Abstract:
A catheter assembly for assessing contact between the catheter assembly and tissue is disclosed. In an exemplary embodiment the assembly includes a catheter shaft and a trio of pressure sensitive capacitive members each of whose electrical resistance varies with a pressure vector applied to the catheter assembly. The assembly also includes at least one measurement terminal to permit the measurement of changes in the electrical characteristics of the pressure sensitive capacitive members. The assembly may optionally include a measurement device to measure resistance, impedance and/or other electrical characteristics. The assembly may utilize a reference electrode secured to the patient&#39;s tissue, which permits the measurement device to measure changes between the reference electrode and the at least one measurement terminal. Optionally, the assembly may include a conductive outer layer. Also disclosed are sensor assemblies, contact sensor, methods of contact sensing, and methods of manufacturing relating to the use of pressure sensitive capacitors.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 11/647,279, filed 29 Dec. 2006 (the &#39;279 application). This application is also related to U.S. application Ser. No. 11/647,314, filed 29 Dec. 2006 (the &#39;314 application); U.S. application Ser. No. 11/647,316, filed 29 Dec. 2006, (the &#39;316 application); U.S. application Ser. No. 11/647,294, filed 29 Dec. 2006 (the &#39;294 application); and U.S. application Ser. No. 11/553,965, filed 27 Oct. 2006 (the &#39;965 application), the &#39;279, the &#39;314, the &#39;316, the &#39;294, and the &#39;965 applications are each hereby incorporated by reference as though fully set forth herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    a. Field of the Invention 
         [0003]    The present invention pertains generally to an electrophysiological device and method for providing energy to biological tissue and, more particularly, to a capacitive contact sensor that is capable of being using with an ablation apparatus to provide greater contact sensitivity during cardiac therapy procedures. 
         [0004]    b. Background Art 
         [0005]    Many medical procedures, including for example, creating lesions with electrical energy, rely on good contact between the medical device and the tissue. In some catheter applications, the point of electrode-tissue contact is as far as about 150 cm away from the point of application of force. This gives rise to functional and theoretical challenges associated with conventional devices, and thus, the ability to accurately assess tissue contact is increasingly important. 
         [0006]    There is a need for contact sensing devices that provide greater contact sensitivity for control of medical treatments. 
         [0007]    There is a need for improved sensor devices that provide greater contact sensitivity, especially in connection with, for example ablation therapy delivery such as radiofrequency (RF) ablation treatment of cardiac tissue. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    Disclosed herein is an electrode assembly having a catheter shaft, at least one tactile sensor; and an electrode at a distal end of the electrode assembly. In an exemplary embodiment, the at least one tactile sensor is located between the catheter shaft and the electrode such that the tactile sensor will detect force that is applied to the electrode. The at least one tactile sensor may comprise two, three, or more sensors; for example, a first sensor that detects forces applied axially to the electrode assembly and a second and third sensor that, cooperatively, with the first sensor, detects forces applied laterally to the electrode assembly. Further, each sensor may be one that detects compression and stretching forces and generates an output signal that distinguishes a compression force applied to the sensor from a stretching force applied to the sensor. The assembly may include an analysis device coupled to the sensors such that it provides directional content information regarding the forces that are applied to the electrode. The output signal of each sensor may be a signal with a magnitude that is proportional to the force applied to the sensor. Optionally, the analysis device may provide information regarding the magnitude and direction of the forces that are applied to the electrode. The sensor(s) may be selected from the group consisting of: a pressure sensitive conductive composite (PSCC) sensor; a capacitance sensor; and a piezoelectric sensor, although a capacitance sensor suite provides a degree of simplicity to such embodiments. The sensor may use a PSCC material. 
         [0009]    Also disclosed is an electrode assembly having a catheter shaft, an ablation electrode at a distal end of the electrode assembly, and a plurality of sensors located between the ablation electrode and the catheter shaft. Each of the plurality of sensors may be in a plane that passes transverse to an axis of the electrode assembly. The plurality of sensors may detect longitudinal compression forces and transverse bending forces applied to the ablation electrode. Preferably, each of the plurality of sensors generates a signal that is indicative of a characteristic selected from the group consisting of: resistance; capacitance; voltage; impedance; and combinations thereof. Preferably, the sensors are selected from the group consisting of: a PSCCosite sensor; a capacitance sensor; a piezoelectric sensor; and combinations thereof. In a particular embodiment, each of the sensors may be one of a capacitive sensor and a quantum tunneling conductive (QTC) composite sensor. Of course, the sensors may comprise a piezoelectric wire. In an optional embodiment, each of the plurality of sensor generates an output signal in proportion to the compression force applied to the sensor, and an output device may provide an indication of a direction of the force applied to the ablation electrode. For example, the output device may provide information on a direction and magnitude of the force applied to the ablation electrode. 
         [0010]    Also disclosed is an ablation catheter for ablating tissue. The catheter has a catheter shaft, an ablation electrode at a distal end of the ablation catheter, and a plurality of sensors located between the ablation electrode and the catheter shaft. The plurality of sensors may be spaced evenly about a circumference of the ablation catheter, wherein each of the plurality of sensor generates an output signal in proportion to the compression force applied to a portion of the ablation electrode. The catheter may include a controller configured to receive each of the output signals from the plurality of sensors, wherein the controller analyzes the output signals and assesses a degree of contact between the ablation electrode and the tissue to be ablated. The ablation electrode is preferably electrically coupled to an ablation energy source such that the controller generates a control signal to activate the ablation energy source when the controller determines that the degree of contact between the ablation electrode and the tissue exceeds a preset contact threshold. Of course, the controller may also generate a control signal to deactivate the ablation energy source when the controller determines that the degree of contact between the ablation electrode and the tissue exceeds a preset maximum value. The sensor may be a PSCC sensor; a capacitance sensor; a piezoelectric sensor; and/or combinations thereof. For example, the sensor may be made in the form of one of capacitive sensors and QTC composite material(s). In one embodiment, the ablation catheter may comprises at least four sensors which are arranged in opposing pairs and are spaced evenly about a circumference of the electrode assembly. 
         [0011]    Also disclosed is a method of sensing contact between a catheter and a tissue. For example, the method may include providing a catheter having a catheter shaft; an ablation electrode; and at least one sensor located between the catheter shaft and the electrode. The catheter may be placed in contact with the tissue such that at least one force is exerted on the ablation electrode. The applied force may generate an output signal from each of the at least one sensors; and further, may generate a signal that is indicative of a degree of contact between the catheter and the tissue. The control signal may be used to inhibit delivery of ablation energy if the degree of contact is below a preset contact threshold. Alternatively, the control signal may generate a control signal that activates delivery of ablation energy if the degree of contact is above a preset contact threshold. The method may also generate a control signal that deactivates delivery of ablation energy if the degree of contact is above a preset maximum value. When two sensors are used, the outputs can be compared such that an assessment may be made to determine whether the force is a lateral force. More particularly, the outputs may be compared in terms of impedance; resistance; capacitance; current and/or voltage. Multiple reference points may also be recorded. For example, the devices being used may be subjected to a first known amount of pressure such that the resulting output signal may be measured. This may be repeated for additional known forces. Then, if a unknown force is applied, the measurement information stored in data may be used to assess the degree of contact. 
         [0012]    An object of the present invention is to provide a contact sensor assembly that can assess contact with tissue based on the degree of pressure that is exerted on the sensor. 
         [0013]    Another object of the present invention is to provide a flexible contact sensor that measures pressure that is being exerted on the sensor based on direct or indirect contact between the sensor and another mass, such as tissue. 
         [0014]    Yet another object of the present invention is to provide a method of contact sensing. 
         [0015]    Yet another object of the present invention is to provide a method of manufacturing a contact sensor. 
         [0016]    An objective of the present invention is to provide a pressure-sensitive, capacitive or conductive composite-based sensor(s) that may be used in connection with ablation energy-dispensing therapy procedures, such as for example RF ablation treatment. 
         [0017]    Another objective of the present invention is to provide a catheter having at least one tactile sensor that can assess whether sufficient contact exists between an ablation electrode and tissue to be ablated before ablation begins. 
         [0018]    Yet another objective of the present invention is to provide a catheter having multiple tactile, or pressure-sensitive, sensors that can assess a direction and magnitude of the forces being applied to a distal portion of the catheter such as a tip electrode. 
         [0019]    Still another objective of the present invention is to provide a tactile force sensor that can measure the force asserted on an electrode by soft tissue. 
         [0020]    Still another objective of the present invention is to provide a tactile force sensor that can assess contact based on resistance measurements using a PSCC sensor. 
         [0021]    Still another objective of the present invention is to provide a tactile force sensor that can assess contact based on capacitance measurements using a capacitance sensor. 
         [0022]    Still another objective of the present invention is to provide a tactile force sensor that can assess contact based on measurements using a piezoelectric sensor. 
         [0023]    An objective of the present invention is to provide a QTC-based sensor that may be used in connection with RF ablation treatment. 
         [0024]    Another object of the present invention is to provide a flexible, contact-sensitive sensor that can be used in a wide variety of tissue environments. 
         [0025]    Yet another objective of this invention is to provide a method for practicing medical procedures using a pressure-sensitive, conductive polymer-based sensor in accordance with the teachings herein. 
         [0026]    An advantage of using a PSCC in a contact sensor is that the design may be significantly less complicated, which permits reduced manufacturing costs and increased reliability. 
         [0027]    The foregoing and other aspects, features, details, utilities, and advantages of the present invention will be apparent from reading the following description and claims, and from reviewing the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIGS. 1A and 1B  are perspective views of a representative embodiment of the present invention, illustrating how the present invention may be used to assess contact with tissue. 
           [0029]      FIG. 2  is a side view drawing of an exemplary catheter having a PSCC sensor. 
           [0030]      FIGS. 3A and 3B  are cross sectional views that demonstrate the contact pressure at the sensor-tissue interface. 
           [0031]      FIGS. 4A and 4B  are cross-sectional views of a preferred embodiment of a catheter having a PSCC sensor. 
           [0032]      FIGS. 5A and 5B  are cross-sectional views of a preferred embodiment in which the PSCC sensor is in the shape of a helix. 
           [0033]      FIGS. 6A and 6B  are cross-sectional views of another preferred embodiment in which the PSCC sensor is located about an inner conductive core. 
           [0034]      FIGS. 7A and 7B  are cross-sectional views of another preferred embodiment in which the PSCC sensor is in the shape of a mesh. 
           [0035]      FIGS. 8A and 8B  are cross-sectional views of another preferred embodiment in which the PSCC sensor is formed as an outer substrate layer. 
           [0036]      FIGS. 9A and 9B  are cross-sectional views of a preferred embodiment of a catheter having a PSCC sensor. 
           [0037]      FIGS. 10A and 10B  are cross-sectional views of another preferred embodiment in which the PSCC sensor is in the shape of a helix. 
           [0038]      FIGS. 11A and 11B  are cross-sectional views of another preferred embodiment in which the PSCC sensor is located about an inner conductive core. 
           [0039]      FIGS. 12A and 12B  are cross-sectional views of another preferred embodiment in which the PSCC sensor is in the shape of a mesh. 
           [0040]      FIGS. 13A and 13B  are cross-sectional views of another preferred embodiment in which the PSCC sensor is formed as an outer substrate layer. 
           [0041]      FIG. 14  is a cross-sectional view of a preferred embodiment having a single tactile sensor. 
           [0042]      FIGS. 15 and 16  are cross-sectional views of a preferred embodiment having two tactile sensors. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0043]    An ablation electrode having at least one tactile sensor is disclosed, together with a method of using and a method of manufacturing the ablation electrode. The present invention utilizes tactile sensors of three basic types: pressure PSCC sensors; capacitance sensors; and piezoelectric sensors. 
         [0044]    As noted hereinabove, when used in this application, the terms “pressure sensitive conductive composite” (and the acronym “PSCC”) are used to indicate a pressure sensitive conductive composite that has unique electrical properties as follows: the electrical resistance of the PSCC varies inversely in proportion to the pressure that is applied to the PSCC. The PSCC material that is most useful with the present invention has a high electrical resistance when not under stress (that is, in a quiescent state), and yet the same PSCC material starts to become conductive under pressure, and indeed, the electrical resistance may fall to less than one ohm (1Ω) when under sufficient pressure. When in a quiescent state, the PSCC material preferably has a resistance that is greater than 100,000 ohms, and more preferably, greater than about 1M ohms, and most preferably, the PSCC material is a non-conductor in its quiescent state (e.g., having a resistance greater than 10M ohms). Preferably, the PSCC material will also meet cytotoxity, hemolysis, systemic toxicity and intracutaneous injection standards. 
         [0045]    The present invention will work with different PSCC materials. For example, U.S. Pat. No. 6,999,821 (which is incorporated by reference herein as if fully set forth below) discloses a conductor-filled polymer that may be useful in the present invention. As disclosed therein, conductor-filled polymers may include presently available materials approved for implantation in a human body such as silicone rubber with embedded metallic, carbon or graphite particles or powder. Silver filled silicone rubbers of the kind manufactured by NuSil or Specialty Silicone Products, modified so as to be approved for implantation, are of potential utility. An example is silver-coated, nickel-filled silicone rubber sold as NuSil R2637. The substrate need not be silicone; for example, it is contemplated that other insulating or weakly conductive materials (e.g., non-conductive elastomers) may be embedded with conductive materials, conductive alloys and/or reduced metal oxides (e.g., using one or more of gold, silver, platinum, iridium, titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicone, tin, chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, palladium, osmium, rhenium, technetium, rhodium, ruthenium, cadmium, copper, zinc, germanium, arsenic, antimony, bismuth, boron, scandium and metals of the lanthanide and actinide series and if appropriate, at least one electroconductive agent). The conductive material may be in the form of powder, grains, fibers or other shaped forms. The oxides can be mixtures comprising sintered powders of an oxycompound. The alloy may be conventional or for example titanium boride. 
         [0046]    Other examples of an acceptable PSCCs for use in the present invention include quantum tunneling composites (as abbreviated herein, “QTC”), such as those available through Peratech Ltd. (of Darlington, UK), including the QTC pill, the QTC substrate and the QTC cables. The QTC materials designed by Peratech Ltd. have variable resistance values that range from greater than 10M ohms (in the absence of stress) to less than 1 ohm when under pressure. Ideally, the QTC would meet cytotoxity, hemolysis, systemic toxicity and intracutaneous injection standards. 
         [0047]    Other examples of PSCC materials that may be used in the present invention include the conductive polymers described and disclosed in U.S. Pat. Nos. 6,646,540 (“Conductive Structures”); 6,495,069 (“Polymer Composition”); and 6,291,568 (“Polymer Composition”); all of the foregoing patents are incorporated by reference as if set forth below in their entireties. These materials are described as having a variable resistance of greater than 10 12  Ohms before any stress is applied to less than 1 ohm when finger pressure is applied. 
         [0048]    As a result of this unique property, PSCC materials may be described as having an ability to transform from an effective insulator to a metal-like conductor when deformed by compression, twisting, or stretching. The electrical response of a PSCC can be tuned appropriately to the spectrum of pressures being applied. Its resistance range often varies from greater than 10 MΩ to less than 1Ω. The transition from insulator to conductor often follows a smooth and repeatable curve, with the resistance dropping monotonically to the pressure applied. Moreover, the effect is reversible in the sense that once the pressure is removed, the electrical resistance is also restored. Thus, a PSCC may be transformed from an insulator to a conductor, and back to an insulator, simply by applying the appropriate pressure. PSCCs have been known to carry large currents (up to 10 Amps) and support large voltages (40 V and higher). 
         [0049]    Preferably, the PSCC being used in connection with the present invention can transform from an insulator (that is, conducting little or no current) to an effective conductor simply by applying a small change in pressure to the PSCC. For example, by applying pressure with a hand, or more particularly, with a finger, a surgeon can transform the PSCC from an insulator to a conductor to permit contact sensing. 
         [0050]    The PSCC used in the present invention may also be chosen or customized to be of a specific pressure sensitivity such that the transformation from an insulator to a conductor occurs over a wide or narrow range of pressure. For example, highly sensitive PSCCs, which register a sharp change in resistance with a small amount of applied pressure, may be preferred for soft contact applications such as the atrial wall. Less sensitive PSCCs, which require more pressure to register the same amount of change in resistance, may be preferred for hard contact applications such as ablation in ventricular walls. 
         [0051]    The unique properties of a PSCC permit the creation of novel and pressure-sensitive current-control devices for evaluating tissue contact. The unique properties also permit the creation of novel and pressure-sensitive sensors to assess contact between the sensors and tissue that may be the subject of ablation. 
         [0052]    Capacitance sensors utilize a probe that senses changes in capacitance to assess contact. Typically, driver electronics are used to convert the changes in capacitance into voltage changes, such that a device can indicate and/or record the resulting voltage change. In its most basic form, a capacitor consists of two conductive plates separated by a dielectric medium. The capacitor stores energy in the form of an electric field, and the ability to store energy is measured in capacitance. A capacitance sensor monitors capacitance which will vary in response to a stimuli such as touch. A force on a capacitance sensor typically reduces the sensor&#39;s ability to store energy, resulting in a measurable change. With a capacitance sensor, the sensor surface is the electrified plate and when pressure is applied, the resulting change in capacitance can be measured and quantified. 
         [0053]    Piezoelectric sensors utilize a piezoelectric material, which generates an electrical voltage when the material is placed under stress. A piezoelectric sensor can be used to measure the voltage that results when a piezoelectric material is placed under strain. Piezoelectric materials can be made in a variety of forms, including for example, piezoelectric wire, piezoelectric film, and piezoelectric tubes. 
         [0054]      FIGS. 1A and 1B  illustrate a sample embodiment of the present invention. As illustrated in  FIGS. 1A and 1B , PSCC contact sensor  105  includes a catheter shaft  90  and a contact surface  100  that extends from catheter shaft  90 . In this embodiment, contact sensor  105  is flexible such that when it comes into contact with tissue  12 , contact sensor  105  is deflected in direction  18  as illustrated in  FIG. 1   b , and the deflection permits the degree of contact between contact sensor  105  and tissue  12  to be assessed. 
         [0055]      FIG. 2  is a close-up of the sample embodiment depicted in  FIGS. 1A and 1B .  FIG. 2  illustrates cross-sectional reference lines A-A and B-B, which will be used to illustrate preferred embodiment of the present invention. 
         [0056]    As illustrated in  FIG. 3A , when the PSCC sensor is in a relatively contact free environment (such as air, or in the flowing blood stream while inside a blood vessel or heart chamber), the PSCC is an insulator. When used for a sensing application, however, the PSCC sensor is placed against tissue as illustrated in  FIG. 3B . As the contact pressure increases, the PSCC becomes conductive and permits the degree of contact to be assessed by the sensing device. Because of the unique properties of a PSCC, only that portion of the PSCC sensor that is in contact with the tissue becomes conductive. Those portions which are not in direct contact with the tissue, such as the region facing the blood, remain non-conductive, thereby mitigating any current leakage that may cause coagulum and thrombus formation. 
         [0057]    The resistance of a PSCC sensor changes anisotropically, based on the variation of the contact pressure on the PSCC sensor. Thus, as illustrated in  FIG. 3B , the contact pressure at the sensor-tissue interface is maximum at the point (or line) of normal incidence and gradually decreases along the arc of contact to zero at the edge of the contact. Because of its ability to detect stress forces in any direction, the sensor can be designed to be omni-directional in use. 
         [0058]      FIGS. 4A and 4B  illustrate a preferred embodiment of the present invention, revealing two cross sectional drawings taken along the reference lines of A-A and B-B as labeled in  FIG. 2 . 
         [0059]    In  FIGS. 4A and 4B , PSCC contact sensor  110  includes a catheter shaft  90  and a contact surface  100  that extends from catheter shaft  90 . Catheter shaft  90  may be either conductive or non-conductive, and preferably, catheter shaft  90  is non-conductive. In this embodiment, the PSCC forms the working surface of the sensor that is used for contact assessment. As depicted in  FIGS. 4A and 4B , PSCC sensor  110  comprises: flexible inner conductive core  111 ; and an outer PSCC substrate layer  112 , which is mechanically and electrically coupled to the flexible inner conductive core  111 . Flexible inner conductive core  111  may include a flat top (like the top of a right cylinder), or optionally it may include a portion of a sphere on its distal end as illustrated in  FIG. 4A . Flexible inner conductive core  111  may be connected to an electrical conductor  114 , which may be connected to an analyzer (not shown). In use, this preferred embodiment is used to assess contact between PSCC sensor  110  and tissue (not shown) to which a reference electrode (not shown) has been attached. PSCC sensor  110  assesses the contact between contact surface  100  and the subject tissue by monitoring the electrical characteristics between two nodes, namely, the reference electrode (not shown) and the flexible inner conductive core  111  (which is preferably measured using electrical conductor  114 ). By way of example, an analyzer (such as an impedance, resistance, capacitance or other electrical measurement device) may be used to measure the electrical characteristics present on electrical conductor  114  relative to the reference electrode (not shown) secured to the tissue being contacted with PSCC sensor  110 . Preferably, the reference electrode is grounded to an electrical ground reference signal. 
         [0060]      FIGS. 5A and 5B  illustrate another preferred embodiment of the present invention, revealing two cross sectional drawings taken along the reference lines of A-A and B-B as labeled in  FIG. 2 . PSCC sensor  120  extends from a catheter shaft  90 , and PSCC sensor  120  comprises: flexible inner conductive coil  121  in the shape of a helix; and a PSCC substrate layer  122  within which the inner conductive coil  121  is located. Flexible inner conductive coil  121  is connected to an electrical conductor  114 , which may be connected to an analyzer (not shown). In use, this preferred embodiment is used to assess contact between PSCC sensor  120  and tissue (not shown) to which a reference electrode (not shown) has been attached. PSCC sensor  120  assesses the contact between contact surface  100  and the subject tissue by monitoring the electrical characteristics between two nodes, namely, the reference electrode (not shown) and the flexible inner conductive coil  121  (which is preferably measured using electrical conductor  114 ). By way of example, an analyzer (such as an impedance, resistance, capacitance or other electrical measurement device) may be used to measure the electrical characteristics present on electrical conductor  114  relative to the reference electrode (not shown) secured to the tissue being contacted with PSCC sensor  120 . Preferably, the reference electrode is grounded to an electrical ground reference signal. 
         [0061]      FIGS. 6A and 6B  illustrate yet another preferred embodiment of the present invention, revealing two cross sectional drawings taken along the reference lines of A-A and B-B as labeled in  FIG. 2 . PSCC sensor  130  extends from a catheter shaft  90 , and PSCC sensor  130  comprises: flexible inner conductive coil  131  in the shape of a helix; an outer PSCC substrate layer  132 ; and an electrically insulative flexible shaft  133  located within the helix of the flexible inner conductive coil  131 . Flexible shaft  133  may optionally include a portion of a sphere on its distal end as shown in  FIG. 6A . Flexible inner conductive coil  131  is connected to an electrical conductor  114 , which may be connected to an analyzer (not shown). In use, this preferred embodiment is used to assess contact between PSCC sensor  130  and tissue (not shown) to which a reference electrode (not shown) has been attached. PSCC sensor  130  assesses the contact between contact surface  100  and the subject tissue by monitoring the electrical characteristics between two nodes, namely, the reference electrode (not shown) and the flexible inner conductive coil  131  (which is preferably measured using electrical conductor  114 ). By way of example, an analyzer (such as an impedance, resistance, capacitance or other electrical measurement device) may be used to measure the electrical characteristics present on electrical conductor  114  relative to the reference electrode (not shown) secured to the tissue being contacted with PSCC sensor  130 . Preferably, the reference electrode is grounded to an electrical ground reference signal. 
         [0062]      FIGS. 7A and 7B  illustrate yet another preferred embodiment of the present invention, revealing two cross sectional drawings taken along the reference lines of A-A and B-B as labeled in  FIG. 2 . PSCC sensor  140  extends from a catheter shaft  90 , and PSCC sensor  140  comprises: flexible inner conductive sheath  141  formed of a mesh; an outer PSCC substrate layer  142 ; and an electrically insulative flexible shaft  143  located interiorly of the flexible inner conductive sheath  141 . Flexible shaft  143  may optionally include a portion of a sphere at its distal end as shown in  FIG. 7A . Flexible sheath  141  is connected to an electrical conductor  114 , which may be connected to an analyzer (not shown). In use, this preferred embodiment is used to assess contact between PSCC sensor  140  and tissue (not shown) to which a reference electrode (not shown) has been attached. PSCC sensor  140  assesses the contact between contact surface  100  and the subject tissue by monitoring the electrical characteristics between two nodes, namely, the reference electrode (not shown) and the flexible sheath  141  (which is preferably measured using electrical conductor  114 ). By way of example, an analyzer (such as an impedance, resistance, capacitance or other electrical measurement device) may be used to measure the electrical characteristics present on electrical conductor  114  relative to the reference electrode (not shown) secured to the tissue being contacted with PSCC sensor  140 . Preferably, the reference electrode is grounded to an electrical ground reference signal. 
         [0063]      FIGS. 8A and 8B  illustrate yet another preferred embodiment of the present invention, revealing two cross sectional drawings taken along the reference lines of A-A and B-B as labeled in  FIG. 2 . PSCC sensor  150  extends from a catheter shaft  90 , and PSCC sensor  150  comprises: an electrically insulative flexible shaft  153 ; a flexible inner conductive layer  151  (formed, for example, as a coating and/or wrap around flexible shaft  153 ); and an outer PSCC substrate layer  152 . Electrically insulative flexible shaft  153  and flexible inner conductive layer  151  may optionally include a portion of a sphere at their respective distal ends (as illustrated in  FIG. 8A ). Flexible inner conductive core  151  is connected to an electrical conductor  114 , which may be connected to an analyzer (not shown). In use, this preferred embodiment is used to assess contact between PSCC sensor  150  and tissue (not shown) to which a reference electrode (not shown) has been attached. PSCC sensor  150  assesses the contact between contact surface  100  and the subject tissue by monitoring the electrical characteristics between two nodes, namely, the reference electrode (not shown) and the flexible inner conductive core  151  (which is preferably measured using electrical conductor  114 ). By way of example, an analyzer (such as an impedance, resistance, capacitance or other electrical measurement device) may be used to measure the electrical characteristics present on electrical conductor  114  relative to the reference electrode (not shown) secured to the tissue being contacted with PSCC sensor  150 . Preferably, the reference electrode is grounded to an electrical ground reference signal. 
         [0064]      FIGS. 9A and 9B  illustrate a preferred embodiment of the present invention, revealing two cross sectional drawings taken along the reference lines of A-A and B-B as labeled in  FIG. 2 .  FIG. 9A  is a variation of the preferred embodiment illustrated in  FIG. 4A . In  FIGS. 9A and 9B , PSCC contact sensor  110 ′ includes a catheter shaft  90  and a contact surface  100  that extends from catheter shaft  90 . Catheter shaft  90  may be either conductive or non-conductive, and preferably, catheter shaft  90  is non-conductive. As depicted in  FIG. 9A , PSCC sensor  110 ′ comprises: flexible inner conductive core  111 ; and an outer PSCC substrate layer  112 , which is mechanically and electrically coupled to the flexible inner conductive core  111 . Flexible inner conductive core  111  may optionally include a portion of a sphere on its distal end, as illustrated in  FIG. 9A . Flexible inner conductive core  111  may be connected to an electrical conductor  114 , which may be connected to an analyzer (not shown). PSCC substrate layer  112  is covered by a conductive outer layer  119 , which may be connected to an electrical conductor  116 ; conductive outer layer  119  may be flexible, rigid, or it may offer an intermediate degree of flexibility. In use, this preferred embodiment is used to assess contact between PSCC sensor  110 ′ and tissue by monitoring the electrical characteristics between two nodes, namely, the conductive outer layer  119  (which is preferably measured using electrical conductor  116 ) and the flexible inner conductive core  111  (which is preferably measured using electrical conductor  114 ). By way of example, an analyzer (such as an impedance, resistance, capacitance or other electrical measurement device) may be used to measure the electrical characteristics present on electrical conductor  114  relative to electrical conductor  116 . 
         [0065]      FIGS. 10A and 10B  illustrate another preferred embodiment of the present invention, revealing two cross sectional drawings taken along the reference lines of A-A and B-B as labeled in  FIG. 2 .  FIG. 10A  is a variation of the preferred embodiment illustrated in  FIG. 5A . PSCC sensor  120 ′ extends from a catheter shaft  90 , and PSCC sensor  120 ′ comprises: flexible inner conductive coil  121  in the shape of a helix; and a PSCC substrate layer  122  within which the inner conductive coil  121  is located. Flexible inner conductive coil  121  is connected to an electrical conductor  114 , which may be connected to an analyzer (not shown). PSCC substrate layer  112  is covered by a conductive outer layer  119 , which may be connected to an electrical conductor  116 ; conductive outer layer  119  may be flexible, rigid, or it may offer an intermediate degree of flexibility. In use, this preferred embodiment is used to assess contact between PSCC sensor  120 ′ and tissue by monitoring the electrical characteristics between two nodes, namely, the conductive outer layer  119  (which is preferably measured using electrical conductor  116 ) and the flexible inner conductive coil  121  (which is preferably measured using electrical conductor  114 ). By way of example, an analyzer (such as an impedance, resistance, capacitance or other electrical measurement device) may be used to measure the electrical characteristics present on electrical conductor  114  relative to electrical conductor  116 . 
         [0066]      FIGS. 11A and 11B  illustrate yet another preferred embodiment of the present invention, revealing two cross sectional drawings taken along the reference lines of A-A and B-B as labeled in  FIG. 2 .  FIG. 11A  is a variation of the preferred embodiment illustrated in  FIG. 6A . PSCC sensor  130 ′ extends from a catheter shaft  90 , and PSCC sensor  130 ′ comprises: flexible inner conductive coil  131  in the shape of a helix; an outer PSCC substrate layer  132 ; and an electrically insulative flexible shaft  133  located within the helix of the flexible inner conductive coil  131 . Flexible shaft  133  may optionally include a portion of a sphere on its distal end as shown in  FIG. 11A  Flexible inner conductive coil  131  is connected to an electrical conductor  114 , which may be connected to an analyzer (not shown). PSCC substrate layer  112  is covered by a conductive outer layer  119 , which may be connected to an electrical conductor  116 ; conductive outer layer  119  may be flexible, rigid, or it may offer an intermediate degree of flexibility. In use, this preferred embodiment is used to assess contact between PSCC sensor  130 ′ and tissue by monitoring the electrical characteristics between two nodes, namely, the conductive outer layer  119  (which is preferably measured using electrical conductor  116 ) and the flexible inner conductive coil  131  (which is preferably measured using electrical conductor  114 ). By way of example, an analyzer (such as an impedance, resistance, capacitance or other electrical measurement device) may be used to measure the electrical characteristics present on electrical conductor  114  relative to electrical conductor  116 . 
         [0067]      FIGS. 12A and 12B  illustrate yet another preferred embodiment of the present invention, revealing two cross sectional drawings taken along the reference lines of A-A and B-B as labeled in  FIG. 2 .  FIG. 12A  is a variation of the preferred embodiment illustrated in  FIG. 7A . PSCC sensor  140 ′ extends from a catheter shaft  90 , and PSCC sensor  140 ′ comprises: flexible inner conductive sheath  141  formed of a mesh; an outer PSCC substrate layer  142 ; and an electrically insulative flexible shaft  143  located interiorly of the flexible inner conductive sheath  141 . Flexible shaft  143  may optionally include a portion of a sphere at its distal end as shown in  FIG. 7A . Flexible sheath  141  is connected to an electrical conductor  114 , which may be connected to an analyzer (not shown). PSCC substrate layer  112  is covered by a conductive outer layer  119 , which may be connected to an electrical conductor  116 ; conductive outer layer  119  may be flexible, rigid, or it may offer an intermediate degree of flexibility. In use, this preferred embodiment is used to assess contact between PSCC sensor  140 ′ and tissue by monitoring the electrical characteristics between two nodes, namely, the conductive outer layer  119  (which is preferably measured using electrical conductor  116 ) and the flexible sheath  141  (which is preferably measured using electrical conductor  114 ). By way of example, an analyzer (such as an impedance, resistance, capacitance or other electrical measurement device) may be used to measure the electrical characteristics present on electrical conductor  114  relative to electrical conductor  116 . 
         [0068]      FIGS. 13A and 13B  illustrate yet another preferred embodiment of the present invention, revealing two cross sectional drawings taken along the reference lines of A-A and B-B as labeled in  FIG. 2 .  FIG. 13A  is a variation of the preferred embodiment illustrated in  FIG. 8A . PSCC sensor  150 ′ extends from a catheter shaft  90 , and PSCC sensor  150 ′ comprises: an electrically insulative flexible shaft  153 ; a flexible inner conductive layer  151  (formed, for example, as a coating and/or wrap around flexible shaft  153 ); and an outer PSCC substrate layer  152 . Electrically insulative flexible shaft  153  and flexible inner conductive layer  151  may optionally include a portion of a sphere at their respective distal ends (as illustrated in  FIG. 13A ). Flexible inner conductive core  151  is connected to an electrical conductor  114 , which may be connected to an analyzer (not shown). PSCC substrate layer  112  is covered by a conductive outer layer  119 , which may be connected to an electrical conductor  116 ; conductive outer layer  119  may be flexible, rigid, or it may offer an intermediate degree of flexibility. In use, this preferred embodiment is used to assess contact between PSCC sensor  150 ′ and tissue by monitoring the electrical characteristics between two nodes, namely, the conductive outer layer  119  (which is preferably measured using electrical conductor  116 ) and the flexible inner conductive core  151  (which is preferably measured using electrical conductor  114 ). By way of example, an analyzer (such as an impedance, resistance, capacitance or other electrical measurement device) may be used to measure the electrical characteristics present on electrical conductor  114  relative to electrical conductor  116 . 
         [0069]    Electrical conductors  114  and  116  may be implemented using a single conductive wire or multiple strands of wire. Preferably, the wires may be made of flexible conductive materials which allow the surface contacting area to be bent and formed into various shapes to provide better contact to the tissue. Acceptable materials include, but are not limited to, stainless steel, nickel titanium (nitinol), tantalum, copper, platinum, iridium, gold, or silver, and combinations thereof. Preferably, the material used to manufacture the conductive element is a bio-compatible electrically conductive material, such as platinum, gold, silver, nickel titanium, and combinations thereof. Other electrically conductive materials coated with bio-compatible materials may also be employed, including for example, gold-plated copper. Finally, it is also contemplated that electrically conductive polymers may also be used provided they are bio-compatible or coated with a bio-compatible material. 
         [0070]    A further embodiment of the present invention is disclosed in connection with  FIG. 14 , namely, an ablation electrode assembly  200 , which includes catheter shaft  290 , ablation electrode  280  and tactile sensor  270  positioned there between. Catheter shaft is typically formed with a non-electrically conductive outer layer and may have one or more lumens internally of the shaft. Ablation electrode  280  may be formed of a wide variety of materials including, but not limited to, stainless steel, nickel titanium (nitinol), tantalum, copper, platinum, iridium, gold, or silver, and combinations thereof. Preferably, the material used to manufacture the ablation electrode is a bio-compatible electrically conductive material, such as platinum, gold, silver, nickel titanium, and combinations thereof. Other electrically conductive materials coated with bio-compatible materials may also be employed, including for example, gold-plated copper. Finally, it is also contemplated that electrically conductive polymers may also be used provided they are bio-compatible or coated with a bio-compatible material. 
         [0071]    In a typical operation, ablation assembly  200  may be used to ablate cardiac tissue, and thus, ablation electrode  280  may be pressed into contact with the myocardium. When the ablation electrode  280  is in sufficient contact with the myocardium, the myocardium exerts a force  210  to the ablation electrode  280 , mostly along the longitudinal axis. The force is delivered by ablation electrode  280  to tactile sensor  270 , which is preferably soft and sufficiently sensitive to measure the small force applied to ablation electrode  280 . 
         [0072]    Tactile sensor  270  may be one of three types of sensors: a PSCC sensor; a capacitance sensor; and a piezoelectric sensor. A PSCC sensor may utilize any number of the PSCC materials and embodiments described above. Preferably the sensor includes, or may be coupled to, a device for measuring the resistance of the tactile sensor  270 . Of course, a capacitance sensor and/or a piezoelectric sensor may be used, in which case the sensor preferably includes, or may be coupled to, a device for measuring the capacitance and/or voltage of the tactile sensor  270 . As described above, the three types of sensors work on different physical principles. For example, a PSCC material responds to pressure such that its resistance (or impedance) changes, and may transform from a non-conductor to a conductor. A capacitance sensor changes it capacitance based on pressure, and similarly a piezoelectric sensor varies its output voltage based on the degree of pressure applied to the surface of the sensor. 
         [0073]    In many applications, the ablation catheter  200  will be placed in contact with a tissue surface such that the ablation catheter is orthogonal to the tissue surface, resulting in an axial force  210  being applied to the ablation electrode  280 . When the force is axial, a single tactile sensor  270  will often be sufficient to assess the contact between ablation electrode  280  and the tissue to be ablated. 
         [0074]    For example, if tactile sensor  270  is a PSCC sensor, then the force  210  will cause the resistance of tactile sensor  270  to drop, and the extent to which it decreases may be used to assess the degree of contact between ablation electrode  280  and the tissue being treated. Similarly, if tactile sensor  270  is a capacitance sensor, then the force  210  will cause the capacitance of tactile sensor  270  to drop, and the extent to which it decreases may be used to assess the degree of contact between ablation electrode  280  and the tissue being treated. If tactile sensor  270  is a piezoelectric sensor, then the force  210  will cause the voltage generated by tactile sensor  270  to change (depending on the configuration, it may increase or decrease), and the extent of the change may be used to assess the degree of contact between ablation electrode  280  and the tissue being treated. 
         [0075]    In other applications, it is possible that the force applied to the catheter is a transverse force, in which case a single tactile sensor  270  as illustrated in ablation electrode  200  may be inadequate to assess the contact. 
         [0076]      FIGS. 15-16  depict an ablation electrode  300  with two tactile sensors  371 ,  372 . First tactile sensor  371  and second tactile sensor  372  are positioned side by side, which is very useful in practice. When axial force  310  is applied as illustrated in  FIG. 15 , both tactile sensors  371 ,  372  are compressed by compressing forces  315 ,  316 , and if axial force  310  is perfectly axial, then tactile sensors will experience approximately equal compressing forces. 
         [0077]    When transverse force  311  is applied as illustrated in  FIG. 16 , tactile sensors  371 ,  372  are affected differently. As illustrated, first tactile sensor  371  will experience a pulling or stretching (that is, tensile) force  317 , whereas second tactile sensor  372  will experience a compression force  318 . Depending on the type of tactile sensor being used, the compression and stretching forces could result in changes that move in opposite directions, and a measurement device (such as a computer or other processor) can deduce useful information about these changes. For example, as illustrated in  FIG. 16 , detecting a tensile force  317  on first tactile sensor  371 , while simultaneously detecting a compression force  318  on second tactile sensor  372 , will permit the measurement device to determine that a lateral force  311  is being applied (based on opposite forces being detected) and further that the direction is downward (or more particularly, in a direction of travel from first tactile sensor  371  to second tactile sensor  372 . Thus, the use of two tactile sensors permits the device to distinguish between an upward lateral force, a downward lateral force, as well as an axial force (relatively equal forces being applied to both tactile sensors). 
         [0078]    Applying the teachings herein, one of ordinary skill would appreciate that additional tactile sensors could be employed in the ablation electrode, in which case, the electrode could glean additional directional content out of the applied forces. By way of example, and without limiting the number of tactile sensors to be used with the present invention, a catheter may be implemented using four tactile sensors, each arranged to be located within a quadrant of the electrode (or in other words, being spaced about a circumference and about 90 degrees apart). Such an arrangement would permit the assessment of forces in at least three directions. 
         [0079]    Of course, the tactile sensors used by the present inventions will also permit one to determine the magnitude of forces being applied to the ablation electrode. Generally, the change effected in the electrical characteristics of the tactile sensor will vary proportionately with the force being applied. When used in this context, the term “proportional” in intended to be construed broadly to encompass all proportionality relationships and constants. 
         [0080]    It is also contemplated that the present invention may monitor the impedance of a tactile sensor, for example, in the event that a measurement device applied an alternating voltage to a PSCC sensor. The teachings above would be easily applied to impedance measurements. 
         [0081]    In operation, any of the devices above could be used to effect an ablation treatment. For example, the ablation device depicted in  FIG. 14  would be placed in contact with a tissue surface to permit the degree of contact to be assessed by measuring one of the resistance, capacitance, voltage, and/or impedance. Based on the measured electrical characteristic, the device could readily generate a signal that is indicative of a degree of contact that exists between the catheter and the tissue. Further, if the measured characteristics were deemed to be associated with a pressure that is below a minimum pressure threshold, a control signal could be generated to preclude ablation (e.g., inhibit the generator&#39;s output of ablation energy). Similarly, if the pressure were deemed to be above a particular threshold (for example, because the resistance of a PSCC material had dropped too low), then a signal could be generated that would inhibit ablation. 
         [0082]    In some circumstances it may be permissible to preclude ablation based on the orientation of the ablation electrode to the tissue. For example, if it is determined that a bending force is being applied to the ablation electrode, then the electrode may not have a desired angle of contact with the surface to be ablated. At such an angle, ablation may not create the proper lesion. Thus, ablation could be inhibited until a proper angle of contact is detected. 
         [0083]    The present invention permits the construction of a flexible, pressure sensitive contact assessment device that can be used in a wide variety of different tissue environments, including for example, tissues having varying degrees of elasticity and contour. 
         [0084]    The present invention permits the construction of a flexible sensor to measure pressure that is applied to the sensor, for example, pressure that may be applied to the sensor by the myocardium. Such sensors may be used to measure the pressure that is applied directly to the sensor, or depending on the configuration of the sensor, it may measure the pressure that is applied to a component that is in contact with the sensor (as may be the case when an additional element is disposed between a PSCC-based sensor and tissue that is exerting pressure on the additional element). In the case where a PSCC-based sensor is positioned within a catheter, the PSCC-based sensor is preferably used to measure pressure that is applied axially to catheter. Of course, the PSCC based sensor could be oriented in order to measure pressure that is applied transversely to the catheter. 
         [0085]    While the preferred embodiments disclosed in the attached figures disclose a contact sensor that is generally cylindrical in shape, the present invention also contemplates that the contact sensor may be formed into various shapes to better fit the contour of the target tissue. In one embodiment, for example, the contact sensor can be made long enough to strap around and form a noose around the pulmonary veins in epicardial applications. Particularly, the conductive element that is coupled to the PSCC (for example, reference numbers  111 ,  121 ,  131 ,  141 , and  151 ) may be formed into a desired shape and then the PSCC layer will be formed over the conductive element in the preferred shape. For example, the contact sensor may be shaped like a spatula for certain applications, including for example, minimally invasive sub-xyphoid epicardial applications, where the spatula shape will permit easy placement and navigation in the pericardial sac. Because PSCC can be made to be a flexible material, it can be used to form electrodes having a great variety of shapes, including a spatula. 
         [0086]    Alternatively, the conductive element that is coupled to the PSCC may be formed using shape-memory retaining material, such as nitinol, which would permit the electrode to be fitted to specific preset geometries, such as the ostium of a pulmonary vein, such that the electrode is shaped to provide a desired contact pressure pattern on the tissue due to the deformation of the wire when pressed against the tissue. 
         [0087]    Similarly, while the reference to insulative shaft (for example,  133 ,  143 , and  153 ) is generally used in connection with a generally cylindrical member, it is contemplated by the present invention that the insulative shaft could be in a geometric shape other than a cylinder, including, for example, a noose, a spatula, or the shape of the ostium of a pulmonary vein. For purposes of this application, the term “insulative shaft” is intended to encompass shapes in addition to a cylindrical shaft. 
         [0088]    Whenever it is desired that the conductive element that is coupled to the PSCC may be formed in the shape of a helix, such as is the case with elements  121 , and  131 , the coil may be chosen to be of a specific stiffness (i.e., having a characteristic spring constant) that would allow the coil to exert a desired amount of pressure on the PSCC when the electrode bends or deflects upon contact with the tissue. One of skill in the art would understand that the degree of desired contact pressure would depend in part upon the elastic property of the tissue being contacted with the electrode. For example, the atrial wall may require less contact pressure than the ventricular wall. Thus, electrodes of varying stiffness can be designed for application in different tissues and different regions of the heart. 
         [0089]    In some embodiments, for example, as depicted in  FIGS. 5 ,  6  and  7 , the conductive element may be mounted on an insulative shaft. The conductive element can be shaped in any number of ways, including for example, a coil, mesh, coating or wrap. The insulative shaft provides additional mechanical support in applications that require greater amounts of axial pressure and torque. The insulative shaft may be made of any electrically insulative material, including, for example, polyurethane. Preferably, the insulative shaft is made of a biocompatible, electrically insulative material. 
         [0090]    The embodiments described above can be used with a processor such that the processor may provide more precise information about the pressures being encountered by the embodiment. In particular, any of the sensors described above may be used with a memory device to record information regarding one or more forces that are applied to the sensor. For example, a first known pressure may be applied to the contact sensor and a first measurement of an electrical characteristic may be made such that the first known pressure may be associated with the first measurement. Similarly, a second known pressure may be applied to the contact sensor and a second measurement of an electrical characteristic may be made such that the second known pressure may be associated with the second measurement. Additional known pressures may be applied and additional corresponding measurements may be made and associated. Then, if an unknown pressure is applied, the processor may use the known pressures and their respective associated measurements to help quantify the unknown pressure, for example by interpolating or extrapolating the value of the unknown pressure from the known pressures. 
         [0091]    While the embodiments above are discussed in the context of applied pressure, the embodiments above can also be used to assess forces relative to contact between tissue and the contact sensor. Pressure is simply a measurement of the force per unit area, and thus, to assess force, the surface area of a contact surface must be known or be capable of being determined or calculated. The force information may be derived from the information available on forces and the contact surface area. 
         [0092]    Though not depicted, it is contemplated that each of the embodiments discussed above may optionally be used in connection with one or more electrically-conductive, outer protective coverings. Preferably, the outer covering is electrically conductive, such as a flexible wire mesh, a conductive fabric, a conductive polymer layer (which can be porous or nonporous), or a metal coating. The outer covering may be used to not only increase the mechanical integrity, but to enhance the contact sensor&#39;s ability to assess the tissue contact (for example, when measuring electrical characteristics using a reference electrode connected to the target tissue). In some cases, the outer covering may be made using a biocompatible material in order to help make the overall assembly biocompatible. 
         [0093]    Though not depicted, it is also contemplated that in certain sensor configurations, it may be desirable to optionally use an electrically non-conductive outer protective covering. In such cases, an outer covering that is electrically insulative, such as a non-conductive polymer layer (which can be porous or nonporous), may be used to increase the mechanical integrity. In some cases, the outer covering may be made using a biocompatible material in order to help make the overall assembly biocompatible. Such an electrically-non-conductive covering may also serve as a pressure transfer element to more evenly distribute pressure to the PSCC member. 
         [0094]    One of ordinary skill will appreciate that while the PSCC materials may be designed to respond to a variety of stresses, the principles and embodiments herein may be adapted to respond to specific stress forces, for example, axial forces, orthogonal forces, twisting forces, compressing forces, stretching forces, etc., without deviating from the scope of the present invention. 
         [0095]    While many of the embodiments above are discussed in the context of a PSCC sensor, the same principles can be applied to devices having tactile sensors of a non-PSCC material. 
         [0096]    Although multiple embodiments of this invention have been described above with a certain degree of particularity, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention. All directional references (e.g., upper, lower, upward, downward, left, right, leftward, rightward, top, bottom, above, below, vertical, horizontal, clockwise, and counterclockwise) are only used for identification purposes to aid the reader&#39;s understanding of the present invention, and do not create limitations, particularly as to the position, orientation, or use of the invention. Joinder references (e.g., attached, coupled, connected, and the like) are to be construed broadly and may include intermediate members between a connection of elements and relative movement between elements. As such, joinder references do not necessarily infer that two elements are directly connected and in fixed relation to each other. It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not limiting. Changes in detail or structure may be made without departing from the spirit of the invention as defined in the appended claims.