Electrosurgical instrument

Abstract

A robotic electrosurgical instrument 300 comprises a shaft and an end effector connected to a distal end of the shaft. The end effector comprises opposing first 311 and second 312 end effector elements. Each end effector element comprises a middle electrode 315, 318 between a pair of lateral electrodes 316, 317, 319, 320, wherein the instrument is operable in a cutting mode in which the middle electrode of the first end effector is connected to a higher potential that at least one lateral electrode in one of the first and second pair of electrodes such that current flows between the first middle electrode and that at least one lateral electrode. The first and second middle electrodes may be reflectionally asymmetric about a plane separating the first and second end effector elements. In the cutting mode, the second middle electrode may be connected to a higher potential than both lateral electrodes in one of the first and second pairs. In the cutting mode, the first and second middle electrodes say have the same polarity. The instrument may also be operable in a sealing mode, where the first pair of lateral electrodes is connected to a higher potential than the second pair of lateral electrodes.

Description

Field The present disclosure relates to a robotic surgical instrument for use in electrosurgical procedures. Background Electrosurgery involves applying a radio frequency current to tissue to achieve a particular effect. Electrosurgical instruments have several uses in surgery. For example, electrosurgical instruments can be used to cut tissue, desiccate tissue (dry it out), and seal vessels within tissue. Depending on the desired effect, different amounts of power per unit volume are delivered to the tissue. For example, to cut tissue using vaporisation, a relatively high power is delivered to the tissue to rapidly heat water in the cells of the tissue so that the water vaporises and the cells rupture. As another example, to desiccate tissue, a relatively low power is supplied to the tissue to slowly heat the water in the cells of the tissue which causes the tissue to dry out. Desiccated tissue can be cut fairly easily by mechanically tearing or pulling the desiccated tissue. Thus, tissue can be cut either from direct vaporisation of the tissue, or via desiccating and tearing the tissue. To seal a vessel in the tissue, a relatively low power is applied to heat internal collagen to around 70 - 100°C and then cool it down to form a seal. In monopolar electrosurgery the high frequency current passes through the patient from an active electrode of the monopolar electrosurgical instrument to a separate return electrode placed on the patient. In bipolar electrosurgery the active and return electrodes are both within the bipolar electrosurgical instrument. The current passes from the active electrode of the bipolar electrosurgical instrument to the return electrode of the bipolar electrosurgical instrument via the tissue between the active and return electrode. It is desirable to increase the effectiveness of the cutting action performed by the electrosurgical instrument. In particular, it is desirable to increase the cutting performance of a robotic bipolar electrosurgical instrument when cutting mesentery and other types of tougher tissue. For example, tissue that has dried out is harder to cut using vaporisation since vaporisation requires the presence of water in the tissue. Increasing the voltage of the cutting instrument can improve the cutting performance of the instrument. However, if the voltage is increased too much then sparking across the electrodes can occur. Sparking can burn the tissue surrounding the electrosurgical instrument. Sparking occurs when the path of least resistance for the current is via an air gap between electrodes, rather than through the tissue. This can happen ifthe tissue impedance increases, or ifthe electrodes are positioned too close to each other. Sparking depends on the dielectric strength of air, which is expressed in terms of volts per distance. If the distance between electrodes is short, then a low voltage must be used to avoid sparking. However, a low voltage is less effective at vaporising tissue. A high voltage, on the other hand, would require the distance between electrodes to be increased, which can reduce the practicality of the instrument and reduce the precision of the cut. Thus, there is a balance to be struck between voltage applied by the electrodes and the distance between the electrodes. Ideally, the electrodes are designed to produce a strong local electric field (V / m) in regions near the surface of the electrode, whilst having a low average electric field between the electrodes so that sparking can be avoided, the distance between electrodes can be kept relatively small, and the voltage delivered to the tissue can be maximised. There is a need for a robotic bipolar electrosurgical instrument that has an improved cutting performance whilst minimising the risk of sparking. Summary According to the present disclosure there is provided a robotic electrosurgical instrument comprising a shaft; and an end effector connected to a distal end of the shaft, the end effector comprising opposing first and second end effector elements, the first end effector element comprising a first middle electrode between a first pair of lateral electrodes, the second end effector element comprising a second middle electrode between a second pair of lateral electrodes; wherein the instrument is operable in a cutting mode in which the first middle electrode is connected to a higher potential than at least one lateral electrode in one of the first and second pair of lateral electrodes such that current flows between the first middle electrode and that at least one lateral electrode. The first and second middle electrodes may be reflectionally asymmetric with respect to each other about a first plane separating the first end effector element from the second end effector element. When the first and second end effector elements are in a closed configuration, a point on at least one of the first and second middle electrodes respectively may be closer to at least one lateral electrode of the end effector element opposing that at least one middle electrode than the pair of lateral electrodes of the end effector element comprising that at least one middle electrode. The first and second middle electrodes may be reflectionally asymmetric with respect to each other about a second plane bisecting each of the first and second end effector elements. The first and second middle electrodes may be shaped and sized such that, when the first and second end effector elements are in a closed configuration, the first plane intersects both the first and second middle electrodes. When the first and second end effector elements are in a closed configuration, a point on one of the first or second middle electrodes may be closer to both lateral electrodes of the end effector element opposing that one middle electrode than the pair of lateral electrodes of the end effector element comprising that one middle electrode. When the first and second end effector elements are in a closed configuration, that one middle electrode may be shaped and sized such that a fourth plane intersects that one middle electrode and said both lateral electrodes of the end effector element opposing that one middle electrode. The first middle electrode and the first pair of lateral electrodes and the second middle electrode and the second pair of lateral electrodes may extend longitudinally along the first and second end effector elements respectively. Each of the first and second middle electrodes may comprise a lateral face having a width that is less than a width of a lateral face of each lateral electrode in the first and second pairs of lateral electrodes. In the cutting mode, the second middle electrode may be connected to a higher potential than at least one lateral electrode in one of the first and second pair of lateral electrodes such that current flows between the second middle electrode and that at least one lateral electrode. In the cutting mode, the first middle electrode may be connected to a higher potential than both lateral electrodes in one of the first and second pair of lateral electrodes such that current flows between the first middle electrode and that pair of lateral electrodes. In the cutting mode, the second middle electrode may be connected to a higher potential than both lateral electrodes in one of the first and second pair of lateral electrodes such that current flows between the second middle electrode and that pair of lateral electrodes. In the cutting mode, each of the first and second middle electrodes may be connected to a higher potential than both the first and second pair of lateral electrodes, such that current flows between the first middle electrode and at least one of the first and second pairs of lateral electrodes, and current flows between the second middle electrode and at least one of the first and second pairs of lateral electrodes. In the cutting mode, the first and second middle electrodes may have the same polarity. In the cutting mode, the first pair of lateral electrodes may be connected to a different potential than the second pair of lateral electrodes. The instrument may be operable in a sealing mode in which the first pair of lateral electrodes is connected to a higher potential than the second pair of lateral electrodes such that current flows between the first pair of lateral electrodes and the second pair of lateral electrodes. The first and second middle electrodes may be cut electrodes. The lateral electrodes in the first and second pair of lateral electrodes may be seal electrodes. The ratio of the width of the lateral face of each of the first and second middle electrode to the width of the lateral face of each lateral electrode in the first and second pairs of lateral electrodes may be in the range 5:1 to 20:1. For each of the first and second pair of lateral electrodes, the lateral electrodes in that pair may form a U-shape. The first plane may be a plane bisecting the opening angle between the first and second end effector elements. When the first and second end effector elements are in a closed configuration, the first middle electrode may be spaced from the second middle electrode by 0.01mm to 0.5mm in a direction parallel to a plane bisecting each of the first and second end effector elements. When the first and second end effector elements are in a closed configuration, the first middle electrode may be offset from the second middle electrode by 0.5mm to 1.5mm in a direction perpendicular to a plane bisecting each of the first and second end effector elements. The end effector may be connected to the distal end of the shaft via an articulation. The articulation may permit the end effector to move relative to the shaft with at least two degrees of freedom. Brief Description of the Drawings The present invention will now be described by way of example with reference to the accompanying drawings. In the drawings: Figure 1 shows an example of surgical robotic system. Figure 2a and 2b shows an example bipolar electrosurgical instrument. Figure 3 shows a cross-section of a bipolar electrosurgical instrument and its electrical connections. Figure 4a and 4b show a cross-section of a bipolar electrosurgical instrument when electrically connected for performing a sealing operation and a cutting operation respectively. Figure 5 shows a cross-section of a bipolar electrosurgical instrument with over-close on the middle electrode. Figure 6 shows a cross-section of a bipolar electrosurgical instrument with lateral spacing between the middle electrodes. Figure 7 shows an exploded view of a bipolar electrosurgical instrument. Detailed Description Figure 1 shows a typical surgical robotic system. A surgical robot 100 comprises a base 102, an arm 104, and a surgical instrument 106. The base supports the robot. The arm extends between the base and the instrument. The arm is articulated by flexible joints 108 along its length. The surgical instrument is attached to the distal end of the robot arm. The surgical instrument comprises a shaft connected to an end effector 110 at its distal end for engaging in a medical procedure. A surgeon controls the surgical robot 100 via a remote surgeon console 112. The surgeon console comprises one or more surgeon input devices 114. These may take the form of a hand controller or foot pedal. The surgeon console also comprises a display 116. A control system 118 connects the surgeon console 112 to the surgical robot 100. The control system receives inputs from the surgeon input device(s) 114 and converts these to control signals to move the joints of the robot arm 104 and instrument 106. The control system sends these control signals to the robot, where the corresponding joints are driven accordingly. Figures 2a and 2b illustrate an example electrosurgical instrument 200 for use in performing electrosurgery. For example, instrument 200 may be a robotic electrosurgical instrument for performing bipolar electrosurgery. The instrument may comprise a base 201 for attaching the instrument to the robot arm. A shaft 202 extends between the base 201 and an end effector 210. The end effector 210 is connected to the distal end of the shaft. In this example, the end effector is connected to the distal end of the shaft via an articulation 203. The articulation is at the distal end of the shaft for articulating the end effector 210. The articulation may permit the end effector to move relative to the shaft. For example, the articulation may move the end effector 210 with at least two degrees of freedom. The end effector 210 comprises a pair of end effector elements. Figures 2a and 2b show the end effector 210 comprising a first end effector element 211 and a second end effector element 212. The first and second end effector elements 211, 212 oppose each other to form a pair of jaws. The first and second end effector elements may be rotatable in opposing rotational directions. The first and second end effector elements may be individually rotatable. For example, the articulation may allow each end effector element to be independently movable. The articulation may comprise joints which permit the end effector 210 to move relative to the shaft 202. The joints of the articulation may be driven by driving elements, such as cables. Each end effector element may be rotated around a joint by movement of a respective driving element. The instrument 200 described herein may conveniently be articulated using driving elements because the instrument does not need to comprise components which are pushed across joints of the articulation to perform a cutting operation. In an example, the articulation may comprise a pitch joint which rotates about a pitch axis. The pitch axis is perpendicular to the longitudinal axis of the shaft. The pitch joint permits the end effector to rotate about the pitch axis relative to the shaft. The articulation may further comprise a first and second yaw joint which each rotate about a respective yaw axis. The first and second yaw joint are distal of the pitch joint. The respective yaw axes of the first and second yaw joints may be parallel. Each yaw axis is perpendicularto both the longitudinal axis of the shaft and the pitch axis, when the shaft and articulation are in a straight configuration. The first yaw joint may be fast with the first end effector element and permit the first end effector element to rotate about the yaw axis of the first yaw joint, relative to the pitch joint and shaft. The second yaw joint may be fast with the second end effector element and permit the second end effector element to rotate about the yaw axis of the second yaw joint, relative to the pitch joint and the shaft. Each joint may be articulated using the driving elements described above. In one example, each end effector element may not be independently moveable. Movement of one end effector element may cause a corresponding movement of the other end effector element. For example, the instrument 200 may comprise a push-pull rod which may move both end effector elements between an open and closed configuration in the same movement. The end effector elements may be articulated using a rack and pinion which is actuated by means of a rod. In another example, one end effector element may be fixed in position with respect to the shaft. The other end effector element may be moveable with respect to the fixed end effector element. For example, the articulation may be in the form of a hinge which allows movement of the moveable end effector element relative to the fixed end effector element. Other means for actuation of the end effector elements are possible. Each of the first and second end effector elements comprise electrodes, depicted generally at 207. The electrodes are used to perform electrosurgery and will be described in more detail below. The electrodes are powered by electrosurgical cables (not shown) that extend through the shaft 202 to the end effector 210. The electrosurgical cables may be constrained to pass from the end effector 210 internally through the articulation to the shaft 202. In an example, the shaft may comprise internal spokes or rods extending from the proximal end of the shaft (e.g. the base 201) through the centre of the shaft to reach the end effector 210 or articulation 203 at the distal end of the shaft. The electrosurgical cables may be fixed to the spokes of the shaft to keep the electrosurgical cables taut. If the instrument comprises an articulation 203 driven by driving elements (e.g. cables), the driving elements may be connected to the spokes of the shaft at each end, along with the electrosurgical cables. Once the electrosurgical cables have passed through the shaft, they may be routed externally to the articulation or end effector. At one end, the electrosurgical cables are connected to electrodes in the end effector. Each electrode is connected to an electrosurgical cable. There may be a separate electrosurgical cable for each electrode. For example, if there are six electrodes in the end effector (three on each jaw), there may be six electrosurgical cables extending through the shaft to the end effector. By powering each electrode with a separate electrosurgical cable, the electrodes can be activated independently of each other. Alternatively, one electrosurgical cable may be used to power multiple electrodes. For example, if the electrodes are activated in pairs, only one electrosurgical cable per pair may be needed to power each electrode in the pair. This can reduce the number of cables extending through the shaft. At the other end, the electrosurgical cables are connected to an electrosurgical generator (not shown). The electrosurgical generator provides electrosurgical energy to power the electrodes via the electrosurgical cables. The electrosurgical generator generates electrosurgical signals for driving the instrument. The electrosurgical generator may generate different current waveforms. The electrosurgical generator may be capable of generating multiple different current waveforms to achieve different surgical effects. For example, electrosurgical generator may be configured to generate COAG and CUT waveforms. The COAG waveform consists of bursts of radio frequency, which when used at a low power setting causes a desiccation effect, and when used at a high-power setting causes a fulguration effect. The CUT waveform is a continuous waveform at higher voltage than COAG, which causes the tissue to be cut. The CUT waveform is typically a constant RMS voltage / current waveform once the desired voltage / current has been reached. The electrosurgical generator may be configured by a user to generate a particular waveform. The electrosurgical generator comprises any suitable means for configuring the waveforms to be generated. Preferably, the maximum voltage delivered by electrosurgical generator to the electrosurgical cables is in the range 250V to 480V. This can reduce the chances of sparking, as described herein. Figure 2a depicts the instrument 200 when the first and second end effector elements are in a closed configuration. When the end effector elements are in a closed configuration, the internal faces of each end effector element directly oppose each other. In this example, in the closed configuration there is a gap between the first and second end effector elements. Generally, having a gap between the first and second end effector elements when in a closed configuration provides space for tissue to be held between the jaws when the instrument is in use, e.g. when performing a sealing operation. The width of the gap may vary depending on the tissue being operated on. In some examples, there may be little or no gap between the first and second end effector elements when they are in a closed configuration. For example, the internal faces of the first and second end effector elements may abut (e.g., be in contact with) one another when in a closed configuration. Thus, the arrangement of the first and second end effector elements in the closed configuration may be variable. Not all the electrodes of each end effector element are visible in figures 2a and 2b. It will be appreciated there may be other gaps between electrodes that are not visible from the side view shown in figure 2a. The details of the arrangements of the electrodes within each end effector element will be described later. Figure 2b depicts the instrument 200 of figure 2a when the jaws of the end effector are in an open configuration. When the first and second end effector elements are in an open configuration, an opening angle a is formed between the first and second end effector elements. The opening angle a of the end effector is the angle between the longitudinal axis of the first end effector element and the longitudinal axis of the second end effector element. In an example, the first and second end effector elements rotate relative to each other about a common joint. In this example, the longitudinal axis of the first end effector element and the longitudinal axis of the second end effector element intersect at the common joint. In the example where one end effector element is fixed with respect to the shaft and one end effector element is moveable with respect to the fixed end effector element about a joint, the longitudinal axis of the fixed end effector element may intersect the longitudinal axis of the moveable end effector element at the joint. The longitudinal axis of the first end effector element is an axis parallel to the length of the first end effector element. The longitudinal axis of the second end effector element is an axis parallel to the length of the second end effector element. In an example, when the first and second end effector elements are in a closed configuration (e.g. as shown in figure 2a), the longitudinal axes of the first and second end effector elements are parallel with respect to each other. In the example shown in figure 2a, when in a closed configuration, the longitudinal axes of the first and second end effector elements are parallel with the longitudinal axis of the shaft 202. The longitudinal axis of the shaft (e.g., axis C in figure 2b) is an axis parallel to the length of the shaft. The longitudinal axis of the shaft extends through the middle of the shaft from the base 201 to the end effector 210. A typical bipolar electrosurgical instrument comprises an electrode designed for performing a cutting action (sometimes referred to as a cut electrode), and at least one other electrode designed to act as the return electrode. Electricity is passed between the cut electrode and the return electrode through the tissue to be operated on to produce the desired effect of cutting. Often the blood vessels surrounding the cutting site need to be sealed to prevent blood flow from interfering with the cutting operation. Some bipolar electrosurgical instruments will therefore also include electrodes designed for sealing the vessels around the cutting site prior to the cutting operation being performed. Such electrodes are sometimes referred to as seal electrodes. In bipolar electrosurgical instruments that can perform both cutting and sealing operations, the seal electrodes are usually positioned on either side of the cut electrode so that the area sealed by the seal electrodes includes the site where the cut is to be performed. Either or both of the electrodes used for the sealing operation may also be used as the return electrode(s) in the cutting operation. The inventors have recognised that the cutting performance of a bipolar electrosurgical instrument can be improved by adding a second electrode designed for performing a cutting operation to the instrument, as will be explained in more detail below. Furthermore, by adapting the arrangement and design of the two electrodes designed for performing a cutting operation, the risk of sparking can be reduced. Figure 3 shows a cross-section of an end effector in an example electrosurgical instrument comprising two electrodes designed for performing a cutting operation. The shaft of the instrument is not shown. The end effector comprises a first end effector element 311 and a second end effector element 312. The first end effector element 311 is depicted as the lower jaw of the end effector in this example. The second end effector element 312 is depicted as the upper jaw of the end effector in this example. The first end effector element 311 comprises a first middle electrode 315 and a first pair of lateral electrodes 316, 317. The lateral electrodes 316, 317 forming the first pair of lateral electrodes are positioned on the same jaw of the end effector as the first middle electrode. The second end effector element 312 comprises a second middle electrode 318 and a second pair of lateral electrodes 319, 320. The lateral electrodes 319, 320 forming the second pair of lateral electrodes are positioned on the same jaw of the end effector as the second middle electrode. The lateral electrodes in each of the first and second pairs of lateral electrodes may be electrically exposed parts of the same conductive component. For example, as shown in figure 7, lateral electrode 319 and lateral electrode 320 in the second pair of lateral electrodes are formed from one conductive component. The one conductive component that forms both lateral electrodes may have a U-shaped cross-section. The tops of the U-shape are the two exposed ends that form each lateral electrode. Lateral electrodes 316 and 317 of the first pair of lateral electrodes of the first end effector element 311 are also shown as being formed from the same conductive component in figure 7. Alternatively, each lateral electrode in the first and / or second pair of lateral electrodes may be formed from a separate conductive component, which may or may not be electrically connected together (for example, if each pair is activated by one electrosurgical cable). Figure 7 depicts an exploded view of the instrument 200, showing its possible construction. In this example, each end effector element comprises a casing, 740, in which the respective electrodes (315 - 320) are housed. Each end effector element also comprises insulation 701 surrounding the electrodes. Each end effector element 211, 212 may be hinged together at the articulation (not shown) to form the jaws of the end effector 210. The details of the electrode and insulation geometry will be discussed later. The first and second middle electrodes are positioned between the first and second pair of lateral electrodes respectively. It will be understood that the term "middle" does not require the first and second middle electrodes to be positioned at the mid-point between the respective first and second pair of lateral electrodes, although they may be positioned at the mid-point. The first and second middle electrodes are "middle" in the sense that there is a lateral electrode on each side of the middle electrode. In other words, the first and second pair of lateral electrodes are positioned either side of the first and second middle electrode respectively. The first and second middle electrodes are configured for performing a cutting operation. In an example, the first and second middle electrodes are first and second cut electrodes. In other words, in an example, the first and second middle electrodes are dedicated electrodes for performing a cutting operation. In this example, the first and second middle electrodes are not used for performing a sealing operation. The first and second middle electrodes may be configured for performing a cutting operation only. The first and second pair of lateral electrodes are configured for performing a sealing operation. In an example, the first and second pair of lateral electrodes are a first and second pair of seal electrodes. In other words, in an example, the first and second pair of lateral electrodes are dedicated electrodes for performing a sealing operation. In this example, the first and second pair of lateral electrodes are not used for performing a cutting operation. The first and second pair of lateral electrodes may be configured for performing a sealing operation only. The first and second middle electrodes and each lateral electrode in the first and second pair of lateral electrodes extend longitudinally along the respective first and second end effector element. For example, in figure 2a and 2b the electrodes are depicted generally at 207 as extending along the longitudinal axes of the first and second end effector elements. As another example, figure 7 shows electrodes 315-320 extending longitudinally along the respective first and second end effector elements. Each electrode has a lateral face. The lateral faces of each electrode can be seen in the cross-sectional view of the end effector shown in figures 3 to 6. For example, the lateral face of the lateral electrode 319 in figure 3 is labelled 319a. The lateral face of the lateral electrodes can also be seen in figure 7. The lateral faces of the other electrodes have not been labelled for clarity. When the end effector is in a closed configuration, the lateral faces of the electrodes oppose each other. The lateral face of each of the first and second middle electrode has a width that is less than a width of the lateral face of each lateral electrode in the first and second pairs of lateral electrodes. The first and second middle electrodes are narrower than the lateral electrodes. When electrically active, the local electric field at the middle electrodes is more concentrated (e.g. around the edges of the middle electrodes), thus increasing the power density at the middle electrodes. This makes them more suitable for performing cutting operations than the lateral electrodes. The lateral electrodes are wider than the first and second middle electrodes. When electrically active, there is a weaker local electric field at the lateral electrodes, and thus a lower power density at the lateral electrodes. This makes them more suited for performing sealing operations, and for acting as the return electrode when performing a cutting operation. In an example, the first and second middle electrodes each have an exposed surface area that is less than an exposed surface area of at least one lateral electrode in one of the first and second pair of lateral electrodes. The exposed surface area of an electrode is the surface area of the electrode that is not covered by insulation. In other words, the exposed surface area of an electrode is the surface area of the electrode that is conductive (e.g., a conductive surface area). The electrosurgical instrument of the present disclosure can operate in two modes: a cutting mode and a sealing mode. Typically, the sealing mode is activated before the cutting mode, although this may not always be the case. When operating in a sealing mode, the electrosurgical generator generates a current waveform suitable for performing a sealing operation (e.g. a COAG waveform). The current waveform is transmitted via the electrosurgical cables to the electrodes configured for performing a sealing operation. For example, as will be explained in more detail below, the current waveform is transmitted via the electrosurgical cables to at least one lateral electrode in each of the first and second pair of lateral electrodes. When operating in the sealing mode, only the electrodes configured for performing a sealing operation receive the current waveform generated by the electrosurgical generator. Sealing of tissue can then be performed by the electrodes configured to perform the sealing operation. When operating in a cutting mode, the electrosurgical generator generates a current waveform suitable for performing a cutting operation (e.g., a CUT waveform). The current waveform is transmitted via the electrosurgical cables to the electrodes configured for performing a cutting operation. For example, as will be explained in more detail below, the current waveform is transmitted via the electrosurgical cables to the first and / or second middle electrodes. When operating in the cutting mode, only the electrodes configured for performing a cutting operation receive the current waveform generated by the electrosurgical generator. Cutting of tissue can then be performed by the electrodes configured to perform the cutting operation. Electrical connection of electrodes In figure 3, the electrical connection of each electrode is shown in a simplified electrical circuit 300. A supply of alternating current is indicated at 303. In the example circuit 300, the first and second middle electrodes 315, 318 are held at the same electric potential. Similarly, the first and second pair of lateral electrodes 316, 317 and 319, 320 are held at the same electric potential. In this example, all of the electrodes are connected in the circuit 300. When operating the instrument shown in figure 3 in a cutting mode, current flows between the first and second middle electrodes and the first and second pair of lateral electrodes. Since the current is alternating, it will be understood that the direction of current flow between the middle electrodes and the lateral electrodes will alternate. When performing a cutting operation, current does not flow between the first middle electrode and the second middle electrode. This is because the first and second middle electrodes have the same polarity. When performing a cutting operation, current does not flow between the lateral electrodes. This is because all the lateral electrodes have the same polarity. Each lateral electrode has the opposite polarity to that of the first and second middle electrodes. Therefore, current flows between the middle electrodes to at least one lateral electrode. In figure 3, both the first and second middle electrodes and both the first and second pairs of lateral electrodes are connected in the circuit 300. However, not all of the electrodes need to be connected at the same time to perform a cutting or a sealing operation. It will be appreciated that whilst each end effector element comprises at least three electrodes, there are a variety of ways in which the electrodes can be electrically activated, as will now be described. To operate the instrument in a cutting mode, at least one middle electrode and at least one lateral electrode need to be electrically active. The end effector elements are usually in a closed configuration when the instrument is operating in the cutting mode. A component is considered "electrically active" if current can flow to the component from power supply 303 to form a circuit that includes the tissue to be operated on. Therefore, to operate in a cutting mode, it is not necessary for both the first and second middle electrodes to be connected in the circuit shown in figure 3. It is further not necessary for both pairs of lateral electrodes to be electrically active. Moreover, it is not necessary for both lateral electrodes in one of the first or second pairs of lateral electrodes to be electrically active. To operate the instrument in a cutting mode, it is only necessary for one of the first or second middle electrodes to be connected to a higher potential than at least one lateral electrode in one of the first and second pair of lateral electrodes such that current flows between the first or second middle electrode and that at least one lateral electrode. In the cutting mode, the at least one lateral electrode acts as a return electrode for receiving the current that has passed through the tissue. Any one or more of the lateral electrodes may act as said return electrode. For example, the instrument shown in figure 3 may operate in a cutting mode in which only one of the middle electrodes 315 or 318 and only one of the lateral electrodes 316, 317, 319 or 320 is connected to power supply 303. The cutting performance of the instrument may be improved if both middle electrodes are electrically active when performing a cutting operation. Suitably, when the instrument is operating in a cutting mode, the first and the second middle electrodes are connected to a higher potential than at least one lateral electrode in one of the first and second pair of lateral electrodes such that current flows between the second middle electrode and that at least one lateral electrode, and the first middle electrode and that at least one lateral electrode. By having both the first and second middle electrodes connected to a higher potential than at least one lateral electrode in the first or second pair of lateral electrodes, an improvement in cutting performance may be seen. This is because the tissue being cut is in electrical contact with a middle electrode on both sides of the tissue, as opposed to only one. This increased electrical contact on the tissue improves the transfer of energy to the tissue. This may result in faster vaporisation of the water in the cells within the tissue. For example, the heating of the tissue from both sides can produce two vaporisation envelopes on either side of the tissue which can join together to form a larger vaporisation envelope with a higher energy density. Thus, vaporisation can occur more rapidly and with a higher energy, thus improving the cutting effect on the tissue. Furthermore, when both middle electrodes are electrically active, a more homogeneous cutting result in the upper and lower surfaces of the tissue may be achieved, compared with when only one middle electrode is electrically active. Water molecules are often unevenly distributed across the tissue being cut, particularly following a sealing operation. By using both electrically active middle electrodes on either side of the tissue, there is a greater chance of the current finding a "wet" path of lower impedance. The increased electrical contact on the tissue may also improve the cutting performance of the instrument via the desiccation and tear method because the tissue may dry out more quickly. As mentioned above, not all the lateral electrodes need to be connected in the circuit when the instrument is operating in a cutting mode. For example, figure 4b shows the instrument operating in a cutting mode in which the first and second middle electrodes 315, 318 are connected to the AC power supply 303 at the same potential and only the first pair of lateral electrodes 316, 317 are connected to the opposite potential. Thus, in the example shown in figure 4b, current flows between the middle electrodes 315, 318 and the lateral electrodes 316, 317 of the first pair only. As mentioned above, the circuit shown in figure 4b would still work if only one of the lateral electrodes 316, 317 in the first pair was connected. Suitably, when operating in the cutting mode, the first middle electrode may be connected to a higher potential than both lateral electrodes in one of the first and second pair of lateral electrodes. For example, the first middle electrode 315 in figure 4b is connected to a higher potential than both lateral electrodes 316, 317 in the first pair of lateral electrodes. Alternatively, the first middle electrode 315 could be connected to a higher potential than both lateral electrodes 319, 320 in the second pair of lateral electrodes. Preferably, the first middle electrode is connected to a higher potential than both lateral electrodes in the second pair of lateral electrodes so that current is encouraged to flow across the tissue from one side of the end effector to the other. Suitably, when operating in the cutting mode, the second middle electrode may be connected to a higher potential than both lateral electrodes in one of the first and second pair of lateral electrodes. For example, instead of the first middle electrode 315 in figure 4b, the second middle electrode 318 may be connected to a higher potential than both lateral electrodes 316, 317 in the first pair of lateral electrodes. Alternatively, the second middle electrode 318 could be connected to a higher potential than both lateral electrodes 319, 320 in the second pair of lateral electrodes. Preferably, the second middle electrode is connected to a higher potential than both lateral electrodes in the first pair of lateral electrodes so that current is encouraged to flow across the tissue from one side of the end effector to the other. In an example, each of the first and second middle electrodes are connected to a higher potential than both lateral electrodes in the pair of lateral electrodes of the end effector element comprising that middle electrode such that, when operating in a cutting mode, current flows across a gap between the first end effector element and the second end effector element. The electrical activation of the electrodes may vary depending on the tissue being cut. For example, for tissue that is relatively soft and / or hydrated, it may be sufficient to electrically activate only one middle electrode and one or two lateral electrodes to make an acceptable cut in the tissue. For tissue that is harder to cut, both middle electrodes may be activated. In cases where not all the lateral electrodes are electrically active during a cutting operation, it may be beneficial to electrically activate opposing lateral electrodes, one from each end effector element, so that the flow of current is concentrated on one side of the tissue. This may produce a cleaner cut than if current is encouraged to flow from a middle electrode to a lateral on either side of the middle electrode. Suitably, when operating in the cutting mode, each of the first and second middle electrodes are connected to a higher potential than both the first and second pair of lateral electrodes, such that current flows between the first middle electrode and at least one of the first and second pairs of lateral electrodes, and current flows between the second middle electrode and at least one of the first and second pairs of lateral electrodes. For example, this may be the case in figure 3 where both the first and second middle electrodes are electrically active, as are both pairs of lateral electrodes. In all of the above examples, when operating in a cutting mode, current is encouraged to flow between at least one middle electrode and at leastone lateral electrode, ratherthan between the middle electrodes. It has been determined that the following metrics affect the cutting performance and risk of sparking in the instrument: • Mean direct electric field - this is the average electric field between an electrode pair. The lower the mean (e.g. average) electric field between an electrode pair, the lower the chances of sparking. This metric depends on the distance between the middle electrodes and the lateral electrodes. If the electric field between two electrodes approaches 3 kV / mm, dielectric breakdown of air is expected and sparks may appear. An example safe value for the mean direct electric field is 0.7 kV / mm. • Maximum local electric field - this is the highest local maxima in an electric field at an electrode. A high local electric field value results in high power density at an electrode, and thus good cutting performance. An example value of a high local electric field is 1.8KV / mm. By encouraging current to flow between the middle electrodes and the lateral electrodes, as opposed to between a middle electrode and the opposing middle electrode, sparking is less likely to occur because there is a greater distance between the electrodes across which current flows, resulting in a lower mean direct electric field between electrodes. In addition, the lateral direction of current flow from a middle electrode to a lateral electrode means that the electric field produced by the middle electrode is concentrated at its edge or edges (depending on which lateral electrodes are electrically active), as opposed to over the whole of its exposed face. This means that there is a high local electric field at the edge or edges of the middle electrode, resulting in a high power density at that point and thus improved cutting performance. Thus, by encouraging current to flow between the middle electrodes and the lateral electrodes, as opposed to between a middle electrode and the opposing middle electrode, the voltage used in the cutting operation can be increased, thus improving the cutting performance of the instrument, without increasing the chances of sparking. Furthermore, since either of the lateral electrodes in each pair of lateral electrodes can act as the return electrode during a cutting operation, there is greater design freedom within the instrument to direct current flow from a middle electrode to a particular lateral electrode than there is if current flows between opposing middle electrodes. This can be exploited to increase cutting efficacy whilst reducing sparking. The exact path taken by the current will depend on the configuration of the end effector, as will be explained in more detail later. In the above examples where the first and second middle electrodes are both electrically active in a cutting operation, they are held at a common potential. However, the first and second middle electrodes need not be connected to the same potential. For example, one middle electrode may be held at 50 volts, whereas the other middle electrode may be held at 80 volts. Importantly, however, when operating in a cutting mode and if using both middle electrodes, both the middle electrodes have the same polarity and are each connected to a higher potential than at least one lateral electrode. This is to ensure that current flows from a middle electrode to a lateral electrode, and not from a middle electrode to a middle electrode. Preferably, the first and second middle electrodes are connected to a common potential to reduce the complexity of the wiring within the electrosurgical instrument. This is important in a robotic electrosurgical instrument in which there is limited space within the instrument to route cables. In particular, the number of relays required to individually activate the electrodes should be kept to a minimum. Preferably, both lateral electrodes within a lateral electrode pair are connected to the same potential to reduce the number of relays required in the instrument to individually activate each lateral electrode. Similarly, the first and second pairs of lateral electrodes may be connected to the same potential. For example, there may be three relays in the circuit 300 shown in figure 3; one to electrically activate the first pair of lateral electrodes, a second to electrically activate the second pair of lateral electrodes, and a third to electrically activate the first and second middle electrodes. The instrument may operate in a sealing mode. Figure 4a shows an example circuit for when the instrument is operating in the sealing mode. As shown in figure 4a, the first and second middle electrodes are not electrically active in the sealing mode. When operating in the sealing mode, one pair of lateral electrodes is connected to a higher potential than the other pair of lateral electrodes such that current flows between the first pair of lateral electrodes and the second pair of lateral electrodes. Either the first or the second pair of lateral electrodes may be connected to the higher potential. Physical arrangement of the electrodes Due to the addition of a second middle electrode, there is increased design freedom for the arrangement of the electrodes within the instrument. The inventors have recognised that the two middle electrodes can be used to create local stress in the tissue at the cut site, thereby improving the cutting performance of the instrument through the desiccate and tear mechanism. The flow of current between the middle electrodes and the lateral electrodes can also be controlled by varying the physical configuration of the electrodes within the instrument. This can be adapted to reduce the chances of sparking occurring within the instrument. The first and second middle electrodes may be reflectionally asymmetric with respect to each other about a first plane separating the first end effector element from the second end effector element. For example, the first and second middle electrodes 315, 318 shown in figure 5 are reflectionally asymmetric with respect to the plane A indicated with a dot-dash line. The first plane may be any plane that separates the first end effector element from the second end effector element. The first plane therefore includes plane C that bisects the opening angle between the first and second end effector elements, as shown in figure 2b. As another example, the first and second middle electrodes 315, 318 shown in figure 6 are reflectionally asymmetric with respect to the plane A indicated with a dot-dash line. The tissue being operated on usually lies in the first plane. In other words, the first and second end effector elements typically clamp tissue in the first plane. In another example, the first and second middle electrodes may be arranged such that, when the first and second end effector elements are in a closed configuration, a point on at least one of the first and second middle electrodes respectively is closer to at least one lateral electrode of the end effector element opposing that at least one middle electrode than the pair of lateral electrodes of the end effector element comprising that at least one middle electrode. For example, in figure 5 the "X" marks a point on the second middle electrode 318 which is closer to the lateral electrodes 316 and 317 of the first end effector element 311 than either lateral electrode 319, 320 of the second pair of lateral electrodes on the second end effector element 312. As another example, in figure 6 the "X" marks a point on the first middle electrode 315 that is closer to the lateral electrode 319 of the second end effector element 312 opposing the first end effector element 311 than the lateral electrodes 316, 317 of the first end effector element 311. By arranging at least one of the first or second middle electrodes in this way, current is encouraged to travel between the at least one first or second middle electrode and the closest lateral electrode of the opposing end effector element through the tissue being operated on, as therefore improving the effectiveness of the instrument. Furthermore, the point on the at least one of the first and second middle electrodes that is closer to the at least one lateral electrode of the end effector element opposing that middle electrode can apply tension to the tissue when the tissue is clamped between the first and second end effector elements along the first plane. Overdose arrangement The first and second middle electrodes may be arranged such that when the first and second end effector elements are in a closed configuration, a point on one of the first or second middle electrodes is closer to both lateral electrodes of the end effector element opposing that one middle electrode than the pair of lateral electrodes of the end effector element comprising that one middle electrode. This can reduce the chances of sparking because there is a greater distance between the middle electrode and the pair of lateral electrodes of the same end effector element, so current is less likely to jump across the air gap between them. For example, in figure 5 the point 'X' on the second middle electrode 318 is closer to both lateral electrodes of the first pair of lateral electrodes 316, 317, than the second pair of lateral electrodes 319, 320. The shortest path for current to flow between the second middle electrode in this case is through the tissue to the lateral electrodes 316, 317. One of the middle electrodes may be shaped and sized such that when the first and second end effector elements are in a closed configuration, a fourth plane intersects that one middle electrode and said both lateral electrodes of the end effector element opposing that one middle electrode. For example, one of the middle electrodes may extend from its respective end effector element to intersect with a plane formed by the lateral faces of the lateral electrodes of the opposing end effector element. In figure 5, the plane formed by the lateral faces of the lateral electrodes of the first end effector element is labelled O. As shown in figure 5, the second middle electrode 318 extends from the second end effector element 312 to intersect the plane O. Plane O is an example of a fourth plane. Not all of the possible fourth planes have been depicted in figure 5. Thus, one of the middle electrodes is shaped and sized so as to impinge on the volume defined by and enclosed by the lateral electrodes of the end effector element opposing the one middle electrode, when the end effector elements are in a closed configuration. The extension of a middle electrode over the plane formed by the lateral faces of the opposing end effector element is termed "over-close". An effect of the over-close of one of the middle electrodes is that the tissue being cut is stretched around the extending middle electrode when the end effector elements are in a closed configuration. This stretching of the tissue can encourage the tissue to tear along the length of the middle electrode, thus improving the cutting performance of the instrument. The middle electrode having over-close may be sprung to the end effector element to which it belongs. For example, the middle electrode with over-close may be attached to a compliant material in the respective end effector element. The compliance of that material may be adjusted so that the middle electrode with overdose does not apply excessive tension to the tissue during a normal grasping operation in which the tissue is not intended to be cut. In an example in which the middle electrode with over-close is sprung, as the end effector elements close together, the tension applied by that middle electrode to the tissue between the end effector elements may reach a maximum value before the end effectors reach the closed configuration. This may prevent the tissue from being unintentionally cut whilst the end effector elements are closing together. The maximum value may be adjusted depending on the compliance of the sprung middle electrode. If the middle electrode with over-close is not sprung, as the end effectors close together the tension applied by that middle electrode to the tissue may continue to increase until the end effector elements have reached the closed configuration. This may allow tissue to be cut more easily once the end effector elements are in the closed configuration. The end effector element that does not comprise the middle electrode with over-close may form a gutter shape, as shown in figure 5. In other words, one end effector element may comprise a recess around the middle electrode of that end effector element. The gutter shape or recess provides space for the tissue to bend around the middle electrode with over-close. The depth of the gutter and the protrusion of the middle electrode from the base of the gutter varies the vertical gap between the first and second middle electrodes when the first and second end effector elements are in the closed configuration. In other words, the depth of the gutter and the protrusion of the middle electrode from the base of the gutter dictates the minimum vertical gap between the first and second end effector elements. The vertical gap between the first and second middle electrodes is the gap between the first and second middle electrodes in a direction parallel to the plane bisecting each of the first and second end effector elements when the first and second end effector elements are in a closed configuration (e.g. plane B). If the gap is too large, then electrical contact may only be made at one of the middle electrodes, thus degrading the cutting performance of the instrument. On the other hand, if there is little or no gap then the tissue cannot be compressed when sealing is performed. In one example, the vertical gap lies in the range 0.01mm to 0.5mm. In another example, the vertical gap lies in the range 0.05mm to 0.3mm. In another example, the vertical gap lies in the range 0.1mm to 0.2mm. In an example, the vertical gap between the first and second middle electrodes is 0.15mm. In the example electrode configuration shown in figures 3 to 5 and described above, the first and second middle electrodes are positioned opposite each other when the first and second end effector elements are in a closed configuration. Thus, when performing a cutting operation, the same section of tissue is contacted by the first and second middle electrodes on either side of that section of tissue. Blunt scissors In some situations, it may be beneficial to laterally offset the first and second middle electrodes from one another so that they are not positioned directly opposite each other when the jaws are in a closed configuration. Figure 6 illustrates an example where the first and second middle electrodes are reflectionally asymmetric with respect to each other about a second plane bisecting each of the first and second end effector elements. The second plane is indicated in figure 5 with the dotted line B. The second plane bisects both the first and the second end effector elements. The first and second middle electrodes are also reflectionally asymmetric with respect to the first plane A. When the first and second middle electrodes are reflectionally asymmetric about the second plane, each of the first and second middle electrodes are positioned closer to one side of the end effector than the other. For example, in figure 6, the first middle electrode 315 is closer to the right-hand side of the end effector than the left-hand side. This influences the path that the current will take between the first middle electrode 315 and the lateral electrodes. The current path can be further influenced by adapting the shape and size of the first and second middle electrodes. In the case where the first and second middle electrodes are reflectionally asymmetric with respect to each other about the second plane, each middle electrode may be shaped and sized such that when the first and second end effector elements are in a closed configuration, the first plane intersects both the first and second middle electrode. In other words, there is a plane A separating the first end effector element from the second end effector element which intersects each of the first and second middle electrode. For this to be the case, the first and second middle electrodes need to be laterally off set from one another, as indicated in figure 6. Otherwise, the first and second middle electrodes might prevent the jaws from closing. The lateral offset of the first and second middle electrodes should be large enough to avoid accidentally tearing the tissue when the cutting mode has not been enabled. In an example, the lateral offset of the first middle electrode and the second middle electrode is in the range 0.1mm to 2.5mm. In another example, the lateral offset may be in the range 0.5mm to 2mm. In another example, the lateral offset may be in the range 0.5mm to 1.5mm. The lateral offset may be 1.5mm. By using the reflectionally asymmetric arrangement of the first and second middle electrodes about the second plane, the tissue may be heated at two portions along its length (where it contacts the first and second middle electrodes). The tissue may then be torn between those two portions. This technique of heating and then tearing the tissue can be beneficial when dealing with tissue that is difficult to cut through using vaporisation only. Other factors affecting the performance of the instrument Ratio of electrode widths One factor that affects the power density at the middle electrodes is the relative width of the middle electrodes compared to the lateral electrodes. The ratio between the power density at the lateral electrodes <5Piaterai and the power density at the middle electrodes <5Pmiddie can be approximated as: ^Pmiddle _ ^lateral ^Plateral ^middle Where Wiaterai is the total width of the electrically active lateral electrodes and Wmiddie is the total width of the electrically active middle electrodes. For example, if the first pair of lateral electrodes are electrically active during a cutting operation and they each have a width of 0.5mm, then Wiaterai = 1mm. Recall that whilst the instrument comprises two middle electrodes and two pairs of lateral electrodes, not all of these electrodes are required to be electrically active during a cutting operation. The above approximation assumes a constant tissue conductivity, minimal electrode protrusion, and flat cross-sections of the electrodes. Thus, if there is a large ratio between the middle electrode width and the lateral electrode width, the power density of the middle electrode can be controlled to be higher than the power density of the lateral electrodes. If the ratio of the widths is too small, an unintended cutting effect can occur at the lateral electrodes instead of the middle electrodes. Preferably, the ratio in total width of the lateral electrodes compared to the middle electrodes is greater than 1. The ratio in total width of the lateral electrodes compared to the middle electrodes may be greater than 5. As an example, the ratio of the width of the lateral face of each lateral electrode in the first and second pairs of lateral electrodes to the width of the lateral face of each of the first and second middle electrode may be in the range 5:1 to 20:1. The width of the middle electrode is variable. In an example, the width of the middle electrode is less than 1mm. In another example, the width of the middle electrodes lies in the range 0.01mm to 0.5mm. In another example, the width of the middle electrode lies in the range 0.05mm to 0.25mm. Preferably, each middle electrode has a width of 0.1mm. This width has been found to reduce the likelihood of producing two separate cuts at each edge of the middle electrode. Having a narrower middle electrode also allows for a greater gap between the middle electrodes and the lateral electrodes without increasing the size of the end effector. In an example, the width of each lateral electrode is in the range 0.5mm to 1.5mm. Preferably, each lateral electrode has a width of 1mm. Shape of the lateral face of the middle electrodes The shape of the lateral face of the middle electrodes also affects the cutting performance of the instrument. Each middle electrode may have a flat lateral face. Each middle electrode may have a tapered lateral face. A flat lateral face, for example if the electrode is square shaped in cross-section, or a tapered lateral face produces significantly higher local electric fields compared with a curved lateral face due to the smaller edge radii of the flat or tapered lateral faces. Insulation The instrument is provided with insulation 501 surrounding parts of the electrodes. The geometry of the insulation can prevent tissue from being in contact with both of the middle electrodes. For example, insulation may result in tissue being pinched which reduces the contact on the middle cut electrodes. If the middle electrodes do not protrude far enough out of the insulation, the cutting performance can be reduced. Preferably, the distance that each middle electrode protrudes from the insulation is in the range 0.08 - 0.11mm. If the protrusion distance is too small, the cutting performance of the instrument is negatively affected. Preferably, the protrusion distance is greater than or equal to 0.1mm. A robotic electrosurgical instrument is described above, in which the electrosurgical instrument is suitable for attachment to a robot arm. It is to be understood that the electrosurgical instrument described above may not attach to a robot arm. The electrosurgical T1 instrument may not be robotically controlled. For example, the electrosurgical instrument described above may be handheld. The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations 5 are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of 10 the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention. 25 09 24

Claims

1. A robotic electrosurgical instrument comprising:a shaft; andan end effector connected to a distal end of the shaft, the end effector comprising opposing first and second end effector elements,the first end effector element comprising a first middle electrode between a first pair of lateral electrodes,the second end effector element comprising a second middle electrode between a second pair of lateral electrodes,wherein, when the first and second end effector elements are in a closed configuration, a point on at least one of the first and second middle electrodes respectively is closer to at least one lateral electrode of the end effector element opposing that at least one middle electrode than the pair of lateral electrodes of the end effector element comprising that at least one middle electrode;wherein the instrument is operable in a cutting mode in which the first middle electrode is connected to a higher potential than at least one lateral electrode in one of the first and second pair of lateral electrodes such that current flows between the first middle electrode and that at least one lateral electrode.

2. The robotic electrosurgical instrument of claim 1, wherein the first and second middle electrodes are reflectionally asymmetric with respect to each other about a first plane separating the first end effector element from the second end effector element.

3. The robotic electrosurgical instrument of any preceding claim, wherein the first and second middle electrodes are reflectionally asymmetric with respect to each other about a second plane bisecting each of the first and second end effector elements.

4. The robotic electrosurgical instrument of claim 2, or claim 3 when dependent on claim 2, wherein the first and second middle electrodes are shaped and sized such that, when the25 09 24first and second end effector elements are in a closed configuration, the first plane intersects both the first and second middle electrodes.

5. The robotic electrosurgical instrument of claim 1 or 2, wherein, when the first and second end effector elements are in a closed configuration, a point on one of the first or second middle electrodes is closer to both lateral electrodes of the end effector element opposing that one middle electrode than the pair of lateral electrodes of the end effector element comprising that one middle electrode.

6. The robotic electrosurgical instrument of claim 5, wherein, when the first and second end effector elements are in a closed configuration, that one middle electrode is shaped and sized such that a fourth plane intersects that one middle electrode and said both lateral electrodes of the end effector element opposing that one middle electrode.

7. The robotic electrosurgical instrument of any preceding claim, wherein the first middle electrode and the first pair of lateral electrodes and the second middle electrode and the second pair of lateral electrodes extend longitudinally along the first and second end effector elements respectively, wherein each of the first and second middle electrodes comprise a lateral face having a width that is less than a width of a lateral face of each lateral electrode in the first and second pairs of lateral electrodes.

8. The robotic electrosurgical instrument of any preceding claim, wherein in the cutting mode, the second middle electrode is connected to a higher potential than at least one lateral electrode in one of the first and second pair of lateral electrodes such that current flows between the second middle electrode and that at least one lateral electrode.

9. The robotic electrosurgical instrument of any preceding claim, wherein in the cutting mode, the first middle electrode is connected to a higher potential than both lateral electrodes in one of the first and second pair of lateral electrodes such that current flows between the first middle electrode and that pair of lateral electrodes.

10. The robotic electrosurgical instrument of any preceding claim, wherein in the cutting mode, the second middle electrode is connected to a higher potential than both lateral25 09 24electrodes in one of the first and second pair of lateral electrodes such that current flows between the second middle electrode and that pair of lateral electrodes.

11. The robotic electrosurgical instrument of any preceding claim, wherein in the cutting mode, each of the first and second middle electrodes are connected to a higher potential than both the first and second pair of lateral electrodes, such that current flows between the first middle electrode and at least one of the first and second pairs of lateral electrodes, and current flows between the second middle electrode and at least one of the first and second pairs of lateral electrodes.

12. The robotic electrosurgical instrument of any preceding claim, wherein in the cutting mode, the first and second middle electrodes have the same polarity.

13. The robotic electrosurgical instrument of any preceding claim, wherein in the cutting mode, the first pair of lateral electrodes is connected to a different potential than the second pair of lateral electrodes.

14. The robotic electrosurgical instrument of any preceding claim, wherein the instrument is operable in a sealing mode in which the first pair of lateral electrodes is connected to a higher potential than the second pair of lateral electrodes such that current flows between the first pair of lateral electrodes and the second pair of lateral electrodes.

15. The robotic electrosurgical instrument as claimed in any preceding claim, wherein the first and second middle electrodes are cut electrodes and the lateral electrodes in the first and second pair of lateral electrodes are seal electrodes.

16. The robotic electrosurgical instrument of claim 7, wherein the ratio of the width of the lateral face of each of the first and second middle electrode to the width of the lateral face of each lateral electrode in the first and second pairs of lateral electrodes is in the range 5:1 to 20:1.

17. The robotic electrosurgical instrument of any preceding claim, wherein for each of the first and second pair of lateral electrodes, the lateral electrodes in that pair form a U-shape.

18. The robotic electrosurgical instrument of claim 2, wherein the first plane is a plane bisecting the opening angle between the first and second end effector elements.

19. The robotic electrosurgical instrument of any preceding claim, wherein, when the first and second end effector elements are in a closed configuration, the first middle electrode is spaced from the second middle electrode by 0.01mm to 0.5mm in a direction parallel to a plane bisecting each of the first and second end effector elements.

20. The robotic electrosurgical instrument of any preceding claim, wherein, when the first and second end effector elements are in a closed configuration, the first middle electrode is offset from the second middle electrode by 0.5mm to 1.5mm in a direction perpendicular to a plane bisecting each of the first and second end effector elements.

21. The robotic electrosurgical instrument of any preceding claim, wherein the end effector is connected to the distal end of the shaft via an articulation, the articulation permitting the end effector to move relative to the shaft with at least two degrees of freedom.25 09 24

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