Abstract:
A medical device includes a handle, a sheath, and an object-engaging unit. The distal end portion of the sheath is strong enough to resist being deformed by the object-engaging unit, yet another portion of the sheath is flexible enough to bend sufficiently.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 10/147,938, filed May 17, 2002 now U.S. Pat. No. 6,939,353, which is a continuation of U.S. patent application Ser. No. 09/548,405, filed Apr. 12, 2000 (now U.S. Pat. No. 6,398,791), which is a continuation-in-part of U.S. patent application Ser. No. 09/478,609, filed Dec. 30, 1999 (now U.S. Pat. No. 6,325,807), which claims the benefit of U.S. provisional application No. 60/138,950, filed Jun. 11, 1999, all of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This invention generally relates to medical devices for manipulating matter in a body. More particularly, the invention relates to a medical device including an object-engaging unit, such as a retrieval device or a surgical blade, and a sheath for introduction of the object-engaging unit directly into a body opening or cavity or into a body through an endoscope or a laparoscope channel, for example. 
     BACKGROUND INFORMATION 
     Medical devices can be used in a body opening, cavity, or tract to manipulate material within the body. Such medical devices may be used through an endoscope or a laparoscope. When inserted into a flexible endoscope, medical devices impair the ability of the flexible endoscope to achieve the desired flexion needed to engage material at a remote body site. 
     One such medical device has a sheath and an object-engaging unit, such as a basket, that is moveable relative to the sheath from a collapsed state within the sheath to another state in which the unit extends past the distal end of the sheath. Typically, the object-engaging unit is in an operational mode when the object-engaging unit is extended beyond the distal end of the sheath. The sheath typically extends from a handle, located at the proximal end (i.e., the end away from the patient) of the sheath, to the object-engaging unit, which is located at the distal end of the sheath (i.e., the end near the patient and that goes into the patient). 
     One purpose of the sheath is to collapse and release the object-engaging unit by sliding the sheath over (to collapse) or away from (to release) the object-engaging unit, or by moving the object-engaging unit into (to collapse) and out of (to release) the sheath. The object-engaging unit is, for example, a grasping forcep-like assembly, a basket assembly, or any type of tissue and/or object manipulating, capturing, and/or retrieving assembly. When the object-engaging unit is enclosed within the sheath, the object-engaging unit is inoperative and in its collapsed or withdrawn state. For example, object-engaging units, such as baskets, assume a collapsed, reduced diameter profile when the basket is enclosed within the distal end of the sheath. When the sheath is retracted relative to the basket or the basket is extended beyond the distal end of the sheath, the basket expands to a relatively larger diameter than when the basket is enclosed and collapsed within the sheath. In the expanded position, the basket is positioned and is operable at least to capture material in the body, such as kidney stones. If the object-engaging unit is, for example, a blade, it would be withdrawn when not needed to cut, and then extended at least partially out beyond the distal end of the sheath to allow cutting with the blade. The sheath also serves other purposes. For example, the sheath serves to encompass and protect the object-engaging unit as it is inserted into the body cavity or channel of an endoscope. The sheath also serves to protect the body cavity from damage that may be introduced by the object-engaging unit itself if it were released, expanded, or extended during passage of the sheath into the body. Also, the sheath must provide sufficient strength and rigidity to allow its insertion into the body or endoscope channel while also providing sufficient flexibility to permit the sheath to navigate through the tortuous channels of the body cavity, opening, or tract. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a medical device comprising a sheath with sufficient flexibility to locate and engage material at a site in a body, such as the renal pelvis. That is, the sheath is sufficiently flexible to navigate the turns and curves of the body tract into which the medical device is introduced in order to manipulate target material at the remote body site. The sheath is also sufficiently flexible to allow a flexible endoscope, into which the sheath is inserted, to achieve the desired flexion. 
     It is another object of the invention to provide a medical device including a sheath that has sufficient strength at the proximal and distal end portions of the sheath to locate, engage, and retrieve target material at a remote body site, and to actuate an object-engaging unit without distorting or deforming the distal end portion of the sheath. 
     It is yet another object of the invention to provide a method of using medical devices including such sheaths to retrieve material at a remote site in a body. 
     In one aspect, the invention relates to a device for use in manipulating material at a site within a body. The device includes a handle and a sheath extending from the handle. The sheath includes a lumen, a proximal end, a distal end, a proximal portion, an intermediate portion, and a distal portion. The intermediate portion of the sheath is more flexible than both the proximal portion and the distal portion of the sheath. The device also includes an object-engaging unit. The object-engaging unit and the sheath are moveable relative to each other to achieve a collapsed state of the object-engaging unit in which the object-engaging unit is collapsed within the lumen of the distal portion of the sheath, and another state in which the object-engaging unit extends from the distal end of the sheath. 
     In one embodiment of the device according to this aspect of the invention, the outside diameter of the intermediate sheath portion is more narrow than the outside diameter of the proximal and distal sheath portions. The intermediate sheath portion, in another embodiment, has one or more fewer layers than the proximal and distal sheath portions, and in this case the outside diameter of the intermediate sheath portion may or may not have the same outside diameter as the proximal and distal sheath portions. 
     Also in accordance with the invention, the intermediate sheath portion has at least two sections, and at least one of the sections has a different number of layers of material than at least one other of the sections. The sections can be arranged in a variety of different patterns such as circumferential, longitudinal, spiral, helical, and/or criss-cross patterns. 
     Also, in accordance with this aspect of the invention, the layers of the sheath in all portions of the sheath comprise various materials, such as fluorinated ethylenepropylene (FEP), polytetrafluoroethylene (PTFE), stainless steel braid, or polyimide. Regardless of the materials or the number of layers of material used to make the proximal, intermediate, and distal sheath portions, the diameter of the sheath lumen is the same or different in the proximal, intermediate, and distal sheath portions. 
     In another aspect, the invention relates to a sheath for a medical device, wherein the sheath comprises a wall extending from a proximal end of the sheath to a distal end of the sheath. The wall defines a lumen extending from the proximal sheath end to the distal sheath end. The wall includes a proximal portion, an intermediate portion, and a distal portion. The intermediate portion is more flexible than the proximal and distal portions. The lumen in the distal portion of the sheath wall is adapted for receiving an object-engaging unit. 
     In still other aspects of the invention, methods of manipulating material in a body include inserting into a body (either directly or through some other mechanism such as an endoscope channel) a medical device including a sheath of the type described above. The steps of the methods include moving the object-engaging unit from a collapsed state to another state in which the object-engaging unit is extended beyond the distal end of the sheath, engaging the material in the body, and ultimately withdrawing the device from the body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. 
         FIG. 1  illustrates a stone lodged in a remote body site. 
         FIG. 2A  illustrates a plan view of an embodiment of a medical device including a sheath according to the invention with an object-engaging unit in a fully extended or open position. 
         FIG. 2B  illustrates the device illustrated in  FIG. 2A  with the object-engaging unit in a collapsed or retracted position within the distal end of the sheath. 
         FIG. 3A  illustrates a plan view of another embodiment of a medical device including a sheath according to the invention with an object-engaging unit in a fully-extended, open position. 
         FIG. 3B  illustrates the device illustrated in  FIG. 3A  with the object-engaging unit in a collapsed or retracted position within the distal end of the sheath. 
         FIG. 4A  illustrates a plan view of an embodiment of the sheath according to the invention. 
         FIG. 4B  illustrates a plan view of a sheath when the intermediate sheath portion is flexed. 
         FIG. 5A  illustrates a cross-section of an embodiment of the distal portion of a sheath according to the invention. 
         FIG. 5B  illustrates a cross-section of an embodiment of the intermediate portion of the sheath illustrated in  FIG. 5A  according to the invention. 
         FIG. 6A  illustrates a cross-section of another embodiment of a proximal portion of the sheath according to the invention. 
         FIG. 6B  illustrates a cross-section of an embodiment of the intermediate portion of the sheath illustrated in  FIG. 6A  according to the invention. 
         FIG. 7A  illustrates a plan view of an embodiment of the sheath according to the invention, the intermediate portion including sections having different numbers of layers of material. 
         FIG. 7B  illustrates a cross-section of the sheath illustrated in  FIG. 7A  taken at line  7 B- 7 B. 
         FIG. 7C  illustrates a cross-section of the sheath illustrated in  FIG. 7A  taken at line  7 C- 7 C. 
         FIG. 7D  illustrates a plan view of another embodiment of the sheath according to the invention, the intermediate portion including sections having different numbers of layers of material. 
         FIG. 7E  illustrates a plan view of another embodiment of the sheath according to the invention, the intermediate portion including sections having different numbers of layers of material. 
         FIG. 7F  illustrates a plan view of another embodiment of the sheath according to the invention, the intermediate portion including sections having different numbers of layers of material. 
         FIG. 8A  is a plan view of an embodiment of a sheath according to the invention including an attenuated sheath wall in the intermediate portion of the sheath. 
         FIG. 8B  illustrates a cross-section of the embodiment of the proximal sheath portion illustrated in  FIG. 8A  taken at line  8   b - 8   b.    
         FIG. 8C  illustrates a cross-section of the embodiment of the intermediate sheath portion illustrated in  FIG. 8A  taken at line  8   c - 8   c.    
         FIG. 8D  illustrates a cross-section of the embodiment of the distal sheath portion illustrated in  FIG. 8A  taken at line  8   d - 8   d.    
         FIG. 9A  shows another embodiment of a sheath according to the invention including an attenuated wall in the intermediate portion of the sheath. 
         FIG. 9B  shows another embodiment of a sheath according to the invention including an attentuated wall in the intermediate portion of the sheath. 
         FIG. 9C  shows another embodiment of a sheath according to the invention including an attentuated wall in the intermediate portion of the sheath. 
         FIG. 9D  shows another embodiment of a sheath according to the invention including an attenuated wall in the intermediate portion of the sheath. 
         FIG. 10A  illustrates a plan view of the long axis of a sheath according to the invention, the intermediate portion of the sheath including numerous slits in the external wall of the sheath. 
         FIG. 10B  shows a cross-section in perspective of the embodiment of the intermediate portion of the sheath illustrated in  FIG. 9A  at  9   b - 9   b.    
         FIG. 10C  shows a cross-section in perspective of another embodiment of the intermediate portion of a sheath according to the invention including numerous depressions in the external wall of the sheath. 
         FIG. 11A  illustrates a first step in a method for retrieving material located in a body cavity using a medical device including a sheath according to the invention. 
         FIG. 11B  illustrates a second step in a method for retrieving material from a body cavity using a medical device including a sheath according to the invention. 
         FIG. 11C  shows a third step in a method for retrieving material from a body cavity using a medical device including a sheath according to the invention. 
     
    
    
     DESCRIPTION 
     One of the problems encountered when an elongated medical device, such as a retrieval device in a sheath, is used to reach a remote location in the body, is the turns and curves in the body tract that must be negotiated in order to reach the remote body location. As illustrated in  FIG. 1 , by way of example, a stone  100  lodged in the renal calyx  42  can not be approached via the ureter  40  without encountering at least one curve  102  illustrated by arrow  104 . The rigid elongate member of a medical device can not effectively negotiate curves, such as the one illustrated, without causing tissue trauma in order to reach the remote tissue location. 
     According to the invention, positioning a flexible sheath portion near the distal end of the sheath, addresses this problem. The embodiments of the invention described below, all have the common feature of a flexible sheath portion positioned near the distal end of the elongated sheath. 
     All of the following embodiments of the invention have at least one thing in common, the sheath of the invention, on one hand, has sufficient flexibility in an area adjacent the distal end portion ( at the “intermediate portion” of the sheath) to allow engagement of a stone, tissue, or other material. On the other hand, the sheath has sufficient strength or rigidity at the most distal end portion of the sheath to enclose the object engaging unit in the inoperative or collapsed state and to permit the object-engaging unit to move in and out of the distal end portion of the sheath without the distal end portion of the sheath deforming or failing. 
     Referring to  FIGS. 2A ,  2 B,  3 A, and  3 B, one embodiment of a medical device, according to the invention, includes a handle portion  16 , an actuator  20 , a sheath  14 , a sheath lumen  15 , an elongated member  18  that passes within the sheath lumen  15 , and an object-engaging unit  10 , such as the grasping forceps illustrated in  FIGS. 2A and 2B . The handle  16 , the sheath  14 , and the object-engaging unit  10  are not shown in their correct size or proportion to each other. The sheath  14  typically is much longer and narrower than the handle  16  or the grasping forceps  10  to allow insertion into a body cavity or tract. 
     Referring to  FIG. 4A , a sheath  14  according to the invention comprises at least three segments: a proximal segment  30 , an intermediate segment  32 , and a distal segment  34 . The proximal segment  30  of the sheath is located nearest to the handle  16 . The distal portion of the sheath  34  is that region of the sheath at the distal end of the medical device opposite to the handle  16 . The intermediate portion  32  of the sheath is positioned distal to the proximal portion  30  of the sheath and proximal to the distal portion  34  of the sheath. The proximal end of the intermediate portion  32  of the sheath is located about 3 inches to 7 inches, preferably 4.75 inches, from the distal end  26  of the sheath. 
     The object-engaging unit  10  is collapsed within the distal portion of the sheath  34  for direct entry into the body or for entry into the body via the operating channel of an endoscope or laparoscope. The object-engaging unit  10  can be a grasping-type device, as illustrated in  FIGS. 2A and 2B . Such grasping type object-engaging units are described in U.S. Ser. No. 09/064,704, the disclosure of which is expressly incorporated by reference herein. Other types of object-engaging units that can be used with the sheath of the invention are retrieval basket-type devices, as illustrated in  FIGS. 3A and 3B , such as those described in U.S. Ser. No. 09/065,158, U.S. Ser. No. 09/027,534, U.S. Ser. No. 09/084,135, U.S. Ser. No. 09/296,327, U.S. Ser. No. 09/268,484, and U.S. Ser. No. 09/369,226, the disclosures of which are expressly incorporated by reference herein. Other suitable object-engaging units are forceps, probes, retractors, elevators, blades, needles, and the like. 
     Referring now to  FIGS. 2A ,  2 B,  3 A and  3 B, the sheath  14 , according to the invention, has at least one lumen  15  therein, and it extends from the handle  16  to a distal sheath end  26 . In one embodiment, an elongated member  18 , such as a cable, coil, shaft, guidewire, or mandril wire, extends within the lumen  15  from the actuating mechanism  20  in the handle portion  16  to a base  13  of the object engaging unit  10  where the elongated member  18  is joined to the object-engaging unit base  13 . Operation of the actuating mechanism  20  by an operator causes the object-engaging unit  10  to move relative to the sheath  14  between a collapsed state within the distal sheath portion  34 , as illustrated in  FIG. 3B , to an extended state outside of the distal end  26  of the sheath  14  where the object-engaging unit  10  is open and extending beyond the distal end  26  of the sheath, as shown in  FIG. 3A . 
     Alternatively, in another embodiment of the invention, the actuating mechanism is joined to the sheath. Operation of the actuating mechanism  20  by an operator in this embodiment causes the sheath  14  to move relative to the object-engaging unit  10  and the elongated member  18 , such that the object-engaging unit  10  is moved between a collapsed position within the distal sheath portion  34 , illustrated in  FIG. 2B  , to an extended position outside of the sheath  14  where the object-engaging unit  10  is open and extended beyond the distal end of the sheath  26  as illustrated in  FIG. 2A . 
     With the object-engaging unit  10  withdrawn into and collapsed within the distal sheath portion  34 , as shown in  FIGS. 2B and 3B , the object-engaging unit  10  is inserted into the body directly, or via an endoscope channel, to a remote internal site in the body where the target material is located. The object-engaging unit  10  is then moved relative to the sheath  14  and placed in the extended position illustrated in  FIGS. 2A and 3A , such that the object-engaging unit  10 , in its operative position, is manipulated by the operator to engage the target material or tissue in the remote body site. The object-engaging unit  10  can then be used to manipulate the material in the body. Following manipulation of the material by the object-engaging unit  10 , the object-engaging unit  10  is at least partially withdrawn into the lumen  15  of the distal sheath portion  34 , as illustrated in  FIG. 3B , by moving the elongated member  18  axially relative to the sheath  14  with the object-engaging unit  10  attached to its distal end, or by moving the sheath  14  axially relative to the object-engaging unit  10 , as illustrated in  FIG. 2B . The medical device, including the object-engaging unit  10  and the sheath  14 , are withdrawn from the body cavity. 
     The overall length of the sheath  14 , according to the invention, can range between 40 inches to 75 inches, preferably about 49.5 inches to 52.5 inches. The proximal portion  30  of the sheath is 35 inches to 55 inches in length, preferably 44.75 inches to 48.25 inches in length, the intermediate portion  32  of the sheath is 3 inches to 6 inches in length, preferably 3.5 to 4.0 inches, and the distal portion  34  of the sheath is 0.10 inches to 1.0 inch in length, preferably 0.75 inches. 
     The outside diameter  11  of the sheath, indicated between the arrows  11  in  FIG. 4A , may or may not be uniform from the proximal end of the sheath  24  to the distal end of the sheath  26 . In general, the outside diameter  11  of the proximal sheath portion  30  may range from 0.0315 inches to 0.0414 inches, the outside diameter  11  of the intermediate sheath portion  32  may range from 0.03 inches to 0.0445 inches, and the outside diameter  11  of the distal sheath portion  34  may range from 0.0315 inches to 0.0445 inches. 
     Sheath flexibility varies along the sheath&#39;s long axis from the proximal end  24  of the sheath to the distal end  26  of the sheath. In one embodiment, according to the invention, the intermediate portion  32  of the sheath  14  is more flexible than either the proximal sheath portion  30  or the distal sheath portion  34 . That is, the proximal sheath portion  30  and the distal sheath portion  34  are more rigid than the intermediate sheath portion  32 . The flexibility of the proximal sheath portion  30  and the distal sheath portion  34  can be the same or can differ. However, the proximal sheath portion  30  and the distal sheath portion  34  are less flexible, i.e., more rigid, than the intermediate sheath portion  32 . The intermediate portion  32  is highly flexible, as illustrated in  FIG. 4B , and can be flexed to as much as 360 degrees from the long axis of the sheath (indicated as reference line  100 ). After the sheath of the invention is inserted into a channel of a flexible endoscope, the flexible endoscope with the sheath may flex as much as 180 degrees to 190 degrees. 
     Enhanced flexibility of the intermediate portion  32  of the sheath  14  is achieved in several ways. Referring to  FIGS. 4A ,  4 B,  5 A,  5 B,  6 A and  6 B, in one embodiment of a sheath  14 , according to the invention, the sheath  14  is constructed with layers of material extending from the luminal side  19  of the sheath wall  12  to the outside  17  of the sheath wall  12 . The wall  12  of the sheath, according to the invention, includes multiple concentric layers of the same or different materials. The intermediate sheath portion  32 , like the proximal sheath portion  30  and the distal sheath portion  34 , includes multiple concentric layers, however, the intermediate sheath portion  32  has at least one fewer layer or at least one different layer of material than the layers of material in the proximal sheath portion  30  and the distal sheath portion  34 . 
     For example, as illustrated by a cross-section of distal sheath  34  in  FIG. 5A , in one embodiment of the sheath, according to the invention, the distal sheath portion  34  and the proximal sheath portion  30  (not shown) are manufactured from four layers of material including, from inside  19  of the sheath wall  12  to outside  17  of the sheath wall  12 , a first layer (a) of polytetrafluoroethylene (PTFE), a second layer (b) of  304  braided stainless steel, a third layer (c) of polyimide, and a fourth layer (d) of fluorinated ethylenepropylene (FEP). Each material is arranged in a concentric layer around the circumference of the sheath wall  12 . In the same sheath, the intermediate sheath portion  34 , illustrated in  FIG. 5B , has one fewer of the layers a-d than the proximal and distal sheath portions. The materials in intermediate sheath portion  32 , from the inside  19  of the sheath wall  12  to the outside  17  of the sheath wall  12 , include a first inner layer (a) of PTFE, a second layer (b) of stainless steel braid, no third layer, and a fourth layer (d) of FEP, each material arranged in a concentric layer around the circumference of the sheath wall  12 . The relative thickness of each layer, illustrated in  FIGS. 5A and 5B , is merely intended to be illustrative and is not limited to that illustrated. The thickness of one or more of the layers of the sheath wall  12  may be greater or lesser than the thickness of one or more of the other layers. Thus, the outer diameter  11  of the intermediate sheath portion  32  may be less than or the same as the outer diameter  11  of proximal sheath portion  30  and distal sheath portion  34 . However, the internal diameter  13  of the intermediate sheath portion  32  is the same as the internal diameter  13  of proximal sheath portion  30  and distal sheath portion  34 . That is, the diameter of the lumen  15  of the sheath  14  is uniform from the proximal end  24  of the sheath to the distal end  26  of the sheath. By maintaining a constant luminal internal diameter  13  throughout the length of the sheath  14 , the object-engaging unit  10  and the elongated member  18  may move in the lumen  15  relative to the sheath  14  without becoming snagged. 
     In an alternate embodiment, the proximal sheath portion  30  and the distal sheath portion  34  are manufactured from a different combination of materials. For example, the first or inner layer is FEP, followed by a second layer of stainless steel braid, followed by a third layer of polyimide, and followed by a fourth, outer layer of PTFE. The intermediate sheath portion  32  of this embodiment of the sheath includes a first inner layer (a) of FEP, a second layer (b) of stainless steel braid, no third layer, and a fourth, outer layer (d) of PTFE. 
     In yet another embodiment of the sheath of the invention, the proximal sheath portion  30 , illustrated in  FIG. 6A , and the distal sheath portion  34  each have five layers, each layer made from the same material. In this embodiment of the sheath, the intermediate sheath portion  32 , illustrated in  FIG. 6B , has four layers of material identical to the material used in the five layers of the proximal  30  and distal  34  sheath portions. Other materials and different number of layers of the sheath are also contemplated by the invention and are not limited to those illustrated in  FIGS. 5A ,  5 B,  6 A and  6 B. Whatever the combination of materials and numbers of layers, the intermediate sheath portion  32  is always more flexible than the proximal sheath portion  30  and distal sheath portion  34 , and the internal diameter  13  of the sheath is uniform from the proximal end  24  of the sheath to the distal end  26  of the sheath. 
     In another embodiment of the sheath, the intermediate sheath portion  32  has at least two sections  33 ,  35 , and at least one of the sections has a different number of layers of material than at least one other of the sections. A section of the sheath is a region of the sheath wall having a different number of layers than an adjacent region of the sheath wall. For example, as illustrated in  FIG. 7A , the wall of the intermediate portion  32  of the sheath  17  can have a section of three layers  35  of material followed by a section of four layers  33  of material, followed by a section of three layers  35  of material, and so on. Alternatively, the wall of the intermediate portion  32  of the sheath can have a section of four layers of material, followed by a section of three layers of material, followed by a section of two layers of material, followed by a section of four layers of material, and so on. This pattern may be repeated along the entire length of the intermediate sheath portion  32  or the pattern may be limited to a portion of the intermediate sheath portion  32 . The number and type of sections can be varied and different than the specific arrangements shown. The sections in the intermediate sheath portion  32  can be arranged circumferentially, spirally in a lattice-like or criss-cross pattern, or longitudinally, as illustrated in  FIGS. 7A ,  7 D,  7 E and  7 F, respectively. 
     In one particular embodiment, the wall of the intermediate sheath portion  32  has a repeating pattern of four layer and three layer sections along its entire length, although other embodiments can have other layers in the sections. In a particular embodiment of the wall of the intermediate portion of the sheath having four layers, one of the four layers is polyimide while the other layers are selected from materials, such as FEP, stainless steel braid, PTFE, polyetheretherketone (PEEK), or Nitinol. In this embodiment, the polyimide layer is absent in the sections of the wall of the sheath having three layers. In other embodiments, a layer of a different material is absent in the wall of the intermediate sheath portion. 
     A sheath, according to the invention, has the advantage of enhanced flexibility while maintaining sheath column and compression strength. For example, in one embodiment of the sheath of the invention, the proximal sheath portion  30  and the distal sheath portion  34  have four layers in the sheath wall while the intermediate sheath portion  32 , has a repeating pattern of a three layer section  35  followed by a four layer section  33 . Alternatively, sections  33  have two, four, or more layers and sections  35  have a different number of layers than sections  33 . 
     In one embodiment of the invention, the arrangement of sections in the intermediate sheath portion  32  may be circumferential as illustrated in  FIGS. 7A ,  7 B and  7 C. In this embodiment, each section  33 ,  35  is a band that wraps 360° around the circumference of the sheath wall. At least one of the sections  35  has fewer layers of material than an adjacent section  33 . 
     In another embodiment, as illustrated in  FIG. 7D , adjacent sections  33 ,  35  in the intermediate sheath portion  32  are spirally arranged. Rather than the sections being circumferentially arranged as illustrated in  FIG. 7A , the sections in this embodiment spiral along all or a portion of the length of the wall of the intermediate sheath portion. 
     In yet another embodiment of the invention, as illustrated in  FIG. 7E , the sections in the intermediate sheath portion  32  are arranged as two intersecting spirals along all or a portion of the intermediate sheath portion  32 . In this embodiment of the intermediate sheath portion  32 , looking at the side of the sheath  17 , spiral sections  33 ,  33 ′ appear as a plurality of “Xs” along all or a portion of the intermediate sheath portion. Sections  35  have a different number of layers of material than sections  33 . 
     In still another embodiment of the invention, as illustrated in  FIG. 7F , adjacent sections  33 , 35  in the intermediate sheath portion  32  are arranged longitudinally, paralleling the long axis of the sheath. In this embodiment of the intermediate sheath portion  32 , one or more longitudinal sections  33 ,  35  are distributed around the circumference of the sheath along all or a portion of the intermediate sheath portion  32 . 
     The embodiments illustrated in  FIGS. 7A-7F  are meant to be illustrative. Other arrangements of sections in the intermediate sheath portion are possible. 
     In another embodiment of the sheath, according to the invention, a first polymer having a certain characteristic stiffness is used to manufacture the sheath wall in the intermediate sheath portion  32  and a second polymer having a different characteristic stiffness than the first polymer can be used in the proximal sheath portion  30  and the distal sheath portion  34  (not shown). For example, a polymer-metal composition may be used in the intermediate sheath portion  32  whereas a more rigid or stiff type of polymer can be used in the proximal sheath portion  30  and the distal sheath portion  34 . In general, any type of material may be used in the intermediate sheath portion  32  that is a suitable material having the appropriate strength, flexibility and biocompatibility characteristics. 
     Other materials that can be included in one or more layers of the sheath are polyetheretherketone (PEEK), or nickel-titanium (Nitinol) braid. 
     Referring now to  FIG. 8A , in another embodiment, according to the invention, the outer diameter  11  of the sheath wall in the intermediate portion  32 , is less than the thickness of the sheath wall at the proximal portion  30  and the thickness of the sheath wall at the distal sheath portion  34 . As illustrated by cross-sections of proximal sheath portion  30  in  FIG. 8B , and distal sheath portion  34  in  FIG. 8D , the thickness of sheath wall  12  is the same in the proximal sheath portion  30  and distal sheath portion  34 . As illustrated by cross-section of intermediate sheath portion  32  in  FIG. 8C , the thickness of the intermediate sheath portion  32  is less than either the proximal sheath portion  30  illustrated in  FIG. 8B  and the distal sheath portion  34  illustrated in  FIG. 8D . The inside diameter  13  of the sheath in the intermediate portion  32  of the sheath, illustrated in  FIG. 8C , is unchanged relative to the inside diameter  13  of proximal sheath portion  30 , illustrated in  FIG. 8B , and distal sheath portion  34 , illustrated in  FIG. 8D . By maintaining a uniform inside diameter  13  from the proximal end  24  of the sheath to the distal end  26  of the sheath, illustrated in  FIG. 8A , an object-engaging unit can slide smoothly in the lumen  15  of the sheath without risk of snagging in dilated or constricted regions of the inner wall  13  of the sheath. 
     In one embodiment, illustrated in  FIG. 8A , the transition in wall  12  thickness along the intermediate portion  32  of the sheath is gradual. In an alternate embodiment, the transition in wall thickness of the intermediate sheath portion  32 , relative to the wall thickness of proximal sheath portion  30  and distal sheath portion  34 , is incremental, i.e., occurs in multiple stages, segments, or steps as illustrated in  FIG. 9A . The number of possible incremental steps is without limit. The increments in wall thickness can be non-uniformly distributed along the long axis of intermediate sheath portion  32  as illustrated in  FIGS. 9B and 9C . The inside diameter  13  of the sheath wall  12  is uniform from the proximal sheath portion  30 , the intermediate sheath portion  32 , to the distal sheath portion  34 . As a result, the diameter of the sheath lumen  15  is unchanged throughout the length of the sheath from the proximal end  24  to the distal end  26  of the sheath  14 . 
     Referring to  FIG. 9D , in another embodiment of the sheath of the invention, the external sheath wall  17  of the intermediate sheath portion  32  is thrown into circumferential folds to achieve an accordion-like effect. This accordion-like effect enhances the flexibility of the intermediate sheath portion  32  relative to the rest of the sheath  14 . The inner diameter of the sheath in the intermediate sheath portion  32  is unchanged relative to the inner diameter of proximal sheath portion  30  and distal sheath portion  34 . The diameter of the lumen  15  of the sheath, therefore, is constant from the proximal end  24  of the sheath to the distal end  26  of the sheath. 
     Referring to  FIGS. 10A-10C , in another embodiment of the invention, flexibility may be added to the intermediate portion  32  of the sheath by modification of the external sheath wall  17  with slits, grooves, or other depressions. Slits, grooves, or depressions can be distributed completely or partially around the circumference of the sheath wall  12  in the intermediate sheath portion  32  as illustrated in  FIG. 10A . As illustrated by a cross-section in perspective of intermediate sheath portion  32  in  FIG. 10B , the modifications in the external sheath wall  17  of the intermediate sheath portion  32  extend part way through the full thickness of the sheath wall  12  and not through the inner sheath wall  19 . Sheath wall modifications, such as slits or grooves, are oriented with their long axis anywhere from parallel to perpendicular to the long axis of the sheath. 
     As illustrated in  FIG. 10C , in yet another embodiment of the sheath of the invention, scattered dimples or other shapes are introduced into the outer sheath wall  17  of the intermediate sheath portion  32 . Like the grooves or slits illustrated in  FIG. 10B , dimples extend from the outer wall  17  partially into the sheath wall  12  but not through the inner sheath wall  19 . Thus, as illustrated in  FIGS. 10B and 10C , the inner diameter  13  of the sheath and, therefore, the sheath lumen  15 , is uniform throughout the length of the sheath beginning at the proximal end  24  of the sheath, extending through the intermediate portion  32  of the sheath, to the distal end  26  of sheath  14 . 
     The surface of the sheath  14 , from the proximal portion  24  to the distal portion  26 , is made or coated with any suitable material with a low friction coefficient, such as a polymeric material, a polyethylene glycol, a photopolyacrylamide-heparin complex, hyaluronic acid, polyvinylpyrrolidine material, and others. These low-friction materials are materials included in the composition of the sheath wall during manufacturing or applied to the surface of the sheath after the sheath is manufactured. 
     The sheath of the invention provides numerous advantages. For example, the flexible intermediate portion  32  of the sheath allows an increase in the deflection capability of the sheath  14 . When inserted into a flexible scope, for example, the flexible intermediate portion  32  of the sheath  14  overcomes the rigidity of the distal end  26  of the sheath that is necessary for the proper actuation of the object-engaging unit  10 . The flexible intermediate portion  32  coincides and deflects with a bending portion of the flexible scope without substantially impairing the capacity of the endoscope to flex. 
     The distal portion  34  of the sheath holds and surrounds the object-engaging unit  10  when it is in the collapsed state or partially collapsed state, without radial deformation of the sheath  14 . The flexible intermediate portion  32  of the sheath allows more steerability than possible with conventional sheaths. 
     In another aspect, the invention relates to a method for manipulating material or tissue in a body, such as a body tract or body canal, with a medical device including a sheath. The sheath, as part of the medical device, may be introduced directly into the body cavity or may be introduced through a channel in an endoscope or laparoscope. Sheaths, according to the invention, provide the requisite rigidity at the proximal sheath portion  30  for effective manipulation by an operator for locating, engaging, and retrieving stones, and the requisite rigidity at the distal sheath portion  34  to enclose the object-engaging unit  10  in the collapsed or partially collapsed state without deformation or distortion of the distal end of the sheath. Sheaths of the invention can be introduced into an endoscope or laparoscope without the need for introducers or other means. When the sheath is inserted into a flexible endoscope, the flexible intermediate portion  32  of the sheath allows the endoscope to flex to reach a remote body site. With the sheath inserted into a channel of the endoscope, the endoscope can flex about 180 to 190 degrees. Sheaths of variable strengths, according to the invention, are used with medical retrieval devices or a variety of other medical devices. Also, the sheaths of the invention can be used in urology, endoscopy, laparoscopy, biliary, and lithotripsy procedures as well as a variety of other applications. 
     Referring to  FIGS. 11A-11C , a method for retrieving material from a patient includes inserting a medical device including a sheath according to the invention into the patient. The medical device including the sheath according to the invention may be directly introduced into the patient or may be introduced via a channel  50  of an endoscope or laparoscope as illustrated in  FIG. 11A . For example, the endoscope  50  is advanced by an operator via the lumen of the ureter  40  until the distal end of the endoscope  50  enters the patient&#39;s body site, such as the renal pelvis  42 , where the target material  100  is located. The medical device, including the object-engaging unit  10  collapsed within the distal end portion  34  of the sheath, is passed into the endoscope  50 , until the distal end  26  of the sheath approaches the distal end  51  of the endoscope  50 . As shown in  FIG. 11B , the flexible intermediate portion  32  of the sheath coincides and deflects with the bending portion of the flexible endoscope  50 . The object-engaging unit  10  is extended from the distal end  26  of the sheath  14  to manipulate the material or tissue  100  within the remote body site  42 . The distal end portion  34  of the sheath is of sufficient strength and rigidity to resist deformation of the sheath as the object-engaging unit  10  is moved relative to the sheath. 
     As shown in  FIG. 11C , after the material or tissue  100  within the remote body site  42  is manipulated, the object-engaging unit  10  is withdrawn partially or completely into the distal end portion  34  of the sheath either by axial movement of the elongated member  18 , attached to the base  13  of the object-engaging unit  10 , or by movement of the sheath  14  over the stationary object-engaging unit  10 . 
     The medical device including the sheath according to the invention can be directly inserted (not shown) into a body cavity without an endoscope or an introducer. The steps are similar to those described above for use of the medical device in an endoscope. The flexible intermediate portion of the sheath  32  deflects against the wall of the body tract as the sheath housing the object-engaging unit is advanced in the body tract. The object-engaging unit extends from the distal end of the sheath in the manner described above for using the medical device in an endoscope. After the material in the body site is manipulated, the object-engaging unit is withdrawn partially or completely into the distal sheath portion. The medical device including the sheath is withdrawn from the body tract. 
     Variations, modifications, and other instrumentations of what is described herein will occur to those of ordinary skill in the art without departing from the spirit and scope of the invention as claimed. Accordingly, the invention is to be defined not by the preceding illustrative description but instead by the spirit and scope of the following claims.