Patent Publication Number: US-11660434-B2

Title: Expandable sheath with interlock dilator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Application No. 62/969,318, filed Feb. 3, 2020, the disclosure of which is incorporated by reference in its entirety. This application is related to U.S. Patent Publication No. 2019/0247627A1 entitled “Expandable Introducer Sheath for Medical Device” which was filed as U.S. application Ser. No. 16/277,378 on Feb. 15, 2019, which is incorporated by reference herein. This application is also related to U.S. Patent Publication No. 2018/0256859A1 entitled “Expandable Introducer Sheath for Medical Device,” which was filed as U.S. application Ser. No. 15/917,042 on Mar. 9, 2018, which is incorporated by reference herein. 
    
    
     BACKGROUND 
     Intracardiac heart pump assemblies can be introduced into the heart either surgically or percutaneously and used to deliver blood from one location in the heart or circulatory system to another location in the heart or circulatory system. For example, when deployed in the heart, an intracardiac pump can pump blood from the left ventricle of the heart into the aorta, or pump blood from the inferior vena cava into the pulmonary artery. Intracardiac pumps can be powered by a motor located outside of the patient&#39;s body (and accompanying drive cable) or by an onboard motor located inside the patient&#39;s body. Some intracardiac blood pump systems can operate in parallel with the native heart to supplement cardiac output and partially or fully unload components of the heart. Examples of such systems include the IMPELLA® family of devices (Abiomed, Inc., Danvers Mass.). 
     In one common approach, an intracardiac blood pump is inserted by a catheterization procedure through the femoral artery using a sheath, such as a peel away introducer sheath. The sheath can alternatively be inserted in other locations such as in the femoral vein or any path for delivery of a pump for supporting either the left or right side of the heart. 
     The introducer sheath can be inserted into the femoral artery through an arteriotomy to create an insertion path for the pump assembly. A portion of the pump assembly is then advanced through an inner lumen of the introducer and into the artery. Once the pump assembly has been inserted, the introducer sheath is peeled away. A repositioning sheath can then be advanced over the pump assembly and into the arteriotomy. Replacing the introducer sheath with the repositioning sheath during insertion of a medical device can reduce limb ischemia and bleeding at the insertion site in the skin (and/or at the insertion site within the vessel) because of better fixation of the sheath to the patient when used with a hemostatic valve. 
     Since commercially available tear away introducer sheaths are not radially expandable, the inner diameter of the introducer sheath must always be large enough to accommodate the largest diameter portion of the pump assembly such as the pump head even if other parts of the pump assembly, such as the catheter, have a significantly smaller diameter. In this example, the introducer creates an opening that has an outer diameter wider than necessary to allow passage of the pump catheter into the vessel. Then, the introducer sheath is peeled or torn away and replaced with a lower-profile repositioning sheath. Removing the introducer sheath by peeling it away presents several challenges. For example, introducers can tear too easily and/or prematurely, leading to bleeding or vascular complications. Some introducers may require excessive force to tear away for removal. If a physician applies too much force, when the introducer finally tears, the physician may inadvertently shift the position of the pump within the heart. This configuration also complicates the design of the hemostatic valve located in the hub of the introducer which also needs to tear. Further, a peel away introducer sheath leads to a larger vessel opening after the system is removed, which can complicate vessel closure. 
     Medical introducers for other applications than inserting heart pumps have expandable sheath bodies which may expand radially to allow passage of percutaneous devices into the patient&#39;s vasculature. These existing expandable introducers are for relatively short-term use and may be designed to prevent thrombosis between the sheath body and an indwelling catheter. 
     These introducers are inserted having inner diameters smaller than the outer diameter of the device being introduced. The introducers expand to allow passage of the device through the sheath and into the vasculature and then may shrink again after the device has passed. In the current state of the industry, these expandable introducers require a distinct expandable feature, e.g., a longitudinal fold or crease or a lumen for injection of a fluid (e.g., saline) to transition from a compressed state to an expanded state. Because these existing expandable introducers are intended for relatively short-term use, clot formation on the outside of the introducer sheath may be unlikely. However, if left in for longer periods of time (e.g., &gt;1 hour, &gt;2 hours, &gt;6 hours, &gt;1 day, &gt;2 days, &gt;1 week), clots may form on the outside surface of the expandable sheath mesh, and risk being dislodged into the blood stream at a later time. Additionally, some commercially available expandable sheaths are completely flexible and therefore do not provide any rigidity within their structure thereby leading to kinking or buckling during insertion or withdrawal of a percutaneous medical device. 
     BRIEF SUMMARY 
     The present technology relates to an expandable introducer sheath with an interlock dilator. More particularly, the present technology provides an expandable sheath with a step feature inside its distal opening, and a dilator with an interlock that includes a catch surface that is configured to engage with the step feature of the expandable sheath. When the step feature engages the catch surface, it resists further relative movement so that the body of the dilator is prevented from exiting the distal end of the expandable sheath. The nature of the interlocking engagement between the step feature and the catch surface allows the dilator to be used to extend and maintain tension on the expandable sheath during insertion into a patient, and then to be retracted from the expandable sheath by simply pulling the dilator in the opposite direction. The present technology also provides a dilator hub with a spring mechanism configured to achieve and maintain a desired tension on the expandable sheath and to prevent overextension of the expandable sheath when the dilator is being inserted into the expandable sheath. 
     One aspect of the present disclosure relates to an apparatus comprising an expandable sheath and a dilator. The expandable sheath comprises a cylindrical or substantially cylindrical expandable frame having a proximal opening, a distal opening, an inner surface, and an outer surface. The expandable sheath further comprises a material covering the outer surface of the expandable frame and a portion of the inner surface of the expandable frame, and forming a step feature within the distal opening, the step feature having a first surface that abuts the inner surface of the expandable frame and that is oriented at a first angle relative to the inner surface of the expandable frame. The dilator comprises a cylindrical or substantially cylindrical body, a tapered tip, and an interlock between the body and the tapered tip. The interlock has a first cylindrical section with a first outer diameter, a second cylindrical section with a second outer diameter that is less than the first diameter, and a catch surface that abuts the first cylindrical section and that is oriented at a second angle relative to the first cylindrical section. The dilator is configured to be inserted into the expandable sheath through the proximal opening of the expandable frame. The catch surface is configured to engage the first surface to resist the body of the dilator from passing out of the expandable frame through the distal opening. 
     In some aspects, the apparatus may further comprise a sheath hub configured to secure the expandable sheath proximate to the distal opening of the expandable frame, and a dilator hub. The dilator hub comprises a dilator insert mold configured to secure the body of the dilator; a spring configured to engage the dilator insert mold, and resist movement of the dilator insert mold within the dilator hub; and one or more latches configured to lock the dilator hub to the sheath hub. 
     In some aspects, the interlock further comprises a tapered section that abuts the second cylindrical section. In some aspects, the tapered section is further configured to engage a portion of the material proximate to the distal opening of the expandable frame. 
     In some aspects, the first angle is ninety degrees. In other aspects, the first angle is less than ninety degrees. 
     In some aspects, the second angle is ninety degrees. In other aspects, the second angle is less than ninety degrees. 
     In some aspects, the step feature has a radial height of between 0.1 mm and 5 mm. 
     In some aspects, the material is a polymer, such as thermoplastic polyurethane. 
     In some aspects, the expandable frame is a braided material, and may comprise strands of nitinol. 
     In some aspects, the expandable sheath further comprises a coating applied to the expandable frame and the material, such as a lubricious coating. 
     In some aspects, the interlock is formed of stainless steel, and may further be coated with a polymer. In other aspects, the interlock is formed of a polymer. 
     In some aspects, the tapered tip is formed of a polymer, such as polyether block amide. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    illustrates a sheath assembly; 
         FIG.  2    illustrates a dilator assembly; 
         FIGS.  3 A and  3 B  illustrate the interlock of the dilator assembly in  FIG.  2   ; 
         FIG.  4    is an isometric phantom view of the dilator hub of the dilator assembly of  FIG.  2   ; 
         FIG.  5    is a cross-sectional side view of the dilator hub of the dilator assembly of  FIG.  4   ; 
         FIG.  6    is a cross-sectional side view of the dilator hub of the dilator assembly of  FIG.  4    being attached to the sheath assembly of  FIG.  1   ; 
         FIG.  7    is a cross-sectional side view of the dilator hub of the dilator assembly of  FIG.  4    locked onto the sheath assembly of  FIG.  1   ; 
         FIG.  8    is a cross-sectional side view of a distal end of the sheath assembly of  FIG.  1    according to aspects of the disclosure. 
         FIG.  9    is a cross-sectional side view of a distal end of the sheath assembly of  FIG.  1    according to aspects of the disclosure. 
         FIG.  10 A  is a cross-sectional side view of a portion of a dilator according to aspects of the disclosure. 
         FIG.  10 B  is a close-up cross-sectional view of the components of  FIG.  10 A  in engagement with the sheath tip of  FIG.  8   . 
         FIG.  11    is a close-up cross-sectional view of the components of  FIG.  10 A  in engagement with the sheath tip of  FIG.  9   . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are described in detail with reference to the figures wherein like reference numerals identify similar or identical elements. It is to be understood that the disclosed embodiments are merely examples of the disclosure, which may be embodied in various forms. Well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present disclosure in virtually any appropriately detailed structure. 
     To provide an overall understanding of the systems, method, and devices described herein, certain illustrative embodiments will be described. Although the embodiments and features described herein are specifically described for use in connection with an intracardiac heart pump system, it will be understood that all the components and other features outlined below may be combined with one another in any suitable manner and may be adapted and applied to other types of medical devices such as electrophysiology study and catheter ablation devices, angioplasty and stenting devices, angiographic catheters, peripherally inserted central catheters, central venous catheters, midline catheters, peripheral catheters, inferior vena cava filters, abdominal aortic aneurysm therapy devices, thrombectomy devices, TAVR delivery systems, cardiac therapy and cardiac assist devices, including balloon pumps, cardiac assist devices implanted using a surgical incision, and any other venous or arterial based introduced catheters and devices. 
     The systems, methods, and devices described herein provide an expandable sheath assembly for the insertion of a medical device (e.g., an intracardiac heart pump) into a blood vessel through a vessel aperture. The expandable sheath assembly comprises a dilator assembly, and a sheath body having an inner surface and an outer surface, the inner surface defining a lumen that extends between proximal and distal ends of the sheath. Optionally, the expandable sheath assembly may include a hemostasis stylet. The expandable sheath assemblies (including the sheath body, dilator assembly, and optional hemostasis stylet) are especially advantageous over existing expandable sheath assemblies for patients with coronary artery disease (CAD) and peripheral artery disease, presenting with calcification and tortuosity of arteries, making delivery of introducer sheaths and catheters difficult. The expandable sheath assemblies herein are easier to insert than traditional assemblies because of their reduced insertion profile, increased flexibility, reduced friction, and reduced risk of kinking under loads. The reduced insertion profile minimizes insertion related complications, minimizes stretching and load on the vessel opening, and minimizes the risk of limb ischemia. The structure of the sheath body described herein provides sufficient axial stiffness for pushability and buckling resistance, while maintaining bending flexibility and kink resistance, and reduces frictional force to prevent “finger trapping.” Moreover, the structures of the sheath body described herein provides an improvement over existing introducer sheath bodies by having a smooth inner surface with a thin coating thickness reducing the force required to expand the sheath (compared to the force required to expand a sheath having a coating without any bias), and/or by having a smooth outer surface reducing the risk of thrombus formation during use over longer durations while at the same time enabling the sheath to expand and contract as desired and reducing friction between the sheath body and devices being inserted through it. Furthermore, the structure of the sheath body described herein interfaces with a dilator assembly, such that the sheath body can be held in place for insertion into a body lumen by having a portion of the sheath body be constrained or entrapped in a longitudinal direction. This constraint or entrapment of the sheath body facilitates the expandable sheath body insertion in combination with a dilator assembly, without damaging the expandable sheath body or altering its properties. 
     The sheath body can expand between different states to accommodate the medical device. For example, the sheath body is elongated in a first smaller diameter state for insertion and relaxed into a second larger diameter state once at a desired location to allow the passage of a portion of a medical device through the lumen, the portion of the medical device having a transverse cross-sectional area larger than a transverse cross-sectional area of the lumen in the first state. In different configurations, the sheath is further expanded between a resting state when the sheath is at its desired location, and a larger diameter state when the medical device is passed through. In any configuration, the expandable sheath assemblies herein do not require additional elements relative to a standard introducer: no external balloon, no fold in the expandable sheath body, no second sheath for delivery. This can be advantageous over existing expandable sheath assemblies by simplifying the use of the expandable sheath assembly (e.g., requiring less steps, taking less time). 
     Moreover, the momentary expansion of the sheath body from the elongated state to the relaxed state (or from the relaxed state to the expanded state) minimizes the size of the opening, e.g., arteriotomy, required when inserting the sheath into the vasculature of the patient. Minimizing the amount of time, the sheath body is in the expanded state also minimizes damage to a vessel wall as a smaller opening would be required to accommodate the sheath body in the relaxed or collapsed state, thereby minimizing thrombotic occlusion of the vessel. A smaller opening also minimizes the time to reach hemostasis after removal of the medical device. Such an expandable sheath does away with the need for the conventional set up of having multiple sheaths, such as a peel away introducer sheath and a repositioning sheath for the introduction of a medical device (e.g., an intracardiac heart pump) into the vessel. Such an expandable sheath also allows such a conventional set-up to be used in conjunction with it, if necessary. Once the expandable sheath is positioned in an opening of a blood vessel of a patient, it maintains access to the vessel even after the medical device is removed, should such access be required for other medical procedures. This increases procedural efficiency of any medical procedure as there is no need to re-gain alternative access or re-insert a second sheath in the same access site. The effective consolidation of the introducer sheath and the repositioning sheath into a single device decreases the costs involved during a medical procedure. Further, since only a single sheath is required to gain arteriotomic access to a vessel, less bleeding would be involved during long term use of a percutaneous medical device such as a heart pump. The integration of the sheath body and dilator assembly with the hemostasis stylet allows for titrated hemostasis at the vessel opening. In some implementations, the hemostasis stylet can be a repositioning sheath, which is also used to control blood flow along the expandable sheath and minimize bleeding. 
     Additionally, the expandable sheath assemblies herein are advantageous over existing expandable sheath assemblies because they maintain guidewire access throughout the full procedure by always allowing the user to remove the pump with the sheath in place. 
     The expandable sheath can be delivered into the patient at a small profile if held in axial tension (drawn down) prior to insertion. This has the following key benefits: i) drawing down to a small insertion profile to minimize insertion related complications (i.e., bleeding, vascular injury, high insertion forces); and ii) maintaining a “soft” sheath body and momentary expansion for interaction at the arteriotomy to allow for small bore closure and minimized bleeding due to minimized vessel recoil during use. 
     Previous expandable sheath delivery systems require a complex mechanism to capture the tip of the sheath, lock the sheath to the sheath hub, and draw the sheath down. This requires user manipulation at least twice, and those manipulations are typically device-specific. As such delivery systems differ from “typical” introducer systems, they may require specific training to use and may lead to use errors. 
     A “typical” introducer system comes packaged as a separate sheath, a separate dilator, and accessories. The user generally then removes the sheath and dilator and separately pre-flushes each with saline to remove air. The user then assembles the introducer system by inserting the dilator into the proximal end of the sheath. The introducer assembly is now ready to use. 
     Described herein are modifications to the tip of the sheath that allow it to “lock” to a dilator via an “interlock” feature. By so locking the dilator to the sheath, the expandable sheath introducer assembly may be inserted into the patient much like a typical introducer but retains the benefits of the expandable introducer sheath described above. This interlocking expandable introducer sheath assembly is easier to manufacture than those described above, while also being easier to use because it is operated like a typical introducer sheath assembly. 
       FIG.  1    shows a sheath assembly  100  in accordance with aspects of the technology. The sheath assembly has a hub  110  that locks the sheath in position once inserted. The hub  110  works in concert with the cap  120  to secure the sheath body  130  in position. The hub  110  also has detents  112  (only one of which is visible) to aid in attaching hub  110  to dilator hub  230  as described further below. The butterfly/suture pad  140  is configured to aid in attaching the sheath assembly  100  to the patient (e.g., by suturing the assembly to the patient). As can be seen, the distal end of the sheath body  130  has a tapered sheath tip  150 . The sheath tip  150  may have a straight linear taper, convex taper, concave taper, or a taper composed of one or more straight, convex, and/or concave sections. The sheath tip  150  may be any suitable length. In some implementations, sheath tip  150  may be between 0.1 mm and 5 mm in length. In the present description, the proximal end of the assembly is at the hub/cap end and the distal end of the assembly is at the tip end. Fluid may be introduced into the assembly via sidearm channel  160 , and fluid flow into the device may be controlled by stopcock  170 . A hemostatic valve (not shown) may also be included within hub  110 , the hemostatic valve being configured to prevent blood from leaking outside of the patient during insertion and/or removal of an intracardiac blood pump or other components. Although any suitable hemostatic valve may be employed, examples are described and illustrated in U.S. Provisional Application No. 62/935,300, which is hereby incorporated by reference. In addition, in some implementations, the hub  110  may include a foam insert (not shown) placed proximal to the hemostatic valve that may be soaked with a lubricant such as silicone so that components will be lubricated as they are inserted through the foam and into the sheath body  130 . 
     The expandable sheath body  130  comprises at least a frame and a coating. A coating may be applied to the outer surface of the sheath body  130  to facilitate passage inside the patient, known as an outer-diameter biased approach. In some implementations, the coating may be a polymer such as the polymer material  312  shown and described with respect to  FIGS.  8 - 10   . This outer-diameter biased coating advantageously provides a smooth outer surface which reduces the risk of clot formation and minimizes friction when inserting a device through the expandable sheath. For example, the use of a smooth outer surface advantageously minimizes the risk of clots forming on the surface of the expandable sheath body  130 , and a corrugated inner surface minimizes the surface area of the expandable sheath in contact with a device being pushed through, thereby minimizing associated friction forces. In some implementations, the corrugated inner surface may be a braided material such as the braided material  314  shown and described with respect to  FIGS.  8 - 10   . In some implementations, an additional lubricious coating may be applied to the inner and/or outer surfaces of sheath body  130 , i.e., covering polymer material  312  and/or braided material  314 . The outer-diameter biased coating further advantageously provides for a thin coating thickness, and a relatively smaller force is required to expand the sheath body  130  compared to a force required to expand a sheath having a coating without any bias. The outer-diameter biased coating also advantageously allows the sheath frame to expand and contract as desired, i.e., the outer-diameter biased coating does not immobilize the frame at a fixed diameter because the thin coating thickness is such that the coating does not encapsulate the portions of the frame where frame elements intersect. For example, for a braided frame having braided elements in an over-under braid pattern and an outer-diameter biased coating, the outer diameter biased coating advantageously is thin enough that it does not encapsulate an overlap of braided elements, i.e., the outer-diameter coating does not extend to the braided elements located under other braided elements in the over-under braided pattern. 
     In some implementations, the expandable sheath frame may have an expansion mechanism that aids the frame in expanding and/or contracting. For example, strands of a braided sheath frame may be configured with a bias to expand and/or contract from a resting position. According to some implementations, the expansion mechanism permits strands to slide relative to each other when the frame expands and contracts. 
     The expandable sheath body  130  and sheath tip  150  may be formed in a variety of ways, including using the configurations and methods of manufacture described in U.S. Patent Publication No. 2019/0247627A1 and/or U.S. Patent Publication No. 2018/0256859A1, which have been incorporated by reference herein. For example, the expandable sheath body  130  (and sheath tip  150 ) can be manufactured using thermal bonding or an outer-diameter biased dipping, which can provide the sheath body  130  with a smooth outer surface while retaining its desired spring-like expandable nature. Specific details of the possible configurations for sheath body  130  and methods of manufacturing them are included in the referenced published applications, and are thus not repeated in full herein. 
     By employing a frame and coating assembly as described above and in the referenced applications, the expandable sheath body  130  can expand and collapse while being resistant to kinking. This enables the sheath body  130  to expand to permit insertion or recovery of the medical device, and then return to its original shape after deformation. In addition, configuring the expandable sheath for compatibility with a dilator assembly and a stylet assembly aids in dilator insertion and removal, and improves hemostasis performance. Advantageously, the combination of a dilator assembly, an expandable sheath, and a hemostasis stylet provide a synergistic system which can be used relatively early in a procedure, e.g., in a catheterization lab rather than later in procedure, e.g., in surgery, when displacement of the pump could have more severe consequences for a patient. Because the system can be used relatively early in a procedure, potential pump migration can be addressed earlier, and vascular injury can be reduced. 
     Such an expandable sheath body  130  can also eliminate the need for the conventional set up of having multiple sheaths, such as a peel away introducer sheath and a repositioning sheath for the introduction of a medical device (e.g., an intracardiac heart pump) into the vessel opening (e.g., arteriotomy). In that regard, once the expandable sheath body  130  is positioned, it maintains access to a vessel even after the medical device is removed, should such access be required for other medical procedures. This increases procedural efficiency of any medical procedure and simplifies the process of inserting a component into the patient, as there is no need to peel away the introducer sheath for the insertion of a repositioning sheath each time access to the vessel opening is required. In addition, since the expandable introducer sheath body  130  need not be removed and replaced by a secondary repositioning sheath, the risk of premature tearing/peeling is essentially eliminated and the risk of shifting the introduced device inadvertently (e.g., by overuse of force) is reduced or eliminated. Furthermore, more accurate repositioning of the medical device can be achieved with the expandable introducer sheath as the expandable introducer sheath is fixed in position once inserted, whereas the insertion of a separate repositioning sheath involves multiple steps that increase the chances that the medical device will unintentionally be moved. Notwithstanding the foregoing, the expandable sheaths described herein may still be used in conjunction with a repositioning sheath. 
       FIG.  2    shows a dilator assembly  200  in accordance with aspects of the technology. The dilator assembly  200  has a dilator hub  230  at its proximal end, a dilator body  210 , an interlock  240 , and a dilator tip  220  at its distal end. As can be seen, dilator tip  220  tapers as it approaches its distal end, to facilitate insertion into the patient&#39;s vasculature. The dilator hub  230  is configured to engage the hub  110  of sheath assembly  100  as described further below. 
       FIGS.  3 A and  3 B  depict a cross-sectional and phantom view of a portion of a dilator assembly  200  in accordance with aspects of the technology. In that regard,  FIG.  3 A  is a cross-sectional side view showing how the interlock  240  attaches to the dilator tip  220  and the dilator body  210 , and  FIG.  3 B  is a close-up isometric phantom view of the same assembly. As illustrated in  FIGS.  3 A and  3 B , the distal end of interlock  240  is connected to the dilator tip  220  via flange  241 . Flange  241  extends into the proximal end of dilator tip  220 . In some implementations, the dilator tip  220  may be molded directly onto flange  241 . The proximal end of interlock  240  is connected to dilator body  210  via a threaded connection. In that regard, the proximal end of interlock has a threaded male connector  242  which is received by a corresponding threaded female connector  212  on the distal end of the dilator body  210 . Threaded male connector  242  and threaded female connector  212  may have any suitable diameter, pitch, specification, etc. For example, threaded male connector  242  and threaded female connector  212  may use a standard metric thread such as M1, M2, etc. Moving proximal to distal, the outer profile of interlock  240  is defined by a tapered waist  246  which begins at or near the outer diameter of dilator body  210  and increases in diameter until it reaches a cylindrical section  247  of constant diameter. Continuing in the distal direction, the cylindrical section  247  is followed by a recess  245  with a smaller outer diameter, and the transition between cylindrical section  247  and recess  245  forms a catch surface  244 . Catch surface  244  and recess  245  are configured to engage with step  316  of sheath tip  150 , as described further below. The length of tapered waist  246 , cylindrical section  247 , and recess  245  may be any suitable length. In some implementations, cylindrical section  247  may be between 0.5 mm and 20 mm in length. The proximal end of the dilator tip  220  has a transitional edge  222 . Transitional edge  222  may be any suitable profile and angle. For example, transitional edge  222  may be a chamfer or a combination of two or more flat edges of different angles, may be curved in a concave or convex direction, or may be composed of one or more straight, convex, and/or concave sections. 
     Dilator tip  220 , interlock  240 , and dilator body  210  may be made of any suitable material. In some implementations, dilator tip  220  may be formed of a flexible material such as polyether block amide (“PEBA”) with a durometer hardness of 40D. In some implementations, dilator tip  220  may be formed of other flexible materials such as PEBA with other hardness ratings, silicone, thermoplastic polyurethane (“TPU”), or thermoplastic elastomer (“TPE”). In some implementations, dilator tip  220  may further include hydrophilic lubricious coating such as polyvinylpyrrolidone (“PVP”) or hyaluronic acid (“HA”), or a hydrophobic coating such as silicone or polytetrafluoroethylene (“PTFE”). In some implementations, dilator tip  220  may have no coating. 
     In some implementations, dilator body  210  may be formed of a semi-rigid material such as PEBA with a durometer hardness of 70D. In some implementations, dilator body  210  may be other semi-rigid materials such as PEBA with other hardness ratings, polyethylene, polypropylene, or polyurethane. In some implementations, heat may be applied to the threaded female connector  212  of dilator body  210  to increase its tensile strength and torque resistance. 
     In some implementations, interlock  240  may be formed of a rigid material such as 304 stainless steel. In some implementations, interlock  240  may be formed of other rigid metals such as 316 stainless steel, or rigid polymers such as polyether ether ketone (“PEEK”), acrylonitrile butadiene styrene (“ABS”), or polycarbonate. In some implementations, interlock  240  may be fully or partially coated, such as with a polymer. In some implementations, interlock  240  may have a coating that is between 0.025 and 0.2 mm. In some implementations, interlock  240  may have a coating with a durometer hardness of between 40A and 70D. In some implementations, interlock  240  may have a coating with a coefficient of friction that is greater than that of stainless steel and/or the material chosen for the dilator tip  220  or dilator body  210 . In some implementations, interlock  240  may have no coating. 
       FIG.  4    shows an isometric view of a dilator hub  230  in accordance with aspects of the technology in which the outer housing  231  (comprised of two halves) is shown in phantom. As can be seen, dilator hub  230  has toothed latches  250  that secure it to the hub  110  of the introducer sheath assembly  100 . The proximal end of dilator body  210  is coupled to a dilator insert mold  280 . Dilator insert mold  280  has a flange  282  configured to engage with a spring  270  that is mounted within the proximal end of dilator hub  230 . The spring  270  is configured to allow the dilator insert mold  280  to move in the proximal direction during attachment of dilator hub  230  with the hub  110  of sheath assembly  100 . In that regard, the spring rate of spring  270  may be selected based on the modulus of elasticity of sheath body  130 , in order to optimize the amount of tension applied to sheath body  130  as dilator hub  230  and hub  110  are pressed together into attachment, and sheath tip  150  is thus pulled in the distal direction. Likewise, the spring  270  may be preloaded to a certain tension or compression in order to optimize the amount of tension applied to sheath body  130  as dilator hub  230  and hub  110  are pressed together into attachment. In that regard, in some implementations, the spring rate of spring  270  may be between 0.1 N/mm and 3 N/mm, and it may have a travel between 1 mm and 20 mm. In some implementations, the force (including any preload) provided by the spring during attachment of dilator hub  230  to hub  110  may be between 5 N and 30 N. 
     As shown in  FIG.  4   , dilator hub  230  also has a lock  260  which is configured to engage with dilator insert mold  280 . When brought into engagement with dilator insert mold  280 , lock  260  will prevent dilator insert mold  280  from moving in the proximal or distal direction. By preventing movement in the proximal direction, lock  260  prevents the spring  270  from compressing as the dilator and sheath are inserted into a patient. Advantageously, by matching the spring rate and preloading of spring  270  with the modulus of elasticity of the sheath body  130 , the sheath body  130  will be properly extended and tensioned when the dilator hub  230  is brought into attachment with hub  110 , and thus lock  260  (once engaged) will maintain the sheath body  130  at this desired point of extension and tension. Lock  260  is configured to “close” or lock automatically upon attachment of dilator hub  230  and sheath body  130 . However, in some implementations, lock  260  may instead be configured such that it must be actuated manually, such as by a button or switch. In addition, as the tension of the sheath body  130  will naturally resist further movement of the dilator body  210  in the distal direction, in some implementations, lock  260  may be configured to only prevent movement in the proximal direction. 
       FIG.  5    is a cross-sectional view of the dilator hub  230  of  FIG.  4   , divided along plane A-A of  FIG.  4   . As can be seen, lock  260  has teeth  261 , and dilator insert mold  280  has teeth  281 .  FIG.  5    shows lock  260  in an “open” position such that dilator insert mold  280  can move within dilator hub  230 . While  FIG.  5    shows a toothed locking mechanism, lock  260  may be utilize any suitable mechanism for preventing movement of dilator insert mold  280 . 
       FIG.  6    is a cross-sectional side view of the dilator hub  230  of  FIG.  4    in the process of being attached to a hub  110  of sheath assembly  100 , in accordance with aspects of the technology. As can be seen from  FIG.  6   , the toothed latches  250  of dilator hub  230  are both in an open position, and have not yet engaged with detents  112  in hub  110 . Likewise, lock  260  is still shown in an “open” position such that dilator insert mold  280  can move within dilator hub  230 . In that regard, dilator insert mold  280  is shown having begun to compress spring  270  in the proximal direction, as will occur when the dilator hub  230  and hub  110  are pressed together into attachment, and sheath tip  150  is thus pulled in the distal direction. 
       FIG.  7    illustrates the dilator hub  230  of  FIG.  4    locked into the hub  110  of sheath assembly  100 , in accordance with aspects of the technology. As can be seen from  FIG.  7   , the toothed latches  250  have engaged with detents  112  in hub  110 , thus providing a clamping force that prevents the dilator hub  230  from being able to be pulled away from hub  110 . In addition, lock  260  is shown in a “closed” position, in which it has moved radially inward within dilator hub  230  such that its teeth  261  engage teeth  281  of the dilator insert mold  280 . The engagement of teeth  261  and  281  provides resistance against dilator insert mold  280  being pushed further into dilator hub  230  in the proximal direction. As discussed above, by tuning of the spring rate and preloading of spring  270  relative to the modulus of elasticity of sheath body  130 , the assembly can be configured such that sheath body  130  is brought to a desired tension at the point that dilator hub  230  locks into hub  110 . Lock  260  can then be applied (e.g., manually or automatically as a result of dilator hub  230  locking into hub  110 ), thus maintaining sheath body  130  at that desired tension and preventing the dilator insert mold  280  from moving in the proximal direction as the dilator and sheath are inserted into a patient. 
       FIG.  8    is a cross-sectional side view of the distal end of sheath assembly  100 , showing an example of how the distal end of sheath body  130  and sheath tip  150  may be configured. The structure of  FIG.  8    will be discussed with respect to three portions,  302 ,  304 , and  306 . In the first portion  302 , the sheath body  130  has a cavity  308  with an inner diameter  310 . The outer surface of first portion  302  is a polymer material  312 . The inner surface of first portion  302  is a braided material  314 . Braided material  314  may be any suitable material, as describe above and in the referenced publications. In some implementations, braided material  314  may be composed of strands of a flexible metal such as Nitinol. As noted above, in some implementations, an additional lubricious coating (not shown) may be applied to the inner and/or outer surfaces of sheath body  130 , i.e., covering polymer material  312  and/or braided material  314 . In some implementations, the polymer material  312  is thermoplastic polyurethane (“TPU”), and it is bonded to the braided material  314  using a thermoforming process. On the inner surface of sheath body  130 , there is a step  316  between the first portion  302  and the second portion  304 . Step  316  forms an angle  324  with the inner surface of the first portion  302 , and creates a cavity  318  with a second inner diameter  320  that is smaller than the inner diameter  310  of cavity  308 . In  FIG.  8   , angle  324  is shown as a right angle, i.e., 90°. However, angle  324  may be any angle that allows for step  316  to suitably engage with catch surface  244  of interlock  240  as described further below. Thus, in some implementations, angle  324  may an obtuse angle, or an acute angle (e.g., as depicted and described with respect to  FIG.  9   , below). Step  316  may be any suitable height. In some implementations, step  316  may be between 0.1 mm and 1 mm. 
     In the second portion  304 , the braided material  314  of the sheath tip  150  is sandwiched between polymer material  312 . As a result, polymer material  312  forms both the inner and outer surfaces of the second portion  304 . In addition, as can be seen, where the sheath body  130  transitions to sheath tip  150 , the outer surface begins tapering down in diameter. This tapering begins near the distal end of the first portion  302 , and the taper continues through the second portion  304  and the third portion  306 . Similarly, the braided material  314  also has both a cylindrical section and a tapered section. As shown in  FIG.  8   , the tapered section of braided material  314  begins at the division between the first portion  302  and the second portion  304 . However, in other implementations, the tapered section of braided material  314  may begin more proximally (i.e., somewhere within the first portion  302 ) or more distally (i.e., somewhere within the second portion  304  or third portion  306 ) than is shown in  FIG.  8   . 
     In the third portion  306 , sheath tip  150  is composed entirely of polymer material  312 . As shown in  FIG.  8   , the inner surface of the third portion  306  has transitional edge  322  at its distal end. Transitional edge  322  is shown in  FIG.  8    as a chamfer. However, transitional edge  322  may be a fillet or any other suitable contour. Further, transitional edge  322  is optional. Thus, in some implementations, the third portion  306  may have a constant inner diameter equal to inner diameter  320 , and transitional edge  322  may be replaced with a squared corner. 
     In some implementations, the surfaces of cavity  318  and/or transitional edge  322  may be textured or otherwise configured to reduce friction and stiction between those surfaces of sheath tip  150  and other devices that pass through it, e.g., the dilator tip  220 , interlock  240 , interventional devices introduced through sheath assembly  100  such as intracardiac heart pumps, etc. Texturing may be applied to the surfaces of cavity  318  and/or transitional edge  322  in any suitable method. For, example, texturing may be applied to sheath tip  150  by forming it using a mandrel which itself has been textured through machining, sand-blasting, shot peening, chemical etching, laser surface texturing, etc. In that regard, in some examples, the surfaces of cavity  318  and/or transitional edge  322  may be cross-hatched, knurled, or dimpled. In some examples, the surfaces of cavity  318  and/or transitional edge  322  may have a pattern composed of dashed or continuous lines, which may extend in any direction, e.g., longitudinally, circumferentially, or any angle therebetween. In some examples, the surfaces of cavity  318  and/or transitional edge  322  may have a pattern of lines that are curvilinear, sinusoidal, saw-toothed, or any combination thereof, and which may extend in any direction, e.g., longitudinally, circumferentially, or any angle therebetween. In some examples, the surfaces of cavity  318  and/or transitional edge  322  may have one or more raised or recessed grooves, which may extend in any direction, e.g., longitudinally, circumferentially, or any angle therebetween. Likewise, in some examples, the surfaces of cavity  318  and/or transitional edge  322  may be coated or comprised of materials that reduce friction or stiction. For example, the surfaces of cavity  318  and/or transitional edge  322  may have a lubricious coating, or polymer material  312  may be a material with a suitably low coefficient of friction, e.g., PTFE. The surfaces of cavity  318  and/or transitional edge  322  may incorporate any combination of the different options described above, including a combination of textured features as well as lubricious coatings and/or low-friction materials. 
       FIG.  9    is a cross-sectional side view of the distal end of sheath assembly  100 , showing an additional example of how the distal end of sheath body  130  and sheath tip  150  may be configured. All features of the implementation of  FIG.  9    are identical to those shown in  FIG.  8   , with the exception of the transition between inner diameter  310  and  320 . In that regard, in  FIG.  9   , the angle  324  between the inner surface of the first portion  302  and step  316  is acute, i.e., less than 90°. Again, angle  324  may be any angle that allows for step  316  to suitably engage with catch surface  244  of interlock  240  as described further below. For example, in some implementations, angle  324  may be an acute angle, e.g., between 30° and 89°. 
       FIG.  10 A  is a cross-sectional view of an implementation of the dilator body  210 , interlock  240 , and dilator tip  220  in accordance with aspects of the technology.  FIG.  10 B  illustrates a close-up cross-sectional view of the components of  FIG.  10 A  in engagement with the sheath tip  150  of  FIG.  8   .  FIGS.  10 A and  10 B  show a generalized embodiment in which the flange  241  and threaded male connector  242  of interlock  240 , and the threaded female connector  212  of dilator body  210  have been omitted. One of ordinary skill in the art will understand that the dilator body  210 , interlock  240 , and dilator tip  220  may be coupled to one another in a variety of ways including, but not limited to, those shown in  FIGS.  3 A and  3 B  above. In that regard, dilator body  210 , interlock  240 , and dilator tip  220  may be bonded, glued, or welded to one another. Likewise, dilator body  210 , interlock  240 , and dilator tip  220  may be coupled using additional fasteners. In some implementations, one or more of dilator body  210 , interlock  240 , and dilator tip  220  may be formed as unitary structures, or joined as a result of overmolding. To arrive at the assembly of  FIG.  10 B , the dilator tip  220  is pushed through the distal end of sheath tip  150 . As discussed above, sheath tip  150  may be configured to expand as the tapered dilator tip  220  is passed through it. Thus, sheath tip  150  may be configured so that it must expand to pass over transitional edge  222  of dilator tip  220 , and then naturally contracts again as it reaches the narrower recess  245  of interlock  240 . As dilator tip  220  continues to be pushed in the distal direction, step  316  of sheath tip  150  will come into contact with catch surface  244  of interlock  240 , as shown in  FIG.  10 B . Catch surface  244  meets up with the surface of recess  245  at an angle  248 . Like angle  324 , angle  248  can be any angle that allows for step  316  to suitably engage with catch surface  244  of interlock  240  as described. Thus, in some implementations, angle  248  may be a right angle. In some implementations, angle  248  may be an acute angle, e.g., between 30° and 89°. In some implementations, angle  248  may be an obtuse angle. In some implementations, angle  248  may be different than angle  324 , such as in  FIG.  10 B . In some implementations, angle  248  may be identical or substantially identical to angle  324 , such as in  FIG.  11   , which illustrates a close-up cross-sectional view of the components of  FIG.  10 A  in engagement with the sheath tip  150  of  FIG.  9   . All features of the implementation of  FIG.  11    are identical to those shown in  FIG.  10 B , with the exception that angle  248  is identical or substantially identical to angle  324  in the implementation of  FIG.  11   . 
     Once the catch surface  244  of interlock  240  engages step  316  of sheath tip  150  as shown in  FIG.  10 B  and  FIG.  11   , pushing the dilator tip  220  further in the distal direction will pull the sheath tip  150  and thus begin to tension the sheath body  130 , causing it to elongate and narrow. Drawing down the sheath body  130  in this way advantageously reduces its insertion profile, which helps to minimize patient complications (e.g., bleeding, vascular injury, high insertion forces). At this point, the dilator assembly  200  may be used to insert the sheath tip  150  and sheath body  130  into the patient&#39;s vasculature. 
     Once the sheath body  130  has been positioned as desired within the patient&#39;s vasculature, the dilator hub  230  may be unlocked from the hub  110  of sheath assembly  100  by pressing toothed latches  250  and pulling the dilator hub  230  in the proximal direction. By continuing to retract the dilator assembly  200  in the proximal direction while sheath tip  150  remains stationary, catch surface  244  will be pulled away from step  316 , and the transitional edge  222  of dilator tip  220  will move past sheath tip  150 , allowing dilator assembly  220  to be fully retracted from the patient. The sheath assembly  100  may then be used to introduce the intracardiac blood pump and/or other components into the patient&#39;s vasculature as discussed further above. Notably, as sheath body  130  will no longer be in tension after dilator assembly  200  has been withdrawn, sheath body  130  will be free to relax into a shorter and wider configuration that aids in insertion of such components. 
     From the foregoing and with reference to the various figure drawings, those skilled in the art will appreciate that certain modifications can also be made to the present disclosure without departing from the scope of the same. While several embodiments of the disclosure have been shown in the drawings, it is not intended that the disclosure be limited thereto, as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.