Abstract:
Methods of manufacturing cellulosic structures, e.g., for use in expandable-collapsible electrode assemblies for diagnostic and/or therepeutic electrophysiology devices, are disclosed. One such preferred method includes providing a mandrel having a head portion and a neck portion, the head portion having an outer circumference greater than the neck portion, dipping the mandrel into a cellulosic substance, curing the cellulosic substance, and separating the mandrel from the cured cellulosic substance.

Description:
FIELD OF THE INVENTION 
     The invention relates, in general, to electrode structures deployable into interior regions of the body, and, in particular, to electrode structures deployable into the heart for diagnosis and treatment of cardiac conditions. 
     BACKGROUND OF THE INVENTION 
     It is known that the effective treatment of cardiac arrhythmias requires creating tissue lesions having a diversity of different geometries and characteristics, depending upon the particular physiology of the arrhythmia to be treated. This recognition is discussed in U.S. patent application Ser. No. 08/631,356, filed Apr. 12, 1996, and Provisional Application Serial Nos. 60/010,223, 60/010,225, and 60/010,354, which were filed on Jan. 19, 1996. These applications are fully incorporated herein by reference for all they disclose and describe. 
     As discussed therein, one proposed solution to the creation of diverse lesion characteristics is to use different forms of ablation energy, e.g., microwave, laser, ultrasound, and chemical ablation. However, these technologies are largely unproven for this purpose. 
     The use of active cooling in association with the transmission of DC or radio frequency (“RF”) ablation energy is known to force the electrode-tissue interface to lower temperature values. As a result, the hottest tissue temperature region is shifted deeper into the tissue, which, in turn, shifts the boundary of the tissue rendered nonviable by ablation deeper into the tissue. An electrode that is actively cooled can be used to transmit more ablation energy into the tissue, compared to the same electrode that is not actively cooled. However, control of active cooling is required to keep maximum tissue temperatures safely below about 100° C., at which tissue desiccation or tissue boiling is known to occur. 
     The treatment of some cardiac arrhythmias requires creating significantly large and deep lesions or lesions having relatively large surface areas with shallow depths. A proposed solution to the creation of these larger lesions is the use of substantially larger electrodes than those commercially available. However, larger electrodes themselves pose problems of size and maneuverability, which weigh against safe and easy introduction of large electrodes through a vein or an artery, and into the heart. 
     In an effort to solve the problems of maneuverability and safe introduction, collapsible ablation structures have been developed. These structures are manipulated to a collapsed position during introduction and maneuvering, and to an expanded position during ablation of the desired heart tissue. Numerous examples of such structures are shown and described in the above-referenced application. A number of the collapsible ablation structures disclosed therein include a balloon with a microporous membrane or coating made of regenerated cellulose that is filled with a hypertonic solution such as saline. In particular, the hypertonic solution acts as both a current carrying means and an inflation medium for expanding the balloon. 
     A balloon coating made of regenerated cellulose is desirable because it is an ion-permeable material, allowing the ionic transfer of electrical energy from an electrode disposed in the balloon interior into a patient&#39;s bloodstream and/or body tissue, while preventing macromolecules, such as blood proteins, from passing into the balloon. 
     The regenerated cellulose coating also acts as a biocompatible barrier between the catheter components and the body tissue, thereby allowing the components to be made from less expensive materials that may be somewhat toxic, e.g., silver or copper. The regenerated cellulose acts as a biocompatible barrier because it increases the diffusional distance to the body tissue and reduces the percentage of metallic surface directly and indirectly exposed to the tissue. 
     A problem with regenerated cellulose is that it is not known to be formable or moldable into a three-dimensional body structure such as that required for proper lesion creation. Also, regenerated cellulose is not known to be formable with operative elements, e.g., temperatures sensors, embedded therein, or formable so as to have a smooth exterior, as required for a tissue-contacting electrode body structure. 
     It would be desirable, therefore, to provide a method for manufacturing a three-dimensional electrode body structure made of regenerated cellulose. 
     During minimally-invasive diagnostic and therapeutic cardiac procedures such as endocardial mapping and ablating, the heart muscles continuously expand and contract with the beating of the heart, i.e., heart diastole and heart systole. When deployed in this environment, an catheter electrode assembly is subject to alternate cycles of contraction and expansion. The surface pressure of the electrode assembly against the moving endocardium can continuously vary, complicating the task of performing the diagnostic and/or therapeutic procedure desired. 
     A need therefore exists for a means for continuously urging the electrode assembly against the endocardium and for maintaining a constant surface pressure, despite contraction and expansion of the heart. 
     A need also exists for a means of evaluating the sufficiency of the surface contact of the electrode assembly with the endocardium so the operating physician will know ahead of time what the potential for success is for the diagnostic or therapeutic procedure to be performed on the heart. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, methods of manufacturing cellulosic structures, such as, e.g., for use in expandable-collapsible electrode assemblies for diagnostic and/or therapeutic electrophysiology devices, are disclosed. One such preferred method includes providing a mandrel having a head portion and a neck portion, the head portion having an outer circumference greater than the neck portion, dipping the mandrel into a cellulosic substance, curing the cellulosic substance, and separating the mandrel from the cured cellulosic substance. 
     According to a separate aspect of the invention, an electrode assembly is provided, which includes an expandable-collapsible body and a biasing device adapted to resiliently urge a distal portion of the body against adjacent body tissue. 
     According to yet another aspect of the invention, an electrode assembly is provided, which includes a regenerated cellulosic body substantially enclosing an interior area, a center support disposed in the interior area, and an electrode disposed on the center support. 
     Other, more particular features and advantages of the inventions are set forth in the following detailed description and drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate both the design and utility of preferred embodiments of the present invention, in which similar elements are referred to by common reference numbers, wherein: 
     FIG. 1A is a perspective view of an embodiment of a system for ablating heart tissue and illustrates an exemplary catheter assembly, including an expandable electrode structure, for ablating heart tissue; 
     FIG. 1B is a perspective view of a lumen guide assembly of the catheter assembly illustrated in FIG. 1A; 
     FIG. 1C is a top view of an embodiment of a pressure-relief mechanism for the expandable electrode structure; 
     FIG. 2 is an enlarged cross-sectional view of an electrode structure constructed in accordance with an embodiment of the invention; 
     FIG. 3 is a partial, cut-away side view of a shaft and a receiver of the electrode structure illustrated in FIG. 2, and illustrates an embodiment of a mechanism for determining the displacement of the shaft; 
     FIG. 4 is a partial, cut-away side view of a shaft and a receiver of the electrode structure illustrated in FIG. 2, and illustrates an alternative embodiment of a mechanism for determining the displacement of the shaft; 
     FIG. 5 is an enlarged cross-sectional view of an electrode structure constructed in accordance with another embodiment of the invention; 
     FIGS. 6A and 6B are an enlarged side elevational view and a top plan view, respectively, of an electrode structure constructed in accordance with a further preferred embodiment of the invention; 
     FIG. 7 is an enlarged cross-sectional view of an electrode structure constructed in accordance with an additional embodiment of the invention; 
     FIGS. 8A and 8B are an enlarged longitudinal cross-sectional view and an enlarged lateral cross-sectional view, respectively, of an electrode structure constructed in accordance with a further preferred embodiment of the invention; 
     FIG. 9A is an enlarged, partially cut-away side elevational view of an electrode structure constructed in accordance with yet another embodiment of the invention; 
     FIG. 9B is a top plan view of an embodiment of a rib support assembly illustrated in FIG. 9A; 
     FIGS. 10A-10C are side elevational views of exemplary mandrels that may be used in manufacturing the electrode structure of the present invention; 
     FIGS. 11A and 11B are a top plan view and a cross-sectional view, respectively, of a balloon support illustrated in FIG. 8A, and illustrate a step in assembling the electrode structure illustrated in FIG. 8A; 
     FIGS. 12A and 12B are a top plan view and a cross-sectional view, respectively, of the balloon support and electrode illustrated in FIG. 8A, and illustrate another step in assembling the electrode structure illustrated in FIG. 8A; 
     FIGS. 13A and 13B are a top plan view and a cross-sectional view, respectively, of the balloon support, electrode, and lumens illustrated in FIG. 8A, and illustrate an additional step in assembling the electrode structure illustrated in FIG. 8A; 
     FIG. 14 is a cross-sectional view of the balloon support, electrode, lumens, and body of the electrode structure illustrated in FIG. 8A, and illustrates a further step in assembling the electrode structure illustrated in FIG. 8A; 
     FIG. 15 is a cross-sectional view of the balloon support, electrode, lumens, body, and distal portion of the steering wire assembly of the electrode structure illustrated in FIG. 8A, and illustrates an additional step in assembling the electrode structure illustrated in FIG. 8A; and 
     FIG. 16 is a cross-sectional view of the electrode structure illustrated in FIG. 8A and a distal portion of the catheter, and illustrates a still further step in assembling the electrode structure illustrated in FIG.  8 A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1A, a steerable catheter  20  for performing ablation on cardiac tissue is shown. The catheter  20  includes a handle  22  and a guide tube  24 . The guide tube  24  has a proximal end  26  attached to the handle  22  and a distal portion  28  that carries an electrode structure  30 . A retractable sheath  29  may cover the electrode structure  30 , when collapsed, to protect the electrode structure. 
     The distal portion  28  of the guide tube  24  may include a series of ring electrodes (not shown) for sensing electrograms to locate the region of the myocardial tissue that is to be ablated. 
     It should be appreciated that the invention is applicable for use in other tissue ablation applications such as, but not by way of limitation, ablating tissue in the prostate, brain, gall bladder, and uterus, and using systems that are not necessarily catheter-based. 
     A cable  31  preferably extends from the rear of the handle  22  and includes plugs  33  for connecting the catheter  20  to a source of ablation energy. The ablation energy is conveyed through wires  35  in the cable  31  to the electrode structure  30  for creating lesions in the myocardial tissue. 
     Although the type of ablation energy used can vary, radio frequency (“RF”) electromagnetic energy is preferably used in the illustrated embodiments. The energy source is a RF generator  37  that preferably delivers up to about 150 watts of power at a frequency of about 350 to 700 kHz, preferably about 500 kHz. 
     A controller  39  is associated with the generator, either as an integrated unit or as a separate interface box for governing the delivery of RF ablation energy to the electrode structure  30 . 
     The handle  22  encloses a steering mechanism  32  for maneuvering the distal portion  28  of the guide  24  through the vasculature of the patient&#39;s body. Left and right steering wires  41 ,  43  (FIGS. 8A,  15 ) extend through the guide tube  24  for interconnecting the steering lever  34  of the steering mechanism  32  to the distal portion  28 . Rotation of the steering lever  34  to the left pulls on the left steering wire  41 , causing the distal portion  28 , including the electrode structure  30 , to bend to the left. Rotation of the steering lever  34  to the right pulls on the right steering wire  43 , causing the distal portion  28  and electrode structure  30  to bend to the right. 
     In use, a physician holds the catheter handle  22  and introduces the catheter  20  through a main vein or artery, typically femoral, into the interior region of the heart near where the myocardial tissue is to be diagnosed and/or treated. The physician then further steers the distal portion  28  of the catheter  20  by means of the steering lever  34 , to place the electrode structure  30  into contact with the tissue that is to be diagnosed and/or treated. 
     With reference to FIG. 2, the electrode structure  30 , which is constructed in accordance with an embodiment of the invention, will now be described. The electrode structure  30  includes an expandable and collapsible body or balloon  38  made of regenerated cellulose. The body  38  preferably includes microsize pores having a size that allows ionic transport, but prevents the ingress of blood cells, infectious agents such as viruses and bacteria, and large biological molecules such as proteins. 
     The electrode body  38  includes a generally spherical head portion  40  and a relatively short, generally tubular neck portion  42 . The relatively short length of the neck portion  42  gives the electrode structure  30  a compact configuration that can be easily maneuvered in the patient&#39;s body. The head portion  40  has a proximal region  44  and a distal region  46 . The distal region  46  is preferably porous because this region is the preferred region of the body  38  for contacting myocardial tissue for ablation. One way to provide a porous distal region  46  is by masking an outer surface of the rest of the body  38  with a non-porous material or primer. The body has an inner surface  50  that surrounds an interior  52 . 
     The body  38  is supported and attached to the distal portion  28  of the catheter  20  by a balloon support assembly  54 . The balloon support assembly  54  includes a generally tubular receiver  56  having a proximal region and a distal region. The proximal region has a reduced-diameter portion  58  that fits snugly within the distal portion  28  of the guide tube  24  and is affixed to an inner wall of the guide tube  24  with an affixant such as cyanoacrylate. 
     Beyond the distal portion  28  of the guide tube  24 , the receiver  56  carries a ring electrode  60  used for pacing myocardial tissue. In an alternative embodiment, the element  60  may be an anchor or connecting collar used to attach the electrode body  38  to the distal portion  28  of the guide tube. The electrode body  38  is attached near the distal portion  28  of the guide tube  24  by sandwiching the neck portion  42  of the body  38  between an inner surface of the ring electrode  60  and an outer surface of the receiver  56 . Mechanical bonding means and/or an adhesive, e.g., cyanoacrylate, are used to ensure the attachment. 
     In the interior  52  of the body  38 , the proximal region of the receiver  56  carries an electrode  62 . A signal wire (not shown) extends through a tubular chamber  64  of the receiver  56  and through the guide tube  24  for electrically coupling the electrode  62  to the RF generator. The receiver  56  slidably receives a tubular plunger or shaft  66  that axially reciprocates within the tubular chamber  64 . The shaft  66  has a proximal region that slides within the tubular chamber  64  and a distal region that includes a support head  68 . The support head  68  of the shaft  66  includes a circular recess. 
     A spring coil  70  is carried along the outside of the shaft  66 . The location of the shaft  66  within the spring  70  permits controlled axial movement of the distal region  46  of the body  38  with respect to the neck portion  42 , and limits lateral movement. 
     Because the body  70  is inherently flexible, it is important for the body  70  to have some lateral and axial stiffness. Maintaining lateral stiffness allows the electrode structure  30  to be remotely steered better and torqued into the desired location within the patient&#39;s body. 
     The spring  70  has a fixed spring constant that does not vary with compression. The spring constant of the coil  70  determines the force required to compress the distal region  46  of the body  38 . The spring constant can be tailored to account for the motion of the heart and maintain the desired amount of surface area of the balloon in contact with the myocardial tissue. Less surface area may be achieved with an axially stiffer spring while larger surface area may be achieved with a less stiff spring. Controlling the surface area in contact with the myocardial tissue controls the characteristics of the resulting lesion(s). The spring  70  may also be electrically conductive so that it functions as an electrode. 
     The body  38 , receiver  56 , reciprocating shaft  66 , and spring  70  include respective longitudinal axes that are coaxial with one another. The resulting axis is represented in the drawings as CL. 
     The distal region  46  of the body  38  includes a hole that, in conjunction with the recess of the shaft head  68 , receives a pacing electrode  72  and temperature sensor  74 . Although the pacing electrode  72  and temperature sensor  74  are shown on the outside of the distal region  46  of the body  38 , in an alternative embodiment, the pacing electrode  72  and temperature sensor  74  may be on the other side of the distal region  46  of the body  38 , within the interior  52  of the body  38 . The temperature sensor  74  is housed by the pacing electrode  72 . An immediate portion of the distal region  46  of the body  38  surrounding the hole is attached to a distal part of the balloon support assembly  54  by sandwiching it between, and affixing it to, the pacing electrode  72  and the shaft head  68 . An affixant such cyanoacrylate is used for affixing this portion of the body  38  to the pacing electrode  72  and shaft head  58 . The temperature sensor  74  may consist of a thermistor, thermocouple, or the like. The pacing electrode  72  a lead wire and the temperature sensor  74  include a pair of lead wires, all of which pass through the balloon support assembly  54  and guide tube  24 , back to the handle  22  for electrical connection to the cable  31 . The controller  39  preferably controls the energy power supply, i.e., generator, in response to the sensed temperature. 
     With reference to FIG. 1A and 1B, the catheter  20  includes an input or infusion lumen  45  and an output or venting lumen  47  for adding and removing an electrically conductive fluid medium to and from the interior  52  of the body  38 . A lumen guide assembly  51  in the handle  22  guides the lumens  45 ,  47  away from the handle where the lumens  45 ,  47  exit the handle  22 . The guide assembly  51  includes hollow guides  53 ,  55  that receive the lumens  45 ,  47  and guide the lumens  45 ,  47  away from the handle  22 . 
     The fluid medium is preferably a hypertonic saline solution having sodium chloride, i.e., about 9% weight by volume. A hypertonic potassium chloride solution may also be used; however, this fluid medium requires close monitoring of ionic transport through the pores to ensure potassium overload does not occur. 
     Each lumen  45 ,  47  forms a fluid pressure transmitting conduit that communicates with the interior  52  of the body  38 . The lumens  45 ,  47  extend from a pressure control device  49  (FIG. 1A) to the interior.  52  of the body  38 , through the guide tube  24 . The pressure control device  49  is used to control the fluid pressure within the interior  52  of the body  38 . 
     Imparting a positive fluid pressure with the pressure control device  49  causes the body  38  to expand or inflate from a normal, low profile condition to an enlarged, expanded operating condition. The inflating body  38  deploys outward, assuming a prescribed three-dimensional shape. The shape can vary, depending upon the pre-molded configuration of the body  38 . The inflation is conducted to the extent that the body  38  is filled and expanded, but not stretched. Due to the pliant nature of the body  38 , the body  38 , when inflated, naturally conforms to the topography of the endocardial surface next to it. It has been recognized by the inventors that a less than fully expanded body condition adapts and conforms better to the surrounding heart tissue. 
     The lumens  45 ,  47  allow air in the interior  52   c  to be purged from the structure  120  and limit the pressure inside the electrode structure  120 . 
     The pressure control device  49  may be controlled to release or vent fluid from the interior  52 , through an inflation or venting lumen. This causes the body  38  to collapse into a deflated condition, and depending on the catheter design, may be retractable back into the catheter. 
     With reference to FIG. 1C, the inflation lumen  45  may include a pressure-relief mechanism  57  between the catheter  22  and the pressure control device  49  to inhibit over inflating the body  38 . The pressure-relief mechanism  57  includes a hollow cylindrical base  59  in which a three-way stopcock  61  is rotatably engaged. The inflation lumen  45  communicates with the hollow cylindrical base  59  through first and second main tubes  63 ,  65 . A valve tube  67  is in communication with the hollow cyclindrical base  59  and carries a pressure-relief valve  69 . The pressure-relief valve  69  is operatively associated with the valve tube  67  through a spring  71  and a mount  73 . The stiffness of the spring  71  determines the relief pressure in the pressure-relief valve  69 . This pressure is preferably between 10-15 psi. When the pressure in the interior  52  exceeds a predetermined threshold, as controlled by the stiffness of the spring  71 , the pressure-relief valve  69  opens, releasing or venting fluid from the interior  52 . The three-way stopcock  61  can be rotated for controlling the communication paths of the mechanism  57 . In an alternative embodiment, the pressure-relief mechanism  57  may be incorporated into the venting lumen  47 . 
     Alternatively, the movable sheath  29  controlled by a retraction mechanism may be used to selectively enclose the body  38  before and after use, during insertion into and retraction from the patient&#39;s body. The retraction mechanism is retracted to free the body  38  for inflation and use. 
     When the body  38  is in its normal, low profile condition, the body  38  maintains a standard 6-10 French size. When in its inflated condition, the same body  38  has an significantly enlarged dimension ranging from approximately 7 mm to 20 mm. 
     The catheter  20  and electrode structure  30  just described is ideally suited for ablating myocardial tissue within the heart. A physician moves the catheter tube  24  through a main vein or artery into a heart chamber, while the expandable-collapsible body  38  of the electrode structure  30  is in its low profile geometry. Once inside the desired heart chamber, the expandable-collapsible body  38  is enlarged into its expanded geometry with the pressure control device  49 , and the distal region  46  containing pores is placed into contact with the targeted region of endocardial tissue, which was preferably determined by a mapping procedure previously performed by the catheter  20  for locating aberrant electrical pathways in the endocardial tissue. 
     Due largely to mass concentration differentials across the pores in the distal region  46 , ions in the medium will pass into the pores because of concentration differential-driven diffusion. Ion diffusion through the pores will continue as long as a concentration gradient is maintained across the body  38 . The ions contained in the pores provide the means to conduct current across the body  38 . 
     RF energy is conveyed from the generator  37  to the electrode  62 , as governed by the controller  39 . When RF voltage is applied to the electrode  62 , electric current is carried by the ions in the fluid medium to the ions within the pores. The RF currents provided by the ions result in no net diffusion of ions, as would occur if a DC voltage were applied. The ions move slightly back and forth during the RF frequency application. This ionic movement and current flow in response to the applied RF field does not require perfusion of liquid in the medium through the pores. 
     The ions convey RF energy through the pores into tissue to a return electrode, which is typically an external patch electrode, i.e., a unipolar arrangement. Alternatively, the transmitted energy can pass through tissue to an adjacent electrode in the heart chamber, i.e, forming a bipolar arrangement. The RF energy heats the tissue, mostly ohmically, forming the desired lesion. 
     The pacing electrodes  60 ,  72  of the electrode structure  30  can be used in association with a conventional pacing apparatus (not shown) for pacing the heart or acquiring electrograms in a conventional fashion. The pacing apparatus is electrically coupled to the electrical wires of the cable to provide a pacing signal to a selected one of the electrodes  60 ,  72 , generating depolarization foci at selected sites within the heart. The electrodes  60 ,  72  may also serve to sense the resulting electrical events for the creation of electrograms. Used in this fashion, the electrode structure  30  can accommodate both pace mapping and entrainment pacing techniques. 
     It is important for the distal region  46  of the body  38  to be in good contact with the myocardial tissue for the above therapeutic and diagnostic procedures, and for accurately determining the temperature of the myocardial tissue to be treated. This is difficult because during these procedures the heart muscles continuously expand and contract with the beating of the heart, i.e., during heart diastole and systole. When expanded, the body  38  is subject to alternate cycles of contraction and expansion. The surface pressure of the distal region  46  against the moving endocardium continuously varies, complicating the task of accurately performing the above-described procedures. The distal region  46  can also slip along the constantly moving endocardial surface. 
     The spring  70 , receiver  56 , and reciprocating shaft  66  form a biasing device that counteracts this phenomenon by continuously urging the distal region  46  of the body  38  in an axial direction against the endocardium and maintaining a constant surface pressure, despite the contraction and expansion of the heart. 
     During heart systole in the heart chamber, the distal region  46  experiences compression. Movement of the distal region  46  towards the receiver  56  axially compresses the spring  70 . When compressed, the spring  70  urges the head  68  of the shaft  66  against the distal region  46 , pushing the distal region  46  forward. The fluid pressure in the interior  52  of the body  38  also provides a radially outward force against the endocardial surface. The spring  70  dampens and resists the movement of the endocardium, holding the distal region  46  against the endocardium. This maintains contact pressure between the distal region  46  and the endocardium during heart systole. 
     When the heart chamber expands, the spring  70  urges the distal region  46  forward, urging the distal region  46  towards its original shape. Thus, the spring  70  maintains contact pressure between the distal region  46  and the surrounding, moving endocardium during heart diastole. 
     Enabling the distal region  46  of the body  38  to compress axially allows a larger surface area to be positioned into contact with myocardial tissue as the geometry of the body surface changes because of the moving heart, and allows the functionality of the electrode structure  30  to be maintained, i.e., the ability to steer, torque, and collapse the electrode structure. 
     Because the spring  70  is a constant force spring, a relatively constant surface pressure is established and maintained between the distal region  46  of the body  38  and the surrounding endocardium when the distal region  46  is compressed. 
     In order to determine the sufficiency of tissue contact obtained by the distal region  46  of the body  38 , the electrode structure may include a mechanism for quantifying the degree of tissue contact with the distal region  46 , i.e. a tissue contact evaluating mechanism. 
     With reference to FIG. 3, the tissue contact evaluating mechanism includes a displacement determining mechanism  76 . The shaft displacement determining mechanism includes an electrical circuit having a variable resistor with a resistance R var , a current source with a current I a , and a measured voltage V a . 
     The resistance R var  of the variable resistor depends on the resistance of R 1  and R 2 , where R 1  is the resistance of the reciprocating shaft  66  and R 2  is the resistance of the receiver  56 , and the displacement of D of the shaft  66 . The resistivities of the receiver  56  and the shaft  66 , R 1 , R 2 , respectively, must be chosen to permit adequate resolution of resistance measurements. Accordingly, the desired resistivities are obtained by constructing the receiver  56  and shaft  66  from conductive metals, or doped thermoplastics or thermosets. The shaft  66  is configured to contact an interior surface of the receiver  56  upon displacement of the shaft  66 . Separate wires of the electrical circuit are attached to the receiver  56  and shaft  66 . 
     Resistances R var  are measured and correlated with different displacements of the shaft  66 . 
     Ohm&#39;s law provides: 
     
       
           V   a   =I   a   R   var . 
       
     
     Thus, displacement of the shaft D is determined by measuring voltage V a , because measured voltage V a  yields resistance R var , which yields displacement D from previous correlations. 
     With reference to FIG. 4, a shaft displacement determining mechanism  77 , constructed in accordance with another embodiment of the invention, will now be described. The shaft displacement determining mechanism  77  includes an electrical circuit having a solenoid  78  with an inductance L, a sinusoidal current I b  having a frequency less than 100 kHz, and a measured voltage V b . The shaft  66  is completely ferromagnetic or includes a substantial ferromagnetic portion so that displacement of the shaft  66  causes a measurable voltage V to develop across the solenoid, and a resulting change in current di over a discrete time period dt. The voltage V across the solenoid is governed by the following equation: 
     
       
           V=L ( di/dt ), 
       
     
     where the inductance L of the solenoid is correlated with numerous displacements D of the shaft so that the displacement D of the shaft  66  can be determined based on L. 
     By measuring the voltage V across the solenoid, the displacement D of the shaft  66  can be determined. 
     Regardless of the means for determining the displacement D of the shaft  46 , the pressure P on the distal region  46  of the body  38  is determined as follows: 
     
       
         
           P=F/A, 
         
       
     
     where F is the compressive force on the distal region  46  of the body  38 , and A is the surface area of the distal region  46 . The surface area A of the distal region in contact with the myocardial tissue depends on the displacement of the shaft. Numerous values for surface area A are correlated for various shaft displacements. Thus, the surface area A will be known for a given shaft displacement D. 
     
       
         Because  F=kD,   
       
     
     where k is the known spring constant of the spring  70 , P can be rewritten as: 
     
       
           P =( kD )/ A   
       
     
     Thus, by measuring the voltage V, the surface area A, and the axial pressure P at the distal region  46  of the body  38  can be determined. The surface area A and axial pressure P can be compared to respective reference values to determine the sufficiency of the tissue contact at the distal region  46 . Knowing the sufficiency of the tissue contact at the distal region  46  is important for a number of reasons such as determining whether the temperature sensor  74  is providing an accurate reading of the myocardial tissue, the electrode structure  30  needs to be re-positioned for pacing, ablating, and/or sensing, and the pacing, ablating and/or sensing potential prior to delivering energy. 
     With reference to FIG. 5, an electrode structure  80 , constructed in accordance with an additional embodiment of the invention will now be described. Where appropriate, like reference numbers with an “a” suffix have been used to indicate like parts of the embodiments for ease of understanding. The electrode structure  80  illustrated in FIG. 3 is different from the electrode structure  30  illustrated in FIG. 2 in that the electrode structure  80  includes an expandable-collapsible body  82  having a different configuration than that of expandable-collapsible body  38 . The body  82  has a generally squashed tear-drop or pear shape. The body  82  includes a head portion  84  having a distal region  86  and proximal region  88 . The distal region  86  of the head portion  84  is generally flat, increasing the surface area of the distal region  86 , compared to distal region  46  of the body  38 , for contacting myocardial tissue for ablation. A larger tissue contact surface area allows larger lesion creating capability. The body  82  also includes a slightly incurved neck portion  90 . The incurved neck portion  90  has a relatively large radius of curvature. The relatively large radius of curvature in the neck portion  90  facilitates collapsing of the body  82  into a low profile for removal of the electrode structure  80  from the body. 
     The body  82  may also include a non-porous mask  92  along the neck portion  90  and the proximal region  88  of the head portion  84  to prevent ionic transport through an area of the body  82  where it is not required. 
     One or more signal wires  93  may be helically wound within the guide tube  24  and/or balloon support assembly  54  to allow for more unrestricted axial movement of the shaft  66   a  and the distal region  86  of the body  82 . A straight wire is stiffer than a helically wound wire and inhibits the axial movement of the shaft  66 a and the distal region  86  of the body  82 . 
     With reference to FIGS. 6A and 6B, in a preferred embodiment, the body  82  includes longitudinally disposed ridges  94  and bulbous regions  96 . The bulbous regions  96  include a support rib assembly  95  having support ribs  97  to structurally reinforce the body  82 . The support ribs  97  are longitudinally disposed with respect to a longitudinal axis CL of the body  82  and are preferably laminated with the body material. This construction facilitates collapsing of the body  82  in a predetermined and repeatable manner. The ridges  94  and bulbous regions  96  cause the body  82  to have a generally summer-squash shape. 
     The support ribs are preferably made of a casing paper such as grade 15254 casing paper sold by the Dexter Corporation of Windsor Locks, Conn. The casing paper is a medium weight hemp fiber tissue possessing multidirectional tensile strength. The casing paper has a basis weight of 25.4 g/m 2 , a wet tensile strength of 1500 g/25 mm and 1200 g/25 mm, a wet grain ratio of 80%, a dry edge elongation of 5.9%, a dry center elongation of 4.4%, and an absorbency of 10 (25 mm water climb). The casing paper has a pore size larger than the pore size of the body material. This prevents the casing paper from interfering with the ion diffusion through the body  82 . 
     When the fluid medium is removed from the interior  52   a  of the body  82 , the body  82  naturally collapses inward at the longitudinally disposed ridges  94 , and the bulbous regions  96  form folds that wrap around the balloon support in an overlapping manner. 
     FIGS. 7-9 illustrate additional embodiments of the electrode structure of the present invention. Where appropriate, like reference numbers with a “b”, “c”, and “d” suffix have been used in the respective figures to indicate like parts of the embodiments for ease of understanding. 
     With reference to FIG.7, an electrode structure  100 , constructed in accordance with an additional embodiment of the invention is shown. The electrode structure  100  includes a tubular hollow balloon support  102 . The balloon support  102  includes a wall  104  having an outer surface  106  and an inner surface  108 . The inner surface  108  surrounds an interior chamber  110 . 
     The balloon support  102  also includes a proximal region and a distal region. The distal region has a reduced-diameter portion  115  and the proximal region has an enlarged-diameter portion  111  that fits snugly within the distal portion  28   b  of the guide tube  24   b.    
     The electrode body  82   b  is attached to the distal portion  28   b  of the guide tube  24   b  by sandwiching the neck portion  90   b  between an inner surface of the distal portion  28   b  of the guide tube  24   b  and an outer surface of the balloon support  102 . The electrode body  82   b  may be further attached at this region with a mechanical bond and/or adhesive. 
     In the interior  52   b  of the body  82   b , the distal region of the balloon support  102  carries an electrode  112 . Signal wires (not shown) electrically couple the electrode  112  to the cable  31 . 
     The body  82   b  and balloon support  102  include respective longitudinal axes that are coaxially aligned, forming a common longitudinal axis CL. 
     A temperature sensor  113  is located in the balloon support wall  104 , partially exposed, near the center of the interior  52   b , for determining the temperature of the fluid medium in the body  82   b . The temperature sensor  113  may comprise a thermocouple, thermistor, or the like with a pair of lead wires (not shown) that pass through the balloon support  102  and guide tube  24   b , back to the handle  22  for electrical connection to the cable  31 . 
     A temperature sensor  113  is placed within the interior of the body or balloon  82   b  to complement the temperature sensor  74   b  positioned within the distal tip pacing electrode  72   b . The temperature sensor  74   b  located at the distal end  86   b  of the body  82   b  provides accurate tissue temperature measurements, particularly when the body  82   b  is positioned such that the distal end  86   b  contacts tissue. Because of the large diameter of the inflated body  82   b , the distal temperature sensor  74   b  does not contact tissue when the body  82   b  is positioned so that one side of the body  82 b contacts tissue. A temperature sensor  113  located within the body  82   b  helps to give a more accurate tissue temperature measurement. The controller regulates delivery of radiofrequency energy to the electrode  112  inside the body  82   b  based on the maximum temperature of the two temperature sensors  74   b ,  113 . This enables more accurate temperature monitoring for all orientations of the body  82   b  relative to tissue. 
     Predictive temperature algorithms previously described may be employed to predict the maximum tissue temperature by comparing temperature measured at the distal tip  86   b  and within the body  82   b . In addition, comparing measured temperature at the distal tip  86   b  and within the body  82   b  provides an estimate of the orientation of the body  82   b  relative to tissue. If the temperature at the distal tip  86   b  is significantly higher than that within the body  82   b , the body  82   b  is end-on relative to tissue. If the temperature within the body  82   b  is higher than that at the distal tip  86   b , the body  82   b  is oriented sideways relative to tissue. 
     A pair of lumens  45   b ,  47   b  extend through the guide tube  24   b  and balloon support  102 , and terminate into respective open distal ends  116 . The balloon support  102  includes a pair of opposing holes near the center of the interior  52   b  that receive the distal ends  116  of the lumens  45   b ,  47   b . The lumens  45   b ,  47   b  convey the fluid medium to and from the interior  52   b  of the body  82   b . Although a pair of lumens  45   b ,  47   b  are shown, the number of lumens and the junction location of the lumens  45   b ,  47   b  with the balloon support  102  may vary. 
     With reference to FIGS. 8A and 8B, an electrode structure  120 , constructed in accordance with a preferred embodiment of the invention, is shown. The electrode structure  120  includes a tubular balloon support  122  having a hollow proximal portion  124  of an enlarged diameter and a distal portion  126  of a reduced diameter. The distal portion  125  of a steering wire assembly  127  including steering wires  41   c ,  43   c  is attached to the balloon support  122  at the proximal portion  124 . The proximal portion  124  includes laterally incurved sections  128  on opposite sides of the balloon support  122 . The laterally incurved sections  128  receive respective electrode wires  129  and lumens  45   c ,  47   c  that extend through the guide tube  24   c  and into the interior  52   c , on the outside of the balloon support  122 . The wires  129  extend from the electrode  112   c  and are connected to the cable  31 . The lumens  45   c ,  47   c  terminate into open distal ends  132  in the interior  52   c . The lumens  45   c ,  47   c  serve the same function as the aforementioned lumens illustrated in FIGS. 1 and 7. 
     With reference to FIG. 9A, an electrode structure  140 , constructed in accordance with a further embodiment of the invention, is shown. The electrode structure  140  includes a body  142  with a similar configuration to the body  82  described above. The body  142  includes a head portion  144  and a neck portion  146 . The head portion includes a proximal region  148  and a distal region  150 . The body  142  also includes an inner surface  152  and an outer surface  154 . 
     The distal region  150  includes a recessed portion  156  made of the same regenerated cellulosic substance as the body  142  and formed integrally therewith. The recessed portion  156  carries the pacing electrode  72   d  and the temperature sensor  74   d . The pacing electrode  72   d  is affixed to the recessed portion  156  along its length and underside, i.e., where the pacing electrode  72   d  contacts the recessed portion  156 , with an affixant such as cyanoacrylate. Providing a recessed portion  156  formed integrally with the body  142  and affixing the recessed portion  156  along its length and underside to the pacing electrode  72   d  ensures that the body  142  does not tear away from the distal portion of the balloon support  102   d . The pacing electrode  72   d , temperature sensor  74   d , and balloon support  102 d may also be formed integrally with the body  142 . 
     A number of elements may also be formed integrally with the body  142 . For example, support ribs  158 , similar to the support ribs  97  described above in conjunction with FIGS. 6A and 6B, may be integrally formed with the body  142 . The support ribs  158  may be made of a separate material such as casing paper, or, similar to the recessed portion  156 , the support ribs  153  may be constructed of the same material as the body  142 . 
     Operative elements other than the pacing electrode  72   d  and the temperature sensor  74   d  may also be integrally formed with or embedded at least partially within the body  142 . For example, temperature sensors  159  and electrodes  160  may be formed integrally with the body  142 . 
     The electrodes  160  are suitable for unipolar or bipolar sensing or pacing. The electrodes  160  are embedded in the body material so that they are able to make electrical contact with body tissue. In other words, if the electrodes  160  are located in the non-porous region, e.g., masked region, the electrodes  160  are at least partially exposed on the outer surface  154  so that they are capable of making tissue contact, and if the electrodes  160  are located in porous region their ability for sensing or pacing is not impaired because the porous region provides good electrically conductive properties. Connection wires  162 ,  163  respectively connect the temperature sensors  159  and electrodes  160  to the cable. 
     Opaque markers  164  may also be integrally formed with the body  142  so that the physician can guide the device under fluoroscopy to the targeted site. Any high-atomic weight material is suitable for this purpose. For example, platinum or platinum-iridium may be used in the markers  164 . Preferred placements of these markers  164  are at the distal tip and the center of the electrode structure  140 , completely embedded within the body  142  or located on the inner surface  152  of the body  142 . 
     With reference to FIGS. 10A-10C, a number of methods for manufacturing a three-dimensional electrode body of regenerated cellulose will now be described. 
     FIG. 10A illustrates a dissolvable mandrel  170  carried by a support frame  172 . The mandrel  170  has a head portion  174  and a neck portion  176 . The head portion has a proximal region  178  and a distal region  180 . The head portion  174  and the neck portion  176  of the mandrel  170  have the same general shape as the head portion  84  and neck portion  90  of the body  82  illustrated in FIG.  5 . 
     The support frame  172  is generally “U” shaped and includes a handle portion  182  that is manipulated by a user&#39;s hands and a support portion  184  that carries the mandrel  170 . 
     The dissolvable mandrel  170  is formed onto the support portion  184  of the support frame  172  by injecting mandrel solution into a two-piece mold. After the mandrel solution solidifies, the molds are split apart, leaving the desired mandrel  170 . The mold may be formed from two mating aluminum, clay, or other material blocks which are milled, pressed, or formed into the desired shape. The two blocks are clamped together during the mandrel forming process and incorporate an injection port for injecting the mandrel solution, i.e., venting ports for preventing bubble formation, and support ports for accommodating the support portion  184  of the support frame  172 . 
     The mandrel  170  preferably has a generally pear or summer squash shape, as illustrated in FIGS. 5-9. To create the body  82  illustrated in FIGS. 6A and 6B, the mandrel  170  must have corresponding longitudinally disposed ridges and bulbous regions. The bulbous regions have longitudinal grooves therein to accommodate the support rib assembly  95 . 
     Similarly, the mandrel  170  may include other specially configured recesses or protrusions to create a desired body geometry. For example, recessed portion  156  illustrated in FIG. 9A may be formed by creating a corresponding recessed portion in the distal region  180  of the mandrel  170 . Ribs similar to the ribs  97  illustrated in FIG. 9A and 9B are preferably integrally formed with the body  142  by providing rib-shaped recesses or grooves in the mandrel  170  so that the formed body  142  includes ribs also made of regenerated cellulosic substance or ribs made of a different material, e.g., casing paper, encased within the cellulosic material. 
     The pacing electrode  72   d  and temperature sensor  74   d  illustrated in FIG. 9A at the distal tip of the regenerated cellulose body  142  may be incorporated into the mandrel  170  so that upon forming the regenerated cellulose body  142  a bond forms between the pacing electrode  72 d and the distal portion of the body  142  surrounding the pacing electrode  72   d . A mask such as a solid strip of flattened wire may be placed over the pacing electrode  72   d  and temperature sensor  74   d  prior to forming the body  142  to prevent these elements from being covered during the body forming process. After the mandrel  170  is dipped into the cellulosic substance, the mask is removed, exposing the pacing electrode  72   d  and temperature sensor  74   d . The bond between the pacing electrode  72   d  and the immediate portion of the body  146  surrounding it may be strengthened by an affixant such as cyanoacrylate, or other mechanical bond, e.g., wrapping these portions together with a wire. 
     Operative elements such as temperature sensors  159 , electrodes  160  and opaque markers  164 , and other elements such as ring electrode/collar  60   d , and balloon support  102   d  illustrated in FIG. 9 may be formed with the body  142 . For example, the operative elements may be readily laminated in the body wall by supporting them by the mandrel  170  during the body forming process, similar to the lamination or encasement of the support rib assembly  95  within the body  142 . The signal wires  162 ,  163  for the temperature sensors  159  and electrodes  160 , respectively, are fed through the mandrel  170  or on the exterior surface of the mandrel  170  towards the neck portion  176 , and ultimately through the guide tube  24  of the catheter  20  for connection to the cable  31 . If any of the operative elements need to be exposed on the exterior surface of the body, a mask may be employed similar to that described above for the pacing electrode  72   d  and temperature sensor  74   d  at the distal tip. 
     The electrode or collar  60   d , and balloon support  102   d  may be incorporated into the mandrel  170  so that the distal region  180  of the mandrel  170  mates with the tip of the balloon support  102   d  and the proximal region of the neck portion  176  mates with the proximal end of the electrode/collar  60   d . Upon formation of the electrode body  142 , a bond is created between the body  142  and the collar  60   d  at the proximal region of the neck portion  146  of the body  142  and the distal tip of the head portion  142  of the body. The electrode  60   d  may also have mechanical bonding means such as threaded fasteners (not shown) for screwing into an interior lumen of the distal portion of the catheter guide tube. The bond between the electrode body and the collar  60   d  and balloon support  102   d  may be strengthened by an affixant such as cyanoacrylate. 
     In a preferred embodiment of the method of manufacturing the electrode body using a dissolvable mandrel, the dissolvable mandrel  170  is made of polyethylene glycol. The polyethylene glycol mandrel  170  has a smooth, waxy exterior. Because of the smooth exterior of the mandrel  170 , an adhesive primer coating is added to the exterior of the mandrel  170  for causing the cellulosic substance to adhere to the mandrel  170  in the dipping steps described below. 
     The primer coating may be applied over the mandrel  170  by dipping or spraying the mandrel  170  in or with a commercially available base primer. Preferably, the mandrel  170  is sprayed with Duro brand all-purpose spray adhesive manufactured by Loctite Corporation, North America Group, of Rock Hill, Conn. It will readily appreciated by those skilled in the art that similar primer coating materials may be used. 
     After applying the primer coating, the mandrel  170  is dipped into a viscose or cellulosic substance, head portion  174  first. The cellulosic substance is viscose (cellulose xanthate), which is sold by Viskase Corporation of Chicago, Ill. Cellulose xanthate is a form of solubilized cellulose derivative that is dissolved in a sodium hydroxide solution. 
     The mandrel  170  is handled at the handle portion  182  of the support frame  172 . The mandrel  170  may be dipped into the viscose solution manually or automatically. Automatic dipping by an automated dipping apparatus at a controlled dipping rate is preferred for achieving more repeatable results. 
     The thickness of the cellulosic substance is controlled by the viscosity of the solution and the dipping rate, and a different viscosity of the solution can be achieved by diluting it with sodium hydroxide solution. A variable wall thickness can be achieved by varying the extraction rate during the dipping process. The slower the extraction rate, the thinner the wall thickness, and the faster the extraction rate, the thicker the wall thickness. 
     Because the shape of the mandrel  170  and gravity causes the cellulosic substance to collect at the distal region  180  of the head portion  174 , after dipping the mandrel, the distal region  180  is preferably tapped on a flat surface to remove substantially all the viscose solution on the distal region  180 . As will be better understood below, this tapping step helps to ensure that the resulting electrode body has a uniform thickness, i.e., not too thick at the distal region of the head portion. 
     After the tapping step, the cellulosic substance remaining on the mandrel  170  is coagulated in a 15% wt./wt. sodium sulfate solution to secure and solidify the viscose solution on the mandrel  170 . 
     Next, the mandrel  170  is re-dipped into the cellulosic substance, neck portion  176  first. The cellulosic substance used in the redipping or second dipping step may be the same or a different cellulosic substance from that used in the first dipping step. Re-dipping the mandrel  170  neck portion  176  first achieves the proper thickness of viscose solution on the mandrel  170 . 
     After re-dipping the mandrel  170 , the viscose solution is regenerated in a weak 0.01% wt./wt. sulfuric acid (H 2 SO 4 ) solution for approximately 45 minutes. The sulfuric acid converts the xanthate of the cellulose xanthate back into the cellulose structure. The term regenerated cellulose refers to cellulose which has been converted from a solubilized cellulose derivative back into a pure cellulose structure. This regeneration process creates micro-size pores in the coating that are large enough to allow ionic transport, yet small enough to prevent the ingress of blood cells, infectious agents such as viruses and bacteria, and large biological molecules such as proteins. It will readily appreciated by those skilled in the art that similar regeneration solutions may be used. 
     Materials other than regenerated cellulose that are mechanically robust and that have suitable characteristics could be used for the coating material. Hydrophilic materials that have effective pore sizes from 500 to 500,00 Daltons with a porosity of 1-10% and which are biocompatible could be effective. Some types of hydrogels, such as those used for disposable contact lenses are good candidate materials. Plastic materials that have additives to make them semiconductive could also be used. The loaded plastic would need to have a resistivity in the range of about 200-2,000 ohm-cm, and would need to be appliable in very thin films to the mandrel  170 . 
     After the cellulose is regenerated, it is rinsed with tap water to remove acid residuals and sulfur compounds. An oxidizing agent, e.g., bleach, may be added to the rinse water to accelerate the removal of sulfur compounds. It will readily appreciated by those skilled in the art that similar oxidizing agents may be used. 
     After the cellulosic substance is regenerated, it is fully cured in a low humidity environmental chamber at approximately 35° C. for approximately one hour, forming a regenerated cellulose body. 
     In order to remove the mandrel  170  regenerated from the cellulosic substance, the mandrel  170  and regenerated cellulosic substance are placed in a hot water bath at approximately 85° C. for approximately  30  minutes. The hot water bath causes the mandrel  170  to dissolve. 
     After the mandrel  170  dissolves, the regenerated cellulose body is removed from the water and dried. 
     Next, the regenerated cellulose body is positioned on a generally cylindrical mandrel having a diameter less than the diameter of the neck portion of the body. The regenerated cellulose body is dipped, neck portion first, into a non-porous masking material up to the distal region of the head portion. The masking material preferably used contains from about 85% wt./wt. to 95% wt./wt., and preferably about 91% wt./wt. to 93% wt./wt., polyester-polyurethane aqueous dispersion such as Bayhydrol PR240™ made by the Bayer Corp., and 5% wt./wt. to 15% wt./wt., and preferably about 4% wt./wt. To 6% wt./wt., polyfunctional aziridine cross linker such as cross linker CX-100™ made by Zeneca Resins in Wilmington, Mass. It will readily appreciated by those skilled in the art that similar masking materials may be used. 
     The masking material is cured by placing the regenerated cellulose body with masking material in an environmental chamber at approximately 110° C. for approximately one hour. The non-porous mask prevents ionic transport of electrical energy through the areas of the regenerated cellulose body covered by the mask. 
     It is preferable to make the regenerated cellulose flexible when dry, and to do so, moisture may be reintroduced into the regenerated cellulose body by placing the body into an environmental chamber and setting the environmental chamber to a high humidity. Alternatively, a small quantity of a material such as glycerol may be applied to the body, and the hydroscopic nature of the glycerol will hydrate the cellulosic substance to create sufficient flexibility. 
     In an alternative embodiment of the method of manufacturing the electrode using a dissolvable mandrel, the dissolvable mandrel  170  is made of a gelatin such as gelatan-type B-VG-100BLOOM made by Vyse Gelation Co. in Schiller Park, Ill. 
     The mandrel  170  is formed in the above-mentioned two-piece mold in a refrigerated environment. The mandrel  170  is preferably maintained in a refrigerated environment until the mandrel  170  is ready to be used for regenerated cellulose body formation. If the mandrel  170  is not refrigerated, water may evaporate from the gelatin mandrel, changing the dimension of the mandrel. 
     The dipping and curing process for the dissolvable gelatin mandrel is the same as that described above for the polyethylene glycol mandrel. 
     Mandrel materials such as polyethylene glycol and gelatin are desirable because they allow the regenerated cellulose body to be formed into a desired three-dimensional body shape. A polyethylene glycol mandrel is preferable over a gelatin mandrel because water evaporation is not a problem with polyethylene glycol. 
     With reference to FIG. 10B, an alternative method of manufacturing an electrode body will now be described. FIG. 10B illustrates an inflatable mandrel balloon  190  on a catheter guide tube  191 , i.e., Swan Ganz catheter. The mandrel  190  is preferably made of latex, but may be made from Teflon or a similar material. The mandrel  190  can be expanded using gas or liquid into the general geometry desired for the body. 
     The dipping and curing process for the inflatable mandrel  190  is the same as that described above for the polyethylene glycol mandrel, except the mandrel balloon  190  is separated from the regenerated cellulose body by deflating the mandrel  190  instead of dissolving the mandrel. 
     In an alternative embodiment of the method of manufacturing the electrode body, a microporous braided structure  192  is provided around a mandrel such as one of the dissolvable mandrels, the balloon mandrel, or a glass mandrel. The braided structure  192  includes a mesh having the desired pore size and porosity. The structure  192  is fabricated from a material such as nylon, polyester, polyethylene, polypropylene, fluorocarbon, fine diameter stainless steel, or similar fiber. 
     If a conductive material such as stainless steel is used, the wire may also be used to deliver RF current from the generator to the body surface. The use of woven materials is advantageous because woven materials are very flexible as small diameter fibers can be used to weave the mesh. By using woven materials, uniformity and consistency in pore size also can be achieved. The three-dimensional structure may be formed from a braided tubing having an open proximal end and distended section with an open distal end, where the open distal end is sewn or welded closed. 
     The mandrel is separated from the braided structure  192  and is dipped into a cellulosic substance such as that previously described and allowed to cure. The wire or fiber separation for the braided structure would be small enough to enable the viscous cellulosic substance to adhere and cure, yet large enough not to interfere with the ionic flow required to produce a current path from the interior of the body to tissue contacting the exterior of the body. 
     After dipping the braided structure  192 , the viscose material is regenerated in a weak 0.01% wt./wt. sulfuric acid (H 2 S 0   4 ) solution for approximately 45 minutes. 
     Alternatively, the mandrel  190  and braided structure  192  are dipped in the cellulosic substance. The cellulosic substance is cured and regenerated. Then, the mandrel  190  is separated from the braided structure  192 . 
     After the cellulose is regenerated, it is rinsed with water, and cured in a low humidity environmental chamber at approximately 35° C. for approximately one hour. 
     Subsequently, a non-porous masking material is applied to the neck portion and proximal region of the head portion of the body in the manner described above and cured at approximately 110° C. for approximately one hour. 
     Alternatively, the dipping and curing process described above with respect to the polyethylene glycol mandrel  170  may be performed. 
     The braided structure gives the electrode body improved tensile strength and burst strength, and reduces the tendency to develop pin holes in the body. 
     The above-described methods of manufacturing a three-dimensional electrode structure produce a three-dimensional electrode body made of regenerated cellulose with a specific geometry that was not achievable in the past. The ability to produce a specific three-dimensional regenerated cellulose body allows all the advantages that regenerated cellulose offers and the advantages of a specific three-dimensional electrode structure, namely, the ability to create lesions having a specific geometry. 
     It will be readily understood by those skilled in the art that other methods may be employed to manufacture the electrode body such as, but not by way of limitation, injecting a cellulosic substance into the interior lumen of a glass mandrel in the shape of the electrode body followed by chemically etching the mandrel so that the cured cellulosic material remains, and dipping the exterior of a glass mandrel in the shape of the electrode body followed by chemically etching the mandrel so that the cured cellulosic material remains. 
     With reference to FIGS. 11-16, a method of assembling the electrode structure illustrated in FIGS. 8A and 8B in accordance with a preferred embodiment of the invention will now be described. 
     With reference to FIGS. 11A and 11B, the temperature sensor  113 c is added to the balloon support  122 . The temperature sensor  113   c  preferably comprises a thermocouple having a pair of lead wires  194  extending therefrom. The temperature sensor  113   c  is affixed within a window  196  of the balloon support  122  with an affixant such as cyanoacrylate so that the temperature sensor  113   c  is at least partially exposed. 
     With reference to FIGS. 12A and 12B, the electrode  112   c  and lead wires  129  are slid over the distal portion  126  of the balloon support  122 . The wires  129  fit into the laterally incurved sections  128 , in opposite side of the balloon support  122  (FIG.  8 B). 
     With reference to FIGS. 13A and 13B, the lumens  45 ,  47  are fit into the laterally incurved sections  128  (FIG.  8 B),over the wires  129 , and affixed there to  122  with an affixant such as cyanoacrylate. 
     With reference to FIG. 14, the body  82   c  and pacing electrode  72   c , including the associated temperature sensor, are added to the balloon support  122 . The balloon support  122  is inserted within the body  82   c  or the body  82   c  is placed over the balloon support  122 . An inner wall of the neck portion  90   c  is affixed to an outer wall of the balloon support  122  with an affixant such as cyanoacrylate. The bond between the pacing electrode  72   c  and the distal region  86   c  of the body  82   c  is created or reaffirmed  122 , if a bond exists, with an affixant such as cyanoacrylate. The bottom portion of pacing electrode  72   c  is affixed to the distal region  126  of the balloon support  122  also with an affixant such as cyanoacrylate. Wires  198  from the pacing electrode  72   c  and associated temperature sensor are threaded through the balloon support  122  for connection to the cable  31 . 
     With reference to FIG. 15, the distal portion  125  of the steering wire assembly  127  is affixed to the proximal portion  124  of the balloon support  122  with an affixant such as cyanoacrylate. 
     Finally, with reference to FIG. 16, the guide tube  24   c  is installed over the proximal portion  124  of the balloon support  122  and the neck portion  90   c  of the body  82   c , and affixed to the outer wall of the neck portion  90   c  with an affixant such as cyanoacrylate. 
     It will be readily understood by those skilled in the art that certain features and elements described above may be incorporated into other embodiments even though not specifically described with respect to that embodiment. For example, the balloon support described in conjunction with FIGS. 7-9 and  11 - 16  may be replaced with or include a biasing device and/or tissue evaluating mechanism such as that described in FIGS. 2-5. Moreover, although this invention has been described in terms of certain preferred embodiments, other embodiments apparent to those of ordinary skill in the art are also within the scope of this invention. Accordingly, the scope of the invention is intended to be defined only by the claims that follow.