). In case abundance of air is essential (this is the case for pressure sensors), a suction pump (23) is connected to the other end.

The heat shrink tube (20) is then subjected to heat, and it shrinks so that the inner diameter is the final catheter outer diameter. As it is peel-able, is it linearly peeled (removed).

The next step is curing of the soft compound. If the soft compound is UV curing, it may be exposed to UV light during filling (figure 9) in order to trigger the curing process. If the soft tube (1) is transparent or partially transparent, UV curing can be continued as the next step.

The last step is to seal off the ends of the soft tube (1), install the tip (13 and 14, figure 6) and the collars (19, figure 8).

An additional last step may be required to ensure that there is no air or gas inside the soft outer tube (1). This involves injection of soft material inside the outer soft tube (1) with subsequent curing of the soft material or an extra shrink operation at the sensor using a peel-able heat shrink with smaller diameter than the heat shrink tube (20).

Tube production, method II

An alternative production method is illustrated in figure 10.

In order to produce the tube it is necessary to fill the inner space with a soft compound (2). In order to do this, a transparent (in case UV curable compound is used for the soft filler (2)) peel-able heat shrink tube (20) is pulled over the electronic part (FPCB with components or wires/FPCB and components). The inner diameter is larger than the final diameter of the soft filler (2) so that pulling is easy to do.

The heat shrink tube is the subjected to heat and the inner diameter shrinks to the final diameter of the soft material (2).

The soft material is then cured and the catheter core is ready.

In the next step, the soft outer tube (1) with an inner diameter larger than the diameter of the soft filler material (2) is pulled over the catheter core.

It is essential that the soft tube (1) has the following properties:

● The inner diameter is larger than the final diameter of the soft catheter core (2). ● The shrink temperature of the heat shrink tube (20) is high enough to melt the soft tube (1).

Next, another peel-able heat shrink (20) is pulled over the soft tube (1). It is essential that the inner diameter of the heat shrink tube is larger than the outer diameter of the soft tube (1).

The heat shrink tube (20) is then subjected to heat and the soft outer tube (1) melts and completely encapsulates the catheter core. If abundance of air is essential, a suction pump (23) can be used to remove air when shrinking. Alternatively, soft material can be injected between the core and the soft outer tube (1) prior to the shrinking of the heat shrink tube (20).

According to the preferred embodiment of the invention the catheter according to the invention includes the following material types:

● The outer tube is polyurethane.

● The soft filler is UV curable silicone.

● The pressure sensor is a bare die resistive half-bridge.

● The temperature sensor is a thermistor.

● The glob-top is epoxy.

● The fibres controlling flexibility and the stiffener plate are peek plastic material. ● The glue used for the tip, the collars and the sealing of the tube (1) ends is a biocompatible UV light curing acrylate.

● The glue used for fixing the stiffener (15) is a hard epoxy.

● The peel-able heat shrink is fluorinated ethylene propylene (FEP) material.

The table below refers to the preferred components and materials of the parts illustrated by reference numbers in the drawings:

Figure 12 shows typical data for a catheter for use in a human patient during sleep. One pressure sensor (POES) is placed in the oesophagus and another (PPH) is placed in the pharynx. The PPH sensor is used as a dual sensor providing both temperature (T0) and pressure (PPH). In addition there is a temperature sensor (T1) placed in the nasopharynx. Signals have been band pass filtered and the temperature curves are the time derivatives of the temperature. On figure 12, the curves are from top to bottom: Temperature variation in the nasopharynx, pressure in the pharynx, temperature in the pharynx and pressure in the oesophagus.

To summarize, the present invention relates to a catheter and method for making the same. The catheter includes sensors for providing measurements within a body, where the catheter includes an outer tube made from a soft material. The tube contains an inner part comprising at least one sensor, where the sensor(s) being electrically connected to an interface at one of the tube ends through a wire inside the tube from said sensor to said interface. The space between the tube wall and the circuitry is filled with a soft, gel-like material so that the pressure and temperature around the sensor corresponds to the temperature and pressure outside the catheter, i.e. in the body to be measured, but the soft material does not conduct the pressure or temperature any essential distance along the catheter.

The sensors may be of different types such as temperature sensors and pressure sensors, and may also be positioned in different positions along the catheter, at least one of said sensors is a temperature sensor. As the pressure sensors are enclosed in the soft filling material they are preferably absolute pressure sensors.

In the inner part, the sensors are preferably mounted on a flexible printed circuit board soldered to wires where the solder pads are distributed lengthwise in the catheter. The circuit board may be of different types e.g. small ones being interconnected along the catheter with conducting wires or wire bundles, or the circuit board may be of a relative flexible type extending along the whole catheter.

The soft outer tube is preferably made from polyurethane, and is preferably transparent or partly transparent to UV light. The soft filler material may be thus UV curable silicone, which allows it to be filled into space between the inner circuitry and the outer tube, and then cured with UV into a more gel like state.

In order to maintain the sensors in the correct position in the catheter cross section, drops of glue may be placed on both sides of at least one sensor to control the radial position of the sensor.

In some cases it may also be preferable to provide the catheter with at least one collar placed outside said catheter in the longitudinal direction positioned close to at least one sensor so that sensor or outer tube in the sensor vicinity does not come in direct contact with surrounding tissue.

The outer tube may be split in sections of different colours in order to mark at least one position along the axis of the catheter. Also the tip of the catheter in the other end from the interface, and thus the end being positioned inside the body, is spherical and formed with a drop of glue.

In order to control the flexibility or stiffness of the catheter, the tube also includes a fibre of a chosen flexibility, the fibre may have a cross sectional shape of the internal fibre chosen so as to make the flexibility of the catheter different in different directions, and/or may vary in the longitudinal direction of the catheter so as to be less flexible in chosen areas.

As discussed above the catheter may be produced by pulling the tube over an inner core of smaller diameter than the inner diameter of the tube, and heating the outer tube and pressing it onto the inner core containing the other components using heat shrink tube, whereby the heat shrink tube is removed. More filling material may then be introduced into the space between them and then cured. Additional shrinking over the sensors may also be done using a suitable diameter heat shrink tube.

References

1. System for monitoring respiratory effort. WO2015181140. Hansen, Tvinnereim. 2. Respiration monitoring. NO20110653. Hansen, Tvinnereim.

3. Tubular catheter for invasive use and manufacturing method therefor.

WO2007139479. Kaellbaeck, Hult, Brodin, Elmqvist.

4. Catheter and method for manufacturing such a catheter. WO2011005165. Kaellbaeck, Thomsen.

5. Flexible conductor carrier for catheter and catheter fitted with a conductor carrier.

CN102665813. Kaellbaeck, Thomsen.

6. Catheter component. WO2013074036 Kaellbaeck, Thomsen, Bagaian.

7. A conductor assembly comprising a resilient tubular outer casing. SE1400311.

Kaellbaeck, Bagaian, Rokosch.

8. Catheter and production method thereof. US7632236. Kaneto, Ohwaki, Ohsawa. 9. Pressure sensing module for a catheter pressure transducer. US 8025623. Millar. 10. Catheter comprising a flexible printed circuit board and production thereof. EP 1671669. Kaneto, Ohwaki.

11. Clausen, I., Sveen, O. Die separation and packaging of a surface micromachined piezoresistive pressure sensor. Sensors and Actuators A 133 (2007) 457-466.

12. Gjersoe, B.G. Anvendelse av sensorer for måling av et individs respirasjonstidevolum. NO301210.

13. Cranen, B., Boves, L. Pressure measurements during speech production using semiconductor miniature pressure transducers: Impact on models for speech production. J. Acoust. Soc. Am.77(4), 1985.

14. Ultraminiature pressure sensor with addressable read-out circuit. US5113868. Wise, K. D., Chau, H-L.

Claims

1. Catheter for providing measurements within a body including an outer tube made from a soft material, the tube containing an inner part comprising at least one sensor, the sensor(s) being electrically connected to an interface at one of the tube ends through a wire inside the tube from said sensor to said interface, wherein the sensors are covered completely in a soft material surrounded by an outer tube.

2. The device as claimed in claim 1, wherein at least one of said sensors is a temperature sensor.

3. The device as claimed in claim 1, where at least one of said sensors is a pressure sensor.

4. The device as claimed in claim 3, wherein said pressure sensor is an absolute pressure sensor.

5. The device as claimed in claim 1 where the inner part includes sensor mounted on a flexible printed circuit board soldered to wires where the solder pads are distributed lengthwise in the catheter.

6. The device as claimed in claim 1 where the inner part includes sensor mounted on a flexible printed circuit board that extends along the entire length of the catheter.

7. A device as claimed in claim 1, wherein the soft outer tube is polyurethane.

8. A device as claimed in claim 7, wherein the outer tube is transparent or partly transparent to UV light.

9. A device as claimed in claim 1, wherein the soft filler material is UV curable silicone.

10. A device as claimed in claim 1, wherein a drop of glue is placed on both sides of at least one sensor to control the radial position of the sensor.

11. A device as claimed in claim 1, wherein at least one collar is placed outside said catheter in the longitudinal direction positioned close to at least one sensor so that sensor cover does not come in direct contact with surrounding tissue.

12. A device as claimed in claim 1, wherein the outer tube is split in sections of different colours in order to mark at least one position along the axis of the catheter.

13. A device as claimed in claim 1, wherein the tip of the catheter is spherical and formed with a drop of glue.

14. A device as claimed in claim1, wherein the wires or flexible printed circuit board is bent in some locations in order to obtain a longitudinal spring effect.

15. A device as claimed in claim 1, wherein the tube also includes a fibre of a chosen flexibility so as to control the flexibility of the catheter.

16. A device as claimed in claim 15, wherein the cross sectional shape of the internal fibre is chosen so as to control the flexibility of the catheter is different in different directions.

17. A device as claimed in claim 15, wherein the flexibility of said fiber varies in the longitudinal direction of the catheter.

18. A method for production of a catheter in claim 1, where the outer tube is pulled over an inner core of smaller diameter than the inner diameter of the tube and where the outer tube is heated and pressed onto the inner core using heat shrink tube, whereby the heat shrink tube is removed.