Abstract:
Medical devices such as catheters can include structure or provision that permit a physician or other health care professional to adjust the stiffness of at least a portion of the medical device. In some instances, the medical device may be adjusted prior to inserting the medical device into a patient. In some cases, the medical device may be adjusted while in use within the patient.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/210,158 filed Aug. 15, 2011, now U.S. Pat. No. 8,303,570, which is a continuation of U.S. application Ser. No. 11/218,857 filed Sep. 2, 2005, now U.S. Pat. No. 7,998,132. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to medical devices such as catheters and relates more particularly to catheters that include structure or provision providing adjustable stiffness. 
     BACKGROUND 
     Medical devices such as catheters may be subject to a number of often conflicting performance requirements such as flexibility and strength. In some instances, improved flexibility may come at the expense of reduced strength. Increased strength may come at the expense of reduced flexibility. Because each patient is unique, there may be a unique balance of performance parameters such as flexibility and strength optimal for a particular patient. 
     While it would certainly be possible to construct a large number of catheters, to accommodate any feasible set of desired performance parameters, this would likely be cost-prohibitive. Moreover, in some instances, a physician may determine in the middle of a procedure that a particular balance of stiffness versus flexibility may be necessary. Therefore, a need remains for medical devices such as catheters that may be adjusted, particularly in situ, with respect to their stiffness. 
     SUMMARY 
     The invention pertains generally to medical devices such as catheters that include structure or provision that permit a physician or other health care professional to adjust the stiffness of at least a portion of the medical device. In some instances, the medical device may be adjusted prior to inserting the medical device into a patient. In some cases, the medical device may be adjusted while in use within the patient. 
     Accordingly, an example embodiment of the invention can be found in an adjustable catheter that includes an elongate polymeric shaft extending from a proximal region of the catheter to a distal region of the catheter and a first spiral-cut hypotube that is disposed within the elongate polymeric shaft. 
     Another example embodiment of the invention can be found in an adjustable catheter having an elongate polymeric shaft defining a lumen that extends from a proximal region of the catheter to a distal region of the catheter. A first inflatable tube that extends from the proximal region to the distal region and that is arranged at least substantially parallel with a longitudinal axis of the catheter is disposed within the lumen. Inflating the first inflatable tube causes the elongate polymeric shaft to increase in stiffness. 
     Another example embodiment of the invention can be found in an adjustable catheter that includes an inner polymeric liner, an outer polymeric liner, and a swellable layer disposed between the inner polymeric liner and the outer polymeric liner. Adding an appropriate fluid to the swellable layer increases the stiffness of the adjustable catheter. 
     Another example embodiment of the invention can be found in an adjustable catheter having an elongate polymeric shaft that extends from a proximal region to a distal region of the catheter. A stiffness-enhancing sheath that is more stiff than the elongate polymeric shaft is slidably disposed over the elongate polymeric shaft. 
     Another example embodiment of the invention can be found in an adjustable catheter that includes an elongate polymeric shaft that extends from a proximal region of the catheter to a distal region of the catheter and that includes a wall. A number of elongate apertures are disposed within the wall such that they extend longitudinally within the elongate polymeric shaft. Each of a number of stiffness-enhancing filaments are slidably disposed in each of the number of elongate apertures. 
     Another example embodiment of the invention can be found in an adjustable catheter having an inner polymeric layer that includes one or more electrically actuated stiffness enhancers. An outer polymeric layer is disposed over the inner polymeric layer. 
     Another example embodiment of the invention can be found in an adjustable catheter that includes an elongate polymeric shaft having a stiffness. The stiffness of the elongate polymeric shaft can be changed by applying a current to the elongate polymeric shaft. 
     The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The Figures, Detailed Description and Examples which follow more particularly exemplify these embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  is a side elevation view of a catheter in accordance with an embodiment of the invention; 
         FIG. 2  is a diagrammatic longitudinal cross-section of a portion of a catheter in accordance with an embodiment of the invention; 
         FIG. 3  is a diagrammatic longitudinal cross-section of a portion of the catheter of  FIG. 2 ; 
         FIG. 4  is a diagrammatic cross-section of a catheter in a relaxed configuration, in accordance with an embodiment of the invention; 
         FIG. 5  is a view of the catheter of  FIG. 4 ; 
         FIG. 6  is another view of the catheter of  FIG. 4 ; 
         FIG. 7  is a side elevation view of a catheter in a deflated configuration, in accordance with an embodiment of the invention; 
         FIG. 8  is a side elevation view of the catheter of  FIG. 7  in an inflated configuration, in accordance with an embodiment of the invention; 
         FIG. 9  is a side elevation view of a catheter in accordance with an embodiment of the invention; 
         FIG. 9A  is a side elevation view of a catheter in accordance with an embodiment of the invention; 
         FIG. 10  is a side elevation view of a catheter in accordance with an embodiment of the invention; 
         FIG. 11  is a side elevation view of a catheter in accordance with an embodiment of the invention; 
         FIG. 12  is a side elevation view of a catheter in accordance with an embodiment of the invention; 
         FIG. 13  is a diagrammatic longitudinal cross-section of a catheter in accordance with an embodiment of the invention; 
         FIG. 14  is a view of the catheter of  FIG. 13 ; 
         FIG. 15  is a side elevation view of a catheter in accordance with an embodiment of the invention; 
         FIG. 16  is a side elevation view of a catheter in accordance with an embodiment of the invention; 
         FIG. 17  is a side elevation view of a catheter in accordance with an embodiment of the invention; 
         FIG. 18  is a perspective view of a catheter in accordance with an embodiment of the invention; 
         FIG. 19  is a diagrammatic longitudinal cross-section of a catheter in a relaxed configuration, in accordance with an embodiment of the invention; and 
         FIG. 20  is a view of the catheter of  FIG. 19  in an actuated configuration, in accordance with an embodiment of the invention. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. 
     DETAILED DESCRIPTION 
     For the following defined terms, these definitions shall be applied, unless a different definition is given in the claims or elsewhere in this specification. 
     All numeric values are herein assumed to be modified by the term “about”, whether or not explicitly indicated. The term “about” generally refers to a range of numbers that one of skill in the art would consider equivalent to the recited value (i.e., having the same function or result). In many instances, the terms “about” may include numbers that are rounded to the nearest significant figure. 
     The recitation of numerical ranges by endpoints includes all numbers within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5). 
     As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
     The following description should be read with reference to the drawings wherein like reference numerals indicate like elements throughout the several views. The drawings, which are not necessarily to scale, depict illustrative embodiments of the claimed invention. 
       FIG. 1  is a plan view of a catheter  10  in accordance with an embodiment of the present invention. The catheter  10  can be any of a variety of different catheters. In some embodiments, the catheter  10  can be an intravascular catheter. Examples of intravascular catheters include balloon catheters, atherectomy catheters, drug delivery catheters, stent delivery catheters, diagnostic catheters and guide catheters. The intravascular catheter  10  can be sized in accordance with its intended use. The catheter  10  can have a length that is in the range of about 100 to 150 centimeters and can have any useful diameter. As illustrated,  FIG. 1  portrays a guide catheter, but the invention is not limited to such. Except as described herein, the intravascular catheter  10  can be manufactured using conventional techniques. 
     In the illustrated embodiment, the intravascular catheter  10  includes an elongate shaft  12  that has a proximal region  14  defining a proximal end  16  and a distal region  18  defining a distal end  20 . A hub and strain relief assembly  22  can be connected to the proximal end  16  of the elongate shaft  12 . The hub and strain relief assembly  22  can be of conventional design and can be attached using conventional techniques. It is also recognized that alternative hub designs can be incorporated into embodiments of the present invention. 
     The elongate shaft  12  can include one or more shaft segments having varying degrees of flexibility. For example, the elongate shaft may include a relatively stiff proximal portion, a relatively flexible distal portion and an intermediate position disposed between the proximal and distal portions having a flexibility that is intermediate to both. 
     In some cases, the elongate shaft  12  may be formed of a single polymeric layer. In some instances, the elongate shaft  12  may include an inner liner such as an inner lubricious layer and an outer layer. In some cases, the elongate shaft  12  may include a reinforcing braid layer disposed between the inner and outer layers. The elongate shaft  12  is considered herein as generically representing a catheter to which various elements can be added to provide the catheter  10  with adjustable stiffness. 
     If the elongate shaft  12  includes an inner liner, the inner liner can include or be formed from a coating of a material having a suitably low coefficient of friction. Examples of suitable materials include perfluoro polymers such as polytetrafluoroethylene (PTFE), better known as TEFLON®, high density polyethylene (HDPE), polyarylene oxides, polyvinylpyrolidones, polyvinylalcohols, hydroxy alkyl cellulosics, algins, saccharides, caprolactones, and the like, and mixtures and combinations thereof. 
     The elongate shaft  12  can include, as an outer layer or layers, any suitable polymer that will provide the desired strength, flexibility or other desired characteristics. Polymers with low durometer or hardness can provide increased flexibility, while polymers with high durometer or hardness can provide increased stiffness. In some embodiments, the polymer material used is a thermoplastic polymer material. Some examples of suitable materials include polyurethane, elastomeric polyamides, block polyamide/ethers (such as PEBAX®), silicones, and co-polymers. The outer polymer layer  32  can be a single polymer, multiple longitudinal sections or layers, or a blend of polymers. By employing careful selection of materials and processing techniques, thermoplastic, solvent soluble, and thermosetting variants of these materials can be employed to achieve the desired results. In some instances, a thermoplastic polymer such as a co-polyester thermoplastic elastomer, for example, available commercially under the ARNITEL® name, can be used. 
       FIG. 2  illustrates an assembly  24  that includes a hypotube  26  disposed within a polymeric layer  28 . Merely for illustrative purposes, the polymeric layer  28  is seen in phantom as a single layer. In some cases, the polymeric layer  28  may represent two or more polymer layers. Any suitable polymers may be employed. It is contemplated that the assembly  24  could also include one or more polymeric layers, such a lubricious layer, within the hypotube  26 . 
     The hypotube  26  can be cut for flexibility purposes. In some instances, such as that illustrated, the hypotube  26  can be a spiral-cut hypotube having spirally-aligned cuts or kerfs  30  separating adjacent bridge portions  32 . The bridge portions  32  permit the hypotube  26  to retain a certain level of strength while the kerfs  30  lend flexibility. The hypotube  26  can be formed of any suitable polymeric or metallic material. In some instances, the hypotube  26  can be formed of stainless steel that has been laser cut. 
     Each of the kerfs  30  can be seen to have a particular width.  FIG. 2  can be assumed as showing the hypotube  26  in a relaxed configuration, i.e. no external forces are being applied to the hypotube  26 . The relative dimensions of the kerfs  30  and the bridge portions  32  will provide the hypotube  26 , and hence, the assembly  24 , with a given balance of flexibility versus strength. 
     In  FIG. 3 , the assembly  24  has been stiffened by reducing the relative size of each of the kerfs  34  while each of the bridge portions  32  remain unchanged. This can be accomplished by, for example, applying a compressive force to the hypotube  26 , as shown by arrow  36 . Alternatively, this can also be accomplished by rotating the hypotube  26 , as shown by arrow  38 . While not expressly illustrated, it should be recognized that applying either a compressive or rotational force to the hypotube  26  may change the diameter of the hypotube  26 . 
     In some instances, as seen for example in  FIGS. 4-6 , a catheter may include two or more coaxially aligned hypotubes.  FIG. 4  is a diagrammatic cross-section of an assembly  40 , showing an inner hypotube  42 , an outer hypotube  44  and a polymeric layer  46 . The polymeric layer  46  can be formed of any suitable polymer. While not expressly illustrated as such, the inner hypotube  42  and the outer hypotube  44  may both be spirally-cut. The inner hypotube  42  and the outer hypotube  44  can each be formed of any suitable polymeric or metallic material. In some instances, the inner hypotube  42  and the outer hypotube  44  can each be formed of stainless steel that has been laser cut. 
     An annular gap  48  can be seen between the inner hypotube  42  and the outer hypotube  44 . It should be noted that  FIG. 4  is not to scale; rather, certain elements have been exaggerated for clarity. The inner hypotube  42  can be considered as having an outer diameter that is somewhat less than an inner diameter of the outer hypotube  44 . The inner hypotube  42 , along with any desired inner layer or layers (not illustrated), forms a lumen  50  suitable for any desired or necessary medical treatment. 
     It will be recognized that the annular gap  48  will permit at least some relative movement between the inner hypotube  42  and the outer hypotube  44  before interference between the two will decrease flexibility of the assembly  40 .  FIG. 4  can be considered as illustrating a relaxed configuration, i.e. no external forces are being applied to any portions of the assembly  40 . 
     In  FIG. 5 , however, the inner hypotube  42  has expanded relative to the outer hypotube  44  such that the annular gap  48  (seen in  FIG. 4 ) has at least substantially disappeared. This can be accomplished, for example, by rotating the inner hypotube  42  to expand the diameter of the inner hypotube  42 . In some instances, the inner hypotube  42  may extend proximally to the proximal region  14  (see  FIG. 1 ), or may be operatively connected to actuation structure that extends proximally to the proximal region  14 , to permit an operator to rotate the inner hypotube  42 . 
     Conversely, as shown in  FIG. 6 , the outer hypotube  44  may be contracted in diameter relative to the inner hypotube  42  such that a new annular gap  52  appears between the outer hypotube  44  and the polymeric layer  46 . This can be accomplished, for example, by rotating the outer hypotube  44  to decrease the diameter of the outer hypotube  44 . In some instances, the outer hypotube  44  may extend proximally to the proximal region  14  (see  FIG. 1 ), or may be operatively connected to actuation structure that extends proximally to the proximal region  14 , to permit an operator to rotate the outer hypotube  44 . 
       FIGS. 7 through 12  illustrate embodiments of the invention in which inflatable elements are deployed within catheters to provide for adjustable stiffness. In  FIG. 7 , a catheter  54  includes an elongate shaft  56 . As discussed previously with respect to  FIG. 1 , the elongate shaft  56  may be a polymeric shaft and may include a single polymeric layer, two polymeric layers, or several polymeric layers, reinforcing layers, and the like. A lumen  58  extends through the interior of the elongate shaft  56 , which can be formed of any suitable polymer or polymers. 
     An elongate inflation tube  60  is deployed within the lumen  58 . In some instances, the elongate inflation tube  60  may be integrally formed within the elongate shaft  56 . In some cases, the elongate inflation tube  60  may be separately formed and subsequently secured within the lumen  58  using any suitable attachment technique. As seen in  FIG. 7 , the elongate inflation tube  60  is deflated. The elongate inflation tube  60  can be formed of any suitable polymer or polymers. 
     Turning to  FIG. 8 , the elongate inflation tube  60  has been inflated. The elongate inflation tube  60  can be seen as extending at least substantially the entire length of the elongate shaft  56 , from a proximal region  62  to a distal region  64 . In some instances, the elongate inflation tube  60  can be considered as extending proximally sufficiently far to be in fluid communication with the hub  22  (see  FIG. 1 ), so that inflation fluid may be introduced into the elongate inflation tube  60 . Any suitable fluid may be used, although saline is an exemplary fluid. Saline is biocompatible, which is important if a rupture occurs. Moreover, as an aqueous solution, saline is largely incompressible. 
     In the illustrated embodiment, the elongate inflation tube  60  has a radial cross-section that is at least substantially circular in shape, and that remains at least substantially constant across the length of the elongate inflation tube  60 . In some instances, it is contemplated that the elongate inflation tube  60  may have a non-circular radial cross-section. For example, the elongate inflation tube  60  may have an ovoid or even polygonal radial cross-section. 
     In some instances, it is contemplated that the elongate inflation tube  60  may have a radial cross-section that changes size across the length thereof. For example, the elongate inflation tube  60  may have a smaller radial cross-section within the distal region  64  and a larger radial cross-section within the proximal region  62 . In some instances, the elongate inflation tube  60  may have two, three or more distinct regions, each region having a distinctive radial cross-section size and/or shape. 
     It can be seen that the elongate inflation tube  60  can have relatively little impact on the flexibility of the elongate shaft  56  when deflated. When the elongate inflation tube  60  is inflated or pressurized, however, the elongate shaft  56  will become relatively less flexible, or relatively more stiff. 
       FIG. 9  shows a catheter  66  that includes an elongate shaft  68 . The elongate shaft  68  may be a polymeric shaft and may include a single polymeric layer, two polymeric layers, or several polymeric layers, reinforcing layers, and the like. A lumen  69  extends through the interior of the elongate shaft  68 , which can be formed of any suitable polymer or polymers. 
     A first elongate inflation tube  70  and a second elongate inflation tube  72  are deployed within the lumen  70 . In some instances, the first elongate inflation tube  70  and the second elongate inflation tube  72  may be integrally formed within the elongate shaft  68 . In some cases, the first elongate inflation tube  70  and the second elongate inflation tube  72  may be separately formed and subsequently secured within the lumen  68  using any suitable attachment technique. Each of the first elongate inflation tube  70  and the second elongate inflation tube  72  may be formed of any suitable material. 
     As illustrated, the first elongate inflation tube  70  and the second elongate inflation tube  72  have been inflated or pressurized, and can be seen as being at least substantially parallel with each other. In some cases, the first elongate inflation tube  70  and the second elongate inflation tube  72  may be arranged at an angle with respect to each other, as seen for example, in  FIG. 9A . Each of the first elongate inflation tube  70  and the second elongate inflation tube  72  can be seen as extending at least substantially the entire length of the elongate shaft  68 , from a proximal region  76  to a distal region  78 . 
     In some instances, the first elongate inflation tube  70  and the second elongate inflation tube  72  can each be considered as extending proximally sufficiently far to be in fluid communication with the hub  22  (see  FIG. 1 ), so that inflation fluid may be introduced. Any suitable fluid may be used, although saline is an exemplary fluid. 
     In  FIG. 10 , a catheter  80  can be seen as including an elongate shaft  82 . The elongate shaft  82  may be a polymeric shaft and may include a single polymeric layer, two polymeric layers, or several polymeric layers, reinforcing layers, and the like. A lumen  84  extends through the interior of the elongate shaft  82 , which can be formed of any suitable polymer or polymers. 
     A first elongate inflation tube  86  and a second elongate inflation tube  88  are deployed within the lumen  84 . In some instances, the first elongate inflation tube  86  and the second elongate inflation tube  88  may be integrally formed within the elongate shaft  82 . In some cases, the first elongate inflation tube  86  and the second elongate inflation tube  88  may be separately formed and subsequently secured within the lumen  68  using any suitable attachment technique. The first elongate inflation tube  86  and the second elongate inflation tube  88  can be formed of any suitable polymer or polymers. 
     As illustrated, the first elongate inflation tube  86  and the second elongate inflation tube  88  have been inflated or pressurized. The second elongate inflation tube  88  can be seen as extending at least substantially the entire length of the elongate shaft  82 , from a distal region  90  to a proximal region  92 . The first elongate inflation tube  86 , however, terminates at a position  94  that is well short of the distal region  90 . In some instances, it may be desirable to be able to temporarily provide additional stiffness to the proximal region  92  while retaining a relatively greater level of flexibility within the distal region  90 . 
     In some instances, the first elongate inflation tube  86  and the second elongate inflation tube  88  can each be considered as extending proximally sufficiently far to be in fluid communication with the hub  22  (see  FIG. 1 ), so that inflation fluid may be introduced. Any suitable fluid may be used, although saline is an exemplary fluid. 
       FIG. 11  shows a catheter  96  having an elongate shaft  98 . The elongate shaft  98  may be a polymeric shaft and may include a single polymeric layer, two polymeric layers, or several polymeric layers, reinforcing layers, and the like. A lumen  100  extends through the interior of the elongate shaft  98 , which can be formed of any suitable polymer or polymers. 
     An elongate annular inflation ring  102  is deployed within the lumen  100 . In some instances, the elongate annular inflation ring  102  may be integrally formed within the elongate shaft  98 . In some cases, the elongate annular inflation ring  102  may be separately formed and subsequently secured within the lumen  100  using any suitable attachment technique. The elongate annular inflation ring  102  can be formed of any suitable polymer or polymers. 
     As seen, the elongate annular inflation ring  102  is inflated or pressurized. The elongate annular inflation ring  102  can extend at least substantially the entire length of the elongate shaft  98 , from a proximal region  104  to a distal region  106 . In some instances, the elongate annular inflation ring  102  can be considered as extending proximally sufficiently far to be in fluid communication with the hub  22  (see  FIG. 1 ), so that inflation fluid may be introduced into the elongate annular inflation ring  102 . Any suitable fluid may be used, although saline is an exemplary fluid. 
       FIG. 12  shows a catheter  108  having an elongate shaft  110 . The elongate shaft  110  may be a polymeric shaft and may include a single polymeric layer, two polymeric layers, or several polymeric layers, reinforcing layers, and the like. A lumen  112  extends through the interior of the elongate shaft  110 , which can be formed of any suitable polymer or polymers. 
     An elongate inflation ring  114  is deployed within the lumen  112 . In some instances, the elongate inflation ring  114  may be integrally formed within the elongate shaft  110 . In some cases, the elongate inflation ring  114  may be separately formed and subsequently secured within the lumen  112  using any suitable attachment technique. The elongate inflation ring  114  can be formed of any suitable polymer or polymers. 
     The elongate annular inflation ring  102  ( FIG. 11 ) has at least a substantially constant dimension. In contrast, the elongate inflation ring  114  has a varying dimension. In some instances, the elongate inflation ring  114  can have a relatively thinner dimension along one side (top, as illustrated) and a relatively thicker dimension along another side (bottom, as illustrated). This can be useful if it is desired to provide relatively greater stiffness along one side of the catheter  108  and relatively reduced stiffness along another side of the catheter  108 . 
     As seen, the elongate inflation ring  114  is inflated or pressurized. The elongate inflation ring  114  can extend at least substantially the entire length of the elongate shaft  110 , from a proximal region  116  to a distal region  118 . In some instances, the elongate inflation ring  114  can be considered as extending proximally sufficiently far to be in fluid communication with the hub  22  (see  FIG. 1 ), so that inflation fluid may be introduced into the elongate inflation ring  114 . Any suitable fluid may be used, although saline is an exemplary fluid. 
       FIGS. 13 and 14  illustrate an embodiment in which a swellable material such as a hydrogel is used to provide a catheter with adjustable stiffness. In  FIG. 13 , a portion of a catheter  120  includes an inner polymer layer  122  and an outer polymer layer  124 . The inner polymer layer  122  and the outer polymer layer  124  can each independently be formed of any suitable polymer or polymers. A gap  126  is disposed between the inner polymer layer  122  and the outer polymer layer  124 . A layer or coating  128  of a swellable material is disposed within the gap  126 . As seen in this Figure, the coating  128  is dry. 
     In  FIG. 14 , the coating  128  of swellable material has been caused to swell, thereby eliminating the gap  126  seen in  FIG. 13 . The coating  128  can be caused to swell by contacting the coating  128  with an appropriate liquid. If, for example, the coating  128  is a hydrogel, it can be caused to swell simply by contacting the coating  128  with water. In some instances, the gap  126  ( FIG. 13 ) can be considered as extending proximally sufficiently far to be in fluid communication with the hub  22  (see  FIG. 1 ), so that an appropriate liquid such as water may be introduced. 
     Examples of suitable swellable materials include hydrophilic polymers. A hydrophilic polymer is a polymer that attracts or binds water molecules when the polymer is placed in contact with an aqueous system. Examples of aqueous systems that can provide water molecules that can bind to a hydrophilic polymer include blood and other bodily fluids. When a hydrophilic polymer comes into contact with such a system, water molecules can bind to the polymer via mechanisms such as hydrogen bonding between the water molecules and substituents or functional groups present within or on the polymer. 
     One class of polymers that can be considered as hydrophilic includes ionomer polymers. An ionomer polymer is a polymer that can be considered as containing covalent bonds between elements within a chain while containing ionic bonds between chains. An ionomer polymer is a polymer that has charged functional groups appended to the polymer chain. The charged functional groups can be positively charged, in which case the polymer can be referred to be a cationomer, or the functional groups can be negatively charged, in which case the polymer can be referred to as an anionomer. 
     An ionomeric polymer can be formed using a variety of negatively charged functional groups. The negatively charged functional group can be added to a previously formed polymer, or the negatively charged functional groups can be part of one or more of the monomers used to form the ionomeric polymer. 
     Examples of suitable negatively charged functional groups include sulfonates and carboxylates. The ionomeric polymer can, in particular, include sulfonate functional groups. These groups are negatively charged and can readily hydrogen bond sufficient amounts of water when brought into contact with a source of water such as an aqueous system. 
     Further examples of suitable materials include nonionic polyether polyurethanes available commercially under the HYDROSLIP® name. Another suitable material includes nonionic aliphatic polyether polyurethanes available commercially under the TECOGEL® name. Examples of other suitable nonionic polymers include polymers such as poly (hydroxy methacrylate), poly (vinyl alcohol), poly (ethylene oxide), poly (n-vinyl-2-pyrolidone), poly (acrylamide) and other similar materials. 
       FIGS. 15 through 17  illustrate embodiments of the invention in which catheters can enjoy adjustable stiffness through the use of external sheaths that may be slidably disposed over the catheters. 
       FIG. 15  shows a catheter  130  including an elongate shaft  132  and a stiffness sheath  134  slidably disposed over the elongate shaft  132 . The elongate shaft  132  may be a polymeric shaft and may include a single polymeric layer, two polymeric layers, or several polymeric layers, reinforcing layers, and the like. Any suitable polymer or polymers can be used. The stiffness sheath  134  may be formed of any suitably stiff polymeric or metallic material. 
     In  FIG. 16 , a catheter  136  includes the elongate shaft  132  as discussed with respect to  FIG. 15 . A first stiffness sheath  138  is slidably disposed over the elongate shaft  132 , while a second stiffness sheath  140  is slidably disposed over the first stiffness sheath  138 . In some instances, each of the first stiffness sheath  138  and the second stiffness sheath  140  may independently be moved either distally or proximally over the elongate shaft  132  to provide a desired degree of stiffness. Each of the first stiffness sheath  138  and the second stiffness sheath  140  may be formed of any suitably stiff polymeric or metallic material. 
     In  FIG. 17 , a catheter  142  includes the elongate shaft  132  as discussed with respect to  FIG. 15 . A tapered or frustoconical-shaped stiffness sheath  144  is slidably disposed over the elongate shaft  132 . The stiffness sheath  144  has a narrow end  146  and a wide end  148  and can provide, as a result, a gradual change in stiffness. The stiffness sheath  144  can be formed of any suitably stiff polymeric or metallic material. 
       FIG. 18  illustrates an embodiment of the invention employing a number of stiffness filaments. A catheter  150  includes an elongate shaft  152 . The elongate shaft  152  may be a polymeric shaft and may include a single polymeric layer, two polymeric layers, or several polymeric layers, reinforcing layers, and the like. A lumen  154  extends through the elongate shaft  152 , which can be formed of any suitable polymer or polymers. 
     The catheter  150  includes a number of elongate apertures  156  disposed within the elongate shaft  152 . It can be seen that the elongate apertures  156  extend longitudinally within the elongate shaft  152 . The elongate apertures  156  can be evenly spaced out about the circumference of the elongate shaft  152 . Any number of elongate apertures  156  may be provided. At least some of the elongate apertures  156  include a stiffness-enhancing filaments  158  slidably deployed within the elongate apertures  156 . 
     Depending on the performance requirements, one or more of the stiffness-enhancing filaments  158  may be inserted into, removed from, or slide within an appropriate and corresponding elongate aperture  156 . In some instances, the stiffness-enhancing filaments  158  may be wires formed of any suitable material such as Nitinol, stainless steel, titanium, aluminum, cobalt chromium or any other suitable metal. 
       FIGS. 19 and 20  illustrate use of an electro-active polymer in providing variable stiffness to a catheter.  FIG. 19  shows a catheter  174  having an elongate shaft  176  that includes one or more polymeric layers. A series of flaps  178  have been cut into the elongate shaft  176 , and extend into a lumen  180 . At least the flaps  178  include an electro-active polymer. It should be noted that the size of the flaps  178  relative to the elongate shaft  176  has been exaggerated for illustrative purposes. In this configuration, which can be considered to be a relaxed configuration, the flaps  178  provide a level of flexibility to the elongate shaft  176 . 
     In  FIG. 20 , a current has been applied. Consequently, the flaps  178  have been actuated from the position seen in  FIG. 19 , in which the flaps  178  extend into lumen  180 , to a position in which the flaps  178  align with the elongate shaft  176  and thereby improve the column strength of the elongate shaft  176 . 
     It should be noted that in some instances, it is contemplated that at least a portion of elongate shaft  12  (see  FIG. 1 ) may be formed from or include a layer of an electrostatically actuatable material such as an electro-active polymer, a polymer including buckytubes, or perhaps a liquid crystal polymer. It is contemplated that such materials may, if subjected to an electrical current, change the relative stiffness of a catheter containing such a material. 
     It should be understood that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of steps without exceeding the scope of the invention. The invention&#39;s scope is, of course, defined in the language in which the appended claims are expressed.