Abstract:
A selectively shapeable medical device comprises an elongate tube, an activation component operatively coupled to the elongate tube, and means for actuating the activation component. The activation component has a first state and a second state different from the first state. The activation component alters a stiffness of at least a portion of the tube coupled to the activation component in the first state and does not alter a stiffness of the portion of the tube in the second state.

Description:
RELATED U.S. PATENT APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 13/632,478, filed filed Oct. 1, 2012, which is a divisional of U.S. patent application Ser. No. 10/661,159 filed Sep. 12, 2003 (now U.S. Pat. No. 8,298,161 B2), which claims the benefit of priority of U.S. Provisional Application No. 60/409,927, filed Sep. 12, 2002, the entirety of each of which is incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to devices, systems, and processes useful for exploration of hollow body structures, particularly those areas accessed through a tortuous, unsupported path. More particularly, the present disclosure relates to a shape-transferring cannula device that creates a custom-contoured access port for insertion and removal of diagnostic, surgical, or interventional instruments to and from a site within the body to which the physician does not have line-of-sight access. 
       BACKGROUND 
       [0003]    Surgical cannulas are well known in the art. Such devices generally include tube-like members that are inserted into openings made in the body so as to line the openings and maintain them against closure. Surgical cannulae can be used for a wide variety of purposes, and their particular construction tends to vary accordingly (see, e.g., U.S. Pat. No. 5,911,714). Flexible endoscopes, endovascular catheters and guidewires, and trocar cannulae such as those used in laparascopic surgery, are examples of such devices. Several U.S. patents recite such devices. See, for example, U.S. Pat. Nos. 5,482,029; 5,681,260; 5,766,163; 5,820,623; 5,921,915; 5,976,074; 5,976,146; 6,007,519; 6,071,234; and 6,206,872. 
         [0004]    All of these devices are in use in one form or another and they are helpful to some extent, but they also pose several problems. Flexible endoscopes and endovascular catheters rely on reaction forces generated by pushing against the tissue of the body cavity being explored to navigate around corners or bends in the anatomy. This approach works reasonably well for small-diameter endovascular catheters that are typically run through arteries well supported by surrounding tissue. In this case the tissue is effectively stiffer than the catheter or guidewire and is able to deflect the catheter&#39;s path upon advancement into the vessel. The approach is much less successful in the case of flexible endoscopes being guided through a patient&#39;s colon or stomach. In these cases the endoscope is either significantly stiffer than the body cavity tissue it is being guided through or, as is the case for the stomach or an insufflated abdomen, the body cavity is sufficiently spacious that the endoscope has no walls at all to guide it. In the case of colonoscopy, the endoscope forces the anatomy to take painful, unnatural shapes. Often, the endoscope buckles and forms “loops” when the colonoscopist attempts to traverse tight corners. Pushing on the end of the flexible endoscope tends to grow the loop rather than advance the endoscope. “Pushing through the loop” relies on the colon to absorb potentially damaging shapes of force to advance the endoscope. In cases of unusually tortuous anatomy, the endoscope may not reach its intended target at all, leaving the patient at risk of undiagnosed and potentially cancerous polyps. 
         [0005]    Endovascular catheters have drawbacks as well. While generally flexible enough to avoid seriously damaging the vessel&#39;s endothelial surface, guidewires are difficult to guide into small side branches of large vessels such as the coronary ostia or into relatively small vessels connecting to relatively large chambers such as the pulmonary veins. Catheters are even more limited in their ability to deal with greatly tortuous vessel anatomy such as the vessels radiating from the brain&#39;s so-called Circle of Willis. 
         [0006]    Ablation and EKG mapping catheters used in cardiological electrophysiology find their intended targets chiefly by trial and error insertion and twisting of a guidewire/catheter accompanied by gross motions of the entire catheter. A need, therefore exists for a cannula system that provides access port for insertion and removal of diagnostic, surgical, or interventional instruments to and from a site within the body to which the physician does not have line-of-sight access. Furthermore, there is a need for cannula systems that can follow a tortuous path through hollow soft-tissue structures without relying on the surrounding tissue to mechanically support and guide its insertion and may be steered and advanced directly to an anatomical point of interest. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a diagrammatic sectional view of the shape transferring cannula system illustrating the major components. 
           [0008]      FIGS. 2A-2D  illustrate diagrammatic sectional representations of a sequence of rigidizing structure stiffening, relaxing, and advancement that enables guiding of the shape transferring cannula. 
           [0009]      FIG. 3  is a perspective view of an embodiment of laterally parallel rigidizing core and sheath linkage structures. 
           [0010]      FIGS. 4A and 4B  illustrate perspective views of captured-link rigidizing linkages. 
           [0011]      FIG. 5  is a perspective view of a cable-rigidized core linkage. 
           [0012]      FIG. 6  shows perspective and sectional views of a cable-rigidized sheath linkage. 
           [0013]      FIG. 7  is a sectional view of an off-axis tensioning mechanism for a cable-rigidized sheath linkage. 
           [0014]      FIG. 8  is a perspective view of an alternate embodiment of a laterally parallel sheath linkage. 
           [0015]      FIG. 9  is a sectional view of a laterally parallel sheath linkage. 
           [0016]      FIG. 10  shows perspective and end views of an open-sided sheath linkage. 
           [0017]      FIG. 11  is a perspective view of a laterally parallel sheath linkage with compliant elements. 
           [0018]      FIGS. 12A-12H  illustrate various views of an alternating advancement mechanism in accordance with the present disclosure. 
           [0019]      FIGS. 13A-13C  is a perspective view of a rotating link cannula structure 
           [0020]      FIGS. 14A-14C  is a diagrammatic view of a cannula structure including a passive element. 
           [0021]      FIG. 15  is a sectional view of a continuous stiffening cannula structure. 
           [0022]      FIG. 16  is a perspective view of a cannula structure formed with normally-rigid, thermally relaxing materials. 
           [0023]      FIGS. 17A and 17B  illustrate sectional views of vacuum-stiffening cannula structure elements. 
           [0024]      FIG. 18  is a sectional view of a pressure-stiffening cannula structure. 
           [0025]      FIGS. 19A and 19B  show a sectional view of normally-rigid, vibrationally relaxing cannula structure elements. 
           [0026]      FIGS. 20A and 20B  illustrate sectional views of a cannula structure including active material elements. 
           [0027]      FIG. 21  is a perspective view of two-axis pivoting links. 
           [0028]      FIG. 22  depicts a catheter with a shape-transferring section. 
           [0029]      FIG. 23  depicts a thermally relaxing normally-rigid structure. 
           [0030]      FIG. 24  depicts a motorized advancement mechanism. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    The present disclosure is directed to a novel shape-transferring cannula system, which provides access to tortuous and unsupported paths. The shape-transferring cannula system and method enables exploration of hollow body structures, and creates a custom-contoured access port for insertion and removal of, for example, diagnostic, surgical, or interventional instruments to and from a site within the body to which the physician does not have line-of-sight access. 
         [0032]    The shape-transferring cannula can follow a tortuous path through hollow soft-tissue structures without relying on the surrounding tissue to mechanically support and guide its insertion. The system includes two parallel rigidizing sections that alternatingly stiffen and relax with respect to one another and alternatingly transfer the path shape traced-out by the articulating tip to one another. A steerable articulated tip is attached to one of the rigidizing sections. The cannula&#39;s custom shape is formed by guiding the articulated tip along a desired path direction, stiffening the attached rigidizing section, and advancing the other rigidizing section along the stiffened section. 
         [0033]    The end of the shape-transferring cannula may be steered and advanced directly to an anatomical point of interest. The user traces a path for the shape-transferring cannula with the steerable tip and in doing so defines the longitudinal shape assumed by the cannula, thus directing the working end of the cannula to a target site without substantially disturbing the length of cannula behind it. The ability to navigate predictably within heart chambers and swap out catheters from a relatively fixed position, for example, greatly improves electrophysiologists&#39; ability to methodically locate and ablate the ectopic foci responsible for atrial fibrillation and other cardiac arrhythmias. 
         [0034]    The ability to localize movements to the user-controlled tip of the cannula is especially valuable when working within particularly sensitive open structures such as the ventricles of the brain or loosely supported, tortuous structures such as the colon that provide very little mechanical support for intubation around corners. 
         [0035]    The shape-transferring cannula system assumes the shape traced by the path of the articulating tip in an incremental fashion, with the core and sheath rigidizing structures transferring the traced path shape back and forth to each other. Having reached the target site, the external sheath can be made flexible and slid out over the rigidized central core. Unlike the lengthy re-intubation procedure for a conventional flexible endoscope, returning to the target site is simply a matter of sliding the sheath or surgical instruments over the core that now acts as a guidewire. Alternately, the sheath may be left rigid and in place once having reached the target site and the core may be made flexible and removed. This leaves the shaped sheath to act as a cannula through which surgical instruments such as snares, ultrasound probes, biopsy probes and other diagnostic devices, electrocautery tools, and the like may be transferred to and from the target site. 
         [0036]    The individual elements of the disclosure have useful applications independent of the full system. For instance, the rigidizing sheath structure may be used on its own as a rigidizing cannula when introduced to a target site by a conventional guidewire, flexible endoscope, or similar introduction element. Unlike conventional rigid cannulae, the rigidizing cannula does not have a predetermined longitudinal shape. Yet, when stiffened, the rigidizing cannula may support reaction forces like a rigid cannula when tools are run down its length, thus protecting sensitive tissue structures. 
         [0037]    The present disclosure will now be described in detail with reference to the following drawings.  FIG. 1  depicts a preferred embodiment of a shape-transferring cannula system Id having two parallel rigidizing core  1  and sheath  2  structures, a steerable articulated tip  3  attached to one of the rigidizing structures, a proximal end  1   a , a distal end  1   b  and a lumen  1   c  through which surgical tools may be introduced or through which the target site may be irrigated or suctioned. The core  1  and sheath  2  are parallel structures that can be coaxial or side-by-side and that may be made rigid or flexible with respect to one another. The core and sheath structures may be unitary materials or continuous structures, or they can be formed of individual, flexibly connected rigid links. The core and sheath structure employs rigidizing cables which, when put into tension, pull the links together to increase friction between links and prevent relative motion between the links. The core&#39;s rigidizing structure is built-up of links such that a convex spherical surface on one link engages a concave surface on an adjacent link. The core&#39;s rigidizing cable runs through each core link&#39;s central orifice, connecting the entire core rigidizing structure. The core link&#39;s central orifice has a diameter D 1  that is in the range from about 0.5 mm to about 30 mm. Those of skill in the art will readily appreciate that the particular application, e.g., device, to which the present disclosure is applied may require a particular diameter D 1 , and it is within the scope of the present disclosure to select an appropriate diameter D 1  for the specific application. For example, a typical endoscope employing structures in accordance with the present disclosure may have an inner diameter from about ¼ inch to about ½ inch, although larger or smaller sizes may also be suitable. 
         [0038]    The system employs a method of incremental advancement to deliver the distal end  1   b  of the cannula to a target site. The core  1  and sheath  2  rigidizing structures are alternatingly advanced, one structure past the other, the stationary structure being made rigid and acting as a guide for the advancing flexible structure. The steerable tip assembly  3  is located on the end of at least one of the two rigidizing structures such as the core  1  as depicted in  FIG. 1 . The steerable tip  3  may be actuated via cables or other tension members, magnetostrictive materials, bimetallic strips or other flexing elements, piezoelectric polymer films or ceramics, shape memory materials such as nickel-titanium shape memory alloys or shape memory polymers, electroactive artificial muscle polymers, or the like. The length of the steerable tip  3  and the length L, described elsewhere herein, are preferably mutually selected to be about the same length, so that the cannula can follow and track the steerable tip (see  FIG. 12G ). 
         [0039]    The overall length of the cannula will vary according to the particular hollow body structure for which it is intended. For instance, used in a colonoscope application the shape-transfer cannula length might range from 100 cm to 180 cm. In a bronchoscope application the shape-transfer cannula length might range from about 30 cm to about 100 cm. In a catheter application, the rigidizing core  1  and sheath  2  components of a shape-transfer cannula might be limited to a relatively small section of the entire catheter length, as depicted in  FIG. 22 . For example, the core  1  and sheath  2  can be provided only at the distalmost end of the device or apparatus that is intended to be steered. In such a case the majority of the cannula&#39;s length might include “passive” conventional extruded catheter material  251  and a non-rigidizing section of core  252 . For example, in accessing particular areas within the heart&#39;s ventricles, the extra control provided by the shape-transferring core  1  and sheath  2  components might only be needed within the ventricles themselves so the length of the rigidizing section R, need only be sufficient to navigate within the ventricles themselves. 
         [0040]    Another aspect of the present disclosure is that the distal, steerable portion of the apparatus has a shape-transforming length L and an outer diameter D, with the ratio L/D being at least about 5 (UD&gt;5), so that there is enough longitudinal length of the shape-transforming portion of the device or apparatus to track the steerable tip  3 . 
         [0041]      FIG. 2  illustrates by way of example a sequence in which the core  1  and sheath  2  alternate sequentially between rigid and flexible such that the entire structure takes the shape traced by the steerable tip  3  as the shape-transferring cannula is inserted into a hollow body structure such as the colon, stomach, lung bronchi, uterus, abdominal cavity, brain ventricle, heart chamber, blood vessel, or the like. In  FIG. 2   a , the sheath  2  is rigid and the core  1  with steerable tip  3  is flexible. The steerable tip  3  initially lies within and is approximately flush with the distal end  1 b of the sheath  2  such that both the sheath  2  and core  1  assume the same longitudinal shape whether curved or straight. To begin the sequence that advances and forms the longitudinal shape of the shaped cannula structure  1   d , in  FIG. 2   b  the core  1  is advanced distally through the rigidized sheath  2 , exposing the length of the steerable tip  3 . The user then directs the exposed steerable tip  3  in the desired direction of insertion and advances the structure with, for example, a squeeze advancement mechanism (which will be discussed later herein) or through a cam mechanism or through any other structure or mechanism that rigidizes and relaxes the core  1  and sheath  2  in proper sequence. Referring to  FIG. 2   c , to advance the entire shape-transferring cannula structure  1   d , the core  1  is made rigid and then the sheath  2  is relaxed and, as shown in  FIG. 2   d , the sheath is advanced over the core  1  and steerable tip  3 . Preferably, the longitudinal relative motion between the two rigidizing elements (i.e. core  1  and sheath  2 ) is limited to the length of the steerable tip  3 , the user-controlled element that serves as the system&#39;s directional guide. The sheath  2  is then made rigid and the core  1  is relaxed and advanced to re-expose the steerable tip  3 . Thus, in sequential fashion, the rigidizing structure portion of the shape-transferring cannula  1   d  takes the shape of the path traced by the steerable tip  3  as guided by the user. 
         [0042]    Other sequences and combinations of stiffening, flexibility, and advancement to achieve the same result are possible within the scope of this disclosure. For instance, the sheath  2  and core  1  may be normally-stiff structures that momentarily become flexible at appropriate times in the shape-transferring cannula advancement sequence. In another example, the sheath  2  and core  1  may be normally-flexible structures that momentarily become rigid at appropriate times to complete the advancement sequence. In another example, the core  1  and sheath  2  may both include a steerable tip  3  providing each structure with both directional control and the momentary rigidity necessary for shape-transference. 
         [0043]      FIG. 3  depicts an alternative embodiment of a shape-transferring cannula system in accordance with the present disclosure. In this embodiment, the shape-transferring core  1  and sheath  2  are not necessarily coaxial structures and may be laterally parallel structures that slidably engage each other longitudinally via engagement features  10 . In general, engagement features in accordance with the present disclosure include structures that permit the core  1  and sheath  2  to slide or otherwise move longitudinally relative to each other. One aspect of engagement features in accordance with the present disclosure that one of the core  1  and sheath  2  provides a rail for advancement of the other of the core and sheath relative thereto. 
         [0044]    At least one of the shape-transferring structures includes a steerable tip  3  with which to guide advancement of the system. Either or both of the shape-transferring structures can contain accessory lumens  11  through which surgical tools may be introduced or through which the target site may be irrigated or suctioned. The engagement feature  10  of either structure can be used as a guide for withdrawing samples or inserting tools that won&#39;t fit through the accessory lumen  11 . Outsized tools may be provided with compatible engagement features such that they track along the guide formed by the rigidizing structure&#39;s engagement features. 
         [0045]    In a preferred embodiment of the present disclosure, the user&#39;s selection of an advancement direction and his actuation of the system, whether manual or powered, causes the entire cycle of core rigidization, sheath relaxation, sheath advancement, sheath rigidization, and core relaxation to occur such that the structure is returned to its initial state with a new longitudinal shape. 
         [0046]    The core and sheath structures may be unitary materials or continuous structures that can be transformed between relatively rigid and relatively flexible or they can be formed of individual, flexibly connected rigid links which become substantially locked together to rigidize the structure. In an embodiment comprised of links, the sheath and core linkage structures are rigidized by temporarily preventing, by any suitable mechanism, substantial relative motion between the links. For example, motion between links may be temporarily stopped or substantially reduced by tightening a tension cable to put the linkage into longitudinal compression, by electrostatic or magnetic forces, by hydraulic or pneumatic actuation, by changes in viscous coupling as with electrorheological or magnetorheological materials, or through any friction modulating means. 
         [0047]    Linkages may be held together by a flexible internal cable or external covering, or by attaching the links to each other while leaving enough freedom of rotation to make the structure longitudinally flexible. More specifically as shown in  FIGS. 4A and 4B , the links may be loosely captured by overlapping ball and cup features in adjacent links such that the cup features  22  overlap past the equators of the adjacent ball features  23 . This arrangement allows two-axis pivoting between links while keeping the linkage intact.  FIG. 4A  illustrates a specific example of a rigidizing mechanism where sheath and core rigidizing linkage structures may include a compression element  20  in each link, such as a loop of nickel-titanium alloy wire or shape memory polymer whose shape-memory transition temperature is higher than normal body temperatures. A slot  24  in the cup  22  may facilitate compression of the cup against the ball  23  when the compression element  20  is actuated. The compression elements  20  in the links may be actuated through electrical or inductive heating or through any suitable means to activate the shape-memory effect such that the compression elements reduce their unstressed diameters, creating local compression between ball and cup, and thus increasing friction between links. 
         [0048]      FIG. 4B  illustrates another example of a rigidizing mechanism employing the same type of captured-link linkage configuration as the previous example. In this embodiment, either the ball  23  or the cup  22  can include active material components  25  made of materials such as electroactive polymer (EAP) that change shape when energized. The active material components may be oriented to expand radially when energized, causing interference between the ball and cup of adjacent links. Alternately, in a normally-rigid structure, the active material components  25  may be oriented to contract radially when energized, relieving interference between the ball and cup features of adjacent links. 
         [0049]    By way of another example, linkages built of links made of dielectric materials may be rigidized electrostatically by building attractive or repulsive charges between links and increasing friction between links. By way of another example, inducing magnetic attraction or repulsion between links containing ferromagnetic materials can stiffen a rigidizing linkage by increasing friction between the links. By way of another example, linkages built with links made of conductive materials may be rigidized by inducing eddy currents that attract links to each other and increase friction between links. 
         [0050]      FIGS. 5 and 6  illustrate a linkage embodiment of parallel rigidizing sheath and core structure that employs rigidizing wires or cables  34  and  40 , which go through the core  1  and sheath  2  respectively. These cables, when put into tension, pull the links together to increase friction between the links and thus prevent relative motion between the links. When not in tension, the rigidizing cables serve to hold the individual links in the rigidizing assembly together. 
         [0051]    As illustrated in  FIG. 5 , the core&#39;s rigidizing structure  30  is built-up of links such that a convex spherical surface  31  on one link engages a concave surface  32  on the adjacent link.  FIG. 5  shows cup-like nesting links  33  with a spherical ball-joint-like interface that allows two-axis pivoting between abutting links, thus making the linkage longitudinally flexible. The core&#39;s rigidizing cable  34  runs through each core link&#39;s central orifice  35 , connecting the entire core rigidizing structure  30 . The steerable tip  3  control cables  36  may also run through each link&#39;s central orifice  35 . The control cables  36  may be mechanical cables transmitting tension or compression, or electrical connections transmitting power or signal to actuate or control the steerable tip  3 . Alternatively, the rigidizing cable  34  and tip-steering cable  36  may be contained within individual lumens of a multilumen housing or within individual housings, keeping them separated and keeping the tip-steering cables  36  from binding when the rigidizing cable  34  is tensioned to stiffen the core structure. The housing material may be chosen for low friction cable movement. One lumen of the multilumen housing might also serve to guide the core  1  along a conventional guidewire for rapid insertion into guidewire accessible anatomy such as the atria and ventricles of the heart. 
         [0052]    Other linkage geometries that allow two-axis pivoting are possible within the scope of this disclosure. For example as in  FIG. 21 , links  240  with male  241  and female  242  pivot features that each rotate on only one axis can be alternated and mounted within each other, within the rigidizing structure, with adjacent pivot features having axes perpendicular to one another. Thus, each pair of links  240  provides two orthogonal pivoting axes to the linkage structure. 
         [0053]      FIG. 6  illustrates a linkage embodiment of the sheath  2  in which the sheath links include a hollow central orifice  41  and pivot on spherical ball-joint like ball  48  and cup  47  surfaces for two-axis pivoting. The sheath&#39;s rigidizing cable  40  runs outside each sheath  2  link&#39;s central orifice  41  allowing the assembled links to form a hollow central lumen  42  that can be occupied by the core  1  structure during cannula advancement as well as by items such as surgical instruments which the sheath lumen  42  can guide to a surgical or diagnostic site. The link&#39;s central orifice  41  has a diameter D 2  that is substantially similar to diameter D 1 , described above. 
         [0054]    The rigidizing sheath links  45  may include at least two cable-guiding features  44  external to the central lumen  42 . The cable  40  and cable-guiding features  44  are configured for low friction sliding. Friction may be further reduced by encasing the cable in a cable housing formed of a material with low-friction properties, such as PTFE, HDPE, and the like, thus separating it from the cable-guiding features  44 . Alternatively, the cable-guiding features themselves could be manufactured from low friction materials different from that of the rest of the links. When the links  45  are assembled into a rigidizing structure  46 , the cable-guiding features  44  form segmented channels running the length of the rigidizing structure  46 . Since the cables  40  do not run down the central axis of the sheath  2 , cables running on opposite sides of the sheath  2  must effectively change length when the sheath  2  bends. Cable segments closer to the center of curvature relative to the structure&#39;s neutral axis will have to shorten. Likewise, cable segments further from the center of curvature relative to the sheath&#39;s  2  neutral axis will have to lengthen. 
         [0055]    An embodiment of a linkage sheath with two cable-guiding channels an equal radial distance from the sheath&#39;s central axis may employ the single cable  40  wrapping around a pulley  43 , which may be a rotating component, sliding surface, or the like, to run back and forth along the length of both cable-guiding channels. As the structure bends, the inner cable path will shorten the same amount as the outer cable path lengthens and cable length will move from the shortening side around the pulley  43  to the lengthening side. Tension on the pulley  43  with respect to the linkage structure  46  tightens the entire cable  40  and stiffens the sheath  2  by increasing friction between the links. Referring to  FIG. 7 , the sheath&#39;s tensioning pulley  43  may be positioned off-axis such that the sheath central lumen  42  is clear and able to receive the core  1  or surgical instruments. A cable-guiding element  50  at the base of the sheath  2  acts to redirect the tensioning cable  40  to an off-axis pulley  43  and away from the sheath&#39;s central lumen  42 . 
         [0056]    Referring to  FIG. 8 , the sheath rigidizing linkage structure  60  may run parallel to the core  1  without being coaxial with it. Each link may include dedicated rigidizing features  61  through which a rigidizing cable  62  may run and at least one lateral orifice  62  which, when multiply assembled in the complete linkage, form a laterally parallel segmented lumen through which the core  1  or surgical tools may run. The lateral orifice  62  has a diameter D 3 , which is substantially similar to diameter D 1 , described above. 
         [0057]      FIG. 9  depicts a laterally parallel sheath linkage. The laterally parallel sheath  60 a may form a lateral lumen  70  capable of forming varying radii of curvature by nesting conical shapes that form the lateral lumen  70 , leaving sufficient mechanical clearance  71  to accommodate an angle .alpha. between adjacent links. The angle .alpha. is preferably in the range from about zero degrees to about 90 degrees. The lateral lumen diameter D 4  is substantially similar to diameter D 1 , described above. 
         [0058]      FIG. 10  depicts an alternate embodiment of a linkage. Linkage  82  with the parallel lateral lumen  70  may include an open side  80  such that objects  81  larger than the lumen diameter D 4  may be introduced to and withdrawn from the surgical or diagnostic site using the combinations of the open sides  80  to retain therein a matching portion  83  on the object  81 . 
         [0059]      FIG. 11  depicts another alternate embodiment of a linkage. Linkage  82   a  with the parallel lateral lumen  70  may employ flexible elements  90  in the lumen portion of each link that partially overlap each adjacent link. The flexible elements serve to form a smoother and larger segmented lumen than would be formed by purely rigid links by flexing when formed into a radius rather than requiring clearance for the entire range of motion between the links. 
         [0060]    Referring to  FIGS. 12A-12G , a mechanism for advancing parallel rigidizing elements may include two opposing racks  91  and  92  that alternatingly advance relative to each other. Referring to  FIG. 12G , the maximum amount of incremental advancement, length ‘L’, is ideally limited to the length of cannula having the steerable tip  3 . A linearly sliding shuttle  93  supports one rack and a housing  94  supports the other rack. The core actuation handle  95  (not shown for clarity in  FIG. 12A  and  FIG. 12B ) and core rack  91  are each pivotally attached to the housing  94 . The sheath actuation handle  96  and sheath rack  92  are each pivotally attached to the shuttle  93 . The core and sheath actuation handles  95  and  96  are attached to the rigidizing cables  40  and  34  of the sheath  2  and core  1 , respectively. Upon actuation by the user, these handles rotate on their pivots  97  and  98  to first relax their respective rigidizing structure, disengage their respective rack from the other, which remains temporarily fixed, and transmit the force which slides the housing  94  and shuttle  93  with respect to one another to advance the shape-transferring cannula. 
         [0061]    Beginning the advancement sequence as shown in FIG,  12 B, spreading the handholds  99  and  100  (core advancement handhold  99  not shown for clarity) apart biases the core handle  95  (not shown for clarity) against its mechanical stop  104  in the housing  94  and rotates the sheath handle  96  on its pivot  98 , first compressing the sheath rigidizing spring  106 , relaxing the sheath linkage  2 , and then disengaging the sheath rack  92  from the currently-fixed core rack  91 . The sheath rack  92  disengages the core rack  91  by rotating on its pivot  105  against the force of the sheath rack bias spring  113 . The sheath rack  92  is rotated away from the core rack  91  by the force of the sheath rack lifter  115 , which extends from the sheath handle  96 , acting against the rack&#39;s lift tab  117 . An initial gap between the sheath rack lifter  115  and rack&#39;s lift tab  117  allows the sheath handle  96  to rotate enough to compress the sheath rigidizing spring  106  and relax the sheath  2  before the sheath rack  92  is disengaged from the core rack  91 . As illustrated in  FIG. 12C , continued spreading of the actuation handles  95  and  96 , with racks  91  and  92  disengaged, translates the handles apart from each other and advances the shuttle  93  and sheath  2  relative to the housing  94  and core  1 . 
         [0062]    As illustrated in  FIG. 12D , releasing the handle spreading pressure allows the sheath rigidizing spring  106  to rotate the sheath handle  96  back to its resting position and re-stiffen the sheath  2  by tensioning the sheath rigidizing cable  40 . The rotation of the sheath handle  96 , in turn, rotates the sheath rack lifter  115  away from the sheath rack  92 , allowing the sheath rack bias spring  113  to rotate the sheath rack  92  towards the core rack  91 . Re-engagement of the racks locks the mechanism in a sheath-forward position shown in  FIG. 12D . 
         [0063]    Continuing the advancement sequence as shown in  FIG. 12E , squeezing the advancement handholds  99  and  100  of the actuation handles  95  and  96  such that they rotate towards each other biases the sheath handle  96  solidly against its mechanical stop  101  on the shuttle  93  and rotates the core handle  95  on its pivot  97 , first compressing the core rigidizing spring  102 , relaxing the core linkage  1 , and then disengaging the core rack  91  from the currently-fixed sheath rack  92 . The core rack  91  disengages the sheath rack  92  by rotating on its pivot  103  against the force of the core rack bias spring  112 . The core rack  91  is rotated away from the sheath rack  92  by the force of the core rack lifter  114 , which extends from the core handle  95 , acting against the rack&#39;s lift tab  116 . An initial gap between the lifter  114  and lift tab  116  allows the core handle  95  to rotate enough to compress the core rigidizing spring  102  and relax the core  1  before the core rack  91  is disengaged from the sheath rack  92 . The racks  91  and  92  being disengaged from each other, continued squeezing as shown in  FIG. 12F  translates the handles  95  and  96  closer together by advancing the housing  94  and core  1  relative to the shuttle  93  and sheath  2 . 
         [0064]    Releasing the squeezing pressure on the advancement handholds  99  and  100  allows the core rigidizing spring  102  to rotate the core handle  95  back to its resting position and re-stiffen the core  1  by tensioning the core rigidizing cable  34 . The rotation of the core handle, in turn, rotates the core rack lifter  114  away from the core rack  91  allowing the core rack bias spring  112  to rotate the core rack  91  towards the sheath rack  92 . Re-engagement of the racks locks the mechanism in the sheath-back position shown on  FIG. 12A . 
         [0065]    Referring to  FIG. 12G , the difference in engaged length of the racks  91  and  92  between the sheath-back position and the sheath-forward position, length ‘L’, as well as the position of the sliding stop structures  110  and  111  in the housing  94  and shuttle  93  define the maximum relative motion for incremental advancement between the sheath  2  and core  1  elements. The amount of incremental advancement, length ‘L’, is preferably limited to the length of the steerable tip  3 . Rack features  107  such as teeth define the increments in which the shape-transferring cannula may be mechanically advanced or retracted. The rack features  107  may be configured to allow only integral advancement of units the length of the entire steerable tip  3  as shown in  FIG. 12G  or, alternately, may be configured allow units of advancement fractions of that length. 
         [0066]    The rack mechanism described above may also withdraw the shape-transferring cannula in controlled increments through a process reversing the advancement sequence. Withdrawal of hand-holds  108  and  109  on the ends of the handles  95  and  96  opposite the advancement ends actuate the mechanism in reverse using the same gripping and spreading finger/thumb motions used to advance the cannula. 
         [0067]    In another embodiment of the disclosure,  FIGS. 20A and 20B  depict rigidizing structures including inner and outer concentric tubes,  221  and  222  respectively, separated by short segments of materials  223  that change shape when energized, such as electroactive polymer (EAP), which changes shape when exposed to electric fields. The inner tube  221  mayor may not have an open lumen. When employing biaxially active materials such as EAP, the active material components are oriented to contract longitudinally and expand radially when energized. The active material components may be employed in a normally-noninterfering configuration or a normally-interfering configuration. In a normally-non-interfering configuration the active material components  223  are each attached to one of the concentric tubes  221  or  222  such that they do not contact the other tube, as shown in  FIG. 20A , when not energized. When energized, the radial expansion of the active material components  223  causes mechanical interference with the other tube, as in  FIG. 20B , thus preventing motion between the opposed surfaces  224  and  225  and effectively locking-in the curvature of the rigidizing structure. According to the present disclosure, one may substitute materials that change shape when exposed to electric current, magnetic fields, light, or other energy sources. The same rigidizing effect may be achieved by replacing normally-non-interfering active material components  223  with non-interfering balloons expandable by gas or liquid fluid pressure. Alternately, such materials may be placed in a normally-interfering configuration between concentric tubes  221  and  222  such that they interfere, as in  FIG. 20B  when not energized and contract radially to the state depicted in  FIG. 20A  when energized. For example, a normally-rigid structure made stiff by normally-interfering EAP components  223  may be made flexible by applying a voltage to the EAP components such that they contract radially to the noninterfering state depicted in  FIG. 20A , relieving the mechanical interference and allowing relative motion between the opposed surfaces  224  and  225  of the concentric tubes  221  and  222 . Similarly, normally-interfering balloons replacing normally-interfering active material components  223  may be collapsed by applying a relative vacuum. 
         [0068]    Referring to  FIG. 13A , in an alternate embodiment of the disclosure, the rigidizing sheath  2  can include rotating wedge links  130 . The wedge links  130  have hollow central axes  131  that form the sheath&#39;s lumen  42  as well as two interface features  132  angled with respect to one another. For example, the angle between the links can be between about zero degrees and about  90  degrees. The perpendicular centerlines  133  of the interface surfaces define axes of rotation between the links. As depicted diagrammatically in  FIGS. 13B and 13C , the wedge links  130  in a sample starting position in  FIG. 13B  rotate with respect to neighboring links  134  at the connecting interface  132  between links. This rotation forms curves as shown in  FIG. 13C  in the sheath  2  structure while maintaining a substantially constant sheath lumen  42  volume. Impeding rotation between links rigidizes the structure. Link rotation can be prevented through any of the ways described above for impeding relative motion between links. The wedge links  130  may be formed as sections of spheres as shown in  FIG. 13A  to avoid creating sharp corners when curves are formed, leaving a relatively smooth and atraumatic outer surface. 
         [0069]      FIGS. 14A-14C  depict another embodiment of the disclosure in which one of the two parallel elements in the shape-transferring cannula is passive. The passive element is more rigid than the relaxed rigidizing structure and more flexible than the stiffened rigidizing structure. The passive element is less mechanically complex than an equivalent rigidizing structure, not requiring rigidizing cables  34  and  40  or other mechanisms to serve the shape-transfer function. Thus a shaped cannula assembly with a passive sheath may be narrower in cross-section than an assembly formed of two rigidizing structures. In  FIG. 14A  the core  1  is relaxed such that it is more flexible than the sheath  2  and has been advanced such that the steerable tip  3  protrudes ahead of the sheath  2 . In  FIG. 14B  the core  1  is stiffened such that it is more rigid than the sheath  2  and the user deflects the steerable tip  3  towards the direction of intended cannula advancement. In  FIG. 14C  the core  1  remains stiffened such that it is more rigid than the sheath  2  and the sheath is then advanced over the core and its steerable tip  3 . The sheath  2  assumes the core&#39;s longitudinal shape including the new bend introduced by the user through the deflected steerable tip  3 . Elements of a passive link structure could be mechanically energized to encourage them to move relative to one another when being advanced past a relatively rigid structure. Mechanical energizing can be achieved by vibrating the passive structure with any suitable device, such as a piezoelectric transducer, voicecoil, or eccentrically weighted motor. 
         [0070]    Embodiments of the disclosure that employ continuous, non-segmented, parallel core and sheath structures can be made smaller in cross-section than mechanically-stiffened linkage structures. Such structures may be constructed such that they become relatively rigid when energized or become relatively flexible when energized. 
         [0071]      FIG. 15  depicts a continuous, parallel shape-transferring core and sheath structure. The core  1  and sheath  2  structures can each include inner  151  and outer  152  flexible tubes containing stiffening material  153  that increases in viscosity or otherwise stiffens when energized. Examples of such substances are electrorheological fluid, which stiffens upon exposure to electrical potential, and magnetorheological fluid, which stiffens upon exposure to magnetic fields. A rigidizing structure configured as a core or as a sheath may be built-up of inner  151  and outer  152  containment tubes with stiffening material  153  sandwiched in between. In the case of a core, the inner tube may be a solid element such as plastic monofilament, lacking a lumen. In the case of a structure employing electrorheological fluid, flexible electrical contacts may line the length of each containment tube or the tube itself may be made of electrically-conductive plastic or other similar material. A section of electrically insulating material  154  may connect the tubes  151  and  152  at their proximal and distal ends, mechanically connecting the tubes  151  and  152  and sealing the electrorheological fluid within. A woven mesh or other similar separating material  155  sandwiched with the electrorheological fluid between the tubes  151  and  152  may act as a baffle, restricting the flow of viscous fluid so as to increase the rigidity of the structure when energized, and as an insulator when an electrical potential is used to energize the elements. The tubes  151  and  152  themselves may contain baffling features such as grooves or threads and may also contain a layer of insulating material, obviating the need for a separating material  155 . A similar structure employing magnetorheological fluid could be constructed with at least one containment tube containing electrical conductors arranged in such a manner as to generate a magnetic field sufficient to rigidize the structure. 
         [0072]    A shape-transferring cannula structure may be constructed of normally-rigid core  1  and sheath  2  elements which, in proper sequence, become flexible when energized and re-stiffen when they return to an un-energized state. Each element can become flexible enough, when energized, to be advanced along a relatively rigid mating structure and then, when de-energized, become rigid enough to mechanically support the advancement of an energized parallel structure. Referring to  FIG. 16 , parallel normally-rigid core  1  and sheath  2  elements may include in their construction thermoplastic, thermoplastic alloys such as Kydex™ (acrylic-PVC alloy), urethane alloys, or similar materials that soften to a flexible state when heated above a transition temperature by embedded heating elements  171  and  172  or any suitable mechanism. The transition temperature can be selected through design and material composition to be somewhat higher than normal body temperatures. The normally-rigid parallel structures may contain heating elements that momentarily increase their temperatures above the flexibility transition temperature. Surrounding body fluid such as blood, saline solution, or lymph can serve as a heat sink to quickly draw heat away and re-stiffen the structures when the momentary heating is ceased. Similarly, as shown in  FIG. 23 , normally-rigid core  1  or sheath  2  structure can include a guidewire  260  with wirewound coils in its construction. The coils  263  can be at least partially potted in a low-temperature flowing material  261  such as wax or polymer which adheres to the coils. The low-temperature flowing material  261  may be contained within a compliant cover  262 . In an un-energized state the flowing material  261  is relatively solid and prevents the coils  263  from moving substantially with respect to one another, thus substantially locking-in the curvature of the structure. When energized through heating, the flowing material  261  softens sufficiently to allow relative motion between coils  263 , thus relaxing the structure. 
         [0073]    Referring to  FIGS. 19A and 19B , shape-transferring cannula can be built of normally-rigid core  1  and sheath  2  structures, each including flexible tubes  212  and  214  respectively, containing substantially stiff materials  213  that relax upon vibration. Such materials can include interlocking particles like sand grains or normally-viscous fluid, such as xanthan gum that becomes less viscous upon agitation. Vibrating each structure, for example with a vibrating element  215  such as a piezoelectric transducer, a voicecoil, or a motor with an eccentrically mounted weight, could temporarily relax it to a flexible state by loosening the interlocking particles or by causing the contained fluid to transition to a less viscous state. Alternatively, the containment tubes  212  and  214  themselves could be constructed of or contain a piezoelectric material such as PVDF (polyvinylidene fluoride) along their length such that each entire tube could actively vibrate when energized with an alternating voltage V. 
         [0074]    In another embodiment of the disclosure,  FIGS. 20A and 20B  depict rigidizing structures including inner and outer concentric tubes,  221  and  222  respectively, separated by short segments of materials  223  that change shape when energized, such as electroactive polymer (EAP), which changes shape when exposed to electric fields. The inner tube  221  mayor may not have an open lumen. When employing biaxially active materials such as EAP, the active material components are oriented to contract longitudinally and expand radially when energized. The active material components may be employed in a normally-noninterfering configuration or a normally-interfering configuration. In a normally-non-interfering configuration the active material components  223  are each attached to one of the concentric tubes  221  or  222  such that they do not contact the other tube, as shown in  FIG. 20A , when not energized. When energized, the radial expansion of the active material components  223  causes mechanical interference with the other tube, as illustrated in  FIG. 20B , thus inhibiting or preventing motion between the opposed surfaces  224  and  225  and effectively locking-in the curvature of the rigidizing structure. The same disclosure may substitute materials that change shape when exposed to electric current, magnetic fields, light, or other energy sources. The same rigidizing effect may be achieved by replacing normally-non-interfering active material components  223  with non-interfering balloons expandable by gas or liquid fluid pressure. Alternately, such materials may be placed in a normally-interfering configuration between concentric tubes  221  and  222  such that they interfere, as in  FIG. 20B  when not energized and contract radially to the state depicted in  FIG. 20A  when energized. For example, a normally-rigid structure made stiff by normally-interfering EAP components  223  may be made flexible by applying a voltage to the EAP components such that they contract radially to the non-interfering state depicted in  FIG. 20A , relieving the mechanical interference and allowing relative motion between the opposed surfaces  224  and  225  of the concentric tubes  221  and  222 . Similarly, normally-interfering balloons replacing normally-interfering active material components  223  may be collapsed by applying a relative vacuum. 
         [0075]    Referring to  FIG. 17A , core and sheath rigidizing  180  structures can include compliant inner and outer tubes,  181  and  182 , containing compression-stiffening particles  183  in the annular space between the opposing tube surfaces. The compression stiffening particles  183  are made of materials such as expanded polystyrene that interlock and form a substantially rigid structure when compressed. Such compression can occur when the space containing the compression-stiffening particles is placed under a relative vacuum P. Alternatively, external pressure may be applied to the material in the annular inter-tubal space to compress and stiffen it. For example, pressure may be applied to the internal concentric tube such that it expands and presses compression-stiffening material in the inter-tube space against the external concentric tube. Referring to  FIG. 17B , core  1  structure can include a compliant tube  184  containing compression-stiffening particles  183 . The structure may be stiffened by putting the tube&#39;s interior under relative vacuum P. 
         [0076]    Referring to  FIG. 18 , a core  1  or sheath  2  structure including links  191  may be rigidized or relaxed via pressure P which can be either positive pressure or relative vacuum. In a normally-rigid configuration, a compliant cover  192  the length of the structure can be stretched taut against the movable links  191  in an equalized pressure environment. The tight covering  192  keeps the links from moving substantially relative to one another, making the rigidizing structure stiff. Application of pressure P underneath the compliant cover  192  expands the cover, allowing the links  191  to rotate relative to one another thereby relaxing the structure. Alternately, in a normally-flexible structure, the compliant cover  192  can loosely cover the links  191  in an equalized pressure environment such that the links can rotate relative to one another. Applying a relative vacuum P inside the compliant cover  192  causes it to compress against the movable links  191 , preventing their rotation relative to one another thereby stiffening the structure. 
         [0077]    The rigidizing structures described above as a paired system may be also employed singly as an alternatingly rigid and compliant support for a steerable catheter such as an endovascular catheter or flexible endoscope. In such cases as depicted in  FIG. 22 , the rigidized structure provides support for the catheter to round corners without the possibility of looping because the flexible element is advanced only when the supporting structure is rigid. Similarly, the relaxed rigidizing support is advanced only along the length of the catheter, using it as a guidewire. 
         [0078]    In another embodiment of the disclosure, a steerable catheter such as an endovascular catheter or flexible endoscope may be aided in advancing around tight corners through alternating between advancement of two parallel structures, using the relatively rigid steerable bending section at the tip to advance through a tight anatomical turn without looping. 
         [0079]    In one embodiment, the sheath is rigidized and the core with an articulating tip is made flexible. The core is advanced and then rigidized. The articulating tip is pointed in the desired direction of path creation. The sheath is relaxed and advanced over the rigid core. 
         [0080]    Referring now to  FIG. 24 , yet further aspects of the present disclosure are illustrated. More specifically,  FIG. 24  illustrates that the handholds, such as handholds  99 ,  100 ,  108 ,  109  illustrated in  FIGS. 12A-12H , can optionally be replaced with a semi- or fully automated systems, to permit the practitioner&#39;s hands to be used for other tasks during the particular procedure performed on a patient. As illustrated in  FIG. 24 , a rack  302  having teeth  304  is pivotally mounted to the arm  306  at a pivot  308 , to which handhold  100  is attached in the embodiment illustrated in  FIG. 12G . A pinion  310  having teeth  316 , which mate with teeth  304 , is rotatably mounted to arm  312 , while a pin or the like  314  holds the rack  302  against the pinion. Thus, rotation of pinion  310 , such as by a rotary motor  318  or the like, causes arm  306  to move in direction X, while the arm  312  can be separately or simultaneously moved along direction X by pulling or pushing on the arm  312 , or the motor  318 , with a suitable linear actuator or motor (not illustrated). Further optionally, the activation of the actuators or motors, including motor  318 , can be automated by controlling them using an automatic controller  320 . By way of example and not of limitation, controller  320  can be a general purpose computer having a memory  322  in which the logic of the sequence of movements of the arms  306 ,  312  can reside. Alternatively, controller  320  can be a PLC controller or other controller as will be readily appreciated by those of skill in the art, which can automatically control the movements of the arms  306 ,  312 . 
         [0081]    While the disclosure has been described in detail with reference to preferred embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the disclosure. Each of the aforementioned documents is incorporated by reference herein in its entirety.