Abstract:
Disclosed are a system and a method for noninvasively and continuously monitoring blood pressure. Also disclosed is a method for making such a device. The system includes a semiconductor chip comprising a transducer array of individual pressure or force sensors and associated circuitry providing control signals to and/or processing signals from these sensors, all of the above integrated in the chip. Also disclosed is a specific sensor structure provided on said chip. The invention further encompasses a system for measuring and/or tracking the blood pressure waveform and for combining the latter with related blood values like the heartbeat, derived from the above or other measuring devices.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a device and a method for noninvasively monitoring blood pressure. The apparatus includes a semiconductor chip comprising a transducer array of individual pressure or force sensors and associated circuitry providing control signals to and/or processing signals from these sensors, integrated in the chip. Also disclosed is a specific sensor structure provided on said chip. The invention also encompasses a system for measuring and/or tracking the blood pressure waveform and combining the latter with related blood values like the heartbeat, derived from the above or other measuring devices. 
       BACKGROUND AND PRIOR ART 
       [0002]    Measurement of blood pressure is one of the most common procedures done during examination of a patient in hospitals. It is usually done with the aid of a cuff attached to the arm, which only gives an indication of two values, namely the systolic and the diastolic pressure. Especially during surgery and treatment at the intensive care unit, a continuous measurement of the blood pressure is required. This is routinely done using an intra-vascular catheter, where the blood pressure is compared to a pressure of the liquid inside the catheter tubing. Since this is an invasive method, it is used only when it is absolutely necessary. However, in many cases a continuous measurement would be beneficial for the medical personnel in the evaluation of the patient&#39;s condition. Furthermore, inserting a catheter into a small child or severely ill person with very weak blood vessels is extremely difficult, even impossible. Thus there exists a need for simple extra-vascular method for measuring the blood pressure giving continuous signal. 
         [0003]    The continuous measurement of blood pressure by use of arterial tonometer transducers is known in the art as disclosed in two Eckerle U.S. Pat. Nos. 4,269,193, 4,802,488, for example. 
         [0004]    Eckerle U.S. Pat. No. 4,802,488, cited above, discloses how intraarterial blood pressure can be measured noninvasively by an electromechanical transducer that includes an array of transducer elements. The transducer extends across an artery with the transducer elements extending across the artery. Diastolic and/or systolic pressure and pulse amplitude values are obtained from the outputs of the transducer elements, which values are stored in computer. Information concerning the subject, i.e. the patient, related to the diameter of the underlying artery including, for example, the subject&#39;s age, weight, arm and wrist diameter may also be entered into the computer, from which information an estimation of the diameter of the underlying artery is obtained. 
         [0005]    Using the set of pulse amplitude values, the particular transducer element or elements located substantially at the center of the measurement area is/are identified and the outputs from only said particular transducer element(s) used for monitoring the subject&#39;s blood pressure and/or for further processing. 
         [0006]    The device and method according to Eckerle U.S. Pat. No. 4,802,488 above appears to be workable and is probably implemented in the device described in the above-cited internet publication. However, looking at the device shown in said publication “Verfahren der Arteriellen Tonometrie”, it becomes clear that the use of this cuff-like, bracelet-type device is limited to so-to-speak normal applications, i.e. applications where there is sufficient space at and around the measurement area and where sterility is of no great concern. In an operating room or even within a human body during surgery, it is hardly conceivable how the described prior art device may be used. 
         [0007]    An internet publication, http://www.dr-kaiser-medizintechnik.de/blutdruck.htm, shows a blood pressure measuring device that appears to incorporate at least part of the technology disclosed in the two U.S. patents cited above. The Colin BP-508 T CS device shown there is of the bracelet-type, looking rather robust, but consequently being of substantial size and requiring a specific position of the patient. 
         [0008]    A similar device is disclosed by L. A. Steiner et al in the journal “Anaesthesia”, 2003, vol. 58, pages 448-454, entitled “Validation of a tonometric noninvasive arterial blood pressure monitor in the intensive care setting”. The CBM-700 shown and described therein is again a rather large hand-cuff device that is to be attached to the wrist or arm of the patient. 
         [0009]    S. Terry, J. S. Eckerle et al disclose in “Silicon Pressure Transducer Arrays for Blood-pressure Measurement”, in “Sensors and Actuators”, A21-A23 (1990), pages 1070-1079, a tonometer transducer array in which several transducers share a common diaphragm. The device is fabricated from silicon using anisotropic etching and includes piezoresistors for signal generation. However, no other electrical or other elements are provided on or in the silicon body. 
         [0010]    The present invention leads to a new approach, providing a remedy to many disadvantages of prior art devices. By reducing size and power consumption of a transducer device significantly, a wide spectrum of new applications is accessible, e.g. intra-body uses during surgery. By shaping the sensor array on such a transducer accordingly, one can improve and simplify blood pressure signal reception and evaluation. By speeding up signal processing, critical situations may be detected early enough to avoid problems in a time-critical environment, e.g. during surgery or after a heart attack. 
         [0011]    Also, reducing power consumption and processing the sensor signals “on chip” may even—when a small power source is included on the chip—allow wireless data transmission and thus provide for a fully independent device for monitoring the blood pressure. Needless to say that this opens a variety of further applications akin to today&#39;s widespread use of cardiac pacemakers. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    This invention is based on detecting the continuous force signal generated by a blood vessel. The origin of this force is the overpressure contained inside the vascular system. One or more force measurement instruments may be placed extravascularily, such as on the skin or the heart surface. 
         [0013]    The force variations are recorded continuously, whereby the continuous blood pressure is extracted from these force variations. The thus derived data can be further used to extract the relative difference between systolic and diastolic pressure. 
         [0014]    If it is necessary to obtain absolute values, often required for the evaluation of the patient&#39;s condition, high and low extremes of the force signal need to be calibrated. This may be done by using a separate measurement device, e.g. a usual handcuff blood pressure meter. 
         [0015]    The present invention now creates a novel approach for such a non-invasive blood measuring device in that it integrates the electromechanical sensor and at least some of the associated circuitry onto a single chip. 
         [0016]    This leads to a number of advantages, including: 
         [0017]    The possibility of making of sensor and circuitry by essentially the same semiconductor manufacturing process, in particular a CMOS process, data transmission speed and reliability are improved and the error probability reduced. It may also results in lower production cost of the whole blood pressure measuring device. 
         [0018]    The produced chip is much smaller and lighter and has a lower power consumption than prior art devices, which opens new possibilities of its use, in particular the use in antiseptic environments as an operation room or even within a human body during surgery. 
         [0019]    By arranging a plurality of sensors in an array adapted to the particular use, even complex measurements can be executed, for example spatially and/or timely distributed measurements to determine the characteristics of blood flow, the “blood wave”, in a blood vessel. 
         [0020]    Spatially and/or timely distributed measurements using plurality of sensors in an array allow also locating and identifying arteries and veins running underneath a tissue, i.e. myocardial tissue on a heart surface, based on directional information from an array and characteristic signal features of arteries and veins. Furthermore, abrupt features, such as blockages due to calcification inside arteries and veins, can be identified based on a map pattern of the blood pressure data. 
         [0021]    These and other advantages and details will be apparent from the subsequent description of an embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0022]    The present invention and its advantages will be better understood when the written description provided herein is taken in conjunction with the drawings wherein: 
           [0023]      FIG. 1  is a top view of a single transducer element; 
           [0024]      FIG. 2  is a cross-sectional view along line A-A′ of the single transducer element in  FIG. 1 ; 
           [0025]      FIG. 3  is a top view of a two-by-two array of transducer elements; 
           [0026]      FIG. 4  is a cross-sectional view along line B-B′ of the two-by-two transducer array in  FIG. 3 ; 
           [0027]      FIG. 5  is a layout of the monolithic integration of a two-by-two transducer array with electronic circuitry; 
           [0028]      FIG. 6  is a block diagram of an integrated transducer chip; 
           [0029]      FIG. 7  is a block diagram of a whole system for measuring and recording deformation of a blood vessel wall; 
           [0030]      FIG. 8 : shows the method of measuring the deformation of a blood vessel wall, 
           [0031]      FIG. 9  is a top view of a single transducer element of a second embodiment; 
           [0032]      FIG. 10  is a cross-sectional view along line C-C′ of the single transducer element in  FIG. 9 ; 
           [0033]      FIG. 11  is a cross-sectional view along line D-D′ of the single transducer element in  FIG. 9 ; 
           [0034]      FIG. 12  is a layout view of a Wheatstone bridge configuration in the crosslinked beam structure in  FIGS. 9 ,  10  and  11 ; and 
           [0035]      FIG. 13  is a top view of a two-by-two array of transducer elements of  FIG. 9 . 
       
    
    
       [0036]    For the sake of clarity, the figures do not necessarily show the correct dimensions, nor are the relations between the dimensions always in a true scale. 
       DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0037]    When describing the details of two embodiments of the present inventions, it should be clear that the following description is directed to persons having a thorough understanding of the technology involved. For background information please refer to the paper by H. Baltes and O. Brand: “CMOS-based Microsensors”, in Sensors and Actuators A 92 (2001), pages 1-9 and to the book “VLSI Technology”, ed. by S. M. Sze, McGraw Hill, New York, 1988, which are both incorporated herein by reference. 
         [0038]      FIGS. 1 and 2  show the structure of an individual transducer element according to the present invention, whereby  FIG. 2  is a cross section of  FIG. 1 . 
         [0039]    The individual transducer element  10  includes several parts. An elastic membrane  11  with a side length of less than 150 μm consists of a top electrode layer with support and protection layers. The support and protection layers of the elastic membrane  11  are made using standard CMOS techniques, e.g. deposited silicon dioxide and oxynitride. The top electrode is a CMOS metal layer, which is deposited aluminum in this embodiment. The thickness of the membrane is about 3 μm. A fluid gap  12  allows the membrane  11  to deflect, the height of the fluid gap  12  being less than 1 μm. This fluid gap  12  is made by etching a material layer or layers through inherent structural layers, i.e. substrate  14  and bottom electrode  13  support layer. In this embodiment, the material layer that is removed to form the fluid gap  12  is deposited aluminium. Rigid bottom electrode  13  has an electrode layer with support and protection layers. These support and protection layers of the bottom electrode  13  are also made using standard CMOS techniques, e.g. deposited silicon dioxide and polysilicon, and thermally oxidized silicon dioxide. The parts  11 ,  12  and  13  are built onto a substrate  14  whose thickness is some hundreds μm. In order to allow for the sacrificial release of the elastic membrane  11  by creating the fluid gap  12 , an opening or several openings  15  are etched through the substrate  14 . 
         [0040]      FIGS. 3 and 4  show a two-by-two array of four transducer elements, whereby  FIG. 4  is a cross section of  FIG. 3  along B-B′. 
         [0041]    An array  20  is formed of single transducer elements  10 . In this particular embodiment, the array  20  is made of two rows and two columns of the single transducer elements  10 . The pitch of the neighbouring transducer elements  10  is less than 200 μm. In the embodiment shown with a single opening  15  through the substrate  14 , this opening  15  is shared by four neighbouring transducer elements  10 . The fluid gaps  12 , cf. also  FIG. 2 , are then formed simultaneously to all transducer elements  10 . 
         [0042]    A second embodiment is shown in  FIG. 13  and will be described in detail further down. 
         [0043]      FIG. 5  is a layout of a monolithically integrated chip of a two-by-two transducer array together with integrated electronic circuitry according to the invention. In the embodiment shown, an array  20  of transducer elements  10  is monolithically integrated onto a single substrate  14  together with the signal readout system  42 . In another embodiment, a transducer array  20   b  of transducer elements  10   b  is integrated. 
         [0044]    When using capacitive transducer elements as in this first embodiment, a typical signal readout system  42  consists of a readout circuit  21 , a signal conditioning circuit  22 , an analog-to-digital converter circuit  23  and an interface circuit  24 . The transducer chip is connected to interfacing system  44  via contact pads  25 . The interface  44 , c.f.  FIGS. 6 and 7 , is in this embodiment an electrical cable. In other embodiments, this may be replaced by a wireless connection. 
         [0045]    In the second embodiment of a transducer array  20   b , described further down in  FIG. 13 , a typical signal readout system  42  consists of the same functional blocks. 
         [0046]      FIG. 6  shows a block diagram of an embodiment of the integrated transducer chip, comprising a 4×4 array of single transducer elements  20 , a readout circuit  21 , a signal conditioning circuit  22 , an analog to digital converter circuit  23  and an interface circuit  24 . The transducers  10  are electrically connected to a readout circuit  21 , which in turn is connected to a signal conditioning circuit  22 . In the preferred embodiment, several transducer elements  10  share one readout circuit  21  and one signal conditioning circuit  22  through a multiplexing scheme, where each transducer element  10  is addressed individually. 
         [0047]    The signal at the output of the signal conditioning circuit  22  is connected to analog to digital converter circuit  23 . In other embodiments several signal conditioning circuits  22  and analog to digital converter circuits  23  may be used in parallel. In the preferred embodiment, the readout circuit  21 , the signal conditioning circuit  22  and the analog to digital converter circuit  23  are realized as a sigma-delta modulator circuits with decimation filtering. An interface circuit  24  is connected to the output of said analog to digital converter circuit  23  to provide a connection to an external device via a said interface  44 . 
         [0048]      FIG. 7  shows a block diagram of an embodiment of a whole measurement and recording system. An interface  44  connects the integrated transducer chip  41  to a computer system  45  which evaluates the transmitted data and provides suitable outputs. 
         [0049]      FIG. 8  finally shows a method of monitoring the blood pressure by measuring the deformation of a blood vessel wall. The sensing device  40  is an assembled structure consisting of the said integrated transducer chip  41 , described in detail above, a base plate for mechanically holding this transducer chip  41 , and some polymer layers for protection and biocompatibility, for example. The sensing device  40  is attached to the surface of an organ  51 , such as the skin or the heart. 
         [0050]    The sensing device  40  somewhat deforms the blood vessel  52  by deforming the surface of the organ  51  in order to sense the movement of the blood vessel wall  53  vertical to elastic membranes  11  of the transducer chip  41  in the sensing device  40 . This movement deflects a membrane  11  of a transducer element  10 . The distance between top electrode in membrane  11  and bottom electrode  13  changes in response to the deflection of the membrane  11 . In this particular embodiment, the change in mutual distance of the electrodes changes the capacitance of the electrode system. Thus the displacement of the vessel wall  53  can be read out as a change in capacitance in transducer element  10 . 
         [0051]      FIGS. 9 ,  10  and  11  show the structure of a second embodiment of an individual transducer element according to the present invention, whereby  FIGS. 10 and 11  are cross sections of  FIG. 9 . 
         [0052]    In this second embodiment, the individual transducer element  10   b  includes several parts. A membrane  11   b , having a side length of less than 250 μm, is suspended over a cross-linked beam structure  16  and connected to it at the center. The membrane  11   b  provides mechanical and electrical protection and is made of standard CMOS deposited silicon dioxide, metal (in this embodiment aluminum), and oxynitride. The membrane  11   b  is about 3 μm thick. The cross-linked beam structure  16  is formed using an implanted n-well of a standard CMOS process. The cross-linked beam structure  16  has a thickness of about 6 μm. 
         [0053]    Close to the support point of each beam in said beam structure  16  are resistors  18  connected with conductor lines  19  to a Wheatstone bridge configuration  17 , shown in  FIG. 12  in detail. The resistors  18  are made by a standard CMOS p-doping process and the conductor lines are CMOS metal, in this embodiment deposited aluminum. A fluid gap  12   b  decouples the cross-linked beam structure  16  from the membrane  11   b  except at the center. The height of the fluid gap  12   b  is less than 1 μm; it is manufactured by etching a material layer or layers through inherent structural layers, i.e. substrate  14  and crosslinked beam structure  16 . In this embodiment, the material layer which is removed to form the fluid gap  12   b , is deposited aluminum. The parts  11   b ,  12   b  and  16  are built onto a substrate  14  whose thickness is some hundred pm. To allow for the sacrificial release of elastic membrane  11   b  by creating the fluid gap  12   b , several openings  15  are etched through the substrate  14  and the cross-linked beam structure  16 . 
         [0054]      FIG. 13  shows a two-by-two array of four transducer elements  10   b . In this second embodiment, an array  20   b  is formed of two rows and two columns of the single transducer elements  10   b . The pitch of the neighbouring transducer elements  10   b  is less than 300 μm. 
         [0055]    In this embodiment, the movement of the blood vessel wall  53  deflects the connected system of a membrane  11   b  and a cross-linked beam structure  16  in transducer element  10   b . The deflection of said cross-linked beam structure  16  changes the electric resistance of the resistors  18  connected into a Wheatstone-bridge configuration  17  with conductor lines  19 . The change in one or several resistors  18  in the Wheatstone-bridge configuration  17  changes the electric voltage output of said Wheatstone bridge. Thus the displacement of the blood vessel wall  53  can be read out as a change in the output voltage of the Wheatstone-bridge  17  in transducer element  10   b.    
         [0056]    In other embodiments, the vertical movement of the blood vessel wall  53  may effect the change other electrical values like inductance or voltage. 
         [0057]    Through an electrical connection of the transducer element  10  to the readout circuit  21 , the change in the electrical measure, capacitance in the first embodiment, is converted to an electric voltage signal. In other embodiments, the signal may be delivered as electric current. In the second embodiment, the transducer element  10   b  provides an electric voltage signal through a readout circuit, as shown in  FIGS. 5 and 6 , embedded in said transducer element  10   b . A connected signal conditioning circuit  22  performs filtering and amplification of said electric voltage signal from said readout circuit  21  and a analog-to-digital converter  23  provides the amplified and filtered data to the interface circuit  24  in digital format. In the preferred embodiment, the interface circuit  24  delivers the data to the interface  44  via contact pads  25 . 
         [0058]    The computer  45 , see  FIG. 7 , receives said data via said interface system  44  and records it as continuous blood pressure data. The computer may also calculate the systolic, diastolic and mean blood pressures and/or the heart stroke volume from the recorded continuous blood pressure data. When directional information is processed from the continuous blood pressure data, it can be used to locate arteries and veins running underneath a tissue, i.e. myocardial tissue on heart surface. Based on the characteristic blood pressure features of arteries and veins, closely together running blood vessels can be identified. Furthermore, abrupt features, such as blockages due to calcification inside arteries and veins, can be identified based on a map pattern of the recorded continuous blood pressure data. 
         [0059]    While the present invention has been described by way of a few examples, these shall not limit the scope of protection since it is obvious to someone skilled in the art that the invention can be easily adapted to match many requirements in the field of blood pressure measuring transducers and systems, including their design and/or manufacturing and integration.