Abstract:
Method and apparatus for applying a thermal conductive medium (TCM) to the inside of a disposable sheath. A uniform amount of TCM is applied in an automated manner to the conductive portion of a disposable sheath to increase conductivity. Application is by way of a rotating tip having a plurality of nozzles through which TCM is pumped under control of a timing circuit. The duration of TCM application and the rotation of the tip is carefully controlled to insure uniform application of a precise amount of TCM. TCM application is avoided at portions of the sheath where thermal conductivity is not desired and at the tip of the sheath which will be coated by TCM as a result of probe insertion into the sheath. By automating the TCM application process, TCM can be applied in precise amounts and in a uniform manner without accidentally coating areas of the sheath where TCM is not desired. In addition the process can be conducted in far less time than is required to manually apply TCM.

Description:
FIELD OF THE INVENTION 
   The present invention is directed to methods and apparatus for coating the inside of at least a portion of a sheath, e.g., a sheath used to cover a medical device, with a thermal conductive medium (TCM). 
   BACKGROUND OF THE INVENTION 
   Medical devices, e.g., probes, are used in a variety of applications. Such probes are frequently inserted into the human body as part of medical procedure. Probes are often relatively expensive durable devices. In many cases, absent the risk of disease transfer and/or infection, probes may be used repeatedly. Unfortunately, a probe&#39;s shape and/or construction can make it difficult to sterilize thoroughly between uses. 
   For purposes of avoiding infection and the problems associated with sterilizing a probe thoroughly a sterile sheath may be used to cover the portion of the probe which is inserted into the human body. After each use, the used sheath is discarded and replaced with a new sterile sheath thereby allowing reuse of the probe at a minimal cost. 
   In cases of medical probes where thermal conductivity is important, e.g., cryogenic probes and/or temperature sensing probes, the sheath should not interfere significantly with the transfer of heat to/from portions of the probe where thermal transfer is intended to occur. 
   One example of a medical probe that may use a sheath is a cryogenic probe such as the probe  100  shown in FIG.  1 . The probe  100  comprises a handle  102 , a hollow tubular cannula  106 , and a cold tip  108 . The cold tip  108  is used to absorb heat from any tissue with which it contacts thereby cooling and potentially freezing the contacted tissue. Thus, in the known probe  100 , heat transfer is to occur at the tip  108 . However, heat transfer is intended to be limited elsewhere to prevent the unintentionally freezing of tissue contacting the cannula  106 . 
   A sterile sheath may be placed over the probe  100  to protect the probe from contamination, and to protect the patient from possible infection. Preferably, the sheath has a thermally conductive region that covers the cold tip  108 , and a nonconductive or less conductive region corresponding to the portion of the sheath intended to surround the cannula  106 . 
   In order to insure good thermal conductivity at points where heat transfer is desired, there should be a snug fit between the sheath and the probe. In order to enhance thermal transfer between the sheath and probe at the desired points a thermal conductive medium (TCM), e.g., thermally conductive grease, may be applied to the inside of the sheath. 
   Known techniques for applying TCM to the interior of a sheath involve manually applying a TCM to the conductive region, e.g., tip, of the sheath using a hand held wand brush. In the known method, the TCM is first applied to the wand brush and then the brush is inserted into the tip of the sheath transferring the TCM to the inside of the sheath&#39;s tip. This method leads to variability in the amount of TCM applied to the sheath and can lead to the problems discussed above associated with TCM lumps and excessive TCM application. In addition to problems relating to variability in TCM application, the known TCM application procedure has the disadvantage of being time consuming to perform. 
   In view of the above discussion, it can be appreciated that there is a need for improved methods and apparatus for applying TCM to specific portions of a sheath. It is desirable that any new TCM application methods produce more reliable and uniform application of TCM to the intended portions of the sheath. It is also desirable that any new TCM application methods reduce the amount of time associated with performing the TCM application process. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to methods and apparatus for applying a thermal conductive medium (TCM) to the inside of a sheath, e.g., a disposable cryosurgical sheath. 
   Generally, the goal is to achieve a uniform thin film of TCM between the thermally conductive portions of the probe and sheath when the sheath is covering the probe. Lumps and/or excessive amounts of TCM on the sheath are undesirable since they can interfere with insertion of the probe into the sheath potentially causing rupturing of the sheath during probe insertion. Lumps and/or excessive amounts of TCM can also cause air to be trapped between the tip of the probe and the sheath interfering with thermal transfer, and/or increase thermal transfer along portions of the probe, e.g., the cannula, where heat transfer is not desired. 
   Normally, a TCM is applied to the interior of the sheath prior to packaging. In this manner, the medical technician need only open the sterile sheath and insert the probe prior to use of the probe. 
   In accordance with the invention, a uniform amount of TCM may be applied in an automated manner to the conductive portion of a disposable sheath, using a TCM applicator device of the present invention, to increase conductivity at desired portions of the sheath. 
   An exemplary TCM application device of the invention includes a TCM pump, tubular shaft, applicator tip, control circuit and a switch. The switch may be manually operated. Alternatively, the switch may be, e.g., a contact switch, that is triggered by placing a sheath over the applicator tip and tubular shaft. In response to activation of the switch, the control circuit beings the pumping of TCM and the applicator shaft and tip are rotated as TCM flows out nozzles in the side of the applicator tip. After TCM is pumped for a fixed amount of time, the pump is stopped by the control circuit while rotation of the applicator shaft and tip is allowed to continue so that the TCM will be spread by the wiping action associated with the rotation. Rotation of the applicator shaft and TIP is stopped after a set amount of time and the sheath is removed. Optionally, the applicator&#39;s shaft and tip may be purged of TCM preparing the device for processing of the next sheath. 
   The control circuit may be implemented as a processor controlled by software with set times for starting/stopping the pump and motor being programmed into memory along with said control software. Alternatively the control circuit can be implemented using fixed electrical circuits with the set periods of time being determined by the electrical components selected to implement the control circuit. 
   In one embodiment, the applicator tip is designed with a mushroom shaped top designed to prevent application of TCM to the tip of the sheath. During insertion of the probe TCM from the sidewalls of the sheath will be transferred by the probe to the tip. Accordingly, by limiting and/or preventing application of TCM to the inside tip of the sheath, excessive amounts of TCM at the tip during use are avoided. 
   Since the amount of TCM applied can be precisely controlled by the time TCM is pumped, application of excessive amounts of TCM can be avoided. In addition, TCM can be applied in a much more uniform manner than can normally be achieved using the known manual TCM application technique. 
   Because the application process of the invention is performed in an automated manner, it has the additional advantage of normally taking less time than the known manual technique of applying TCM. 
   Numerous features and advantages of the present invention will be apparent in view of the detailed description set forth below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a known cryogenic medical probe. 
       FIG. 2  is a diagram of a sheath and an exemplary TCM applicator implemented in accordance with the present invention. 
       FIG. 3  is a diagram of an exemplary TCM applicator tip suitable for use with the TCM applicator illustrated in FIG.  2 . 
       FIG. 4  is a flow chart illustrating the steps of a TCM application method of the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention is directed to methods and apparatus for applying a thermal conductive medium (TCM), e.g., conductive grease, to portions of the interior of a sheath, e.g., a sheath for a disposable cryosurgical probe. The TCM may be applied in order to increase the thermal conductivity between the disposable sheath and the probe at specific desired locations, e.g., the tip. 
     FIG. 2  illustrates an exemplary disposable cryosurgical sheath  202  and an exemplary TCM applicator  212  implemented in accordance with the present invention. The disposable sheath  202  is shown for purposes of explaining the invention. The sheath  202  includes a tip  204 , cannula cover  206  and main body  208 . The tip  204  may be made out of a thermally conductive material, e.g., copper, while the cannula cover  206  and main body  208  may be made of a flexible insulating material, e.g., a latex material. For purposes of applying TCM to inside portions of the sheath  202 , the sheath is placed onto the applicator  212  by moving it in the direction indicated by arrow  210 . 
   The Applicator  212  comprises a tubular applicator tip  214 , tubular applicator shaft  220  and main housing  224 . The tubular design of the applicator tip  214  and applicator shaft  220  provides a hollow channel through the center of the shaft  220  and nozzle  214  through which TCM and/or air can pass. 
   The tubular applicator tip  214  has a closed tip end  213 , an open shaft end  216  and a tubular nozzle portion  215  connecting the tip end and shaft end together. The closed tip end  213  may be mushroom shaped with radial edges protruding over the narrower tubular nozzle portion  215  to prevent application of TCM to the very end to the sheath&#39;s tip. 
   The shaft end  216  of the applicator tip  214  can be attached to the applicator shaft  220  by threads or other non-permanent attachment methods which allow for easy removal, cleaning and/or replacement of the applicator tip  214 . Alternatively, the applicator tip  214  can be permanently attached to, and/or integrated with, the applicator shaft  220 . 
   A plurality of nozzles  217  extend axially along the length of the tip&#39;s nozzle portion  215 . Each nozzle  217  extends through the sidewall of the tip&#39;s nozzle portion into the hollow channel extending through the center of the tubular nozzle  215 . Nozzles  217  may be implemented as holes in the tip  214 . TCM entering the nozzles  217  by way of the tip&#39;s hollow channel will be expelled onto portions of the interior surface of the sheath  202 . In various implementations nozzle holes having a diameter D in the range between and including 0.142″ through 0.144″ (0.142″≦D ≦0.144″) were found to be suitable. Nozzles outside the exemplary range may also be used. 
   When the sheath is placed onto the applicator  212  air can be trapped between the sheath  202  and  22  applicator shaft  220  and/or nozzle  214 . In order to remove such air prior to TCM application, the tubular applicator shaft  220  may include an air bleeder hole  218 . Air trapped at or near the tip of the sheath  202  during insertion onto the applicator  212  can pass through the nozzles  215  and out through the bleeder hole  218  by way of the hollow channel extending though the center of the applicator shaft and nozzle. Removing the air from the tip portion  214  of the sheath  202  in this manner prior to application of the TCM, facilitates proper positioning of the sheath on the applicator device  212 . 
   The applicator housing  224  comprises a gear and motor assembly  234 , a control module  226 , and a TCM dispenser  236 . The TCM dispenser includes a TCM storage container  240  which is coupled to a pump  238 . When activated pump  238  pumps TCM from the storage container  240  into the shaft&#39;s channel and out through the tip&#39;s nozzles  217 . 
   The applicator shaft  220  is mounted to the gear and motor assembly  234  which, when activated, causes the shaft  220  and tip  214  mounted thereon to rotate. The control module may include a contact switch  222  which protrudes through the top of the applicator housing  224 , and a timing and control circuit  228 . Alternatively, instead of a contact switch, a manually operated switch may be used as a start switch. In such an embodiment, an operator manually activates the start switch to begin the TCM application process. The timing and control circuit  228  controls the rotation of the applicator shaft  220  and thus tip  214  by way of a motor control signal supplied to the gear and motor assembly  234 . It also controls application of TCM by enabling/disabling a TCM pump  238  included in the dispenser  238  by way of a pump control signal. 
   Contact switch  222  is activated as a result of contact with the sheath  202  when the sheath is properly placed over the applicator tip and shaft for TCM application. In response to activation of the switch  222 , timing and control circuit  228  processed to control motor and pump operation to insure proper application of TCM as will be discussed further below with reference to FIG.  4 . 
   The design of the TCM applicator tip  214  will now be discussed further with regard to FIG.  3 .  FIG. 3  illustrates an exemplary tip  214  in detail. As illustrated, a solid mushroom shaped head  311  is position at the tip end  213  of the applicator tip  214 . The mushroom shaped head  311  protrudes radially outward beyond the sidewall of the tubular nozzle portion  215  of the tip  214 . Nozzles  217  are uniformly spaced in the axial direction of the tip  214  by a distance S, e.g., 0.125 inches apart. Nozzles  217  extend along a central portion of the tip&#39;s nozzle region  215 . A base  302  is positioned at the shaft end  216  of the tip  302  for purposes of attachment to the shaft. As discussed above, the base  302  may be threaded so that it can be screwed to the shaft  220 . 
   From a TCM application standpoint, the applicatior tip  214  can be divided into three regions or portions, an upper solid tip region  306 , a TCM application region  308  corresponding to the area where nozzles  217  are located, and a lower tip region  310 . 
   Notably, as a result of the applicator tip&#39;s design, application of TCM to the inside tip of the sheath may be avoided. During use, as a probe is inserted into the sheath, a small amount of TCM will be scraped from the sidewall of the sheath and forced up into the tip. Accordingly, by carefully controlling the application of TCM to the sheath&#39;s sidewall with the understanding that some of the TCM will be transferred to the inside of the sheath&#39;s tip during use, a high degree of uniformity can be achieved in TCM coating while avoiding lumps and/or other insertion problems. 
     FIG. 4  illustrates an exemplary method  400  for applying TCM to an inside portion of a disposable sheath in accordance with the present invention. The method starts in step  402  with a sheath being selected for TCM application. Then in step  404  the selected sheath is placed over the applicator tip  214  and shaft  220 . As the sheath is placed over the shaft  220  it will come into contact with contact switch  222 . In response to the start switch  222  being activated, in step  408 , the timing and control circuit  228  turns the TCM dispenser pump on. This causes TCM to start traveling up the applicator shaft  220  due to the pumping action. Then, in step  412  the motor  234  is turned on. An optional waiting period, shown as step  410 , may be inserted between the time pumping action is started and motor  234  is turned on. Such a waiting period can be useful, e.g., in embodiments where the shaft  220  and applicator tip  214  are purged after each use. 
   Activation of the motor  234  causes the applicator tip  214  and shaft  220  to rotate as TCM is pushed out the nozzles  217 . After a set period of time, in step  414 , the timing and control circuit  228  shuts off the pump  238 . Then, in step  416 , the control circuit waits for another set period of time before proceeding to step  418  wherein the motor  234  is turned off. Once the motor is turned off, the sheath is removed in step  420 . 
   In optional step  422  the timing and control circuit  228  activates the pump to purge the applicator shaft of TCM. This can be done by, e.g., operating the pump  238  to create a vacuum in the shaft  220  to suck out the remaining TCM. After the shaft is purged, the TCM application process stops in step  424  leaving the applicator  212  ready for the next sheath to be processed. 
   By initiating motor operation after TCM dispensing begins, the risk of damaging the sheath from the rotating action of the applicator tip and shaft is reduced. In addition, by continuing to rotate the applicator tip and shaft after TCM pumping has been stopped serves to distribute the TCM applied near the tip through the wiping action of the rotating tip and applicator shaft. 
   In one particular exemplary embodiment a specific amount of TCM, e.g., 0.040-0.045 grams, was applied around the inside of the conductive region of disposable sheath  202  to form a thin layer, e.g., 0.0030 inches deep and having a high degree of uniformity. 
   In the above described manner, the applicator  212  of the present invention can be used to spread TCM on the inner circumference of the conductive region of a disposable sheath evenly, and quickly with far greater accuracy then the known manual application technique. In addition, application of TCM to the tip of the sheath can be avoided and/or minimized allowing the application to occur during probe insertion. 
   Ten experimental applications were preformed using 80 psi of pressure to pump the TCM, and a 0.143 diameter tip. During the tests, the applicator dispensed an average of 0.040 grams of TCM during a 12 second application period, with the range of application amounts being between 0.037-0.043 grams. The resulting thickness of applied TCM to the interior of the sheath at the intended locations varied from a minimum of 0.0022″ to a maximum of 0.0040″. The slight variance between applications during testing shows a high degree of repeatability which is important from a quality control standpoint. Furthermore, using the TCM applicator of the invention produced TCM coatings which were, in most cases, considerably thinner and more uniform than could be achieved using the known manual application technique. 
   The automated TCM application process described above has the advantage of providing a uniform TCM coating, avoiding lumps, and insuring that TCM application is limited to intended portions of a sheath. 
   Numerous variations on the above described methods and apparatus are possible without departing from the scope of the invention. For example, application and removal of sheaths to/from the described TCM applicator device  212  can, and in one embodiment is, automated using a robotic device.