Patent Publication Number: US-10772487-B2

Title: Method and apparatus for advancing a probe

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/990,931, filed Jan. 24, 2011, which is a National Stage application of PCT Application No. PCT/AU2009/000555, filed on May 5, 2009, which claims priority to Australian Patent Application No. 2008902195, filed on May 5, 2008. 
    
    
     TECHNICAL FIELD 
     Described embodiments relate to methods and apparatus for use in advancing a probe. In particular, embodiments may be used for advancing a probe across a surface or within a tract, such as biological tract. 
     BACKGROUND 
     It can be difficult to explore tracts, tight spaces or areas not readily accessible to a person. This is particularly so where adequate control of advancement of a probe can be problematic. For example, intestinal tracts are often relatively long and form a convoluted path, which is difficult for a probe to traverse without the aid of some form of device assisting the advancement of the probe. 
     Tracts such as intestinal and vascular tracts may be beneficially explored using a probe for medical purposes. 
     It is desired to address or ameliorate one or more shortcomings or disadvantages associated with existing methods and/or apparatus for advancing probes, or to at least provide a useful alternative thereto. 
     SUMMARY 
     Some embodiments relate to apparatus comprising:
         an elongate flexible tube sized to be received within a tract and having a proximal end and a distal end;   a drive mechanism coupled to the proximal end of the tube; and   a liquid column extending from the proximal end to the distal end;   wherein the drive mechanism is configured to cause movement of the liquid column within the tube to impart forward momentum to the tube and thereby promote advancement of at least the distal end of the tube within the tract when at least the distal end is received within a part of the tract.       

     The liquid column may be part of a liquid volume enclosed by the tube and drive mechanism. The tube may have periodic perturbations formed on an external surface of the tube along at least part of the distal end. The periodic perturbations may extend circumferentially around the tube and may have a radial variance of a same order of magnitude as a radial thickness of a wall of the tube. 
     An external surface of the tube may be contoured to enhance resistance to movement of the tube in a reverse direction. An internal surface of the tube may be contoured to enhance resistance to movement of the column through the tube in the forward direction. The external and internal surfaces of the tube (i.e. periodic perturbations) may be formed in a proximally swept fir tree pattern. Internal periodic perturbations may be formed along at least a section of the tube that is distal of the proximal end. 
     A liquid of the liquid column may have a density of about the same as or greater than the density of water, so that the liquid compresses minimally when the liquid column is acted upon by the drive mechanism. 
     The drive mechanism may be configured to impart a specific speed profile to a proximal end of the liquid column to enhance forward movement of the tube within the tract. The speed profile may comprise one or more of:
         a gradual acceleration portion at a first part of a forward movement of the liquid column;   a sharp deceleration portion at a second part of the forward movement of the liquid column following the first part of the forward movement;   a sharp acceleration portion at a first part of a rearward movement of the liquid column; and   a gradual deceleration portion at a second part of the rearward movement of the liquid column following the first part of the rearward movement.       

     The drive mechanism may comprise a piston and a drive member, such as a shaft, configured to cause repeated advancement and retraction of the liquid column within the tube. The drive mechanism may be configured to cause the piston to sharply decelerate toward the end of each stroke of the piston and/or to sharply accelerate away from the end of each stroke of the piston. 
     The apparatus may further comprise a flexible membrane within the tube at the distal end for enclosing a distal end of the fluid column. The distal end of the tube may house a compressive fluid volume (e.g. air or another low density inert gas) bounded by the tube, the flexible membrane and another membrane positioned distally of the flexible membrane. The other membrane may also be flexible, with both membranes being elastically deformable in response to advancement of the liquid column. 
     An internal diameter of the tube may narrow in the distal direction. This narrowing may be stepped and/or gradual. This narrowing may assist in minimising loss of pressure in the liquid column towards the distal end while the drive mechanism moves the liquid column. The tube wall may be reinforced by some form of reinforcing means to help the tube resist expanding or collapsing in response to pressure differences created by the action of the drive mechanism. 
     A probe may be located at the distal end of the tube. The probe may house an imaging device for capturing images of an area in front of the probe. A plurality of conduits may extend along the tube and be coupled to the probe, for example to send and/or receive signals to and/or from the probe. The conduits may be disposed within the tube along at least part of the tube. At least one of the conduits may extend in a spiral along at least part of the tube. In some embodiments, a secondary lumen may extend within a primary lumen defined by the tube and one or more of the conduits may extend within the secondary lumen along at least part of the tube. In some embodiments, one or more of the conduits may be embedded within the tube wall along at least part of a length of the tube. 
     The tract within which the tube is sized to extend may be a digestive tract or a vascular tract, for example. Alternatively, the tract may be a non-biological structure or area, such as a pipe, conduit, container or other structure that may be difficult or dangerous for a person to access and/or inspect. 
     Further embodiments relate to a method of advancing a probe, the method comprising:
         positioning a distal end of an elongate flexible tube at least partly within a lower end of a tract, the tube being sized to be received within the tract and having a liquid column extending from a proximal end of the tube to the distal end, wherein the probe is located at the distal end of the tube; and   operating a drive mechanism to cause advancement of the liquid column within the tube to impart forward momentum to the tube and thereby promote advancement of at least the distal end of the tube within the tract.       

     The operating may comprise imparting a specific speed profile to a proximal end of the liquid column to enhance forward movement of the tube within the tract. The speed profile may comprise at least one of:
         a gradual acceleration portion of a first part of a forward movement of the liquid column;   a sharp deceleration portion of a second part of the forward movement of the liquid column following the first part of the forward movement;   a sharp acceleration portion of a first part of a rearward movement of the liquid column; and   a gradual deceleration portion at of a second part of the rearward movement of the liquid column following the first part of the rearward movement.       

     The operating may comprise operating a piston and a drive shaft to cause repeated advancement and retraction of the liquid column within the tube. The operating may cause the piston to sharply decelerate toward the end of each stroke of the piston (i.e. just prior to the point of maximum stroke). The operating may cause the piston to sharply accelerate away from the end of each stroke of the piston (i.e. just after the point of maximum stroke). 
     The method may further comprise providing contours along the outside of the tube to resist movement of the tube in a reverse direction within the tract, and may comprise providing contours along the inside of the tube to resist movement of the liquid column through the tube in a distal direction. 
     The probe may comprise an imaging device, and the method may further comprise capturing images within the tract using the imaging device. The method may further comprise transmitting image data corresponding to the captured images to a system configured to process and display the images. Conduits, including at least one electrical conduit, may extend along the tube to perform at least one of sending and receiving signals to and from the probe, and the transmitting may be performed using the at least one electrical conduit. 
     Some embodiments relate to an advancement method comprising inducing reciprocating movement of a liquid column extending within an elongate member from one end of the member to an opposite end of the member to impart forward movement of the member along a length of the elongate member. 
     Some embodiments relate to apparatus comprising a probe positioned at one end of an elongate member and a drive mechanism at an opposite end of the elongate member, the elongate member housing a liquid column extending from the one end to the opposite end, wherein the drive mechanism causes reciprocating movement of the liquid column within the elongate member to impart forward movement to the probe. 
     Some embodiments relate to a replaceable self-advancing tube assembly comprising an elongate flexible tube, a liquid chamber disposed at a proximal end of the tube and a probe disposed at a distal end of the tube, the tube having a liquid column extending between the liquid chamber and the distal end. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments are described in detail below, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic block diagram of a system for use in advancing a probe within a tract; 
         FIG. 2  is a graph of an illustrative speed profile to be imparted to a liquid column; 
         FIG. 3  is a schematic representation of advancement apparatus to be used to advance a probe; 
         FIG. 4A  is a schematic side-sectional view of a proximal portion of a tube forming part of the advancement apparatus of  FIG. 3 ; 
         FIG. 4B  is schematic side-sectional view of a distal part of the advancement apparatus of  FIG. 3 ; 
         FIG. 5A  is a schematic representation illustrative of a flexible membrane positioned toward a distal end of the advancement apparatus, with the membrane shown in a relaxed position; 
         FIG. 5B  is a schematic diagram illustrative of the membrane of  FIG. 5A , with the membrane shown in a deformed position; 
         FIG. 6  is a schematic diagram of a system for advancing a probe according to some embodiments; 
         FIG. 7A  is a partial side sectional view of a tube according to some embodiments; 
         FIG. 7B  is a cross-sectional view of the tube of  FIG. 7A , taken along line  7 - 7 ; 
         FIG. 8A  is a side view of a tube according to some embodiments; 
         FIG. 8B  is a cross-sectional view of the tube of  FIG. 8A , taken along line  8 - 8 ; 
         FIG. 9A  is a side view of a tube according to some embodiments; 
         FIG. 9B  is a cross-sectional view of the tube of  FIG. 9A , taken along line  9 - 9 ; 
         FIG. 10A  is a side view of a tube according to some embodiments; 
         FIG. 10B  is a cross-sectional view of the tube of  FIG. 10A , taken along line  10 - 10 ; 
         FIG. 11A  is a side view of a tube according to some embodiments; 
         FIG. 11B  is a cross-sectional view of the tube of  FIG. 11A , taken along line  11 - 11 ; 
         FIG. 12A  is a partial side sectional view of a tube according to some embodiments; 
         FIG. 12B  is a cross-sectional view of the tube of  FIG. 12A , taken along line  12 - 12 ; 
         FIG. 13A  is a partial side sectional view of a tube according to some embodiments; 
         FIG. 13B  is a cross-sectional view of the tube of  FIG. 13A , taken along line  13 - 13 ; 
         FIG. 13C  is an alternative cross-sectional view of the tube of  FIG. 13A , taken along line  13 - 13 ; 
         FIG. 14A  is a partial side sectional view of a tube according to some embodiments; 
         FIG. 14B  is a cross-sectional view of the tube of  FIG. 14A , taken along line  14 - 14 ; 
         FIG. 15A  is a partial side sectional view of a tube according to some embodiments; 
         FIG. 15B  is a cross-sectional view of the tube of  FIG. 15A , taken along line  15 - 15 ; 
         FIGS. 16A and 16B  are schematic representations of a piston moving within a chamber according to some embodiments of a drive mechanism; 
         FIGS. 17A and 17B  are schematic representations of a piston moving within a chamber according to some embodiments of a drive mechanism; 
         FIG. 18  is a schematic representation of a piston acting on a flexible membrane of a fluid chamber according to some embodiments of a drive mechanism; 
         FIG. 19  is a schematic representation of a piston of circular cross-section that is eccentrically rotatable to displace a membrane of a fluid chamber according to some embodiments of a drive mechanism; 
         FIG. 20  is a schematic representation of a fluid chamber having a piston movable within the chamber under the control of electromagnetic elements, according to some embodiments of a drive mechanism; 
         FIG. 21  is a schematic representation of a distal biasing chamber according to some embodiments; 
         FIG. 22  is a schematic representation of a distal biasing chamber according to some embodiments; 
         FIG. 23  is a schematic representation of a distal biasing chamber according to some embodiments; 
         FIG. 24  is a schematic representation of a distal biasing chamber according to some embodiments; 
         FIG. 25  is a schematic representation of a distal biasing chamber according to some embodiments, shown in an uncompressed state; 
         FIG. 26  is a schematic representation of the distal biasing chamber of  FIG. 25  in a compressed state; 
         FIG. 27  is a schematic representation of a distal biasing chamber according to some embodiments, shown in an uncompressed state; 
         FIG. 28  is a schematic representation of the distal biasing chamber of  FIG. 27  in a compressed state; 
         FIG. 29  is a schematic representation of a distal biasing chamber according to some embodiments; 
         FIG. 30  is a schematic representation of a distal biasing chamber according to some embodiments; 
         FIG. 31  is a schematic representation of a distal biasing chamber according to some embodiments; 
         FIG. 32  is a schematic representation of a distal biasing chamber according to some embodiments, shown in an uncompressed state; 
         FIG. 33  is a schematic representation of the distal biasing chamber of  FIG. 32  in a compressed state; 
         FIG. 34  is a schematic representation of a distal biasing chamber according to some embodiments, shown in an uncompressed state; 
         FIG. 35  is a schematic representation of the distal biasing chamber of FIG.  34  in a compressed state; 
         FIG. 36  is a schematic representation of a distal biasing chamber according to some embodiments; 
         FIG. 37A  is a partial side-sectional view of part of a tube according to some embodiments, showing periodic perturbations along an external surface of the tube; 
         FIG. 37B  is a partial side-sectional view of part of a tube according to some embodiments, showing periodic perturbations along an external surface of the tube; 
         FIG. 38A  is a partial side-sectional view of part of a tube according to some embodiments, showing periodic perturbations along an external surface of the tube; 
         FIG. 38B  is a partial side-sectional view of part of a tube according to some embodiments, showing periodic perturbations along an external surface of the tube; 
         FIG. 39A  is a partial side-sectional view of part of a tube according to some embodiments, showing periodic perturbations along an external surface of the tube; 
         FIG. 39B  is a partial side-sectional view of part of a tube according to some embodiments, showing periodic perturbations along an external surface of the tube; 
         FIG. 40A  is a partial side-sectional view of a tube according to some embodiments, showing periodic perturbations along an internal surface of the tube; 
         FIG. 40B  is a partial side-sectional view of a tube according to some embodiments, showing periodic perturbations along an internal surface of the tube; 
         FIG. 41A  is a partial side-sectional view of a tube according to some embodiments, showing periodic perturbations along an internal surface of the tube; 
         FIG. 41B  is a partial side-sectional view of a tube according to some embodiments, showing periodic perturbations along an internal surface of the tube; 
         FIG. 42A  is a partial side-sectional view of a tube according to some embodiments, showing periodic perturbations along an internal surface of the tube; 
         FIG. 42B  is a partial side-sectional view of a tube according to some embodiments, showing periodic perturbations along an internal surface of the tube; 
         FIG. 43A  is a partial side-sectional view of a part of a tube according to some embodiments, showing periodic perturbations along both the internal and external surfaces of the tube; 
         FIG. 43B  is a partial side-sectional view of a part of a tube according to some embodiments, showing periodic perturbations along both the internal and external surfaces of the tube; and 
         FIG. 44  is a partial side-sectional view of a part of a tube according to some embodiments, showing periodic perturbations formed along internal and external surfaces of a tube in sections that are spaced along the tube. 
     
    
    
     DETAILED DESCRIPTION 
     The described embodiments relate generally to methods and apparatus for use in advancing a probe. As different kinds of probes may be used with the described embodiments, this description will focus primarily on apparatus and methods for advancing the probe within a tract, passage or area. The described methods and apparatus employ an elongate flexible tube defining a lumen and sized to be received within the tract, passage or area and having a proximal end and a distal end. A drive mechanism is coupled to the proximal end of the tube and a liquid column extends within the lumen from the proximal end to the distal end of the tube. The drive mechanism is configured to cause movement of the liquid column within the tube to impart forward movement to the tube, which promotes advancement of at least the distal end of the tube within the tract, passage or area when at least the distal end is supported by a part of the tract, passage or area. 
     Generally, the movement of the liquid column within the lumen imparts momentum to the inner wall of the tube along most of the length of the tube by friction and/or turbulence. For example, for a tube of about 3 metres in length, the movement of the liquid column within the tube will impart some movement to the tube relative to an underlying surface or passage along most of the 3 metre length of the tube, except for those sections close to the drive mechanism or not supported by the underlying surface of passage. 
     As used herein, the terms “proximal” and “distal” are intended to have relative positional meanings. Generally, the term “distal” is intended to indicate a position or direction generally toward an end of the tube which is to be advanced within the tract ahead of the rest of the tube. The term “proximal” is intended to indicate a position or direction generally opposite to that of “distal” and may indicate a position or direction toward an end of the tube to which the drive mechanism is coupled. The described embodiments are generally concerned with advancement of the probe in a distal direction. 
     Referring in particular to  FIG. 1 , a system  100  for advancing a probe  160  is described in further detail. System  100  comprises advancement apparatus  110  responsive to a control module  115  to advance the probe  160  within a tract  180  or other area when the probe  160  is placed within the tract  180  or other area. 
     Advancement apparatus  110  comprises a drive mechanism  130  coupled to a proximal end  142  of an elongate flexible tube  140 . The tube has a distal end  144  at which the probe  160  is located. Drive mechanism  130  is responsive to control signals received from control module  115  to operate some form of drive means, such as a drive shaft that drives a piston, to cause reciprocating (back and forth) movement of a liquid column  156  within the tube  140 . 
     Flexible tube  140  defines a primary internal lumen  141  within which liquid column  156  extends. This primary lumen  141  extends from adjacent drive mechanism  130  to distal end  144  and the liquid column  156  extends substantially the full length of lumen  141 . The liquid column  156  may not extend right to the probe  160  in order to allow for a distal biasing means (described below) to be positioned proximally at probe  160  to bias liquid column  156  in a proximal direction once it has been distally advanced. Liquid column  156  comprises part of a liquid volume that is enclosed by tube  140 , the distal biasing means and a fluid chamber of the drive mechanism  130 . Examples of distal biasing means are shown and described below in relation to  FIGS. 21 to 36 . 
     Elongate flexible tube  140  may have a diameter and length selected to suit a particular exploratory application. The material of tube  140  may be similarly selected to suit a particular application. For example, where advancement apparatus  110  is employed to advance a probe within a biological tract, such as a gastrointestinal tract, the tube may have a maximum external diameter of about 5 mm to about 15 mm (possibly closer to 7 mm) and may have a length of about 1 metre up to about 10 metres, possibly about 3 metres to about 6 metres. A tube length of about 3 to 4 metres may be suitable for advancing probe  160  within an intestinal tract (i.e. into the small intestine) via the anus. 
     The material of the tube when used to explore an intestinal tract (i.e. for gastrointestinal endoscopy) may be formed of a suitable flexible and medically inert material, such as suitable polyvinylchloride (PVC), silicone, latex or rubber materials. The material of tube  140  should allow tube  140  to be bendable to be able to be formed in a loop of a relatively small minimum diameter (depending on the application) without the wall of the tube  140  kinking or collapsing or otherwise deforming to decrease the internal cross-sectional of the tube  140 . For this purpose, the tube wall may be reinforced for increased structural integrity. For endoscopy applications, the minimum loop diameter may be about 2 cm and may range from about 1 cm to about 5 cm, for example. 
     For medical or veterinary applications in which it is desired to explore a vascular tract (i.e. for angioscopy), the tube diameter and length may be commensurately smaller, for example about 3 mm to about 10 mm (possibly closer to 5 mm) in diameter and about 0.8 to about 3 m in length, with probe  160  also being selected to have a suitably small diameter. 
     For exploration applications of a more industrial nature, such as for exploring pipes, ducts, containers, passages, tracts or other areas that are inconvenient, unsafe or difficult for a person to access, tube  140  may be formed of a more rugged material, at least on its external surface, to avoid or reduce damage to the tube as it passes along potentially abrasive surfaces. In some applications, the tube  140  needs to be relatively flexible and to be able to gain some purchase on a surface, structure or object across which the tube  140  is intended to travel. Thus, periodic perturbations formed along an external surface of the tube  140 , as described in further detail below with reference to  FIGS. 37A to 44 , may assist in frictionally engaging the surface or structure across which tube  140  is intended to travel. 
     System  100  may comprise a computer system  120  to provide control, signal processing and user interface functions in relation to advancement of the probe  160 . Thus, computer system  120  may comprise control module  115 , which may be provided in the form of hardware, software or a combination of both. Although not shown, computer system  120  comprises at least one processor and memory configured to perform the functions described herein. 
     Computer system  120  may comprise a user interface module  124 . Computer system  120  may also comprise a signal processing module  122  for receiving and processing signals from probe  160 , such as signals corresponding to image data or status or feedback signals. Signal processing module  122  may interface with user interface module  124  in order to provide images captured by probe  160  on a display (not shown) so that a user of system  100  may obtain visual feedback as probe  160  progresses. 
     User interface module  124  may be configured to allow settings and/or functions of signal processing module  122  and control module  115  to be modified or tailored to suit a particular environment, application or circumstance. 
     Each of modules  115 ,  122  and  124  may be executable as program code stored in memory accessible to at least one processor and may be supplemented by suitable software and/or hardware components, such as input-output components, operating system components, computer peripheral devices, etc. 
     Supplemental to drive mechanism  130 , ancillary equipment  135  may be provided under the control of control module  115  to provide power, signals and/or substances to probe  160 . For example, ancillary equipment  135  may provide electrical power to one or more light sources, such as light emitting diodes (LED) positioned at a distal face of probe  160 , for example, via at least one electrical conduit extending along tube  140 . Additionally, where probe  160  comprises an image-capturing device having a charge-coupled device (CCD) or other suitably small imaging device, the at least one electrical conduit may also be used to power such an image-capturing device. 
     Ancillary equipment  135  may further comprise a source of purified air and/or water to be provided to probe  160  along one or more further conduits extending along tube  140 . For this purpose, ancillary equipment  135  may comprise a suitable compressor to pressurize the air, water or other substance to be provided to probe  160 . Probe  160  may, depending on the application, use an air vent positioned at its distal extremity to insufflate a tract, such as a vascular or intestinal tract. The probe  160  may also dispense water from an opening in its distal surface to clean an area in front of the imaging device, for example. 
     Ancillary equipment  135  may be partially or entirely under the control of control module  115 , which in turn may be controlled by a user via a user interface module  124 , or it may be separately controlled, for example by manual manipulation of suitable components of the ancillary equipment, to provide the necessary interaction with probe  160 . Depending on the application, ancillary equipment  135  may also comprise a mechanism for controlling capture of a material adjacent probe  160 , for example to biopsy the material or otherwise subject it to later analysis. For this purpose, ancillary equipment  135  may mechanically, pneumatically and/or electrically communicate with probe  160  via a further suction conduit and/or control cable conduit extending along tube  140 . 
     System  100  as shown in  FIG. 1  may employ wireless data gathering of image data captured by the imaging device in probe  160 , with such data being received by a suitable antenna associated with computer system  120  to provide the image data directly to data processing module  124  for processing. Alternatively or additionally, control signals may be wirelessly received from or transmitted to probe  160  responsive to control module  115  and/or ancillary equipment  135  using a suitable short range low power radio transceiver. 
     In order to advance probe  160 , drive mechanism  130  imparts a specific speed profile to the liquid column  156  within lumen  141  in a repetitive manner. An example of such a speed profile is depicted in the graph of velocity vs. time shown in  FIG. 2 . The movement of liquid column  156  imparted by drive mechanism  130  may be divided into a forward movement section  30  and a reverse movement section  34 , with each such section  31 ,  34  being divided into two parts or phases. The forward movement section  30  is divided into a first phase  31 , in which the drive mechanism  130  imparts a gradual acceleration to a proximal end of the liquid column. A second phase  32  immediately following the first phase involves the drive mechanism  130  imparting a sharp deceleration up until the liquid column  156  momentarily comes to rest at a rest position  33 , which corresponds to the liquid  156  being moved to its distal-most position (corresponding to the point of maximum stroke) within tube  140 . The reverse movement section  34  may then comprise a first phase  35  of sharp acceleration in the proximal direction, followed immediately by a second phase  36  of gradual deceleration in the proximal direction, which continues until the liquid column  156  is again momentarily at rest at its proximal-most position, as indicated by reference numeral  37 . 
     Although the first and second phases  31 ,  32 ,  35  and  36  of the forward and rearward movement sections  30 ,  34 , are shown in  FIG. 2  as having constant change in velocity (i.e. constant acceleration) in each phase, such changes in velocity need not be linear. Rather, a velocity profile involving a sharp inversion (i.e. from a small but positive acceleration to a larger negative acceleration or vice versa) is considered to be effective for imparting a transfer of momentum from the liquid column  156  to the tube  140  in the forward (i.e. distal) direction. 
     If it is desired to retract the probe  160 , the speed profile may be inverted to have a sharp acceleration and deceleration on either side of the proximal-most rest position indicated by reference numeral  37 . For example, a sharp acceleration phase would be followed by a gradual deceleration phase in the forward movement section and a gradual acceleration phase would be immediately followed by a sharp deceleration phase in the rearward movement section. 
     In some embodiments, the sharp velocity inversion may be employed in only the forward movement section  30  or only the reverse movement section  34 , with the other movement section having relatively gradual changes in velocity. 
     Although the drive mechanism can be operated to impart a desired speed profile to a proximal end of the liquid column  156 , because movement of the liquid column  156  relies on pressure differences created by the drive mechanism and communicated to the liquid column  156  for the proximal end  142  to the distal end  144 , there may be some pressure loss over the length of the liquid column  156 . Thus the speed profile imparted by the drive mechanism  130  to the liquid column  156  at the proximal end  142  may not be the same speed profile as is experienced by the liquid column  156  at the distal end  144 . In order to minimize or reduce the loss of pressure across the length of tube  140 , the generally cylindrical wall of tube  140  may be reinforced to resist expansion or collapsing of the tube wall in response to pressure differences induced along the liquid column  156 . Additionally, an internal diameter of lumen  141  may be gradually reduced over the length of tube  140  from a first internal diameter at the proximal end  142  to a lesser second internal diameter at the distal end  144 . This reduction in diameter may be achieved in a smooth or stepped manner. For example, stepped reductions may comprise reductions of, say 0.05 mm or 0.1 mm every 15, 20, 25 or 30 cm along the tube  140 . This diametrical reduction may be linear or non-linear along the length of tube  140 . In this context, the reduction in internal diameter along the length of tube  140  is independent of any periodic variation in internal lumen diameter due to periodic perturbations, such as are described below in relation to  FIGS. 37A to 44 . 
     Pressure loss along tube  140  may be minimized by using a liquid that has a density at room temperature and at internal body temperatures about the same as or greater than that of water at such temperatures. Liquids of such densities generally do not appreciably compress under the relatively small pressure exerted by drive mechanism  130 . Thus, water, such as purified or demineralised water for example, may be used as the liquid of liquid column  156 . 
     In use of the system  100 , most of the length of tube  140  may be coiled, curled or held slack so that it can straighten gradually as the distal end  144  and probe  160  are positioned in and advance within the tract  180  or other area. Thus, as probe  160  advances under the operation of drive mechanism  130 , more and more of tube  140  will be received within the tract  180 . Once all of the slack in tube  140  is taken up and that part of tube  140  that is outside of the tract  180  cannot advance any further, probe  160  will have reached the limit to which it can extend within the tract  180 . 
     Once the endoscopy, angioscopy or other form of exploration is completed, probe  160  can be withdrawn from the tract  180  by gently manually pulling on that part of tube  140  which remains outside of tract  180 . This may be assisted and/or substituted by operating drive mechanism  130  to provide an inverted speed profile to liquid column  156  tending to impart a reverse motion and retract tube  140  in a generally proximal direction. 
     Advancement apparatus  110  is shown and described in further detail in relation to  FIGS. 3, 4A and 4B . As shown in  FIG. 3 , advancement apparatus  110  comprises drive mechanism  130  coupled to proximal end  142  of tube  140 . Probe  160  is coupled to distal end  144  of tube  140 . Drive mechanism  130  may comprise a drive piston  352  that is movable in a reciprocating manner in relation to a chamber  351  defined by a chamber wall  350 . Movement of piston  352  within wall  350  can pressurize and depressurize liquid, such as water, within chamber  351 , either forcing liquid out of chamber  351  through an opening  356  or drawing it back into chamber  351 . Various alternative embodiments of drive mechanism  130  are shown and described below in relation to  FIGS. 16A to 20 . 
     Drive mechanism  130  may comprise a drive wheel  322  mounted to contact and act upon a drive member  324  coupled to a drive shaft  354  which drives piston  352 . Drive wheel  322  and drive member  324  are arranged so that rotation of drive wheel  322  in a clockwise or anticlockwise direction causes linear movement of drive member  324  in a proximal or distal direction, respectively. Drive wheel  322  may be securely positioned within a mounting bracket  310  for mounting to a surface and/or structure (not shown) via one or more fasteners received through slots  312  formed in mounting bracket  310 . Drive member  324  rests on a support  326  fixedly coupled to mounting bracket  310 . Drive member  324  is slidable relative to support  326  with relatively little friction. 
     In some embodiments, drive member  324  and/or drive shaft  354  may be removably attached to piston  352  so that chamber  350  and all parts distal thereof (including tube  140  and probe  160 ) can be replaced after one or more uses or due to performance deterioration. 
     Drive wheel  322  may be rotated under the control of a stepper motor (not shown) comprised in drive mechanism  130 . Control of the stepper motor may be performed by control module  115  using a suitable driver program such as is commonly available with commercially available stepper motors. Control module  115  may be configured to cause the stepper motor to rotate drive wheel  322  so as to impart the desired speed profile to the proximal end of liquid column  156  by advancement and retraction of piston  352  within wall  350 . 
     As shown in  FIGS. 3 and 4A , advancement mechanism  110  may comprise a Y-type junction  330  coupled between outlet  356  of drive chamber  351  and one end of tube  140 . The Y-type junction  330  acts as a means for allowing one or more conduits  340 ,  342  to pass or be merged into a proximal part of tube  140  so that such conduits extend within lumen  141  and are coextensive with liquid column  156  along most of the length of tube  140 . Y-type junction  330  has a proximal end  332  coupled for fluid communication with drive chamber  351  via opening  356 . Proximal end  332  forms a first limb of Y-type junction  330 , while a second limb  334  extends at an acute angle away from proximal end  332  as shown in  FIG. 3 . Y-type junction  330  has a distal end  336  through which passes the liquid column  156  and the fluid conduits  340 ,  342 . 
     Conduit  340  may define a secondary lumen through which other conduits pass in order to communicate signals and/or substances between ancillary equipment  135  and probe  160 . Such conduits may include, for example, air and/or water passages, electrical conduits for signal transmission, control cables, a biopsy tube, etc. Conduit  342  may comprise electrical conduits, for example to provide a voltage to one or more light sources exposed at a distal face  162  of probe  160 . Conduit  342  may be bonded to conduit  340  so as to extend in a spiral therealong as both conduits  340  and  342  extend within lumen  141  of tube  140 . Liquid column  156  extends within lumen  141  in the spaces  376  not taken up by conduits  340 ,  342 . 
     As shown in  FIGS. 4A and 4B , hollow fluid connectors  410 ,  412 ,  414  and  416  may be used to couple different sections of advancement apparatus  110  together. For example, a first connector  410  couples proximal end  332  of Y-type junction  330  to a tube  440  that is coupled to wall  350  around opening  356 . A second connector  412  couples a distal end  336  of Y-type junction  330  to a proximal end  142  of tube  140 . A third connector couples a distal end of tube  140  to a distal tube section  450  which in turn is coupled to a flexible section  460  via a fourth connector  416 . Flexible section  460  may be directly coupled to probe  160  and may be directionally controlled, for example by use of control cables extending within the conduits  340  and/or  342 . 
     Distal end section  450  includes a membrane  454  sealing a distal end of liquid column  156  by sealing against an inner wall of distal tube section  452  and sealing against outer walls of conduits  340 ,  342 . A generally cylindrical sealing section  455  may also be provided to prevent fluid from liquid column  156  entering into flexible section  460 . 
     Flexible section  460  may define an internal lumen or plenum  464  through which conduits  340 ,  342  pass to be coupled to probe  160 . Flexible section  460  has a flexible wall  462  defining the plenum  464 . Flexible wall  462  is coupled to fourth connector  416  at a proximal end of flexible wall  462  and to the probe  160  at a distal end of flexible wall  462 . 
     As shown in  FIG. 4B , probe  160  may house an imaging device  474  and one or more light sources  472 , such as LEDS, positioned at the distal face  162  in order to shine light distally and capture images of the area illuminated by light sources  472 . 
     Referring now to  FIGS. 5A and 5B , an alternative form of distal end section  450  is shown and described. Alternative distal end section  550  is shown schematically in  FIGS. 5A and 5B  and is not to scale. Distal end section  550  comprises a flexible membrane  554  sealingly coupled to an inner surface of cylindrical wall  552  and extending inwardly in a cone shape in a distal direction to be coupled circumferentially and sealingly around conduit  340 . Flexible membrane  554  is positioned so that liquid column  156  is disposed generally proximally of flexible membrane  554 , with a second fluid volume  556 , such as air, being disposed distally of flexible  554 . Second fluid volume  556  should be a compressible fluid volume so that, when liquid column  156  is moved distally due to the action of drive mechanism  130 , flexible membrane  554  can deform, as shown in  FIG. 5B , and compress second fluid volume  556  somewhat. This compression of second fluid volume  556  and elastic deformation of flexible membrane  554  provides a biasing function because the deformation and compression tend to push back on liquid column  156  in a proximal direction following distal movement of liquid  156 . 
     Referring now to  FIG. 6 , an alternative schematic representation of system  100  is provided. System  100  as depicted in  FIG. 6  has similar features and functions to those described above in relation to  FIG. 1 . In addition, computer system  120  comprises a display  612  for displaying captured images, an input device  614 , such as a keyboard, and a user control device  616 , such as a joy stick, for interfacing with control module  115 . Ancillary equipment  135 , which may be integrated with a computer system  120 , is used to provide air and/or water and/or suction for a biopsy tube, if appropriate. Control module  115  may be configured to translate input from user input control device  616  into control signals to be provided to a directionally controllable flexible section  662  coupled intermediate probe  160  and a distal end section (such as is shown and described in relation to  FIG. 4B, 5A, 5B or 21 to 36 ) in order to change the position of probe  160 . 
     Conduits  340 ,  342  are provided within tube  140  to provide suitable control and/or feedback functions to flexible section  662  and probe  160 . Alternatively or in addition, other conduits or control means may be provided to directionally control probe  160 . As shown in  FIG. 6 , distal end section  450  (or  550 ,  2150 ,  2250 ,  2350 ,  2450 ,  2550 ,  2750 ,  2950 ,  3050 ,  3150 ,  3250 ,  3450  or  3650 ), flexible section  662  and probe  160  form a distal portion  644  at a distal end of tube  140 . Versions of system  100  shown in  FIG. 6  may be suited for endoscopy or angioscopy, for example. 
     Referring now to  FIGS. 7A and 7B , a tube  740  according to some specific embodiments of tube  140  is shown and described. Tube  740  has a generally cylindrical wall  750  defining a lumen  741  through which liquid column  156  and optionally conduits  340 ,  342  extend. Longitudinal reinforcing members  752  may be embedded or otherwise disposed within wall  750 , spaced circumferentially around wall  750 . Alternatively or in addition, reinforcing members  752  may comprise conduits for coupling to probe  160  to provide the conduit functions described above. 
     Referring now to  FIGS. 8A and 8B , a tube  840  according to some specific embodiments of tube  140  is shown and described. Tube  840  has a substantially cylindrical wall  850  defining a lumen  841  and has a plurality of reinforcing members  852  disposed circumferentially around the outside of wall  850 . Reinforcing members  852  may be adhered or otherwise bonded to an external surface of wall  850  in a suitably flexible manner to resist changes in diameter of wall  850 , while allowing tube  840  to curve as necessary while passing along a tract. Reinforcing members  852  are thus similar in function and purpose to reinforcing members  752  of tube  740 . 
     Tubes  940 ,  1040  and  1140 , as shown in  FIGS. 9A, 9B, 10A, 10B, 11A and 11B , also use respective reinforcing members  952 ,  1052  and  1152  in order to provide structural integrity to the wall of the tube to resist collapsing or expansion of the tube wall due to pressure changes, while allowing adequate flexion to allow flexible passage through a convoluted tract.  FIGS. 9A and 9B  show the reinforcing members  952  formed in a spiral around and along an outside of wall  950  that defines a central lumen  941 . 
     Tube  1040  is similar to tubes  840  and  940 , in that tube  1040  combines longitudinal and spiral reinforcing members  1052 , thus combining the features of tubes  840  and  940 . Reinforcing members  1052  are disposed around the outside of wall  1050  which defines a central lumen  1041 . 
     Tube  1140  is similar to tube  940  except that reinforcing members  1152  are formed in separate spirals that cross each other as they travel around wall  1150 . Reinforcing members  1152  are therefore oppositely angled with respect to their spiral forms. Such spiral forms may have different angles relative to the longitudinal axis of tube  1140  and may therefore have differently spaced coils. Wall  1150  defines a central lumen  1141  which, like lumens  741 ,  841 ,  941  and  1041 , allows passage of liquid column  156  therewithin. 
     In some embodiments, reinforcing members  752 ,  852 ,  952 ,  1052  and  1152  may comprise one or more conduits for coupling to probe  160  to provide the conduit functions described above. Thus, such reinforcing members may provide a dual function. For reinforcing members  852 ,  952 ,  1052  and  1152  disposed around the outside of the tube wall, such members may be bonded to the outside of the wall, for example by a suitable adhesive or ultrasonic welding or by overlay of an adhesive layer or coating. For medical applications, such adhesive or bonding materials should be suitably medically inert. In some embodiments, reinforcing members  952 ,  1052  and  1152  may act as periodic perturbations along the exterior of the tube wall for increasing frictional engagement of the tube with a surrounding area to a degree sufficient to enhance the ability of the tube to progress within the tract or other area under the action of drive mechanism  130 . 
       FIGS. 12A, 12B, 13A, 13B, 13C, 14A, 14B, 15A and 15B  illustrate various specific embodiments of tube  140  with respect to the arrangement of conduits extending within the lumen  141  of tube  140 . As shown in  FIGS. 12A and 12B , a tube  1240  may have multiple conduits  1262  extending within a lumen  1241  defined by tube  1250 . Conduits  1262  may extend in an arrangement involving multiple conduits  1262  spiraling around a central conduit  1262 , which may be larger in diameter (e.g. to house further conduits) than the spiraling conduits  1262 . Conduits  1262  may take up most of the space within lumen  1241 , while leaving sufficient space for liquid column  156  to be movable within the remaining spaces  376 . 
     As shown in  FIGS. 13A, 13B and 13C , tube  1340  has a generally cylindrical outer wall  1350  defining at least one lumen  1341 . At least one dividing membrane  1364  extends within lumen  1341  to divide the internal cross-sectional area defined by wall  1350  into two or more sections, such as are illustrated in  FIGS. 13B and 13C .  FIG. 13B  illustrates a tube  1340   a  in which a dividing membrane  1364  divides lumen  1341  into a section along which conduits  1362  pass and another portion along which liquid column  156  is free to pass.  FIG. 13C  illustrates an alternative cross-section of  FIG. 13A , where a tube  1340   b  has at least two dividing membranes  1364  which divide lumen  1341  into four sections, two of which are used to house conduit  1362 , while the remaining two portions of lumen  1341  allow free movement of liquid column  156  therealong. 
     As shown in  FIGS. 14A and 14B , a tube  1440  according to some embodiments has a wall  1450  defining a lumen  1441  that is a primary lumen within which passes a secondary conduit  1464  defining a secondary lumen. This secondary conduit  1464  houses a plurality of conduits  1462 , contained within the generally cylindrical form of the secondary lumen. The secondary conduit  1464  may be adhered or otherwise bonded to or integrally formed with an internal surface of wall  1450 . 
     Referring now to  FIGS. 15A and 15B , a tube  1540  according to further embodiments is shown, having a wall  1550  defining a lumen  1541 . Lumen  1541  is a primary lumen through which extends a secondary conduit  1564  defining a secondary lumen similar to secondary conduit  1464 , except that it is positioned centrally within primary lumen  1541 . Secondary conduit  1564  houses a plurality of conduits  1562  within a generally cylindrical tube. Secondary conduit  1564  may comprise a tube that is positioned centrally within primary lumen  1541  by means of a series of spaced positioning elements, such as locating ribs, extending inwardly from wall  1550  in a manner that does not appreciably obstruct movement of liquid column  156  within primary lumen  1541 . 
     Referring now to  FIGS. 16A, 16B, 17A, 17B, 18, 19 and 20 , various embodiments of drive mechanism  130  are illustrated schematically. As shown in  FIGS. 16A and 16B , drive mechanism  130  may comprise a simple piston  1652  and drive shaft  1654  arranged to move piston  1652  back and forth within a chamber  1651  defined by a wall  1650 . As piston  1652  repeatedly moves back and forth within wall  1650 , liquid in chamber  1651  is repeatedly forced out of an opening  1656  formed in wall  1650  and then drawn back into chamber  1651  through opening  1656 . Piston  1652  sealingly engages wall  1650  so that liquid in chamber  1651  does not pass proximally of piston  1652 . 
     The drive mechanism arrangement depicted in  FIGS. 17A and 17B  is substantially similar to that shown in  FIGS. 16A and 16B , except that a longitudinally compressible/extensible bellows or sylphon  1770  is arranged to extend between a distal part of wall  1650  and piston  1652 , thereby defining a fluid volume  1751  bounded by the piston  1652  at one end, the accordion-like walls of sylphon  1770  and the walls  1650  that define the distal opening  1656 . Sylphon  1770  obviates the need for sealing engagement of piston  1652  with wall  1650 , for example where such engagement might entail an undesirable amount of friction or may be difficult to seal properly. In some embodiments, sylphon  1770  may be substituted by another flexible membrane that is also flexibly compressible but that is less structured than sylphon  1770 . 
     Referring now to  FIG. 18 , further embodiments of drive mechanism  130  are described, which employ an elastically deformable flexible membrane  1870  forming one wall of a housing enclosing a liquid volume  1851 . A housing wall  1850  cooperates with flexible membrane  1870  to enclose liquid volume  1851 . A drive shaft  1854  coupled to a flat or somewhat curved piston  1852  is used to push inwardly on flexible membrane  1870  to thereby expel liquid from liquid volume  1851  out of an opening  1856  in the wall  1850  of the housing. Upon release (i.e. retraction) of the drive shaft  1854 , flexible membrane  1870  is allowed to at least partially return to a position from which it is resiliently deflected, thereby increasing the amount of liquid in liquid volume  1851  by creating suction and thereby drawing liquid back through opening  1856 . Drive shaft  1854  is operated by the drive mechanism to repeatedly deflect flexible membrane  1870  to move liquid column  156  back and forth within lumen  141 . In some embodiments, drive shaft  1854  may be coupled to flexible membrane  1870  so that retraction of the drive shaft  1854  causes the flexible membrane  1870  to more strongly return to its relaxed position (or at least a less deflected position), thereby creating greater suction than may be achievable due to the flexible membrane  1870  alone. 
     The drive mechanism schematically illustrated in  FIG. 19  operates on a similar principle to the drive mechanism illustrated in  FIG. 18 , except that instead of a pushing rod and piston, a cylindrical piston is eccentrically rotated about a drive shaft  1954  to cyclically inwardly deflect a resilient flexible membrane  1970 , thereby decreasing the volume of liquid  1951  within a housing defined by wall  1950  and flexible membrane  1970 . As piston  1952  rotates around drive shaft  1954 , liquid is pushed outward and sucked inward through opening  1956  formed in wall  1950 . In some embodiments, piston  1952  may have an oblong, noncircular (but curved) shape to impart a specific speed profile to liquid column  156 . For example, piston  1952  may be more bulb-shaped or have a relatively flat face, rather than circular, but still rotate eccentrically around drive shaft  1954 . 
     Referring now to  FIG. 20 , a further alternative drive mechanism is shown that uses electromagnetic elements  2054  positioned outside a wall  2050  that defines a chamber  2051 . A piston  2052  is movable under the control of electromagnetic elements  2054  so as to push liquid out of chamber  2051  through opening  2056  formed in wall  2050  and to subsequently suck liquid back into chamber  2051 . Piston  2052  is formed of a suitable material to enable electromagnetic control using elements  2054  and, like the drive mechanism of embodiments described above in relation to  FIGS. 16A, 16B, 17A and 17B , either uses a sealing engagement of piston  2052  with wall  2050  or a sylphon to obviate such sealing engagement. 
     The drive mechanism embodiments described above in relation to  FIGS. 16A to 20  provide only some examples of possible mechanisms for creating reciprocating movement of liquid column  156  within tube  140 . Further embodiments may be employed, for example involving pneumatic, hydraulic, electrical or mechanical means to create repeated positive and negative pressure differences within and along liquid column  156 , tending to cause reciprocating movement thereof in a manner that is suitably controllable to impart a desired speed profile to liquid column  156 . 
     Referring now to  FIGS. 21 to 36 , various embodiments of a distal biasing section are shown and described. Similar to distal biasing section  550 , these embodiments use various different means or mechanisms to bias the liquid column  156  back in the proximal direction once it has been advanced distally. This may also assist in avoiding collapse of the tube wall as the liquid column is sucked proximally under the negative pressure by drive mechanism  130 . Accordingly, the distal biasing sections shown in  FIGS. 21 to 36  are all intended to be positioned distally of the liquid column  156 , but proximally of probe  160  and they are intended to be positioned within a tube wall, either provided by tube  140  or a tube section adjacent or contiguous with tube  140 . 
     Distal biasing chamber  2150  shown in  FIG. 21  has the most basic construction, consisting mainly of a cylindrical wall  2152  with a movable element  2154 , such as a piston, movable within a chamber  2156 . At its proximal face, element  2154  is exposed to the distal end of liquid column  156  and, in response to distal movement of liquid column  156  is pushed distally. Chamber  2156  comprises a compressive fluid volume, such as air, and is enclosed by wall  2152  and a distal end provided by another distally positioned structure (not shown). Element  2154  sealingly engages wall  2152  so that liquid from liquid column  156  does not pass into chamber  2156 . The pressure increase in chamber  2156  as a result of distal movement of element  2154  provides a proximally directed force on element  2154  to return it in the proximal direction as liquid column  156  is sucked proximally by the action of drive mechanism  130 . 
     The distal biasing chamber  2250  of  FIG. 22  operates in an identical manner to that of  FIG. 21 , except that wall  2252  defines more restricted end passages at the proximal and distal ends. Movable member  2254  moves within wall  2252  to compress chamber  2256  in response to distal movement of liquid column  156 . 
     Distal biasing chamber  2350  shown in  FIG. 23  operates identically to that shown in  FIG. 21 , except that it has a distal end wall  2380  that, together with movable element  2350  at wall  2352 , defines an enclosed chamber  2356  comprising a compressible fluid, such as air. 
     Distal biasing chamber  2450  shown in  FIG. 24  is similar to that of  FIG. 23 , except that a flexible membrane  2480  is provided as the distal end wall. Together with movable element  2454  and wall  2452 , flexible membrane  2480  defines an enclosed chamber  2456  comprising a compressible fluid, such as air. Flexible membrane  2480  expands and contracts, depending on the pressure within chamber  2456  and may assist in biasing movable element  2454  in the proximal direction. 
     Distal biasing chamber  2550  shown in  FIGS. 25 and 26  are similar to that shown in  FIG. 22 , except that movable element  2554  is biased distally by a spring  2580  housed within wall  2552 . Spring  2580  compresses when movable element  2554  progresses distally and therefore tends to bias movable element  2554  in the proximal direction. Spring  2580  sits within a chamber  2556  defined distally of movable element  2554 . 
     Distal biasing chamber  2750  shown in  FIGS. 27 and 28  is identical to that shown in  FIGS. 25 and 26 , except that instead of a spring, a resiliently deflectable mesh or sponge  2780  is provided within a chamber  2756  defined by wall  2752  distally of movable element  2754 , 
     Distal biasing chamber  2950  shown in  FIG. 29  is similar to those described above, but has a sylphon  2970  coupled to a proximal side of movable element  2954  to define a proximal chamber  2958  that is expandable in response to distal movement of liquid column  156 , but that tends to retract according to the shape memory of the sylphon and/or increased pressure in distal fluid volume  2956 , thereby biasing the movable element  2954  in the proximal direction. Sylphon  2970  is coupled at the proximal end of distal biasing chamber  2950  to a wall  2952 . Compressive distal fluid volume  2956  is provided distally of movable element  2954  to further bias movable element  2954  in the proximal direction. 
     Distal biasing chamber  3050  shown in  FIG. 30  employs a first sylphon  3070  in a similar manner to that shown in  FIG. 29  and a second sylphon  3071  disposed within a distal chamber  3056  defined by wall  3052 . The opposite shape memories of first and second sylphons  3070  and  3071  tend to bias movable element  3054  in the proximal direction. Distal biasing chamber  3150  shown in  FIG. 31  is identical to that shown in  FIG. 30 , except that its distal end wall is substituted by a resiliently deflectable flexible membrane  3180 . 
     Distal biasing chamber  3250  shown in  FIGS. 32 and 33  represents a combination of the spring and sylphon features shown and described in relation to  FIGS. 25, 26 and 29 . Distal biasing chamber  3450  shown in  FIGS. 34 and 35  represents a combination of the sylphon and sponge/mesh features and functions described above in relation to  FIGS. 27, 28 and 29 . All of  FIGS. 32 to 35  employ a proximally disposed sylphon  3270 / 3470  defining a proximal chamber  3258 / 3458  and coupled to a movable element  3254 / 3454 , with a biasing element, such as a spring  3280  or sponge or mesh  3480  positioned distally of the movable element  3254 / 3454  within wall  3252 / 3452 . 
     Distal biasing chamber  3650  shown in  FIG. 36  has a wall  3652  that defines an internal compressible fluid chamber  3656  between a resiliently deflectable proximal flexible membrane  3654  and a resiliently deflectable distal flexible membrane  3680 . Both of the flexible membranes  3654  and  3680  may deflect distally in response to distal movement of the liquid column  156  and will tend to return to a rest position in which they are not distally displaced, thereby tending to bias liquid column  156  in the proximal direction. 
     Referring to  FIGS. 37A, 37B, 38A, 38B, 39A and 39B , various embodiments of tube  140  are described. Each of the embodiments has a nominal wall thickness X relative to which periodic perturbations are formed along an external surface of the tube. The periodic perturbations have a maximum amplitude Y and a separation Z. As shown in these Figures, the periodic perturbations are formed to have a pattern generally resembling a fir-tree or the serrations on a saw blade. However, in some embodiments the periodic perturbations may be more rounded and/or not proximally swept (as in the case of the fir-tree pattern). 
     As shown in  FIG. 37A , the minimum thickness of the wall of tube  3740  is X with the thickness of the wall varying along the periodic perturbations between X and X+Y. Tube  3745  shown in  FIG. 37B  has a wall thickness varying between the nominal thickness X and X−Y. 
       FIGS. 38A and 38B  show a slightly different fir-tree pattern than  FIGS. 37A and 37B , without an undercut, but are otherwise substantially the same, with tube  3845  having a larger nominal thickness X than tube  3840 . 
     Tube  3940  shown in  FIG. 39A  has a greater spacing Z between the periodic perturbations, with the thickness of the wall varying between the nominal thickness X and X+Y. Tube  3945  shown in  FIG. 39B  is the same as  FIG. 39A , but with a larger nominal thickness X and the wall thickness varying between X and X−Y. 
     In the described and depicted embodiments, the separations of the periodic perturbations may be anywhere between say about 2 mm and about 50 mm. The variation in thickness (i.e. amplitude) Y may be in the order of 0.5 mm to about 5 mm, depending on the exploration application for which the tube is to be used. The nominal wall thickness X may be about 0.5 mm to about 10 mm, depending again on the application. In some embodiments, variation of the wall thickness may be based on proportions of amplitude Y (or M, described below), for example the thickness may vary between X+½Y and X−½Y or between X+⅓Y and X−⅔Y. 
     Referring now to  FIGS. 40A, 40B, 41A, 41B, 42A and 42B , various embodiments of tube  140  are depicted and described in which periodic perturbations are provided on an internal wall of the tube. The nominal thickness L of the tube wall may vary, together with the amplitude M and period N of the periodic perturbations. The various embodiments depicted have a generally proximally swept fir-tree pattern, which may also be described as a saw-tooth pattern, although rounded and/or non-proximally-swept perturbations may also be employed. Tube  4040  is shown in  FIG. 40A  with the wall thickness varying between the nominal thickness L and L+M. In  FIG. 40B , tube  4045  has a nominal wall thickness varying between L and L−M. The tubes  4140  and  4145  shown in  FIGS. 41A and 41B  are substantially the same as tubes  4040  and  4045 , except for the sharper undercut of the fir-tree pattern shown in the latter figures. Tube  4240  shown in  FIG. 42A  has a nominal wall thickness L that varies between L and L+M. Tube  4245  has a nominal thickness L that varies between L and L−M, as shown in  FIG. 42B . In some embodiments, variation of the wall thickness may be based on proportions of amplitude M, as described above. 
     As shown in  FIGS. 43A and 43B , embodiments of tube  140  include tubes  4340  and  4345 , representing combinations of tube embodiments  38 A,  38 B,  41 A and  41 B, described above. Tube  4340  has a nominal thickness X, with the thickness varying between X and X+Y+M. The spacing Z of the external periodic perturbations may be different from the spacing N of the internal periodic perturbations. Additionally, the internal and external periodic perturbations need not have the same saw-toothed or fir-tree shape. Specifically, one of the internal or external periodic perturbations may be saw-toothed, while the other may be more rounded and more spaced apart. Tube  4345  shown in  FIG. 43B  is similar to tube  4340 , except that it has a greater nominal thickness X, with the thickness varying between X and X−Y−M. In some embodiments, variation of the wall thickness may be based on proportions of amplitude M and/or Y, as described above. 
       FIG. 44  shows a schematic representation of a tube  4440  according to some embodiments in which a first section  4441  of the tube may have internal periodic perturbations, while a second section of the tube  4440  may have external periodic perturbations. The first and second sections of the tube may be separated by a section  4442  that does not contain any internal or external periodic perturbations. 
     According to the described embodiments, some embodiments of tube  140  may involve periodic perturbations along part or a substantial portion of an internal or external surface of the wall of tube  140 . Such periodic perturbations on the internal surface of the tube wall can assist in providing greater resistance to advancement of liquid column  156 , because of the proximally swept shape of the perturbations in some embodiments, thereby improving momentum transfer from liquid column  156  to tube  140  in the distal direction. The periodic perturbations formed on the external wall of tube  140  may similarly assist in advancing the tube  140  by providing a greater resistance to movement of tube  140  in the proximal direction than in the distal direction so that retraction of liquid column  156  results in a small tube movement in the rearward direction compared with the tube movement achieved in the forward direction. 
     The different embodiments of tube  140  described herein may be combined, for example so as to provide periodic perturbations in combination with reinforcing members such as those extending externally along the tube wall or within the tube wall. In particular, the extension of conduits, such as conduits  340 ,  342 , within lumen  141  can be combined with internal and/or external periodic perturbations in the tube wall and/or may be combined with external or embedded longitudinal or spiral reinforcing members. 
     Described embodiments of tube  140  may be formed by a moulding process, for example, using suitable materials as described above. 
     The embodiments described herein and illustrated in the drawings are intended to be provided by way of example and without limitation. Accordingly, the described embodiments are intended to be non-limiting and should be interpreted accordingly.