Patent Publication Number: US-2021169366-A1

Title: Catheter contact force sensor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority under the Paris Convention as well as 35 U.S.C. §§ 119 and 120 to prior filed U.S. Provisional Patent Application No. 62/943,572 filed on Dec. 4, 2019 which is hereby incorporated by reference as set forth in full herein 
    
    
     FIELD 
     The present disclosure relates to instruments for diagnostic and surgical purposes that measure force, pressure, mechanical tension, and/or mechanical compression, and more particularly to catheter-based probes for diagnostic and/or surgical procedures in the heart. 
     BACKGROUND 
     Cardiac arrhythmias, such as atrial fibrillation, occur when regions of cardiac tissue abnormally conduct electric signals to adjacent tissue, thereby disrupting the normal cardiac cycle and causing asynchronous rhythm. 
     Procedures for treating arrhythmia include surgically disrupting the origin of the signals causing the arrhythmia, as well as disrupting the conducting pathway for such signals. By selectively ablating cardiac tissue by application of energy via a catheter, it is sometimes possible to block or modify the propagation of unwanted electrical signals from one portion of the heart to another. The ablation process destroys the unwanted electrical pathways by formation of non-conducting lesions. 
     Various attempts in the art to verify electrode contact with the tissue have been attempted or suggested including utilizing one more pressure transducers (U.S. Pat. Nos. 6,695,808 and 6,241,724), utilizing an electrode to measure electrical activity in the heart (U.S. Pat. No. 6,915,149), utilizing an electromechanical movement sensor (U.S. Patent Application Publication 2007/0100332), measuring impedance between a tip electrode and a return electrode (U.S. Pat. Nos. 5,935,079, 5,836,990, and 5,447,529), and utilizing fluoroscopic imaging (U.S. Patent Publication 2005/0203597). 
     U.S. Patent Application Publication No. 2009/0093806 to Govari et al., which is hereby incorporated by reference in its entirety into this application as if set forth in full and attached in the appendix to priority application U.S. 62/943,572, describes another application of contact pressure measurement, in which deformation in response to pressure on a resilient member located at the distal end of a catheter is measured using a sensor. 
     U.S. Pat. No. 9,168,004 to Gliner, et al., which is hereby incorporated by reference in its entirety into this application as if set forth in full and attached in the appendix to priority application U.S. 62/943,572, describes using machine learning based on impedance between two electrodes to determine catheter electrode contact. 
     U.S. Patent Publication 2018/0256110 to Govari et al., which is hereby incorporated by reference in its entirety into this application as if set forth in full and attached in the appendix to priority application U.S. 62/943,572, describes a flexible probe having a pair of spring coils and a transducer having a transmitter circuit and a receiver circuit situated on either side of the spring coils. The spring coils deform in response to pressure exerted to the distal tip of the probe to cause the transmitter and receiver circuits to move in relation to each other. 
     SUMMARY 
     There is provided, in accordance with some embodiments of the present disclosure a spring assembly usable with an intravascular catheter. The spring assembly can include a tubular body, a first mounting surface configured to engage a substantially planar electrical circuit, a second mounting surface configured to engage a substantially planar electrical circuit, and a compressible framework extending between the first and second mounting surfaces. The tubular body extends along a longitudinal axis. The tubular body is sized to traverse vasculature. The first mounting surface is positioned in the tubular body, perpendicular to the longitudinal axis, and facing a first direction parallel to the longitudinal axis. The second mounting surface is positioned in the tubular body, perpendicular to the longitudinal axis, and facing toward the first mounting surface in a second direction opposite the first direction. 
     In some embodiments, the spring assembly further includes a first opening in the tubular body and a second opening in the tubular body. The first opening is circumscribed by the first mounting surface and the compressible framework. The second opening is circumscribed by the second mounting surface and the compressible framework. The first and second openings can be at least partially collapsible in response to compression of the compressible framework. 
     In some embodiments, the spring assembly further includes one or more openings circumscribed by the compressible framework. The one or more openings circumscribed by the compressible framework can be at least partially collapsible in response to compression of the compressible framework. 
     In some embodiments, the spring assembly further includes third, fourth, fifth, and sixth mounting surfaces each respectively configured to engage a substantially planar electrical circuit and positioned in the tubular body. The first, third, and fifth mounting surfaces are coplanar in a first plane perpendicular to the longitudinal axis. The second, fourth, and sixth mounting surfaces are coplanar in a second plane perpendicular to the longitudinal axis. The third and fourth mounting surfaces are facing toward each other. The fifth and sixth mounting surfaces are facing toward each other. 
     In some embodiments, the spring assembly includes first, second, third, fourth, and fifth openings. The first opening in the tubular body is circumscribed by the first mounting surface and the compressible framework. The second opening in the tubular body is circumscribed by the second mounting surface and the compressible framework. The third opening in the tubular body is circumscribed by the third mounting surface and the compressible framework. The fourth opening in the tubular body is circumscribed by the fourth mounting surface and the compressible framework. The fifth opening in the tubular body is circumscribed by the fifth mounting surface and the compressible framework. The sixth opening in the tubular body is circumscribed by the sixth mounting surface and the compressible framework. The spring assembly can further include three additional openings each respectively circumscribed by the compressible framework. 
     In some embodiments including the first, second, third, fourth, fifth, and sixth mounting surfaces, each mounting surface has substantially similar dimensions to each of the remaining mounting surfaces. 
     In some embodiments including the first, third, and fifth mounting surfaces, the first, third, and fifth mounting surfaces are spaced 120° from each other as measured rotationally in about the longitudinal axis. 
     In some embodiments including the second, fourth, and sixth mounting surfaces, the second, fourth, and sixth mounting surfaces are spaced 120° from each other as measured rotationally about the longitudinal axis. 
     In some embodiments, the first mounting surface includes a plurality of T-shaped indentations thereon, and/or the second mounting surface includes a plurality of T-shaped indentations thereon. Some or all of the mounting surfaces include a plurality of T-shaped indentations thereon. 
     In some embodiments, the spring assembly further includes multiple engagement extensions extending in the first direction from a first end of the tubular body and multiple engagement extensions extending in the second direction from a second end of the tubular body. Each engagement extension on either side of the tubular body includes a protrusion extending therefrom in a circumferential direction about the longitudinal axis. 
     In some embodiments including multiple engagement extensions, the spring assembly includes exactly three engagement extensions extending in the first direction from the first end of the tubular body and exactly three engagement extensions extending in the second direction from the second end of the tubular body. 
     In some embodiments, the spring assembly further includes a first electrical circuit and a second electrical circuit. The first electrical circuit is affixed to the first mounting surface. The first electrical circuit includes a first inductive coil. The second electrical circuit is affixed to the second mounting surface. The second electrical circuit includes a second inductive coil. 
     In some embodiments including the first, second, third, fourth, fifth, and sixth mounting surface and a first and second electrical circuit, the spring assembly further includes a third, fourth, fifth, and sixth electrical circuit. The first electrical circuit is affixed to the first mounting surface. The first electrical circuit includes the first inductive coil. The second electrical circuit is affixed to the second mounting surface. The second electrical circuit includes a second inductive coil. The third electrical circuit is affixed to the third mounting surface. The third electrical circuit includes a third inductive coil. The fourth electrical circuit is affixed to the fourth mounting surface. The fourth electrical circuit includes a fourth inductive coil. The fifth electrical circuit is affixed to the fifth mounting surface. The fifth electrical circuit includes a fifth inductive coil. The sixth electrical circuit is affixed to the sixth mounting surface. The sixth electrical circuit includes a sixth inductive coil. 
     In some embodiments including the first, second, third, fourth, fifth, and sixth electrical circuits, the spring assembly can further include a first, second, third, and fourth electrical circuit segment, each extending over an outer surface of the tubular body. The first electrical circuit segment joins the first electrical circuit to the third electrical circuit. The second electrical circuit segment joins the third electrical circuit to the fifth electrical circuit. The third electrical circuit segment joins the second electrical circuit to the fourth electrical circuit. The fourth electrical circuit segment joins the fourth electrical circuit to the sixth electrical circuit. 
     There is further provided, in accordance with some embodiments of the present disclosure, a force probe including a tubular segment, a first mounting surface in the tubular segment, a second mounting surface in the tubular segment, a compressible framework extending between the first and second mounting surfaces, a catheter, and an atraumatic probe tip. The tubular segment extends along a longitudinal axis. The tubular segment is sized to traverse vasculature. The first mounting surface is configured to engage a substantially planar electrical circuit. The first mounting surface positioned in a sidewall of the tubular segment, perpendicular to the longitudinal axis, and facing a first direction parallel to the longitudinal axis. The second mounting surface is configured to engage a substantially planar electrical circuit. The second mounting surface positioned in a sidewall of the tubular segment, perpendicular to the longitudinal axis, and facing toward the first mounting surface in a second direction opposite the first direction. The catheter is affixed to a first end of the tubular segment. The atraumatic probe tip is affixed to the second end of the tubular segment. 
     There is further provided, in accordance with some embodiments of the present disclosure an intravascular force probe including a proximal tube, an atraumatic distal end, a first inductive coil, a second inductive coil, and a compressible framework. The proximal portion includes an elongated body sized to traverse vasculature. The proximal tube and the atraumatic distal end define a longitudinal axis along which the force probe extends. The first inductive coil and the second inductive coil are each affixed between the proximal tube and the atraumatic distal end. The first inductive coil is confined in a first plane perpendicular to the longitudinal axis. The second inductive coil is confined in a second plane perpendicular to the longitudinal axis. The compressible framework is compressible parallel to the longitudinal axis to move the first inductive coil and the second inductive coil toward each other. 
     In some embodiments, the intravascular force probe further includes a catheter coupler affixed to the proximal tube and the atraumatic distal end. The catheter coupler includes the compressible framework. The catheter coupler structurally supports the first inductive coil and the second inductive coil. 
     In some embodiments, the intravascular force probe further includes a first connecting conductor electrically connecting the first inductive coil. The first connecting conductor extends circumferentially about the longitudinal axis within the first plane and is positioned outside an outer surface of the catheter coupler. 
     In some embodiments, the intravascular force probe further includes a third, fourth, fifth, and sixth inductive coil. The third and fifth inductive coils are confined in the first plane. The fourth and sixth inductive coils are confined in the second plane. 
     In some embodiments, the compressible framework is bendable to define a bend in the longitudinal axis, thereby causing the first plane and the second plane to be non-parallel. 
     In some embodiments, the intravascular force probe further includes a generator and an electrical measurement tool. The generator is electrically connected to the first inductive coil. The electrical measurement tool is electrically connected to the second inductive coil. 
     In some embodiments, the intravascular force probe further includes an electrical diagnostic system configured to receive a first electrical signal corresponding to a first distance between the first inductive coil and the second inductive coil, receive a second electrical signal corresponding to a second distance between the third inductive coil and the fourth inductive coil, receive a third electrical signal corresponding to a third distance between the fifth inductive coil and the sixth inductive coil, and determine a three dimensional force vector representing a force applied to the atraumatic distal end, the force vector determined based at least in part on the first electrical signal, the second electrical signal, and the third electrical signal. 
     There is further provided, in accordance with some embodiments of the present disclosure a catheter including a proximal tube, an atraumatic distal end, a first inductive coil, a second inductive coil, a compressible framework, a catheter coupler, and an elongated catheter tube. 
     The proximal tube and atraumatic distal end define a longitudinal axis. 
     The first inductive coil is affixed between the proximal tube and the atraumatic distal end. The first inductive coil is confined in a first plane perpendicular to the longitudinal axis. 
     The second inductive coil is affixed between the proximal tube and the atraumatic distal end and confined in a second plane perpendicular to the longitudinal axis. 
     The compressible framework is compressible parallel to the longitudinal axis to move the first inductive coil and the second inductive coil toward each other. 
     The catheter coupler is affixed to the proximal tube and the atraumatic distal end. The catheter coupler includes the compressible framework and structurally supports the first inductive coil and the second inductive coil. 
     The elongated catheter tube is affixed to the proximal tube, surrounds the first inductive coil, surrounds the second inductive coil, surrounds the compressible framework, surrounds the catheter coupler, and extends to a proximal end of the catheter. 
     The present disclosure will be more fully understood from the following detailed description of embodiments thereof, taken together with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a spring assembly in accordance with some embodiments of the present disclosure; 
         FIG. 2  is an illustration in an elevated profile view of a catheter coupler of the spring assembly in accordance with some embodiments of the present disclosure; 
         FIG. 3  is an illustration in a cross-sectional view of the catheter coupler in accordance with some embodiments of the present disclosure; 
         FIG. 4  is an illustration of the spring assembly in an elevated profile view in accordance with some embodiments of the present disclosure; 
         FIG. 5  is an illustration of an electrical circuit of the spring assembly in accordance with some embodiments of the present disclosure; 
         FIGS. 6A and 6B  are illustration of a sequence for opening an electrical circuit for insertion into the spring assembly in accordance with some embodiments of the present disclosure; 
         FIG. 7  is an illustration of an electrical circuit having connecting segments shaped for strain relief in accordance with some embodiments of the present disclosure; 
         FIG. 8  is an illustration of another electrical circuit of the spring assembly in accordance with some embodiments of the present disclosure; 
         FIG. 9  is an illustration of components of a force probe of a partially assembled catheter including the spring assembly in accordance with some embodiments of the present disclosure; 
         FIG. 10  is another illustration of components of the force probe of a partially assembled catheter including the spring assembly in accordance with some embodiments of the present disclosure; and 
         FIG. 11  is an illustration of a medical treatment utilizing the catheter and force probe in accordance with some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     As used herein, the terms “about” or “approximately” for any numerical values or ranges indicate a suitable dimensional tolerance that allows the part or collection of components to function for its intended purpose as described herein. More specifically, “about” or “approximately” may refer to the range of values ±10% of the recited value, e.g. “about 90%” may refer to the range of values from 81% to 99%. In addition, as used herein, the terms “patient,” “host,” “user,” and “subject” refer to any human or animal subject and are not intended to limit the systems or methods to human use, although use of the subject invention in a human patient represents a preferred embodiment. 
     As used herein, the term “computing system” is intended to include stand-alone machines or devices and/or a combination of machines, components, modules, systems, servers, processors, memory, detectors, user interfaces, computing device interfaces, network interfaces, hardware elements, software elements, firmware elements, and other computer-related units. By way of example, but not limitation, a computing system can include one or more of a general-purpose computer, a special-purpose computer, a processor, a portable electronic device, a portable electronic medical instrument, a stationary or semi-stationary electronic medical instrument, or other electronic data processing apparatus. 
     As used herein, the terms “component,” “module,” “system,” “server,” “processor,” “memory,” and the like are intended to include one or more computer-related units, such as but not limited to hardware, firmware, a combination of hardware and software, software, or software in execution. For example, a component may be, but is not limited to being, a process running on a processor, an object, an executable, a thread of execution, a program, and/or a computer. By way of illustration, both an application running on a computing device and the computing device can be a component. One or more components can reside within a process and/or thread of execution and a component may be localized on one computer and/or distributed between two or more computers. In addition, these components can execute from various computer readable media having various data structures stored thereon. The components may communicate by way of local and/or remote processes such as in accordance with a signal having one or more data packets, such as data from one component interacting with another component in a local system, distributed system, and/or across a network such as the Internet with other systems by way of the signal. Computer readable medium can be non-transitory. Non-transitory computer-readable media include, but are not limited to, random access memory (RAM), read-only memory (ROM), electronically erasable programmable ROM (EEPROM), flash memory or other memory technology, compact disc ROM (CD-ROM), digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other tangible, physical medium which can be used to store computer readable instructions and/or data. 
     As used herein, the term “trace” includes a conductive path in an electrical circuit such as a path integral to a printed circuit, an individual wire, a conductor within a ribbon cable, or other such structure as appreciated and understood by a person of ordinary skill in the art according to the teachings of the present disclosure. 
     As used herein, the terms “tubular” and “tube” are not limited to a structure that is a right cylinder or strictly circumferential in cross-section or of a uniform cross-section throughout its length. For example, the tubular structure or system is generally illustrated as a substantially right cylindrical structure. However, the tubular system may have a tapered outer surface, a curved outer surface, and/or a partially flat outer surface without departing from the scope of the present disclosure. 
       FIG. 1  illustrates a spring assembly  100  including a catheter coupler  190 , a distal circuit  180  and a proximal circuit  110 . The spring assembly  100  is sized to traverse vasculature; accordingly, all components and subcomponents thereof are sized to traverse vasculature. The catheter coupler  190  is compressible to cause the distal circuit  180  and an opposite portion  114  of the proximal circuit  110  to move in relation to each other. The distal circuit  180  and proximal circuit  110  are configured to provide an electrical signal indicative of one or more distances (or change in distance) at one more or locations between the distal circuit  180  and the opposite portion  114  of the proximal circuit  110 . 
     The catheter coupler  190  has a tubular body defining a longitudinal axis L-L extending therethrough. The catheter coupler  190  is sized to traverse vasculature. The catheter coupler  190  includes a compressible framework  196  that can be compressed when force is applied to the spring assembly  100  along a longitudinal axis L-L. The compressible framework  196  can also be deflected by a force non-parallel to the longitudinal axis, causing the coupler  190  to bend. Because the longitudinal axis L-L is defined by the tubular body of the coupler  190 , a bend in the coupler  190  thereby creates a bend in the longitudinal axis L-L. 
     The coupler  190  includes distal mounting surfaces  206 ,  210 ,  224  to which the distal circuit  180  is affixed and proximal mounting surfaces  204 ,  208 ,  222  to which a portion  114  of the proximal circuit  110  is affixed. The mounting surfaces  204 ,  206 ,  208 ,  210 ,  222 ,  224  are each configured to engage a substantially planar electrical circuit such as the distal circuit  180  and the opposite portion  114  of the proximal circuit  110 . Each mounting surface  204 ,  206 ,  208 ,  210 ,  222 ,  224  is substantially perpendicular to the longitudinal axis L-L. Each of the proximal mounting surfaces  204 ,  208 ,  222  faces toward a respective mounting surface of the distal mounting surfaces  206 ,  210 ,  224  such that each of the proximal mounting surfaces  204 ,  208 ,  222  face a first direction parallel to the longitudinal axis and each of the distal mounting surfaces faces toward a respective proximal mounting surface  204 ,  208 ,  222  in a second direction opposite the first direction and parallel to the longitudinal axis. Each of the proximal mounting surfaces  204 ,  208 ,  222  can be sized and shaped as a mirror image of the facing distal mounting surface  206 ,  210 ,  224 . 
     The coupler  190  can include three distal mounting surfaces  206 ,  210 ,  224  that are coplanar in a first plane P 1  and three proximal mounting surfaces  204 ,  208 ,  222  that are coplanar in a second plane P 2 . The first and second planes P 1 , P 2  are each perpendicular to the longitudinal axis L-L. A first distal surface  206  is positioned opposite a second proximal surface  204 ; a third distal surface  210  is positioned opposite a fourth proximal surface  208 ; and a fifth distal surface  224  is positioned opposite a sixth proximal surface  222 . The first, third, and fifth surfaces  206 ,  210 ,  224  are coplanar in the first plane P 1 . The second, fourth, and sixth surfaces  204 ,  208 ,  222  are coplanar in the second plane P 2 . The first, second, third, fourth, fifth, and sixth mounting surfaces  204 ,  206 ,  208 ,  210 ,  222 ,  224  have substantially similar dimensions to each other. The first, third, and fifth mounting surfaces  206 ,  210 ,  224  are spaced 120° from each other as measured rotationally about the longitudinal axis. The second, fourth, and sixth mounting surfaces  204 ,  208 ,  222  are spaced 120° from each other as measured rotationally about the longitudinal axis. 
     Each mounting surface  204 ,  206 ,  208 ,  210 ,  222 ,  224  is positioned in a sidewall of the tubular body of the coupler  190 . 
     The compressible framework  196  extends from the proximal mounting surfaces  204 ,  208 ,  222  to the distal mounting surfaces  206 ,  210 ,  224 . 
     The catheter coupler  190  is further configured to join two portions of a catheter  14  (see  FIG. 11 ). The catheter coupler  190  has distal engagement extensions  194  extending to a distal end of the tubular body and proximal engagement extensions  192  extending to a proximal end of the tubular body. 
     The distal circuit  180  and the opposite portion  114  of the proximal circuit  110  can include inductive coils positioned opposite each other. Each inductive coil can be positioned opposite another inductive coil with the compressible framework  196  therebetween. Opposite coils can be configured to act as an inductive distance sensor with one or more coils (preferably on the portion  114  of the proximal circuit  110 ) acting as a transmitter and one or more opposite coils (preferably on the distal circuit  180 ) acting as a receiver. A force probe including the spring assembly  100  can include a generator electrically connected to coil(s) acting as a transmitter and an electrical measurement tool electrically connected to coil(s) acting as a receiver. 
     The distal circuit  180  can include three inductive coils  184  (see  FIG. 8 ) and the opposite portion  114  of the proximal circuit  110  can include three inductive coils  116 ,  118 ,  120  (see  FIG. 5 ). Each of the inductive coils  184  on the distal circuit  180  can be positioned opposite a respective inductive coil  116 ,  118 ,  120  on the proximal circuit  110 . Inductive coils  184  on the distal circuit  180  can be confined to a plane perpendicular to the longitudinal axis L-L by virtue of being affixed to the distal mounting surfaces  206 ,  210 ,  224  of the coupler  190  (the distal mounting surfaces being coplanar in the first plane P 1 ) and also by virtue of, for example, planar coils. Inductive coils  116 ,  118 ,  120  on the opposite portion  114  of the proximal circuit  110  can be confined to a plane perpendicular to the longitudinal axis L-L by virtue of being affixed to the proximal mounting surface  204 ,  208 ,  222  of the coupler  190  (the proximal mounting surfaces being coplanar in the second plane) and also by virtue of, for example, planar coils. 
     An intravascular force probe can include six inductive coils  184 ,  116 ,  118 ,  120  paired to form three inductive distance sensors. The force probe can further include an electrical diagnostic system configured to receive first, second, and third electrical signals corresponding to first, second, and third distances between first, second, and third coil pairs respectively. The electrical diagnostic system can further be configured to determine a three-dimensional force vector representing a force applied to a tip of the force probe based at least in part on the first, second, and third electrical signals. 
     The coupler  190  can structurally support the inductive coils  184 ,  116 ,  118 ,  120 . 
     The proximal circuit  110  can further include additional circuit sections  124 ,  134 ,  142  for additional sensors and/or connections to an energy source. (See  FIG. 5 .) 
       FIG. 2  is an illustration of the catheter coupler  190  in an elevated profile view.  FIG. 3  is an illustration of a cross section of the catheter coupler  190  of  FIG. 2 . Referring collectively to  FIGS. 2 and 3 , the tubular body of the coupler  190  includes openings  232 ,  234 ,  236  in the area of the compressible framework  196 . As illustrated, the tubular body includes three proximal openings  232  each circumscribed by one of the proximal mounting surfaces  204 ,  208 ,  222  and struts  226  of the compressible framework  196 . As illustrated, the tubular body includes three distal openings  234  each circumscribed by one of the distal mounting surfaces  206 ,  210 ,  224  and struts  226  of the compressible framework. As illustrated, the tubular body includes three central openings  236  each circumscribed by struts  226  of the compressible framework  196 . 
     Struts  226  and openings  232 ,  234 ,  236  in the compressible framework can be alternatively configured to allow compression and/or bending of the compressible framework  196  when the spring assembly  100  is manipulated as part of a catheter force probe. The struts  236  can be, sized, shaped, positioned, and otherwise configured to provide a predetermined travel length (in the direction of the longitudinal axis L-L and/or off-axis) and/or a predetermined spring constant. The spring constant and/or travel length can be determined based on clinical working range required for a specific application or range of applications. The struts  236  can be designed to result in a specific travel length and spring constant based on the clinical working range of contact force (e.g., 500 g or less) required in specific applications. 
     Some or all of the openings  232 ,  234 ,  236  in the tubular body can collapse when the compressible framework  196  is compressed. Openings  232 ,  234  circumscribed in part by one of the mounting surfaces  204 ,  206 ,  208 ,  210 ,  222 ,  224  can collapse when the compressible framework  196  is compressed. Additionally, or alternatively, openings  236  circumscribed by struts  226  of the compressible framework  196  can collapse when the compressible framework  196  is compressed. 
     Each mounting surface  204 ,  206 ,  208 ,  210 ,  222 ,  224  can be configured to receive a liquid adhesive suitable for adhering the distal and proximal circuits  180 ,  110 . Each mounting surface  204 ,  206 ,  208 ,  210 ,  222 ,  224  can include T-shaped indentations  228  into which adhesive can flow. The T-shaped indentations  228  can provide improved securement of adhesion to the mounting surfaces  204 ,  206 ,  208 ,  210 ,  222 ,  224  compared to smooth planar mounting surfaces. Each mounting surface  204 ,  206 ,  208 ,  210 ,  222 ,  224  can otherwise be treated to improve adhesion as appreciated and understood by a person of ordinary skill in the art according to the teachings of the present disclosure. 
     The coupler  190  can further include multiple engagement extensions  194  extending from a first end of the tubular body, parallel to the longitudinal axis L-L and multiple engagement extensions  192  extending from a second end of the tubular body, parallel to the longitudinal axis L-L. Each engagement extension  192 ,  194  on either side of the tubular body can include a protrusion  192 ,  195  extending therefrom in a circumferential direction about the longitudinal axis. The spring assembly can include exactly three engagement extensions  192  extending from the first end of the tubular body and exactly three engagement extensions  194  extending from the second end of the tubular body. 
       FIG. 4  is an elevated profile view of a distal portion of the spring assembly  100  to provide a view of the position of the distal circuit  180  and opposite portion  114  of the proximal circuit  110  within the spring assembly  100 . In this view, four electrical circuit segments  132 ,  136 ,  186  can be seen extending outside an outer surface of the tubular body of the coupler  190 . 
     The distal electrical circuit  180  is illustrated in greater detail in  FIG. 8 . Referring collectively to  FIGS. 4 and 8 , the distal electrical circuit  180  includes two segments  186  each extending over an outer surface of the tubular body of the coupler  190 . The two segments  186  each connect two larger portions  178  of the distal electrical circuit  180  such that three larger portions  178  of the distal electrical circuit  180  are connected by the two segments  186 . Inductive coils  184  of the distal circuit  180  are positioned in the larger portions  178  of the distal circuit  180 . 
     The proximal circuit  110  is illustrated in greater detail in  FIG. 5 . Referring collectively to  FIGS. 4 and 5 , the portion  114  of the proximal circuit  110  positioned opposite the distal circuit  180  includes two connecting segments  132 ,  136 . The first connecting segment  132  of the proximal circuit  110  joins a first larger portion  164  to a second larger portion  162 . The second segment  136  of the proximal circuit  110  joins the second larger portion  162  to a third larger portion  160 . 
     The larger portions  160 ,  162 ,  164 ,  178  can each be shaped to extend across each respective mounting surface  204 ,  206 ,  208 ,  210 ,  222 ,  224  into a lumen  212  defined by the inner surface of the coupler  190  when the respective circuits  180 ,  114  are mounted to the coupler  190 . The connecting segments  132 ,  136 ,  186  are positioned to extend over an outer surface of the coupler  190 . Each segment  132 ,  136 ,  186  and its pair of adjacent larger portions  160 ,  162 ,  164 ,  178  forms a notch into which portions of the strut framework  196  between mounting surfaces  204 ,  206 ,  208 ,  210 ,  222 ,  224  can be positioned. Configured as such, the circuits  180 ,  114  can be aligned parallel to each other in planes P 1 , P 2  ( FIG. 1 ) perpendicular to the longitudinal axis L-L. Circuits  114 ,  180  can be positioned closer to each other compared to some other known force sensor designs. 
       FIG. 5  is an illustration of a flexible circuit  110  that can be used as the proximal circuit  110  in the spring assembly  100 . The flexible circuit  110  can be employed within a catheter to provide signals concerning force to a processor at a physician console. The flexible circuit  110  can also include circuitry to provide signals concerning location. The flexible circuit  110  includes a substantially planar substrate  112  including a substantially circular portion  114 . The circular portion  114  is shaped to be mounted in the catheter coupler  190  opposite the distal circuit  180  as illustrated in  FIGS. 1 and 4 . The substrate  112  includes additional portions  124 ,  134 ,  142  shaped to wrap around the longitudinal axis L-L. The additional portions  124 ,  134 ,  142  can have a substantially rectangular shape as illustrated or other shape. Portions  114 ,  124 ,  134 ,  142  can be joined by connector segments  144 ,  146 ,  148 . The width of the rectangular portions  124 ,  134 ,  142  and associated connector segments  146 ,  148  can be sized such that when wrapped within or around a catheter, sleeve, sheath, or other such tubular structure, the additional portions  124 ,  134 ,  142  almost completely circumscribe the longitudinal axis L-L. The substrate  112  can be formed of a suitable material that is non-conductive and is capable of resisting high temperatures, e.g. polyimide, polyamide, or liquid crystal polymer (LCP). Alternatively, some or all of the portions  114 ,  124 ,  134 ,  142  and segments  144 ,  146 ,  148  can be constructed on a substrate separate from substrate  112  and joined to form circuit  110  by processes known to a person of ordinary skill in the art. 
     The circular portion  114  of the circuit  110  includes the three larger portions  160 ,  162 ,  164  and two connecting segments  132 ,  136  as illustrated and described in relation to  FIG. 4 . Each of the three larger portions  160 ,  162 ,  164  is an annular sector of the circular portion  114 . The annular sectors  160 ,  162 ,  164  can have a substantially identical shape to each other. The first annular sector  164  is separated from the third annular sector  160  by an opening  122  in the circular portion  114 . The segments  132 ,  136  can be flexible so that the circular portion  114  can be opened for insertion into the coupler  190  and positioned to a final position as illustrated in  FIG. 4 . 
       FIG. 6A  is an illustration of the circular portion  114  in an elevated view in a relaxed shape as illustrated in  FIG. 5 .  FIG. 6B  is an illustration in an elevated view of the circular portion  114  opened for insertion into the coupler  190 . 
     The entire circular portion  114  can be flexible. Alternatively, the annular sectors  160 ,  162 ,  164  can be rigid, not substantially elastically deformable. Regardless as to whether annular sectors  160 ,  162 ,  164  are rigid or flexible when manufactured, when affixed to coupler  190 , the annular sectors  160 ,  162 ,  164  can be sufficiently rigid to maintain position of the inductive coils in the second plane P 2  (see  FIG. 1 ) during shipping, handling pre-treatment, and manipulation during treatment. Segments  132 ,  136  can be sufficiently flexible to allow the circular portion  114  to be installed into the coupler  190 . 
     Segments  132 ,  136  can be rigidly affixed to coupler  190 . Alternatively, segments  132 ,  136  can be flexible, providing strain relief between the annular sectors  178  when the annular sectors  178  are affixed to the coupler. For instance, segments  132 ,  136  can include an S configuration that can be positioned flat against the outer diameter of the coupler  190 . 
       FIG. 7  illustrates is an illustration of a circular circuit  114   a  in an elevated view having segments  132   a,    136   a  that extend radially outwardly. The segments  132   a,    136   a  are shaped to provide strain relief. When the circular circuit  114   a  is affixed to the coupler  190 , the segments  132   a,    136   a  are separated from the outer surface of the coupler  190  over at least a portion of the length of each respective segment  132   a,    136   a.  The segments  132 ,  136  can be alternatively shaped such that the segments are able to lengthen as the coupler  190  bends. 
     Each annular sector  160 ,  162 ,  164  includes an inductive coil  116 ,  118 ,  120 . The coils  116 ,  118 ,  120  can be discrete from each other, as shown, or they can each be connected to one or both of the others. As shown, each coil  116 ,  118 ,  120  on circular portion  114  includes approximately five turns. However, because signal strength is a function of the number of turns, the number of turns can be maximized based on the size of each segment and the pitch that the lithographic process can accomplish. 
     Each segment  132 ,  136  can include a connecting trace  166 ,  176  electrically connected to one or more of the inductive coils  116 ,  120 . A trace  166 ,  176  in a connecting segment  132 ,  136  can extend circumferentially about the longitudinal axis at a position outside an outer surface of the catheter coupler. As illustrated in  FIGS. 4 and 5 , such a trace  166 ,  176  can be confined in a plane with the inductive coils  116 ,  118 ,  120  on the circular portion  114  of the proximal circuit  110  when the proximal circuit  114  is affixed to the coupler  190 . Alternatively, if the connective segment  132 ,  136  in which the connecting trace  166 ,  176  is positioned is non-planar (e.g. is shaped for strain relief), the trace  166 ,  176  can have a non-planar shape, following the shape of the respective connecting segment  132 ,  136 . 
     The circuit  110  includes a connector segment  144  joining the circular portion  114  to a rectangular portion  142  of the circuit. The connector segment  144  can be sufficiently flexible to bend from a planar shape to include a curve turning at about 90°. Alternatively, the circuit  110  can include a 90° curve in the segment  144  as fabricated. When each inductive coil  116 ,  118 ,  120  is discrete, the segment  144  can include three traces  166 ,  174 ,  176 , each trace electrically connected to a respective inductive coil  116 ,  118 ,  120  in the circular portion  114  of the circuit  110 . When two or more of the coils  116 ,  118 ,  120  are electrically connected, the segment  144  can include one or two traces. 
     The rectangular portion  142  connected directly to the circular portion  114  includes solder joints  168 . Where the three coils are discrete from each other, each respective extension  166 ,  174 , and  176  is joined to a separate solder joint  168 . Alternatively, extensions  166 ,  174 ,  176  can join two or more coils  116 ,  118 ,  120 , in which case, fewer solder joints  168  can be included in the rectangular portion  142 . Where the coils  116 ,  118 ,  120  are discrete from each other, the signals generated in each of the coils may be used to provide additional details of force, such as an indication of an off-center force or an off-axis direction of the force. 
     The distal circuit  180 , and the circular portion  114  of the proximal circuit  110  are sufficient to serve as distance measurement transducers that when assembled in the coupler  190  can provide electrical signals indicative of deflection of the coupler. Structural properties of the compressible framework  196  and geometry of the catheter force probe can be known such that a deflection and/or compression of the compressible framework  196  is associated with a force or force vector applied to the catheter force probe. 
     Planar coils or traces used to measure signals relating to location (i.e., location coils or traces) can be incorporated into additional rectangular portions  124 ,  134  of the circuit  110 . Alternatively, the rectangular portions  124 ,  134  with coils can be omitted if only force sensing is desired. 
     A left rectangular coil  128  can be incorporated with a left portion  124  and a right coil  140  can be incorporated with a right portion  134  (where right and left are oriented in relation to the illustration in  FIG. 5 ). The circuit  110  can include extensions  156 ,  154  positioned in a joining segment  146  joining the left portion  124  to the central rectangular portion  142 . The extensions  156 ,  154  connect the left rectangular coil  128  to solder pads  168  in the central rectangular portion  142 . Likewise, the circuit  110  can include extensions positioned in the opposite joining segment  148  and connecting the right rectangular coil  140  to the solder pads  168 . As shown, each coil  128 ,  140  on the rectangular portions  124 ,  134  includes approximately five turns. However, because signal strength is a function of the number of turns, the number of turns may be maximized based on the size of the rectangular portions  124 ,  134 , and the pitch that the lithographic process can accomplish. 
     The wind of coils  116 ,  118 ,  120 ,  128 ,  140  can be clockwise (i.e., have a clockwise orientation) or counterclockwise. The orientation of coils can be selected as appreciated and understood by a person of ordinary skill in the art to perform sensor functionality described herein. 
     Substrate  112  can be a single layer. Alternatively, the substrate  112  can include between two and ten layers, e.g., four layers. In this manner the coils can be thickened by adding layers. However, thickening by layers can result in increased non-linearity of signal yield. As another alternative, the circuit  110  can include additional portions (not illustrated) each including one or more inductive coils. The additional portions can each be connected respectively to an illustrated portion  114 ,  124 ,  134  and positioned underneath the respective illustrated portion. The circuit  110  can have sufficient flexibility at connection(s) between the additional portions and the illustrated portions to bend 360° for placement of the additional portions underneath the respective illustrated portion  114 ,  124 ,  134 . For instance, tabs  126 ,  136 ,  150 ,  152  extending from the left and right rectangular portions  124 ,  134  can be folded segments joining to additional left and right rectangular portions each respectively positioned under the illustrated left and right rectangular portions  124 ,  134 . Coils within overlapping portions can be aligned. By stacking substrates, panelization density due to increased area may increase, and the yield of the combined coil may not suffer from increased non-linearity as extensively compared to coils stacked within a substrate. 
       FIG. 8  is an illustration of a circular circuit  180  that can be used as the distal circuit  180  of the spring assembly  100 . The circuit  180  includes the three larger portions  178  and two segments  186  as illustrated and described in relation to  FIG. 4 . Each of the three larger portions  178  is an annular sector of the circuit  180 . Two of the annular sectors  178  are separated by an opening  188  in the circuit  180 . 
     The segments  186  can be flexible so that the circuit  180  can be opened for insertion into the coupler  190  and positioned as illustrated in  FIG. 4 . The entire circuit  180  can be flexible. Alternatively, the annular sectors  178  can be rigid, not substantially elastically deformable. Regardless as to whether annular sectors  178  are rigid or flexible when manufactured, when affixed to coupler  190 , the annular sectors  178  can be sufficiently rigid to maintain position of the inductive coils in the second plane P 2  (see  FIG. 1 ) during shipping, handling pre-treatment, and manipulation during treatment. Segments  186  can be sufficiently flexible to allow the circuit  180  to be installed into the coupler  190 . Segments  186  can be rigidly affixed to coupler  190 . Alternatively, segments  186  can be flexible, providing strain relief between the annular sectors  178 . 
     The circuit  180  can be opened similar to the circular portion  114  illustrated in  FIGS. 6A and 6B . The segments  186  can be shaped for strain relief similar to the connecting segments  132   a,    136   a  illustrated in  FIG. 7  and otherwise described in relation to the connecting segments  132 ,  136  of the circular portion  114 ,  114   a.    
     Each annular sector  178  includes an inductive coil  184 . The coils  184  can be discrete from each other, as shown, or they can each be connected to one or both of the others. As shown, each coil  184  includes approximately five turns. However, because signal strength is a function of the number of turns, the number of turns can be maximized based on the size of each segment and the pitch that the lithographic process can accomplish. 
     Each segment  186  can include a trace electrically connected to one or more of the inductive coils  116 ,  120 . 
     The distal circuit  180  can further include an extension  181  through which traces to the coils  184  can be routed. The extension  181  can bend to extend longitudinally past the proximal end of the coupler  190  and ultimately to solder joints (not shown). The extension  181  can be adhered or otherwise joined to the proximal circuit  110  such that traces extending through the extension are connected to solder joints  168  on the proximal circuit  110 . Alternatively, the distal circuit  180  can include a solder pad portion and/or additional inductive coil portions such as corresponding portions  142 ,  124 ,  134  of the proximal circuit  110 . As another alternative, where location sensing is not incorporated with the spring assembly, both the proximal circuit  110  and the distal circuit  180  can be configured substantially similar to the example distal circuit  180  illustrated in  FIG. 8  with solder joints connecting to traces in the extension  181 . 
       FIGS. 9 and 10  show a catheter  14  (see  FIG. 11 ) at two different steps of its assembly, illustrating components of a force probe including the spring assembly  100 .  FIG. 9  shows the proximal flexible circuit  110  as assembled to coupler  190  and a coupling sleeve  200 . The circular portion  114  of the proximal circuit  110  and the distal circuit  180  are each adhered to the coupler  190  as illustrated in  FIGS. 1 and 4 . 
     As illustrated in  FIG. 10 , the force probe can include a distal portion  18 , which can include ablation electrode(s)  32  and irrigation apertures  214 . The distal end  32  of the distal portion can be atraumatic. The force probe can further include a tubular proximal portion, coupling sleeve  200  sized to be positioned within an elongated catheter body  216  and shaped such that rectangular portions  124 ,  134 ,  142  can be mounted thereon. The force probe can include a pair of planar inductive coils affixed between the proximal portion  200  and the atraumatic distal end  32  such that each coil in the pair is in a plane perpendicular to the longitudinal axis L-L. The force probe can include the compressible framework  196  which is compressible parallel to the longitudinal axis to move the inductive coils in the pair toward each other. 
     The force probe can further include catheter coupler  190 . The coupler can be attached to the spring assembly  100  via the engagement extensions  194  extending from a distal end of the coupler  190 . Protrusions  195  of the distal engagement extensions  194  each engage a complementary feature of the tip  18 . The coupler  190  can be attached to the tubular proximal portion  200  via the engagement extensions  192  extending from a proximal end of the coupler  190 . Protrusions  193  of the proximal engagement extensions  192  each engage a complementary feature of the tubular proximal portion  200 . 
     The catheter  14  can include a cable bundle  198  including a set of cables connecting to the force probe, which, although not visible, are connected to solder joints  168  on the associated rectangular portion  142  of the proximal circuit  110  and thus to the various coils or traces on the proximal circuit  110 . The cable bundle  198  also includes cables connected to traces to coils  184  of the distal circuit  180 . 
     Rectangular portions  124 ,  134 ,  142  of the proximal circuit  110  are affixed to substantially planar surfaces  202  of the coupling sleeve  200  either direly adhered, or by virtue of attachment of an underlying layered substrate adhered to the surface  202 . So assembled, these portions  124 ,  134 ,  142  of flexible circuit  110  may be viewed as having a triangular cross section as viewed in plane A-A. perpendicular to the longitudinal axis L-L. Connecting segments  146 ,  148  can be adhered to arcuate surfaces of sleeve  200  positioned at corners of the triangular cross section of the planar portions  124 ,  134 ,  142  as viewed in plane A-A indicated in  FIG. 9 . 
     Accordingly, as assembled, flexible circuit  110 , may be readily inserted into an outer tube or sleeve  216  ( FIG. 11 ) that provides an outer surface of catheter  14  and that defines the inner diameter within which componentry (e.g., flexible circuits  110 ,  180 , coupler  190 , sleeve  200 ) of catheter  14  resides. All of the components describe herein are disposed inside a sleeve  216  of approximately 15 French or smaller. To help prevent soft spots under sleeve  216  that result from gaps between the substantially planar outer surfaces of rectangular portions  124 ,  134 ,  142  of the proximal circuit  110  on the one hand, and the curvature of sleeve  216  on the other hand, these gaps may be filled by including additional material, e.g., adhesives and/or polyimide layers on the rectangular portions  124 ,  134 ,  142 . The additional material may be fabricated separately from flexible circuit  110  and adhered thereto, or they may be an integral portion of flexible circuit  110 , formed during the same lithographic process as the remainder of flexible circuit  110 . 
     Flexible circuit  110  may be assembled into catheter  14  as follows. First, flexible circuit  110  may be provided. If portions of the circuit  110  are to be folded to form layered substrates, such portions can be folded accordingly. Adhesive can be applied to annular sectors  160 ,  162 ,  164  and/or proximal mounting surfaces  204 ,  208 ,  222  of the coupler  190 . Annular sectors  160 ,  162 ,  164  can be spread apart by flexing the connecting segments  132 ,  136  to widen the opening  122  ( FIGS. 6A and 6B ). Each annular sector  160 ,  162 ,  164  can be respectively wedged into an opening  232  in the tubular body of the coupler  190  circumscribed by a proximal mounting surface  204 ,  208 ,  222  and struts  226  of the compressible framework  196 . The connecting segments  132 ,  136  can be bent and/or twisted as the annular sectors  160 ,  162 ,  164  are moved into position. The connecting segments  132 ,  136  can relaxed to a non-stressed shape once the sectors  160 ,  162 ,  164  are in position. Adhesive can cure to join each annular sector  160 ,  162 ,  164  of the proximal circuit  110  to a respective mounting surface  222 ,  208 ,  204  of the coupler  190 . Annular sectors  160 ,  162 ,  164  can be coplanar when adhered to the coupler  190 . Adhesive can be applied to a backside of the rectangular portions  124 ,  142 ,  134  and/or the connecting segments  146 ,  148  therebetween. The segment  144  connecting the circular portion  114  of the proximal circuit  110  to the portion  142  having solder joints  168  thereon can be bent at about 90°. The rectangular segments  124 ,  142 ,  134  can be wrapped about the coupling sleeve  200 . Adhesive can cure to affix the rectangular segments  124 ,  142 ,  134  and connecting segments  146 ,  148  to the sleeve  200  as illustrated in  FIGS. 9 and 10 . 
       FIG. 11  is a pictorial system  10  for evaluating electrical activity and performing ablative procedures on a heart  12  of a living subject. The treatment illustrated represents an example of an application where the spring assembly  100  can be used as an intravascular force probe. 
     The system  10  illustrated in  FIG. 11  includes a catheter  14 , which is percutaneously inserted by an operator  16  through the patient&#39;s vascular system into a chamber or vascular structure of the heart  12 . The operator  16 , who is typically a physician, brings the catheter&#39;s distal tip  18  into contact with the heart wall, for example, at an ablation target site. Areas determined to be abnormal, for example by evaluation of electrical activation maps of the heart, can be ablated by application of thermal energy, e.g., by passage of radiofrequency electrical current through wires in the catheter to one or more electrodes at the distal tip  18 , which apply the radiofrequency energy to target tissue. The energy is absorbed in the tissue, heating it to a point (typically above 50° C.) at which point it permanently loses its electrical excitability. This procedure creates non-conducting lesions in the cardiac tissue, which disrupt the abnormal electrical pathway causing the arrhythmia. Such principles can be applied to different heart chambers to diagnose and treat many different types of cardiac arrhythmias. 
     The catheter  14  can include a handle  20 , having suitable controls to enable the operator  16  to steer, position and orient the distal end of the catheter as desired for the ablation. To aid the operator  16 , distal portion  18  of catheter  14 , or portions proximate thereto, contain the spring assembly  100  which acts as a force sensor to provide feedback to the operator as to force of contact (e.g. force vector) between the distal portion  18  and tissue. The catheter  14  can also contains position sensors, e.g., traces or coils (discussed below), that provide signals to a processor  22 , located in a console  24  to provide feedback as to the position of the distal portion  18  in relation to internal anatomy of the patient. 
     Ablation energy and electrical signals can be conveyed to and from the heart  12  through one or more ablation electrodes  32  located at or near the distal tip  18  via cable  38  to the console  24 . Pacing signals and other control signals may be conveyed from the console  24  through the cable  38  and the electrodes  32  to the heart  12 . 
     Wire connections  35  link the console  24  with body surface electrodes  30  and other components of a positioning sub-system for measuring location and orientation coordinates of the catheter  14 . The processor  22  or another processor may be an element of the positioning subsystem. The electrodes  32  and the body surface electrodes  30  may be used to measure tissue impedance at the ablation site as taught in U.S. Pat. No. 7,536,218, issued to Govari et al., which is hereby incorporated by reference in its entirety into this application as if set forth in full and attached in the appendix to priority application U.S. 62/943,572. A temperature sensor (not shown), typically a thermocouple or thermistor, may be mounted on or near each of the electrodes  32 . 
     The console  24  typically contains one or more ablation power generators  25 . The catheter  14  may be adapted to conduct ablative energy to the heart using a known ablation technique, e.g., radiofrequency energy, ultrasound energy, cryogenic energy, and laser-produced light energy. Such methods are disclosed in commonly assigned U.S. Pat. Nos. 6,814,733, 6,997,924, and 7,156,816, which are hereby incorporated by reference in their entirety into this application as if set forth in full and attached in the appendix to priority application U.S. 62/943,572. 
     The positioning subsystem may also include a magnetic position tracking arrangement that determines the position and orientation of the catheter  14  by generating magnetic fields in a predefined working volume and sensing these fields at the catheter, using coils or traces disposed within the catheter, typically proximate to the tip. A positioning subsystem is described in U.S. Pat. No. 7,756,576, which is hereby incorporated by reference in its entirety into this application as if set forth in full and attached in the appendix to priority application U.S. 62/943,572, and in the above-noted U.S. Pat. No. 7,536,218. 
     Operator  16  may observe and regulate the functions of the catheter  14  via console  24 . Console  24  includes a processor, preferably a computer with appropriate signal processing circuits. The processor is coupled to drive a monitor  29 . The signal processing circuits typically receive, amplify, filter and digitize signals from the catheter  14 , including signals generated by sensors such as electrical, temperature and contact force sensors, and a plurality of location sensing coils or traces located distally in the catheter  14 . The digitized signals are received and used by the console  24  and the positioning system to compute the position and orientation of the catheter  14 , and to analyze the electrical signals from the electrodes and the contact force sensors. 
     The console  24  can include an electrical diagnostic system including a processor and a memory in communication with the processor with instructions thereon that when executed by the processor cause the processor to receive a first electrical signal corresponding to a first distance between a first pair of inductive coils  184 ,  116  in the spring assembly  100 , receive a second electrical signal corresponding to a second distance between a second pair of inductive coils  184 ,  118  in the spring assembly  100 , receive a third electrical signal corresponding to a third distance between a third pair of inductive coils  184 ,  120  in the spring assembly, and determine a three dimensional force vector representing a force applied to the atraumatic distal end of the catheter  14 , the force vector determined based in at least in part on the first electrical signal, the second electrical signal, and the third electrical signal. 
     The subject matter disclosed herein concerns structures including the spring assembly  100  within the catheter  14  that may be used to provide feedback to a user of an ablation catheter (e.g., electrophysiologist), the feedback concerning the force exerted on the catheter&#39;s tip and any electrodes disposed thereon. The feedback can further include catheter location. These structures reside within the small inner diameter of the catheter (e.g., often equal to or less than about 0.1 inch) yet overcome various design constraints related thereto to provide the feedback reliably. For example, metal coils may be used to detect location within a magnetic field and/or relative position in relation to another coil. Generally, larger and thicker coils with more turns are easier to detect than smaller and thinner coils with fewer turns and the size of the coils is limited by the interior geometry of the catheter  14 . Further, when such coils are fabricated as traces on a circuit board or flexible circuit via a lithographic process, the process limits the trace pitch. Although the option is available to increase thickness of the circuit by additional layers lithographically, this option has two disadvantages. First, it is expensive because fabrication costs are proportional to the number of layers. That is, other things being equal, a flexible circuit having more layers costs more to fabricate than one having fewer layers. Second, non-linearity of yield is also proportional to the number of layers. That is, the yield from the coils is compromised because non-linearity of the yield increases with the number of traces. These design challenges are compounded by inclusion of additional structures proximate to the location traces including the bendable coupler  190  and irrigation structure. Further, cross-talk interference that may arise from packing the structures in a tight space should be accounted for. So too should the need for ease of assembly and wiring for safe products and positive patient outcomes. 
     The descriptions contained herein are examples of embodiments of the invention and are not intended in any way to limit the scope of the invention. As described herein, the invention contemplates many variations and modifications of a catheter, spring assembly, and force probe including alternative materials for component parts, alternative geometrical configurations, alternative methods of construction, and alternative methods of use. Modifications and variations apparent to those having ordinary skill in the art according to the teachings of this disclosure are intended to be within the scope of the claims which follow.