Patent Publication Number: US-10321950-B2

Title: Managing tissue treatment

Description:
RELATED APPLICATION 
     This application is related to concurrently filed U.S. application Ser. No. 14/660,627, entitled “Managing Tissue Treatment”, now U.S. Patent Application Publication No. 2016/0270841, which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Various embodiments are directed to surgical systems that may be utilized in electrosurgical and/or ultrasonic devices to manage the delivery of energy to tissue to optimize tissue treatment. 
     Electrosurgical devices for applying electrical energy to tissue in order to treat and/or destroy the tissue are commonly used in surgical procedures. An electrosurgical device may comprise a handle and an instrument having a distally-mounted end effector (e.g., one or more electrodes). The end effector can be positioned against the tissue such that electrical current is introduced into the tissue. Electrosurgical devices can be configured for bipolar or monopolar operation. During bipolar operation, current is introduced into and returned from the tissue by active and return electrodes, respectively, of the end effector. During monopolar operation, current is introduced into the tissue by an active electrode of the end effector and returned through a return electrode (e.g., a grounding pad) separately located on a patient&#39;s body. Heat generated by the current flow through the tissue may form hemostatic seals within the tissue and/or between tissues and thus may be particularly useful for sealing blood vessels, for example. The end effector of an electrosurgical device may also comprise a cutting member that is movable relative to the tissue and the electrodes to transect the tissue. 
     Electrical energy applied by an electrosurgical device can be transmitted to the instrument by a generator in communication with the handle. The electrical energy may be in the form of radio frequency (“RF”) energy. RF energy is a form of electrical energy that may be in the frequency range of 300 kHz to 1 MHz. During its operation, an electrosurgical device can transmit low frequency RF energy through tissue, which causes ionic agitation, or friction, in effect resistive heating, thereby increasing the temperature of the tissue. Because a sharp boundary may be created between the affected tissue and the surrounding tissue, surgeons can operate with a high level of precision and control, without sacrificing un-targeted adjacent tissue. The low operating temperatures of RF energy may be useful for removing, shrinking, or sculpting soft tissue while simultaneously sealing blood vessels. RF energy may work particularly well on connective tissue, which is primarily comprised of collagen and shrinks when contacted by heat. 
     Ultrasonic surgical devices, such as ultrasonic scalpels, are another type of powered surgical devices used in surgical procedures. Depending upon specific device configurations and operational parameters, ultrasonic surgical devices can provide substantially simultaneous transection of tissue and homeostasis by coagulation, desirably minimizing patient trauma. An ultrasonic surgical device may comprise a handle containing an ultrasonic transducer, and an instrument coupled to the ultrasonic transducer having a distally-mounted end effector (e.g., a blade tip) to cut and seal tissue. In some cases, the instrument may be permanently affixed to the handle. In other cases, the instrument may be detachable from the handle, as in the case of a disposable instrument or an instrument that is interchangeable between different handles. The end effector transmits ultrasonic energy to tissue brought into contact with the end effector to realize cutting and sealing action. Ultrasonic surgical devices of this nature can be configured for open surgical use, laparoscopic, or endoscopic surgical procedures including robotic-assisted procedures. 
     Ultrasonic energy cuts and coagulates tissue using frictional heating and can be transmitted to the end effector by an ultrasonic generator in communication with the transducer. Vibrating at high frequencies (e.g., 55,500 times per second), the ultrasonic blade denatures protein in the tissue to form a sticky coagulum. Pressure exerted on tissue by the blade surface collapses blood vessels and allows the coagulum to form a hemostatic seal. A clinician can control the cutting speed and coagulation by the force applied to the tissue by the end effector, the time over which the force is applied and the selected excursion level of the end effector. 
     SUMMARY 
     Various embodiments are directed to systems and methods for providing segmented power curves to tissue. A surgical generator may receive an indication of a first impedance of tissue to be treated by a surgical end effector. The generator may determine that the first impedance is within an impedance range corresponding to a first power curve segment of the segmented power curve and provide a drive signal to the end effector according to a first power curve. The generator may further receive an indication of a second impedance of the tissue to be treated by the end effector, determine that the second impedance is within an impedance range corresponding to a second power curve segment of the segmented power curve, and provide the drive signal to the end effector according to a second power curve. 
     In some embodiments, the generator may provide a drive signal to the end effector according to a first power curve, determine that an impedance of tissue treated by the end effector has moved from an impedance range corresponding to a first power curve segment to an impedance range corresponding to a second power curve segment, and provide the drive signal to the end effector according to the second power curve. 
    
    
     
       FIGURES 
       The novel features of the various embodiments are set forth with particularity in the appended claims. The described embodiments, however, both as to organization and methods of operation, may be best understood by reference to the following description, taken in conjunction with the accompanying drawings in which: 
         FIG. 1  illustrates one embodiment of a surgical system comprising a generator and various surgical devices usable therewith. 
         FIG. 2  illustrates one embodiment of an example ultrasonic device that may be used for transection and/or sealing. 
         FIG. 3  illustrates one embodiment of the end effector of the example ultrasonic device of  FIG. 2 . 
         FIG. 4  illustrates one embodiment of a clamp arm assembly that may be employed with the ultrasonic device of  FIG. 2 . 
         FIG. 5  is a schematic diagram of a tissue impedance module of the generator of  FIG. 1  coupled to the blade and the clamp arm assembly of  FIGS. 3 and 4  with tissue located therebetween. 
         FIG. 6  illustrates one embodiment of an example electrosurgical device that may also be used for transection and sealing. 
         FIGS. 7, 8, and 9  illustrate one embodiment of the end effector shown in  FIG. 6 . 
         FIGS. 10, 11, 12, 13 and 13A  illustrate one embodiment of an alternative end effector  132 ′ that may be used with the electrosurgical device shown in  FIG. 6 . 
         FIG. 14  illustrates one embodiment of the surgical system of  FIG. 1 . 
         FIG. 15  shows a perspective view of one example embodiment of a surgical system comprising a cordless electrical energy surgical instrument with an integral generator. 
         FIG. 16  shows a side-view of a handle of one embodiment of the surgical instrument of  FIG. 9  with half of the handle body removed to illustrate various components therein. 
         FIG. 17  shows one embodiment of an RF drive and control circuit. 
         FIG. 18  shows one embodiment of the main components of a control circuit. 
         FIG. 19  illustrates one embodiment of a chart showing example power curves. 
         FIG. 20  illustrates one embodiment of a process flow for applying one or more power curves to a tissue bite. 
         FIG. 21  illustrates one embodiment of a chart showing example power curves that may be used in conjunction with the process flow of  FIG. 20 . 
         FIG. 22  illustrates one embodiment of a chart showing example common shape power curves that may be used in conjunction with the process flow of  FIG. 20 . 
         FIGS. 23-25  illustrate process flows describing routines that may be executed by a digital device of the generator to generally implement the process flow of  FIG. 20  described above. 
         FIG. 26  illustrates one embodiment of a process flow for applying one or more power curves to a tissue bite. 
         FIG. 27  illustrates one embodiment of a segmented power curve. 
         FIG. 28  illustrates an alternative embodiment of a segmented power curve. 
         FIG. 29  illustrates a plot showing an implementation of the segmented power curve of  FIG. 28  according to one embodiment. 
         FIG. 30  illustrates a state diagram showing one embodiment of a state diagram that may be implemented by a surgical system to execute a segmented power curve. 
         FIG. 31  illustrates a flow chart showing one embodiment of a process flow that may be executed by a surgical system to execute a segmented power curve. 
         FIG. 32  illustrates a flow chart showing one embodiment of a process flow that may be executed by a surgical system (e.g., a generator  102 ,  220  thereof) to execute a segmented power curve using a look-up table. 
         FIG. 33  illustrates a flow chart showing one embodiment of a process flow for modifying a drive signal when a previous energy cycle has already been applied to a tissue bite. 
         FIG. 34  illustrates a flow chart showing another embodiment of a process flow for modifying a drive signal when a previous energy cycle has already been applied to a tissue bit. 
     
    
    
     DESCRIPTION 
     Before explaining various embodiments of surgical devices and generators in detail, it should be noted that the illustrative embodiments are not limited in application or use to the details of construction and arrangement of parts illustrated in the accompanying drawings and description. The illustrative embodiments may be implemented or incorporated in other embodiments, variations and modifications, and may be practiced or carried out in various ways. Further, unless otherwise indicated, the terms and expressions employed herein have been chosen for the purpose of describing the illustrative embodiments for the convenience of the reader and are not for the purpose of limitation thereof. Also, it will be appreciated that one or more of the following-described embodiments, expressions of embodiments and/or examples, can be combined with any one or more of the other following-described embodiments, expressions of embodiments and/or examples. 
     In some embodiments, a surgical system may be configured to provide a drive signal to an end effector of an ultrasonic or electrosurgical instrument according to a segmented power curve. A segmented power curve may be a power curve having designated impedance ranges or segments. Each segment may be associated with one or more power curves. When tissue treated by the end effector exhibits a tissue impedance in a first power curve segment associated with a first power curve, then the surgical system may apply the first power curve. When the tissue treated by the end effector exhibits a tissue impedance in a second power curve segment associated with the second power curve, the surgical system may apply the second power curve. The second power curve, in some embodiments, may be less aggressive than the first power curve. In some examples, the second power curve corresponds to a constant power (e.g., 0-5 Watts). In this way, the tissue may rest or coast during application of the second power curve. 
     In some embodiments, the surgical system may be configured to modify an energy cycle provided to tissue by an ultrasonic or electrosurgical end effector if a clinician requests a second or subsequent energy cycle on the same tissue bite. When an energy cycle is requested, the generator may determine whether the end effector is positioned to treat the same tissue that was treated by the previous energy cycle. For example, when an energy cycle is requested, the generator may be configured to determine whether jaws of the end effector have been opened since the completion of the previous energy cycle (e.g., whether the jaw aperture or distance between the jaw members has increased). If the jaws have not been opened since the previous energy cycle, it may indicate that the end effector is positioned to treat the same tissue bite. If the end effector is positioned to treat the same tissue bite, the generator may be configured to modify the requested energy cycle. For example, the generator may apply a less aggressive power curve and/or modify parameters of the applied power curve or curves to reduce the amount of power provided. In this way, the generator may reduce the risk of damaging tissue by overtreatment. 
     It will be appreciated that the terms “proximal” and “distal” are used herein with reference to a clinician gripping a surgical device. Thus, an end effector is distal with respect to the more proximal portion of the surgical device gripped by the clinician. It will be further appreciated that, for convenience and clarity, spatial terms such as “top” and “bottom” may also be used herein with respect to the clinician gripping the surgical device. However, surgical devices are used in many orientations and positions, and these terms are not intended to be limiting and absolute. 
       FIG. 1  illustrates one embodiment of a surgical system  100  comprising a generator  102  configurable for use with surgical devices. According to various embodiments, the generator  102  may be configurable for use with surgical devices of different types, including, for example, the ultrasonic surgical device  104  and electrosurgical or RF surgical device  106 . Although in the embodiment of  FIG. 1  the generator  102  is shown separate from the surgical devices  104 ,  106 , in certain embodiments the generator  102  may be formed integrally with either of the surgical devices  104 ,  106  to form a unitary surgical system. 
       FIG. 2  illustrates one embodiment of an example ultrasonic device  104  that may be used for transection and/or sealing. The device  104  may comprise a handle  116  which may comprise an ultrasonic transducer  114 . The transducer  114  may be in electrical communication with the generator  102 , for example, via a cable  112  (e.g., a multi-conductor cable). The transducer  114  may comprise piezoceramic elements, or other elements or components suitable for converting the electrical energy of a drive signal into mechanical vibrations. When activated by the generator  102 , the ultrasonic transducer  114  may cause longitudinal vibration. The vibration may be transmitted through an instrument portion  124  of the device  104  (e.g., via a waveguide embedded in an outer sheath) to an end effector  126  of the instrument portion  124 . 
       FIG. 3  illustrates one embodiment of the end effector  126  of the example ultrasonic device  104 . The end effector  126  may comprise a blade  151  that may be coupled to the ultrasonic transducer  114  via the wave guide (not shown). When driven by the transducer  114 , the blade  151  may vibrate and, when brought into contact with tissue, may cut and/or coagulate the tissue, as described herein. According to various embodiments, and as illustrated in  FIG. 3 , the end effector  126  may also comprise a clamp arm  155  that may be configured for cooperative action with the blade  151  of the end effector  126 . With the blade  151 , the clamp arm  155  may comprise a set of jaws  140 . The clamp arm  155  may be pivotally connected at a distal end of a shaft  153  of the instrument portion  124 . The clamp arm  155  may include a clamp arm tissue pad  163 , which may be formed from TEFLON® or other suitable low-friction material. The pad  163  may be mounted for cooperation with the blade  151 , with pivotal movement of the clamp arm  155  positioning the clamp pad  163  in substantially parallel relationship to, and in contact with, the blade  151 . By this construction, a tissue bite to be clamped may be grasped between the tissue pad  163  and the blade  151 . In some embodiments, a strain gauge  156  or other pressure sensor may be positioned on the clamp arm  155 , for example, between the clamp pad  163  and clamp arm  155 , to measure the pressure exerted on tissue held between the clamp arm  155  and the blade  151 . Also, in some embodiments, the clamp arm  155  may comprise a temperature sensor  158  for sensing a temperature of tissue between the clamp arm  155  and the blade  151 . The temperature sensor  158  may be, for example, a thermocouple, a resistive temperature device, an infrared sensor, a bimetallic device, etc. 
     The tissue pad  163  may be provided with a sawtooth-like configuration including a plurality of axially spaced, proximally extending gripping teeth  161  to enhance the gripping of tissue in cooperation with the blade  151 . The clamp arm  155  may transition from the open position shown in  FIG. 3  to a closed position (with the clamp arm  155  in contact with or proximity to the blade  151 ) in any suitable manner. For example, the handle  116  may comprise a jaw closure trigger  138 . When actuated by a clinician, the jaw closure trigger  138  may pivot the clamp arm  155  in any suitable manner. For example, the jaw closure trigger  138  may be coupled to a jaw closure member  141  extending through the shaft  124  to the clamp arm  155 . Proximal motion of the jaw closure trigger  138  may cause corresponding proximal motion of the jaw closure member  141 , which may pull the clamp arm  155  towards the blade. 
     The generator  102  may be activated to provide the drive signal to the transducer  114  in any suitable manner. For example, the generator  102  may comprise a foot switch  120  coupled to the generator  102  via a footswitch cable  122  ( FIG. 14 ). A clinician may activate the transducer  114 , and thereby the blade  151  by depressing the foot switch  120 . In addition, or instead of the foot switch  120  some embodiments of the device  104  may utilize one or more switches or buttons positioned on the handle  116  that, when activated, may cause the generator  102  to activate the transducer  114 . In some embodiments, the handle  116  may comprise a pair of buttons  136   a ,  136   b  positioned relative to the closure trigger  138  to allow the clinician to operate the buttons  136   a ,  136   b  with an index finger, for example, while gripping the closure trigger  138 . In other embodiments, the buttons  136   a ,  136   b  may be replaced with a single similarly located button. Also, for example, one or more additional buttons, such as  136   c , may be positioned on an upper portion of the handle  116 . For example, the button  136   c  may be configured to, when depressed, cause the generator  102  to provide a pulsed output. The pulses may be provided at any suitable frequency and grouping, for example. In certain embodiments, the power level of the pulses may be the power levels set utilizing buttons  136   a ,  136   b , as described above. Also, in some embodiments, the generator  102  may be activated based on the position of the jaw closure trigger  138 , (e.g., as the clinician depresses the jaw closure trigger  138  to close the jaws  140 , ultrasonic energy may be applied). 
     The various buttons  136   a ,  136   b ,  136   c  may be hardwired and/or programmable to, when depressed, bring about various effects on the drive signal provided to the transducer  114 . For example, in some embodiments, the state of the buttons  136   a ,  136   b  may be communicated to the generator  102 . In response to the state of the buttons, the generator  102  may determine an operating mode of the device  104 , expressed as the form of the drive signal provided by the generator  102 . When the button  136   a  is depressed, for example, the ultrasonic generator  102  may provide a maximum drive signal to the transducer  114 , causing it to produce maximum ultrasonic energy output. Depressing button  136   b  may cause the generator  102  to provide a user-selectable drive signal to the transducer  114 , causing it to produce less than the maximum ultrasonic energy output. 
     It will be appreciated that the ultrasonic device  104  may comprise any combination of the buttons  136   a ,  136   b ,  136   c . For example, the device  104  could be configured to have only two buttons: a button  136   a  for producing maximum ultrasonic energy output and a button  136   c  for producing a pulsed output at either the maximum or less than maximum power level per. In this way, the drive signal output configuration of the generator  102  could be 5 continuous signals and 5 or 4 or 3 or 2 or 1 pulsed signals. In certain embodiments, the specific drive signal configuration may be controlled based upon, for example, EEPROM settings in the generator  102  and/or user power level selection(s). 
     In certain embodiments, a two-position switch may be provided as an alternative to a button  136   c . For example, a device  104  may include a button  136   a  for producing a continuous output at a maximum power level and a two-position button  136   b . In a first detented position, button  136   b  may produce a continuous output at a less than maximum power level, and in a second detented position the button  136   b  may produce a pulsed output (e.g., at either a maximum or less than maximum power level, depending upon the EEPROM settings). 
     In some embodiments, the end effector  126  may also comprise a pair of electrodes  159 ,  157 . The electrodes  159 ,  157  may be in communication with the generator  102 , for example, via the cable  128 . The electrodes  159 ,  157  may be used, for example, to measure an impedance of a tissue bite present between the clamp arm  155  and the blade  151 . The generator  102  may provide a signal (e.g., a non-therapeutic signal) to the electrodes  159 ,  157 . The impedance of the tissue bite may be found, for example, by monitoring the current, voltage, etc. of the signal. In some embodiments, the non-therapeutic signal provided to the electrodes  159 ,  157  may be provided by the surgical device  106  itself. 
       FIG. 4  illustrates one embodiment of the clamp arm assembly  451  that may be employed with the ultrasonic device  104 . In the illustrated embodiment, the clamp arm assembly  451  comprises a conductive jacket  472  mounted to a base  449 . The conductive jacket  472  is the electrically conductive portion of the clamp arm assembly  451  that forms the second, e.g., return, electrode. In one implementation, the clamp arm  155  ( FIG. 3 ) may form the base  449  on which the conductive jacket  472  is mounted. In various embodiments, the conductive jacket  472  may comprise a center portion  473  and at least one downwardly-extending sidewall  474  which can extend below the bottom surface  475  of the base  449 . In the illustrated embodiment, the conductive jacket  472  has two sidewalls  474  extending downwardly on opposite sides of the base  449 . In other embodiments, the center portion  473  may comprise at least one aperture  476  which can be configured to receive a projection  477  extending from the base  449 . In such embodiments, the projections  477  can be press-fit within the apertures  476  in order to secure the conductive jacket  472  to the base  449 . In other embodiments, the projections  477  can be deformed after they are inserted into the apertures  476 . In various embodiments, fasteners can be used to secure the conductive jacket  472  to the base  449 . 
     In various embodiments, the clamp arm assembly  451  comprises a non-electrically conductive or insulating material, such as plastic and/or rubber, for example, positioned intermediate the conductive jacket  472  and the base  449 . The electrically insulating material can prevent current from flowing, or shorting, between the conductive jacket  472  and the base  449 . In various embodiments, the base  449  may comprise at least one aperture  478 , which can be configured to receive a pivot pin (not illustrated). The pivot pin can be configured to pivotably mount the base  449  to the shaft  153  ( FIG. 3 ), for example, such that the clamp arm assembly  451  can be rotated between open and closed positions relative to the shaft  153 . In the illustrated embodiment, the base  449  includes two apertures  478  positioned on opposite sides of the base  449 . In one embodiment, a pivot pin may be formed of or may comprise a non-electrically conductive or insulating material, such as plastic and/or rubber, for example, which can be configured to prevent current from flowing into the shaft  153  even if the base  449  is in electrical contact with the conductive jacket  472 , for example. Additional clamp arm assemblies comprising various embodiments of electrodes may be employed. Examples of such clamp arm assemblies are described in commonly-owned and contemporaneously-filed U.S. patent application Ser. Nos. 12/503,769, 12/503,770, and 12/503,766, each of which is incorporated herein by reference in its entirety. 
       FIG. 5  is a schematic diagram of the tissue impedance module  402  coupled to the blade  151  and the clamp arm assembly  451  with tissue  414  located therebetween. With reference now to  FIGS. 1-3 , the generator  102  may comprise a tissue impedance module  402  configured for monitoring the impedance of the tissue  414  (Z t ) located between the blade  151  and the clamp arm assembly  451  during the tissue transection process. The tissue impedance module  402  is coupled to the ultrasonic device  104  by way of the cable  112 . The cable  112  includes a first “energizing” conductor  112   a  connected to the blade  151  (e.g., positive [+] electrode) and a second “return” conductor  112   b  connected to the conductive jacket  472  (e.g., negative [−] electrode) of the clamp arm assembly  451 . In some embodiments, the generator  102  may provide a drive signal to the transducer on the conductors  112   a ,  112   b  and/or over additional conductors included in the cable  112 . In one embodiment, RF voltage v rf  is applied to the blade  151  to cause RF current i rf  to flow through the tissue  414 . The second conductor  112   b  provides the return path for the current i rf  back to the tissue impedance module  402 . The distal end of the return conductor  112   b  is connected to the conductive jacket  472  such that the current i rf  can flow from the blade  151 , through the tissue  414  positioned intermediate the conductive jacket  472  and the blade  151  and the conductive jacket  472  to the return conductor  112   b . The impedance module  402  connects in circuit, by way of the first and second conductors  112   a, b . In one embodiment, the RF energy may be applied to the blade  151  through the ultrasonic transducer  114  and the waveguide (not shown). In some embodiments, the RF energy applied to the tissue  414  for purposes of measuring the tissue impedance Z t  is a low level subtherapeutic signal that does not contribute in a significant manner, or at all, to the treatment of the tissue  414 . 
       FIG. 6  illustrates one embodiment of an example electrosurgical device  106  that may also be used for transection and sealing. According to various embodiments, the transection and sealing device  106  may comprise a handle assembly  130 , an elongated shaft  165  and an end effector  132 . The shaft  165  may be rigid, as shown, (e.g., for laparoscopic and/or open surgical application) or flexible, (e.g., for endoscopic application). In various embodiments, the shaft  165  may comprise one or more articulation points. The end effector  132  may comprise jaws  144  having a first jaw member  167  and a second jaw member  169 . A translating member  173  may extend within the shaft  165  from the end effector  132  to the handle  130 . At the handle  130 , the shaft  165  may be directly or indirectly coupled to a jaw closure trigger  142  ( FIG. 6 ). 
     The jaw members  167 ,  169  of the end effector  132  may comprise respective electrodes  177 ,  179 . The electrodes  177 ,  179  may be connected to the generator  102  via electrical leads  187   a ,  187   b  ( FIG. 7 ) extending from the end effector  132  through the shaft  165  and handle  130  and ultimately to the generator  102  (e.g., by a multi-conductor cable  128 ). The generator  102  may provide a drive signal to the electrodes  177 ,  179  to bring about a therapeutic effect to tissue present within the jaw members  167 ,  169 . The electrodes  177 ,  179  may comprise an active electrode and a return electrode, wherein the active electrode and the return electrode can be positioned against, or adjacent to, the tissue to be treated such that current can flow from the active electrode to the return electrode through the tissue. As illustrated in  FIG. 6 , the end effector  132  is shown with the jaw members  167 ,  169  in an open position. 
       FIGS. 7, 8, and 9  illustrate one embodiment of the end effector  132  shown in  FIG. 6 . To close the jaws  144  of the end effector  132 , a clinician may cause the jaw closure trigger  142  to pivot along arrow  183  ( FIG. 6 ) from a first position to a second position. This may cause the jaws  144  to open and close according to any suitable method. For example, motion of the jaw closure trigger  142  may, in turn, cause the translating member  173  to translate within a bore  185  of the shaft  165 . A distal portion of the translating member  173  may be coupled to a reciprocating member  197  such that distal and proximal motion of the translating member  173  causes corresponding distal and proximal motion of the reciprocating member. The reciprocating member  197  may have shoulder portions  191   a ,  191   b , while the jaw members  167 ,  169  may have corresponding cam surfaces  189   a ,  189   b . As the reciprocating member  197  is translated distally from the position shown in  FIG. 8  to the position shown in  FIG. 9 , the shoulder portions  191   a ,  191   b  may contact the cam surfaces  189   a ,  189   b , causing the jaw members  167 ,  169  to transition to the closed position. Also, in various embodiments, the blade  175  may be positioned at a distal end of the reciprocating member  197 . As the reciprocating member  197  extends to the fully distal position shown in  FIG. 9 , the blade  175  may be pushed through any tissue present between the jaw members  167 ,  169 , in the process, severing it. In some embodiments, a strain gauge  166  or any other suitable pressure sensor may be placed on the jaw member  167  and/or the jaw member  169  to measure the pressure placed on tissue by the respective jaw members  167 ,  169  during tissue treatment ( FIG. 8 ). Also, in some embodiments, one or both of the jaw members  167 ,  169  may comprise a temperature sensor  168  for sensing a temperature of tissue between the jaw members  167 ,  169  ( FIG. 8 ). The temperature sensor  168  may be, for example, a thermocouple, a resistive temperature device, an infrared sensor, a bimetallic device, etc. 
     In use, a clinician may place the end effector  132  and close the jaws  144  around a tissue bite to be acted upon, for example, by pivoting the jaw closure trigger  142  along arrow  183  as described. Once the tissue bite is secure between the jaws  144 , the clinician may initiate the provision of RF or other electro-surgical energy by the generator  102  and through the electrodes  177 ,  179 . The provision of RF energy may be accomplished in any suitable way. For example, the clinician may activate the foot switch  120  ( FIG. 14 ) of the generator  102  to initiate the provision of RF energy. Also, for example, the handle  130  may comprise one or more switches  181  that may be actuated by the clinician to cause the generator  102  to begin providing RF energy. Additionally, in some embodiments, RF energy may be provided based on the position of the jaw closure trigger  142 . For example, when the trigger  142  is fully depressed (indicating that the jaws  144  are closed), RF energy may be provided. Also, according to various embodiments, the blade  175  may be advanced during closure of the jaws  144  or may be separately advanced by the clinician after closure of the jaws  144  (e.g., after a RF energy has been applied to the tissue). 
       FIGS. 10, 11, 12, 13 and 13A  illustrate another embodiment of an electrosurgical device  4106 . Referring to  FIG. 13A , the device  4106  may comprise a handle assembly  4130 , a shaft  4165  and an end effector  4132 . The shaft  4165  may couple the end effector  4132  to the handle assembly  4130 . In some examples, the shaft  4165  may comprise an outer sheath  4158 . The end effector  4132 , may be used with the electrosurgical device  4106  and/or may be used that may be used with the electrosurgical device  106  shown in  FIG. 6 . The end effector  4132  comprises first and second jaw members  4167 ,  4169 . The jaw members  4167 ,  4169  may comprise respective electrodes  4177 ,  4179  for providing electrosurgical energy to tissue between the jaw members  4167 ,  4169 . A knife  4137  may be alternately extendible and retractable through respective slots  4127 ,  4129  in the jaw members  4167 ,  4169 . In some examples, the knife  4137  may omit shoulder portions for actuating the jaw members, as described above with respect to the end effector  132 . For example, the jaw members  4167 ,  4169  may be actuated by a jaw actuator independent of the blade  4137 , such as the outer sheath  4158 . In some examples, one or more of the jaw members  4167 ,  4169  may comprise teeth  4143  for gripping tissue ( FIG. 12 ). Also, one or more of the jaw members  4167 ,  4169  may comprise a pin  4133  positioned to ride within the slot  4127  of the opposite jaw member  4167  to maintain alignment of the jaw members  4167 ,  4169  during the closing and firing process ( FIG. 12 ). 
     The handle assembly  4130  may comprise a jaw closure trigger  4142 . When actuated by a clinician, the jaw closure trigger  4142  may retract a jaw actuator, such as, for example, the outer sheath  4158 , to close jaw members  4167 ,  4169 . The jaw closure trigger  4142  be coupled to a clamp arm  4454 . The clamp arm  4454  may be coupled to a yoke  4456 . When the jaw closure trigger  4142  is actuated in the direction indicated by arrow  4183 , the yoke  4456  moves proximally and compresses a clamp spring  4406 . Compression of the clamp spring  4406  retracts the jaw actuator to transition the jaw members  4167 ,  4169  from an open position to a closed position. For example, the jaw actuator may comprise one or more pins or shoulder portions, similar to  191   a ,  191   b , which may contact one or more cam surfaces, similar to  189   a ,  189   b , on the jaw members  4167 ,  4169 . In some examples, the jaw actuator may comprise one or more pins that ride in cam slots of the respective jaw members  4167 ,  4169 . 
     The handle assembly  4130  may also comprise a firing trigger  4402 . The firing trigger  4402  may be actuatable independent of the jaw closure trigger  4142 . The firing trigger  4402  may be coupled to a reciprocating member  4404 . The reciprocating member  4404  may be coupled to the knife  4137 . For example, the knife  4137  may be positioned at a distal portion of the reciprocating member ( FIG. 11 ). The firing trigger  4402  may comprise a rack gear  4412  positioned to couple with a combination gear  4460 . The combination gear  4460  comprises a gear  4410  and a gear  4414  coupled on a common axis. The gear  4414  may be in contact with the rack gear  4412 . To actuate the firing trigger  4402 , a clinician may depress the firing trigger  4402  proximally. This may cause the firing trigger to pivot about pivot point  4442 . Actuation of the firing trigger  4402  in the direction indicated by arrow  4183  may translate the rack gear, causing counter-clockwise rotation of the combination gear  4460 . The gear  4410  may interface with a rack gear  4408  coupled to the reciprocating member  4404 . Counter-clockwise rotation of the gear  4410  may translate the rack gear  4408  and the reciprocating member  4404  distally, advancing the knife  4137  within the  4127 ,  4129  described herein. An energy button  4416  may be depressed or otherwise actuated to begin delivery of energy to the electrodes  4167 ,  4169 . The energy button  4416  may be any suitable type of button, switch, or similar device. 
     The electrosurgical instrument  4106  may also comprise a closure switch  4450 . The closure trigger  4142  may have a corresponding feature  4452  such that when the closure trigger  4142  is actuated in a proximal direction, the feature  4452  contacts and actuates the switch  4450 . Actuation of the switch  4450  may indicate jaw closure. The feature  4452  is illustrated as a protrusion extending proximally from a body of the jaw closure trigger  4142 . Any suitable feature of the trigger  4142 , however, may be positioned to contact the closure switch  4450 . Additional details of electrosurgical devices similar to the  4106  and other devices that may be used in conjunction with the various features described herein are provided in U.S. patent application Ser. No. 14/075,863, filed on Nov. 8, 2013, which is incorporated herein by reference in its entirety. 
       FIG. 14  is a diagram of the surgical system  100  of  FIG. 1 . In various embodiments, the generator  102  may comprise several separate functional elements, such as modules and/or blocks. Different functional elements or modules may be configured for driving the different kinds of surgical devices  104 ,  106 . For example an ultrasonic generator module  108  may drive an ultrasonic device, such as the ultrasonic device  104 . An electrosurgery/RF generator module  110  may drive an electrosurgical device  106 ,  4106 . For example, the respective modules  108 ,  110  may generate respective drive signals for driving the surgical devices  104 ,  106 ,  4106 . In various embodiments, the ultrasonic generator module  108  and/or the electrosurgery/RF generator module  110  each may be formed integrally with the generator  102 . Alternatively, one or more of the modules  108 ,  110  may be provided as a separate circuit module electrically coupled to the generator  102 . (The modules  108  and  110  are shown in phantom to illustrate this option.) Also, in some embodiments, the electrosurgery/RF generator module  110  may be formed integrally with the ultrasonic generator module  108 , or vice versa. 
     In accordance with the described embodiments, the ultrasonic generator module  108  may produce a drive signal or signals of particular voltages, currents, and frequencies, e.g. 55,500 cycles per second (Hz). The drive signal or signals may be provided to the ultrasonic device  104 , and specifically to the transducer  114 , which may operate, for example, as described above. In some embodiments, the generator  102  may be configured to produce a drive signal of a particular voltage, current, and/or frequency output signal that can be stepped with high resolution, accuracy, and repeatability. Optionally, the tissue impedance module  402  may be included separately or formed integrally with the ultrasonic generator module  108  to measure tissue impedance when utilizing an ultrasonic device  104 , for example, as described herein above. 
     In accordance with the described embodiments, the electrosurgery/RF generator module  110  may generate a drive signal or signals with output power sufficient to perform bipolar electrosurgery using radio frequency (RF) energy. In bipolar electrosurgery applications, the drive signal may be provided, for example, to the electrodes of the electrosurgical device  106 ,  4106 , for example, as described above. Accordingly, the generator  102  may be configured for therapeutic purposes by applying electrical energy to the tissue sufficient for treating the tissue (e.g., coagulation, cauterization, tissue welding, etc.). 
     The generator  102  may comprise an input device  145  located, for example, on a front panel of the generator  102  console. The input device  145  may comprise any suitable device that generates signals suitable for programming the operation of the generator  102 . In operation, the user can program or otherwise control operation of the generator  102  using the input device  145 . The input device  145  may comprise any suitable device that generates signals that can be used by the generator (e.g., by one or more processors contained in the generator) to control the operation of the generator  102  (e.g., operation of the ultrasonic generator module  108  and/or electrosurgery/RF generator module  110 ). In various embodiments, the input device  145  includes one or more of buttons, switches, thumbwheels, keyboard, keypad, touch screen monitor, pointing device, remote connection to a general purpose or dedicated computer. In other embodiments, the input device  145  may comprise a suitable user interface, such as one or more user interface screens displayed on a touch screen monitor, for example. Accordingly, by way of the input device  145 , the user can set or program various operating parameters of the generator, such as, for example, current (I), voltage (V), frequency (f), and/or period (T) of a drive signal or signals generated by the ultrasonic generator module  108  and/or electrosurgery/RF generator module  110 . 
     The generator  102  may also comprise an output device  147  ( FIG. 1 ) located, for example, on a front panel of the generator  102  console. The output device  147  includes one or more devices for providing a sensory feedback to a user. Such devices may comprise, for example, visual feedback devices (e.g., an LCD display screen, LED indicators), audio feedback devices (e.g., a speaker, a buzzer) or tactile feedback devices (e.g., haptic actuators). Although certain modules and/or blocks of the generator  102  may be described by way of example, it can be appreciated that a greater or lesser number of modules and/or blocks may be used and still fall within the scope of the embodiments. Further, although various embodiments may be described in terms of modules and/or blocks to facilitate description, such modules and/or blocks may be implemented by one or more hardware components, e.g., processors, Digital Signal Processors (DSPs), Programmable Logic Devices (PLDs), Application Specific Integrated Circuits (ASICs), circuits, registers and/or software components, e.g., programs, subroutines, logic and/or combinations of hardware and software components. 
     In some embodiments, the ultrasonic generator drive module  108  and electrosurgery/RF drive module  110  may comprise one or more embedded applications implemented as firmware, software, hardware, or any combination thereof. The modules  108 ,  110  may comprise various executable modules such as software, programs, data, drivers, application program interfaces (APIs), and so forth. The firmware may be stored in nonvolatile memory (NVM), such as in bit-masked read-only memory (ROM) or flash memory. In various implementations, storing the firmware in ROM may preserve flash memory. The NVM may comprise other types of memory including, for example, programmable ROM (PROM), erasable programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), or battery backed random-access memory (RAM) such as dynamic RAM (DRAM), Double-Data-Rate DRAM (DDRAM), and/or synchronous DRAM (SDRAM). 
     In some embodiments, the modules  108 ,  110  comprise a hardware component implemented as a processor for executing program instructions for monitoring various measurable characteristics of the devices  104 ,  106 ,  4106  and generating a corresponding output drive signal or signals for operating the devices  104 ,  106 ,  4106 . In embodiments in which the generator  102  is used in conjunction with the ultrasonic device  104 , the drive signal may drive the ultrasonic transducer  114  in cutting and/or coagulation operating modes. Electrical characteristics of the device  104  and/or tissue may be measured and used to control operational aspects of the generator  102  and/or provided as feedback to the user. In embodiments in which the generator  102  is used in conjunction with an electrosurgical device  106 ,  4106 , the drive signal may supply electrical energy (e.g., RF energy) to the end effector  132  in cutting, coagulation and/or desiccation modes. Electrical characteristics of the device  106 ,  4106  and/or tissue may be measured and used to control operational aspects of the generator  102  and/or provided as feedback to the user. In various embodiments, as previously discussed, the hardware components may be implemented as DSP, PLD, ASIC, circuits, and/or registers. In some embodiments, the processor may be configured to store and execute computer software program instructions to generate the step function output signals for driving various components of the devices  104 ,  106 ,  4106 , such as the ultrasonic transducer  114  and the end effectors  126 ,  132 . 
     The surgical devices  104 ,  106 ,  4106  described herein may be incorporated into unitary surgical systems comprising both a surgical device, such as  104 ,  106 , and  4106 , and an integral generator.  FIGS. 15-18  show various embodiments of a surgical system  200  comprising an example unitary electrosurgical system  200 .  FIG. 15  shows a perspective view of one example embodiment of a surgical system  200  comprising a cordless electrical energy surgical instrument  210  with an integral generator (not shown in  FIG. 15 ). The electrosurgical system  200  is similar to the surgical system  100  (e.g., utilized with the electrosurgical device  106 ,  4106 ). The electrosurgical system  200  can be configured to supply energy, such as electrical energy, ultrasonic energy, heat energy, or any combination thereof, to the tissue of a patient either independently or simultaneously as described in connection with  FIGS. 1-14 , for example. The electrosurgical instrument  210  may utilize the end effector  132  and elongated shaft  165  described herein with respect to  FIGS. 6-9  in conjunction with a cordless proximal handle  212 . In one example embodiment, the handle  212  includes the integral generator circuit  220  (see  FIG. 16 ). The generator circuit  220 , sometimes referred to herein as a generator  220 , performs a function substantially similar to that of generator  102 . In one example embodiment, the generator circuit  220  is coupled to a controller or control circuit (e.g.,  281  in  FIG. 17 ). In the illustrated embodiment, the control circuit is integrated into the generator circuit  220 . In other embodiments, the control circuit may be separate from the generator circuit  220 . 
     In one example embodiment, various electrodes in the end effector  126  (e.g.,  177 ,  179 ) may be coupled to the generator circuit  220 . The control circuit  281  may be used to activate the generator  220 , which may serve as an electrical source. In various embodiments, the generator  220  may comprise an RF source, an ultrasonic source, a direct current source, a microwave source, and/or any other suitable type of thermogenic energy source, for example. For example, a direct current source may be utilized to power a heating element that could treat tissue. In one example embodiment, a button  228  may be provided to activate the generator circuit  220  to provide energy to the end effector  126 . 
       FIG. 16  shows a side-view of one example embodiment of the handle  212  of the cordless surgical instrument  210  with half of a first handle body removed to illustrate various components within the second handle body  234 . The handle  212  may comprise a lever arm  221  (e.g., a trigger) which may be pulled along a path (similar to  183 ) around a pivot point. The lever arm  221  may be coupled to an axially moveable member  278  disposed within the shaft  165  by a shuttle operably engaged to an extension of lever arm  221 . In one example embodiment, the lever arm  221  defines a shepherd&#39;s hook shape comprising a distal trigger hook  221   a  and a proximal trigger portion  221   b . As illustrated, the distal trigger hook  221   a  may have a first length while the proximal trigger portion  221   b  may have a second length with the second length greater than the first length. 
     In one example embodiment, the cordless electrosurgical instrument comprises a battery  237 . The battery  237  provides electrical energy to the generator circuit  220 . The battery  237  may be any battery suitable for driving the generator circuit  220  at the desired energy levels. In one example embodiment, the battery  237  is a 1030 mAhr, triple-cell Lithium Ion Polymer battery. The battery may be fully charged prior to use in a surgical procedure, and may hold a voltage of about 12.6V. The battery  237  may have two fuses fitted to the cordless electrosurgical instrument  210 , arranged in line with each battery terminal. In one example embodiment, a charging port  239  is provided to connect the battery  237  to a DC current source (not shown). 
     The generator circuit  220  may be configured in any suitable manner. In some embodiments, the generator circuit comprises an RF drive and control circuit  240  and a controller circuit  282 .  FIG. 17  shows one embodiment of an RF drive and control circuit  240 .  FIG. 17  is a part schematic part block diagram showing the RF drive and control circuitry  240  used in this embodiment to generate and control the RF electrical energy supplied to the end effector  126 . In this embodiment, the drive circuitry  240  is a resonant mode RF amplifier comprising a parallel resonant network on the RF amplifier output and the control circuitry operates to control the operating frequency of the electrosurgical drive signal so that it is maintained at the resonant frequency of the drive circuit, which in turn controls the amount of power supplied to the end effector  126 . The way that this is achieved will become apparent from the following description. 
     As shown in  FIG. 17 , the RF drive and control circuit  240  comprises the above described battery  237  are arranged to supply, in this example, about 0V and about 12V rails. An input capacitor (C in )  242  is connected between the 0V and the 12V for providing a low source impedance. A pair of FET switches  243 - 1  and  243 - 2  (both of which are N-channel in this embodiment to reduce power losses) is connected in series between the 0V rail and the 12V rail. FET gate drive circuitry  245  is provided that generates two drive signals—one for driving each of the two FET&#39;s  243 . The FET gate drive circuitry  245  generates drive signals that causes the upper FET ( 243 - 1 ) to be on when the lower FET ( 243 - 2 ) is off and vice versa. This causes the node  247  to be alternately connected to the 12V rail (when the FET  243 - 1  is switched on) and the 0V rail (when the FET  243 - 2  is switched on).  FIG. 8B  also shows the internal parasitic diodes  248 - 1  and  248 - 2  of the corresponding FET&#39;s  243 , which conduct during any periods that the FET&#39;s  243  are open. 
     As shown in  FIG. 17 , the node  247  is connected to an inductor-inductor resonant circuit  250  formed by inductor L s    252  and inductor L m    254 . The FET gate driving circuitry  245  is arranged to generate drive signals at a drive frequency (f d ) that opens and crosses the FET switches  243  at the resonant frequency of the parallel resonant circuit  250 . As a result of the resonant characteristic of the resonant circuit  250 , the square wave voltage at node  247  will cause a substantially sinusoidal current at the drive frequency (f d ) to flow within the resonant circuit  250 . As illustrated in  FIG. 17 , the inductor L m    254  is the primary of a transformer  255 , the secondary of which is formed by inductor L sec    256 . The inductor L sec    256  of the transformer  255  secondary is connected to an inductor-capacitor-capacitor parallel resonant circuit  257  formed by inductor L 2    258 , capacitor C 4    260 , and capacitor C 2    262 . The transformer  255  up-converts the drive voltage (V d ) across the inductor L m    254  to the voltage that is applied to the output parallel resonant circuit  257 . The load voltage (V L ) is output by the parallel resonant circuit  257  and is applied to the load (represented by the load resistance R load    259  in  FIG. 8B ) corresponding to the impedance of the forceps&#39; jaws and any tissue or vessel gripped by the end effector  126 . As shown in  FIG. 8B , a pair of DC blocking capacitors C bl1    280 - 1  and C bl2    280 - 2  is provided to prevent any DC signal being applied to the load  259 . 
     In one embodiment, the transformer  255  may be implemented with a Core Diameter (mm), Wire Diameter (mm), and Gap between secondary windings in accordance with the following specifications: 
     Core Diameter, D (mm) 
     D=19.9×10-3 
     Wire diameter, W (mm) for 22 AWG wire 
     W=7.366×10-4 
     Gap between secondary windings, in gap=0.125 
     G=gap/25.4 
     In this embodiment, the amount of electrical power supplied to the end effector  126  is controlled by varying the frequency of the switching signals used to switch the FET&#39;s  243 . This works because the resonant circuit  250  acts as a frequency dependent (loss less) attenuator. The closer the drive signal is to the resonant frequency of the resonant circuit  250 , the less the drive signal is attenuated. Similarly, as the frequency of the drive signal is moved away from the resonant frequency of the circuit  250 , the more the drive signal is attenuated and so the power supplied to the load reduces. In this embodiment, the frequency of the switching signals generated by the FET gate drive circuitry  245  is controlled by a controller  281  based on a desired power to be delivered to the load  259  and measurements of the load voltage (V L ) and of the load current (I L ) obtained by conventional voltage sensing circuitry  283  and current sensing circuitry  285 . The way that the controller  281  operates will be described in more detail below. 
     In one embodiment, the voltage sensing circuitry  283  and the current sensing circuitry  285  may be implemented with high bandwidth, high speed rail-to-rail amplifiers (e.g., LMH6643 by National Semiconductor). Such amplifiers, however, consume a relatively high current when they are operational. Accordingly, a power save circuit may be provided to reduce the supply voltage of the amplifiers when they are not being used in the voltage sensing circuitry  283  and the current sensing circuitry  285 . In one-embodiment, a step-down regulator (e.g., LT1502 by Linear Technologies) may be employed by the power save circuit to reduce the supply voltage of the rail-to-rail amplifiers and thus extend the life of the battery  237 . 
       FIG. 18  shows the main components of the controller  281 , according to one embodiment. In the embodiment illustrated in  FIG. 18 , the controller  281  is a microprocessor based controller and so most of the components illustrated in  FIG. 8 c    are software based components. Nevertheless, a hardware based controller  281  may be used instead. As shown, the controller  281  includes synchronous I,Q sampling circuitry  291  that receives the sensed voltage and current signals from the sensing circuitry  283  and  285  and obtains corresponding samples which are passed to a power, V rms  and I rms  calculation module  293 . The calculation module  293  uses the received samples to calculate the RMS voltage and RMS current applied to the load  259  ( FIG. 8B ; end effector  126  and tissue/vessel gripped thereby) and from them the power that is presently being supplied to the load  259 . The determined values are then passed to a frequency control module  295  and a medical device control module  297 . The medical device control module  297  uses the values to determine the present impedance of the load  259  and based on this determined impedance and a pre-defined algorithm, determines what set point power (P set ) should be applied to the frequency control module  295 . The medical device control module  297  is in turn controlled by signals received from a user input module  299  that receives inputs from the user (for example pressing buttons  228  or activating the control levers  221  on the handle  212 ) and also controls output devices (lights, a display, speaker or the like) on the handle  212  via a user output module  261 . 
     The frequency control module  295  uses the values obtained from the calculation module  293  and the power set point (P set ) obtained from the medical device control module  297  and predefined system limits (to be explained below), to determine whether or not to increase or decrease the applied frequency. The result of this decision is then passed to a square wave generation module  263  which, in this embodiment, increments or decrements the frequency of a square wave signal that it generates by 1 kHz, depending on the received decision. As those skilled in the art will appreciate, in an alternative embodiment, the frequency control module  295  may determine not only whether to increase or decrease the frequency, but also the amount of frequency change required. In this case, the square wave generation module  263  would generate the corresponding square wave signal with the desired frequency shift. In this embodiment, the square wave signal generated by the square wave generation module  263  is output to the FET gate drive circuitry  245 , which amplifies the signal and then applies it to the FET  243 - 1 . The FET gate drive circuitry  245  also inverts the signal applied to the FET  243 - 1  and applies the inverted signal to the FET  243 - 2 . 
     In various embodiments, the surgical devices  104 ,  106 ,  4106 ,  200  may be configured to provide to the generator  102 ,  220 , an indication of the state of the jaw or clamp arm. This may allow the generator  102 ,  220  to sense when the clinician has released a tissue bite after treatment. For example, after providing an energy cycle to a surgical device  104 ,  106 ,  4106 ,  200 , the generator  102 ,  220  may determine whether the clinician has released the treated tissue from between the jaw members or clamp arm and blade based on whether the generator  102 ,  220  has received an indication that the jaw members or clamp arms have been opened since the energy cycle (e.g., whether the jaw aperture has increased). 
     The surgical devices  104 ,  106 ,  4106 ,  200  may be configured to provide the generator  102 ,  220  with an indication of the state of the jaw members or clamp arm in any suitable manner. For example, referring to the ultrasonic surgical device  104 , a proximity sensor  191  may be positioned to provide the generator  102 ,  220  with an indication when the clamp arm  155  is opened or closed ( FIG. 3 ). Any suitable proximity sensor may be used such as, for example, a capacitive proximity sensor, an optical proximity sensor, a magnetic proximity sensor, a mechanical switch, a Hall effect sensor, etc. Although the proximity sensor  191  is shown coupled to the shaft  153 , the proximity sensor  191  may be placed at any position on the shaft  153 , clamp arm  155  or other component that moves relative to the clamp arm  155  or blade  151 . Additionally, or alternatively, the generator  102 ,  220  may be in communication with the electrodes  157 ,  159 . For example, a closed circuit between the electrodes  157 ,  159  may indicate that tissue is present while an open circuit between the electrodes  157 ,  159  may indicate that no tissue is present. Additionally or alternatively, the generator  102 ,  220  may be in communication with the pressure sensor  156 . For example, an increase in pressure may indicate that the clamp arm  155  is closed or closing while a decrease in pressure may indicate that the clamp arm  155  is open or opening. Additionally or alternatively, the generator  102 ,  220  may be in communication with the temperature sensor  156 . For example, a decrease in temperature may indicate that tissue has been removed from between the clamp arm  155  and the blade  151 . In some embodiments, a sensor  193  may be positioned in the handle  116  to sense the position of the trigger  138  ( FIG. 2 ) or of any transmission components between the trigger  138  and the clamp arm  155 . The sensor  193  may be any suitable type of sensor including, for example, a proximity sensor, a switch, etc. For example, the position of the trigger  138  and transmission components may correspond to the position of the clamp arm  155 . 
     Referring to the various surgical devices  104 ,  106 ,  4106 ,  200 , the position of the jaw members (or clamp arm) may be determined in a similar manner. For example, a proximity sensor  195  may be positioned at the end effector  132  ( FIG. 8 ) to sense when the jaw members  167 ,  169  are in proximity to one another (e.g., closed). Proximity or switch sensors may also be positioned in the respective handle assemblies. The handle assembly  130  ( FIG. 6 ) shows an example of a proximity sensor  199  positioned therein. The proximity sensor  199  may be positioned to sense actuation of the jaw closure trigger  142  and/or an intermediate component directly or indirectly in contact with the jaw closure trigger  142 . In addition to or instead of the proximity sensor  199 , one or more similar proximity sensors (not shown) may be positioned in the shaft  165  ( FIG. 6 ),  4165  ( FIG. 13A ). For example, a shaft-positioned proximity sensor may sense a position of the closure trigger  142 ,  4142 , jaw members  167 ,  169 ,  4167 ,  4169 , reciprocating member  197 ,  139 ,  173 ,  4404 , etc. In some examples, a proximity sensor may comprise a switch, such as  4450  ( FIG. 13A ). 
     In addition to or instead of using proximity sensors, the generator  102 ,  220  may be in communication with one or more of the pressure sensor  166 , and/or the temperature sensor  168  to sense the closure state of the jaw members  167 ,  169 , for example, as described herein with respect to the ultrasonic device  104 . In some examples, the generator  102 ,  220  may also determine whether the jaws are closed by sending a signal to the electrodes  177 ,  179 . If a closed circuit is detected, it may indicate that the jaw members  167 ,  169  are closed either on each other or on tissue. 
     According to various embodiments, the generator  102 ,  220  may provide power to a tissue bite according to one or more power curves. A power curve may define a relationship between power delivered to the tissue and the impedance of the tissue. For example as the impedance of the tissue changes (e.g., increases) during coagulation, the power provided by the generator  102 ,  220  may also change (e.g., decrease) according to the applied power curve. Power curves may be applied in any suitable manner. For example, the generator  102 ,  220 , may manipulate characteristics of the drive signal provided to the surgical device to bring about the level of power indicated by the power curve. Also, in some examples, a power curve may be implemented in terms of other drive signal characteristics. For example, a power curve may specify voltages, currents, frequencies, duty cycles, etc. corresponding to tissue impedance. 
     Different power curves may be particularly suited, or ill-suited, to different types and/or sizes of tissue bites. Aggressive power curves (e.g., power curves calling for high power levels) may be suited for large tissue bites. When applied to smaller tissue bites, such as small vessels, more aggressive power curves may lead to exterior searing. Exterior searing may reduce the coagulation/weld quality at the exterior and can also prevent complete coagulation of interior portions of the tissue. Similarly, less aggressive power curves may fail to achieve hemostasis when applied to larger tissue bites (e.g., larger bundles). 
       FIG. 19  illustrates one embodiment of a chart  1300  showing example power curves  1306 ,  1308 ,  1310 . The chart  1300  comprises an impedance axis  1302  illustrating increasing potential tissue impedances from left to right. A power axis  1304  illustrates increasing power from down to up. Each of the power curves  1306 ,  1308 ,  1310  may define a set of power levels, on the power axis  1304 , corresponding to a plurality of potential sensed tissue impedances, in the impedance axis  1302 . In general, power curves may take different shapes, and this is illustrated in  FIG. 19 . Power curve  1306  is shown with a step-wise shape, while power curves  1308 ,  1310  are shown with curved shapes. It will be appreciated that power curves utilized by various embodiments may take any usable continuous or non-continuous shape. The rate of power delivery or aggressiveness of a power curve may be indicated by its position on the chart  1300 . For example, power curves that deliver higher power for a given tissue impedance may be considered more aggressive. Accordingly, between two power curves, the curve positioned highest on the power axis  1304  may be the more aggressive. It will be appreciated that some power curves may overlap. 
     The aggressiveness of two power curves may be compared according to any suitable method. For example, a first power curve may be considered more aggressive than a second power curve over a given range of potential tissue impedances if the first power curve has a higher delivered power over the range. Delivered power over the range of potential tissue impedances may be measured in any suitable manner. For example, the delivered power over the range may be represented by an area under the power curve over the range or, when a power curve is expressed discretely, a sum of the power values for the power curve over the set of potential tissue impedances. 
     According to various embodiments, the algorithms described herein may be used with any kind of surgical device (e.g., ultrasonic device  104 , electrosurgical device  106 ,  4106 ,  200 ). In embodiments utilizing an ultrasonic device  104 , tissue impedance readings may be taken utilizing electrodes  157 ,  159  and/or utilizing the clamp arm assembly  451  described herein with respect to  FIGS. 4 and 5 . With an electrosurgical device, such as  106 ,  200 ,  4106 , tissue impedance readings may be taken utilizing first and second electrodes  177 ,  179 . 
     In some embodiments, an electrosurgical device  104  may comprise a positive temperature coefficient (PTC) material positioned between one or both of the electrodes  177 ,  179 ,  4177 ,  4179  and the tissue bite. The PTC material may have an impedance profile that remains relatively low and relatively constant until it reaches a threshold or trigger temperature, at which point the impedance of the PTC material may increase. In use, the PTC material may be placed in contact with the tissue while power is applied. The trigger temperature of the PTC material may be selected such that it corresponds to a tissue temperature indicating the completion of welding or coagulation. Accordingly, as a welding or coagulation process is completed, the impedance of the PTC material may increase, bringing about a corresponding decrease in power actually provided to the tissue. 
     During the coagulation process, tissue impedance may generally increase. In some embodiments, tissue impedance may display a sudden impedance increase indicating successful coagulation. The increase may be due to physiological changes in the tissue, a PTC material reaching its trigger threshold, etc., and may occur at any point in the coagulation process. The amount of energy that may be required to bring about the sudden impedance increase may be related to the thermal mass of the tissue being acted upon. The thermal mass of any given tissue bite, in turn, may be related to the type and amount of tissue in the bite. 
     Various embodiments may utilize this sudden increase in tissue impedance to select an appropriate power curve for a given tissue bite. For example, the generator  102 ,  220  may select and apply successively more aggressive power curves until the tissue impedance reaches an impedance threshold indicating that the sudden increase has occurred. For example, reaching the impedance threshold may indicate that coagulation is progressing appropriately with the currently applied power curve. The impedance threshold may be a tissue impedance value, a rate of change of tissue impedance, and/or a combination of impedance and rate of change. For example, the impedance threshold may be met when a certain impedance value and/or rate of change are observed. According to various embodiments, different power curves may have different impedance thresholds, as described herein. 
       FIG. 20  illustrates one embodiment of a process flow  1330  for applying one or more power curves to a tissue bite. The process flow  1330  may be applied as all or a part of an energy cycle performed by a surgical device on tissue. In the process flow  1330 , any suitable number of power curves may be used. The power curves may be successively applied in order of aggressiveness until one of the power curves drives the tissue to the impedance threshold. When the tissue is driven to the impedance threshold the energy cycle may be terminated. At  1332 , the generator  102 ,  220  may apply a first power curve. According to various embodiments, the first power curve may be selected to deliver power at a relatively low rate. For example, the first power curve may be selected to avoid tissue searing with the smallest and most vulnerable expected tissue bites. 
     The first power curve may be applied to the tissue in any suitable manner. For example, the generator  102 ,  220  may generate a drive signal implementing the first power curve. The power curve may be implemented by modulating the power of the drive signal. The power of the drive signal may be modulated in any suitable manner. For example, the voltage and/or current of the signal may be modulated. Also, in various embodiments, the drive signal may be pulsed. For example, the generator  102 ,  220  may modulate the average power by changing the pulse width, duty cycle, etc. of the drive signal. The drive signal may be provided to the first and second electrodes  177 ,  179 ,  417 ,  4179  of the electrosurgical device  106 . In some embodiments the drive signal implementing the first power curve may be provided to an ultrasonic transducer  114  of the ultrasonic device  104  described above. 
     While applying the first power curve, the generator  102 ,  220  may monitor the total energy provided to the tissue. The impedance of the tissue may be compared to the impedance threshold at one or more energy thresholds. There may be any suitable number of energy thresholds, which may be selected according to any suitable methodology. For example, the energy thresholds may be selected to correspond to known points where different tissue types achieve the impedance threshold. At  1334 , the generator  102 ,  220  may determine whether the total energy delivered to the tissue has met or exceeded a first energy threshold. If the total energy has not yet reached the first energy threshold, the generator  102 ,  220  may continue to apply the first power curve at  1332 . 
     If the total energy has reached the first energy threshold, the generator  102 ,  220  may determine whether the impedance threshold has been reached ( 1336 ). As described above, the impedance threshold may be a predetermined rate of impedance change (e.g., increase) a predetermined impedance, or combination of the two. If the impedance threshold is reached, the generator  102 ,  220  may continue to apply the first power curve at  1332 . For example, reaching the impedance threshold in the first power curve may indicate that the aggressiveness of the first power curve is sufficient to bring about suitable coagulation or welding. 
     In the event that the impedance threshold is not reached at  1336 , the generator  102 ,  220  may increment to the next most aggressive power curve at  1338  and apply the power curve as the current power curve at  1332 . When the next energy threshold is reached at  1334 , the generator  102 ,  220  again may determine whether the impedance threshold is reached at  1336 . If it is not reached, the generator  102 ,  220  may again increment to the next most aggressive power curve at  1338  and deliver that power curve at  1332 . 
     The process flow  1330  may continue until terminated. For example, the process flow  1330  may be terminated when the impedance threshold is reached at  1336 . Also, for example, the process flow  1330  may terminate upon the exhaustion of all available power curves. Any suitable number of power curves may be used. If the most aggressive power curve fails to drive the tissue to the impedance threshold, the generator  102 ,  220  may continue to apply the most aggressive power curve until the energy cycle is otherwise terminated (e.g., by a clinician or upon reaching a final energy threshold). 
     According to various embodiments, the process flow  1330  may continue until tissue impedance reaches a termination impedance threshold. The termination impedance threshold may indicate that coagulation and/or welding is complete. For example, the termination impedance threshold may be based on one or more of tissue impedance, tissue temperature, tissue capacitance, tissue inductance, elapsed time, etc. These may be a single termination impedance threshold or, in various embodiments, different power curves may have different termination impedance thresholds. According to various embodiments, different power curves may utilize different impedance thresholds. For example, the process flow  1330  may transition from a first to a second power curve if the first power curve has failed to drive the tissue to a first tissue impedance threshold and may, subsequently, shift from the second to a third power curve if the second power curve has failed to drive the tissue to a second impedance threshold. 
       FIG. 21  illustrates one embodiment of a chart  1380  showing example power curves  1382 ,  1384 ,  1386 ,  1388  that may be used in conjunction with the process flow  1330 . Although four power curves  1382 ,  1384 ,  1386 ,  1388  are shown, it will be appreciated that any suitable number of power curves may be utilized. Power curve  1382  may represent the least aggressive power curve and may be applied first. If the impedance threshold is not reached at the first energy threshold, then the generator  102 ,  220  may provide the second power curve  1384 . The other power curves  1386 ,  1388  may be utilized, as needed, for example in the manner described above. 
     As illustrated in  FIG. 21 , the power curves  1382 ,  1384 ,  1386 ,  1388  are of different shapes. It will be appreciated, however, that some or all of a set of power curves implemented by the process flow  1330  may be of the same shape.  FIG. 22  illustrates one embodiment of a chart showing example common shape power curves  1392 ,  1394 ,  1396 ,  1398  that may be used in conjunction with the process flow  1330 . According to various embodiments, common shape power curves, such as  1392 ,  1394 ,  1396 ,  1398  may be constant multiples of one another. Accordingly, the generator  102 ,  220  may implement the common shape power curves  1392 ,  1394 ,  1396 ,  1398  by applying different multiples to a single power curve. For example, the curve  1394  may be implemented by multiplying the curve  1392  by a first constant multiplier. The curve  1396  may be generated by multiplying the curve  1392  by a second constant multiplier. Likewise, the curve  1398  may be generated by multiplying the curve  1392  by a third constant multiplier. Accordingly, in various embodiments, the generator  102 ,  220  may increment to a next most aggressive power curve at  1338  by changing the constant multiplier. 
     According to various embodiments, the process flow  1330  may be implemented by a digital device (e.g., a processor, digital signal processor, field programmable gate array (FPGA), etc.) of the generator  102 ,  220 .  FIGS. 23-25  illustrate process flows describing routines that may be executed by a digital device of the generator  102 ,  220  to generally implement the process flow  1330  described above.  FIG. 23  illustrates one embodiment of a routine  1340  for preparing the generator  102 ,  220  to act upon a new tissue bite. The activation or start of the new tissue bite may be initiated at  1342 . At  1344 , the digital device may point to a first power curve. The first power curve, as described above, may be the least aggressive power curve to be implemented as a part of the process flow  1330 . Pointing to the first power curve may comprise pointing to a deterministic formula indicating the first power curve, pointing to a look-up table representing the first power curve, pointing to a first power curve multiplier, etc. 
     At  1346 , the digital device may reset an impedance threshold flag. As described below, setting the impedance threshold flag may indicate that the impedance threshold has been met. Accordingly, resetting the flag may indicate that the impedance threshold has not been met, as may be appropriate at the outset of the process flow  1330 . At  1348 , the digital device may continue to the next routine  1350 . 
       FIG. 24  illustrates one embodiment of a routine  1350  that may be performed by the digital device to monitor tissue impedance. At  1352 , load or tissue impedance may be measured. Tissue impedance may be measured according to any suitable method and utilizing any suitable hardware. For example, according to various embodiments, tissue impedance may be calculated according to Ohm&#39;s law utilizing the current and voltage provided to the tissue. At  1354 , the digital device may calculate a rate of change of the impedance. The impedance rate of change may likewise be calculated according to any suitable manner. For example, the digital device may maintain prior values of tissue impedance and calculate a rate of change by comparing a current tissue impedance value or values with the prior values. Also, it will be appreciated that the routine  1350  assumes that the impedance threshold is a rate of change. In embodiments where the impedance threshold is not a rate of change,  1354  may be omitted. If the tissue impedance rate of change (or impedance itself) is greater than the threshold ( 1356 ), then the impedance threshold flag may be set. The digital device may continue to the next routing at  1360 . 
       FIG. 25  illustrates one embodiment of a routine  1362  that may be performed by the digital device to provide one or more power curves to a tissue bite. At  1364 , power may be delivered to the tissue, for example, as described above with respect to  1334  of  FIG. 20 . The digital device may direct the delivery of the power curve, for example, by applying the power curve to find a corresponding power for each sensed tissue impedance, modulating the corresponding power onto a drive signal provided to the first and second electrodes  177 ,  179 , the transducer  114 , etc. 
     At  1366 , the digital device may calculate the total accumulated energy delivered to the tissue. For example, the digital device may monitor the total time of power curve delivery and the power delivered at each time. Total energy may be calculated from these values. At  1368 , the digital device may determine whether the total energy is greater than or equal to a next energy threshold, for example, similar to the manner described above with respect to  1334  of  FIG. 20 . If the next energy threshold is not met, the current power curve may continue to be applied at  1378  and  1364 . 
     If the next energy threshold is met at  1368 , then at  1370 , the digital device may determine whether the impedance threshold flag is set. The state of the impedance threshold flag may indicate whether the impedance threshold has been met. For example, the impedance threshold flag may have been set by the routine  1350  if the impedance threshold has been met. If the impedance flag is not set (e.g., the impedance threshold is not met), then the digital device may determine, at  1372 , whether any more aggressive power curves remain to be implemented. If so, the digital device may point the routine  1362  to the next, more aggressive power curve at  1374 . The routine  1362  may continue ( 1378 ) to deliver power according to the new power curve at  1364 . If all available power curves have been applied, then the digital device may disable calculating and checking of accumulated energy for the remainder of the tissue operation at  1376 . 
     If the impedance flag is set at  1370  (e.g., the impedance threshold has been met), then the digital device may disable calculating and checking of accumulated energy for the remainder of the tissue operation at  1376 . It will be appreciated that, in some embodiments, accumulated energy calculation may be continued, while  1370 ,  1372 ,  1374 , and  1376  may be discontinued. For example, the generator  102 ,  220  and/or digital device may implement an automated shut-off when accumulated energy reaches a predetermined value. 
       FIG. 26  illustrates one embodiment of a process flow  1400  for applying one or more power curves to a tissue bite. For example, the process flow  1400  may be implemented by the generator  102 ,  220  (e.g., the digital device of the generator  102 ,  220 ). At  1402 , the generator  102 ,  220  may deliver a power curve to the tissue. The power curve may be derived by applying a multiplier to a first power curve. At  1404 , the generator  102 ,  220  may determine if the impedance threshold has been met. If the impedance threshold has not been met, the generator  102 ,  220  may increase the multiplier (e.g., as a function of the total applied energy). This may have the effect of increasing the aggressiveness of the applied power curve. It will be appreciated that the multiplier may be increased periodically or continuously. For example, the generator  102 ,  220  may check the impedance threshold ( 1404 ) and increase the multiplier ( 1406 ) at a predetermined periodic interval. In various embodiments, the generator  102 ,  220  may continuously check the impedance threshold ( 1404 ) and increase the multiplier ( 1406 ). Increasing the multiplier as a function of total applied energy may be accomplished in any suitable manner. For example, the generator  102 ,  220  may apply a deterministic equation that receives total received energy as input and provides a corresponding multiplier value as output. Also, for example, the generator  102 ,  220  may store a look-up table that comprises a list of potential values for total applied energy and corresponding multiplier values. According to various embodiments, the generator  102 ,  220  may provide a pulsed drive signal to tissue (e.g., via one of the surgical devices  104 ,  106 ,  200 ,  4106 ). According to various embodiments, when the impedance threshold is met, the multiplier may be held constant. The generator  102 ,  220  may continue to apply power, for example, until a termination impedance threshold is reached. The termination impedance threshold may be constant, or may depend on the final value of the multiplier. 
     According to various embodiments, a surgical system may be configured to apply energy to tissue according to a segmented power curve. According to a segmented power curve, the surgical system (e.g., a generator  102 ,  220  thereof) may apply a first power curve when tissue impedance is in one of a set of first power curve segments and a second power curve when tissue impedance is in one of a set of second power curve segments. The first and second power curves may have any suitable shape, for example, as described herein. In various embodiments, the second power curve may be less aggressive than the first power curve over a range of impedances. For example, the second power curve may be less aggressive than the first power curve over the range of impedances actually exhibited by the tissue during the course of an energy cycle (e.g., as received by the generator  102 ,  220 ). In some examples, the second power curve may be less aggressive than the first power curve over the range of impedances expected to be exhibited by tissue in general, by tissue of the specific type being treated, etc. In some embodiments, the second power curve is a constant (e.g., 0-5 Watts or another suitable value). In this way, when tissue impedance is in one of the first power curve segments, the generator  102 ,  220  may provide a high energy level that advances the coagulation of the tissue quickly and drives tissue impedance up. When tissue impedance passes into a second power curve segment, the generator  102 ,  220  may reduce the energy level (e.g., to between 0 and 5 Watts). This may slow the rate of impedance increase, which can lead to better sealing outcomes. 
     In some embodiments, first and second power curve segments may be described by a ratio of the sum of first power curve segment widths (e.g., in ohms) to the sum of second power curve segment widths over all tissue impedances or a range of tissue impedances. The ratio may describe the tissue impedance range over which the segmented power curve is actually applied (e.g., as measured by the surgical system) or a range over which the segmented power curve is expected to be applied. The expected range may be determined, for example, based on impedances measured over prior applications of the segmented power curve or similar segmented power curves. In some examples, the ratio may be taken over a single first power curve segment and a single second power curve segment. A segmented power curve for driving tissue impedance up quickly may have a relatively high ratio of first power curve segment width to second power curve segment width. A segmented power curve for driving tissue impedance up more slowly may have a lower ratio. Any suitable ratio may be used in a segmented drive signal. In some examples, the ratio may have a value between 1/10 and 10. In some examples, the ratio may have a value between ⅓ and 3. 
     Additionally, first and second power curve segments may be placed according to any suitable pattern. In some embodiments, first and second power curve segments are equally spaced across tissue impedances. First power curve segments, for example, may have a constant first width. Second power curve segments may have a constant second width. In some embodiments, first power curve segment widths may vary across tissue impedances, as may second power curve segment widths. Also, first and second power curve segments may be irregularly spaced according to any suitable pattern. 
       FIG. 27  illustrates one embodiment of a segmented power curve  3000 . In  FIG. 27 , the horizontal axis  3002  represents tissue impedance (Z) while the vertical axis  3004  represents power ( 3006 ), current ( 3008 ) and voltage ( 3010 ) delivered to tissue according to the segmented power curve  3000 . First power curve segments  3013  are ranges of tissue impedance at which the generator  102 ,  220  applies a first power curve. Second power curve segments  3012  are ranges of tissue impedance at which the generator  102 ,  220  applies a second power curve. For the example power curve  3000 , the first power curve provides a maximum power of 200 Watts when the tissue impedance is below fifty ohms and then gradually reduces its power level as tissue impedance increases. The second power curve is a constant 5 Watts. Below about 80 ohms, the segmented power curve  3000  is in a first power curve segment  3013 . Between about 80 ohms and 320 ohms, the first and second power curve segments  3013 ,  3012  alternate regularly with a ratio of first power curve segment widths  3013  to second power curve segment widths  3012  that is about one (1). 
       FIG. 28  illustrates an alternative embodiment of a segmented power curve  3020 . The segmented power curve  3020  uses the same first and second power curves as the example segmented power curve  3000 . The segmented power curve  3020 , however, utilizes a larger number of smaller segments  3012 ,  3013 . Below about 60 ohms, the segmented power curve  3020  is in a first power curve segment  3013 . Between about 60 ohms and 320 ohms, the first and second power curve segments  3013 ,  3012  alternate regularly with a ratio of the first power curve segment widths  3013  to the second power curve segment widths  3012  that is about one third (⅓). 
       FIG. 29  illustrates a plot  3050  showing an implementation of the segmented power curve  3020  according to one embodiment. In the plot  3050 , the horizontal axis  3052  represents time while the vertical axis  3054  shows power  3058  and tissue impedance  3056 . An energy cycle begins when time is equal to zero (e.g., the generator  102 ,  220  begins to apply energy to tissue). Between zero (0) and about 0.45 seconds, tissue impedance  3056  remains below about 60 ohms and the surgical system remains in the initial first power curve segment  3013  where a maximum power of 200 Watts is provided (e.g., power  3058 ). Between zero (0) and about 0.45 seconds, tissue impedance  3056  initially falls and then rises, forming what is sometimes referred to the “bathtub” portion of the impedance plot  3056 . After about 0.45 seconds, tissue impedance  3056  exceeds about 60 ohms, causing a transition into an initial second power curve segment  3012 . As the power is cut according to the second power curve, the impedance drops below about 60 ohms and back into the initial first power curve segment  3013 . The impedance  3056  continues to rise (in first power curve segments  3013 ) and fall (in second power curve segments  3012 ), but trending towards higher impedance peaks in the first power curve segments  3013 . This may continue until an end-of-cycle event occurs (e.g., a threshold impedance is reached). 
       FIG. 30  illustrates a state diagram showing one embodiment of a state diagram that may be implemented by a surgical system (e.g., a generator  102 ,  220  thereof) to execute a segmented power curve, such as  3000 ,  3020 . During execution of the segmented power curve, the generator  102 ,  220  may be in have a first power curve state  3032  and a second power curve state  3034 . When in the first power curve state  3032 , the generator  102 ,  220  may provide a first power curve  3036  that may be any suitable power curve including, for example, those described herein. When in the second power curve state, the generator  102 ,  220  may provide a second power curve  3038 . The second power curve  3038  may be any suitable power curve including, for example, those described herein. In various embodiments, the second power curve  3038  may be a constant power level, as shown in  FIGS. 27 and 28 . The constant power level may be any suitable power level including, for example, between 0 Watts and 5 Watts. Although the second power curve  3038  is a constant, other examples may use any suitable power curve in the second power curve state. In some embodiments, the second power curve  3038  is less aggressive than the first power curve  3032 . 
     The generator  102 ,  220  may transition  3040  between the first power curve state  3032  and the second power curve state  3034  according to a state transition function  3042 . Referring to the state transition function  3042 , the horizontal axis  3002  corresponds to tissue impedance while the vertical axis  3044  corresponds to the state of the generator  120 . The high (H) state on the axis  3044  corresponds to the first power curve state  3032 , while the low (L) state on the axis  3044  corresponds to the second power curve state  3034 . For example, impedance ranges for which the state transition function  3042  is high may correspond to the first power curve segments  2012  in  FIGS. 27 and 28 . Impedance ranges for which the state transition function  3042  is low may correspond to second power curve segments  3012 . The high/low ratio of the state transition function may correspond to the ratio of first power curve segment widths to second power curve segment widths described above. To apply a segmented power curve, the generator  102 ,  220  may monitor tissue impedance and find a value for the state transition function  3042 . If the state transition function  3042  is high, the generator  102 ,  220  may apply the first power curve  3036 . If the state transition function  3042  is low, the generator  102 ,  220  may apply the second power curve  3038 . 
       FIG. 31  illustrates a flow chart showing one embodiment of a process flow  3050  that may be executed by a surgical system (e.g., a generator  102 ,  220  thereof) to execute a segmented power curve, such as  3000 ,  3020 . For example, the process flow  3050  may represent an energy cycle. At the beginning of the energy cycle, at  3054 , the generator  102 ,  220  may provide the first power curve to tissue. At  3056 , the generator  102 ,  220  may receive a measurement of the impedance of tissue being acted upon by the first power curve. In some embodiments utilizing an ultrasonic surgical device, such as  104 , the impedance may be an impedance of the transducer. At  3058 , the generator  102 ,  220  may determine whether the received impedance measurement indicates a second power curve segment. If no, the generator  102 ,  220  may continue to provide the first power curve at  3054 . If yes, the generator  102 ,  220  may provide the second power curve at  3060 . Whether starting from action  3054  or from action  3060 , the generator  102 ,  220  may periodically execute  3056  and  3058  to determine whether to switch power curves. The process flow  3050  may conclude, for example, when the tissue impedance or other property indicates the end of the energy cycle. 
     In some examples, a segmented power curve may implemented utilizing a look-up table or other suitable mechanism for storing tissue impedances and corresponding drive signal characteristics. For example,  FIG. 32  illustrates a flow chart showing one embodiment of a process flow  3070  that may be executed by a surgical system (e.g., a generator  102 ,  220  thereof) to execute a segmented power curve, such as  3000 ,  3020 , using a look-up table. Instead of alternately applying first and second power curves, the generator  102 ,  220  may apply a consolidated segmented power curve indicated by a look-up table. At  3072 , the generator  102 ,  220  may receive an indication of the impedance of tissue being treated by the end effector. At  3074 , the generator  102 ,  220  may receive a corresponding drive signal characteristic (e.g., power, voltage, current, etc.). For example, the generator  102 ,  220  may store a look-up table or other mechanism that stores tissue impedances and corresponding drive signal characteristics. Drive signal characteristics indicated by the look-up table may incorporate both the first power curve and the second power curve described herein. For example, for impedances corresponding to a first power curve segment, the look-up table may return a drive signal characteristic consistent with the first power curve. For impedances corresponding to a second power curve segment, the look-up table may return a drive signal characteristic consistent with the second power curve. At  3076 , the generator  102  may apply the received drive signal characteristic to the end effector. The process flow  3070  may be executed, for example, during the provision of an energy cycle. In some examples, it may be executed continuously until termination of the energy cycle. 
     In various embodiments, the surgical system (e.g., the generator  102 ,  220  thereof) may be programmed to modify the signal provided to tissue if a clinician requests a second or subsequent energy cycle on the same tissue bite. An energy cycle may represent a single instance in which energy is applied to a tissue. For example,  FIGS. 20, 26, 30 and 31  illustrate flow charts and/or state diagrams setting forth energy delivery algorithms that may be applied in an energy cycle. An energy cycle may conclude when energy provision to the tissue ceases, for example, when the tissue impedance or other indicator reaches a threshold value or an energy delivery algorithm otherwise terminates. 
     Many energy cycles are designed to operate on tissue that has not yet been treated. Some clinicians, however, prefer to apply multiple energy cycles to the same piece of tissue in contact with the end effector (e.g., tissue bite). Applying a second or subsequent energy cycle to the same tissue bite, however, can sometimes damage tissue by applying excessive energy. Accordingly, the generator  102 ,  220  may be programmed to modify a drive signal provided to electrosurgical electrodes and/or an ultrasonic transducer if the generator determines that the end effector is in contact with tissue that has already been treated by a previous energy cycle. 
       FIG. 33  illustrates a flow chart  3100  showing one embodiment of a process flow  3100  for modifying a drive signal when a previous energy cycle has already been applied to tissue. The process flow  3100  may be executed by a surgical system with any suitable type of surgical device including, for example, the ultrasonic surgical device  104  and/or the electrosurgical devices  106 ,  4106 ,  200  described herein. At  3102 , the surgical system (e.g., a generator  102 ,  220  thereof) may receive an indication of an energy cycle request from a clinician. The clinician may request an energy cycle in any suitable manner. For example, referring to the ultrasonic surgical device  104 , the clinician may request an energy cycle by actuating one or more of buttons  136   a ,  136   b ,  136   c  ( FIG. 2 ). In some examples, the clinician may request an energy cycle by actuating a button on the surgical device (e.g.,  181 ,  4412 ,  136   a ,  136   b ,  136   c ). 
     At  3104 , the generator  102 ,  220  may determine whether the end effector of the surgical device is in contact with tissue that has already been treated. The generator  102 ,  220  may make this determination in any suitable way. For example, the generator  102 ,  220  may determine whether the end effector has released the last tissue to be treated. Release may be sensed in any suitable manner. For example, the generator  102 ,  220  may determine whether electrosurgical jaws  167 ,  169  or an ultrasonic clamp arm  155  have been opened, as described herein. In some examples, the generator  102 ,  220  may monitor a property of the end effector, such as temperature. If the temperature of the end effector is above a threshold value, it may indicate that the tissue present has already been treated. In some examples, the generator  102 ,  220  may monitor an impedance between electrodes in the end effector, such as  157 ,  159  of the end effector  126 , electrodes  177 ,  179  of the end effector  132 , the electrodes  4177 ,  4179  of the end effector  4132 , etc. For example, after an energy cycle, the generator  102 ,  220  may provide a non-therapeutic signal to the electrodes and monitor whether an open circuit is detected. An open circuit may be detected, for example, if the impedance exceeds a threshold value, if the current between the electrodes falls below a threshold value, etc. If an open circuit is detected after an energy cycle, the generator  102 ,  220  may determine that the end effector is no longer in contact with the tissue that was treated during the energy cycle. If no open circuit is detected after the energy cycle, the generator  102 ,  220  may determine that the end effector is still in contact with the tissue that was treated during the energy cycle. In some examples, the generator  102 ,  220  may monitor a pressure exerted between members of the end effector (e.g., between jaw members  167 ,  169  or  4167 ,  4169 , between a clamp arm  155  and blade  151 , etc.). The pressure may be sensed, for example, by one or more pressure sensors  156 ,  166 , as described herein. If the generator  102 ,  220  determines there no reduction in pressure below a threshold pressure level since the previous energy cycle was completed, it may determine that the tissue contacting the end effector has been previously treated. 
     If the tissue at the end effector has not been treated, then the generator  102 ,  220  may, at  3108 , apply the requested energy cycle. If the tissue at the end effector has been treated, then the generator  102 ,  220  may, at  3106 , apply a modified energy cycle. For example, because the tissue has already been treated, the generator  102 ,  220  may modify the requested energy cycle to reduce the total energy delivered during the energy cycle. 
     The energy cycle may be modified in any suitable manner. In some examples, the generator  102 ,  220  may reduce average power levels provided during the energy cycle, for example, by reducing the current, voltage, duty cycle, or any other suitable property. In some examples, the generator  102 ,  220  may change the value of a threshold property, such as impedance, that indicates the end of the algorithm. For example, the threshold property may be modified in a manner that tends to reduce the total energy delivered during the energy cycle. In some examples, the generator  102 ,  220  may modify the energy cycle by applying one or more maximum values. For example, the generator  102 ,  220  may apply a maximum total energy to be delivered during the energy cycle. If the maximum total energy is exceeded, then the generator  102 ,  220  may terminate the energy cycle. That is, if the energy cycle has not otherwise terminated before the expiration of the maximum time, then the generator  102 ,  220  may terminate the energy cycle. Also, for example, where the energy cycle involves the application of a power curve, modifying the energy cycle may comprise reducing the power levels delivered over all or a portion of the power curve, for example, by applying a multiplier. Where the energy cycle involves applying a segmented power curve, as described herein, modifying the energy cycle routine may comprise, modifying the first and/or second power curves, reducing the ratio of first power curve segment widths to second power curve segments widths, etc. Also, for example, the generator  120 ,  220  may apply a maximum power, voltage, current, etc. The generator  120 ,  220  may be programmed to prevent the power, voltage, current, etc., from exceeding respective maximum values during the energy cycle. 
       FIG. 34  illustrates a flow chart showing another embodiment of a process flow  3200  for modifying a drive signal when a previous energy cycle has already been applied to a tissue bit. The process flow  3200  may be executed by a surgical system including any suitable type of surgical device including, for example, the ultrasonic surgical device  104  and the electrosurgical devices  106 ,  4106 ,  200  described herein. At  3202 , the surgical system (e.g., a generator  102 ,  220  thereof) may receive an indication of jaw closure. When the surgical system uses an ultrasonic device, the indication of jaw closure may be an indication that the clamp arm  155  has been pivoted against the blade  151 . When the surgical system uses an electrosurgical device, the indication of jaw closure may be an indication that the jaw members  167 ,  169  have been moved to a closed position. The indication of jaw closure may come from any suitable sensor or mechanism including, for example, proximity sensors  191 ,  193 ,  195 ,  199 ,  4450 , pressure sensors  156 ,  166 , temperature sensors  156 , electrodes  157 ,  159 ,  177 ,  179 , etc. 
     At  3204 , the generator  102 ,  220  may receive an indication of an energy cycle request, similar to  3102  described herein. At  3206 , the generator  102 ,  220  may provide a drive signal to the surgical device (e.g., ultrasonic surgical device  104  or electrosurgical device  106 ,  4106 ,  200 ) according to the requested energy cycle. At  3208 , the energy cycle may cease. For example, the energy cycle may terminate, at which point the generator  102 ,  220  may discontinue the drive signal. The generator  102 ,  220  may then await a next request for an energy cycle. At  3210 , the generator  102 ,  220  may receive a next request for an energy cycle, for example, as described herein. At  3212 , the generator  102 ,  220  may determine whether the jaws have opened since the provision of the first energy cycle (or, with the ultrasonic device  104 , whether the clamp arm  155  has pivoted away from the blade  151 ). If the jaws have opened, it may indicate that the clinician has released the tissue bite treated by the previous energy cycle. Accordingly, if the jaws have opened, the generator  102 ,  220  may, at  3206 , provide the drive signal according to the requested energy cycle. If the jaws have not opened, it may indicate that the clinician intends to apply an energy cycle to the same tissue bite that was previously treated. Accordingly, at  3214 , the generator  102 ,  220  may provide a drive signal according to a modified energy cycle, as described herein. 
     Although the various embodiments of the devices have been described herein in connection with certain disclosed embodiments, many modifications and variations to those embodiments may be implemented. For example, different types of end effectors may be employed. Also, where materials are disclosed for certain components, other materials may be used. The foregoing description and following claims are intended to cover all such modification and variations. 
     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. 
     Various aspects of the subject matter described herein are set out in the following numbered clauses: 
     1. A surgical system, the system comprising: 
     an end effector; and 
     a generator programmed to:
         receive an indication of a first impedance of tissue to be treated by the end effector;   determine that the first impedance is within an impedance range corresponding to a first power curve segment of the segmented power curve;   provide a drive signal to the end effector according to a first power curve;   receive an indication of a second impedance of the tissue to be treated by the end effector;   determine that the second impedance is within an impedance range corresponding to a second power curve segment of the segmented power curve;   provide the drive signal to the end effector according to a second power curve.       

     2. The surgical system of clause 1, wherein the second power curve is less aggressive than the first power curve. 
     3. The surgical system of clause 1, wherein the second power curve has a lower delivered power over a range of tissue impedance indications received by the generator, wherein the range of tissue impedance indications comprises the indication of the first impedance and the indication of the second impedance. 
     4. The surgical system of clause 1, wherein the second power curve is a constant power. 
     5. The surgical system of clause 4, wherein the second power curve is a constant power between 0 Watts and 5 Watts. 
     6. The surgical system of clause 1, wherein a ratio of a width of the first power curve segment to a width of the second power curve segment is between 1/10 and 10. 
     7. The surgical system of clause 6, wherein the ratio of a width of the first power curve segment to a width of the second power curve segment is between ⅓ and 3. 
     8. The surgical system of clause 1, wherein the ratio of a width of the first power curve segment to a width of the second power curve segment is about 1. 
     9. The surgical system of clause 1, further comprising a transducer mechanically coupled to the end effector, providing the drive signal to the end effector comprises providing the drive signal to the transducer. 
     10. The surgical system of clause 1, wherein the end effector further comprises a first electrode and a second electrode, and wherein providing the drive signal to the end effector comprises providing the drive signal to the first electrode and the second electrode. 
     11. The surgical system of clause 1, wherein the generator is further programmed to: 
     receive a request for an energy cycle routine to the end effector; 
     determine that the at least one jaw member has not been in the open position since a conclusion of a previous energy cycle; and 
     apply a modified energy cycle routine to the end effector, wherein applying the modified energy cycle routine delivers less energy than applying the energy cycle routine. 
     12. A surgical system, the system comprising: 
     an end effector; and 
     a generator to provide a drive signal to the end effector, wherein the generator is programmed to:
         provide a drive signal to the end effector according to a first power curve;   determine that an impedance of tissue treated by the end effector has moved from an impedance range corresponding to a first power curve segment to an impedance range corresponding to a second power curve segment; and   provide the drive signal to the end effector according to the second power curve.       

     13. The system of clause 12, wherein determining that an impedance of the tissue treated by the end effector has moved from an impedance range corresponding to a first power curve segment to an impedance range corresponding to a second power curve segment comprises: 
     receiving an indication of a first impedance of the tissue treated by the end effector; and 
     evaluating a state transfer function based on the first impedance, wherein a result of the state transfer function indicates that the first impedance is within an impedance range corresponding to a second power curve segment. 
     14. The system of clause 12, wherein determining that an impedance of the tissue treated by the end effector has moved from an impedance range corresponding to a first power curve segment to an impedance range corresponding to a second power curve segment comprises: 
     receiving an indication of a first impedance of the tissue treated by the end effector; and 
     retrieving a drive signal characteristic corresponding to the first impedance, and wherein providing the drive signal to the end effector according to the second power curve comprises applying the drive signal characteristic to the drive signal. 
     15. The surgical system of clause 12, wherein the second power curve is less aggressive than the first power curve. 
     16. The surgical system of clause 12, wherein the second power curve has a lower delivered power over a range of tissue impedance indications received by the generator, wherein the range of tissue impedance indications comprises the indication of the first impedance and the indication of the second impedance. 
     17. The surgical system of clause 12, wherein the second power curve is a constant power. 
     18. The surgical system of clause 17, wherein the second power curve is a constant power between 0 Watts and 5 Watts. 
     19. The surgical system of clause 12, wherein a ratio of a width of the first power curve segment to a width of the second power curve segment is between 1/10 and 10. 
     20. The surgical system of clause 19, wherein the ratio of a width of the first power curve segment to a width of the second power curve segment is between ⅓ and 3. 
     21. The surgical system of clause 12, wherein the ratio of a width of the first power curve segment to a width of the second power curve segment is about 1. 
     22. The surgical system of clause 12, wherein the generator is further programmed to: 
     receive a request for an energy cycle routine to the end effector; 
     determine that the at least one jaw member has not been in the open position since a conclusion of a previous energy cycle; and 
     apply a modified energy cycle routine to the end effector, wherein applying the modified energy cycle routine delivers less energy than applying the energy cycle routine.