Patent Publication Number: US-2021186637-A1

Title: Bending structure and flexible tube for medical manipulator

Description:
FIELD OF THE INVENTION 
     The present invention relates to a flexible tube and a bending structure applicable to a bendable part of a medical manipulator such as a surgical robot. 
     BACKGROUND OF THE INVENTION 
     In recent medical treatment, a medical manipulator such as robot forceps for a surgical robot and manual forceps becomes broadly widened in order to enable to lighten burdens on both a patient and a doctor at the time of a surgery. 
     The medical manipulator such as the robot forceps and manual forceps allows a doctor to insert an arm as well as an endoscope camera through a small wound of a patient and perform a surgery with feeling as if forceps are actually manipulated while capturing a surgical field with eyes through a  3 D monitor. 
     As such a medical manipulator, there is one which provides an arm with a joint function by means of a bendable part to secure a high degree of freedom and allow more fine surgical operation like Patent document 1. 
     In the medical manipulator, a coiled spring is used for the bendable part of the arm so that the coiled spring is bent by drawing drive wires passing through an inside thereof. 
     The arm of the medical manipulator is desired to be reduced in size in order to make a wound of a patient smaller and lighten mental and physical burdens. Accordingly, the bendable part used in the arm is also desired to be reduced in size. 
     In the technique of Patent document 1, however, the bendable part is composed of the coiled spring and therefore is limited on the seize reduction for necessity of securing load bearing and bendability. 
     Such a problem is existed in not only the above-mentioned medical manipulator such as the robot forceps and the manual forceps but also other types of medical manipulators such as an endoscope camera.
     PATENT DOCUMENT 1: JP 2014-38075 A   

     SUMMARY OF THE INVENTION 
     A problem to be solved is that there is a limit on securing load bearing and bendability while conducting size reduction. 
     In order to conduct size reduction and provide superior load bearing and bendability, the present invention is most characterized by a flexible tube through which drive wires for a medical manipulator are passed in an axial direction and being configured to be bent according to operation of the drive wires, comprising, a corrugated tube portion having a corrugated portion in which crests and troughs are alternately arranged in the axial direction and being bendable according to expansion and contraction of the crests and the troughs; and through portions provided on the corrugated portion to pass the drive wires in the axial direction. 
     Since the corrugated tube portion is bent according to the expansion and contraction of the crests and the troughs, the present invention makes it possible to obtain the flexible tube having superior load bearing and bendability while conducting size reduction. 
     Further, the present invention uses the corrugated tube portion as a guide for the drive wires by passing the drive wires through the through portions provided on the corrugated portion that comprises the crests and the troughs of the corrugated tube portion, and therefore the drive wires are retained at appropriate positions to stably and accurately conduct bending motion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrating robot forceps having a flexible tube according to an embodiment 1 of the present invention; 
         FIG. 2  is a front view of the robot forceps of  FIG. 1 ; 
         FIG. 3  is a sectional view of the robot forceps of  FIG. 1 ; 
         FIG. 4  is a perspective view of the partly omitted robot forceps of  FIG. 1 ; 
         FIG. 5  is a side view of the partly omitted robot forceps of  FIG. 1 ; 
         FIG. 6  is a sectional view of the partly omitted robot forceps of  FIG. 1 ; 
         FIG. 7  is a perspective view of the flexible tube of the robot forceps of  FIG. 1 ; 
         FIG. 8  is a front view of the flexible tube of  FIG. 7 ; 
         FIG. 9(A)  is a sectional view of the flexible tube of  FIG. 7  and  FIG. 9(B)  is an enlarged view of a IX part of  FIG. 9(A) ; 
         FIG. 10  is a sectional view of the flexible tube at the time of bending; 
         FIG. 11(A)  is a graph illustrating a relationship between loads and bending angles of the flexible tube and  FIG. 11(B)  is a schematic view illustrating bending directions; 
         FIG. 12  is a perspective view illustrating a flexible tube according to an embodiment 2 of the present invention; 
         FIG. 13  is a side view of the flexible tube of  FIG. 12 ; 
         FIG. 14  is a sectional view of the flexible tube of  FIG. 12 ; 
         FIG. 15  is a sectional view illustrating robot forceps that uses a bending structure according to an embodiment 3 of the present invention; 
         FIG. 16  is a perspective view of the partly omitted robot forceps of  FIG. 15 ; 
         FIGS. 17(A) and 17(B)  are sectional views illustrating the bending structure of  FIG. 15  in which  FIG. 17(A)  illustrates a normal state and  FIG. 17(B)  illustrates a bending state; 
         FIG. 18  is a graph illustrating relationships between loads and bending angles of bending structures; 
         FIG. 19  is a perspective view of partly omitted robot forceps using a bending structure according to an embodiment 4 of the present invention; 
         FIG. 20  is a sectional view of the robot forceps of  FIG. 19 ; 
         FIG. 21  is a plan view of an elastic member according to a modification of the embodiment 4; 
         FIG. 22  is a plan view of an elastic member according to another modification of the embodiment 4; 
         FIG. 23  is a perspective view of partly omitted robot forceps using a bending structure according to an embodiment 5 of the present invention; 
         FIG. 24  is a sectional view of the robot forceps of  FIG. 23 ; and 
         FIG. 25  is a perspective view of an elastic member used for the bending structure of  FIG. 23 . 
     
    
    
     EMBODIMENT FOR CARRYING OUT THE INVENTION 
     The present invention accomplishes the object of conducting size reduction and providing superior load bearing and bendability by a flexible tube having through portions for passing drive wires that are formed with respect to a corrugated portion of a corrugated tube portion in which crests and troughs are alternately arranged in an axial direction. 
     The through portions are preferably provided in a circumferential direction of the corrugated tube portion and preferably have a constant distance from an axial center of the corrugated tube portion in a radial direction. 
     The through portions may be insertion holes each provided on a web between a crest and a trough of the corrugated tube portion being adjacent to each other in an axial direction. The through portions may be, however, insertion holes, cutouts, recessed portions or the like provided on the crests or the troughs. 
     The insertion hole may be positioned in the middle of an outer diameter at the crest and an inner diameter at the trough. 
     Further, an elastic member may be provided in the flexible tube to form a bending structure. The elastic member is configured to be arranged in the corrugated tube portion, have higher rigidity in the axial direction than the corrugated tube portion, and be bendable as well as the corrugated tube portion. 
     The elastic member may employ a variety of shapes and be, for example, a coiled spring, a solid cylinder, a hollow cylinder or the like located on an axial center portion of the corrugated tube portion. 
       FIG. 1  is a perspective view illustrating robot forceps having a flexible tube according to the embodiment 1 of the present invention,  FIG. 2  is a front view of the same, and  FIG. 3  is a sectional view of the same. 
     Robot forceps  1  composes a front end of a robot arm of a surgical robot as a medical manipulator. In addition, the robot forceps  1  are an example of a medical manipulator. 
     It should be noted that a medical manipulator to which the flexible tube  3  is applicable is not limited particularly as long as, regardless of whether being attached to a surgical robot, one is manipulated by a hand of a doctor or the like and has a bendable part which performs bending motion. 
     The medical manipulator, therefore, also includes an endoscope camera, manual forceps and the like that are not attached to the surgical robot. 
     The robot forceps  1  of the present embodiment comprises a shaft part  5 , a bendable part  7 , and a grasping unit  9 . 
     The shaft part  5  is formed into, for example, a cylindrical shape. Inside the shaft part  5 , passings are drive wires  11  for driving the bendable part  7  and a push/pull cable  13  for driving the grasping unit  9 . At a front end of the shaft part  5 , the grasping unit  9  is provided through the bendable part  7 . 
     The driving wires  11  are sufficient to be cord members, may be, for example, stranded wires, NiTi (Nickel-titanium) solid wires, piano wires, articulated rods, chains, strings, stitches, ropes or the like, but are not limited thereto. 
     The bendable part  7  comprises a flexible tube  3  according to the present embodiment. The bendable part  7  (flexible tube  3 ) passes the drive wires  11  and the push/pull cable  13  therethrough in an axial direction and is bendable according to operation of the driving wires  11 . The axial direction means a direction along an axial center of the flexible tube  3 , is not necessarily a direction being strictly parallel to the axial center, but also includes a direction slightly inclining relatively to the axial center. 
     In addition, the push/pull cable  13  is provided on an axial center portion of the bendable part  7  (flexible tube  3 ). The four driving wires  11  are provided so as to be located at 90 degrees in a circumferential direction according to the present embodiment, and are outwardly displaced and located relatively to the push/pull cable  13  in a radial direction, respectively. The details of the flexible tube  3  will be explained later. In addition, the radial direction is a radial direction of the flexible tube  3 . 
     The grasping unit  9  has a pair of grasping parts  9   b  which are openably pivotally supported with a base part  9   a  attached to a front end of the bendable part  7 . To the base part  9   a , the drive wires  11  passing through the bendable part  7  are connected. 
     The grasping unit  9 , therefore, is capable of orienting the grasping parts  9   b  to a desired direction while bending the bendable part  7  by operation of the driving wires  11 . 
     To the grasping parts  9   b , groove portions  9   c  are provided so as to be inclined relatively to the axial direction in a closed state of the grasping parts. Projections  9   e  of a movable piece  9   d  slidably engage with the groove portions  9   c  of the grasping parts  9   b . The movable piece  9   d  is arranged in a through-hole  9   f  of the base part  9   a  of the grasping unit  9  movably in the axial direction and is connected to the push/pull cable  13  passing through the bendable part  7 . 
     The grasping parts  9   b , therefore, are configured to be opened and closed by the movable piece  9   d  moving in the axial direction according to reciprocating movement (push/pull movement) of the push/pull cable  13 . It should be noted that the driving of the grasping unit  9  to open and close the grasping parts  9   b  is not limited to use of the push/pull cable  13  and may be used an air tube or drive cables. 
       FIG. 4  is a perspective view of the partly omitted robot forceps  1  of  FIG. 1 ,  FIG. 5  is a side view of the same, and  FIG. 6  is a sectional view of the same.  FIG. 7  is a perspective view of the flexible tube  3 , and  FIG. 8  is a sideview of the same. Further,  FIG. 9(A)  is a sectional view of the flexible tube of  FIG. 1  and  FIG. 9(B)  is an enlarged view of a IX part of  FIG. 9(A) .  FIG. 10  is a sectional view of the flexible tube at the time of bending. 
     As illustrated in  FIGS. 1-10 , the flexible tube  3  is a bellows made of metal such as nickel and is formed into a tubular shape. It should be noted that the material of the flexible tube  3  may be appropriately employed according to required characteristics, manufacturing method or the like. 
     The flexible tube  3 , as the bendable part  7  of the robot forceps  1 , resiliently supports the grasping unit  9  relatively to the shaft part  5 . According to the present embodiment, the flexible tube  3  comprises end tube portions  15 , and a corrugated tube portion  17 . 
     The end tube portions  15  are circular ring portions located at respective end of the flexible tube  3 . The end tube portions  15  are respectively fitted to the front end side of the shaft part  5  and the base part  9   a  side of the grasping unit  9  of the robot forceps  1  to allow the flexible tube  3  to be attached to the robot forceps  1  side. 
     According to the present embodiment, the end tube portions  15  are fitted to a first connection part  19  and a second connection part  21  fixed to the front end of the shaft part  5  and the base part  9   a  of the grasping unit  9 . 
     The first and second connection parts  19 ,  21  respectively composes parts of the front end of the shaft part  5  and the base part  9   a  of the grasping unit  9  and are formed of resin, metal or the like into cylindrical shapes. 
     In the first connection part  19 , the driving wires  11  pass through through-holes  19   a  in the axial direction. In the second connection part  21 , front end portions of the driving wires  11  are fixed into fixing holes  21   a . Further, on an axial center portion of the first connection part  19 , a cable passing hole  19   b  is provided to pass the push/pull cable  13  therethrough. 
     Between the end tube portions  15  of the flexible tube  3 , the corrugated tube portion  17  is integrally provided. 
     The corrugated tube portion  17  is formed into a hollow circular tubular shape continuously transitioning from the end tube portions  15 . In addition, the corrugated tube portion  17  and the end tube portions  15  may be configured to have the same thickness or different thicknesses. Further, thickness may vary among crests  17   a , troughs  17   b , and webs  17   c  of the corrugated tube portion  17  to be explained later. 
     The corrugated tube portion  17  has a corrugated portion  18  formed into a corrugated shape in which the crests  17   a  and the troughs  17   b  are alternately arranged according to variation in the diameter in the axial direction and being bendable according to expansion and contraction of the crests  17   a  and the troughs  17   b.    
     In addition, the corrugated tube portion  17  may have a tubular shape such as square tube. In the case of the square tube, however, it is preferably that the corrugated tube portion  17  has a plan view such as foursquare, regular hexagon, or regular octagon being point symmetrical around an axial center of the corrugated tube portion in order to suppress anisotropy as explained later. 
     The crests  17   a  and the troughs  17   b  of the corrugated portion  18  have sectional shapes curved into arc shapes. Outer diameters of the crests  17   a  are constant and the same as outer diameters of the end tube portions  15 . Pitches between the crests  17   a  and inner diameters of the troughs  17   b  are constant. The outer diameters of the crests  17   a , the pitches between the crests  17   a , and the inner diameters of the troughs  17   b  may be, however, varied in the axial direction. 
     Radii of curvatures of the crests  17  and the troughs  17   b  are the same as each other according to the present embodiment. The radii of curvatures may be, however, different from each other. 
     An interposition between the crest  17   a  and the trough  17   b  being adjacent to each other is the web  17   c  being flat in the radial direction. On the web  17   c , an insertion hole  17   d  is formed as a through portion. Accordingly, the insertion holes  17   d  are formed on the corrugated portion  18  in the present invention. It should be noted that the insertion holes  17   d  may be formed on the crests  17   a  or the troughs  17   b  having the curved shape. 
     The corrugated shape of the corrugated portion  18  of the corrugated tube portion  17  is not limited particularly and may be formed into a sine wave, a triangular wave, a rectangular wave, or a sawtooth wave as a whole by, for example, setting of the sectional shapes of the crests  17   a , the troughs  17   b , and the webs  17   c.    
     The insertion holes  17   d  are provided on each web  17   c  in the circumferential direction of the corrugated tube portion. According to the present embodiment, since the four drive wires  11  are provided respectively at 90 degrees in the circumferential direction, the four insertion holes  17   d  are provided at 90 degrees in the circumferential direction on each web  17   c  accordingly. 
     Between the webs  17   c  adjacent to each other in the axial direction, the insertion holes  17   d  are communicated with each other in the axial direction and the drive wires  11  are passed through the communicating insertion holes  17   d . With the passing, the flexible tube  3  passes the drive wires  11  therethrough in the axial direction as the through portions and functions as a guide to retain the drive wires at given positions. 
     It should be noted that the through portions may be cutouts or recessed portions radially recessed from an inner periphery or outer periphery of the main body  15  of the flexible tube  3  instead of the insertion holes  17   d . The flexible tube  3  may, therefore, axially pass the drive wires  11  alongside the through portions being the recessed portions or the like on the inner periphery or outer periphery. 
     Further, each insertion hole  17   d  is positioned in the middle of the outer diameter at the crest  17   a  and the inner diameter at the trough  17   b  on the web  17   c . The insertion hole  17   d  may be, however, displaced radially inwardly or outwardly with respect to the middle of the outer diameter and the inner diameter. Further, distances from the axial center of the main body  15  to the respective insertion holes  17   d  may be appropriately set according to the characteristics of the flexible tube  3  and, for example, may or may not be constant. 
     A shape of the insertion hole  17   d  is circular, the diameter of which is larger than the diameter of the drive wire  11 . The difference between the diameters permits the expansion and contraction of the crests  17   a  and the trough  17   b  at the time of the bending of the flexible tube  3 . It should be noted that the shape of the insertion hole  17   d  is not limited to the circular shape and may be another shape such as rectangular shape as long as the expansion and contraction of the crests  17   a  and the trough  17   b  are permitted. 
     In the flexible tube  3  as the bendable part  7 , the movable side located on the grasping unit  9  side bends relatively to the stationary side located on the shaft part  5  side as illustrated in  FIG. 10  by drawing any one of the drive wires  11  ( FIG. 11(B) ) when a doctor manipulates the robot forceps  1 . Then, a number of the drive wires  11  are combined to be drawn, thereby to allow the flexible tube to bend omnidirectionally at 360 degrees. 
     When drawing any one of the drive wires  11  to conduct the bending, the flexible tube  3  is compressed at the crests  17   a  and the troughs  17   b  on an inner portion of the bending relative to a neutral axis and is extended at the crests  17   a  and the troughs  17   b  on an outer portion of the bending relative to the neutral axis. 
     Namely, the crests  17   a  and the troughs  17   b  on the inner portion of the bending deforms so as to shrink a spread in the axial direction, and the crests  17   a  and the troughs  17   b  on the outer portion of the bending deforms so as to enlarge a spread in the axial direction. 
     By deforming in this way, the flexible tube  3  bends as a whole. Such bending motion is performed omnidirectionally at 360 degrees in the same way as the above without a change in the bending state, thereby to suppress anisotropy. 
     Further, at the time of the bending, the flexible tube  3  passes the drive wires  11  through the insertion holes  17   d  to retain the drive wires at the appropriate positions, so that the flexible tube  3  stably and accurately conducts the bending motion according to the manipulation of the doctor. 
     Furthermore, although the drive wires  11  bend according to the bending of the flexible tube  3 , the drive wires pass through each web  17   c  that displaces so as to incline according to the bending of the flexible tube  3  at this time, thereby to secure stability of the manipulation. 
       FIG. 11(A)  is a graph illustrating a relationship between loads and bending angles of the flexible tube  3  according to the embodiment 1 and  FIG. 11(B)  is a schematic view illustrating bending directions. 
     In  FIG. 11(A) , loads are plotted when any one of the drive wires  11  of  FIG. 11(B)  is operated to bend the flexible tube  3  toward said any one of the drive wires  11  (0 degree, 90 degrees, 180 degrees, or 270 degrees in  FIG. 11(B) ) from 0 degree to 90 degrees in a bending angle. 
     As illustrated in  FIG. 11(A) , the linearity of the increase in load relative to the increase from 0 degree to 90 degrees in a bending angle is high and therefore the load bearing and the bendability are superior. 
     As mentioned above, the flexible tube  3  according to the present embodiment is provided with the corrugated tube portion  17  having the corrugated portion  18  in which the crests  17   a  and the troughs  17   b  are alternately arranged in the axial direction and being bendable according to the expansion and contraction of the crests  17   a  and the troughs  17   b , and the insertion holes  17   d  as the through portions provided on the corrugated portion  18  to pass the drive wires  11  in the axial direction. 
     Since the corrugated tube portion  17  is bent according to the expansion and contraction of the crests  17   a  and the troughs  17   b , therefore, the present embodiment heightens the linearity of the load bearing of the bending angles and the loads. The present embodiment makes it possible to obtain the flexible tube  3  having the superior load bearing and bendability while conducting size reduction. 
     Further, the present embodiment substantially uniforms the bending state of the crests  17   a  and the troughs  17   b  regardless of the bending direction and suppresses the anisotropy of the bending. 
     As a result, the flexible tube  3  stably and accurately conducts the bending motion according to the operation of the doctor. 
     Furthermore, the present embodiment passes the drive wires  11  through the corrugated portion  18  of the corrugated tube portion  17 , thereby to use the corrugated tube portion  17  as the guide for the drive wires  11 . 
     The present embodiment, therefore, retains the drive wires  11  at the appropriate positions to more stably and accurately conduct the bending motion. 
     Further, the flexible tube  3  has the high airtightness to prevent the inside thereof from being contaminated. 
     Further, the flexible tube  3  provides superior torsional rigidity. 
     According to the present embodiment, the insertion hole  17   d  is formed on the web  17   c  between the crest  17   a  and the trough  17   b , so that the drive wires  11  are passed through the webs  17   c  inclined according to the bending of the flexible tube  3  to secure the stability of the operation of the drive wires  11 . 
       FIG. 12  is a perspective view illustrating a flexible tube according to the embodiment 2 of the present invention,  FIG. 13  is a side view of the same, and  FIG. 14  is a sectional view of the same. In addition, components in the embodiment 2 corresponding to those in the embodiment 1 are represented with the same numerals to eliminate duplicate explanation. 
     In the flexible tube  3  according to the present embodiment, a corrugated shape of a corrugated tube portion  17  is altered. 
     Namely, each crest  17   a  of the corrugated tube portion  17  has a sectional shape being a wedge shape in which a web  17   ca  on one side in an axial direction and a web  17   cb  on the other side are connected to each other. Each trough  17   b  has a sectional shape being a wedge shape in which, with reverse of the one side and the other side in the axial direction relative to the case of the crest  17   a , the web  17   cb  on one side and the web  17   ca  on the other side are connected to each other. 
     The sectional shapes of the webs  17   ca  and  17   cb  are substantially the same shape curved in a cubic curve. In addition, the web  17   cb  is inclined relatively to the web  17   ca.    
     With the inclination and the curved shape, part of the web  17   cb  is located within a length of the web  17   ca  in the axial direction. Namely, the part of the web  17   cb  and the part of the web  17   ca  overlap each other in a radial direction. 
     The corrugated tube portion  17  of the present embodiment is, therefore, reduced in length as a whole. 
     Further, in each web  17   ca , part on a radial inner side and part on a radial outer side overlap each other in the radial direction. By that amount, the corrugated tube portion  17  is reduced in length in the axial length. 
     Accordingly, the present embodiment reduces the corrugated tube portion  17  in size in the axial direction. In addition, the present embodiment also provides the same effect as the embodiment 1. 
       FIG. 15  is a sectional view illustrating robot forceps that uses a bending structure having a flexible tube according to the embodiment 3 of the present invention, and  FIG. 16  is a perspective view of the partly omitted robot forceps of  FIG. 15 . Further,  FIG. 17  is a set of sectional views illustrating the bending structure of  FIG. 15  in which  FIG. 17(A)  illustrates a normal state and  FIG. 17(B)  illustrates a bending state. In addition, components in the embodiment 3 corresponding to those in the embodiment 1 are represented with the same numerals to eliminate duplicate explanation. 
     According to the present embodiment, an elastic member  23  is arranged in the flexible tube  3  of the embodiment 1 to form a bending structure  25 . 
     The elastic member  23  is a coiled spring made of metal, in particular a close contact coiled spring. In addition, the close contact coiled spring means a coiled spring in which coils are in closely contact with each other in a free state. As the elastic member  23 , a non-close contact coiled spring may be used, the non-close contact coiled spring having a gap between coils in a free state. 
     The elastic member  23  of the present embodiment has a sectional shape of an element wire of the coiled spring being circular. The sectional shape of the element wire of the coiled spring may be, however, another shape such as rectangular or oval shape. 
     The elastic member  23  is arranged on the axial center portion of the flexible tube  3  so as to define a cable insertion hole  23   a  through which a push/pull cable  13  passes on an inner periphery. An outer periphery of the elastic member  23  has a gap with respect to troughs  17   b  of the flexible tube  3 . 
     In an axial direction, the elastic member  23  extends over at least a whole corrugated tube portion  17  of the flexible tube  3 , rigidity against compression of which is set higher than that of the flexible tube  3 . Accordingly, the elastic member  23  is capable of preventing the flexible tube  3  from being unexpectedly compressed in the axial direction. 
     Further, the elastic member  23  is bendable according to the corrugated tube portion  17  and has a function to adjust load characteristics of the flexible tube  3  according to load characteristics in a bending direction. 
       FIG. 18  is a graph illustrating relationships between loads and bending angles of the bending structures  25  according to the embodiment 3 and a comparative example. 
     As the comparative example, the relationship between the loads and the bending angles of the flexible tube  3  of the embodiment 1 is illustrated. 
     The embodiment 3, similar to the comparative example, loads are plotted when the bending structure  25  is bent from 0 degree to 90 degrees in the bending angle. 
     The embodiment 3 heightens the loads all over the bending angles relatively to the comparative example while maintaining linearity of increase of the loads relative to increase from 0 degrees to 90 degrees in the bending angle, so that load bearing and bendability are superior. 
     As mentioned above, the bending structure  25  of the present embodiment is provided with the elastic member  23  being arranged in the corrugated tube portion  17  of the flexible tube  3 , having the higher rigidity in the axial direction than the corrugated tube portion  17 , and being bendable according to the bending of the corrugated tube portion  17 . 
     The bending structure  25  of the present embodiment is, therefore, capable of preventing the flexible tube  23  from being unexpectedly compressed. 
     Accordingly, although behavior of the bendable part  7  in response to operation of driving wires  11  may be unstable if the flexible tube  3  is unexpectedly compressed, the present embodiment suppresses such unstable behavior. Further, a path length is not varied at the time of the bending, so that operation of the grasping unit  9  is stable. 
     Further, the bending structure  25  of the present embodiment adjusts the load characteristics of the flexible tube  3  according to the load characteristics in the bending direction of the elastic member  23 . 
     In addition, the present embodiment provides the same effects as the embodiment 1. 
     It should be noted that the elastic member  23  is applicable to the embodiment 2. 
       FIG. 19  is a perspective view of partly omitted robot forceps that is provided with a bending structure according to the embodiment 4 of the present invention, and  FIG. 20  is a sectional view of the same. In addition, components in the embodiment 4 corresponding to those in the embodiment 3 are represented with the same numerals to eliminate duplicate explanation. 
     A bending structure  25  of the present embodiment is what an elastic member  23  is made solid cylindrical. The others are the same components as the embodiment 3. 
     The elastic member  23  is formed of elastic material such as rubber into a solid cylinder. With this, the elastic member  23  is configured to have higher rigidity in an axial direction than a main body  15  of a flexible tube  3  and be bendable according to bending of the flexible tube  3 . 
     In addition, in the present embodiment, since the solid cylindrical elastic member  23  is located on an axial center portion of the flexible tube  3 , drive wires or the like are preferably employed instead of a push/pull cable  13  to drive a grasping unit  9 . 
       FIG. 21  a plan view illustrating an elastic member  23  according to a modification, and  FIG. 22  is a plan view illustrating an elastic member  23  according to another modification. 
     The modification of  FIG. 21  forms groove portions  23   b  on an outer periphery of the solid cylindrical elastic member  23 , the groove portions being recessed in a radial direction. The groove portions  23   b  are provided along the elastic member  23  in the axial direction, to guide drive wires  24  employed instead of the push/pull cable  13  for driving the grasping unit  9 . 
     It should be noted that the number and the arrangement of the drive wires  24  are appropriately altered according to the structure of the grasping unit  9 , and accordingly the number and the arrangement of the groove portions  23   b  are appropriately altered. 
     The modification of  FIG. 22  provides the solid cylindrical elastic member  23  with a slit  23   c  being recessed from an outer periphery to the vicinity of an axial center portion in the radial direction. The slit  23   c  is provided along the elastic member  23  in the axial direction to guide the push/pull cable  13  in the axial center portion of the elastic member  23 . 
     In addition, the slit  23   c , as illustrated with a two-dot chain line, may be configured to be slightly narrower than a diameter of the push/pull cable  13  from the outer periphery of the elastic member  23  before the axial center portion and have the same diameter as the push/pull cable  13  at the axial center portion. Further, the slit  23   c  may be provided so as to pass across the axial center portion of the elastic member  23 . 
     The embodiment 4 and the modifications provide the same effects as the embodiment 3. 
       FIG. 23  is a perspective view of partly omitted robot forceps being provided with a bending structure according to the embodiment 5 of the present invention,  FIG. 24  is a sectional view of the same, and  FIG. 25  is a perspective view illustrating an elastic member used for the bending structure of  FIG. 24 . In addition, components in the embodiment 5 corresponding to those in the embodiment 3 are represented with the same numerals to eliminate duplicate explanation. 
     A bending structure  25  of the present embodiment is what an elastic member  23  is made into a hollow cylinder. The others are the same components as the embodiment 3. 
     The elastic member  23  is made of super elastic alloy and comprises end tube portions  27   a ,  27   b , ring portions  29 , tube connecting portions  31   a ,  31   b , and tube slits  33 . In addition, the super elastic alloy may be NiTi alloy (Nickel-titanium alloy), titanium-base alloy such as gummetal (registered trademark), Cu—Al—Mn alloy (copper-base alloy), Fe—Mn—Al alloy (iron-base alloy) or the like. 
     The end tube portions  27   a ,  27   b  are rings provided at respective end portions. Between the end tube portions  27   a ,  27   b , the ring portions  29  are located. 
     The ring portions  29  are successively parallelly provided at regular intervals in an axial direction. Spreads in the axial direction of the ring portions  29  are constant according to the embodiment. The spreads in the axial direction of the ring portions  29  may be, however, gradually reduced from a stationary side located on a shaft portion  5  side to a movable side located on a grasping unit  9  side. 
     The adjacent ring portions  29  are connected by the tube connecting portions  31   a ,  31   b  at parts in a circumferential direction. The ring portions  29  at respective ends are connected by the tube connecting portions  31   a ,  31   b  to the end tube portions  27   a ,  27   b.    
     The tube connecting portions  31   a ,  31   b  are provided integrally to the ring portions  29  and connects the ring portions  29  being adjacent to each other in the axial direction at two parts in the circumferential direction, the two parts opposing each other in a radial direction. 
     In each ring portion  29 , the tube connecting portions  31   a ,  31   b  located on one side (base end side) in the axial direction and the tube connecting portions  31   a ,  31   b  located on the other side (front end side) in the axial direction are arranged to be displaced by 180/N degrees in the circumferential direction. 
     The displacement of the tube connecting portions  31   a ,  31   b  means displacement between center lines of the tube connecting portions  31   a ,  31   b  (the same shall apply hereinafter). N is an integer equal to or more than 2. According to the present embodiment, N=2 is set and the tube connecting portions  31   a ,  31   b  are arranged to be displaced by 90 degrees. 
     It should be noted that the displacement between the tube connecting portions  31   a ,  31   b  may be 60 degrees or the like, but is preferably 90 degrees. This reduces the number of the ring portions  29  required to bend the flexible tube  3  and makes the entire length compact. 
     Each tube connecting portion  31   a ,  31   b  is a rectangular plate extended in the axial direction and has a slight curvature according to the ring portion  29 . Widths of the tube connecting portions  31   a ,  31   b  in the circumferential direction are constant according to the present embodiment and may be gradually reduced from the stationary side located on the shaft portion  5  side to the movable side located on the grasping unit  9  side. 
     If the widths of the tube connecting portions  31   a ,  31   b  in the circumferential direction are gradually reduced toward the movable side, the spread of the ring portions  29  in the axial direction may be smaller than the maximum width of the connecting tube portion  31   a ,  31   b  in the circumferential direction. In this case, the minimum width of the tube connecting portion  31   a ,  31   b  in the circumferential direction is preferably equal to the spread of the ring portions  29  in the axial direction. 
     Both ends of the tube connecting portions  31   a ,  31   b  in the axial direction transition through arc portions  35  to the ring portions  29 . Accordingly, the tube connecting portions  31   a ,  31   b  and the ring portions  29  are tangentially continued to each other. 
     In addition, the tube connecting portions  31   a ,  31   b  and the ring portions  29  are transition to each other with no step on respective inner and outer peripheries in the radial direction of the ring portions  29 . The tube connecting portions  31   a ,  31   b  may have, however, form to be thicker or thinner than the ring portions  29  to have steps. 
     The tube connecting portions  31   a ,  31   b  bend so that one side of a neutral axis as a boundary in the circumferential direction is compressed and the other side in the circumferential direction is extended, to allow the flexible tube  3  to be bent. According to the present embodiment, the tube connecting portions  31   a ,  31   b  displaced by 90 degrees in the circumferential direction are bent to allow bending in two different orthogonal directions. 
     On both sides of each tube connecting portion  31   a ,  31   b  in the circumferential direction, the tube slits  33  are provided to allow the bending of the flexible tube  3  based on the bending of the tube connecting portions  31   a ,  31   b.    
     Namely, the tube slits  33  are defined on both sides of the tube connecting portions  31   a ,  31   b  in the circumferential direction between the ring portions  29  being adjacent to each other in the axial direction. Each tube slit  33  is a rectangular shape with rounded corners according to the shapes of the ring portions  29  and the tube connecting portions  31   a ,  31   b.    
     The embodiment 5 also provides the same effects as the embodiment 3. 
     Further, according to the embodiment 5, the elastic member  23  made of the super elastic alloy is formed by connecting the ring portions  29  to each other with the tube connecting portions  31   a ,  31   b  in the axis direction and is bendable according to the bending of the tube connecting portions  31   a ,  31   b , thereby to conduct size reduction and provide the superior load bearing and bendability. 
     Based on the characteristics, the present embodiment improves the characteristics of the whole bending structure  25 . 
     Further, the elastic member  23 , with the structure to connect the ring portions  29  by the tube connecting portions  31   a ,  31   b , is superior in torsional rigidity. Accordingly, the present embodiment improves the torsional rigidity of the whole bending structure  25 .