Patent Publication Number: US-2019183506-A1

Title: Surgical instrument battery comprising a plurality of cells

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application claiming priority under 35 U.S.C. § 120 to U.S. patent application Ser. No. 13/961,236, entitled SURGICAL INSTRUMENT BATTERY COMPRISING A PLURALITY OF CELLS, filed Aug. 7, 2013, now U.S. Patent Application Publication No. 2013/0324981, which is a continuation application claiming priority under 35 U.S.C. § 120 to U.S. patent application Ser. No. 12/884,838, entitled SURGICAL INSTRUMENTS AND BATTERIES FOR SURGICAL INSTRUMENTS, filed Sep. 17, 2010, which issued on Mar. 22, 2016 as U.S. Pat. No. 9,289,212, the entire disclosures of which are hereby incorporated by reference herein. 
    
    
     BACKGROUND 
     A growing number of surgical instruments are powered by one or more battery cells. Such instruments include a variety of electrically powered implements and may be used in a variety of surgical environments. For example, battery-powered surgical instruments may include motor-driven implements (cutters, graspers, staplers, etc.) and/or non-motor driven implements (e.g., RF cutter/coagulators, ultrasonic cutter/coagulators, laser cutter/coagulators, etc.). Battery-powered instruments are also used now in various different surgical environments including, for example, endoscopic environments, laparoscopic environments, open environments, etc. 
     Battery-powered surgical instruments often utilize primary cells, which are pre-charged and often intended for a single discharge (e.g., one use). This avoids the difficulties associated with re-sterilizing and recharging secondary, rechargeable cells. Primary cells, however, present additional challenges related to shipping, storage and disposal. 
     SUMMARY 
     Various embodiments may be directed to a surgical instrument comprising an end effector and a handle operatively coupled to the end effector. The handle may comprise a trigger to actuate the end effector and may also define a first cavity having a first asymmetrical cross-sectional shape and a second cavity having a second asymmetrical cross-sectional shape. A first battery pack may be positioned within the first cavity and may be in electrical contact with at least one of the handle and the end effector. The first battery pack may comprise: a first casing having a cross-sectional shape corresponding to the first asymmetrical cross-sectional shape, and a first plurality of cells electrically coupled to one another and positioned within the first casing. A second battery pack may be positioned within the second cavity and may be in electrical contact with at least one of the handle and the end effector. The second battery pack may comprise: a second casing having a cross-sectional shape corresponding to the second asymmetrical cross-sectional shape, and a second plurality of cells electrically coupled to one another and positioned within the second casing. 
     Also, various embodiments may be directed to a surgical system comprising a battery pack. The battery pack may comprise a casing and a plurality of cells positioned within the casing. At least a portion of the plurality of cells may not be electrically connected to one another. The battery pack may also comprise a first switch having an open position and a closed position. In the closed position, the first switch may electrically interconnect the plurality of cells. The first switch may be mechanically biased to the open position. The battery pack may further comprise a discharge switch having an open position and a closed position. The discharge switch may be positioned to, when in the closed position, electrically connect an anode of the battery pack to a cathode of the battery pack. The discharge switch may be mechanically biased to the closed position, and may be held in the open position by a portion of the casing. 
     According to various embodiments, the battery pack may comprise a plurality of cells, where at least a portion of the plurality of cells are not electrically connected to one another. The battery pack may further comprise a casing defining an interior cavity having at least one interior cavity wall. The at least one interior cavity wall may comprise a first electrode electrically connected to an anode of the battery pack and a second electrode electrically connected to a cathode of the battery pack. The battery pack may further comprise a battery drain positioned within the interior cavity. The battery drain may comprises first and second contacts electrically connected to one another and in contact with the at least one interior cavity wall. The battery drain may be positionable at a first position within the interior cavity where the first and second contacts are not in electrical contact with the first and second electrodes and at a second position where the first contact is in electrical contact with the first electrode and the second contact is in electrical contact with the second electrode. 
     Additionally, various embodiments may be directed to a surgical instrument comprising an end effector, a handle operatively coupled to the end effector, and a battery pack. The handle may comprise a trigger to actuate the end effector, and may define a cavity. The battery pack may be positioned within the cavity and may be in electrical contact with at least one of the handle and the end effector. Further, the battery pack may comprise a casing; a plurality of cells; and a movable tab. The movable tab may have a first position where it electrically separates at least a portion of the plurality of cells, and a second position where it does not electrically separate the plurality of cells. 
    
    
     
       DRAWINGS 
       The features of the various embodiments are set forth with particularity in the appended claims. The various embodiments, however, both as to organization and methods of operation, together with advantages thereof, may best be understood by reference to the following description, taken in conjunction with the accompanying drawings as follows: 
         FIGS. 1 and 2  are perspective views of one embodiment of a surgical cutting and fastening instrument; 
         FIG. 3  is an exploded view of one embodiment of the end effector of the surgical cutting and fastening instrument of  FIGS. 1 and 2 . 
         FIGS. 4 and 5  are exploded views of one embodiment of the end effector and shaft of the surgical cutting and fastening instrument of  FIGS. 1 and 2 . 
         FIG. 6  is a side view of one embodiment the end effector of the surgical cutting and fastening instrument of  FIGS. 1 and 2 . 
         FIG. 7  is an exploded view of one embodiment of a motor-driven endocutter. 
         FIGS. 8 and 9  are partial perspective views of one embodiment of the handle of the endocutter of  FIG. 7 . 
         FIG. 10  is a side view of one embodiment of the handle of the endocutter of  FIG. 7 . 
         FIG. 11  is a schematic diagram of one embodiment of an electrical circuit of a surgical cutting and fastening instrument. 
         FIG. 12  is a side-view of a handle of one embodiment of a power-assist motorized endocutter. 
         FIG. 13  is a side-view of a handle of another embodiment of a power-assist motorized endocutter. 
         FIGS. 14 and 15  show one embodiment of a closure trigger locking mechanism. 
         FIG. 16  shows another embodiment of a closure trigger locking mechanism 
         FIGS. 17-22  show another embodiment of a closure trigger locking mechanism. 
         FIGS. 23A-B  show one embodiment of a universal joint (“u-joint”) that may be employed at the articulation point of a surgical instrument. 
         FIGS. 24A-B  show one embodiment of a torsion cable that may be employed at an articulation point of a surgical instrument. 
         FIGS. 25-31  illustrate another embodiment of a motorized, two-stroke surgical cutting and fastening instrument with power assist. 
         FIGS. 32-36  illustrate one embodiment of a two-stroke, motorized surgical cutting and fastening instrument with power assist. 
         FIGS. 37-40  illustrate one embodiment of a motorized surgical cutting and fastening instrument with such a tactile position feedback system. 
         FIGS. 41 and 42  illustrate two states of one embodiment of a variable sensor that may be used as the run motor sensor. 
         FIG. 43  illustrates one embodiment of a surgical instrument comprising a pair of asymmetrical battery packs. 
         FIG. 44  illustrates one embodiment of a battery pack outside of the handle of the surgical instrument of  FIG. 43 . 
         FIG. 45  illustrates one embodiment of a handle of the surgical instrument of  FIG. 43  illustrating cavities for receiving battery packs. 
         FIG. 46  illustrates one embodiment of the battery pack of  FIG. 44  showing a positive electrode contact and a negative electrode contact. 
         FIG. 47  illustrates one embodiment of the battery pack of  FIG. 44  in conjunction with a discharge plug. 
         FIG. 48  illustrates a schematic diagram of one embodiment of a surgical instrument and a battery pack. 
         FIG. 49  illustrates an alternate embodiment of the battery pack and surgical instrument shown in  FIG. 48 . 
         FIG. 50  illustrates another embodiment of the battery pack of  FIG. 48 . 
         FIGS. 51-53  illustrate one mechanical embodiment of a battery pack implementing the schematic of the battery pack shown in  FIG. 48 . 
         FIGS. 54-59  illustrate another mechanical embodiment of a battery pack  800  implementing the schematic of the battery pack shown in  FIG. 48 . 
         FIGS. 60 and 61  illustrates one embodiment of the battery drain of  FIGS. 57-58  removed from the casing. 
     
    
    
     DESCRIPTION 
     Various embodiments are directed to battery powered surgical instruments and batteries comprising features for facilitating shipping, storage and disposal. For example, according to one embodiment, a battery pack may comprise a plurality of cells mechanically and electrically coupled together within a casing having an asymmetric cross-sectional shape. The number and type of cells within the casing may be selected to reduce the power of potential accidental discharges below a threshold level. A surgical instrument for use with the battery pack may comprise a handle defining a plurality of cavities. Each cavity may have an asymmetric cross-sectional shape and at least one of the cavities may have an asymmetric cross-section shape sized to receive the battery pack. An additional cavity and/or cavities may house additional battery packs. According to various embodiments, grouping multiple cells within a single casing may reduce inconveniences associated with loading each cell into the handle individually. At the same time, limiting the number of cells grouped together may reduce safety hazards during shipping, storage and disposal. 
     According to various embodiments, a surgical instrument may utilize one or more battery packs, each comprising a plurality of cells and at least one switch for electrically connecting the plurality of cells. The switch may have an open position, where the cells are electrically disconnected from one another, and a closed position where the cells are electrically connected to one another. The switch may transition from the open position to the closed position when the battery pack is installed in a surgical instrument. In this way, surgical instrument may utilize power associated with a multi-cell battery. At the same time, however, the battery pack may be shipped with the switch in the open position to mitigate the available energy for a short and/or arc and, thereby, mitigate safety hazards during shipping, storage and disposal. In certain embodiments, batteries and cells described herein may have discharge switches for connecting a load across the terminals of the battery or cell to discharge the battery. For example, the discharge switch may be closed prior to disposal. In this way, the battery may discharged either prior to disposal or shortly thereafter. Accordingly, battery safety hazards due to disposal may be mitigated. 
     Prior to describing embodiments of the cells, batteries, battery packs, and associated surgical instruments, a detailed description of an example embodiments of a battery powered surgical instrument is provided. Although the surgical instruments described herein comprise motorized implements for cutting and stapling, it will be appreciated that the battery configurations described herein may be used with any suitable type of electrical surgical instrument including, for example, cutters, claspers, staplers, RF cutter/coagulators, ultrasonic cutter/coagulators, laser cutter/coagulators, etc. 
       FIGS. 1 and 2  are perspective views of one embodiment of a surgical cutting and fastening instrument  10 . The illustrated embodiment is an endoscopic instrument and, in general, the embodiments of the instrument  10  described herein are endoscopic surgical cutting and fastening instruments. It should be noted, however, that according to other embodiments, the instrument may be a non-endoscopic surgical cutting and fastening instrument, such as a laparoscopic or open surgical instrument. 
     The surgical instrument  10  depicted in  FIGS. 1 and 2  comprises a handle  6 , a shaft  8 , and an articulating end effector  12  pivotally connected to the shaft  8  at an articulation pivot  14 . An articulation control  16  may be provided adjacent to the handle  6  to effect rotation of the end effector  12  about the articulation pivot  14 . In the illustrated embodiment, the end effector  12  is configured to act as an endocutter for clamping, severing and stapling tissue, although, in other embodiments, different types of end effectors may be used, such as end effectors for other types of surgical devices, such as graspers, cutters, staplers, clip appliers, access devices, drug/gene therapy devices, ultrasound, RF or laser devices, etc. 
     The handle  6  of the instrument  10  may include a closure trigger  18  and a firing trigger  20  for actuating the end effector  12 . It will be appreciated that instruments having end effectors directed to different surgical tasks may have different numbers or types of triggers or other suitable controls for operating the end effector  12 . The end effector  12  is shown separated from the handle  6  by a preferably elongate shaft  8 . In one embodiment, a clinician or operator of the instrument  10  may articulate the end effector  12  relative to the shaft  8  by utilizing the articulation control  16 , as described in more detail in pending U.S. patent application Ser. No. 11/329,020, filed Jan. 10, 2006, entitled SURGICAL INSTRUMENT HAVING AN ARTICULATING END EFFECTOR, by Geoffrey C. Hueil et al., now U.S. Pat. No. 7,670,334, which is incorporated herein by reference. 
     The end effector  12  includes in this example, among other things, a staple channel  22  and a pivotally translatable clamping member, such as an anvil  24 , which are maintained at a spacing that assures effective stapling and severing of tissue clamped in the end effector  12 . The handle  6  includes a pistol grip  26  towards which a closure trigger  18  is pivotally drawn by the clinician to cause clamping or closing of the anvil  24  toward the staple channel  22  of the end effector  12  to thereby clamp tissue positioned between the anvil  24  and channel  22 . The firing trigger  20  is farther outboard of the closure trigger  18 . Once the closure trigger  18  is locked in the closure position as further described below, the firing trigger  20  may rotate slightly toward the pistol grip  26  so that it can be reached by the operator using one hand. Then the operator may pivotally draw the firing trigger  20  toward the pistol grip  26  to cause the stapling and severing of clamped tissue in the end effector  12 . In other embodiments, different types of clamping members besides the anvil  24  could be used, such as, for example, an opposing jaw, etc. 
     It will be appreciated that the terms “proximal” and “distal” are used herein with reference to a clinician gripping the handle  6  of an instrument  10 . Thus, the end effector  12  is distal with respect to the more proximal handle  6 . It will be further appreciated that, for convenience and clarity, spatial terms such as “vertical” and “horizontal” are used herein with respect to the drawings. However, surgical instruments are used in many orientations and positions, and these terms are not intended to be limiting and absolute. 
     The closure trigger  18  may be actuated first. Once the clinician is satisfied with the positioning of the end effector  12 , the clinician may draw back the closure trigger  18  to its fully closed, locked position proximate to the pistol grip  26 . The firing trigger  20  may then be actuated. The firing trigger  20  returns to the open position (shown in  FIGS. 1 and 2 ) when the clinician removes pressure, as described more fully below. A release button  160  on the handle  6 , and in this example, on the pistol grip  26  of the handle  6 , when depressed may release the locked closure trigger  18 . 
       FIG. 3  is an exploded view of one embodiment of the end effector  12 . As shown in the illustrated embodiment, the end effector  12  may include, in addition to the previously-mentioned channel  22  and anvil  24 , a cutting instrument  32 , a sled  33 , a staple cartridge  34  that is removably seated in the channel  22 , and a helical screw shaft  36 . The cutting instrument  32  may be, for example, a knife. The anvil  24  may be pivotably opened and closed at a pivot point  25  connected to the proximate end of the channel  22 . The anvil  24  may also include a tab  27  at its proximate end that is inserted into a component of the mechanical closure system (described further below) to open and close the anvil  24 . When the closure trigger  18  is actuated, that is, drawn in by a user of the instrument  10 , the anvil  24  may pivot about the pivot point  25  into the clamped or closed position. If clamping of the end effector  12  is satisfactory, the operator may actuate the firing trigger  20 , which, as explained in more detail below, causes the knife  32  and sled  33  to travel longitudinally along the channel  22 , thereby cutting tissue clamped within the end effector  12 . The movement of the sled  33  along the channel  22  causes the staples of the staple cartridge  34  to be driven through the severed tissue and against the closed anvil  24 , which turns the staples to fasten the severed tissue. U.S. Pat. No. 6,978,921, which issued on Dec. 27, 2005, entitled SURGICAL STAPLING INSTRUMENT INCORPORATING AN E-BEAM FIRING MECHANISM, which is incorporated herein by reference, provides more details about such two-stroke cutting and fastening instruments. According to various embodiments, the sled  33  may be an integral part of the cartridge  34 , such that when the knife  32  retracts following the cutting operation, the sled  33  does not retract. 
     It should be noted that although the embodiments of the instrument  10  described herein employ an end effector  12  that staples the severed tissue, in other embodiments different techniques for fastening or sealing the severed tissue may be used. For example, end effectors that use RF energy or adhesives to fasten the severed tissue may also be used. U.S. Pat. No. 5,810,811, which issued Sep. 22, 1998, entitled ELECTROSURGICAL HEMOSTATIC DEVICE, which is incorporated herein by reference, discloses a cutting instrument that uses RF energy to fasten the severed tissue. U.S. patent application Ser. No. 11/267,811, entitled SURGICAL STAPLING INSTRUMENTS STRUCTURED FOR DELIVERY OF MEDICAL AGENTS, now U.S. Pat. No. 7,673,783 and U.S. patent application Ser. No. 11/267,383, entitled SURGICAL STAPLING INSTRUMENTS STRUCTURED FOR PUMP-ASSISTED DELIVERY OF MEDICAL AGENTS, now U.S. Pat. No. 7,607,557, both of which are also incorporated herein by reference, disclose cutting instruments that use adhesives to fasten the severed tissue. Accordingly, although the description herein refers to cutting/stapling operations and the like below, it should be recognized that this is an example embodiment and is not meant to be limiting. Other tissue-fastening techniques may also be used. 
       FIGS. 4 and 5  are exploded views and  FIG. 6  is a side view of one embodiment of the end effector  12  and shaft  8 . As shown in the illustrated embodiment, the shaft  8  may include a proximate closure tube  40  and a distal closure tube  42  pivotably linked by a pivot links  44 . The distal closure tube  42  includes an opening  45  into which the tab  27  on the anvil  24  is inserted in order to open and close the anvil  24 , as further described below. Disposed inside the closure tubes  40 ,  42  may be a proximate spine tube  46 . Disposed inside the proximate spine tube  46  may be a main rotational (or proximate) drive shaft  48  that communicates with a secondary (or distal) drive shaft  50  via a bevel gear assembly  52 . The secondary drive shaft  50  is connected to a drive gear  54  that engages a proximate drive gear  56  of the helical screw shaft  36 . When the main drive shaft  48  is caused to rotate by actuation of the firing trigger  20  (as explained in more detail below), the bevel gear assembly  52   a - c  causes the secondary drive shaft  50  to rotate, which in turn, because of the engagement of the drive gears  54 ,  56 , causes the helical screw shaft  36  to rotate, which causes the knife/sled driving member  32  to travel longitudinally along the channel  22  to cut any tissue clamped within the end effector  12 . The vertical bevel gear  52   b  may sit and pivot in an opening  57  in the distal end of the proximate spine tube  46 . A distal spine tube  58  may be used to enclose the secondary drive shaft  50  and the drive gears  54 ,  56 . Collectively, the main drive shaft  48 , the secondary drive shaft  50 , and the articulation assembly (e.g., the bevel gear assembly  52   a - c ) are sometimes referred to herein as the “main drive shaft assembly.” 
     A bearing  38  is threaded on the helical drive screw  36 . The bearing  36  is also connected to the knife  32 . When the helical drive screw  36  forward rotates, the bearing  38  traverses the helical drive screw  36  distally, driving the cutting instrument  32  and, in the process, the sled  33  to perform the cutting/stapling operation. The sled  33  may be made of, for example, plastic, and may have a sloped distal surface. As the sled  33  traverses the channel  22 , the sloped forward surface may push up or drive the staples in the staple cartridge  34  through the clamped tissue and against the anvil  24 . The anvil  24  turns the staples, thereby stapling the severed tissue. When the knife  32  is retracted, the knife  32  and sled  33  may become disengaged, thereby leaving the sled  33  at the distal end of the channel  22 . 
     Because of the lack of user feedback for the cutting/stapling operation, there is a general lack of acceptance among physicians of motor-driven surgical instruments where the cutting/stapling operation is actuated by merely pressing a button. In contrast, various embodiments may provide a motor-driven endocutter with user-feedback of the deployment, force, and/or position of the cutting instrument in the end effector. 
       FIGS. 7-10  illustrate one embodiment of a motor-driven endocutter, and in particular the handle  6  thereof, that provides user-feedback regarding the deployment and loading force of the cutting instrument in the end effector. In addition, the embodiment may use power provided by the user in retracting the firing trigger  20  to power the device (a so-called “power assist” mode). As shown in the illustrated embodiment, the handle  6  includes exterior lower side pieces  59 ,  60  and exterior upper side pieces  61 ,  62  that fit together to form, in general, the exterior of the handle  6 . A battery  64 , such as a Li ion battery, may be provided in the pistol grip portion  26  of the handle  6 . Although the battery  64  is illustrated as containing a single cell, it will be appreciated that the battery  64 , in some embodiments, may include multiple cells connected together. The battery  64  may power a motor  65  disposed in an upper portion of the pistol grip portion  26  of the handle  6 . According to various embodiments, the motor  65  may be a DC brushed driving motor having a maximum rotation of, approximately, 5000 RPM. The motor  65  may drive a 90° bevel gear assembly  66  comprising a first bevel gear  68  and a second bevel gear  70 . The bevel gear assembly  66  may drive a planetary gear assembly  72 . The planetary gear assembly  72  may include a pinion gear  74  connected to a drive shaft  76 . The pinion gear  74  may drive a mating ring gear  78  that drives a helical gear drum  80  via a drive shaft  82 . A ring  84  may be threaded on the helical gear drum  80 . Thus, when the motor  65  rotates, the ring  84  is caused to travel along the helical gear drum  80  by means of the interposed bevel gear assembly  66 , planetary gear assembly  72  and ring gear  78 . 
     The handle  6  may also include a run motor sensor  110  in communication with the firing trigger  20  to detect when the firing trigger  20  has been drawn in (or “closed”) toward the pistol grip portion  26  of the handle  6  by the operator to thereby actuate the cutting/stapling operation by the end effector  12 . The sensor  110  may be a proportional sensor such as, for example, a rheostat or variable resistor. When the firing trigger  20  is drawn in, the sensor  110  detects the movement, and sends an electrical signal indicative of the voltage (or power) to be supplied to the motor  65 . When the sensor  110  is a variable resistor or the like, the rotation of the motor  65  may be generally proportional to the amount of movement of the firing trigger  20 . That is, if the operator only draws or closes the firing trigger  20  in a little bit, the rotation of the motor  65  is relatively low. When the firing trigger  20  is fully drawn in (or in the fully closed position), the rotation of the motor  65  is at its maximum. In other words, the harder the user pulls on the firing trigger  20 , the more voltage is applied to the motor  65 , causing greater rates of rotation. 
     The handle  6  may include a middle handle piece  104  adjacent to the upper portion of the firing trigger  20 . The handle  6  also may comprise a bias spring  112  connected between posts on the middle handle piece  104  and the firing trigger  20 . The bias spring  112  may bias the firing trigger  20  to its fully open position. In that way, when the operator releases the firing trigger  20 , the bias spring  112  will pull the firing trigger  20  to its open position, thereby removing actuation of the sensor  110 , thereby stopping rotation of the motor  65 . Moreover, by virtue of the bias spring  112 , any time a user closes the firing trigger  20 , the user will experience resistance to the closing operation, thereby providing the user with feedback as to the amount of rotation exerted by the motor  65 . Further, the operator could stop retracting the firing trigger  20  to thereby remove force from the sensor  100 , to thereby stop the motor  65 . As such, the user may stop the deployment of the end effector  12 , thereby providing a measure of control of the cutting/fastening operation to the operator. 
     The distal end of the helical gear drum  80  includes a distal drive shaft  120  that drives a ring gear  122 , which mates with a pinion gear  124 . The pinion gear  124  is connected to the main drive shaft  48  of the main drive shaft assembly. In that way, rotation of the motor  65  causes the main drive shaft assembly to rotate, which causes actuation of the end effector  12 , as described above. 
     The ring  84  threaded on the helical gear drum  80  may include a post  86  that is disposed within a slot  88  of a slotted arm  90 . The slotted arm  90  has an opening  92  its opposite end  94  that receives a pivot pin  96  that is connected between the handle exterior side pieces  59 ,  60 . The pivot pin  96  is also disposed through an opening  100  in the firing trigger  20  and an opening  102  in the middle handle piece  104 . 
     In addition, the handle  6  may include a reverse motor (or end-of-stroke sensor)  130  and a stop motor (or beginning-of-stroke) sensor  142 . In various embodiments, the reverse motor sensor  130  may be a limit switch located at the distal end of the helical gear drum  80  such that the ring  84  threaded on the helical gear drum  80  contacts and trips the reverse motor sensor  130  when the ring  84  reaches the distal end of the helical gear drum  80 . The reverse motor sensor  130 , when activated, sends a signal to the motor  65  to reverse its rotation direction, thereby withdrawing the knife  32  of the end effector  12  following the cutting operation. 
     The stop motor sensor  142  may be, for example, a normally-closed limit switch. In various embodiments, it may be located at the proximate end of the helical gear drum  80  so that the ring  84  trips the switch  142  when the ring  84  reaches the proximate end of the helical gear drum  80 . 
     In operation, when an operator of the instrument  10  pulls back the firing trigger  20 , the sensor  110  detects the deployment of the firing trigger  20  and sends a signal to the motor  65  to cause forward rotation of the motor  65  at, for example, a rate proportional to how hard the operator pulls back the firing trigger  20 . The forward rotation of the motor  65  in turn causes the ring gear  78  at the distal end of the planetary gear assembly  72  to rotate, thereby causing the helical gear drum  80  to rotate, causing the ring  84  threaded on the helical gear drum  80  to travel distally along the helical gear drum  80 . The rotation of the helical gear drum  80  also drives the main drive shaft assembly as described above, which in turn causes deployment of the knife  32  in the end effector  12 . That is, the knife  32  and sled  33  are caused to traverse the channel  22  longitudinally, thereby cutting tissue clamped in the end effector  12 . Also, the stapling operation of the end effector  12  is caused to happen in embodiments where a stapling-type end effector is used. 
     By the time the cutting/stapling operation of the end effector  12  is complete, the ring  84  on the helical gear drum  80  will have reached the distal end of the helical gear drum  80 , thereby causing the reverse motor sensor  130  to be tripped, which sends a signal to the motor  65  to cause the motor  65  to reverse its rotation. This in turn causes the knife  32  to retract, and also causes the ring  84  on the helical gear drum  80  to move back to the proximate end of the helical gear drum  80 . 
     The middle handle piece  104  includes a backside shoulder  106  that engages the slotted arm  90  as best shown in  FIGS. 8 and 9 . The middle handle piece  104  also has a forward motion stop  107  that engages the firing trigger  20 . The movement of the slotted arm  90  is controlled, as explained above, by rotation of the motor  65 . When the slotted arm  90  rotates CCW as the ring  84  travels from the proximate end of the helical gear drum  80  to the distal end, the middle handle piece  104  will be free to rotate CCW. Thus, as the user draws in the firing trigger  20 , the firing trigger  20  will engage the forward motion stop  107  of the middle handle piece  104 , causing the middle handle piece  104  to rotate CCW. Due to the backside shoulder  106  engaging the slotted arm  90 , however, the middle handle piece  104  will only be able to rotate CCW as far as the slotted arm  90  permits. In that way, if the motor  65  should stop rotating for some reason, the slotted arm  90  will stop rotating, and the user will not be able to further draw in the firing trigger  20  because the middle handle piece  104  will not be free to rotate CCW due to the slotted arm  90 . 
       FIGS. 41 and 42  illustrate two states of one embodiment of a variable sensor that may be used as the run motor sensor  110 . The sensor  110  may include a face portion  280 , a first electrode (A)  282 , a second electrode (B)  284 , and a compressible dielectric material  286  (e.g., EAP) between the electrodes  282 ,  284 . The sensor  110  may be positioned such that the face portion  280  contacts the firing trigger  20  when retracted. Accordingly, when the firing trigger  20  is retracted, the dielectric material  286  is compressed, as shown in  FIG. 42 , such that the electrodes  282 ,  284  are closer together. Since the distance “b” between the electrodes  282 ,  284  is directly related to the impedance between the electrodes  282 ,  284 , the greater the distance the more impedance, and the closer the distance the less impedance. In that way, the amount that the dielectric material  286  is compressed due to retraction of the firing trigger  20  (denoted as force “F” in  FIG. 42 ) is proportional to the impedance between the electrodes  282 ,  284 , which can be used to proportionally control the motor  65 . 
     Components of an example closure system for closing (or clamping) the anvil  24  of the end effector  12  by retracting the closure trigger  18  are also shown in  FIGS. 7-10 . In the illustrated embodiment, the closure system includes a yoke  250  connected to the closure trigger  18  by a pin  251  that is inserted through aligned openings in both the closure trigger  18  and the yoke  250 . A pivot pin  252 , about which the closure trigger  18  pivots, is inserted through another opening in the closure trigger  18  which is offset from where the pin  251  is inserted through the closure trigger  18 . Thus, retraction of the closure trigger  18  causes the upper part of the closure trigger  18 , to which the yoke  250  is attached via the pin  251 , to rotate CCW. The distal end of the yoke  250  is connected, via a pin  254 , to a first closure bracket  256 . The first closure bracket  256  connects to a second closure bracket  258 . Collectively, the closure brackets  256 ,  258  define an opening in which the proximate end of the proximate closure tube  40  (see  FIG. 4 ) is seated and held such that longitudinal movement of the closure brackets  256 ,  258  causes longitudinal motion by the proximate closure tube  40 . The instrument  10  also includes a closure rod  260  disposed inside the proximate closure tube  40 . The closure rod  260  may include a window  261  into which a post  263  on one of the handle exterior pieces, such as exterior lower side piece  59  in the illustrated embodiment, is disposed to fixedly connect the closure rod  260  to the handle  6 . In that way, the proximate closure tube  40  is capable of moving longitudinally relative to the closure rod  260 . The closure rod  260  may also include a distal collar  267  that fits into a cavity  269  in proximate spine tube  46  and is retained therein by a cap  271  (see  FIG. 4 ). 
     In operation, when the yoke  250  rotates due to retraction of the closure trigger  18 , the closure brackets  256 ,  258  cause the proximate closure tube  40  to move distally (e.g., away from the handle end of the instrument  10 ), which causes the distal closure tube  42  to move distally, which causes the anvil  24  to rotate about the pivot point  25  into the clamped or closed position. When the closure trigger  18  is unlocked from the locked position, the proximate closure tube  40  is caused to slide proximally, which causes the distal closure tube  42  to slide proximally, which by virtue of the tab  27  being inserted in the window  45  of the distal closure tube  42 , causes the anvil  24  to pivot about the pivot point  25  into the open or unclamped position. In that way, by retracting and locking the closure trigger  18 , an operator may clamp tissue between the anvil  24  and channel  22 , and may unclamp the tissue following the cutting/stapling operation by unlocking the closure trigger  18  from the locked position. 
       FIG. 11  is a schematic diagram of one embodiment of an electrical circuit of the instrument  10 . When an operator initially pulls in the firing trigger  20  after locking the closure trigger  18 , the sensor  110  is activated, allowing current to flow therethrough. If the normally-open reverse motor sensor switch  130  is open (meaning the end of the end effector stroke has not been reached), current will flow to a single pole, double throw relay  132 . Since the reverse motor sensor switch  130  is not closed, the coil  134  of the relay  132  will not be energized, so the relay  132  will be in its non-energized state. The circuit also includes a cartridge lockout sensor switch  136 . If the end effector  12  includes a staple cartridge  34 , the sensor switch  136  will be in the closed state, allowing current to flow. Otherwise, if the end effector  12  does not include a staple cartridge  34 , the sensor switch  136  will be open, thereby preventing the battery  64  from powering the motor  65 . 
     When the staple cartridge  34  is present, the sensor switch  136  is closed, which energizes a single pole, single throw relay  138 . When the relay  138  is energized, current flows through the relay  138 , through the variable resistor sensor  110 , and to the motor  65  via a double pole, double throw relay  140 , thereby powering the motor  65  and allowing it to rotate in the forward direction. 
     When the end effector  12  reaches the end of its stroke, the reverse motor sensor  130  will be activated, thereby closing the switch  130  and energizing the relay  132 . This causes the relay  132  to assume its energized state (not shown in  FIG. 11 ), which causes current to bypass the cartridge lockout sensor switch  136  and variable resistor  110 , and instead causes current to flow to both the normally-closed double pole, double throw relay  140  and back to the motor  65 , but in a manner, via the relay  140 , that causes the motor  65  to reverse its rotational direction. 
     Because the stop motor sensor switch  142  is normally-closed, current will flow back to the relay  132  to keep it energized until the switch  142  opens. When the knife  32  is fully retracted, the stop motor sensor switch  142  is activated, causing the switch  142  to open, thereby removing power from the motor  65 . 
     In other embodiments, rather than a proportional-type sensor  110 , an on-off type sensor could be used. In such embodiments, the rate of rotation of the motor  65  would not be proportional to the force applied by the operator. Rather, the motor  65  would generally rotate at a constant rate. But the operator would still experience force feedback because the firing trigger  20  is geared into the gear drive train. 
       FIG. 12  is a side-view of the handle  6  of a power-assist motorized endocutter according to another embodiment. The embodiment of  FIG. 12  is similar to that of  FIGS. 7-10  except that in the embodiment of  FIG. 12 , there is no slotted arm  90  connected to the ring  84  threaded on the helical gear drum  80 . Instead, in the embodiment of  FIG. 12 , the ring  84  includes a sensor portion  114  that moves with the ring  84  as the ring  84  advances down (and back) on the helical gear drum  80 . The sensor portion  114  includes a notch  116 . The reverse motor sensor  130  may be located at the distal end of the notch  116  and the stop motor sensor  142  may be located at the proximate end of the notch  116 . As the ring  84  moves down the helical gear drum  80  (and back), the sensor portion  114  moves with it. Further, as shown in  FIG. 12 , the middle piece  104  may have an arm  118  that extends into the notch  116 . 
     In operation, as an operator of the instrument  10  retracts in the firing trigger  20  toward the pistol grip  26 , the run motor sensor  110  detects the motion and sends a signal to power the motor  65 , which causes, among other things, the helical gear drum  80  to rotate. As the helical gear drum  80  rotates, the ring  84  threaded on the helical gear drum  80  advances (or retracts, depending on the rotation). Also, due to the pulling in of the firing trigger  20 , the middle piece  104  is caused to rotate CCW with the firing trigger  20  due to the forward motion stop  107  that engages the firing trigger  20 . The CCW rotation of the middle piece  104  cause the arm  118  to rotate CCW with the sensor portion  114  of the ring  84  such that the arm  118  stays disposed in the notch  116 . When the ring  84  reaches the distal end of the helical gear drum  80 , the arm  118  will contact and thereby trip the reverse motor sensor  130 . Similarly, when the ring  84  reaches the proximate end of the helical gear drum  80 , the arm  118  will contact and thereby trip the stop motor sensor  142 . Such actions may reverse and stop the motor  65 , respectively, as described above. 
       FIG. 13  is a side-view of the handle  6  of a power-assist motorized endocutter according to another embodiment. The embodiment of  FIG. 13  is similar to that of  FIGS. 7-10  except that in the embodiment of  FIG. 13 , there is no slot in the arm  90 . Instead, the ring  84  threaded on the helical gear drum  80  includes a vertical channel  126 . Instead of a slot, the arm  90  includes a post  128  that is disposed in the channel  126 . As the helical gear drum  80  rotates, the ring  84  threaded on the helical gear drum  80  advances (or retracts, depending on the rotation). The arm  90  rotates CCW as the ring  84  advances due to the post  128  being disposed in the channel  126 , as shown in  FIG. 13 . 
     As mentioned above, in using a two-stroke motorized instrument, the operator first pulls back and locks the closure trigger  18 .  FIGS. 14 and 15  show one embodiment of a closure trigger  18  locking mechanism for locking the closure trigger  18  to the pistol grip portion  26  of the handle  6 . In the illustrated embodiment, the pistol grip portion  26  includes a hook  150  that is biased to rotate CCW about a pivot point  151  by a torsion spring  152 . Also, the closure trigger  18  includes a closure bar  154 . As the operator draws in the closure trigger  18 , the closure bar  154  engages a sloped portion  156  of the hook  150 , thereby rotating the hook  150  upward (or CW in  FIGS. 14-15 ) until the closure bar  154  completely passes the sloped portion  156  into a recessed notch  158  of the hook  150 , which locks the closure trigger  18  in place. The operator may release the closure trigger  18  by pushing down on a slide button release  160  on the back or opposite side of the pistol grip portion  26 . Pushing down the slide button release  160  rotates the hook  150  CW such that the closure bar  154  is released from the recessed notch  158 . 
       FIG. 16  shows another closure trigger locking mechanism according to various embodiments. In the embodiment of  FIG. 16 , the closure trigger  18  includes a wedge  160  having an arrow-head portion  161 . The arrow-head portion  161  is biased downward (or CW) by a leaf spring  162 . The wedge  160  and leaf spring  162  may be made from, for example, molded plastic. When the closure trigger  18  is retracted, the arrow-head portion  161  is inserted through an opening  164  in the pistol grip portion  26  of the handle  6 . A lower chamfered surface  166  of the arrow-head portion  161  engages a lower sidewall  168  of the opening  164 , forcing the arrow-head portion  161  to rotate CCW. Eventually the lower chamfered surface  166  fully passes the lower sidewall  168 , removing the CCW force on the arrow-head portion  161 , causing the lower sidewall  168  to slip into a locked position in a notch  170  behind the arrow-head portion  161 . 
     To unlock the closure trigger  18 , a user presses down on a button  172  on the opposite side of the closure trigger  18 , causing the arrow-head portion  161  to rotate CCW and allowing the arrow-head portion  161  to slide out of the opening  164 . 
       FIGS. 17-22  show another embodiment of a closure trigger locking mechanism. As shown in this embodiment, the closure trigger  18  includes a flexible longitudinal arm  176  that includes a lateral pin  178  extending therefrom. The arm  176  and pin  178  may be made from molded plastic, for example. The pistol grip portion  26  of the handle  6  includes an opening  180  with a laterally extending wedge  182  disposed therein. When the closure trigger  18  is retracted, the pin  178  engages the wedge  182 , and the pin  178  is forced downward (e.g., the arm  176  is rotated CW) by the lower surface  184  of the wedge  182 , as shown in  FIGS. 17 and 18 . When the pin  178  fully passes the lower surface  184 , the CW force on the arm  176  is removed, and the pin  178  is rotated CCW such that the pin  178  comes to rest in a notch  186  behind the wedge  182 , as shown in  FIG. 19 , thereby locking the closure trigger  18 . The pin  178  is further held in place in the locked position by a flexible stop  188  extending from the wedge  184 . 
     To unlock the closure trigger  18 , the operator may further squeeze the closure trigger  18 , causing the pin  178  to engage a sloped backwall  190  of the opening  180 , forcing the pin  178  upward past the flexible stop  188 , as shown in  FIGS. 20 and 21 . The pin  178  is then free to travel out an upper channel  192  in the opening  180  such that the closure trigger  18  is no longer locked to the pistol grip portion  26 , as shown in  FIG. 22 . 
       FIGS. 23A-B  show a universal joint (“u-joint”)  195  that may be employed at the articulation point of a surgical instrument, such as the instrument  10 . The second piece  195 - 2  of the u-joint  195  rotates in a horizontal plane in which the first piece  195 - 1  lies.  FIG. 23A  shows the u-joint  195  in a linear (180°) orientation and  FIG. 23B  shows the u-joint  195  at approximately a 150° orientation. The u-joint  195  may be used instead of the bevel gears  52   a - c  (see  FIG. 4 , for example) at the articulation point  14  of the main drive shaft assembly to articulate the end effector  12 .  FIGS. 24A-B  show a torsion cable  197  that may be used in lieu of both the bevel gears  52   a - c  and the u-joint  195  to realize articulation of the end effector  12 . 
       FIGS. 25-31  illustrate another embodiment of a motorized, two-stroke surgical cutting and fastening instrument  10  with power assist. The embodiment of  FIGS. 25-31  is similar to that of  FIGS. 6-10  except that instead of the helical gear drum  80 , the embodiment of  FIGS. 25-31  includes an alternative gear drive assembly. The embodiment of  FIGS. 25-31  includes a gear box assembly  200  including a number of gears disposed in a frame  201 , wherein the gears are connected between the planetary gear  72  and the pinion gear  124  at the proximate end of the drive shaft  48 . As explained further below, the gear box assembly  200  provides feedback to the user via the firing trigger  20  regarding the deployment and loading force of the end effector  12 . Also, the user may provide power to the system via the gear box assembly  200  to assist the deployment of the end effector  12 . In that sense, like the embodiments described above, the embodiment of  FIGS. 25-31  is another power assist, motorized instrument  10  that provides feedback to the user regarding the loading force experienced by the cutting instrument  32 . 
     In the illustrated embodiment, the firing trigger  20  includes two pieces: a main body portion  202  and a stiffening portion  204 . The main body portion  202  may be made of plastic, for example, and the stiffening portion  204  may be made out of a more rigid material, such as metal. In the illustrated embodiment, the stiffening portion  204  is adjacent to the main body portion  202 , but according to other embodiments, the stiffening portion  204  could be disposed inside the main body portion  202 . A pivot pin  207  may be inserted through openings in the firing trigger pieces  202 ,  204  and may be the point about which the firing trigger  20  rotates. In addition, a spring  222  may bias the firing trigger  20  to rotate in a CCW direction. The spring  222  may have a distal end connected to a pin  224  that is connected to the pieces  202 ,  204  of the firing trigger  20 . The proximate end of the spring  222  may be connected to one of the handle exterior lower side pieces  59 ,  60 . 
     In the illustrated embodiment, both the main body portion  202  and the stiffening portion  204  include gear portions  206 ,  208  (respectively) at their upper end portions. The gear portions  206 ,  208  engage a gear in the gear box assembly  200 , as explained below, to drive the main drive shaft assembly and to provide feedback to the user regarding the deployment of the end effector  12 . 
     The gear box assembly  200  may include as shown, in the illustrated embodiment, six (6) gears. A first gear  210  of the gear box assembly  200  engages the gear portions  206 ,  208  of the firing trigger  20 . In addition, the first gear  210  engages a smaller second gear  212 , the smaller second gear  212  being coaxial with a large third gear  214 . The third gear  214  engages a smaller fourth gear  216 , the smaller fourth gear  216  being coaxial with a fifth gear  218 . The fifth gear  218  is a 90° bevel gear that engages a mating 90° bevel gear  220  (best shown in  FIG. 31 ) that is connected to the pinion gear  124  that drives the main drive shaft  48 . 
     In operation, when the user retracts the firing trigger  20 , a run motor sensor (not shown) is activated, which may provide a signal to the motor  65  to rotate at a rate proportional to the extent or force with which the operator is retracting the firing trigger  20 . This causes the motor  65  to rotate at a speed proportional to the signal from the sensor. The sensor is not shown for this embodiment, but it could be similar to the run motor sensor  110  described above. The sensor could be located in the handle  6  such that it is depressed when the firing trigger  20  is retracted. Also, instead of a proportional-type sensor, an on/off type sensor may be used. 
     Rotation of the motor  65  causes the bevel gears  66 ,  70  to rotate, which causes the planetary gear  72  to rotate, which causes, via the drive shaft  76 , the ring gear  122  to rotate. The ring gear  122  meshes with the pinion gear  124 , which is connected to the main drive shaft  48 . Thus, rotation of the pinion gear  124  drives the main drive shaft  48 , which causes actuation of the cutting/stapling operation of the end effector  12 . 
     Forward rotation of the pinion gear  124  in turn causes the bevel gear  220  to rotate, which causes, by way of the rest of the gears of the gear box assembly  200 , the first gear  210  to rotate. The first gear  210  engages the gear portions  206 ,  208  of the firing trigger  20 , thereby causing the firing trigger  20  to rotate CCW when the motor  65  provides forward drive for the end effector  12  (and to rotate CCW when the motor  65  rotates in reverse to retract the end effector  12 ). In that way, the user experiences feedback regarding loading force and deployment of the end effector  12  by way of the user&#39;s grip on the firing trigger  20 . Thus, when the user retracts the firing trigger  20 , the operator will experience a resistance related to the load force experienced by the end effector  12 . Similarly, when the operator releases the firing trigger  20  after the cutting/stapling operation so that it can return to its original position, the user will experience a CW rotation force from the firing trigger  20  that is generally proportional to the reverse speed of the motor  65 . 
     It should also be noted that in this embodiment the user can apply force (either in lieu of or in addition to the force from the motor  65 ) to actuate the main drive shaft assembly (and hence the cutting/stapling operation of the end effector  12 ) through retracting the firing trigger  20 . That is, retracting the firing trigger  20  causes the gear portions  206 ,  208  to rotate CCW, which causes the gears of the gear box assembly  200  to rotate, thereby causing the pinion gear  124  to rotate, which causes the main drive shaft  48  to rotate. 
     Although not shown in  FIGS. 25-31 , the instrument  10  may further include reverse motor and stop motor sensors. As described above, the reverse motor and stop motor sensors may detect, respectively, the end of the cutting stroke (full deployment of the knife  32  and sled  33 ) and the end of retraction operation (full retraction of the knife  32 ). A circuit similar to that described above in connection with  FIG. 11  may be used to appropriately power the motor  65 . 
       FIGS. 32-36  illustrate another embodiment of a two-stroke, motorized surgical cutting and fastening instrument  10  with power assist. The embodiment of  FIGS. 32-36  is similar to that of  FIGS. 25-31  except that in the embodiment of  FIGS. 32-36 , the firing trigger  20  includes a lower portion  228  and an upper portion  230 . Both portions  228 ,  230  are connected to and pivot about a pivot pin  207  that is disposed through each portion  228 ,  230 . The upper portion  230  includes a gear portion  232  that engages the first gear  210  of the gear box assembly  200 . The spring  222  is connected to the upper portion  230  such that the upper portion is biased to rotate in the CW direction. The upper portion  230  may also include a lower arm  234  that contacts an upper surface of the lower portion  228  of the firing trigger  20  such that when the upper portion  230  is caused to rotate CW the lower portion  228  also rotates CW, and when the lower portion  228  rotates CCW the upper portion  230  also rotates CCW. Similarly, the lower portion  228  includes a rotational stop  238  that engages a lower shoulder of the upper portion  230 . In that way, when the upper portion  230  is caused to rotate CCW the lower portion  228  also rotates CCW, and when the lower portion  228  rotates CW the upper portion  230  also rotates CW. 
     The illustrated embodiment also includes the run motor sensor  110  that communicates a signal to the motor  65  that, in various embodiments, may cause the motor  65  to rotate at a speed proportional to the force applied by the operator when retracting the firing trigger  20 . The sensor  110  may be, for example, a rheostat or some other variable resistance sensor, as explained herein. In addition, the instrument  10  may include a reverse motor sensor  130  that is tripped or switched when contacted by a front face  242  of the upper portion  230  of the firing trigger  20 . When activated, the reverse motor sensor  130  sends a signal to the motor  65  to reverse direction. Also, the instrument  10  may include a stop motor sensor  142  that is tripped or actuated when contacted by the lower portion  228  of the firing trigger  20 . When activated, the stop motor sensor  142  sends a signal to stop the reverse rotation of the motor  65 . 
     In operation, when an operator retracts the closure trigger  18  into the locked position, the firing trigger  20  is retracted slightly (through mechanisms known in the art, including U.S. Pat. No. 6,978,921, which issued Dec. 27, 2005, entitled SURGICAL STAPLING INSTRUMENT INCORPORATING AN E-BEAM FIRING MECHANISM and U.S. Pat. No. 6,905,057, which issued Jun. 14, 2005, entitled SURGICAL STAPLING INSTRUMENT INCORPORATING A FIRING MECHANISM HAVING A LINKED RACK TRANSMISSION, both of which are incorporated herein by reference) so that the user can grasp the firing trigger  20  to initiate the cutting/stapling operation, as shown in  FIGS. 32 and 33 . At that point, as shown in  FIG. 33 , the gear portion  232  of the upper portion  230  of the firing trigger  20  moves into engagement with the first gear  210  of the gear box assembly  200 . When the operator retracts the firing trigger  20 , according to various embodiments, the firing trigger  20  may rotate a small amount, such as five degrees, before tripping the run motor sensor  110 , as shown in  FIG. 34 . Activation of the sensor  110  causes the motor  65  to forward rotate at a rate proportional to the retraction force applied by the operator. The forward rotation of the motor  65  causes, as described above, the main drive shaft  48  to rotate, which causes the knife  32  in the end effector  12  to be deployed (e.g., begin traversing the channel  22 ). Rotation of the pinion gear  124 , which is connected to the main drive shaft  48 , causes the gears  210 - 220  in the gear box assembly  200  to rotate. Since the first gear  210  is in engagement with the gear portion  232  of the upper portion  230  of the firing trigger  20 , the upper portion  230  is caused to rotate CCW, which causes the lower portion  228  to also rotate CCW. 
     When the knife  32  is fully deployed (e.g., at the end of the cutting stroke), the front face  242  of the upper portion  230  trips the reverse motor sensor  130 , which sends a signal to the motor  65  to reverse rotational direction. This causes the main drive shaft assembly to reverse rotational direction to retract the knife  32 . Reverse rotation of the main drive shaft assembly causes the gears  210 - 220  in the gear box assembly to reverse direction, which causes the upper portion  230  of the firing trigger  20  to rotate CW, which causes the lower portion  228  of the firing trigger  20  to rotate CW until the front face  242  of the upper portion  230  trips or actuates the stop motor sensor  142  when the knife  32  is fully retracted, which causes the motor  65  to stop. In that way, the user experiences feedback regarding deployment of the end effector  12  by way of the user&#39;s grip on the firing trigger  20 . Thus, when the user retracts the firing trigger  20 , the operator will experience a resistance related to the deployment of the end effector  12  and, in particular, to the loading force experienced by the knife  32 . Similarly, when the operator releases the firing trigger  20  after the cutting/stapling operation so that it can return to its original position, the user will experience a CW rotation force from the firing trigger  20  that is generally proportional to the reverse speed of the motor  65 . 
     It should also be noted that in this embodiment the user can apply force (either in lieu of or in addition to the force from the motor  65 ) to actuate the main drive shaft assembly (and hence the cutting/stapling operation of the end effector  12 ) through retracting the firing trigger  20 . That is, retracting the firing trigger  20  causes the gear portion  232  of the upper portion  230  to rotate CCW, which causes the gears of the gear box assembly  200  to rotate, thereby causing the pinion gear  124  to rotate, which causes the main drive shaft assembly to rotate. 
     The above-described embodiments employed power-assist user feedback systems, with or without adaptive control (e.g., using a sensor  110 ,  130 , and  142  outside of the closed loop system of the motor, gear drive train, and end effector) for a two-stroke, motorized surgical cutting and fastening instrument. That is, force applied by the user in retracting the firing trigger  20  may be added to the force applied by the motor  65  by virtue of the firing trigger  20  being geared into (either directly or indirectly) the gear drive train between the motor  65  and the main drive shaft  48 . In other embodiments, the user may be provided with tactile feedback regarding the position of the knife  32  in the end effector  12 , but without having the firing trigger  20  geared into the gear drive train.  FIGS. 37-40  illustrate one embodiment of a motorized surgical cutting and fastening instrument  10  with such a tactile position feedback system. 
     In the illustrated embodiment of  FIGS. 37-40 , the firing trigger  20  may have a lower portion  228  and an upper portion  230 , similar to the instrument  10  shown in  FIGS. 32-36 . Unlike the embodiment of  FIGS. 32-36 , however, the upper portion  230  does not have a gear portion that mates with part of the gear drive train. Instead, the instrument  10  includes a second motor  265  with a threaded rod  266  threaded therein. The threaded rod  266  reciprocates longitudinally in and out of the motor  265  as the motor  265  rotates, depending on the direction of rotation. The instrument  10  also includes an encoder  268  that is responsive to the rotations of the main drive shaft  48  for translating the incremental angular motion of the main drive shaft  48  (or other component of the main drive assembly) into a corresponding series of digital signals, for example. In the illustrated embodiment, the pinion gear  124  includes a proximate drive shaft  270  that connects to the encoder  268 . 
     The instrument  10  also includes a control circuit (not shown), which may be implemented using a microcontroller or some other type of integrated circuit, that receives the digital signals from the encoder  268 . Based on the signals from the encoder  268 , the control circuit may calculate the stage of deployment of the knife  32  in the end effector  12 . That is, the control circuit can calculate if the knife  32  is fully deployed, fully retracted, or at an intermittent stage. Based on the calculation of the stage of deployment of the end effector  12 , the control circuit may send a signal to the second motor  265  to control its rotation to thereby control the reciprocating movement of the threaded rod  266 . 
     In operation, as shown in  FIG. 37 , when the closure trigger  18  is not locked into the clamped position, the firing trigger  20  rotated away from the pistol grip portion  26  of the handle  6  such that the front face  242  of the upper portion  230  of the firing trigger  20  is not in contact with the proximate end of the threaded rod  266 . When the operator retracts the closure trigger  18  and locks it in the clamped position, the firing trigger  20  rotates slightly towards the closure trigger  18  so that the operator can grasp the firing trigger  20 , as shown in  FIG. 38 . In this position, the front face  242  of the upper portion  230  contacts the proximate end of the threaded rod  266 . 
     As the user then retracts the firing trigger  20 , after an initial rotational amount (e.g., 5 degrees of rotation) the run motor sensor  110  may be activated such that, as explained above, the sensor  110  sends a signal to the motor  65  to cause it to rotate at a forward speed proportional to the amount of retraction force applied by the operator to the firing trigger  20 . Forward rotation of the motor  65  causes the main drive shaft  48  to rotate via the gear drive train, which causes the knife  32  and sled  33  to travel down the channel  22  and sever tissue clamped in the end effector  12 . The control circuit receives the output signals from the encoder  268  regarding the incremental rotations of the main drive shaft assembly and sends a signal to the second motor  265  to cause the second motor  265  to rotate, which causes the threaded rod  266  to retract into the motor  265 . This allows the upper portion  230  of the firing trigger  20  to rotate CCW, which allows the lower portion  228  of the firing trigger to also rotate CCW. In that way, because the reciprocating movement of the threaded rod  266  is related to the rotations of the main drive shaft assembly, the operator of the instrument  10 , by way of his/her grip on the firing trigger  20 , experiences tactile feedback as to the position of the end effector  12 . The retraction force applied by the operator, however, does not directly affect the drive of the main drive shaft assembly because the firing trigger  20  is not geared into the gear drive train in this embodiment. 
     By virtue of tracking the incremental rotations of the main drive shaft assembly via the output signals from the encoder  268 , the control circuit can calculate when the knife  32  is fully deployed (e.g., fully extended). At this point, the control circuit may send a signal to the motor  65  to reverse direction to cause retraction of the knife  32 . The reverse direction of the motor  65  causes the rotation of the main drive shaft assembly to reverse direction, which is also detected by the encoder  268 . Based on the reverse rotation detected by the encoder  268 , the control circuit sends a signal to the second motor  265  to cause it to reverse rotational direction such that the threaded rod  266  starts to extend longitudinally from the motor  265 . This motion forces the upper portion  230  of the firing trigger  20  to rotate CW, which causes the lower portion  228  to rotate CW. In that way, the operator may experience a CW force from the firing trigger  20 , which provides feedback to the operator as to the retraction position of the knife  32  in the end effector  12 . The control circuit can determine when the knife  32  is fully retracted. At this point, the control circuit may send a signal to the motor  65  to stop rotation. 
     According to other embodiments, rather than having the control circuit determine the position of the knife  32 , reverse motor and stop motor sensors may be used, as described above. In addition, rather than using a proportional sensor  110  to control the rotation of the motor  65 , an on/off switch or sensor can be used. In such an embodiment, the operator would not be able to control the rate of rotation of the motor  65 . Rather, it would rotate at a preprogrammed rate. 
       FIGS. 43-61  herein describe embodiments of batteries and battery configurations for use with powered surgical devices. The batteries and battery configurations described below may be utilized with any suitable powered surgical instrument including for example, the instrument embodiments described above. In addition to or instead of the functionality of the embodiments described herein above surgical instruments utilizing the batteries and battery configurations of  FIGS. 43-61  may comprise end effectors for cutting, clasping, laser cutting and/or coagulation, RF cutting and/or coagulation, ultrasonic cutting and/or coagulation, etc. Additional details regarding surgical instruments and battery units are described in U.S. patent application Ser. No. 12/884,995, entitled, POWER CONTROL ARRANGEMENTS FOR SURGICAL INSTRUMENTS AND BATTERIES, filed Sep. 17, 2010, now U.S. Pat. No. 8,632,525, which is incorporated herein by reference in its entirety. 
       FIG. 43  illustrates one embodiment of a surgical instrument  500  comprising a pair of asymmetrically-shaped battery packs  506 . The instrument  500  may comprise a handle  502 , a trigger  504  and an end effector  501 . According to various embodiments, the handle  502 , trigger  504  and end effector  501  may operate in a manner similar to that of the various handles  6 , triggers,  18 ,  20  and end effectors  12  described herein. In addition to or instead of the functionality described herein above, the end effector  501  may comprise surgical implements for cutting, clasping, laser cutting and/or coagulation, RF cutting and/or coagulation, ultrasonic cutting and/or coagulation, etc. 
     The handle  502  of the instrument  502  may house battery packs  506 , as shown. The battery packs  506  may be electrically connected to and may provide power to a circuit  514  of the instrument  500 . The circuit may be located in the handle  502 , as shown, in the end effector  501 , or in any combination of locations within the instrument  500 . In use, the circuit  514  may power the operation of at least one surgical implement at the end effector  501 . For example, the circuit  514  may comprise an electric motor for operating an electrically powered cutter, clasper, or other mechanical device. In addition to, or instead of a motor, the circuit  514  may comprise suitable circuit components for implementing an RF, ultrasonic, or other type of non-motor-powered surgical implement. 
       FIG. 44  illustrates one embodiment of a battery pack  506  outside of the handle  502 . The battery pack  506  may have an asymmetric cross-sectional shape. For example, in the embodiment pictured in  FIG. 44 , the battery pack  506  has a half-ovaloid shape. It will be appreciated that other asymmetric cross-sectional shapes could be used. As illustrated, the battery pack  506  comprises three cells  508 . The cells  508  may be any suitable type of cell including, for example, lithium-ion cells such as the CR123-type cell and/or the CR2-type cell. The cells  508  may be electrically connected to one another in series or parallel. The number of cells  508  may be chosen to the power of any accidental discharge from the battery pack  506 . For example, the number of connected cells  508  may be selected such that the cumulative energy available to an arc or short is less than the energy required to ignite common shipping and/or packing materials. According to various embodiments, this value may be defined by appropriate government regulations. 
       FIG. 45  illustrates one embodiment of the handle  502  illustrating cavities  510 ,  512  for receiving the battery packs  506 . The cavities  510 ,  512  may have an asymmetric cross-sectional shape corresponding to the cross-sectional shape of the battery packs  506 . This may allow the battery packs  506  to be received within the cavities  510 ,  512 , as illustrated in  FIG. 43 . An interior portion  529  of the cavity  510  is also shown in  FIG. 45 . A wall  530  may comprise contacts  532 ,  534 . The contacts  532 ,  534  may be connected to the circuit  514  and may be configured to connect the battery pack  506  to the circuit  514  when the battery pack  506  is installed in the cavity  510 . It will be appreciated that cavity  512  may comprise a similar interior portion and similar contacts. Illustration of these elements is omitted in  FIG. 45 , however, for clarity. 
       FIG. 46  illustrates one embodiment of the battery pack of  506  showing positive electrode contact  518  and negative electrode contact  520 . Upon insertion of the battery pack  506  within the cavity  510 , the electrode contacts  518  and  520  may connect to the contacts  532 ,  534 , illustrated in  FIG. 45 , to establish a connection between the battery pack  506  and the circuit  514 . The electrode contacts  518 ,  520  are illustrated on a first end  522  of the battery pack  506 . It will be appreciated, however, that the electrode contacts  518 ,  520  may be positioned on any other surface of the battery pack  506  including, for example, the end  524 , flat face  526  and/or curved face  528 . Accordingly, the contacts  532 ,  534  may be positioned on a corresponding surface of the interior portion  529  of the cavity  510 . 
     The asymmetric cross sectional shape of the battery packs  506  and the cavities  510 ,  512  may ensure that the battery packs  506  are inserted into the instrument  500  with the correct polarity. For example, due to its asymmetric cross-sectional shape, the end  522  of the battery pack  506  may fit into the cavity  510  of the handle  502  in only one orientation, ensuring that the correct electrodes  518 ,  520 ,  532 ,  534  are in contact with one another. Similarly, the end  522  of the battery pack  506  may fit into the cavity  512  in only one orientation. Because the cross-sectional shape of the cavity  512  is reciprocal to that of the cavity  510 , the orientation of the electrode contacts  518 ,  520  may be reversed in the cavity  512  relative to the cavity  510 . Accordingly, when the cavities  510 ,  512  have reciprocal cross-sectional shapes, as illustrated, the position of the contacts (not shown) within the cavity  512  may also be reversed to ensure correct polarity. 
     The clinician may be relied upon to recognize that the end  522  of the battery pack  506  with the electrodes  518 ,  520  is properly inserted into the cavities  510 ,  512 . According to various embodiments, however, the form of the battery pack  506  may be manipulated to make it difficult or impossible for the end  524  of the battery pack  506  to be inserted into the cavities  510 ,  512 . For example, in  FIG. 46 , the battery pack  506  is shown with an optional flange  522  at the end  524 . The flange  522  may extend beyond the battery pack  506  to ensure that the end  524  cannot be inserted into one of the cavities  510 ,  512 . Although the instrument  500  illustrated utilizes two battery packs  506  and defines two cavities  510 ,  512 , it will be appreciated that more or additional battery packs and corresponding cavities may be used. 
       FIG. 47  illustrates one embodiment of the battery pack  506  in conjunction with a discharge plug  540 . The discharge plug  540  may be attached to the end  522  of the battery pack  506 , for example, after use of the battery pack  506  is complete. In certain embodiments, the discharge plug  540  may have a cross-sectional area slightly larger than that of the battery pack  506  and may slide over the end  522 . The discharge plug may comprise electrode contacts  542 ,  546  electrically connected to one another via a resistive element  546 . The resistive element  546  may be any suitable resistive element having any suitable electrical resistance and/or impedance. With the discharge in place, the electrode contacts  542 ,  546  may contact positive and negative electrode contacts  518 ,  520 . This may place the resistive element  546  in series with the battery pack  506 , causing the battery to drain. In this way, the battery pack  506  may be drained either prior to or during disposal, reducing hazard disposal. 
       FIG. 48  illustrates a schematic diagram of one embodiment of a surgical instrument  602  and a battery pack  600 . The surgical instrument  602  may operate in a manner similar to that of the surgical instruments  10 ,  500  described herein above. For example, the instrument  602  may be any suitable type of surgical instrument utilizing battery power including, for example, instruments having motorized implements for cutting, motorized implements for stapling, RF implements for cutting and/or coagulating, ultrasonic implements for cutting and/or coagulating, laser implements for cutting/coagulating, etc. The surgical instrument  602  may comprise a pair of electrodes  604 ,  606 , which, when the battery pack  600  is connected to the surgical instrument  603 , may connect with a pair of electrodes  608 ,  610  of the battery pack  600 . 
     The battery pack  600  may comprise a plurality of cells  612 . The cells  612  may be any suitable type of cell. According to various embodiments, the cells may be lithium-ion cells such as the CR123-type cell and/or the CR2-type cell. A switch  614  may have an open position and a closed position. The switch  614  may be any suitable type of mechanical or solid state switch. When the switch  614  is in the open position, the cells  612  may be electrically disconnected from one another. When the switch  614  is in the closed position, the cells  612  may be electrically connected to one another. For example, in  FIG. 48 , the cells  612  are shown connected in parallel. In various embodiments, however, the cells  612  may be connected in series or in any other desirable configuration. The switch  614  may be engaged to the closed position at the time that the battery pack  600  is connected to the surgical instrument  602 . For example, the switch  614  may be manually engaged by a clinician using the surgical instrument  602  either before or after the battery pack  600  is connected to the instrument  602 . Also, according to various embodiments, the switch  614  may be engaged to the closed position automatically when the battery pack  600  is connected to the instrument  602  (e.g., by placing at least a portion of the battery pack  600  within the surgical instrument  602 ). 
     The battery pack  600  may also comprise a discharge system  616 . The discharge system  616  may comprise a discharge switch  618  and a resistive element  620 . The resistive element  620  may be any suitable resistive element having any suitable electrical resistance and/or impedance. The discharge switch  618  may have an open position and a closed position. When the discharge switch is in the open position, the resistive element  620  may not be electrically connected to the battery pack. When the discharge switch  618  is in the closed position, the resistive element  620  may be electrically connected across the cells  612  of the battery pack  600 . In this way, the cells  612  may drain when the discharge switch is closed  618 . The discharge switch  620  may be any type of mechanical or solid state switch. The discharge switch  618  may be manually or automatically transitioned from the open to the closed position, for example, upon installation of the battery pack  614  to the instrument  602  or upon removal of the surgical instrument  602  from the instrument  602 . In some embodiments, the cells  612  may deliver sufficient power and/or the resistive element  620  may be designed such that discharge switch  618  may be closed while the instrument  602  is in use. 
       FIG. 49  illustrates an alternate embodiment of the battery pack  600  and surgical instrument  602  shown in  FIG. 48 . As illustrated in  FIG. 49 , the switch  614  may comprise at least one open portion  622  and at least one contactor  624 . As illustrated, the at least one contactor  624  may be a part of the surgical instrument  602 . In this way, the cells  612  may be electrically connected to one another when the battery pack  600  is installed to the surgical instrument  602 , bringing the at least one connector portion in electrical contact with the at least one open portion  622 . 
       FIG. 50  illustrates another embodiment of the battery pack  600  of  FIG. 48 . As illustrated in  FIG. 50 , the switch  614 , is implemented with an open portion  634 , a contactor  636  and a movable tab  631 . The contactor  636  may be mechanically biased against the open portion  634 , for example, by a spring  630 . The movable tab  631  may be positioned between the open portion  634  and the contactor  636 . The movable tab  631  may be made from an insulating material, such as plastic. In this way, the cells  612  may not be electrically connected to one another when the movable tab  631  is in place. When the battery pack  600  is ready for use, the tab  631  may be removed, for example, by the clinician. When the tab  631  is removed, the contactor  636  may be mechanically pushed into electrical contact with the open portion  634 , resulting in the electrical connection of the cells  612  to one another. According to various embodiments, the tab  631  may comprise a portion  632  configured to be received by a corresponding portion  638  of the surgical instrument. When the battery pack  600  is installed to the instrument, the portion  638  of the surgical instrument  602  may contact the tab  631 , tending to remove it from between the open portion  634  and the contactor  636 . The tab  631  may be made from a polymer or any suitable electrically insulating material. Also, according to various embodiments, the tab  631  may have a thickness of about  1  mil. 
       FIGS. 51-53  illustrate one mechanical embodiment of a battery pack  700  implementing the schematic of the battery pack  600  shown in  FIG. 48 . The battery pack  700  comprises a casing  707  having therein a battery  703  comprising a plurality of cells that can be interconnected to one another by connecting contacts  708 ,  710 . A discharge switch  712  may, when in the closed position, connect a resistive element  714  across the terminals of the cells  703 , causing them to discharge. The battery pack  700  may comprise a pair of contacts  706 ,  704  positioned on a switch platform  716 . The contactors  706 ,  704  may have an open position shown in  FIG. 51  and a closed position. In the closed position, the contactors  706 ,  704  may be placed in electrical contact with the contacts  708 ,  710 , causing the cells  703  to be interconnected to one another. Collectively, the switch platform  716 , contacts  708 ,  710 , and contactors  704 ,  706  may form a switch. According to various embodiment, when the switch is closed (e.g., the contactors  704 ,  706  are in contact with the contacts  708 ,  710 ), the cells of the battery  703  may electrically interconnected. 
     The switch platform  716  may be coupled to a clutch  705  comprising a pair of locking mechanisms  702 . In the position shown in  FIG. 51 , the clutch (including locking mechanisms  702 ) is engaged, holding the switch platform  716  in the open position. The battery pack  700  may also comprise a discharge switch  712 . In a closed position, the discharge switch  712  may switch a resistive element  714  across the anode and the cathode of the cells  703 , causing the cells to discharge. As illustrated in  FIG. 51 , the discharge switch may be mechanically biased to the closed position by a spring  718 . The bias of the spring  718 , however, may be overcome by a stopper  720  in contact with a movable portion or panel  722  of the casing  707 . 
       FIG. 52  illustrates a configuration of the battery pack  700  of  FIG. 51  upon insertion into a surgical instrument  750 , illustrated in cross-section. The battery pack  700  may be inserted into a cavity  754  defined by the instrument  750 . The cavity  754  may be positioned at any portion of the instrument  750  including, in various embodiments, at a handle portion. The cavity  754  may comprise a pair of contacts  756 ,  758  that may be aligned with contactors  706 ,  708 . The battery pack  700  may be inserted into the instrument  750  in the direction of arrow  753 . As the battery pack  700  is inserted, contactors  706 ,  704  may come into contact with the contacts  756 ,  758 . This may force the contactors  706 ,  708 , and the switch platform  716  toward the contacts  708 ,  710  such that the contactors  706 ,  704  are in electrical communication with the contacts  710 ,  708  and the contacts  756 ,  758 , which may cause the cells of the battery  703  to be interconnected and connected to the instrument  750 . 
     According to various embodiments, pressure from the contacts  756 ,  758  may overcome the force of the clutch  705 , disengaging the lock mechanisms  702 , allowing the switch platform  716  to translate towards the contacts  708 ,  710 . Also, in various embodiments, a portion of an interior of the cavity  754  may comprise one or more keyed portions  760 ,  764  that are aligned with one or more receptacles  762 ,  766  associated (e.g., mechanically or electronically) with the lock mechanisms  702 . When the keyed portions  760 ,  764  come into contact with the receptacles  762 ,  766 , the clutch  705  lock mechanisms  702  may be enabled to disengage, allowing the switch platform  716  to assume the position illustrated in  FIG. 52 . According to various embodiments, after the switch platform  716  assumes the position illustrated in  FIG. 52 , the lock mechanisms  702  may re-engage, locking the switch platform  716  in place. According to various embodiments, this may make it difficult for the battery pack  700  to lose electrical connectivity with the instrument  750  after insertion. 
     The interior of the cavity  754  may also comprise a feature  752  (e.g., an extension), for contacting the panel  722 . For example, as the battery pack  700  is inserted into the cavity  754 , the extension  752  may contact the panel  722 , sliding it in the direction of arrow  755  and allowing the stopper  720  to protrude through the casing  707  (e.g., because of the biasing of the spring  718 ). According to various embodiments, the stopper  720  may contact the interior wall of the cavity  754 , preventing the discharge switch  712  from being closed.  FIG. 53  illustrated one embodiment of the battery pack  700  after removal from the surgical instrument  750 . The switch platform  716  may be locked by the lock mechanisms  702  into the same position shown in  FIG. 52 . Also, within the interior wall of the cavity  754 , the stopper  720  may protrude from the casing  707  by an amount suitable to close the discharge switch  712 . This may cause the battery  703  to discharge. 
       FIGS. 54-61  illustrate another mechanical embodiment of a battery pack  800  implementing the schematic of the battery pack  600  shown in  FIG. 48 . The battery pack  800  may comprise a casing  802  defining an interior cavity  810 . The casing  802  may be covered by a cap  804  that may be secured to the casing  802  utilizing one or more mechanical latches  806 ,  808 .  FIG. 55  illustrates one embodiment of the battery pack  800  with the cap  804  removed to show a plurality of cells  812  within. Any suitable number and/or type of cells  812  may be used. For example, CR123 and/or CR2 cells may be used.  FIG. 56  illustrates one embodiment of the battery pack  800  with a portion of the casing  802  removed to reveal the cells  812 . 
       FIG. 57  illustrates a cross-sectional view of one embodiment of the battery pack  800  including a battery drain  814 . The battery drain  814  may be positioned within the interior cavity  810  and may be slidable within the interior cavity  810  in the directions of arrow  815 . The drain  814  may comprise at least two contacts  818 ,  816 . A portion of the contacts  818 ,  816  may touch wall  826  of the interior cavity  810 . According to various embodiments, the contacts  816 ,  818  may be biased to exert a force against the walls  826  in order to resist movement of the drain  814  in the direction of the arrows  815 . Also, in some embodiments, the walls  826  may define one or more protrusions or catch members  828  shaped to be received by a portion of one or more of the contacts  816 ,  818  to hold the drain  814  at a first position, as shown in  FIG. 57 . Additionally, the walls  826  may define one or more electrodes  824 . The electrodes  824  may be wired to the cells  812 , such that making an electrical connection across the electrodes  824  may short the positive and negative electrodes of the cells  812 . 
     The contacts  816 ,  818  of the drain  814  may be coupled at a base portion  820  of the drain  814 . According to various embodiments, the contacts  816 ,  818  may be electrically shorted to one another, or may be electrically connected to one another via a resistive element  822 .  FIG. 58  illustrates one embodiment of the battery pack  800  being installed to a surgical instrument  850 . The surgical instrument  850  may comprise an extending member  852  configured to be received into the interior cavity  810 . The extending member  854  may comprise one or more electrodes  854  positioned to contact electrodes  855  of the battery pack  800  when the member  854  is completely installed. In this way, the cells  812  of the battery pack  800  may provide electrical power to the instrument  830  via the electrodes  854 ,  855 . 
     As the member  852  is inserted into the interior cavity  810 , it may contact the battery drain  820  and force it along the interior cavity  810  in the direction of the arrow  857 . For example, the force provided to the battery drain  820  by the member  852  may overcome the drain&#39;s resistance to movement provided by the contacts  816 ,  818 , for example, in conjunction with the catch members  828 . When completely installed, as shown in  FIG. 59 , the member  852  may push the drain  814  into the cavity  810  until the contacts  816 ,  818  come into electrical contact with the electrodes  824 . This may either short the cells  812  or electrically connect them across the resistive element  822 . When the battery pack  800  is uninstalled from the instrument  850 , the member  852  may be removed from the cavity  810 . The drain  814 , however, may remain in the position shown in  FIG. 59 . In this way, the cells  812  may drain any remaining charge across the resistive element  822  either before or during disposal. This may, for example, minimize the power of any accidental discharges during disposal. 
       FIGS. 60 and 61  illustrates one embodiment of the battery drain  814  removed from the casing  802 . As illustrated, the drain  814  may comprise two sets of contacts  818 ,  816  and  818 ′,  816 ′. The base  820  may define a central portion  830  between the two sets of contacts  816 ,  818 ,  816 ′,  818 ′. According to various embodiments, the central portion  830  may be configured to contact the member  852 , as illustrated in  FIGS. 58-59 . Referring now to  FIG. 61 , resistive elements  822  are shown mounted to the base  820 . The resistive elements  822  may be elements of any suitable resistance value and any suitable mechanical configuration. For example, as illustrated in  FIG. 61 , the resistive elements  822  may comprise one or more surface-mount components. 
     It is to be understood that at least some of the figures and descriptions herein have been simplified to illustrate elements that are relevant for a clear understanding of the disclosure, while eliminating, for purposes of clarity, other elements. Those of ordinary skill in the art will recognize, however, that these and other elements may be desirable. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the disclosure, a discussion of such elements is not provided herein. 
     While several embodiments have been described, it should be apparent, however, that various modifications, alterations and adaptations to those embodiments may occur to persons skilled in the art with the attainment of some or all of the advantages of the disclosure. For example, according to various embodiments, a single component may be replaced by multiple components, and multiple components may be replaced by a single component, to perform a given function or functions. This application is therefore intended to cover all such modifications, alterations and adaptations without departing from the scope and spirit of the disclosure as defined by the appended claims. 
     Any patent, publication, or other disclosure material, in whole or in part, that is said to be incorporated by reference herein is incorporated herein only to the extent that the incorporated materials does not conflict with existing definitions, statements, or other disclosure material set forth in this disclosure. As such, and to the extent necessary, the disclosure as explicitly set forth herein supersedes any conflicting material incorporated herein by reference. Any material, or portion thereof, that is said to be incorporated by reference herein, but which conflicts with existing definitions, statements, or other disclosure material set forth herein will only be incorporated to the extent that no conflict arises between that incorporated material and the existing disclosure material.