Field of the Invention

The present invention relates to a coaxial cable and particularly, although not exclusively, to a coaxial cable which is usable to reduce trajectory error in use with surgical instruments.

Background

An endoscope is a medical device with an elongate shaft which can be inserted into a body for imaging and treatment. In many endoscopes, the elongate shaft is flexible and has a lumen or channel therethrough (a working channel or instrument channel) through which a surgical instrument may be delivered to a treatment site deep inside the body. As the elongate shaft is flexible, the surgical instrument also needs to be flexible.

The elongate shaft may be steerable, and in particular a distal tip of the shaft may be controllable to ensure that the distal end of the working channel is suitably loctaleated at or near a treatment site. When the treatment site is located, the surgical instrument is advanced through the working channel until the instrument protrudes from the distal end.

The inventors have realised that many instruments which are used in this way exhibit plastic behaviour. That is, when the instrument protrudes from the distal end of the working channel, the instrument maintains the shape of the working channel at its distal end. For example, if the working channel is curved in order to reach the treatment site, the instrument will also be curved as it protrudes from the distal end. This results in what is known as ‘trajectory error’, where the instrument does not follow the path which is required by the physician in order to reach the treatment site but instead continues following the curved path of the distal end of the working channel. To minimise the impact of trajectory error, the physician must readjust the position and angle of the working channel, which may not always be possible in the constraints of the region around a treatment site.

The present invention has been devised in light of the above considerations.

Summary of the Invention

According to a first aspect of the present invention, there is provided a coaxial cable for conveying radiofrequency (RF) and/or microwave frequency energy, the coaxial cable comprising: an inner conductor; an outer conductor; and a dielectric material separating the inner conductor and the outer conductor; wherein at least a portion of the inner conductor comprises a resiliently deformable material. ‘Resiliently deformable’ here means that the elasticity, that is, the limit of elastic recoverable deformation (or, simply, elastic deformation), for the portion of the inner conductor is higher than that of materials used in conventional coaxial cables. In particular, the resilient deformable material is more elastic than copper. That is, at least a portion of the inner conductor comprises a resilient deformable material with a higher flexural strength, or yield point, than conventional inner conductor materials (in particular, copper), such that the inner conductor can undergo larger deformations before exhibiting plastic behaviour (i.e, retaining the deformed shape). As a result, the inner conductor returns to its original un-stressed shape (e.g., straight) after being subjected to large stresses and strains. A coaxial cable according to the present invention is particular advantageous for electrosurgery, as it can be passed through a working channel and return to its original un-stressed shape (e.g., straight) as it emerges from the distal end of the working channel, thereby reducing or eliminating trajectory error and aiding a user in performing electrosurgery by helping to ensure that an instrument can be accurately delivered to a treatment site.

Optionally, the resiliently deformable material may be configured such that a portion of the coaxial cable comprising the resiliently deformable material may be deformable to a bend radius of 5 to 20mm (e.g. about 15 mm), and may be resilient so as to return to within +/-20 degrees (e.g. within +/-10 degrees) of linear over a length (i.e., a length of the coaxial cable) of 10 to 55 mm (e.g. about 40mm) when the deforming force is removed. This ensures that the coaxial cable can be used to accurately reach target tissue during surgery. For example, the coaxial cable may be passed through a working channel section having a bend radius of 10 mm and a 200° bend angle (i.e., a U-shaped bend with a radius of curvature or bend radius of 10 mm), and the coaxial cable will have a trajectory error of less than 5° (i.e., a deviation from the longitudinal axis of the working channel of less than 5°) at a distance of 55 mm from an end face of the working channel. An example of such a working channel arrangement is described below with respect to Figs. 3, 7 and 8. A coaxial cable which is resiliently deformable or elastic in this range has been found to provide an optimal trade-off between elasticity, stiffness, electrical/ablation performance and accuracy (that is, accuracy of reaching target tissue) and can, for example, reach a 2cm area of target tissue at the 30 mm distance.

Optionally, the resiliently deformable material may be configured such that a portion of the coaxial cable comprising the resiliently deformable material may have a deflection force of 4N over a 30mm deflection distance as measured by a three-point bend test. For example, the three-point bend test may be as described below with respect to Fig. 4, and/or methods as described in the ISO 178:2019 or ASTM E111- 17 standards on material testing.

In particular, the portion of the coaxial cable comprising the resiliently deformable material may have a stiffness which is less than the stiffness of a coaxial cable with an outer diameter of 1 .9 mm. The coaxial cable with such a stiffness is able to be delivered to a desired treatment site easily, and the stiffness of the coaxial cable does not negatively affect or impact the catheter or working channel through which the coaxial cable is delivered. For example, a stiffer coaxial cable may displace the catheter as it is passed therethrough, meaning that it may be more difficult to reach the desired treatment site.

In some examples, the resiliently deformable material may comprise a section of the inner conductor which is provided as a braided metal structure to provide increased elasticity.

Preferably, the portion of the inner conductor comprising a resiliently deformable material may be able to be deformed to a minimum bend radius of less than 20 mm, for example around 10 mm, such as 5 mm or more, over an angle of up to 200 degrees, and return to its original shape after deformation. In some embodiments, the entire length of the inner conductor may comprise the resiliently deformable material, which may make manufacturing a coaxial cable according to the present invention relatively simple.

Optionally, the inner conductor may comprise a first portion having a first elasticity and a second portion having a second elasticity, wherein the second elasticity is higher than the first elasticity, and wherein the second portion comprises the resiliently deformably material. For example, the first portion may be a proximal portion of the inner conductor, and the second portion may be a distalmost portion of the inner conductor. For example, the distalmost section of the inner conductor may comprise the resiliently deformable material, that is, the distal section which is extended from a distal end of a working channel when used in electrosurgery to ensure that trajectory error is eliminated for the portion of the coaxial cable for which it is most important; and the proximal portion of the inner conductor may be made of a conventional material such as copper or silver. For example, the distal section of the inner conductor may be connected to a proximal section by welding, or via a collar which is crimped over the two parts of the inner conductor to fix them in relative position. This arrangement may make the coaxial cable cheaper to manufacture by reducing the amount of resiliently deformable material which is required. For example, the second portion, the distalmost section, of the inner conductor may have a length of 100 mm or less, such as in a range of 50-60 mm, for example. For example, the distal portion of the coaxial cable may be deformable to a bend radius of 10 mm, and is able to return to within 20 degrees of linear over a length (i.e., a length of the coaxial cable) of 55 mm when the deforming force is removed, in the manner described herein, and/or may have a deflection force of less than 4N over a 30 mm deflection distance (or a stiffness which is less than the stiffness of a coaxial cable with an outer diameter of 1 .9 mm) as measured by a three-point bend test, in the manner described herein.

Optionally, the resiliently deformable material may comprise a shape memory metal (also known as shape memory alloy, or SMA), for example nitinol. Shape memory materials are particularly suitable for this use as they exhibit superelasticity (or pseudoeleasticity), which is a result of transformation between martensite and austenite phases of the crystal lattice. This property therefore provides a large degree of resilient deformability (that is, a high elastic deformation limit).

Alternatively, the resilient deformable material may comprise a metal having a high yield strength (e.g., higher than conventional materials, such as copper), for example spring steel or the like.

In some examples, the inner conductor may comprise a core made from the resiliently deformable material, and an outer conductive layer. For example, the resiliently deformable material may comprise a conductive coating; that is, the outer conductive layer may be a conductive coating directly applied to the core of resiliently deformable material. In particular, the outer conductive layer or coating may have a higher conductivity than the resiliently deformable material to ensure that the inner conductor can efficiently convey RF and/or microwave energy. For example, the outer conductive layer or coating may be copper, silver, or gold. Optionally, the outer conductive layer or coating may have a thickness of at least 10 micrometres, for example up to 0.05 mm, or between 3 micrometres and 20 micrometres to ensure that energy (e.g. microwave energy) is efficiently conveyed along the coaxial cable via the skin effect.

Optionally, the diameter of the coaxial cable may be 5 mm or less, for example 2 mm or less. By being provided with a diameter in this range, it may be ensured that the comparatively low limit of elastic recoverable deformation of the other materials in the coaxial cable (e.g., the outer conductor and dielectric material) do not unduly limit the elastic limit of the coaxial cable as a whole. That is, by being provided with a diameter in this range, the resilient deformability of the inner conductor ensures that the coaxial cable as a whole is resiliently deformable.

Optionally, at least a portion of the outer conductor may also comprise a resiliently deformable material (e.g., a material with increased elasticity, for example relative to copper, as described above). For example, the outer conductor may comprise a braided metal structure made of a conventional material (such as copper or silver) which additionally comprises strips or lengths of a resiliently deformable material embedded within, or attached to the outside of the outer conductor. Alternatively, the resiliently deformably material could be added to a portion of the outer conductor in the form of an outer wrap. The outer wrap may comprise a resiliently deformable metal (e.g., nitinol or spring steel) or a rigid polymer, for example. According to a second aspect of the present invention, there is provided an electrosurgical instrument comprising a coaxial cable according to the first aspect of the present invention; and a radiating tip disposed at a distal end of the coaxial cable to receive the RF and/or microwave frequency energy, the radiating tip comprising an elongate conductor electrically connected to the inner conductor and extending in a longitudinal direction to form a radiating element. For example, in some embodiments the elongate conductor may be a distal portion of the inner conductor that extends beyond a distal end of the outer conductor. In this way, the second aspect of the present invention provides an electrosurgical instrument which can be accurately delivered to a treatment site (e.g. through the working channel of a scoping device) with improved trajectory error in directing the radiating tip, as discussed above.

According to a third aspect of the present invention there is provided an electrosurgical apparatus for treating biological tissue, the electrosurgical apparatus comprising an electrosurgical generator arranged to supply microwave energy; and an electrosurgical instrument according to the second aspect, connected to receive the microwave energy from the electrosurgical generator.

Optionally, the electrosurgical apparatus may further comprise a surgical scoping device that comprises a flexible insertion cord having an instrument channel, wherein the electrosurgical instrument is dimensioned to fit within the instrument channel.

The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

Summary of the Figures

Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Fig. 1 is a schematic diagram of an electrosurgical system that is an embodiment of the present invention;

Fig. 2 is a schematic cross-sectional side view of an electrosurgical instrument that may be used with embodiments of the present invention;

Fig. 3a is a schematic view of a conventional coaxial cable passing through a working channel;

Fig. 3b is a schematic view of a coaxial cable according to an embodiment of the present invention passing through a working channel;

Fig. 4 shows a schematic view of a coaxial cable undergoing a three-point bend test;

Fig. 5 shows a schematic cross-section side view of a coaxial cable that is an embodiment of the present invention;

Fig. 6 shows a schematic cross-section side view of a second coaxial cable that is another embodiment of the present invention; and

Figs. 7 and 8 are diagrams of a trajectory error testing apparatus.

Detailed Description of the Invention

Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

Fig. 1 is a schematic diagram of a complete electrosurgical system 100 that is capable of supplying RF and/or microwave energy to the distal end of an invasive electrosurgical instrument. The system 100 comprises a generator 102 for controllably supplying RF and/or microwave energy. A suitable generator for this purpose is described in WO 2012/076844, which is incorporated herein by reference. The generator may be arranged to monitor reflected signals received back from the instrument in order to determine an appropriate power level for delivery. For example, the generator may be arranged to calculate an impedance seen at the distal end of the instrument in order to determine an optimal delivery power level.

The generator 102 is connected to an interface joint 106 by an interface cable 104. If needed, the interface joint 106 can house an instrument control mechanism that is operable by sliding a trigger 110, e.g. to control longitudinal (back and forth) movement of one or more control wires or push rods (not shown). If there is a plurality of control wires, there may be multiple sliding triggers on the interface joint to provide full control. The function of the interface joint 106 is to combine the inputs from the generator 102 and instrument control mechanism into a single flexible shaft 112, which extends from the distal end of the interface joint 106. In other embodiments, other types of input may also be connected to the interface joint 106. For example, in some embodiments a fluid supply may be connected to the interface joint 106, so that fluid may be delivered to the instrument. The flexible shaft 112 is insertable through the entire length of an instrument (working) channel of an endoscope 114.

The flexible shaft 112 has a distal assembly 118 (not drawn to scale in Fig. 1) that is shaped to pass through the instrument channel of the endoscope 114 and protrude (e.g. inside the patient) at the distal end of the endoscope’s tube. The distal end assembly includes an active tip for delivering microwave energy into biological tissue. An example tip configuration is discussed in more detail below.

The structure of the distal assembly 118 may be arranged to have a maximum outer diameter suitable for passing through the working channel. Typically, the diameter of a working channel in a surgical scoping device such as an endoscope is less than 4.0 mm, e.g. any one of 2.0 mm, 2.8 mm, 3.2 mm, 3.7 mm, 3.8mm. The length of the flexible shaft 112 can be equal to or greater than 0.3 m, e.g. 2 m or more. In other examples, the distal assembly 118 may be mounted at the distal end of the flexible shaft 112 after the shaft has been inserted through the working channel (and before the instrument cord is introduced into the patient). Alternatively, the flexible shaft 112 can be inserted into the working channel from the distal end before making its proximal connections. In these arrangements, the distal end assembly 118 can be permitted to have dimensions greater than the working channel of the surgical scoping device 114.

The system described above is one way of introducing the instrument into a patient’s body. Other techniques are possible. For example, the instrument may also be inserted using a catheter.

Fig. 2 shows a cross-sectional side view of an electrosurgical instrument 200 that may be used with embodiments of the present invention. The distal end of the electrosurgical instrument may correspond, for example, to the distal assembly 118 discussed above. The electrosurgical instrument 200 includes a coaxial feed cable 202 that is connectable at its proximal end to a generator (such as generator 102) in order to convey microwave energy. The coaxial feed cable 202 may be the interface cable 104 discussed above, which passes through the flexible shaft 112. The coaxial feed cable 202 comprises an inner conductor 204 and an outer conductor 206 which are separated by a dielectric material 208. The coaxial feed cable 202 is preferably low loss for microwave energy. A choke (not shown) may be provided on the coaxial feed cable 204 to inhibit back propagation of microwave energy reflected from the distal end and therefore limit backward heating along the device. The coaxial feed cable 202 further includes a flexible outer sheath 210 disposed around the outer conductor 206 to protect the coaxial feed cable 204. The outer sheath 210 may be made of an insulating material to electrically isolate the outer conductor 206 from its surroundings. The outer sheath 210 may be made of, or coated with, a non-stick material such as PTFE to prevent tissue from sticking to the instrument.

A radiating tip 212 is formed at the distal end 214 of the coaxial feed cable 202. The dashed line 215 in Fig. 2 illustrates an interface between the coaxial feed cable 202 and the radiating tip 212. The radiating tip 212 is arranged to receive microwave energy conveyed by the coaxial feed cable 202, and deliver the energy into biological tissue. The outer conductor 206 of the coaxial feed cable 202 terminates at the distal end 214 of the coaxial feed cable 202, i.e. the outer conductor 206 does not extend into the radiating tip 212. The radiating tip 212 includes a distal portion 216 of the inner conductor 204 which extends beyond the distal end of the coaxial feed cable 202. In particular, the distal portion 216 of the inner conductor 204 extends beyond a distal end of the outer conductor 206.

A proximal tuning element 218 made of a conductive material (e.g. metal) is electrically connected to the distal portion 216 of the inner conductor 204 near a proximal end of the radiating tip 212. The proximal tuning element 218 has a cylindrical shape, and includes a channel 220 through which the distal portion 216 of the inner conductor 204 passes. A diameter of the channel 220 is substantially the same as an outer diameter of the inner conductor 204, such that the inner conductor 204 is in contact with the proximal tuning element 218 inside the channel 220. The proximal tuning element 218 may be further secured to the inner conductor 204, e.g. using a conductive adhesive (e.g. conductive epoxy) or by soldering or welding. The proximal tuning element 218 is centred on the inner conductor 204. In other words, a central axis of the cylindrical proximal tuning element 218 is collinear with the longitudinal axis of the inner conductor 204. In this manner, the proximal tuning element 218 is disposed around the distal portion 216 of the inner conductor 204 in a manner that is symmetrical about the longitudinal axis of the inner conductor 204.

A distal tuning element 222 made of a conductive material (e.g. metal) is electrically connected to the distal portion 216 of the inner conductor 204 near a distal end of the radiating tip 212. Thus, the distal tuning element 222 is located further along the inner conductor 204 than the proximal tuning element 218. The distal tuning element 222 is spaced apart from the proximal tuning element by a length of the distal portion 216 of the inner conductor 204. Like the proximal tuning element 218, the distal tuning element has a cylindrical shape and includes a channel 224. As can be seen in Fig. 2, the distal portion 216 of the inner conductor 204 extends into the channel 224. The distal portion 216 of the inner conductor 204 terminates at a distal end of the channel 224, i.e. it does not protrude beyond the distal tuning element 222. In this manner, a distal end of the inner conductor 204 lies flush with a distal face of the distal tuning element 222. A diameter of the channel 224 is substantially the same as the outer diameter of the inner conductor 204, such that the inner conductor 204 is in contact with the distal tuning element 222 inside the channel 224. The distal tuning element 222 may be further secured to the inner conductor 204, e.g. using a conductive adhesive (e.g. conductive epoxy) or by soldering or welding. Like the proximal tuning element 218, the distal tuning element 222 is mounted so that it is centred on the inner conductor 204.

Both the proximal tuning element 218 and the distal tuning element 222 have the same outer diameter. The outer diameter of the proximal tuning element 218 and the distal tuning element 222 may be slightly less than the outer diameter of the electrosurgical instrument 200. In the example shown, the distal tuning element 222 is longer than the proximal tuning element 218 in the longitudinal direction of the instrument. In other words, the length of inner conductor 204 in channel 224 in the distal tuning element 222 is greater than the length of inner conductor 204 in channel 220 in the proximal tuning element 218. For example, the distal tuning element 222 may be approximately twice as long as the proximal tuning element 218. By making the distal tuning element 222 longer than the proximal tuning element 218, it is possible to concentrate microwave emission around the distal end of the radiating tip 212. A distal portion 226 of the dielectric material 208 extends beyond the distal end 214 of the coaxial feed cable 202 into the radiating tip 212. The distal portion 226 of the dielectric material 208 acts as a spacer between the proximal tuning element 218 and the distal end 214 of the coaxial feed cable 202. In some embodiments (not shown), the dielectric material 208 may terminate at the distal end 214 of the coaxial feed cable 202, and a separate spacer may be provided between the distal end 214 of the coaxial feed cable 202 and the proximal tuning element 218. A dielectric spacer 228 is provided in the radiating tip 212 between the proximal tuning element 218 and the distal tuning element 222. The dielectric spacer 228 is a cylindrical piece of dielectric material, having a central channel extending therethrough. Thus, the dielectric spacer 228 may be a tube of dielectric material. The distal portion 214 of the inner conductor 204 extends through the channel in the dielectric spacer 228. A proximal face of the dielectric spacer 228 is in contact with the proximal tuning element 218, and a distal face of the dielectric spacer 228 is in contact with the distal tuning element 222. The dielectric spacer 228 has approximately the same outer diameter as the proximal and distal tuning elements 218, 222.

A protective sheath 230 is provided on the outside of the radiating tip 212. The protective sheath 230 covers the dielectric spacer 228 and the proximal and distal tuning elements 218, 222 to form an outer surface of the radiating tip 212. The protective sheath 230 may be a tube made of an insulating material. The protective sheath 230 may serve to insulate the radiating tip 212 and protect it from the environment. The protective sheath 230 may be made of or coated with a non-stick material (e.g. PTFE) to prevent tissue from sticking to it. An outer diameter of the protective sheath 230 is substantially the same as the outer diameter of the coaxial feed cable 202, so that the instrument has a smooth outer surface, i.e. the radiating tip 212 has an outer surface that is flush with an outer surface of the coaxial feed cable 202 at the interface 215. In some embodiments (not shown) the protective sheath 230 may be a continuation of the outer sheath 210 of the coaxial feed cable 202. Together, the distal portion 226 of the dielectric material 208, the dielectric spacer 228 and the protective sheath 230 form a dielectric body of the radiating tip 212.

The radiating tip 212 further includes a distal tip 232 located at its distal end. The distal tip 232 may be pointed in order to facilitate insertion of the radiating tip 212 into target tissue. However, in other embodiments (not shown), the distal tip may be rounded or flat. The distal tip 232 may be made of a dielectric material, e.g. the same as dielectric material 208. In some embodiments, the material of the distal tip 232 may be selected to improve impedance matching with target tissue, in order to improve the efficiency with which the EM energy is delivered to the target tissue. The distal tip 232 may be made of, or covered with a non-stick material (e.g. PTFE) to prevent tissue from sticking to it.

The following are example dimensions of electrosurgical instrument 200:

- distance from the interface 215 to the distal end of the distal portion 216 of the inner conductor 204: 5.75 mm;

- outer diameter of proximal tuning element 218 and distal tuning element 222: 1 .5 mm;

- length of proximal tuning element 218: 0.5 mm; - length of distal tuning element 222: 1 .0 mm;

- spacing between proximal tuning element 218 and distal tuning element 222: 3.75 mm;

- spacing between the proximal tuning element 218 and the interface 215: 0.5 mm; and

- outer diameter of electrosurgical instrument 200: 1 .85 mm.

The radiating tip 212 may act as a microwave monopole antenna when microwave energy is conveyed to the radiating tip 212. In particular, microwave energy may be radiated from the distal portion 216 of the inner conductor 202, so that microwave energy can be delivered into surrounding biological tissue. The proximal and distal tuning elements 218, 222 act to shape the radiation profile of the radiating tip 212, and improve impedance matching between the instrument and surrounding target tissue.

Fig. 3a shows a schematic view of a conventional coaxial cable 310 passing through a working channel section 300. Arrow 302 indicates the direction in which the coaxial cable 310 is inserted through the working channel section 300 (i.e. generally from left to right as view in Fig. 3a). It will be appreciated that the working channel section 300 depicted in Fig. 3a is only a short section to demonstrate the principles of the present invention, and in electrosurgical systems (e.g. as shown in Fig. 1) the working channel may be much longer (i.e. the length of a scoping device, for example up to around 1500 mm in length).

The working channel section 300 is generally U-shaped (such that the bending angle is 180 degrees), and it can be seen that as the coaxial cable 310 is passed through the channel in the direction indicated by arrow 302, a distal portion 315 of the coaxial cable 310 has adopted the shape of the working channel, such that it emerges from the distal end of the working channel section 300 with a curved shape. Dashed line 305 is drawn perpendicular to the end face of the working channel section 300, and it can be seen that the distal portion 315 of the coaxial cable 310 deviates significantly from this axis. This is known as the trajectory error, or shape memory error. This effect arises due to the plasticity of the coaxial cable 310, meaning that it retains the shape of the working channel section 300 and does not return to its original, unstressed shape. If the coaxial cable 310 was to be used for electrosurgery (e.g. with a probe tip mounted at the distal end) and delivered to the treatment area through a scoping device (e.g. as shown above in Fig. 1), then this trajectory error could result in the surgeon missing the intended tissue treatment site. In order to avoid this, the working channel needs to be repositioned, or the surgeon needs to anticipate the likely trajectory error prior to insertion of the coaxial cable 310. Either case adds complication and risk to surgery.

Fig. 3b shows a schematic view of a coaxial cable 320 according to an embodiment of the present invention passing through a work channel section 300. The working channel section 300 is configured in generally the same manner as discussed above with respect to Fig. 3a, and again it will be appreciated that the working channel section 300 is only an example, and in electrosurgical systems the working channel may be much longer (i.e. the length of a scoping device, for example up to around 1500 mm in length). The radius of curvature or bend radius of the working channel section 300 may be greater 5 mm, for example 20 mm or less. Although the working channel section 300 is generally U-shaped (such that the bending angle is 180 degrees), it can be seen that as the coaxial cable 320 is passed through the channel in the direction indicated by arrow 302, a distal portion 325 of the coaxial cable 320 returns has returned to its original, unstressed shape (i.e. the coaxial cable 320 is straight as it emerges from the distal end of the working channel 300). Dashed line 305 is perpendicular to the end face of the working channel section 300, and it can be seen that the distal portion 325 of the coaxial cable 320 generally follows the same, straight path. Using the coaxial cable 320, which is an embodiment of the present invention, has therefore substantially reduced the trajectory error when compared with a known coaxial cable 310 as shown in Fig. 3a. This means that during electrosurgery a surgeon can be certain that when the coaxial cable 320 emerges from the distal end of a working channel, the coaxial cable 320 (e.g. with a probe tip mounted at the distal end) will be directed where the surgeon intends, making it easier to accurately reach the intended tissue treatment site.

The working channel section 300 may be used to test and characterize different coaxial cables in order to determine whether or not the coaxial cable is suitable for embodiments of the present invention. In particular, a coaxial cable may be passed through the working channel section 300, and a deviation of the coaxial cable from the line 305 perpendicular to the distal end face of the working channel section 300 (that is, the angle 306 between the distal portion 315 and the line 305) may be measured at a distance of up to 55 mm from the distal end face of the working channel section 300 (that is the distance 307 shown in Fig. 3a).

Specifically, in certain embodiments or examples, a coaxial cable may be selected which has a trajectory error of less than 5 degrees from the perpendicular line 305 when measured at a distance of 55 mm from the distal end of the working channel section 300 (shown by distance 307), where the working channel section 300 has a radius of curvature or bend radius of 10 mm. This may allow precise delivery of an electrosurgical instrument or distal assembly into target tissue (for example, allowing the device to hit a 2 cm diameter area of target tissue). A trajectory error in this range presents a good trade-off between trajectory error, stiffness, electrical/ablation performance and accuracy. An arrangement for testing the trajectory error in this way is described below with respect to Figs. 7 and 8.

Fig. 4 shows a schematic view of a three-point bend test apparatus 350, testing a coaxial cable 370 according to an embodiment of the present invention. The three-point bend test apparatus 350 may be used to assess the stiffness of a given coaxial cable. For example, the coaxial cable 370 portion may have a length of between 200 mm and 300 mm.

The three-point bend test apparatus 350 comprises two supports 351 a, 351 b, upon which the coaxial cable 370 portion is rested for testing, and a central loading edge 352. The two supports 351 a, 351 b may be spaced apart such that the coaxial cable 370 is supported at two points which are 50 mm apart, and the central loading edge 352 is positioned to contact the coaxial cable 370 at the centre point between the supports 351a, 351 b on the opposite side of the coaxial cable. The loading edge 352 is moveable towards the cable (e.g. downwards), in a direction indicated by arrow 353, in order to apply a force to the coaxial cable 370 which deflects or deforms the coaxial cable 370. The force which is required to deflect the coaxial cable 370 by a predetermined distance (e.g., 30 mm) may be used to compare the stiffness of different coaxial cable or material samples.

For example, the three-point bend test apparatus 350 may conform with ISO 178:2019 or ASTM E111-17 standards on material testing.

Specifically, a material may be particularly suitable for use in embodiments of the present invention if the stiffness of the resulting coaxial cable is lower than that of a coaxial cable with an outer diameter of 1 .9 mm which comprises 1 mm diameter SS316 stainless steel wire as an inner conductor, so as to ensure that a coaxial cable does not unduly deform or deflect a working channel as the coaxial cable passes through.

Fig. 5 shows a schematic cross-section side view of a coaxial cable 400 that is an embodiment of the present invention. The coaxial cable 400 is connectable at its proximal end to a generator (such as generator 102 shown in Fig. 1) in order to convey microwave and/or RF energy. The coaxial cable 400 may be the interface cable 104, for example, passing through the flexible shaft 112.

The coaxial cable 400 comprises an inner conductor 405 and an outer conductor 410 which are separated by a dielectric material 420. The coaxial cable 400 further includes a flexible outer sheath 430 disposed around the outer conductor 410 to protect the coaxial cable 400. The outer sheath 430 may be made of an insulating material to electrically isolate the outer conductor 410 from its surroundings. The outer sheath 430 may be made of, or coated with, a non-stick material such as PTFE to prevent tissue from sticking to the instrument. A radiating tip may be connected to or formed at the distal end of the coaxial cable 400, such that the radiating tip is arranged to receive microwave and/or RF energy conveyed by the coaxial cable 400, and deliver the energy into biological tissue.

The inner conductor 405 comprises a core section 406 made of a shape memory material such as a shape memory metal or a shape memory alloy, for example, nitinol. This ensures that the coaxial cable 400 maintains elasticity as it is passed through a working channel to reach a treatment site, such that when the cable emerges from the distal end of the working channel it tends to its original, unstressed shape (i.e. straight) and so reduces or removes the trajectory error, as discussed above with respect to Fig. 3. In particular, the core section 406 has a high yield point or elastic limit such that it does not undergo plastic deformation as it is passed through the working channel, which may have many twists and bends. Shape memory metals are particularly suitable for this purpose as they exhibit superelasticity (or pseudoeleasticity), which is a result of transformation between martensite and austenite phases of the crystal lattice. This allows the core section 406 to be subjected to large stresses and strains, while maintaining elasticity.

The core section 406 of the inner conductor 405 has a coating 407 of a material with a higher conductivity than that of the core section 406. For example, the coating 407 may be a coating of metal, which may be the same metal as the outer conductor 410, for example copper or silver. The coating 407 thereby ensures that the coaxial cable 400 is able to efficiently convey microwave and/or RF energy. The coating 407 has a suitable thickness to ensure that energy is efficiently conveyed along the coaxial cable 400 via the skin effect. For example, the coating 407 may have a thickness of at least 10 pm, such as 30 pm. Of course, in other embodiments, the conductivity of the core section 406 (e.g. of the shape memory material) may be large enough that a coating is not required, and the inner conductor 405 may comprise only of the core section 406.

Fig. 6 shows a schematic cross-section side view of a second coaxial cable 500 that is another embodiment of the present invention. In this embodiment, the coaxial cable 500 comprises a proximal section 510 and a distal section 520. The proximal section 510 is configured generally in the same manner as a conventional coaxial cable, having an inner conductor 512 and an outer conductor 514 which are spaced by a dielectric material 516. At a proximal end, the proximal section 510 is connectable to a generator (such as generator 102 shown in Fig. 1) in order to convey microwave and/or RF energy.

The distal section 520 is configured generally in the same manner as coaxial cable 400 discussed above with respect to Fig. 5. That is, the distal section 520 has an inner conductor 522 comprising a core section 524 made of a shape memory material such as a shape memory metal or a shape memory alloy; for example, nitinol. The distal section 520 also comprises an outer conductor 528 which is separated from the inner conductor 522 by a dielectric material 530.

In one example, the inner conductor 522 may have an outer diameter of 0.55 mm, including a coating 526 thickness of between 3 pm and 15 pm (e.g., by plating the core section 524, the depth of which may be acceptable due to the skin depth at frequencies of 2.45 GHz and 5.8 GHz), the dielectric material 516 may have an outer diameter of 1 .5 mm, and the outer conductor 514 may have an outer diameter of 1 . 7 mm. In some examples, the coating 526 may have a thickness of up to 0.05 mm (e.g., by cladding the core section 524).

By providing the inner conductor 522 which a core section 524 made of a shape memory material, the distal section 520 of the coaxial cable 500 maintains elasticity as the coaxial cable 500 is passed through a working channel to reach a treatment site. As a result, when the coaxial cable 500 emerges from the distal end of the working channel, the distal section 520 tends to its original, unstressed shape (i.e. straight) and so reduces or removes trajectory error, as discussed above with respect to Fig. 3. In particular, the core section 524 has a high yield point or elastic limit such that it does not undergo plastic deformation as it is passed through the working channel, which may have many twists and bends when its distal end is positioned at or near the treatment site. Shape memory metals are particularly suitable for this purpose as they exhibit superelasticity (or pseudoeleasticity), which is a result of transformation between martensite and austenite phases of the crystal lattice. This allows the core section 524 to be subjected to large stresses and strains, while maintaining elasticity. The distal section 520 may have a length which corresponds with the distance to which the coaxial cable 500 is intended to be extended beyond the distal end of the working channel. For example, the distal section 520 may have a length of 100 mm or less, such as 50 mm or less.

In this embodiment the core section 524 has a coating 526 of a material with a higher conductivity than that of the core section. For example, the coating 526 may be a coating of metal, which may be the same metal as the outer conductor 528, for example copper or silver. The coating 526 thereby ensures that the distal section 520 of the coaxial cable 500 is able to efficiently convey microwave and/or RF energy. The coating 526 has a suitable thickness to ensure that energy is efficiently conveyed along the coaxial cable 500 via the skin effect. For example, the coating 526 may have a thickness of at least 10 pm, such as 30 pm. Of course, in other embodiments, the conductivity of the core section 524 (e.g. of the shape memory material) may be large enough that a coating is not required, and the inner conductor 522 may comprise only the core section 524.

The proximal section 510 and the distal section 520 of the coaxial cable 500 may be joined together by welding, or a collar may hold the two sections together, for example. In some embodiments, the outer conductor and the dielectric material may extend from the proximal section 510 into the distal section 520 such that only the inner conductor differs between the two sections. In such embodiments, the inner conductor of the proximal section 510 and the inner conductor of the distal section 520 may be electrically connected by welding, or a collar may hold the two conductors together, for example.

A radiating tip may be connected to or formed at the distal end of the coaxial cable 500, such that the radiating tip is arranged to receive microwave and/or RF energy conveyed by the coaxial cable 500, and deliver the energy into biological tissue. By providing the coaxial cable 500 with a distal section 520 wherein the inner conductor comprises a shape memory metal in the manner discussed, an operator of the device (e.g. a surgeon) can be more certain of the trajectory of the radiating tip as it emerges from the distal end of a working channel, aiding accurate placement of the radiating tip at the intended treatment site.

The coaxial cable 500 further includes a flexible outer sheath 540 which extends over each of the proximal section 510 and distal section 520, disposed around the outer conductor to protect the coaxial cable 500. The outer sheath 540 may be made of an insulating material to electrically isolate the outer conductor 514, 528 from its surroundings. The outer sheath 540 may be made of, or coated with, a nonstick material such as PTFE to prevent tissue from sticking to the instrument.

Fig. 7 shows a diagram of a trajectory error testing apparatus 600 for testing the elasticity of a coaxial cable 650. The testing apparatus 600 is generally similar to the working channel section 300 as described above with respect to Fig. 3, with additional features allowing the elasticity of the coaxial cable 650 to be determined.

The trajectory error testing apparatus 600 generally comprises a plurality of channels 610a, 610b, 610c, 61 Od each connected with a bend segment 620. Spaced at a distance of 40 mm but could be configured to a distance of up to 55mm from the distal end of the bend segment 620 is an angle gauge 630, which allows the deviation of the coaxial cable 650 from the linear (c.f., the line 305 shown in Fig. 3) to be determined. In particular, the 0° angle of the angle gauge 630 is arranged to lie on a line which extends from the longitudinal axis of the bend segment 620, that is, along a line perpendicular to the end face of the channel (c.f., the line 305 shown in Fig. 3). In the apparatus 600, the plurality of channels 610a-610 and the bend segment have a channel diameter of 2 mm, and the coaxial cable 650 has an outer diameter of 1 .9 mm. The bend segment 620 is configured, in combination with the first channel 610a, to have a radius of curvature or bend radius of 15 mm (the distance shown by arrow 625). The plurality of channels 610a-610d are provided such that the elasticity of the coaxial cable 650 can be determined after passing through a range of different angles. Specifically, the first channel 610a, in combination with the bend segment 620, allows an angle of 180° (that is, a U-bend) to be tested; the second channel 610b, in combination with the bend segment 620, allows an angle of 150° to be tested; the third channel 610c, in combination with the bend segment 620, allows an angle of 120° to be tested; and the fourth channel 61 Od, in combination with the bend segment 620, allows an angle of 90° to be tested.

It can be seen in Fig. 7 that the coaxial cable 650 passes over the angle gauge 630 at an angle between 0° and 10°.

Fig. 8 shows the trajectory error testing apparatus 600 being used to test a second coaxial cable 660. In this example, the coaxial cable 660 comprises a 0.5 mm Ag-Cu alloy. It can be seen in Fig. 8 that the coaxial cable 660 passes over the angle gauge 630 at an angle between 20° and 30°, which demonstrates that the coaxial cable 660 is less elastic than the coaxial cable 650 shown in Fig. 7,

The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example +/- 10%.