Patent Publication Number: US-2021187254-A1

Title: Contact Force Spring with Mechanical Stops

Description:
FIELD OF THE INVENTION 
     The present invention relates to medical equipment, and in particular, but not exclusively to, contact force springs. 
     BACKGROUND 
     In some diagnostic and therapeutic techniques, a catheter is inserted into a chamber of the heart and brought into contact with the inner heart wall. In such procedures, it is generally important that the distal tip of the catheter engages the endocardium with sufficient pressure to ensure good contact. Excessive pressure, however, may cause undesired damage to the heart tissue and even perforation of the heart wall. 
     For example, in intracardiac radio-frequency (RF) ablation, a catheter having an electrode at its distal tip is inserted through the patient&#39;s vascular system into a chamber of the heart. The electrode is brought into contact with a site (or sites) on the endocardium, and RF energy is applied through the catheter to the electrode in order to ablate the heart tissue at the site. Proper contact between the electrode and the endocardium during ablation is necessary in order to achieve the desired therapeutic effect without excessive damage to the tissue. 
     US Patent Publication 2011/0263934 of Aeby, et al., describes a catheter for diagnosis or treatment of a vessel or organ is provided in which a flexible elongated body includes a tri-axial force sensor formed of a housing and a plurality of optical fibers associated with the housing that measure changes in the intensity of light reflected from the lateral surfaces of the housing resulting from deformation caused by forces applied to a distal extremity of the housing. A controller receives an output of the optical fibers and computes a multi-dimensional force vector corresponding to the contact force. 
     US Patent Publication 2011/0130648 of Beeckler, et al., describes a medical probe, consisting of a flexible insertion tube, having a distal end for insertion into a body cavity of a patient, and a distal tip, which is disposed at the distal end of the flexible insertion tube is configured to be brought into contact with tissue in the body cavity. The probe also includes a coupling member, which couples the distal tip to the distal end of the insertion tube and which consists of a tubular piece of an elastic material having a plurality of intertwined helical cuts therethrough along a portion of a length of the piece. 
     US Patent Publication 2016/0339207 of Beeckler, et al., describes a catheter having a catheter shaft that has a more uniform construction throughout its length and is able to provide more than one deflection curvature. The catheter shaft includes a flexible outer tubular member, and a less flexible inner tubular member extending through the outer tubular member in a proximal section of the catheter shaft, wherein the inner tubular member is afforded longitudinal movement relative to the outer tubular member. The catheter also includes at least one puller wire extending through the inner tubular member to deflect a distal deflection section of the catheter shaft, wherein longitudinal movement of the inner tubular member relative to the outer tubular member enables an operator to select and set a deflection curvature of the distal deflection section. 
     SUMMARY 
     There is provided in accordance with an embodiment of the present disclosure, a catheter apparatus, including an elongated deflectable element, a distal assembly, a force sensor disposed between the elongated deflectable element and the distal assembly, and including a spring including a tube with at least one helical cut extending around a circumference of the tube, the at least one helical cut including deviations extending in a longitudinal direction of the tube, the deviations being configured to prevent overstretching and overbending of the spring. 
     Further in accordance with an embodiment of the present disclosure respective ones of the deviations include respective opposing sigmoid curves. 
     Still further in accordance with an embodiment of the present disclosure respective ones of the deviations of the helical cuts define respective mechanical stops, each mechanical stop including opposing surfaces which are configured to come into contact with each other to prevent overstretching and overbending of the spring. 
     Additionally, in accordance with an embodiment of the present disclosure respective ones of the mechanical stops are configured to engage simultaneously so that a force applied on the spring is shared among the respective mechanical stops to prevent sequential failure of the respective mechanical stops. 
     Moreover, in accordance with an embodiment of the present disclosure the at least one helical cut includes multiple helical cuts, respective ones of the mechanical stops of each of the helical cuts being configured to engage simultaneously. 
     Further in accordance with an embodiment of the present disclosure the at least one helical cut includes multiple helical cuts, the mechanical stops of each of the helical cuts being configured to engage simultaneously. 
     Still further in accordance with an embodiment of the present disclosure respective ones of the mechanical stops include respective T-shape elements disposed in respective T-shape openings. 
     Additionally, in accordance with an embodiment of the present disclosure respective ones of the mechanical stops include respective L-shape elements disposed in respective L-shape openings. 
     Moreover, in accordance with an embodiment of the present disclosure respective ones of the mechanical stops include respective loops and sockets. 
     Further in accordance with an embodiment of the present disclosure the at least one helical cut includes multiple helical cuts, the tube including three of the helical cuts. 
     Still further in accordance with an embodiment of the present disclosure, the apparatus includes a proximal coupler having a proximal and distal end, wherein the elongated deflectable element has a distal end connected to the proximal end of the proximal coupler, the tube and the distal end of the proximal coupler including complementary bayonet connecting features connecting the distal end of the proximal coupler with the tube. 
     Additionally, in accordance with an embodiment of the present disclosure, the apparatus includes a distal coupler, wherein the tube has a distal end including holes disposed around the circumference of the tube, the distal end of the tube being connected to the distal coupler via an adhesive which extends into respective ones of the holes. 
     Moreover, in accordance with an embodiment of the present disclosure the tube includes a distal edge with openings therein, the distal coupler including protrusions configured for engaging the openings to prevent rotation of the distal coupler with respect to the tube. 
     Further in accordance with an embodiment of the present disclosure the openings include U-shaped openings. 
     Still further in accordance with an embodiment of the present disclosure, the apparatus includes a distal coupler, wherein the tube includes a distal edge with openings therein, the distal coupler including protrusions configured for engaging the openings to prevent rotation of the distal coupler with respect to the tube. 
     Additionally, in accordance with an embodiment of the present disclosure the openings include U-shaped openings. 
     Moreover, in accordance with an embodiment of the present disclosure the distal assembly includes an expandable distal assembly. 
     Further in accordance with an embodiment of the present disclosure the expandable distal assembly includes an inflatable balloon. 
     Still further in accordance with an embodiment of the present disclosure the force sensor includes a transmitting coil and at least one receiving coil disposed on the tube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood from the following detailed description, taken in conjunction with the drawings in which: 
         FIG. 1  is a schematic view of a catheter constructed and operative in accordance with an embodiment of the present invention; 
         FIGS. 2A-B  are schematic views of the catheter of  FIG. 1  with an outer sleeve removed; 
         FIG. 3  is a schematic view of the catheter of  FIG. 1  with several elements removed; 
         FIG. 4  is a partially exploded view of the catheter as shown in  FIG. 3 ; 
         FIGS. 5A-C  are schematic views of a proximal coupler of the catheter of  FIG. 1 ; 
         FIGS. 6A-B  are schematic views of the distal coupler of the catheter of  FIG. 1 ; 
         FIGS. 7A-B  are schematic views of a spring of the catheter of  FIG. 1 ; 
         FIGS. 8A-C  are schematic views of a force sensor of the catheter of  FIG. 1 ; and 
         FIG. 9  is a schematic view of a spring constructed and operative in accordance with an alternative embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Overview 
     An assembly, such as a balloon, at the tip of a contact force sensor of a catheter presents a unique challenge to a contact force spring of the force sensor. The spring experiences significantly higher tensile forces as the assembly is withdrawn into the catheter sheath, or other such maneuvers. In addition to the withdrawal forces, even during normal use side and other forces exerted on the spring are higher than those exerted with a focal catheter due to the length of the assembly, e.g., a balloon. 
     For example, the contact force spring may be formed from one or more helices cut around a tube. The spring functions very well when attached to a focal catheter. However, in the case of other catheters such as a balloon catheter, the higher forces on the spring, typically when the balloon is withdrawn into its sheath, or pulling the balloon on a Haemostatic valve, can permanently damage the spring, for example, the ends of the helical cuts may open too much. 
     Embodiments of the present invention solve the above problems by adding deviations in the helical cut(s) in a spring to provide mechanical stops which prevent overbending and overstretching of the spring. The mechanical stops are designed to engage once the spring has been extended by a preset amount, and the engagement prevents plastic deformation, which is irreversible, of the spring. The mechanical stops are generally designed to prevent overstretching while still allowing for compression of the spring so that the spring can still perform its main function in measuring force. Alternate shapes of mechanical stops are possible. 
     In some embodiments, the mechanical stops are designed to engage simultaneously rather than sequentially in order to share the load evenly, otherwise one (or more) mechanical stop(s) will take the entire load until it (or they) fails, at which point the next mechanical stop(s) would take the load, etc. 
     In some embodiments, a catheter includes an elongated deflectable element, a distal assembly (which may include an expandable distal assembly, e.g., including an inflatable balloon), a force sensor disposed between the elongated deflectable element and the distal assembly. The force sensor includes a tube with helical cuts extending around a circumference of the tube. Each helical cut includes deviations extending in a longitudinal direction of the tube. The deviations prevent overstretching and overbending of the spring. The force sensor may include position coils disposed on the tube. 
     The tube may include any suitable number of helical cuts, for example, two, three, or more than three. The term “helical cut”, as used in the specification and claims, is defined as a helical cut extending more than one turn around the tube, or extending at least half of a turn around the tube. 
     Respective deviations of respective helical cuts define respective mechanical stops. Each mechanical stop includes opposing surfaces which come into contact with each other to prevent overstretching and overbending of the spring. 
     In some embodiments, respective mechanical stops are designed to engage simultaneously so that a force applied on the spring is shared among the respective mechanical stops to prevent sequential failure of the respective mechanical stops. In some embodiments, at least two mechanical stops of each helical cut are designed to engage simultaneously. In some embodiments, all the mechanical stops of each helical cut are designed to engage simultaneously. 
     In some embodiment the deviations include respective opposing sigmoid curves. In some embodiments, respective mechanical stops include respective loops and sockets. In some embodiments, respective mechanical stops include respective T-shape elements placed in respective T-shape openings. In some embodiment, respective mechanical stops include respective L-shape elements placed in respective L-shape openings. 
     In some embodiments, the catheter includes a proximal coupler having a proximal end connected to the distal end of the elongated deflectable element. The tube and the distal end of the proximal coupler include complementary bayonet connecting features connecting the distal end of the proximal coupler with the tube. 
     In some embodiments, the catheter includes a distal coupler and the distal end of the tube includes holes around its circumference. The distal end of the tube is connected to the distal coupler via an adhesive (such as epoxy) which extends into the holes to promote adhesion between the distal coupler and the tube. 
     In some embodiments, the distal edge of the tube includes openings (e.g., U-shape openings) for engaging protrusions of the distal coupler to prevent rotation of the distal coupler with respect to the tube. 
     System Description 
     Reference is now made to  FIG. 1 , which is a schematic view of a catheter  10  constructed and operative in accordance with an embodiment of the present invention. The catheter  10  includes an elongated deflectable element  12 , a distal assembly  14  and an outer sleeve  16  disposed about a longitudinal axis L-L (which will be used to reference various internal and external components of catheter  10 ). The distal assembly  14  may include any suitable distal assembly, for example, a lasso catheter assembly or a focal catheter assembly. In some embodiments, the distal assembly  14  includes an expandable distal assembly  18 , which may include an inflatable balloon  20  or a basket, by way of example only. The elongated deflectable element  12  includes lumens (not shown) in which to carry electrical connections, irrigation channels, puller wires, and the like. The distal assembly  14  may also include multiple electrodes  22  (only two labeled for the sake of simplicity) disposed thereon for use in mapping and/or ablation or any other suitable function. 
     Reference is now made to  FIGS. 2A-B , which are schematic views of different sides of the catheter  10  of  FIG. 1  with the outer sleeve  16  of  FIG. 1  removed. The catheter  10  includes a proximal coupler  24 , a distal coupler  26 , and a force sensor  28 . 
     The force sensor  28  is disposed between the elongated deflectable element  12  and the distal assembly  14 , and more specifically disposed between the proximal coupler  24  and the distal coupler  26 . 
     The force sensor  28  includes a spring  38  including a tube  40  with one or more helical cuts  42  extending around a circumference of the tube  40 . As used herein, the circumference includes both the outer circumferential surface  41 A and the inner circumferential surface  41 B of the tubular member  40 . Each helical cut  42  includes deviations  44  extending in a longitudinal direction of the tube  40 . The deviations  44  are configured to prevent overstretching and overbending of the spring  38 . The spring  38  is described in more detail with reference to  FIGS. 7A-8C . 
     The inflatable balloon  20  is mounted on the distal coupler  26  with the distal coupler  26  extending until a nose  30  in the center of the distal end of the inflatable balloon  20 . Therefore, the distal coupler  26  couples the inflatable balloon  20  with the force sensor  28 . The distal coupler  26  is described in more detail with reference to  FIGS. 4, 6A -B. 
     The proximal coupler  24  couples the force sensor  28  with a distal end  32  of the elongated deflectable element  12 . The catheter  10  includes a position sensor  34  ( FIG. 2B ), such as a single, dual, and/or triple-axis coil. The position sensor  34  is mounted on the proximal coupler  24  in the examples of  FIG. 2B . The catheter  10  also includes a solder pad  36  ( FIG. 2A ) disposed on the proximal coupler  24  to which various electrical connections from components at the distal end of the catheter  10  and electrical connections from the proximal end of the catheter  10  may be connected. The proximal coupler  24  is described in more detail with reference to  FIGS. 5A-C . 
     Reference is now made to  FIGS. 3 and 4 .  FIG. 3  is a schematic view of the catheter  10  of  FIG. 1  with several elements removed.  FIG. 3  shows the catheter  10  with the distal assembly  14  and the sleeve  16  of  FIG. 1  removed, and with the solder pad  36  and the position sensor  34  of  FIGS. 2A and 2B  removed.  FIG. 4  is a partially exploded view of the catheter  10  as shown in  FIG. 3 .  FIG. 4  shows the distal coupler  26  pulled away from the spring  38  to show how the distal coupler  26  and the spring  38  are connected to each other. 
     The distal end  32  of the elongated deflectable element  12  is connected to a proximal end  46  of the proximal coupler  24 . A proximal end  52  of the tube  40  and a distal end  48  of the proximal coupler  24  include complementary bayonet connecting features  50  connecting the distal end  48  of the proximal coupler with the proximal end  52  of the tube  40 . The bayonet connecting features  50  are described in more detail with reference to  FIGS. 5A-C  and  FIGS. 7A-B . 
       FIG. 4  shows that a distal end  54  of the tube  40  comprises holes  56  (only some labeled for the sake of simplicity) disposed around the circumference of the tube  40 . The distal end  54  of the tube  40  is connected to the distal coupler  26  distal coupler via an adhesive, e.g., epoxy, which extends into respective ones of the holes  56  thereby improving a bond between the distal end  54  of the tube  40  and an inner proximal surface  58  of the distal coupler  26 . The holes  56  are described in more detail with reference to  FIGS. 7A-B . 
     A distal edge  60  of the tube  40  includes openings  62  (only some labeled for the sake of simplicity) disposed around the circumference of the distal edge  60 . The inner proximal surface  58  of the distal coupler  26  includes protrusions  64  (only some labeled for the sake of simplicity), disposed circumferentially around the inner proximal surface  58 , and configured for engaging the openings  62  (or slots disposed around the perimeter of the generally tubular member  38 ) to prevent rotation of the distal coupler  26  with respect to the tube  40 . The openings  62  and the protrusions  64  may be any suitable shape. In some embodiments, the openings  62  include U-shaped openings as shown in the example of  FIG. 4  or rectangular shape openings. The openings  62  and the protrusions  64  are described in more detail with reference to  FIG. 6B  and  FIGS. 7A-B . 
     Reference is now made to  FIGS. 5A-C , which are schematic views of various sides of the proximal coupler  24  of the catheter  10  of  FIG. 1 . The proximal coupler  24  has a generally cylindrical shape with a central lumen  70  for passing various elements therein such as wires and an irrigation tube (not shown). The distal end  48  includes the bayonet connecting features  50  which may have any suitable shape, for example, but not limited to an L-shape recess. In some embodiments the distal end  48  includes three bayonet connecting features  50 . However, the distal end  48  may include more or less than three bayonet connecting features  50 . The proximal coupler  24  includes a recess  66  ( FIG. 5B ) for accepting the solder pad  36  ( FIG. 2A ) therein in addition to allowing a passage for wires connected to solder pad  36  to pass into coupler  24 . Coupler  24  also includes two flat surfaces  68  ( FIGS. 5A and 5C ) for accepting the position sensor  34  ( FIG. 2B ) thereon. The proximal coupler  24  may be formed from any suitable material or combination of materials, for example, but not limited to, polycarbonate with or without glass filler, polyether ether ketone (PEEK) with or without glass filler, or polyetherimide (PEI) with or without glass filler. The proximal coupler  24  may have any suitable outer diameter, for example, in a range of 1 mm to 10 mm. 
     Reference is now made to  FIGS. 6A-B , which are schematic views of the distal coupler  26  of the catheter  10  of  FIG. 1 .  FIG. 6A  shows that the distal coupler  26  includes openings  72  for irrigation and/or feeding wires to the electrodes  22  ( FIG. 1 ) of the distal assembly  14  ( FIG. 1 ). In some embodiments, the openings  72  may be disposed around a circumference of the distal coupler  26  in a proximal portion of the distal coupler  26  and/or in a distal portion of the distal coupler  26 . The distal coupler  26  may include any suitable number of openings  72 , for example, in a range between 1 and 12 openings  72 , such as 8 openings. 
       FIG. 6B  shows the inner proximal surface  58  and its protrusions  64  (only some labeled for the sake of simplicity). The distal coupler  26  may include any suitable number of protrusions  64  to engage with the openings  62  ( FIG. 4 ) of the tube  40  ( FIG. 4 ). In some embodiments, the distal coupler  26  includes eight protrusions  64 , but may be in the range of 1 to 30 protrusions  64 , by way of example. The protrusions  64  may have any suitable dimensions, in the order of 0.05 mm to 10 mm and may depend on the number of protrusions  64 . The distal coupler  26  may be formed from any suitable material or combination of materials, for example, but not limited to, polycarbonate with or without glass filler, polyether ether ketone (PEEK) with or without glass filler, or polyetherimide (PEI) with or without glass filler. The distal coupler  26  may have any suitable outer diameter, for example, in a range of 1 mm to 10 mm. 
     Reference is now made to  FIGS. 7A-B , which are schematic views of the spring  38  of the catheter  10  of  FIG. 1 . As previously mentioned, the spring  38  includes the tube  40  with helical cuts  42  extending around a circumference of the tube  40 . As seen in  FIG. 7A , two generally parallel helical cuts extend through the tubular member  38  while traversing along the longitudinal axis L-L to define a helical shaped spring member  45  (between the gaps  42 ) which is separated from the tubular member  38  by the gaps or cuts  42 . As shown in  FIG. 7A , three helical spring members  45  are provided. The helical cuts  42  provide the coupler  38  with its ability to bend and compress while provide a resistance to bending and compression. Each helical cut  42  includes deviations  44  extending in a longitudinal direction (approximately parallel to the axis L-L) of the tube  40 . The deviations  44  are configured to prevent overstretching and overbending of the spring  45  by having a portion of the spring  45  configured to interlock with another portion of the tubular member  38  via a suitable interlock  44 . 
     The helical cuts  42  extend from the outer surface of the tube  40  through to the inner surface of the tube  40 . The helical cuts  42  may be right-handed helices or left-handed helices. The pitch of each helical cut  42  may be any suitable value, for example in the range of 0.2 mm to 5 mm. The helical cuts  42  may have any suitable width, for example, in the range of 0.05 mm to 0.5 mm, such as 0.1 mm. The spring  38  may include any suitable number of helical cuts  42 , for example, two helical cuts  42  forming a double helix, or three helical cuts  42  forming a triple helix, and so on. The helical cuts  42  are generally circular helices disposed about a longitudinal axis L-L extending through the center of member  38 . Each of the helical cuts  42  may extend around the circumference for any suitable amount of turns, including extending for less than one turn but at least half a turn. For example, any of the helical cuts  42  may extend 1.5 turns, 1 turn, two-thirds of a turn (as shown in  FIGS. 7A and 7B ), or half a turn. In the example of  FIGS. 7A-B , respective ones of the helical cuts  42  extend from respective ones of the bayonet connecting features  50  up and around the circumference of the tube  40 . 
     Respective ones of the deviations  44  of respective ones of the helical cuts  42  define respective mechanical stops  74  (only some labeled for the sake of simplicity). Each mechanical stop  74  includes opposing surfaces  76  (only some labeled for the sake of simplicity) which are configured to come into contact with each other to prevent overstretching and overbending of the spring  38 . In some embodiments, the opposing surfaces  76  of respective ones of the mechanical stops  74  are configured to engage simultaneously so that a force applied on the spring  38  is shared among the respective mechanical stops  74  to prevent sequential failure of the respective mechanical stops  74 . In some embodiments, at least two, and generally all, of the mechanical stops  74  of each helical cut  42  are configured to engage simultaneously. In some embodiments, respective ones of the mechanical stops  74  include respective loops  78  and sockets  80  (only one labeled for the sake of simplicity). 
     In some embodiments, respective one of the deviations  44  include respective opposing sigmoid curves  82  (only two labeled for the sake of simplicity) with characteristic S-shapes so that the deviations  44 , and the mechanical stops  74 , are formed by an S-shape and a reversed S-shape as shown in  FIGS. 7A-B . In some embodiments, respective ones of the mechanical stops  74  include respective T-shape elements  84  disposed in respective T-shape openings  86 . 
     The bayonet connecting features  50  of the tube  40  are generally L-shaped and are configured to connect with the bayonet connecting features  50  of the proximal coupler  24  ( FIGS. 5A-C ). The tube  40  may include any suitable number of bayonet connecting features  50 , for example, one, two, three or more. 
     The holes  56  (only some labeled for the sake of simplicity) in the distal end  54  of the tube  40  are disposed around the circumference of the tube  40 . The tube  40  may include any suitable number of holes  56 , for example, two or more. The maximum number of holes  56  is generally restricted by the available space on the tube  40  above the helical cuts  42 . Each hole  56  may have any suitable diameter, for example, in the range of 0.05 mm to 0.5 mm. 
     In some embodiments, the tube  40  includes eight openings  62  (only some labeled for the sake of simplicity), but may have any suitable number of openings  62 , for example, in the range of 1 to 30 openings  62 . The openings  62  may have any suitable dimensions, in the order of 0.05 mm to 10 mm and may depend on the number of openings  62 . 
     The lower surface of the tube  40  may include notches  90  (only some labeled for the sake of simplicity) to enhance adhesive connection between the lower surface of the tube  40  and other elements of the catheter  10 , such as proximal coupler  24 . 
     The helical cuts  42 , holes  56 , and openings  62  may be formed by any suitable method, for example, by laser machining, electric discharge machining, or conventional machining. The tube  40  may have any suitable outer diameter, for example, in the range of 1 mm to 10 mm. The tube  40  may have any suitable wall thickness, for example, in the range of 0.1 mm to 3 mm. The tube may be formed from any suitable material, for example, Nitinol, beta titanium, beryllium copper, or phosphor bronze. 
     Reference is now made to  FIGS. 8A-C , which are schematic views of the force sensor  28  of the catheter  10  of  FIG. 1 . The force sensor  28  includes the spring  38  and a transmitting coil  92  disposed on the top and receiving coils  93  disposed on the bottom of the tube  40 . The receiving coils  93  receive a transmitted signal from the transmitting coil  92 . The received signal is representative of a distance between the transmitting coil  92  and the receiving coils  93  thereby providing an indication of a force applied to the force sensor  28 . Any suitable transmitting coil  92 , and receiving coils  93  may be used, for example, the coils described in US Patent Publication 2018/0256247 of Govari, et al. which is incorporated by reference with a copy provided in the Appendix. In some embodiments, the positioning of the transmitting coil  92  and the receiving coils  93  may be reversed or in any suitable position. In some embodiments, the force sensor  28  may include a single receiving coil and multiple transmitting coils, or any suitable combination thereof. 
     Reference is now made to  FIG. 9 , which is a schematic view of a spring  94  constructed and operative in accordance with an alternative embodiment of the present invention. The spring  94  includes mechanical stops  96 . Respective mechanical stops  96  include respective L-shape elements  98  disposed in respective L-shape openings  100 . 
     As used herein, the terms “about” or “approximately” for any numerical values or ranges indicate a suitable dimensional tolerance that allows the part or collection of components to function for its intended purpose as described herein. More specifically, “about” or “approximately” may refer to the range of values ±20% of the recited value, e.g. “about 90%” may refer to the range of values from 72% to 108%. 
     Various features of the invention which are, for clarity, described in the contexts of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment may also be provided separately or in any suitable sub-combination. 
     The embodiments described above are cited by way of example, and the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.