Patent Publication Number: US-2022226053-A1

Title: Interface structure

Description:
BACKGROUND 
     It is known to use robots for assisting and performing surgery.  FIG. 1  illustrates a typical surgical robot  100  which consists of a base  108 , an arm  102 , and an instrument  105 . The base supports the robot, and is itself attached rigidly to, for example, the operating theatre floor, the operating theatre ceiling or a trolley. The arm extends between the base and the instrument. The arm is articulated by means of multiple flexible joints  103  along its length, which are used to locate the surgical instrument in a desired location relative to the patient. The surgical instrument is attached to the distal end  104  of the robot arm. The surgical instrument penetrates the body of the patient  101  at a port  107  so as to access the surgical site. At its distal end, the instrument comprises an end effector  106  for engaging in a medical procedure. 
       FIG. 2  illustrates a typical surgical instrument  200  for performing robotic laparoscopic surgery. The surgical instrument comprises a base  201  by means of which the surgical instrument connects to the robot arm. A shaft  202  extends between the base  201  and an articulation  203 . The articulation  203  terminates in an end effector  204 . In  FIG. 2 , a pair of serrated jaws are illustrated as the end effector  204 . The articulation  203  permits the end effector  204  to move relative to the shaft  202 . It is desirable for at least two degrees of freedom to be provided to the motion of the end effector  204  by means of the articulation. 
     A surgeon utilises many instruments during the course of a typical laparoscopy operation. For this reason, it is desirable for the instruments to be detachable from and attachable to the end of the robot arm with an ease and speed which enables instruments to be exchanged mid-operation. It is therefore desirable to minimise the time taken and maximise the ease with which one instrument is detached from a robot arm and a different instrument is attached. 
     The operating theatre is a sterile environment. The surgical robotic system must be sterile to the extent it is exposed to the patient. Surgical instruments are sterilised prior to use in an operation, however the robot arm is not sterilised prior to use. Instead, a sterile drape is placed over the whole of the surgical robot prior to the operation. In this way, the patient is not exposed to the non-sterile surgical robot arm. When exchanging instruments mid-operation, it is desirable for the sterile barrier to be maintained. 
     SUMMARY 
     According to an aspect of the invention, there is provided an interface structure for detachably interfacing a surgical robot arm to a surgical instrument, the interface structure comprising:
         a main body; and   a drive transfer element comprising a first portion and a second portion, the first portion being releasably engageable with a portion of the surgical robot arm and the second portion being releasably engageable with a portion of the surgical instrument;   the drive transfer element being movable relative to the main body so as to enable transfer of drive between the surgical robot arm and the surgical instrument.       

     Suitably the drive transfer element is slidably movable relative to the main body. 
     Suitably the main body comprises a first side for facing the surgical robot arm and a second side for facing the surgical instrument, the first portion being disposed towards the first side and/or the second portion being disposed towards the second side. 
     Suitably the main body comprises an aperture connecting the first side and the second side, and the drive transfer element is configured so as to obstruct fluid flow through the aperture so as to maintain a sterile barrier between the surgical robot arm and the surgical instrument. 
     Suitably the interface structure is configured so that the drive transfer element obstructs fluid flow through the aperture as the drive transfer element moves relative to the main body. 
     Suitably the interface structure is configured so that the drive transfer element obstructs fluid flow through the aperture across the extent of movement of the drive transfer element relative to the main body. 
     Suitably the drive transfer element covers the aperture. 
     Suitably the main body defines a path along which the drive transfer element is movable. 
     Suitably the main body defines a linear path along which the drive transfer element is movable. 
     Suitably the path defined by the main body comprises a slot within which the drive transfer element is receivable and along which the drive transfer element is movable. 
     Suitably the path defined by the main body comprises a channel within which the drive transfer element is receivable and along which the drive transfer element is movable. 
     Suitably the channel is formed as a cavity or recess. The cavity or recess may be within the main body. 
     Suitably the interface structure comprises a cover attachable to the main body, the channel being defined between a portion of the main body and a portion of the cover. 
     Suitably the cover is attachable to one of the first side and the second side of the main body. 
     Suitably the drive transfer element comprises a central portion, the central portion comprising the first portion and the second portion, and an extending portion which extends away from the central portion. 
     Suitably the extending portion covers the aperture. Suitably the central portion and the extending portion cover the aperture. 
     Suitably the extending portion is configured so that at an end of a range of motion of the drive transfer element relative to the main body, a distal end of the extending portion remains covered by the cover. 
     Suitably the drive transfer element is a rigid element. 
     Suitably the drive transfer element comprises a portion of movable material to which at least one of the first portion and second portion are attachable, the movable material being movable so as to accommodate movement of the drive transfer element relative to the main body whilst permitting the maintenance of the sterile barrier between the surgical robot arm and the surgical instrument. 
     Suitably the portion of movable material comprises a resilient material. 
     Suitably the interface structure comprises a roller, and the portion of movable material is windable onto the roller. 
     Suitably the interface structure comprises a plurality of drive transfer elements. 
     Suitably the interface structure comprises a first drive transfer element which is movable relative to the main body along a first path. Suitably the first path is a linear path. Suitably the interface structure comprises a second drive transfer element which is movable relative to the main body along a second path. Suitably the second path is a linear path. 
     Suitably the interface structure comprises a first drive transfer element movable relative to the main body along a first linear path, and a second drive transfer element movable relative to the main body along a second linear path, the first path and second path being parallel to one another. 
     Suitably the first path is parallel to a shaft of a surgical instrument when the interface structure is interfaced with the surgical instrument. 
     Suitably at least one of the first path and the second path is aligned with a shaft of a surgical instrument when the interface structure is interfaced with the surgical instrument. 
     Suitably the interface structure comprises three drive transfer elements, each drive transfer element being movable relative to the main body along a respective path, and each of the respective paths being parallel to one another. 
     Suitably the main body of the interface structure comprises a deformable material. Suitably the main body of the interface structure comprises a resilient material. 
     The material of the main body disposed between the drive transfer elements may comprise an unconstrained portion. The unconstrained portion may permit movement of the drive transfer elements relative to one another. 
     Suitably the material of the main body disposed between the drive transfer elements comprises a fabric material. 
     Suitably the main body of the interface structure comprises a rigid material. 
     Suitably the paths of the first and second drive transfer elements are in the same plane. Suitably the path of the third drive transfer element is in the same plane as the paths of the first and second drive transfer elements. 
     The first path may be in a first plane. The second path may be in a second plane. The first plane and the second plane may be different to one another. The third path may be in a third plane. The third plane may be different to the first plane and/or the second plane. Suitably the third plane is co-planar to the first plane or the second plane. Suitably at least two of the first plane, the second plane and the third plane are co-planar. 
     Suitably the first path is of a different length to the second path. Suitably the third path is the same length as at least one of the first path and the second path. 
     Suitably the main body comprises a first retention portion for retaining the interface structure on at least one of the surgical robot arm and the surgical instrument. Suitably the retention portion is arranged to engage with a second retention portion on the at least one of the surgical robot arm and the surgical instrument. 
     Suitably the main body comprises a first retention portion for retaining the interface structure on at least one of the surgical robot arm and the surgical instrument, the retention portion being engageable with a second retention portion on the at least one of the surgical robot arm and the surgical instrument. 
     Suitably the interface structure is retainable on the surgical robot arm to minimise relative movement between the main body of the interface structure and the surgical robot arm. Suitably the interface structure is retainable on the surgical instrument to minimise relative movement between the main body of the interface structure and the surgical instrument. 
     Suitably the first portion of the drive transfer element comprises one of a recess and a protrusion. Suitably the second portion of the drive transfer element comprises one of a recess and a protrusion. Suitably where the first portion comprises a recess, the second portion comprises a protrusion. Suitably where the first portion comprises a protrusion, the second portion comprises a recess. 
     Suitably the first portion of the drive transfer element comprises a recess for engagement with a protruding fin on the surgical robot arm and the second portion of the drive transfer element comprises a protrusion for engagement with a cup on the surgical instrument. Suitably the protrusion comprises a chamfer or rounded portion at its distal end. The chamfer or rounded portion may ease engagement of the protrusion with the cup. 
     Suitably the protrusion comprises a cavity in communication with the recess. The recess may be continuous with the cavity in the protrusion, such that a fin receivable into the recess is permitted to extend into the cavity. Suitably the protrusion of the second portion comprises a cavity in communication with the recess of the first portion. The recess of the first portion may be continuous with the cavity in the protrusion of the second portion, such that a fin receivable into the recess is permitted to extend into the cavity. 
     Suitably the main body comprises an alignment feature on at least one of the first side and the second side for aiding alignment of the interface structure with the surgical robot arm and/or the surgical instrument during engagement. 
     Suitably the alignment feature comprises a stud and/or aligning recess. The alignment feature may be engageable with a corresponding alignment feature on the surgical robot arm and/or on the surgical instrument. 
     Suitably a surgical drape extends from the interface structure. 
     Suitably the surgical drape extends from a periphery of the interface structure. Suitably the interface structure comprises a lip adjacent a periphery of the interface structure, and the surgical drape extends from the lip. The interface structure and the surgical drape may be integrally formed. 
     According to an aspect of the invention, there is provided a robotic surgical system comprising an interface structure as described above and a surgical robot arm, the surgical robot arm comprising a base and a plurality of articulations connecting the base to a drive assembly interface at or towards a distal end of the surgical robot arm, the plurality of articulations enabling the drive assembly interface to be articulated relative to the base; the interface structure being attachable to the drive assembly interface. 
     According to an aspect of the invention, there is provided a robotic surgical system comprising an interface structure as described above and a surgical instrument for use in robotic surgery, the surgical instrument comprising a shaft, a surgical end effector at or towards a distal end of the shaft and an instrument interface at or towards a proximal end of the shaft; the interface structure being attachable to the instrument interface. 
     Any one or more features of any aspect above may be combined with any one or more features of that aspect and/or any other aspect above. These have not been written out in full here for the sake of brevity. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The present invention will now be described by way of example with reference to the accompanying drawings. In the drawings: 
         FIG. 1  illustrates a surgical robot performing a surgical procedure; 
         FIG. 2  illustrates a known surgical instrument; 
         FIG. 3  illustrates a surgical robot; 
         FIG. 4  illustrates a drive assembly interface of a surgical robot arm; 
         FIG. 5  illustrates an instrument interface of a surgical instrument; 
         FIG. 6  illustrates the drive assembly interface of a robot arm with attached instrument; 
         FIG. 7 a    illustrates one side of an interface structure; 
         FIG. 7 b    illustrates the other side of the interface structure of  FIG. 7   a;    
         FIG. 8  illustrates an axial cross-sectional view of an instrument interfaced with a drive assembly via the interface structure of  FIG. 7   a;    
         FIG. 9  illustrates a side cross-sectional view of an instrument interfaced with a drive assembly via the interface structure of  FIG. 7   a;    
         FIG. 10 a    schematically illustrates a side view of an alternative interface structure; 
         FIG. 10 b    schematically illustrates a plan view of the alternative interface structure shown in  FIG. 10   a;    
         FIG. 11 a    schematically illustrates an end view of drive transfer elements of an alternative interface structure; and 
         FIG. 11 b    schematically illustrates a perspective view of drive transfer elements of the alternative interface structure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  illustrates a surgical robot having an arm  300  which extends from a base  301 . The arm comprises a number of rigid limbs  302 . The limbs are coupled by revolute joints  303 . The most proximal limb  302   a  is coupled to the base by a proximal joint  303   a . It and the other limbs are coupled in series by further ones of the joints  303 . Suitably, a wrist  304  is made up of four individual revolute joints. The wrist  304  couples one limb ( 302   b ) to the most distal limb ( 302   c ) of the arm. The most distal limb  302   c  carries an attachment  305  for a surgical instrument  306 . Each joint  303  of the arm has one or more motors  307  which can be operated to cause rotational motion at the respective joint, and one or more position and/or torque sensors  308  which provide information regarding the current configuration and/or load at that joint. Suitably, the motors are arranged proximally of the joints whose motion they drive, so as to improve weight distribution. For clarity, only some of the motors and sensors are shown in  FIG. 3 . The arm may be generally as described in our co-pending patent application PCT/GB2014/053523. 
     The arm terminates in the attachment  305  for interfacing with the instrument  306 . Suitably, the instrument  306  takes the form described with respect to  FIG. 2 . The instrument has a diameter less than 8 mm. Suitably, the instrument has a 5 mm diameter. The instrument may have a diameter which is less than 5 mm. The instrument diameter may be the diameter of the shaft. The instrument diameter may be the diameter of the profile of the articulation. Suitably, the diameter of the profile of the articulation matches or is narrower than the diameter of the shaft. The attachment  305  comprises a drive assembly for driving articulation of the instrument. Movable interface elements of the drive assembly interface mechanically engage corresponding movable interface elements of the instrument interface in order to transfer drive from the robot arm to the instrument. One instrument is exchanged for another several times during a typical operation. Thus, the instrument is attachable to and detachable from the robot arm during the operation. Features of the drive assembly interface and the instrument interface aid their alignment when brought into engagement with each other, so as to reduce the accuracy with which they need to be aligned by the user. 
     The instrument  306  comprises an end effector for performing an operation. The end effector may take any suitable form. For example, the end effector may be smooth jaws, serrated jaws, a gripper, a pair of shears, a needle for suturing, a camera, a laser, a knife, a stapler, a cauteriser, a suctioner. 
     As described with respect to  FIG. 2 , the instrument comprises an articulation between the instrument shaft and the end effector. The articulation comprises several joints which permit the end effector to move relative to the shaft of the instrument. The joints in the articulation are actuated by driving elements, such as cables. These driving elements are secured at the other end of the instrument shaft to the interface elements of the instrument interface. Thus, the robot arm transfers drive to the end effector as follows: movement of a drive assembly interface element moves an instrument interface element which moves a driving element which moves a joint of the articulation which moves the end effector. 
     Controllers for the motors, torque sensors and encoders are distributed within the robot arm. The controllers are connected via a communication bus to a control unit  309 . The control unit  309  comprises a processor  310  and a memory  311 . The memory  311  stores in a non-transient way software that is executable by the processor to control the operation of the motors  307  to cause the arm  300  to operate in the manner described herein. In particular, the software can control the processor  310  to cause the motors (for example via distributed controllers) to drive in dependence on inputs from the sensors  308  and from a surgeon command interface  312 . The control unit  309  is coupled to the motors  307  for driving them in accordance with outputs generated by execution of the software. The control unit  309  is coupled to the sensors  308  for receiving sensed input from the sensors, and to the command interface  312  for receiving input from it. The respective couplings may, for example, each be electrical or optical cables, or may be provided by a wireless connection. The command interface  312  comprises one or more input devices whereby a user can request motion of the end effector in a desired way. The input devices could, for example, be manually operable mechanical input devices such as control handles or joysticks, or contactless input devices such as optical gesture sensors. The software stored in the memory  311  is configured to respond to those inputs and cause the joints of the arm and instrument to move accordingly, in compliance with a pre-determined control strategy. The control strategy may include safety features which moderate the motion of the arm and instrument in response to command inputs. Thus, in summary, a surgeon at the command interface  312  can control the instrument  306  to move in such a way as to perform a desired surgical procedure. The control unit  309  and/or the command interface  312  may be remote from the arm  300 . 
       FIGS. 4 and 5  illustrate an exemplary mechanical interconnection of the drive assembly interface and the instrument interface in order to transfer drive from the robot arm to the instrument.  FIG. 4  illustrates an exemplary drive assembly interface  400  at the end of a robot arm  404 . The drive assembly interface  400  comprises a plurality of drive assembly interface elements  401 ,  402 ,  403 . The drive assembly interface elements protrude from surfaces  406 ,  407 ,  408  on the drive assembly interface  400 . The protrusion of the drive assembly interface elements from the drive assembly interface  400  permits engagement of the drive assembly interface elements with corresponding instrument interface elements, as described below. The protrusions are in the form of fins in the illustrated example. In other implementations, other types of protrusion can be provided. The drive assembly interface elements suitably comprise a stiff material, such as a metal. Suitably the protrusion is formed from a stiff material, such as a metal. Preferably the drive assembly interface element is formed from a stiff material, such as a metal. 
     The protrusions (the fins in the illustrated example) comprise a chamfer  414  at their distal ends. The chamfer provides for ease of engagement of the protrusions in corresponding recesses, as described below. In other examples the distal ends of the protrusions can be provided with a rounded corner. The edges of the chamfered portions can be rounded. 
     The fins extend through the surfaces  406 ,  407 ,  408 . The portions of the fins that protrude from the surfaces are perpendicular to the plane of the surfaces. In other examples the fins can protrude in a direction that is within a range of 10 degrees from perpendicular. Preferably the direction in which the fins extend is within a range of 5 degrees or within a range of 2 degrees from perpendicular. 
       FIG. 4  illustrates three drive assembly interface elements. In other examples, there may be greater than or fewer than three drive assembly interface elements. The drive assembly interface elements  401 ,  402 ,  403  are movable within the drive assembly interface  400  along linear paths  409 ,  410 ,  411 . The paths can be parallel with one another. Suitably at least two of the paths are parallel. The paths need not be precisely parallel with one another. There may be some tolerance in how closely aligned the paths need to be. For example, the paths may be within 10 degrees of each other. The paths may extend in respective directions within a 10 degree range. Preferably the paths are within 5 degrees of each other, or within 2 degrees or 1 degree of each other. The paths may extend in respective directions within a 5 degree range, or preferably a 2 degree or 1 degree range. 
     Aligning the paths in this manner can assist in providing corresponding mechanisms more compactly. For instance, the mechanisms can be arranged to move alongside one another, permitting the mechanisms to be arranged more closely together. 
     In the illustrated example, the linear paths  409 ,  410 ,  411  are disposed on two parallel planes. The central linear path  410  is disposed on a plane  407  set into the drive assembly interface  400  compared to that in which the outer two linear paths  409 ,  411  are disposed. This arrangement permits a more compact interface between the drive assembly interface  400  and an instrument interface  500 , as will be described below. 
     In other implementations, the three linear paths  409 ,  410 ,  411  can be disposed on the same plane, or all on different planes. In another example, the outer two linear paths  409 ,  411  are disposed on a plane set into the drive assembly interface  400  compared to that in which the central linear path  410  is disposed. In implementations utilising differing numbers of drive assembly interface elements, different configurations of planes on which the paths are disposed are possible. 
     The drive assembly interface  400  comprises a recessed portion  412  for receiving a portion of the instrument. This arrangement can permit a more compact configuration when the instrument is mounted onto the robot arm. 
     Referring now to  FIG. 5 , the shaft  501  of the instrument terminates in the instrument interface  500 . The instrument interface  500  comprises a plurality of instrument interface elements (one of which is shown at  502  in  FIG. 5 ; these can more clearly be seen in  FIG. 6  at  502 ,  507 ,  509 ). The instrument interface elements suitably comprise a stiff material, such as a metal. Suitably the instrument interface element is formed from a stiff material, such as a metal. Pairs of driving elements (one such pair is shown at  503 ,  504 ) extend into the instrument interface  500  from the end of the shaft  501 . Each pair of driving elements terminates in one of the instrument interface elements. In the example shown in  FIG. 5 , the driving element pair  503 ,  504  terminates in instrument interface element  502 ; likewise, other driving element pairs terminate in corresponding instrument interface elements. 
     In the illustrated example there are three driving element pairs that terminate in three instrument interface elements. In other examples, there may be greater than or fewer than three instrument interface elements. There may be greater than or fewer than three driving element pairs. In  FIG. 5  there is a one-to-one relationship between instrument interface elements and driving element pairs. In other examples, there may be any other coupling relationship between the instrument interface elements and driving element pairs. For example, a single instrument interface element may drive more than one pair of driving elements. In another example, more than one instrument interface element may drive a single pair of driving elements. 
     Each instrument interface element  502 ,  507 ,  509  comprises a recess, or cup  505 , which is the portion of the instrument interface element engageable with the drive assembly interface element. 
     The instrument interface elements are displaceable within the instrument interface. In the example shown, the instrument interface elements are slideable along rails. Instrument interface element  502  is slideable along rail  506 . Instrument interface element  507  is slideable along rail  508 . Instrument interface element  509  is slideable along rail  510 . Each instrument interface element is displaceable along a direction parallel to the direction of elongation of the pair of driving elements which that instrument interface element holds captive. Each instrument interface element is displaceable in a direction parallel to the longitudinal axis  512  of the instrument shaft  501 . When the instrument interface element moves along its respective rail, it causes a corresponding movement to the driving element pair secured to it. Thus, moving an instrument interface element drives motion of a driving element pair and hence motion of a joint of the instrument. 
     Drive assembly interface  400  mates with instrument interface  500 . The instrument interface  500  comprises structure for receiving the drive assembly interface elements  401 ,  402 ,  403 . Specifically, the instrument interface elements  507 ,  502 ,  509  receive drive assembly interface elements  401 ,  402 ,  403 . In the example shown, each instrument interface element comprises a socket or cup  505  for receiving the fin of the corresponding drive assembly interface element. The socket  505  of one instrument interface element  502  receives a fin of the corresponding drive assembly interface element  402 . Similarly, sockets of the other instrument interface elements receive fins of the other drive assembly interface elements. 
     Each drive assembly interface element is displaceable within the drive assembly. This displacement is driven. For example, the displacement may be driven by a motor and lead screw arrangement. In the example shown, the drive assembly interface elements are slideable along drive rails  802 ,  804 . Each drive assembly interface element is coupled to one drive rail. Referring to  FIG. 8 , the right-hand drive assembly interface element  403  is coupled to a right-hand drive rail  804 . The central drive assembly interface element  402  is coupled to a left-hand drive rail  802 . Whilst not shown, the left-hand drive assembly interface element  401  is coupled to the left-hand drive rail  802 . In other configurations, the central drive assembly interface element is coupled instead to the right-hand drive rail  804 , or to both of the left-hand drive rail  802  and the right-hand drive rail  804 . 
     Coupling the central drive assembly interface element  402  to both the left-hand drive rail and to the right-hand drive rail assists in stabilising the central drive assembly interface element. For the arrangement illustrated in  FIG. 8 , where the central drive assembly interface element is elongate in the vertical direction of  FIG. 8 , this can reduce bending or rotation of the central drive assembly interface element as it moves and drives the corresponding drive transfer element. 
     Each drive assembly interface element is displaceable along a direction parallel to the longitudinal axis  413  of the terminal link of the robot arm. When the drive assembly interface element moves along its rail, it causes a corresponding movement to the instrument interface element to which it is engaged. Thus, driving motion of a drive assembly interface element drives motion of an instrument interface element which drives articulation of the end effector of the instrument. 
     The portions of the fins that protrude from the surfaces comprise front and rear faces aligned in the directions of movement of the drive assembly interface elements. Here, front and rear refer to movement in one direction, when the front face will face the direction of movement and the rear face will face away from the direction of movement. When the drive assembly interface element moves in the opposite direction, the front face will face away from the direction of movement and the rear face will face the direction of movement. 
     The front and rear faces of the drive assembly interface elements are transverse to the direction in which the drive assembly interface elements are drivably movable. The front and rear faces of the drive assembly interface elements are parallel to the direction in which the fins protrude from the surfaces. The front and rear faces need not be exactly parallel to this direction, but are preferably within a range of 10 degrees, or within a range of 5 degrees, or more preferably within a range of 2 degrees of this direction. 
     The socket  505  comprises an interior face that is transverse to the direction in which the instrument interface elements are movable. The interior face need not be exactly transverse to this direction, but is preferably within a range of 10 degrees, or within a range of 5 degrees, or more preferably within a range of 2 degrees of being transverse to this direction. 
     In the illustrated example the interior face of the instrument interface elements and the front and rear faces of the drive assembly interface elements are parallel to one another. This can assist in the transferal of drive between the elements. 
     In  FIGS. 4 and 5  there is a one-to-one relationship between instrument interface elements and drive assembly interface elements. In other examples, there may be any other coupling relationship between the instrument interface elements and drive assembly interface elements. For example, a single drive assembly interface element may drive more than one instrument interface element. In another example, more than one drive assembly interface element may drive a single instrument interface element. 
       FIG. 6  illustrates the instrument being placed into engagement with the robot arm. When drive assembly interface element  401  is held captive by instrument interface element  507 , drive assembly interface element  402  is held captive by instrument interface element  502 , and drive assembly interface element  403  is held captive by instrument interface element  509 , the instrument interface elements and the drive assembly interface elements are all displaceable in the same direction. This direction is parallel to both the longitudinal axis  413  of the terminal link of the robot arm  404  and the longitudinal axis  512  of the instrument shaft  501 . 
     During an operation or surgical procedure, the surgical robot is shrouded in a sterile drape to provide a sterile barrier between the non-sterile surgical robot and the sterile operating environment. The surgical instrument is sterilised before being attached to the surgical robot. The sterile drape is typically constructed of a plastic sheet, for example made of polyester, polypropylene, polyethylene or polytetrafluoroethylene (PTFE). Suitably, the drape is flexible and/or deformable. 
     The sterile drape does not pass directly between the drive assembly interface  400  and the instrument interface  500 . The drape comprises an interface structure  700  for interfacing between the drive assembly interface  400  and the instrument interface  500 .  FIGS. 7 a  and 7 b    show an exemplary interface structure  700  in isolation. The interface structure  700  is also shown in  FIG. 6  attached to the drive assembly interface  400  and to the instrument interface  500 . The interface structure  700  may be integrally formed with the drape. Alternatively, the interface structure  700  may be formed separately from the drape and subsequently attached to the drape. Either way, the interface structure  700  is sterile. One side  701  of the interface structure  700  directly contacts the drive assembly interface. The other side  702  of the interface structure  700  directly contacts the instrument interface. Thus, the interface structure  700  prevents the non-sterile drive assembly interface from directly touching the sterile instrument interface and hence maintains the sterile barrier between the two components. 
     The interface structure  700  comprises a main body  704  and drive transfer elements  706 ,  707 ,  708 . The drive transfer elements are movable relative to the main body. Conveniently, when the interface structure  700  is attached to the surgical robot arm, the main body  704  lies parallel to the surface(s) of the drive assembly interface  400 . Suitably in this attached configuration, the main body  704  is aligned with the drive assembly interface. 
     The main body  704  comprises a first side  701  which faces the robot arm when the instrument is attached to the robot arm. Specifically, the first side  701  faces the drive assembly  400 . The main body  704  comprises a second side  702  opposite to the first side. The second side  702  faces the instrument when the instrument is attached to the robot arm. Specifically, the second side  702  faces the instrument interface  500 . Suitably both the first side  701  and the second side  702  are substantially flat. The first side and the second side need not be completely flat. Being substantially flat, or flat over at least a portion of its surface (for example over at least 10% of its surface, over at least 20% of its surface, over at least 30% of its surface, preferably over at least 40% of its surface or more preferably over at least 50% of its surface) permits the interface structure  700  to be compactly sandwiched between the instrument and the robot arm when the instrument is attached to the robot arm. 
     Being flat can include having flat portions in different planes. For example, as illustrated in  FIG. 4 , the drive assembly interface  400  can have portions which are flat, but disposed generally over two planes, as described above. Suitably the interface structure  700  is configured to correspond to the general surface features of the drive assembly interface so as to compactly engage therewith, reducing or minimising gaps or space between the interface structure and the drive assembly interface. 
     The main body  704  comprises an aperture. In the interface structure  700  illustrated in  FIGS. 7 a , 7 b    and  8 , an aperture is located generally central to the main body  704 , though it need not be located in this position. In the illustrated example, the main body  704  comprises three apertures: a first aperture  816 , a second aperture  817  and a third aperture  818  (as can be seen in  FIG. 8 ). The apertures  816 ,  817 ,  818  provide for communication between the first side  701  and the second side  702  though the main body  704 . 
     A cover  710  is provided which covers a portion of the main body  704 . The cover covers the part of the main body that comprises the apertures. In the illustrated implementation, the cover  710  is located on the second side  702  of the main body  704 . In other examples, the cover can be located on the first side  701  of the main body, or covers can be located on both sides of the main body. The cover  710  is attached to the main body  704 . Suitably the cover  710  is fixed to the main body  704 . The cover can be attached to the main body by adhesive, or by any other convenient means or method of attachment. 
     The cover  710  comprises further apertures, or slots. In the illustrated example, the cover  710  comprises a first slot  726 , a second slot  727  and a third slot  728 . The slots communicate with the apertures in the main body  704 . The first slot  726  is aligned with the first aperture  816 ; the second slot  727  is aligned with the second aperture  817 ; the third slot  728  is aligned with the third aperture  818 . Thus the slots in the cover  710  provide fluid flow paths between the first side and the second side  702  of the main body. 
     The apertures  816 ,  817 ,  818  in the main body  704  define paths along which the drive transfer elements are movable. In the example illustrated in  FIGS. 7 a  and 7 b   , the paths are linear paths. The first aperture  816  defines a first path; the second aperture  817  defines a second path; the third aperture  818  defines a third path. 
     The main body  704  and the cover  710  define therebetween channels along which drive transfer elements are movable. Suitably the drive transfer elements are slideable within the channels. Referring to  FIG. 8 , a lip  819  adjacent an aperture in the main body  704  and a corresponding lip  820  adjacent an aperture in the cover  710  define a channel  821  between the lips. The main body  704  and the cover  710  define two channels per aperture, one to either side of the aperture. The channels extend along the length of the apertures. 
     As mentioned above, the interface structure  700  comprises drive transfer elements. In the example illustrated in  FIGS. 7 a  and 7 b   , the interface structure comprises three drive transfer elements: a first drive transfer element  706 , a second drive transfer element  707  and a third drive transfer element  708 . The first drive transfer element  706  is slidably received in the first slot  726 . The second drive transfer element  707  is slidably received in the second slot  727 . The third drive transfer element  708  is slidably received in the third slot  728 . Each drive transfer element is slidably movable along its respective slot. 
     The drive transfer elements comprise a central portion and an extending portion which extends away from the central portion. With reference to the first drive transfer element  706 , the central portion  736  comprises a protrusion. The extending portion  716  comprises a flat plate that extends from the central portion  736 . The extending portion  716  is elongate in two opposite directions which, when the first drive transfer element  706  is located in the first slot  726 , are aligned with the directions in which the first slot  726  extends. In directions transverse to these directions, i.e. in directions transverse to the extent of the slots, the first drive transfer element comprises a first lip  846  as can be seen from  FIG. 8 . The first lip  846  is receivable into channels to either side of the first aperture  816 . Similarly, a second lip  847  on the second drive transfer element  707  is receivable into channels to either side of the second aperture  817 . A third lip  848  is receivable into channels to either side of the third aperture  818 . Suitably the drive transfer elements are rigid. 
     The drive transfer elements extending along the slots restricts the fluid flow path through the apertures. The drive transfer elements extending into the channels adjacent the apertures restricts the fluid flow path through the apertures. In this way the drive transfer elements restrict the fluid flow path around the drive transfer elements. 
     Suitably the inter-engagement between the drive transfer elements  706 ,  707 ,  708  and the main body  704  is such as to restrict the fluid flow path between the drive transfer elements and the main body. This inter-engagement is, for example, by a portion of the drive transfer elements being retained adjacent the main body, such as by being retained in the slots, or by being retained in the channels. 
     The first slot  726  comprises a first end  730  and a second end  731  opposite the first end, along the length of the first slot. The extending portion  716  of the first drive transfer element  706  comprises a first extension  732  and a second extension  734 . The length of the first extension  732  from the central portion  736  of the first drive transfer element  706  is L 1 . The length of the second extension  734  from the central portion  736  of the first drive transfer element  706  is L 2 . 
     At the furthest extent of movement of the first drive transfer element  706  towards the second end  731  of the first slot  726 , the distance between the central portion  736  and the first end  730  is D 1 . At the furthest extent of movement of the first drive transfer element  706  towards the first end  730  of the first slot  726 , the distance between the central portion  736  and the second end  731  is D 2 . 
     The length of the first extension L 1  is at least the same as the distance D 1 . Suitably L 1  is greater than D 1 , for example to provide an overlap between the first extension and the main body and/or between the first extension and the cover. The length of the second extension L 2  is at least the same as the distance D 2 . Suitably L 2  is greater than D 2 , for example to provide an overlap between the first extension and the main body and/or between the first extension and the cover. In this way, the extending portion  716  (comprising the first extension  732  and the second extension  734 ) covers the aperture. In other words, it covers the space between the central portion and the ends of the slots. Providing the extension portions  732 ,  734  to be the same length as, or greater than, the potential gap means that the gap will remain covered throughout the extent of movement of the drive transfer element within the slot. 
     The second drive transfer element  707  and the third drive transfer element  708  are similarly configured. For example, the second drive transfer element  707  comprises a third extension  735 . Thus each aperture or slot remains covered throughout the whole extent of movement of the respective drive transfer element. 
     Referring to  FIGS. 7 a  and 7 b   , the slots are not all of equal length. The second slot  727  is shorter than the first slot  726  and the third slot  728 . The slots need not be sized in this particular way. Each slot can be sized as desired to account for or permit the required movement of the respective drive transfer element. In this example the central drive assembly interface element  402  is configured to move along a shorter linear path  410  than the linear paths  409 ,  411  along which the left-hand drive assembly interface element  401  and the right-hand drive assembly interface element  403  are configured to move. Correspondingly the first slot  726  and the third slot  728  are longer than the second slot  727 . In the illustrated example the first drive transfer element and the third drive transfer element have a relative movement with respect to the main body of ±5.1 mm (i.e. 10.2 mm from one end to the other). The second drive transfer element has a relative movement with respect to the main body of ±3 mm (i.e. 6 mm from one end to the other). The relative movements need not be the same as these. In some examples the relative movement of the first and third drive transfer elements is longer or shorter than this. The relative movement of the second drive transfer element can be longer or shorter than this. The ratio of relative movements need not be this ratio, but could be greater or less than this ratio. 
     As can be seen from  FIG. 9 , the ends  910  of the slot in the cover are further apart than the ends  911  of the aperture in the main body. The slot in the cover is longer than the respective aperture in the main body. This additional length permits the socket  502  to protrude at least partially within the slot in the cover without reducing the travel of the drive transfer element within the aperture. Suitably the additional length of the slot compared to the aperture is at least equal to the width of that portion of the socket disposed between the drive transfer element and the end  910  of the slot. Suitably the additional length of the slot compared to the aperture is at least equal to twice the width of that portion of the socket disposed between the drive transfer element and the end  910  of the slot, so as to avoid reducing the travel of the drive transfer element within the aperture at either end of the range of movement. 
     The protrusion of the socket  502  at least partially within the slot in the cover permits better coupling between the socket and the drive transfer element. The protrusion of the socket at least partially within the slot in the cover permits better coupling between the socket and the fin. The coupling is improved by providing a greater overlap between the socket and the fin in the direction of drive transfer. 
     In the illustrated example the slots are aligned at one end. An end of the first slot proximal to an indent  712  in the interface structure  700  is aligned with an end of the second slot proximal to the indent  712  and an end of the third slot proximal to the indent  712 . When the drive transfer elements are moved, for example by being driven, to their furthest extent towards the indent  712 , each of the drive transfer elements will be aligned with the others. Where the ends of the slots, or the drive transfer elements, are aligned, they may be at the same distance as one another along a length of the interface structure. 
     In other examples, the length of the slots need not match the length of the linear paths. Suitably the slots are at least as long as the linear paths. 
     This arrangement assists in restricting fluid flow through the aperture or slot. Restricting this fluid flow assists in maintaining a sterile barrier. Thus when attached to a surgical robot arm, and/or to a surgical instrument, the interface structure can assist in maintaining the sterile barrier between the arm and the instrument. 
     As mentioned above, the central portion  736  of the first drive transfer element  706  comprises a protrusion to the second side  702  of the interface structure  700 . As can be seen from  FIG. 7 a   , each of the drive transfer elements comprises a central portion which comprises a protrusion to the second side  702  of the interface structure  700 . In this example, the central portions of the drive transfer elements comprise recesses to the first side  701  of the interface structure  700  (visible in  FIG. 7 b   ) for engagement with the fins of the respective drive assembly interface elements. 
     In other examples, the central portions of the drive transfer elements can be arranged the other way round. In other words, recesses can be provided towards the second side and protrusions can be provided towards the first side. Alternatively, any combination of protrusions and recesses can be provided. This can include one drive transfer element comprising either both a protrusion towards the first side and a protrusion towards the second side, or a recess towards the first side and a recess towards the second side. The configuration adopted will suitably match that of the drive assembly interface  400  and the instrument interface  500 . In other words, where a drive assembly interface element comprises a protruding fin, the central portion of the respective drive transfer element towards the first side will comprise a recess for receiving the fin. Where the drive assembly interface element comprises a recess, the central portion of the respective drive transfer element towards the first side will comprise a protrusion for engaging with the recess. Similarly, where the instrument interface element comprises a protruding fin, the central portion of the respective drive transfer element towards the second side will comprise a recess for receiving the fin. Where the instrument interface element comprises a recess, the central portion of the respective drive transfer element towards the second side will comprise a protrusion for engaging with the recess. 
     Suitably the drive transfer elements comprise a plastic material. Preferably the drive transfer elements are able to deform slightly so as to accommodate interfacing with the drive assembly interface elements and/or the instrument interface elements. Preferably the drive transfer elements engage with the drive assembly interface elements by an interference fit, such as a light interference fit. Suitably the drive transfer elements engage with the instrument interface elements by an interference fit, such as a light interference fit. 
     Generally, each drive transfer element comprises a first portion and a second portion. The central portion suitably comprises the first portion and the second portion. The first portion is engageable with the robot arm. For example, the first portion is engageable with the drive assembly interface, such as being engageable with a drive assembly interface element. The second portion is engageable with the instrument. For example, the second portion is engageable with the instrument interface, such as being engageable with an instrument interface element. 
     To put it another way, at least one of the first portion and the second portion can be a drive transfer element recess, or a recess in the drive transfer element. At least one of the first portion and the second portion can be a drive transfer element protrusion, or a protruding portion of the drive transfer element. Preferably, the drive transfer element comprises both a drive transfer element recess and a drive transfer element protrusion. 
     The drive transfer element recess is engageable with an interface protrusion, such as a protrusion on a drive assembly interface element or on an instrument interface element. The drive transfer element protrusion is engageable with an interface recess, such as a recess in a drive assembly interface element or in an instrument interface element. 
     Referring to the illustrated example, the first portion comprises a recess and the second portion comprises a protrusion. The protrusion of the second portion comprises a chamfer and/or rounded edge to ease engagement of the protrusion with a cup, such as a cup on an instrument interface element, into which the protrusion is receivable. In the illustrated example, as best seen from  FIGS. 7 a    and  9  the protrusion of the second portion has a V-shape in cross-section. This aids in engaging the protrusion with the cup. The V-shape of the protrusion can accommodate misalignment between the protrusion and the cup as the instrument is attached to the interface structure. The recess of the first portion comprises a flared and/or rounded edge adjacent the opening into the recess to ease engagement with a protrusion or fin, such as a protrusion or fin on a drive assembly interface element, with which the recess is engageable. 
     Preferably, the first portion and the drive assembly interface element comprise cooperating surfaces which are complementary to one another. Preferably, the second portion and the instrument interface element comprise cooperating surfaces which are complementary to one another. Referring to  FIG. 9 , an interior of the cup of the instrument interface element is shaped to be complementary to an exterior of the protrusion on the drive transfer element, and an interior of the drive transfer element is shaped to be complementary to an exterior of the protrusion of the drive assembly interface element. 
     Where one of the first portion and the second portion is a recess and the other of the first portion and the second portion is a protrusion, the recess  901  can communicate with a cavity  902  in the protrusion  903 , as can be seen from  FIG. 9 . A fin  930  receivable into the recess can be receivable into the cavity through the recess. This can provide a more stable engagement between the drive assembly and the interface structure, between the interface structure and the instrument, and/or between the drive assembly and the instrument. 
     The drive transfer element comprises an outer edge or wall  904 . The outer wall  904  faces to the left in  FIG. 9 . The outer wall  904  faces a direction in which the drive transfer element can be driven by the drive assembly interface element. The outer wall  904  contacts, or engages with, a socket inner wall  905  of the socket or cup  502  of the instrument interface element. The socket inner wall  905  faces an opposing direction, such as an opposite direction, to that faced by the outer wall  904 . In the illustrated example, the outer wall  904  and the socket inner wall  905  face in opposite directions. The drive transfer element comprises an inner edge or wall  906 . The inner wall  906  faces to the right in  FIG. 9 . The inner wall  906  opposes a direction in which the drive transfer element can be driven by the drive assembly interface element. For example, referring to  FIG. 9 , the inner wall  906  is opposite to a direction in which the drive transfer element can be driven (the drive transfer element can be driven to the left, and the inner wall  906  faces to the right). The inner wall  906  contacts, or engages with, a portion of the drive assembly interface element. In the illustrated example the drive assembly interface element comprises a protrusion or fin  930  which contacts the inner wall  906  of the drive transfer element. Thus drive is transferable from the drive assembly interface element  930  to the inner wall  906  of the drive transfer element, and from the outer wall  904  of the drive transfer element to the socket inner wall  905 . 
     The outer wall  904  and the socket inner wall  905  overlap one another in the direction in which the drive transfer element is movable, i.e. in the direction in which drive is transferrable between the drive assembly interface element and the instrument interface element. Suitably the outer wall  904  overlaps the whole of the socket inner wall  905 . 
     Increasing, or maximising, the overlap between the outer wall  904  and the socket inner wall  905 , in other words increasing the area of overlap, can reduce, or minimise, the pressure on the drive transfer element. Thus a greater overlap reduces pressure between the drive assembly interface element and the instrument interface element. Pressure could be reduced by increasing the width of the overlap. This might, however, cause the width of the interfaces to increase, all other things remaining equal. Preferably, where the width of overlap is increased, this is accommodated within the interfaces to avoid needing to increase the overall width of the interfaces. 
     Pressure could be reduced by increasing the height, or vertical (with respect to  FIG. 9 ) extent, of the overlap. This might, however, cause the height of the interfaces to increase, all other things remaining equal. Preferably, where the height of overlap is increased, this is accommodated within the interfaces to avoid needing to increase the overall height of the interfaces. One way of achieving this is to provide the drive assembly interface element and instrument interface element at relative positions such that the vertical overlap is increased without affecting the overall height of the structure. Providing the cup  502  (which in the example illustrated in  FIG. 9  is located on the instrument interface element, but which, as described above, could alternatively be located on the drive assembly interface element) so as to at least partially protrude within the slot, as described above, can lead to an increase in the area of overlap without necessarily causing the overall height to increase. 
     In a similar manner, another outer wall (for example a second outer wall) of the drive transfer element faces to the right (in  FIG. 9 ). The second outer wall faces another direction in which the drive transfer element can be driven by the drive assembly interface element. The second outer wall contacts, or engages with, another socket inner wall (for example a second socket inner wall) of the socket  502 . The second socket inner wall faces an opposing direction, such as an opposite direction, to that faced by the second outer wall. The drive transfer element comprises a second inner edge or wall. The second inner wall opposes the other direction in which the drive transfer element can be driven by the drive assembly interface element. The second inner wall contacts, or engages with, a portion of the drive assembly interface element. Thus drive is transferable from the drive assembly interface element to the second inner wall of the drive transfer element, and from the second outer wall of the drive transfer element to the second socket inner wall. 
     The second outer wall and the second socket inner wall overlap one another in the direction (to the right in  FIG. 9 ) in which the drive transfer element is movable, i.e. in the direction in which drive is transferrable between the drive assembly interface element and the instrument interface element. Suitably the second outer wall overlaps the whole of the second socket inner wall. 
     The outer wall and the socket inner wall are, in the example illustrated in  FIG. 9 , parallel to one another. The second outer wall and the second socket inner wall are, in the example illustrated in  FIG. 9 , parallel to one another. In other examples, either or both pairs of walls need not be parallel to one another. 
     The inner wall and a wall of the drive assembly interface element arranged to be adjacent the inner wall are, in the example illustrated in  FIG. 9 , parallel to one another. The second inner wall and a wall of the drive assembly interface element arranged to be adjacent the second inner wall are, in the example illustrated in  FIG. 9 , parallel to one another. In other examples, either or both of these pairs of walls need not be parallel to one another. 
     The overlap of the outer wall and the socket inner wall (and/or of the second outer wall and the second socket inner wall) permits drive to be transferred by a compressive force, such as a substantially compressive force. Where the pairs of walls are parallel to one another, and the walls are transverse to the direction in which drive is transferable, the force will be a compressive force. An increasing deviation from this arrangement will result in a reduction in the compressive component of the force, and an increase in other components of the force, for example bending or shear components. 
     In other examples, drive need not be transferred via overlapping portions of the walls. Drive can be transferable, at least in part, via a portion of the drive transfer element which does not overlap with the drive assembly interface element in the direction of drive transfer. For example, drive can be transferable via the V-shaped portion of the drive transfer element (or, more generally, via a rounded and/or chamfered portion of the drive transfer element). This arrangement may provide a greater positional tolerance between the drive transfer element and the instrument interface element whilst still being able to transfer drive. An interference fit, though preferable also in this arrangement, would again not be necessary. In such an arrangement, a force which is vertical in the orientation of  FIG. 9  is likely to be advantageously provided to assist in keeping the drive assembly interface elements and the instrument interface elements in an engaged configuration as drive is transferred. 
     The size or width of an opening (in a direction in which drive is transferrable) of the drive transfer element recess  901  is larger than that of the cavity  902 . Referring to  FIG. 9 , both the recess and the cavity are provided centrally in the drive transfer element in a left-right direction, i.e. one in which the drive transfer element is movable. Since the recess has a larger width than the cavity, the inner walls of the recess are not co-planar with the inner walls of the cavity. The inner walls of the recess are outwardly offset from the inner walls of the cavity. 
     This offsetting of the internal walls of the drive transfer element recess and the cavity can permit the interface protrusion to be engaged within the cavity without being engaged by the walls of the drive transfer element recess. This arrangement can assist in providing drive transfer through the walls of the cavity. In turn, this can assist in reducing the bending moment on the drive transfer element when being driven. This arrangement can reduce the components of the drive transfer force other than the compressive component. 
     Suitably the drive transfer element protrusion comprises a chamfer or rounded portion at or towards its distal end to ease engagement of the drive transfer element protrusion with the interface recess. Suitably the drive transfer element recess comprises a chamfer or rounded portion at its opening to ease engagement of the interface protrusion with the drive transfer element recess. Referring to  FIG. 9 , the offsetting of the walls of the recess from the walls of the cavity provide a lip between the recess and the cavity. Suitably the lip is angled or rounded to ease engagement of the interface protrusion with the cavity through the drive transfer element recess and past the lip. 
     The drive assembly interface element comprises an elongate protruding portion and a drive assembly interface element body  920 . The elongate protruding portion is receivable into the drive transfer element recess  901 . A strengthening and/or stiffening portion  921  is provided on the drive assembly interface element proximal to the drive assembly interface element body  920 . Referring to  FIG. 9 , the strengthening or stiffening portion is a buttress portion. In other examples the strengthening or stiffening portion is a strut, or other abutment or fillet, or a gusset. The strengthening or stiffening portion can be any combination of these. The strengthening or stiffening portion can be made of a stronger and/or stiffer material than the drive assembly interface element body, for example titanium. Preferably the strengthening or stiffening portion  921  is provided on the sides of the drive assembly interface element towards and/or away from a direction in which the drive assembly interface element is movable. The strengthening or stiffening portion, such as the buttress portion, suitably resists bending of the drive assembly interface element, for example as the drive assembly interface element is moved or driven. Suitably the drive assembly interface element is strong enough and/or stiff enough to withstand a force of at least 80N without breaking. Suitably the drive assembly interface element is strong enough and/or stiff enough to withstand a force of at least 130N without breaking. Suitably the drive assembly interface element can resist a force of at least 80N, and preferably of at least 130N. 
     The interface structure comprises a first fastener  740  for retaining the interface structure  700  on the robot arm when the interface structure is mounted, or attached, to the robot arm. The drive assembly interface  400  comprises a retention lip  440 . The first fastener  740  is engageable with the retention lip  440 . The first fastener  740  comprises a ridge  742 . During attachment of the interface structure  700  to the drive assembly  400 , the ridge  742  passes over the retention lip  440 . The first fastener is resilient to permit flexing so that the ridge  742  can pass over the retention lip  440 . Once the first fastener has passed the retention lip, a flat portion  743  at the rear of the first fastener (in the direction of attachment) abuts a front portion of the retention lip (again, in the direction of attachment) and resists movement of the interface structure  700  in a direction away from the robot arm along the longitudinal axis  413  of the distal end  404  of the arm. In this way, the interface structure  700  is retained in position attached to the drive assembly interface  400 . To remove the interface structure  700  from the robot arm, the first fastener can be released. The first fastener  740  is releasable by resiliently deforming the first fastener so as to lift the ridge  742  over the retention lip  440 . In the example illustrated in  FIGS. 7 a  and 7 b   , the first fastener comprises a tab  744 . The tab  744  permits a user to lift the first fastener so as to disengage the ridge  742  from the retention lip  440 . The tab  744  need not be provided in all examples. The engagement of the first fastener with the retention lip can provide tactile feedback that the interface structure is correctly or properly attached to the robot arm. 
     Additional retention features are provided on an edge  750  of the interface structure  700  in the illustrated example. As illustrated in  FIG. 7 b   , one edge  750  of the interface structure comprises on an internal face thereof two lugs  751 ,  752 . The lugs  751 ,  752  protrude inwardly from the internal face of the edge  750  of the interface structure  700 . Cooperating retention features are provided on an outer edge of the drive assembly interface  400 . Two passages  451 ,  452  are provided on the outer edge of the drive assembly interface  400  which communicate with a retention channel  453 . In the illustrated example a common retention channel communicates with both passages, but this need not be the case. In alternatives, each passage can communicate with a respective retention channel. The passages  451 ,  452  and the retention channel  453  are formed as recesses in the outer edge of the drive assembly interface  400 . 
     As the interface structure  700  is mounted to the drive assembly interface  400 , the lugs  751 ,  752  will pass through the passages  451 ,  452  and into the retention channel  453 . The interface structure  700  can be moved along the longitudinal axis  413  of the distal end of the arm  404 . The retention channel  453  is parallel to the longitudinal axis  413  of the distal end of the arm. The movement of the interface structure in this direction (i.e. parallel to the longitudinal axis  413 ) moves the lugs along the retention channel  453  away from the openings to the passages  451 ,  452 . At the same time, the first fastener  740  is moved to engage with the retention lip  440 . When the lugs  751 ,  752  are moved away from the openings to the passages  451 ,  452 , the interface structure will be restricted to move along the longitudinal axis  413  of the arm  404 . The lugs  751 ,  752  will abut an upper edge  454  of the retention channel  453  to restrict movement of the interface structure  700  away from the drive assembly interface  400  in a direction transverse to the longitudinal axis  413 . In other words, the engagement of the lugs in the retention channel will prevent or restrict the interface structure from being lifted off the drive assembly. 
     As can be seen from  FIG. 7 b   , in this example the lugs  751 ,  752  comprise an upright portion  753 ,  754 . As the interface structure is moved along the longitudinal axis  413  of the distal end of the arm  404  so as to engage the lugs in the retention channel  453 , the front face of the upright portions  753 ,  754  will move into abutment with faces  455 ,  456  adjacent the passages  451 ,  452 . This abutment between the upright portions  753 ,  754  and the faces  455 ,  456  serves to limit the movement of the interface structure, and provides tactile feedback that the limit of travel has been reached. The upright portions  753 ,  754  need not be provided in every example. 
     This combination of retention features of the interface structure  700 , i.e. the first fastener  740  and the lugs  751 ,  752 , restrict the removal of the interface structure  700  from the robot arm. 
     The interface structure is suitably configured to fully engage with the drive assembly interface whilst being moved a distance along the longitudinal axis  413  of the arm that is the same as or less than the distance of travel of a drive transfer element permitted by the shortest slot (i.e. in the example above, a distance of up to 6 mm). The drive transfer elements of the interface structure engage with the drive assembly interface elements as the interface structure is mounted on the drive assembly. As the main body of the interface structure is moved relative to the drive assembly so as to engage the retention features of the interface structure with those of the drive assembly, the drive transfer elements are restricted in movement by virtue of being engaged with the drive assembly elements. 
     Thus as the interface structure is moved into engagement, the drive transfer elements will move relative to the main body. Restricting the possible extent of travel of the main body relative to the drive assembly interface to the same as or less than the extent of travel of the drive transfer element with the shortest travel can prevent that drive transfer element from being urged past its extent of travel. This can reduce potential damage to the interface structure, and assist in maintaining the sterile barrier. 
     In one example, prior to attaching the interface structure to the drive assembly interface, the drive assembly interface elements are driven to a desired position, such as an interfacing position. Suitably the interfacing position, or the desired position, is for engaging the drive assembly interface elements with respective instrument interface elements. This desired position is suitably with the drive assembly interface elements at one end of their respective travel, for instance towards the end of the drive assembly interface away from the proximal end of the robot arm. The interface structure can be arranged so that the drive transfer elements are correspondingly at cooperating positions within their respective travel, for instance with one of the drive transfer elements (suitably the drive transfer element with the shortest extent of travel) being at one end of its respective travel. In this way, the engagement of the drive transfer elements with the drive assembly interface elements is reliably effected. This method of engagement can be done without needing to drive or otherwise move the drive transfer elements and/or the drive assembly interface elements back and forth to effect engagement. 
     Engaging the interface structure with the drive assembly in this way can mean that the main body of the interface structure is then able to move relative to the drive assembly interface by up to the full travel of the drive transfer element with the shortest travel. 
     To determine whether the drive assembly interface elements are in, or have been driven to, the desired position, in one example the drive assembly comprises a sensor. Preferably the drive assembly comprises a plurality of sensors, a respective one for each drive assembly interface element. The sensor is configured to sense the position of the respective drive assembly interface element. The sensor senses, or determines, when that drive assembly interface element passes a threshold position, such as a pre-determined or known position along the extent of travel of that drive assembly interface element. Suitably the sensor comprises at least one of a magnetic sensor, such as a Hall sensor, a light sensor, a capacitive sensor, an inductive sensor, an acoustic sensor, and a microswitch. Any suitable position-determining sensor can be used. The sensor can be a proximity sensor. The sensor can be a position sensor associated with the drive assembly interface element. 
     The sensor, in the example schematically illustrated in  FIG. 9 , comprises two parts. A first part  912  of the sensor is provided in a body of the drive assembly. A second part  913  of the sensor is provided on the drive assembly interface element. As the drive assembly interface element is moved, the first part  912  moves relative to the second part  913 . The first part  912  and the second part  913  are configured to interact with one another. In one example the first part  912  comprises a magnetic sensor and the second part  913  comprises a magnet. The magnetic sensor is configured to sense the magnet. The magnetic sensor is configured to output a first signal when the magnet is proximal to the magnetic sensor and a second signal when the magnet is distal from the magnetic sensor. Suitably the magnetic sensor is configured, or calibrated, so that the output changes from the second signal to the first signal when the magnet, and hence the drive assembly interface element, is less than a predetermined distance from the magnetic sensor. Thus when the magnetic sensor outputs the first signal, it can be determined that the drive assembly interface element is adjacent the magnetic sensor. The first part  912  of the sensor is located in the drive assembly so that the drive assembly interface element is adjacent the first part  912  of the sensor when it is in the desired position. 
     The drive assembly comprises a communication unit  914  for communicating with the control unit  309 . The communication unit can be a wired and/or a wireless unit, and/or can couple the sensor, for example the first part  912  of the sensor, to the control unit  309  via a communication bus. Instead of or as well as the sensor determining the position of the drive assembly interface element and/or determining whether it is in its interfacing position, the processor is configured to receive signals, such as the first signal and the second signal, from the sensor and in dependence on the received signals to determine the position of the drive assembly interface element and/or to determine whether it is in its interfacing position. The processor can make such determinations in dependence on one or more of an algorithm and a reference table (such as a look-up table), which may be in a local memory  311  or remote memory. 
     Suitably where the sensor comprises a passive and an active part, the second part  913  comprises the passive part, such as the magnet, and the first part  912  comprises the active part, such as the magnetic sensor. The first part  912  can more easily be connected, for example by wires, to a power source and/or to the communication unit  914 . 
     In some examples there is some play or tolerance in the location or position of the drive transfer elements. The play or tolerance may be provided by a small flexibility or deformation in the material of the drive transfer elements. There might be some play or tolerance in the location of the drive transfer elements in a direction along the length of the main body, and/or transverse to this direction. There may be some play or tolerance in the location of the drive transfer elements perpendicular to the main body. This play or tolerance is suitably small compared to the extent of movement of the drive transfer elements, for example to maintain positional determinability of the drive transfer elements. The play or tolerance can be less than 1 mm, for example less than 0.5 mm or preferably less than 0.25 mm. 
     The play or tolerance can, in one example, be provided by the distance between the channels to either side of an aperture in the main body of the interface structure being slightly greater than the width of the extending portion that is arranged to slide within the channels. The play or tolerance can, in one example, be provided by the height of a channel to one side of an aperture in the main body of the interface structure being slightly greater than a height or thickness or the extending portion that is arrange to slide within the channel. As an example, where the distance between the channels and/or the height of the channel or channels exceeds the respective width and/or thickness of the extending portion by 0.2 mm, there is a play or tolerance of 0.2 mm provided. Other values for this play or tolerance are possible. 
     As described above, there are provided two lugs on one side of the interface structure. Similarly, two lugs can be provided on the other side of the interface structure, as illustrated in  FIG. 7 b   . Correspondingly the outer edge of the other side of the drive assembly interface can be provided with passages and a retention channel for receiving the lugs. In other examples a differing number of lugs can be provided on each inner side of the interface structure  700 . The numbers of lugs on each side need not be the same. Preferably there is at least one lug on each side of the interface structure, though in some examples a lug need only be provided on one side of the interface structure. Providing lugs on both sides can assist in retaining the interface structure on the arm. Such a retention can be more stable and/or effective where at least one lug is provided on each side of the interface structure. 
     Suitably the number of passages on the outer edges of the drive assembly interface  400  correspond to the number of lugs on the inner edges of the interface structure, though this need not be the case. The number of passages on the outer edges of the drive assembly interface  400  is suitably at least the same as the number of lugs on the corresponding side of the interface structure. 
     As mentioned above, the retention channel can be common to all passages. In other examples a retention channel can communicate with fewer than the total number of passages on the respective side of the drive assembly interface. Each retention channel can communicate with one or more passage. 
     In some examples, the passages and/or the retention channels can comprise raised portions over which the lugs pass as the interface structure is attached to the robot arm. The raised portions may comprise detents. Such raised portions can provide tactile feedback that the interface structure is properly or correctly attached. The raised portions can provide additional resistance to inadvertent removal of the interface structure from the robot arm. 
     With the arrangement described above, the interface structure  700  is arranged to be mounted to the drive assembly interface  400  by placing it onto the drive assembly interface and then sliding it towards the robot arm (i.e. generally towards the right in the orientation of  FIG. 4 ). 
     The interface structure  700  comprises a front face  760 . The interface structure comprises an indent  712  towards the front face. The front face is shaped to accommodate the indent. Similarly, the drive assembly interface  400  comprises a corresponding indent  412 . The drive assembly interface comprises a front face  460 . As the interface structure is attached to the drive assembly interface, the indent  712  of the interface structure will at least partially pass into the indent  412  of the drive assembly interface. As the interface structure is slid along the longitudinal axis  413  of the distal end of the arm, the inner side of the front face  760  of the interface structure will abut the front face  460  of the drive assembly interface. This can restrict the interface structure from being slid too far, and can help ensure that it is correctly or properly mounted to the drive assembly interface. The indent  712  in the interface structure can act, for example together with the indent  412  of the drive assembly, as an alignment feature to assist in the alignment of the interface structure and the drive assembly. 
     The indent  712  in the interface structure  700  can permit a more compact arrangement. The indent is shaped and configured, or sized, to receive the shaft  501  of the instrument when the instrument is attached to the interface structure. More particularly, the indent is shaped and configured, or sized, to receive a shaft attachment  610  located at the proximal end of the shaft  501  to the instrument interface  500 . The provision of the indent  712 , and the corresponding indent  412  in the drive assembly interface  400  permits the instrument to be mounted to the robot arm with the longitudinal axis  512  of the instrument shaft  501  closer to the longitudinal axis  413  of the distal end of the robot arm  404 . Preferably the instrument is mountable to the robot arm so that the longitudinal axis  512  of the instrument shaft  501  is collinear with the longitudinal axis  413  of the distal end of the robot arm  404 . 
     Another feature which can permit a more compact arrangement is the arrangement of the drive transfer elements on different planes. Referring again to  FIG. 7 a   , the second drive transfer element  707  is movable along a plane lower (in the perspective of the figure) than that in which the first and third drive transfer elements  706 ,  708  are movable. This offsetting of the drive transfer elements of the interface structure  700  permits a corresponding offsetting of the drive assembly interface elements and the instrument interface elements. Thus the drive assembly interface  400  and the instrument interface  500  can be configured to be more compact in a direction lateral to the direction in which the drive interface elements are movable when the interface structure is attached to the robot arm and the instrument. In other words, locating the central drive transfer element off-plane with respect to the outer drive transfer elements (in either direction) can permit the outer drive transfer elements (for example the axes of movement of the outer drive transfer elements) to be located closer together. This can result in a more compact arrangement. 
     The provision of the second, central, drive transfer element on a lower plane also assists in reducing the bending moment as the drive transfer element is driven, by bringing its axis of movement closer towards the axis of movement of the corresponding drive assembly interface element. 
     The retention features of the interface structure  700 , for example at least one of the first fastener and the lug, are shaped and/or configured such that when the surgical instrument is detached from the surgical robot arm, the interface structure is retained on the surgical robot arm. The interface structure can be engageable with the instrument interface by a second fastener (not shown). The force required to disengage the second fastener is less than the force required to disengage the first fastener and/or the lugs. The interface structure is more securely attached to the surgical robot arm than to the surgical instrument. Thus, the interface structure and the drape to which it is incorporated, remain attached to the surgical robot arm during instrument exchange. This is important in order to reduce the time taken to change instruments, since the interface structure does not need to be re-attached to the robot arm following detachment of an instrument. It is also important in order to reduce the likelihood of the drape tearing when changing instruments, which would cause the sterile operating environment to become contaminated with the non-sterile environment on the robot arm side of the drape. 
     The main body  704  of the interface structure  700  is rigid in the illustrated example. In other examples it need not be rigid. At least a portion of the main body  704  can be of a resilient and/or deformable material. At least a portion of the main body can be flexible. A portion of the main body can be a flexible material such as a fabric. A portion of the main body can be unconstrained. The resilience, flexibility and/or unconstrained nature of the portion of the main body can permit and/or accommodate relative movement between the drive transfer elements. 
     Suitably a portion of the main body between the apertures is resilient and/or deformable, for example flexible. Suitably the main body can be formed in whole or in part of a resilient and/or deformable material. The resilient and/or deformable material can comprise one or more of silicone, latex, vinyl, butyl, nitrile, neoprene, and a polymer. The resilient and/or deformable material suitably comprises a material with a low modulus and low hysteresis. The resilient and/or deformable material suitably comprises a material with a good strain to failure. 
     In another example, illustrated schematically in  FIG. 10 , the interface structure comprises one or more movable portions  1010 . The movable portion is flexible and/or elastic. For example, the movable portion is a material such as a fabric. Preferably the material is water-resistant to assist in providing the sterile barrier between the robot arm and the instrument. The material can be constructed of a plastic sheet, for example made of polyester, polypropylene, polyethylene or polytetrafluoroethylene (PTFE). The movable portion  1010  reduces the likelihood that the material of the interface structure ruckles and/or controls the extent to which the material of the interface structure ruckles, though it need not do this in all examples. The movable portion is arranged to control the manner in which material of the interface structure moves as the drive transfer elements  1001 ,  1002 ,  1003  move. This can permit control of, and/or reaction to, the tension within the material of the interface structure. 
     The first portion and/or the second portion is attached to the movable portion. In other examples, the first portion can be attached to one movable portion. The second portion can be attached to another movable portion. The flexible and/or elastic nature of the movable portion can assist in accommodating movement of the first and/or the second portions relative to the main body. 
     In the illustrated example, two reels  1011 ,  1012  are provided. Each reel is configured to hold and retain an amount of material. Material can be rolled onto one or both reels to take up slack in the material between the reels. Material can be rolled off one or both reels to relieve tension in the material between the reels. Material can be rolled onto or off the reels to accommodate movement of the drive transfer elements. 
     Referring to  FIG. 10 a   , the material between the reels moves to the left. This is, for example, because the drive transfer element attached to that material (not shown) is driven to the left by the drive assembly. As a drive assembly interface element to which that drive transfer element is engaged moves to the left, so will the material held by the drive transfer element. The right-hand reel  1011  will rotate clockwise, as indicted by the arrow, to feed material from the right-hand reel  1011 . This means that material between the drive transfer element and the right-hand reel  1011  is not exposed to a high tension that might otherwise cause a rupture in the material, and/or disrupt operation of the interface structure and/or the instrument interface. The left-hand reel  1012  can rotate anti-clockwise, as indicated by the arrow, to roll material onto the left-hand reel  1012 . This means that material between the drive transfer element and the left-hand reel  1012  does not become loose. Similarly, if the drive transfer element moves to the right, material will be fed from the left-hand reel  1012 . Material can be taken up by the right-hand reel  1011 . Either or both of the left-hand reel  1012  and the right-hand reel  1011  need not take up slack in the material. However, maintaining the material taut can assist in covering the aperture and in maintaining the sterile barrier. 
     Referring now to  FIG. 10 b   , where three drive transfer elements  1001 ,  1002 ,  1003  are provided adjacent one another, three pairs of reels are provided. This permits each of the three drive transfer elements to move independently of one another without such independent movement causing tension to increase in the material of the interface structure. For example, the provision of a pair of reels for each drive transfer element can reduce the extent to which the material between the reels, i.e. the movable portion, is exposed to tension, shear forces and/or rupture. This may be compared to an arrangement in which a single pair of reels is provided for a plurality of drive transfer elements, and the positioning of the material is based, for example, on an average such as a weighted average of the positions of the plurality of drive transfer elements. 
     In the illustrated example, an uppermost (in the orientation of  FIG. 10 b   ) drive transfer element  1001  is moved to the right (as indicated by the arrow), a middle drive transfer element  1002  is moved to the left (as indicated by the arrow) and a lower drive transfer element  1003  is moved to the right (as indicated by the arrow). A first right-hand reel  1013 , that of the uppermost section, takes up material of the movable portion and so has a greater reel diameter. A first left-hand reel  1014 , that of the uppermost section, feeds material of the movable portion from the reel and so has a smaller reel diameter. A second right-hand reel  1015 , that of the middle section, feeds material of the movable portion from the reel and so has a smaller reel diameter. A second left-hand reel  1016 , that of the middle section, takes up material of the movable portion and so has a greater reel diameter. A third right-hand reel  1017 , that of the lower section, takes up material of the movable portion and so has a greater reel diameter. A third left-hand reel  1018 , that of the lower section, feeds material of the movable portion from the reel and so has a smaller reel diameter. 
     It will be understood that where the number and/or arrangement of the drive transfer elements differs from the illustrated example, the number and/or arrangement of the pairs of reels can similarly differ. 
     Provision of a reel can assist in reducing the length of the interface structure compared to provision of rigid drive transfer elements. Provision of a reel can ensure that the sterile barrier is maintained whilst reducing the length of the interface structure needed. This is because the reel can take up material that might otherwise have projected past (overlapped) the end of the slot when the central portion is adjacent that end of the slot. 
     Material of the interface structure, such as the movable portion, can be taken up and/or fed from a reel by driving the respective reel about its axis. Material of the interface structure can be taken up and/or fed from a reel by resiliently biasing the respective reel about its axis. In one example each reel is resiliently biased and is also driven. 
     Resiliently biasing a reel can assist in keeping tension within the material of the interface structure consistent. When tension is lowered (by, for example, a drive transfer element moving towards the relevant reel), the biasing of the reel will cause the reel to rotate so as to take up material. When tension is increased (by, for example, a drive transfer element moving away from the relevant reel), the biasing of the reel will permit the reel to rotate to as to feed material from the reel. 
     The resilience of the resilient biasing can be determined to provide for a desired tension or range of tension in the material of the interface structure. The resilient biasing is, in one example, provided by a spring coupled to the respective reel. 
     Driving of the reels can be accomplished by coupling a motor, such as an electric motor, to each reel. Driving the reels can permit tension to be released and/or slack taken up at a desired speed. For example, driving the reels can permit tension to be released and/or slack taken up at a higher speed than might occur with resilient biasing. Driving the reels can permit tension to be controlled more accurately than by relying on resilient biasing, or on resilient biasing alone. 
     In one example, one of a pair of reels is coupled to a motor for driving that reel, and the other of the pair of reels is resiliently biased. The resilient biasing adapts to the tension in the material whilst the motor is driven so as to achieve a desired tension. This arrangement permits control of the tension in the material of the interface structure. 
     A first tension sensor  1021  (shown schematically in  FIG. 10 a   ) is coupled to the right-hand reel  1011 ,  1013 ,  1015 ,  1017 . The first tension sensor is configured to sense tension in the material between the drive transfer element and the right-hand reel. The first tension sensor is suitably coupled to a rotational axis of the right-hand reel. A second tension sensor  1022  (shown schematically in  FIG. 10 a   ) is coupled to the left-hand reel  1012 ,  1014 ,  1016 ,  1018 . The second tension sensor is configured to sense tension in the material between the drive transfer element and the left-hand reel. The second tension sensor is suitably coupled to a rotational axis of the left-hand reel. Tension sensed by either or both of the first tension sensor and the second tension sensor is used to determine how to drive either or both of the right-hand reel and the left-hand reel. In other words, either or both of the right-hand reel and the left-hand reel is controlled in dependence on tension sensed by either or both of the first tension sensor and the second tension sensor. 
     The provision of the first tension sensor and the second tension sensor can permit a comparison of the tension sensed by each of the first and second tension sensors. This comparison can be used to detect rupture or other damage in the material. For example, if the tension sensed at both of a pair of reels reduces as a drive transfer element moves, it can be determined that the material between the reels has ruptured. 
     In some examples, only one tension sensor need be provided for each of a pair of reels. 
     In an alternative configuration of the interface structure, the aperture in the main body can be a single aperture. In this configuration where a single drive transfer element is provided, it can engage with the main body of the interface structure as described above. Where two or more drive transfer elements are provided within a single aperture of the main body, the adjacent edges of the drive transfer elements can be provided with tongue and groove features to enable the drive transfer elements to engage with one another. This can assist in restricting fluid flow paths between the drive transfer elements. It can also therefore assist in maintaining the sterile barrier. 
     Taking as an example a configuration in which three drive transfer elements are provided, with the first drive transfer element being provided to one side, the third drive transfer element being provided to the other side, and the second drive transfer element being provided between the first and the third drive transfer elements, tongue and groove type engagements can be provided between the first and the second drive transfer elements and between the second and the third drive transfer elements. The first and third drive transfer elements can engage with the main body of the interface structure as described above. In this configuration, the first drive transfer element can comprise (on its side adjacent the second drive transfer element) one of a first tongue and a first groove. The second drive transfer element can comprise (on its side adjacent the first drive transfer element) the other of the first tongue and the first groove. The first tongue is engageable with the first groove. The second drive transfer element can comprise (on its side adjacent the third drive transfer element) one of a second tongue and a second groove. The third drive transfer element can comprise (on its side adjacent the second drive transfer element) the other of the second tongue and the second groove. The second tongue is engageable with the second groove. This arrangement can permit the first drive transfer element to slide along the second drive transfer element and the second drive transfer element to slide along the third drive transfer element. This approach thus allows relative movement between adjacent drive transfer elements whilst still restricting fluid flow paths between the drive transfer elements. 
     An example of such an arrangement is shown in  FIGS. 11 a  and 11 b   .  FIG. 11 a    schematically illustrates an end view of three drive transfer elements.  FIG. 11 b    schematically illustrates a perspective view of the three drive transfer elements of  FIG. 11 a   . The first drive transfer element  1101  comprises an engagement feature such as a lip (not shown) to engage with an edge of the aperture as described above. The first drive transfer element  1101  comprises the first groove  1104  on the side adjacent the second drive transfer element  1102 . The second drive transfer element comprise the first tongue  1105  on the side adjacent the first drive transfer element  1101 . The first tongue is engageable with the first groove so as to engage the first drive transfer element with the second drive transfer element. The second drive transfer element  1102  comprises the second tongue  1106  on the side adjacent the third drive transfer element  1103 . The third drive transfer element comprises the second groove  1107  on the side adjacent the second drive transfer element. The second tongue is engageable with the second groove so as to engage the second drive transfer element with the third drive transfer element. The other side of the third drive transfer element  1103  comprises an engagement feature such as a lip (not shown) to engage with an edge of the aperture as described above. Protrusions  1108  are also schematically shown (these have been omitted from  FIG. 11 b    for clarity). The protrusions are for engaging with recesses as described above. Recesses could instead be provided. Any combination of protrusions and recesses could be provided. 
     The outer boundary of the interface structure terminates in a sterile drape (not shown). The sterile drape shrouds the surgical robot arm. The inner boundary of the interface structure may terminate in a sterile membrane (not shown) which extends over the hollow interior to isolate the sterile environment from the non-sterile drive assembly. 
     The interface structure may be packaged with the drape, for example in a flat configuration. 
     Suitably, the interface structure  700  is fastened to the drive assembly as the robot arm is being shrouded in the sterile drape as part of the set-up procedure prior to the operation beginning. An instrument is subsequently fastened to the interface structure  700 . At some point during the operation, the instrument is exchanged for another instrument. A different instrument can then be attached to the interface structure. Providing the interface structure, and retaining the interface structure on the robot arm when removing an instrument means that instruments can be quickly and easily detached from and attached to the robot arm during an operation without exposing the patient to a non-sterile environment. 
     The instrument could be used for non-surgical purposes. For example, it could be used in a cosmetic procedure. The interface structure may be used for non-surgical purposes. The barrier provided by the interface structure can be a barrier to fluid flow and/or a barrier to particulate matter, for example particulate matter entrained in a flow of fluid such as air. 
     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 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 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.