Patent Publication Number: US-2020297197-A1

Title: Method and apparatus for instrument propulsion

Description:
TECHNICAL FIELD 
     Embodiments generally relate to propulsion tube units and propulsion devices for progressing instruments along passages, and associated methods of use. For example, the instruments may include, tools, sensors, probes and/or monitoring equipment for medical use (such as endoscopy) or industrial use (such as mining). The described embodiments may also be suitable for applications in other fields to progress an instrument along a passage. 
     BACKGROUND 
     There are a number of existing methods and apparatus for progressing instruments along passages including applications in mining and in medicine, such as endoscopy. There are a number of difficulties with progressing conventional endoscopic equipment along a tract or lumen in a patient, and these difficulties may carry associated risks of causing damage to the patient. 
     It is desired to address or ameliorate one or more shortcomings or disadvantages associated with existing propulsion devices for progressing instruments along passages, or to at least provide a useful alternative. 
     Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application. 
     Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. 
     SUMMARY 
     Some embodiments relate to a propulsion device for progressing an instrument along a passage, the propulsion device comprising: 
     an elongate tube comprising a first end and a second end opposite the first end, the tube defining a channel configured to accommodate a liquid, a first end of the channel being closed at or near the first end of the tube and a second end of the channel being defined by the second end of the tube; and 
     a pressure actuator in communication with the second end of the channel and configured to selectively adjust a pressure of the liquid in the channel to alternatingly:
         reduce the pressure to induce cavitation and form gas bubbles in the liquid; and   increase the pressure to collapse some or all of the gas bubbles back into the liquid, thereby accelerating at least part of the liquid towards the first end of the tube and transferring momentum to the tube to progress the tube along the passage.       

     Some embodiments relate to a propulsion tube unit comprising: 
     an elongate tube comprising a first end and a second end opposite the first end, the tube defining a channel configured to accommodate a liquid, a first end of the channel being closed at or near the first end of the tube and a second end of the channel being defined by the second end of the tube; and 
     a piston assembly connected to the second end of the tube, the piston assembly comprising:
         a body defining a bore in fluid communication with the channel of the tube; and   a movable piston disposed within the bore and configured to seal against an internal surface of the bore,       

     wherein the piston assembly and the tube cooperate to define a sealed vessel containing a selected mass of liquid and a selected mass of gas. 
     The piston assembly may be configured for cooperation with an actuator to effect movement of the piston to selectively adjust a pressure of the liquid in the channel to alternatingly: reduce the pressure to induce cavitation and form gas bubbles in the liquid; and increase the pressure to collapse some or all of the gas bubbles back into the liquid, thereby accelerating at least part of the liquid towards the first end of the tube and transferring momentum to the tube to progress the tube along the passage. 
     In some embodiments, the propulsion device or propulsion tube unit may comprise one or more mechanisms configured to promote cavitation in one or more regions of the channel when the pressure is reduced, wherein the one or more regions extend along at least part of a length of the channel. The one or more mechanisms may be configured to promote cavitation in a plurality of regions spaced along at least part of the length of the channel. The one or more mechanisms may comprise a surface variation on an internal surface of the channel. 
     The surface variation may comprises a coating. The coating may comprise a hydrophobic material. The coating may comprise a catalytic material. The coating may comprise one or more coatings selected from: octadecyltrichlorosilane, silane compounds, Parylene C, flouropolymers, PTFE (Teflon™), manganese oxide polystyrene (MnO2/PS), nano-composite zinc oxide polystyrene (ZnO/PS), nano-composite precipitated calcium carbonate, fluorinated acrylate oligomers, urethane, acrylic, polyvinylpyrrolidone (PVP), polyethylene oxide, combinations of hydroxyethylmethacrylate, and acrylamides, or other hydrophobic compounds. 
     The surface variation may comprise a topographical variation. The topographical variation may have a surface roughness in the range of about 0.1 μm to 500 μm, about 0.5 μm to 100 μm, or about 1 μm to 10 μm, for example. 
     The topographical variation may comprises a scratched or pitted surface. The topographical variation may define a plurality of V-shaped channels. A characteristic angle of the V-shaped channels may be in the range of about 10° to 90°, about 30° to 60°, or about 40° to 50°, for example. An average width of the V-shaped channels may be in the range of about 1 μm to 10 μm, or about 2 μm to 4 μm, for example. 
     The topographical variation may define a plurality of conical pits. A characteristic angle of the conical pits may be in the range of about 10° to 90°, about 30° to 60°, or about 40° to 50°, for example. An average width of the conical pits may be in the range of about 1 μm to 10 μm, or about 2 μm to 4 μm, for example. 
     The topographical variation may define a plurality of protrusions. An average height of the protrusions may be in the range of about 0.1 μm to 1 mm, about 1 μm to 500 μm, or about 10 μm to 100 μm, for example. An average width of the protrusions may be in the range of about 0.1 μm to 500 μm, about 0.5 μm to 100 μm, or about 1 μm to 10 μm, for example. An average distance between adjacent protrusions may be in the range of about 0.1 μm to 500 μm, about 0.5 μm to 100 μm, or about 1 μm to 10 μm, for example. 
     In some embodiments, the protrusions may comprise nanowires or hollow nanotubes which may be formed of materials such as carbon or silicon, for example. 
     For nanowire, the width of the protrusions may be in the range of about 10 nm to 500 nm, about 20 nm to 300 nm, or about 100 nm to 200 nm; the length or height of the protrusions  835  may be in the range of about 0.1 μm to 100 μm, about 1 μm to 50 μm, or about 10 μm to 20 μm; and the average spacing between protrusions may be in the range of about 10 nm to 10 μm, about 10 nm to 100 nm, or about 100 nm to 1 μm, for example. 
     For nanotubes, the width of the protrusions may be in the range of about 10 nm to 100 nm, about 10 nm to 50 nm, or about 20 nm to 40 nm; the length or height of the protrusions may be in the range of about 1 μm to 50 μm, about 5 μm to 30 μm, or about 10 μm to 20 μm; the pore size (or internal diameter) of the protrusions may be in the range of about 1 μm to 40 μm, about 5 μm to 30 μm, or about 10 μm to 20 μm; and the average spacing between protrusions may be in the range of about 10 nm to 10 μm, about 10 nm to 100 nm, or about 100 nm to 1 μm, for example. 
     The topographical variation may define a porous surface. The porous surface may comprise a foam, sintered material or other porous material, for example. An average pore size of the porous surface may be in the range of about 10 nm to 200 μm, about 20 nm to 250 nm, about 50 nm to 150 nm, about 10 μm to about 200 μm, or about 50 μm to about 100 μm, for example. The porous surface may comprise a layer of porous material. The thickness of the porous layer may be in the range of about 10 μm to 1 mm, or about 50 μm to 100 μm, for example. 
     The one or more mechanisms may comprise a variation in a thermal conductivity of a wall of the tube along the length of the channel. The thermal conductivity of the wall may vary along the length of the channel over a range of about 0.25 Wm −1  K −1  to 240 Wm-1 K-1. 
     The one or more mechanisms may comprise one or more acoustic transducers. The one or more of the acoustic transducers may be disposed within a wall of the tube. The one or more of the acoustic transducers may be disposed outside of a wall of the tube. An operating frequency of the acoustic transducers may be in the range of about 1 kHz to 100 kHz. A power associated with insonation energy directed to a lumen of the channel by the acoustic transducers may be in the range of about 10 mW to 100 mW. 
     In some embodiments, the propulsion device may be configured for progressing a medical instrument along a lumen within a patient. 
     In some embodiments, the channel may be a continuous enclosed channel extending from the first end of the tube to the second end of the tube. The tube may be reinforced against expansion or contraction due to internal pressure changes. The tube may be formed of a material suitable for sterilisation. 
     In some embodiments, the propulsion device may comprise a plurality of tubes according to any one of the described embodiments extending side by side. 
     In some embodiments, the pressure actuator may comprise a flexible membrane defining a sealed chamber and a driving mechanism configured to deform the flexible membrane to selectively adjust the pressure of the liquid in the channel. 
     In some embodiments, the pressure actuator may comprise a piston assembly including a moveable piston disposed within a bore of the piston assembly; and a driving mechanism configured to drive the piston of the piston assembly to selectively adjust the pressure of the liquid in the channel. The piston assembly may be connected to the tube to form a sealed tube unit containing the liquid, and the piston assembly may be removably coupleable to the driving mechanism. 
     Some embodiments relate to a propulsion tube unit comprising one or more of the tubes according to any one of the described embodiments; and 
     a piston assembly connected to the second end of the tube, the piston assembly comprising:
         a body defining a bore in fluid communication with the channel of each of the one or more tubes; and   a movable piston disposed within the bore and configured to seal against an internal surface of the bore.       

     Some embodiments relate to a propulsion tube unit comprising: one of the tubes according to any one of the described embodiments; and 
     a movable piston disposed within the channel at or near the second end of the tube and configured to seal against an internal surface of the channel. 
     In some embodiments, the piston assembly and the one or more tubes may cooperate to define a sealed vessel containing a selected mass of liquid and a selected mass of gas. The selected masses of liquid and gas may be chosen for a particular length and diameter of the tube. The liquid and gas may be held at a predetermined pressure not significantly higher than a typical channel pressure during operation. 
     Some embodiments relate to a drive console comprising: 
     a housing defining a socket configured to receive and engage a propulsion tube unit according to any one of the described embodiments; 
     an actuator configured to engage the piston; and 
     a controller configured to operate the actuator to move the piston to selectively adjust a pressure within the channel of the tube. 
     Some embodiments relate to a method of progressing an instrument along a passage, the method comprising selectively adjusting a pressure of a liquid within a tube connected to the instrument to successively induce cavitation of gas bubbles in the liquid and subsequently collapse the gas bubbles back into the liquid to accelerate the liquid within the tube, transfer momentum from the liquid to the tube, and progress the tube along the passageway. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Exemplary embodiments will now be described in detail with respect to the drawings, in which: 
         FIG. 1  is a schematic diagram of a propulsion device according to some embodiments; 
         FIGS. 2A to 2F  are a series of longitudinal sections of a portion of tube of a propulsion device showing a cycle of nucleation and cavitation of gas bubbles within a liquid contained within the tube, and subsequent collapse of the gas bubbles back into the liquid, according to some embodiments; 
         FIGS. 3A to 3G  are a series of longitudinal sections of a portion of tube of a propulsion device showing a cycle of nucleation and cavitation of gas bubbles within a liquid contained within the tube, and subsequent collapse of the gas bubbles back into the liquid, according to some embodiments; 
         FIG. 4  is a longitudinal section of a portion of tube of a propulsion device illustrating mechanisms for promoting bubble nucleation and/or coalescence in certain regions within the tube, according to some embodiments; 
         FIG. 5  is a longitudinal section of a portion of tube of a propulsion device illustrating mechanisms for promoting bubble nucleation and/or coalescence in certain regions within the tube, according to some embodiments; 
         FIG. 6  is a longitudinal section of a portion of tube of a propulsion device illustrating mechanisms for promoting bubble nucleation and/or coalescence in certain regions within the tube, according to some embodiments; 
         FIG. 7  is an illustration of a topographical surface variation for promoting bubble nucleation, according to some embodiments; 
         FIGS. 8A to 8E  show a series of illustrations of different types of protrusions for promoting bubble nucleation, according to some embodiments; 
         FIG. 9  is an illustration of a porous surface for promoting bubble nucleation, according to some embodiments; 
         FIGS. 10A to 10C  show a series of illustrations of different types of large scale topographical surface variations for enhancing momentum transfer between the liquid and the tube, according to some embodiments; 
         FIG. 11  shows exemplary displacement and velocity profiles illustrating the movement of a piston of a pressure actuator, according to some embodiments; 
         FIG. 12  shows an exemplary pressure cycle illustrating the pressure applied to the liquid in the tube, according to some embodiments; 
         FIGS. 13A and 13B  show cross-sections of two devices with a plurality of tubes illustrating different arrangements of the tubes, according to some embodiments; 
         FIG. 14  shows a schematic diagram of part of a propulsion device with a removable tube and piston assembly, according to some embodiments; 
         FIG. 15  shows a front panel of a drive console of the propulsion device of  FIG. 14 ; 
         FIG. 16  shows an endoscopic system including the propulsion device of  FIG. 14 , according to some embodiments; and 
         FIG. 17  shows a propulsion device with an alternative tube unit according to some embodiments. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments generally relate to propulsion devices for progressing instruments along passages, and associated methods of use. For example, the instruments may include, tools, sensors, probes and/or monitoring equipment for medical use (such as endoscopy) or industrial use (such as mining). The described embodiments may also be suitable for applications in other fields to progress an instrument along a passage. 
     Referring to  FIG. 1 , a propulsion device  100  is shown according to some embodiments. The propulsion device  100  comprises an elongate tube  110  defining a lumen or channel  120  configured to accommodate a liquid  130 , and a pressure actuator  140  configured to selectively adjust a pressure of the liquid  130  in the channel  120 , such as by varying the pressure, optionally varying the pressure continuously. 
     A first or distal end  122  of the channel  120  is closed at or near a first or distal end  112  of the tube  110 . The distal end  112  of the tube  110  is shown disposed in a channel or lumen  101  of a passage  103  in  FIG. 1 . 
     In some embodiments, the tube  110  may be configured to be inserted into and through a biological tract, such as a lumen  101  of a passage  103  of a patient. Examples of such biological tracts include the oesophagus, stomach, bowel, colon, small intestine, large intestine, duodenum, or any one or more passages of the gastro-intestinal system. In some embodiments, the tube  110  may be configured for insertion into and through another passage  103  in a patient, such as blood vessels, veins or arteries, for example. In some embodiments, the tube  110  may be configured for human medical applications or veterinary applications. In some embodiments, the tube  110  may be configured for industrial applications, such as for use in plumbing pipes, wall cavities, cable tracks, machinery, mining or wellbores, for example. 
     In some embodiments, the tube  110  may be configured to be accommodated within an insertion tube of an endoscope, and the insertion tube configured to be inserted into a passage  103 , such as a passage in a patient. An example of such an arrangement is illustrated in  FIG. 16 . The tube  110  of the propulsion device  100  may be accommodated within a propulsion tube channel (not shown) within the insertion tube. In some embodiments, the propulsion tube channel may be concentric or coaxial with the outer diameter of the insertion tube, and may extend along a central longitudinal axis of the insertion tube. In some embodiments, the propulsion tube channel may be radially offset from the central axis of the insertion tube. 
     The pressure actuator  140  is in communication with a second or proximal end  124  of the channel  120  at or near a second or proximal end  114  of the tube  110  opposite the distal end  112 . The channel  120  may comprise a continuous enclosed channel extending from the first end  112  of the tube  110  to the second end  114  of the tube  110 . 
     The pressure actuator  140  may comprise any suitable device configured to selectively adjust a pressure of the liquid  130  in the channel  120 , such as a reciprocating piston, for example. In some embodiments, the pressure actuator  140  may comprise a piston driven by a motor, such as a linear motor, controlled by a controller (not shown). 
     The pressure actuator  140  may be configured to gradually reduce the pressure within the channel  120  to induce cavitation and form gas bubbles in the liquid  130 , and then to suddenly increase the pressure to compress and collapse the gas bubbles back into the liquid  130 , thereby accelerating at least part of the liquid  130  towards the first end  112  of the tube  110 , such that momentum is transferred from the liquid to the tube  110  to progress the tube  110  along a passage. 
     In some embodiments, when the channel  120  is at an initial or base pressure, there may be volumes of gas and liquid  130  within the channel  120 , and the pressure actuator  140  may be controlled to increase the pressure to compress and dissolve part or all of the gas into the liquid  130 . In some embodiments, the channel  120  may be entirely filled with the liquid  130 , and the pressure actuator  140  may reduce the pressure to induce cavitation of gas out of the liquid  130 . In various embodiments, the base pressure may be set: at or near atmospheric pressure; significantly higher than atmospheric pressure; or significantly lower than atmospheric pressure. 
     The pressure actuator  140  may be configured to adjust the pressure in a repeating cycle to induce cavitation of gas bubbles out of the liquid  130  and subsequently compress part or all of the gas back into the liquid  130 . In various applications, the difference between the maximum pressure in the channel  120  and the minimum pressure in the channel  120  may be in the range of about 10 kPa to 100 MPa, about 10 kPa to 100 kPa, about 100 kPa to 1 MPa, about 1 MPa to about 10 MPa, or about 10 MPa to about 100 MPa, for example. In some embodiments, the maximum pressure may be above, below or close to atmospheric pressure. In some embodiments, the minimum pressure may be above, below or close to atmospheric pressure but having a non-zero difference from the maximum pressure. 
     For example, for gastro-intestinal applications, the channel pressure may vary from 100 kPa below atmospheric pressure to 1000 kPa above atmospheric pressure; for cardiovascular applications, the channel pressure may vary from 20 kPa below atmospheric pressure to 300 kPa above atmospheric pressure; for industrial applications, the channel pressure may vary from 1000 kPa below atmospheric pressure to 10000 kPa above atmospheric pressure. 
     The liquid  130  in the channel  120  may comprise any one or more of: a pure liquid, a solution, a gas/liquid solution (i.e., a gas dissolved in liquid), a mixture of gas and liquid, a mixture of liquid and solid particles, such as a suspension, and a mixture of two or more miscible or immiscible liquids, for example. In some embodiments, the volumetric ratio of gas to liquid at atmospheric pressure may be in the range of about 0.1% to 10%, about 0.5% to 5%, or about 1% to 2%, for example. 
     The liquid  130  may comprise any suitable liquids, gases, solid particles or solutions, such as: water, ethanol, carbon dioxide, nitrogen, air, nitric oxide, argon, salts, sodium chloride, potassium formate, acids, acetic acid, or lithium metatungstate, for example. 
     Different liquids may be suitable for different applications. For example, in medical applications, preferred liquids may be biocompatible, non-toxic (or have very low toxicity), non-pyrogenic, non-inflammatory, not highly osmotic, relatively inert, and be suitable for operation at relatively low pressures and at temperatures similar to the typical temperature of a patient. For example, water, ethanol, carbon dioxide, nitrogen, air, nitric oxide, argon. 
     In industrial applications where biocompatibility is not required, liquids with higher densities may be preferred, such as aqueous solutions of inert inorganic compounds, for example. One suitable high density liquid may be an aqueous solution of lithium metatungstate which has high density, low viscosity, and good thermal stability. 
     In various embodiments, the tube  110  may be formed of different materials depending on their suitability for a given application. For example, for medical applications, the tube  110  may be formed of a non-toxic material which is sufficiently flexible to bend around corners or turns in a passage within the body of a patient. 
     Some examples of materials which may be used to form the tube  110  in different applications include: polymers, plastics, polyethylene, high density polyethylene, polytetrafluoroethylene, vinyl, nylon, rubber, elastomers, resins or composite materials comprising textiles impregnated with polymers, elastomers or resins. Polymers containing voids (foams) in the internal structure may also be used to increase flexibility such as extruded polytetrafluoroethylene (ePTFE). Composite layering of these materials may also be used to increase strength maintain flexibility and resist internal pressure or kinking. 
     A wall  118  of the tube  110  should have a strength and thickness sufficient to withstand the expected range of pressure differentials for a given application. In some embodiments, the tube  110  or tube wall  118  may be reinforced to mitigate against expansion and/or contraction of the tube due to pressure changes. Any suitable reinforcing material may be used, such as high strength fibres or ultra-high molecular weight polyethylene, for example. 
     Referring to  FIGS. 2A to 2F , a segment of the tube  110  of the propulsion device  100  is shown according to some embodiments, illustrating the cavitation process in a series of diagrams. 
     Referring to  FIG. 2A , at an initial or base pressure, the channel  120  may be substantially or entirely filled with the liquid  130  with little or no gas within the channel  120 . (Although, in some embodiments, there may be a significant volume of gas present in the channel at the base pressure). 
     When the pressure in the channel  120  is gradually reduced by the pressure actuator  140 , gas bubbles  133  may begin to form in the liquid  130  within the channel  120 , as shown in  FIG. 2B . The gas bubbles  133  may comprise gas that was previously dissolved in the liquid  130  and/or vapour (i.e., a gas phase of the liquid  130 ). The bubbles  133  may form through homogeneous nucleation or through heterogeneous nucleation in the liquid  130  on nucleation sites, such as particles suspended in the liquid  130  and/or at nucleation sites on an inner surface  126  of the tube  110 . 
     As the pressure is reduced further, the bubbles  133  may grow in volume to form larger bubbles  133   c , as shown in  FIG. 2C , and new bubbles  133  may continue to be formed through nucleation. Some of the bubbles  133 ,  133   c  may coalesce to form even larger bubbles  133   d , as shown in  FIG. 2D . 
     Under certain conditions, the bubbles  133  may coalesce to form a large bubble  133   e  which spans a lumen of the channel  120 , as shown in  FIG. 2E . That is, the spanning bubble  133   e  may take up the entire lumen of the channel  120  in a region of the channel  120  such that different portions of the liquid  130  are separated on either side of the bubble  133   e . It may be desirable to encourage or promote the formation of such spanning bubbles  133   e  in the channel  120 , as this may enhance or increase the propulsive effect by increasing the acceleration of the liquid  130  during the sudden increase of pressure, and thus increasing the kinetic energy imparted to the liquid  130  and the momentum transferred to the tube  110 . 
     When the pressure is increased (i.e., during compression) the liquid  130  is accelerated in a distal direction (i.e., towards the first or distal end  122  of the channel  120 ), as indicated by arrows  201  in  FIG. 2F . Due to the relatively high compressibility of the gas bubbles  133 , which is orders of magnitude higher than the relatively low compressibility of the liquid  130 , the liquid  130  is allowed to accelerate quickly and compress the bubbles  133 , as shown in  FIG. 2F . 
     When the bubbles  133  are compressed, they experience a sudden increase in pressure and density, and collapse (i.e., dissolve and/or condense) back into the liquid  130 , as shown in  FIG. 2A . The rate of dissolution/collapse of the bubbles  133  into the liquid  130  may be increased by increasing the total surface area of the gas-liquid interfaces. Therefore, it may be desirable to encourage or promote the formation of many bubbles  133 , and preferably many spanning bubbles  133   e.    
     There are a number of ways in which the likelihood of the formation of spanning bubbles  133   e  may be increased, several of which are discussed below. For example, in some embodiments, one or more additives may be included in the liquid  130  to enhance bubble coalescence. In some embodiments, the internal diameter of the channel  120  may be selected to be relatively small so that only relatively small bubble volumes are required to span the lumen. However, the internal diameter of the lumen should still be large enough to allow the liquid  130  to flow along the channel  120  when the pressure is suddenly increased (i.e., not be too limited by capillary resistance). In some embodiments, the propulsion device  100  may comprise a plurality of tubes  110  extending side by side with each other, and each defining a channel  120 . This may allow the internal diameter of each channel  120  to be relatively small while maintaining a relatively high total mass of liquid  130  within the tubes  110 . 
     In some embodiments, cavitation, bubble nucleation and/or bubble coalescence may be enhanced, encouraged or promoted in certain regions of the channel  120 . 
     In some embodiments, the propulsion device  100  may comprise one or more mechanisms configured to promote cavitation, bubble nucleation and/or bubble coalescence in one or more regions of the channel when the pressure is reduced. The one or more regions may extend along at least part of a length of the channel  120 . For example, the one or more mechanisms may be configured to promote cavitation, bubble nucleation and/or bubble coalescence in a plurality of regions spaced along the length of the channel  120 . 
     In some embodiments, each region where cavitation is promoted may extend along part of the channel length by a distance of between about 10% and 400% of an internal diameter of the channel  120 , optionally about 30% and 300%, optionally about 50% and 200%. In some embodiments, a distance between adjacent regions where cavitation is promoted may be greater than the internal diameter of the channel  120  by a factor of about 2 to 50, about 5 to 30, or about 10 to 20, for example. 
     Referring to  FIGS. 3A to 3G , a segment of the tube  110  of the propulsion device  100  is shown according to some embodiments, illustrating the cavitation process in a series of diagrams. The cavitation process is similar to that described in relation to  FIGS. 2A to 2F ; however, the tube  110  shown in  FIGS. 3A to 3G  also includes one or more mechanisms  330  configured to promote cavitation, bubble nucleation and/or bubble coalescence in one or more regions of the channel  120  when the pressure is reduced. 
     Referring to  FIG. 3A , at the base pressure, the channel  120  may be substantially or entirely filled with the liquid  130 , with little or no gas within the channel  120 . 
     When the pressure in the channel  120  is gradually reduced by the pressure actuator  140 , gas bubbles  133  may begin to form in the liquid  130  within the channel  120 , as shown in  FIG. 3B . Some bubbles  133  may form randomly throughout the liquid  130 ; however, the likelihood of bubbles  133  forming will be higher in the regions of the cavitation-promoting mechanisms  330 . 
     As the pressure is reduced further, the bubbles  133  may grow in volume to form larger bubbles  133   c , as shown in  FIG. 3C , and new bubbles  133  may continue to be formed through nucleation. Some of the bubbles  133 ,  133   c  may coalesce to form even larger bubbles  133   d , as shown in  FIG. 3D . 
     The bubbles  133  may coalesce to form lumen-spanning bubbles  133   e  which span the entire diameter of a lumen of the channel  120 , as shown in  FIG. 3E . The formation of lumen-spanning bubbles  133   e  may be more likely in the regions of the mechanisms  330  due to a greater number or size of bubbles being formed and/or enhanced bubble coalescence. 
     When the pressure is increased, the liquid  130  is accelerated in a distal direction (i.e., towards the first or distal end  122  of the channel  120 ), as indicated by arrows  301  in  FIG. 3F , and the bubbles  133  are compressed and reduce in volume, as shown in  FIG. 3G . 
     When the bubbles  133  are compressed, they experience a sudden increase in pressure and density, and collapse (i.e., dissolve and/or condense) back into the liquid  130 , as shown in  FIG. 3A . 
     The mechanisms  330  may comprise any suitable means for enhancing, promoting, encouraging or increasing the likelihood of cavitation, bubble nucleation and/or bubble coalescence. 
     Referring to  FIG. 4 , in some embodiments, the one or more mechanisms  330  may comprise a variation in a thermal conductivity and/or thermal mass of a wall  118  of the tube along the length of the channel  120 . This variation in thermal conductivity and/or thermal mass could be achieved by including wall portions  430  at different locations along the length of the channel  120  comprising a material having a higher thermal conductivity and/or thermal mass than the rest of the wall  118 . For example, in some embodiments, the wall  118  may be formed of an extruded polymer, and metal particles could be impregnated in certain portions of the wall  118  to create the wall portions  430  of relatively higher thermal mass and thermal conductivity. 
     The difference in thermal conductivity and/or thermal mass between the wall portions  430  and the rest of the wall  118  may result in a higher likelihood of cavitation and bubble nucleation in the region of the wall portions  430  compared with the rest of the channel  120 . 
     In some embodiments, the thermal conductivity of the wall  118  may vary along the length of the channel over a range of about 0.25 Wm −1  K −1  to 240 Wm −1  K −1 . In some embodiments, the thermal conductivity of the wall portions  430  may be higher than the rest of the wall  118  by a factor of at least 10, at least 100, at least 500, or at least 1000. For example, in some embodiments, the thermal conductivity of the wall portions  430  may be in the range of about 100 Wm −1  K 1  to 300 Wm −1  K 1 , about 150 Wm −1  K −1  to 250 Wm −1  K −1 , or about 200 Wm −1  K −1 , while the thermal conductivity of the rest of the wall  118  may be in the range of about 0.1 Wm −1  K 1  to 10 Wm −1  K −1 , or about 0.5 Wm −1  K −1  to 1 Wm −1  K −1 . 
     Referring to  FIG. 5 , in some embodiments, the one or more mechanisms  330  may comprise one or more acoustic transducers  530 . The acoustic transducers  530  may be connected to a controller via one or more cables  535  and configured to emit acoustic energy with an amplitude and frequency which promotes cavitation, bubble nucleation and/or bubble coalescence. 
     The acoustic transducers  530  may be coupled to the external or internal surface of the tube  110 , disposed outside of the  118  wall of the tube  110 , or in some embodiments, may be disposed or embedded within the wall  118  of the tube  110 . In some embodiments, the acoustic transducers  530  may comprise piezoelectric patch transducers. 
     An operating frequency of the acoustic transducers  530  may be in the range of about 1 kHz to 100 kHz or about 10 kHz to 25 kHz, for example. The operating frequency of the acoustic transducers  530  may be selected to be higher than the Blake threshold for the mechanical nucleation of gas bubbles of at least 1 micrometre in systems with a gas saturation coefficient approaching 1 (i.e fully saturated). The threshold increases with increasing frequency and decreasing gas saturation (for reference, see Acoustic cavitation prediction, R. E. Apfel, The Journal of the Acoustical Society of America 69, 1624 (1981).) 
     Acoustic insonation energy may be directed into a lumen of the channel by the acoustic transducers  530  to promote, enhance or assist in inducing cavitation in the liquid  130 . In some embodiments, the characteristics of the insonation field may comprise: a pressure variation in the range of about 10 MPa to 100 Mpa, a pulse duration in the range of about 0.2 ms 10 ms, and a total power in the range of about 10 mW to 100 mW, for example. In some embodiments, the acoustic transducers  530  may be operated with a pressure of about 100 kPa, a displacement of about 25 μm and a frequency of about 21 kHz. 
     In some embodiments, the mechanisms  330  may comprise one or more lasers configured to induce cavitation in the liquid  130 . For example, in some embodiments, the mechanisms  330  may comprise microdiode laser modules embedded in the wall  118  of the tube  110 . The laser modules may be activated in a pulse of 10 ms to 20 ms duration to co-inside with the low pressure phase of the pressure cycle, to promote, enhance or assist in inducing nucleation of gas bubbles  133 . 
     In some embodiments, the mechanisms  330  may comprise one or more pairs of electrical conductors disposed within the lumen of the channel  120  and arranged with a close separation in the range of about 0.1 mm to 0.5 mm, such that an electrical current can discharge from one conductor to the other through the liquid  130  causing ionisation of the liquid  130  and subsequent gas nucleation. The conductive pairs may be arranged in a circular configuration and imbedded in the wall of the non-conductive polymer tube. The conductive pairs may be connected to an electrical power source via conductive wires running along the length of the tube  110 . The power source may comprise a high capacity, high voltage, low current discharge circuit which can be timed to discharge at the lowest point of the pressure cycle produced by the pressure actuator  140 . The supplied voltage may be in the range of about 100V to 200V. The current may be in the range of about 1 mA to 10 mA. 
     Referring to  FIG. 6 , in some embodiments, the one or more mechanisms  330  may comprise a surface variation  630  on the internal surface  126  of the tube  110 . That is, a surface variation portion  630  of the internal surface  126 , which is different to the rest of the internal surface  126  and configured to promote or encourage bubble nucleation. 
     In some embodiments, the surface variation  630  may comprise a coating applied to part of the internal surface  126  of the tube  110 . In some embodiments, the surface variation  630  may comprise a coating of a catalytic material, such as octadecyltrichlorosilane (for promoting CO 2  nucleation) or other similar compounds, for example. In some embodiments, the surface variation  630  may comprise a hydrophobic coating, such as silane (silicone hydride) compounds, Parylene C, or flouropolymers such as PTFE (Teflon™), manganese oxide polystyrene (MnO2/PS), nano-composite zinc oxide polystyrene (ZnO/PS), nano-composite precipitated calcium carbonate or fluorinated acrylate oligomers, for example. 
     In some embodiments, the rest of the internal surface  126  may be formed of or coated with a hydrophilic material, such as urethane, acrylic, polyvinylpyrrolidone (PVP), polyethylene oxide, combinations of hydroxyethylmethacrylate, or acrylamides, for example, or another material suitable for discouraging bubble nucleation on the rest of the internal surface  126  (i.e. away from the surface variations  630 ). 
     In some embodiments, the surface variation  630  may comprise a topographical variation. Relatively small topographical variations (e.g., at length scales in the order of 1 μm-100 μm) may provide nucleation sites to encourage or promote bubble nucleation and growth. For example, the topographical variation may comprise a change in surface roughness, a microporous surface, a scratched or pitted surface, a plurality of protrusions, projecting fibres, nanotubes, pits, channels, ridges, fins, recesses, cavities or other geometrical variation. The topographical variations may be formed by moulding, scratching, cutting, knurling, etching, abrasion, or impression, for example. In some embodiments, porous particulates such as ceramics may be embedded in the wall  118  of the tube  110  at the inner surface  126  to provide nucleation sites. 
     In some embodiments, the topographical variation may extend across the entire internal surface  126  of the tube  110 . In some embodiments, the tube  110  may be formed with the topographical variation extending across the entire internal surface  126 , and then certain portions of the internal surface  126  may be smoothed (for example, with a polymer coating), leaving the exposed/unsmoothed portions of the topographical variation to form the surface variations  630 . For example, the wall  118  of the tube  110  may be formed of a porous material, and then certain portions of the inner surface  126  may be sealed leaving the exposed/unsealed portions of the inner surface  126  to form the surface variations  630 . 
     The surface variations  630  may comprise any suitable topographical variations for a given application. A number of suitable topographical variations are described below. 
     In some embodiments, the topographical variation may define a plurality of V-shaped channels. The V-shaped channels may be aligned in parallel with each other, or may be randomly oriented and intersect each other. 
     A characteristic angle of the V-shaped channels (i.e., the angle of the apex of the V-shape) may be in the range of about 10° to 90°, about 30° to 60°, or about 40° to 50°, for example. An average width of the V-shaped channels may be in the range of about 1 μm to 10 μm, or about 2 μm to 4 μm, for example. An average depth of the V-shaped channels may be in the range of about 1 μm to 10 μm, or about 2 μm to 4 μm, for example. 
     Referring to  FIG. 7 , in some embodiments, a surface variation  730  may comprise a partially randomised pattern of intersecting V-shaped channels  737 . This may be achieved by abrasion using Diamond particulates with a nominal size of 2 μm. The Diamond particulates may have sharp V-shaped vertices and may be sintered on to a metal rod for rotary application to the internal surface  126 . The metal rod may be applied to the internal surface  126  with a rotary oscillation to produce the surface variations  730 . An Atomic Force micrograph of a typical random V-shaped scratch pattern achieved using this method is shown in  FIG. 7 . 
     In some embodiments, the topographical variation may define a plurality of conical pits. The conical pits may be arranged randomly or in a periodic array. 
     A characteristic angle of the conical pits (i.e., the angle of the apex of the conical pits) may be in the range of about 10° to 90°, about 30° to 60°, or about 40° to 50°, for example. An average width of the conical pits may be in the range of about 1 μm to 10 μm, or about 2 μm to 4 μm, for example. An average depth of the conical pits may be in the range of about 1 μm to 10 μm, or about 2 μm to 4 μm, for example. 
     In some embodiments, the topographical variation may define a plurality of protrusions. The protrusions may define any suitable shape and, in some embodiments, may define a plurality of different shapes. The protrusions may be arranged randomly or in a periodic array. 
     An average height of the protrusions may be in the range of about 0.1 μm to 1 mm, about 1 μm to 500 μm, or about 10 μm to 100 μm, for example. An average width of the protrusions may be in the range of about 0.1 μm to 500 μm, about 0.5 μm to 100 μm, or about 1 μm to 10 μm, for example. An average distance between adjacent protrusions may be in the range of about 0.1 μm to 500 μm, about 0.5 μm to 100 μm, or about 1 μm to 10 μm, for example. 
     Referring to  FIGS. 8A to 8E , some examples of surface variations  830  are shown according to some embodiments. The surface variations  830  each define a plurality of protrusions  835 . In some embodiments, the protrusions  835  may define fins or ridges  835  separated by channels  837 . 
     In some embodiments, the protrusions  835  may comprise nanowires or hollow nanotubes which may be formed of materials such as carbon or silicon, for example. For nanowire, the width of the protrusions  835  may be in the range of about 10 nm to 500 nm, about 20 nm to 300 nm, or about 100 nm to 200 nm; the length or height of the protrusions  835  may be in the range of about 0.1 μm to 100 μm, about 1 μm to 50 μm, or about 10 μm to 20 μm; and the average spacing between protrusions  835  may be in the range of about 10 nm to 10 μm, about 10 nm to 100 nm, or about 100 nm to 1 μm, for example. For nanotubes, the width of the protrusions  835  may be in the range of about 10 nm to 100 nm, about 10 nm to 50 nm, or about 20 nm to 40 nm; the length or height of the protrusions  835  may be in the range of about 1 μm to 50 μm, about 5 μm to 30 μm, or about 10 μm to 20 μm; the pore size (or internal diameter) of the protrusions  835  may be in the range of about 1 μm to 40 μm, about 5 μm to 30 μm, or about 10 μm to 20 μm; and the average spacing between protrusions  835  may be in the range of about 10 nm to 10 μm, about 10 nm to 100 nm, or about 100 nm to 1 μm, for example. 
     In some embodiments, the topographical variation may define a porous surface, such as a foam, sintered material or other porous material, for example. An average pore size of the porous surface may be in the range of about 10 nm to 200 μm, about 20 nm to 250 nm, about 50 nm to 150 nm, about 10 μm to about 200 μm, or about 50 μm to about 100 μm, for example. The porous surface may comprise a layer of porous material. The thickness of the porous layer may be in the range of about 10 μm to 1 mm, or about 50 μm to 100 μm, for example. 
     Referring to  FIG. 9 , a surface variation  930  comprising a porous layer  933  is shown according to some embodiments. The porous layer  933  may be formed of sintered particles  935  with diameters ranging from about 10 μm to about 100 μm, for example. 
     In some embodiments, the topographical variation may have a surface roughness in the range of about 0.1 μm to 500 μm, about 0.5 μm to 100 μm, or about 1 μm to 10 μm, for example. 
     In some embodiments, one or more additives may be included in the liquid  13 —to promote cavitation, bubble nucleation and/or bubble coalescence. For example, additives may be included to alter the density, viscosity, pH-level, gas solubility, coalescence characteristics or surface tension of the liquid  130 . 
     The solubility and coalescence characteristics of each fluid gas combination may be dependent on factors which may be controlled, such as temperature and pH. In the case of CO 2 , it is thought that the pH of the solution should ideally be adjusted to be between 6 and 6.5 for optimum effect. If the pH is above 6.5 it may be difficult to induce bubble nucleation due to the high solubility of the gas in water. In some embodiments, where CO 2  is used as the gas, the pH of the solution may be decreased to a level between 6 and 6.5 with the addition of acetic acid producing to promote nucleation and coalescence of CO 2  bubbles in the liquid. 
     In some applications, the mechanical work of the pressure actuator  140  acting on the liquid  130  may produce heating of the liquid  130 , which may decrease the solubility of the gas  133  in the liquid  130 . In some embodiments, the propulsion device  100  may include a heat sink (not shown) to draw excess heat away from the liquid  130 . For example, the heat sink may comprise a metal heat sink disposed at or near the proximal end  114  of the tube  110 , which may be disposed within or adjacent to the pressure actuator  140 . The heat sink may be cooled by air convection, refrigeration, or radiation. 
     In some embodiments, the thermal conductivity of the liquid  130  may be sufficient for heat to be transferred through the liquid  130  along the tube  110  to the heat sink. In some embodiments, salts such as Potassium Formate may be added to water to increase the thermal conductivity and density of the liquid  130  without significantly increasing the viscosity or boiling point. 
     In some embodiments, the thermal mass and conductivity of the tube  110  itself may be sufficient for heat to be transferred along the tube  110  to the heat sink. In some embodiments, the tube wall  118  may comprise one or more heat conductors, such as a metallic film or wire, to transfer heat along the tube  110  to the heat sink. 
     In some embodiments, the liquid  130  may comprise a particularly dense liquid and/or one or more additives may be included in the liquid  130  to increase the density or inertia of the liquid  130  in order to increase the momentum developed when the liquid  130  is accelerated and thus to increase the momentum transferred to the tube  110  to progress the tube  110  along the passage. 
     In some embodiments, such as for medical use, the liquid  130  may comprise water combined with one or more additives, such as: ethanol to reduce surface tension and viscosity; citric acid or acetic acid to reduce the pH-level; or salts such as sodium chloride to increase the density. 
     In some embodiments, the internal surface  126  of the tube  110  may define a relatively large scale topographical variation (for example, with length scales in the range of about 5% to 10% of the internal diameter of the tube  110 ) configured to enhance momentum transfer from the liquid  130  to the tube  110  during the sudden pressure increase. 
     Referring to  FIGS. 10A to 10C , segments of tube  110  are shown illustrating some examples of large scale topographical variations defined by the internal surface  126 , according to some embodiments. The internal surface  126  may define a plurality of periodic annular ridges  1010  swept back in a proximal direction (towards the second or proximal end  124  of the channel  120 ). The ridges  1010  appear as a swept fir tree pattern, or proximally swept teeth in cross-section as shown in  FIGS. 10A to 10C . 
     The proximally swept annular ridges  1010  may provide a fluid diode effect, whereby there is a greater resistance to fluid flow in the distal direction and relatively less resistance to fluid flow in the proximal direction. This effect may enhance momentum transfer from the liquid  130  to the tube  110  during the sudden pressure increase. 
     In some embodiments, the annular ridges  1010  may not be proximally swept, and a fluid diode effect may be achieved with a different type of topographical variation or, in some embodiments, not at all. 
     As described above, when the channel  120  accommodates a volume of liquid  130  and a separate volume of gas  133  in an initial or rest state, the pressure actuator  140  may be configured to increase the channel pressure to dissolve the gas  133  into the liquid  130  (this may be referred to as a pressure increase phase), and subsequently decrease the channel pressure to induce nucleation and cavitation of gas bubbles  133  in the liquid  130  (this may be referred to as a pressure decrease phase). Alternatively, when the channel  120  accommodates only the liquid  130  in the initial or rest state, the pressure actuator  140  may be configured to decrease the channel pressure to induce nucleation and cavitation of gas bubbles  133  in the liquid  130  (the pressure decrease phase), and subsequently increase the channel pressure to collapse the gas bubbles  133  (through condensation or dissolution) into the liquid  130  (the pressure increase phase). 
     In some embodiments, the pressure increase phase may be substantially similar in duration to the pressure decrease phase. In some embodiments, the duration of the pressure increase phase may be significantly shorter than the duration of the pressure decrease phase. 
     In some embodiments, the pressure actuator  140  may be configured to increase the pressure over a period of time which is between about 1% and 50% of a period of time over which the pressure is reduced, optionally between about 5% and 30%, optionally between about 10% and 20%, for example. 
     As described above, the pressure actuator  140  may comprise any suitable apparatus for varying the channel pressure in the manner described. In some embodiments, the pressure actuator  140  may comprise a flexible diaphragm with a mechanism configured to deflect or deform the diaphragm to change the volume of the system and control the channel pressure. In some embodiments, the pressure actuator  140  may comprise a reciprocating piston driven by a motor, such as an electric motor or linear motor, for example. 
     Referring to  FIG. 11 , an exemplary displacement profile x(t) and corresponding velocity profile v(t) are shown illustrating the movement of the pressure actuator  140 , in the form of a piston, over time, according to some embodiments. 
     The displacement and velocity profiles show a pressure increase phase  1110  followed by a pressure decrease phase  1120 . During the pressure increase phase  1110  (corresponding to a compression stroke of the piston), the piston undergoes a rapid acceleration  1112  which is made possible by the highly compressible nature of the gas bubbles  133 . 
     Once the gas bubbles  133  collapse back into the liquid  130 , there is a sudden deceleration  1114  of the piston due to the relatively incompressible nature of the liquid  130  (i.e., greatly less compressible than the gas  133 ). The sudden deceleration  1114  of the piston and liquid  130  results in a large impulse and transfer of momentum from the liquid  130  to the tube  110  and a consequent propulsive effect which acts to progress the tube  110  along the passage  103 . 
     After the deceleration  1114  of the piston, once the channel pressure has reached a maximum the pressure decrease phase  1120  begins as the piston is withdrawn. The withdrawal stroke (pressure decrease phase  1120 ) may be significantly slower than the compression stroke (pressure increase phase  1110 ) due to the time required for nucleation and cavitation of the gas bubbles  133  to occur. The channel pressure is then decreased to a minimum. The movement of the piston may then be repeated in a similar manner to repeat the pressure variation cycle. 
     The pressure actuator  140  may be configured to repeatedly increase and decrease the channel pressure to impart momentum to the tube  110  with multiple impulses, each impulse being associated with corresponding pressure increase phases. In some embodiments, the channel pressure may be varied by the pressure actuator  140  in a periodic or cyclic manner with a repeating pressure cycle (i.e., pressure increase followed by pressure decrease). In some embodiments, the pressure actuator  140  may be configured to vary the channel pressure according to a repeating pressure cycle with a frequency in the range of about 0.1 Hz to 10 Hz, about 0.5 Hz to 5 Hz, about 0.5 Hz to 1.5 Hz, about 2 Hz to 4 Hz, or about 3 Hz, for example. 
     In some embodiments, the pressure actuator  140  may be configured to operate in a reverse cycle to adjust the channel pressure to impart a reverse impulse to the tube  110  to move the instrument in a proximal direction. This reverse pressure cycle may be used to withdraw the instrument from the passage. 
     Referring to  FIG. 12 , an exemplary pressure/time profile is shown, according to some embodiments, illustrating the changes in channel pressure required to compress the gas bubbles  133  into the liquid  130  when the pressure is increased, and subsequently induce cavitation of gas bubbles  133  in the liquid  130  when the pressure is reduced. The pressure scale is shown in kilopascals (kPa) above atmospheric pressure and the time scale is shown in seconds. The channel pressure is reduced gradually over a period of about 0.3 s, and then suddenly increased over a period of about 0.05 s. This pressurisation cycle is repeated at a frequency of about 3 Hz. 
     As discussed previously, in some embodiments, it may be desirable for the channel  120  to be relatively small in order to increase the likelihood of spanning bubbles  133   e  forming before compression. The internal diameter of the channel  120  may be in the range of 0.1 mm to 10 mm, 0.1 mm to 1 mm, 0.1 mm to 0.5 mm, 1 mm to 7 mm, or 2 mm to 5 mm, for example. In some embodiments, the propulsion device  100  may comprise a plurality of the tubes  110  extending side by side as illustrated by the cross-sections of example tube configurations shown in  FIGS. 13A and 13B . 
     In some embodiments, the tubes  110  may be arranged around an instrument channel  1301  configured to receive a probe such as an endoscope, for example as illustrated in  FIG. 13B . In some embodiments, the tubes  110  may be arranged in a bundle for insertion into a lumen of a probe such as an endoscope, for example as illustrated in  FIG. 13A . In some embodiments, the tubes  110  may be integrally formed as part of a probe such as an endoscope, with instrument channels (e.g. video tract, lighting, irrigation, suction, steering, biopsy and other instrument channels) extending alongside the tubes  110 . 
     In some embodiments, the propulsion device  100  may comprise a first tube  110  within a second tube  110 , with the liquid  130  and gas  133  contained in an annular channel  120  defined between the two tubes  110 . An inner lumen of the first tube  110  may also contain liquid  130  and gas  133 , or alternatively, in some embodiments, the inner lumen of the first tube  110  may define an instrument channel. 
     The tube  110  or tubes  110  may be formed of a flexible material with sufficient strength and stiffness to withstand the expected forces for a given application. For medical applications, some suitable materials may comprise: high to ultra-high molecular weight polyethylene or other biocompatible polymers, for example. In some embodiments, the tube  110  or tubes  110  may be formed of composite materials, such as a polyethylene spiral with polyurethane and silicone elastomer coatings, for example. 
     The dimensions of the tubes  110  may vary for different applications. For example, for a medical endoscope, such as a gastro-intestinal endoscope, a single tube propulsion device may comprise a tube  110  with an external diameter of 8 mm and an internal diameter of 6 mm, or an external diameter of 6 mm and an internal diameter of 4.5 mm, whereas a multi-tube propulsion device may comprise 4 tubes  110 , each having an external diameter of 3 mm and an internal diameter of 2 mm. In some embodiments, the tube  110  of a single tube propulsion device  100  or the tubes  110  of a multi-tube propulsion device  100  may have an internal diameter in the range of 1 mm to 5 mm, for example, and an external diameter in the range of 0.5 mm to 15 mm, 1 mm to 10 mm, 2 mm to 8 mm or 4 mm to 6 mm, for example. The lengths of medical endoscopes are typically in the range of about 1 m to 5 m, or about 3 m to 4 m, for example. In some embodiments, such as for gastro-intestinal endoscopy, the tube(s)  110  may have a length in the range of 3 m to 4 m, 1 m to 5 m, or even greater than 5 m, such as 5 m to 15 m, or 7 m to 9 m, for veterinary applications, for example. In some embodiments, such as for arterial endoscopy, the tube(s) may have a length in the range of 0.5 m to 2 m, 0.7 m to 1.5 m or 0.9 m to 1.2 m, for example. In some embodiments, such as for industrial endoscopes, the dimensions of the tubes may be much larger. 
     For medical applications, it will usually be important for the propulsion device  100  to be sterile. To that end, it may be desirable for at least part of the device  100  to comprise a disposable component which can be provided in a sterile package and discarded after use. Referring to  FIG. 14 , a propulsion device  1400  is shown according to some embodiments. The propulsion device  1400  comprises generally similar features to those described in relation to propulsion device  100  and are referred to with like numbers. A pressure actuator  1440  and proximal end  1414  of a tube  1410  defining a channel  1420  are shown. It will be understood that the tube  1410  extends to a distal end (not shown) as described in relation to the propulsion device  100  of  FIG. 1 . The tube  1410  may be referred to as a propulsion tube, and may comprise similar features to tube  110  described above. In some embodiments, tube  1410  may comprise tube  110 , or a bundle of tubes  110  as described in relation to  FIG. 13A or 13B . 
     The pressure actuator  1440  comprises a housing  1442 , a driving mechanism  1444  (in the form of a motor), an actuation rod  1446  and a socket  1448  defined in a side of the housing  1442 . The pressure actuator  1440  further comprises a piston assembly  1450  comprising a body  1452  defining a cylinder  1454 , a piston  1456  disposed in the cylinder, and a piston seal  158  to seal the piston  1456  against an internal bore  1460  of the cylinder  1454 . The piston  1456  and cylinder  1454  act together to form a piston pump. However, in some embodiments, a different type of pump or compressor may be used to adjust the channel pressure in the tube  1410 , for example, a diaphragm pump, as described below in relation to  FIG. 17 . 
     The piston assembly  1450  is attached to the tube  1410  to form a tube unit  1401 . The tube unit  1401  may be manufactured and filled with a predetermined mass of liquid  130  and a predetermined mass of gas  133  sealed within the channel  1420  of the tube  1410  at a predetermined pressure. The tube unit  1401  may then be packaged and sterilised separately from the housing  1442  (including the socket  1448  and driving mechanism  1444 ) so that the housing  1442  can be resterilised and reused, while the tube unit  1401  can be manufactured and sterilised as a disposable unit to be discarded after use. 
     This arrangement may make it easier to sterilise the fluid  130 ,  133  and tube unit  1401  together rather than having to fill the tube  1410  with sterile fluid  130 ,  133  in a sterile environment such as an operating theatre. 
     The piston assembly  1450  is removably coupled to the housing  1442  (i.e., removable from the socket  1448 ). The socket  1448  may comprise an internal cylindrical wall  1486  that helps define the socket  1448  and accommodate the piston assembly  1450  in the socket  1448 . 
     The body  1452  defines a first opening  1462  and a second opening  1464  with the cylinder  1454  defining an open passage between the first and second openings  1462 ,  1464 . The proximal end  1414  of the tube  1410  is connected to the body  1452  of the piston assembly  1450  at the second opening  1464 , such that the channel  1420  is in fluid communication with the cylinder  1454 . The internal diameter or bore of the cylinder  1454  may be significantly larger than the internal diameter of the tube  1410  so that a relatively shorter stroke length is required to affect the desired pressure changes in the tube  1410 . For example, the ratio between the internal diameters of the tube  1410  and the cylinder  1454  may be in the range of 0.01 to 0.5, 0.05 to 0.4, 0.1 to 0.3, or 0.1 to 0.2. 
     The internal diameter of the cylinder  1454  may gradually taper down to the internal diameter of the tube  1410  at the second opening  1464 . In some embodiments, the second opening  1464  may be offset from a central axis of the body  1452 , and may be disposed at or near a top of the cylinder  1454  when the pressure actuator  1440  is disposed in a horizontal configuration. This may reduce the likelihood of gas bubbles, which may be formed in the cylinder  1454  during cavitation, being trapped in the cylinder, and instead, allow the bubbles to rise up towards the second opening  1464  and into the tube  1410  due to gravity. 
     The pressure actuator  1440  is configured to move the piston  1456  back and forth along the length of the cylinder  1454  to adjust the channel pressure, such as by varying the channel pressure in the tube  1410 . A compression stroke, or pressure increase stroke, moves the piston  1456  towards the tube  1410  and pushes fluid from the cylinder  1454  and into the tube  1410 , thereby increasing the channel pressure in the tube  1410 . A return stroke, or withdrawal or pressure decrease stroke, moves the piston  1456  away from the tube  1410  and allows fluid to flow back into the cylinder  1454  from the tube  1410 , thereby decreasing the channel pressure in the tube  1410 . 
     The motor  1444  and actuation rod  1446  are disposed in the housing  1442 , such that when the piston assembly  1450  is disposed in the socket  1448 , the actuation rod  1446  is aligned with the first opening  1462  of the body  1452  and can pass through the first opening  1462  to contact and move the piston  1456  within the cylinder  1454 . In some embodiments, the channel pressure within the tube  1410  may be sufficient to move the piston  1456  through the return stroke when the actuation rod  1446  is withdrawn from the cylinder  1454 . In some embodiments, the piston assembly  1450  may further comprise a biasing member  1470 , such as a spring, to bias the piston  1456  against the actuation rod  1446  and/or away from the tube  1410 , such that the piston  1456  is pushed back through the return stroke by the biasing member  1470  when the actuation rod  1446  is withdrawn from the cylinder  1454 . For example, the biasing member  1470  may comprise a stainless steel spring and/or a helical spring. In some embodiments, the actuation rod  1446  may be removably couplable to the piston  1456  itself to allow the actuation rod  1446  to pull the piston  1456  back as well as pushing the piston  1456  forward. 
     The piston assembly  1450  may further comprise a locking ring  1466  to restrict the piston  1456  from being removed from the cylinder  1454  through the first opening  1462 . In some embodiments, the driving mechanism  1444  may comprise one or more electromagnets configured to drive the piston  1456  directly rather than via a motor and actuation rod. 
     The body  1452  may further define one or more locking lugs  1468  configured to engage the socket  1448  to couple the piston assembly  1450  to the housing  1442 . The socket  1448  may also comprise one or more external flanges  1488  configured to engage the lugs  1468  to secure the piston assembly  1450  in the socket  1448 . In this way, the piston assembly  1450  is configured to be removably coupled to the housing  1442 , such that the piston assembly  1450  and tube  1410  can be manufactured together as a single disposable tube unit, while the housing  1442  and motor  1444  can be reused with a new tube unit for each new operation. The locking lugs  1468  may alternatively be referred to as tabs or radial projections, for example. 
     The tube unit may be assembled with liquid  130  and gas  133  disposed in the channel  1420  (either at atmospheric pressure or at a higher pressure depending on the application) and connected to the piston assembly  1450  to seal liquid  130  and gas  133  within the tube unit. In some embodiments, the body  1452  may be fixed to the proximal  1414  end of the tube  1410  and the piston  1456  subsequently placed in the cylinder  1454  and locked in with the locking ring  1466  to seal the liquid  130  and gas  133  in the channel  1420  and cylinder  1454 . The seal  1458  may comprise one or more gaskets such as o-rings, which may be seated in one or more corresponding gasket seats defined in the piston  1456 , or alternatively in the inner surface of the cylinder  1454 . 
     In some embodiments, the body  1452  of the piston assembly  1450  may include an inlet valve  1490  for filling cylinder  1454  and the channel  1420  of the tube  1410  with a predetermined mass of a selected liquid  130  and a predetermined mass of a selected gas  133 . The body  1452  may also include an outlet valve  1492  to allow air to be released from the channel  1420  and cylinder  1454  while they are being filled with the liquid  130  and gas  133 . 
     The valves  1490 ,  1492  may be located at one end of the body  1452  near the second opening and may be configured to maintain pressure within the cylinder  1454  and channel  1420 . In some embodiments, the valves  1490 ,  1492  may comprise spring plunger valves. The inlet valve  1490  may be located relatively nearer the second opening  1464  and the outlet valve  1492  may be located relatively farther from the second opening  1464 , as shown in  FIG. 14 . 
     To fill the tube unit  1401  with the gas  133  and liquid  130 , the body  1452  may be held upside down, or arranged with the valves  1490 ,  1492  disposed above the second opening, with most or substantially all of the volume of the channel  1420  and cylinder  1454  at a lower level than the outlet valve  1492 . This is to encourage excess air to rise towards the outlet valve  1492  when the channel  1420  and cylinder  1454  are being filled with the liquid  130 . The air may be sucked from the outlet valve  1492  via a vacuum line or other suction. 
     In some cases, the liquid  130  and gas  133  may be mixed together in a pressure vessel, such that the gas  133  is fully dissolved in the liquid  130  in a saturated solution, in which case the gas/liquid solution can be introduced to the tube unit  1401  via the inlet valve  1490  as the air is removed via the outlet valve  1492 . If the gas  133  and liquid  130  are to be introduced separately, it may be preferable to first remove as much air as possible from the channel  1420  and cylinder  1454  via the outlet valve  1492 ; before injecting the liquid  130  into the channel  1420  and cylinder  1454  via the inlet valve  1490 ; removing any remaining air via the outlet valve  1492 ; and then injecting the gas  133  into the channel  1420  and cylinder  1454  via the inlet valve  1492 . 
     Alternatively, the tube  1410  could be formed with an open distal end; the liquid  130  and gas  133  could be drawn along the channel  1420  and into the cylinder  1454  as the air is withdrawn from the cylinder  1454 ; and then the distal end of the tube  1410  could be closed with a plug and steel swage to hold the plug in the channel  1420  and seal the tube  1410 . However, it may be preferable to form the tube  1410  with a closed distal end to avoid having to close it with a plug or other means. 
     Once the tube unit is fully assembled with the liquid  130  and gas  133  sealed inside the channel  1420  and cylinder  1454 , the tube unit may be packaged and sterilised with gamma radiation, for example. Together, the tube  1410  and piston assembly  1450  may define a sealed vessel containing a selected mass of liquid  130  and a selected mass of gas  133 . In some embodiments, a gas tight closure may be fitted to the body  1452  of the piston assembly  1450  during packaging to close the first opening  1462  of the cylinder  1454  and to assist in maintaining a selected tube channel pressure until use. The body  1452  may comprise an engaging portion (not shown) defining one or more recesses, notches or projections to engage the closure and form a gas tight seal. 
     In some embodiments, the pressure actuator  1440  may comprise a diaphragm pump instead of a piston pump to control the channel pressure in the tube  1410 . Referring to  FIG. 17 , the propulsion device  1400  is shown with an alternative tube unit  1701 , comprising a diaphragm pump assembly  1750  instead of the piston assembly  1450  described above. In all other respects, the tube unit  1701  may be substantially similar to the tube unit  1401  described above with similar features indicated with like reference numerals. 
     The diaphragm pump assembly  1750  comprises a body  1752  defining a chamber  1754  extending between a first opening  1762  and a second opening  1764 , and a diaphragm  1770  which closes or covers the first opening  1762  of the chamber  1754 . The proximal end  1414  of the tube  1410  is connected to the body  1752  of the diaphragm pump assembly  1750  at the second opening  1764 , such that the channel  1420  is in fluid communication with the chamber  1754 . The body  1752  may further define one or more lugs  1768  configured to engage the flanges  1488  of the socket  1448  to couple the diaphragm pump assembly  1750  to the housing  1442 . 
     In some embodiments, the body  1752  of the diaphragm pump assembly  1750  may include an inlet valve  1790  and outlet valve  1792 , which may be configured in a similar manner to valves  1490  and  1492  as described in relation to tube unit  1401  and body  1452 . 
     The diaphragm  1770  may be formed separately and held in place over the first opening  1762  of the chamber  1754  by a clamp  1772 . For example, the clamp  1772  may comprise a threaded locking ring configured to threadedly engage the body  1752  thereby clamping a periphery of the diaphragm  1770  between the body  1752  and the clamp  1772 , as shown in  FIG. 17 . In other embodiments, the diaphragm  1770  may be integrally formed with the body  1752 , for example, using a composite moulding process. 
     The diaphragm  1770  comprises a resiliently deformable membrane which may be deformed by an actuator to change the volume of the chamber  1754  in fluid communication with the channel  1420  of the tube  1410 . A central portion  1774  of the diaphragm  1770  may be removably coupled to the actuation rod  1446  of the driving mechanism  1444 . The diaphragm  1770  includes a resiliently deformable portion  1776  surrounding the central portion  1774  allowing the central portion  1774  of the diaphragm to be moved back and forth relative to the body  1752  along an axis  1780  which is substantially normal (perpendicular) to a surface of the central portion  1774 . For example, parallel to or in alignment with the axial motion of the actuation rod  1446  of the driving mechanism or linear motor  1444 . 
     As the central portion  1774  of the diaphragm  1770  moves back and forth between a compressed position  1778   a  (shown in dashed lines) and a withdrawn position  1778   b  (shown in solid lines), the volume of the chamber  1754  is changed. Thus, the channel pressure in the tube  1410  can be adjusted and controlled by controlling the position of the actuation rod  1446  and central portion  1774  of the diaphragm  1770 . 
     The diaphragm  1770  may be round or rotationally symmetric, but could define any suitable shape for a resiliently deformable membrane. The chamber  1754  is illustrated as a cylinder in  FIG. 17 , but may define any suitable shape for providing the desired range of channel pressure. In some embodiments, the chamber  1754  may be relatively short and taper towards the second end  1764  allowing for a relatively wide diaphragm  1770  and relatively narrow diameter of the second opening  1764 , to allow a greater range of channel pressures for relatively little axial movement of the diaphragm. 
     In some embodiments, different tube units for different medical applications may be fitted with similar piston assemblies to allow each of the different tube units to be used with a common housing  1442  and motor  1444 . In some embodiments, a plurality of tubes  1410  may be connected to a single piston assembly  1450  with the channel  1420  of each tube  1410  being in fluid communication with the cylinder  1454  of the piston assembly  1450 . 
     In some embodiments, the housing  1442  may comprise a drive console or drive unit  1500  as shown in  FIG. 15 . The drive console  1500  may comprise a power switch  1502  to control the supply of power to the drive console  1500  from a power source  1560 . 
     The socket  1448  may comprise one or more circumferential flanges  1488  extending part way around a circumference of the socket and extending radially inward to retain the lugs  1468  of the body  1452  in the socket  1448 . The lugs  1468  are shown in dashed lines in  FIG. 15 , projecting radially away from the body  1452  to be accommodated within or under the flanges  1488 . The lugs  1468  also extend circumferentially around part of the body  1452 . 
     Both the lugs  1468  and flanges  1488  are arranged such that there are gaps between the flanges  1488  to allow passage of the lugs  1468  and gaps between the lugs  1468  to allow passage of the flanges  1488  when coupling or decoupling the piston assembly  1450  to or from the socket  1448 . To couple the piston assembly  1450  to the housing  1442 , the body  1452  is inserted into the socket  1448  with the lugs  1468  aligned with the gaps between the flanges  1488 , then the body  1452  is rotated to engage the lugs  1468  in a snug fit in a space defined between the flanges  1488  and a surface (not shown) of the housing  1442  that is opposed to and directly underlies the flanges  1488 . 
     In some embodiments, the lugs  1468  and/or flanges  1488  may comprise a resilient click lock, clip or latch to secure the body  1452  against rotation in the connected alignment with the lugs  1468  engaged with the flanges  1488 . The lugs  1468 , and/or flanges  1488  may also comprise a stopper to restrict rotation of the piston assembly  1450  beyond the angle at which the lugs  1468  are fully engaged with the flanges  1488 . 
     To decouple the piston assembly  1450  from the housing  1442 , the body  1450  is rotated to disengage the lugs  1468  from the flanges  1488  with the lugs  1468  aligned with the gaps between the flanges  1488 . Then the piston assembly  1450  can be removed from the socket  1448 . 
     In some embodiments, the body  1450  may comprise an indicator tab  1480  to indicate the correct orientation when coupling the piston assembly  1450  to the socket  1448 . The flanges  1488  may define a complimentary cut-out or recess  1482  configured to allow passage of the indicator tab  1480  when the piston assembly  1450  is correctly oriented for insertion into the socket  1448 . Once inserted into the socket  1448 , the body  1450  may be rotated, with the indicator tab passing under one or more of the flanges  1488 , until the lugs  1468  are fully engaged with the flanges  1488 . In some embodiments, the housing  1442  may comprise an indicia or marking to indicate the position of the indicator tab  1480  when the lugs  1468  are fully engaged with the flanges  1488 . 
     The drive console  1500  may comprise a connection indicator light  1504  configured to light up when the piston assembly  1450  is connected to the drive console  1500 . The drive console  1500  may comprise a sensor (not shown) to detect when the piston assembly  1450  is connected to the socket  1448  and/or when the lugs  1468  are fully engaged with the flanges  1488 . When the sensor detects connection of the piston assembly  1450  to the drive console  1500 , it may trigger a signal or complete an electrical circuit to turn on the connection indicator light  1504 . 
     The drive console may comprise an operating indicator light or running indicator light  1506  configured to light up when the pressure actuator  1440  is in operation. The indicator light  1506  may be included in or linked to an electrical circuit controlling the supply of power to the motor  1444 , such that the indicator light  1506  is turned on when the motor  1444  is in operation. 
     In some embodiments, the drive console  1500  may include a connection terminal  1508  configured to receive a connector of a signal cable from an external controller, such as a foot switch, for controlling operation of the pressure actuator  1440 . In some embodiments, the drive console  1500  may include a display or user interface  1510  to provide information to a user regarding operations of the propulsion device  1400  and/or to allow the user to control operations of the propulsion device  1400 . In some embodiments, the drive console  1500  may comprise a computer and/or controller  1550  configured to control operations of the propulsion device  1400 . 
     The computer  1550  may be connected to the user interface  1510  to provide information about the operations of the propulsion device  1400  and, in some embodiments, may receive inputs from the user interface to select certain operating parameters. The user interface  1510  may comprise an intelligent display graphic user interface, and the computer  1550  may comprise a programmable microprocessor to control functions of the drive console  1500  and driving mechanism  1444 . The power source  1560  may be connected to the drive console  1500  and computer  1550 , and the computer  1550  may control the supply of power to various components of the drive console  1500 . 
     Referring to  FIG. 16 , an endoscopic system  1600  is shown according to some embodiments. The endoscopic system  1600  comprises an endoscope  1601  having an insertion tube  1610  for insertion into a patient; an endoscope console  1620  for controlling operations of the endoscope; an endoscope handpiece  1630  for further and/or alternative control of operations of the endoscope  1601 ; a propulsion device  1400  for progressing the endoscope  1601  and insertion tube  1610  along a passage within a patient; and a power source (not shown) to supply power to the drive console  1500  and endoscope console  1620 . 
     The propulsion device  1400  comprises a propulsion tube  1410  for insertion into the insertion tube  1610  as described above and a drive console  1500  to control operations of the propulsion device  1400 . 
     The endoscopic system  1600  may further comprise a monitor  1640  configured to display images received from a camera of the endoscope via the endoscope console  1620 . 
     The propulsion device  1400  may be operated to provide a propulsive force to the endoscope  1601  and insertion tube  1610  via momentum transfer in the propulsion tube  1410 , as described above. The propulsive force may be used to progress the endoscope  1601 , insertion tube  1610  and propulsion tube  1410  along a passage within a patient. 
     As the momentum is transferred to the propulsion tube  1410  along its length, there may be a reduced risk of the insertion tube  1610  getting stuck or reduced resistance as it navigates turns of the passage (e.g. turns of the gastrointestinal tract), as can often occur with conventional push-type endoscopes. This method of propulsion may also reduce friction at each turn as the endoscope progresses along the passage, as it provides an alternative to simply pushing the endoscope against each turn to further progress the endoscope, as is done with conventional push endoscopes. 
     In some embodiments, the propulsion device  1400  may be able to progress the endoscope  1601  along the passage at advancement speeds of about 1.5 cm/s, for example. Depending on various operational circumstances, conditions, and/or requirements, the advancement speed may be varied in the range of 0.1 cm/s to 2 cm/s, or 0.5 cm/s to 1 cm/s, for example. In some applications, the time-pressure profile may be reversed to move the tube  1410  backwards along the passage, for example, to assist in withdrawing the tube  1410  from the passage. The propulsion device  1400  may also allow for an improved completion rate for intestinal endoscopy, by allowing the endoscope  1601  to be progressed further or entirely along the length of the intestines to allow the full extent of the small intestine to be examined. The propulsion device  1400  may also allow access to the entire gastro-intestinal tract via endoscopy. 
     In various embodiments, the propulsion device  100 ,  1400 ,  1700  may be configured for progressing along a passage any one or more of: an instrument, probe, sensor, camera, monitoring device, tool, surgical tool, mining tool, drilling tool, endoscope, enteroscope, duodenoscope, borescope, robot tether, and industrial endoscope, for example. The propulsion device  100 ,  1400 ,  1700  may be configured to assist in progressing an instrument, sensor or tool along any one or more of: a passage, mine shaft, well bore, pipe, sewer, wall cavity, and passage in a patient, such as a lumen of a biological passage, artery or tract. 
     It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.