Patent Publication Number: US-2021170091-A1

Title: Cannulas and cannula assemblies for hemodialysis

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to cannulas for cannulating a fistula vein of a patient. 
     BACKGROUND 
     Patients suffering from renal damage or failure typically undergo hemodialysis on a regular basis for removing toxins that accumulate in the blood. Hemodialysis involves filtering of wastes, toxins and water from blood, a function that is normally performed by the kidneys. Hemodialysis helps control blood pressure and maintain balance of important minerals in the blood, such as sodium, potassium, and calcium. An arteriovenous (AV) fistula may be grafted into the body (e.g., forearm or upper arm) of the patient to allow a cannula to be inserted for drawing blood out of the patient and delivering blood back to the patient. An AV fistula is a connection of an artery to a vein of the patient, generally performed by a vascular surgeon. The AV fistula causes extra pressure and extra blood to flow into the vein, causing an increase in size and strength of the vein. The larger vein (referred to herein as “fistula vein”) provides easy and reliable access for a cannula to be inserted therein. A cannula can be inserted repeatedly into such a fistula vein over numerous hemodialysis sessions. This would not be possible with normal veins because repeated needle insertions can collapse the vein when suction is applied in the cannula for drawing blood out of the vein. Traditionally, hemodialysis fistulas were formed using an invasive surgical procedure. Recently, percutaneous fistula vein formation procedures have been used to form a fistula vein with the proximal radial artery in the forearm via a minimally invasive procedure. 
     Patients (e.g., end stage renal failure patients) are generally required to regularly visit a medical provider or a hospital to undergo dialysis. This is inconvenient for such patients because they may also suffer from limited mobility and other health issues, and also increases their financial burden. Home dialysis machines are now available that allow patients to perform dialysis in the comfort of their homes. However, in the absence of a medical provider, the patients have to insert a cannula into the fistula vein themselves or a home caregiver. Currently available cannulas are difficult to insert into fistula veins and can also damage the fistula vein if not inserted properly. Moreover, the patients may have to insert two cannulas into the fistula vein, one for drawing blood and one for returning cleaned blood, making the process even more difficult. 
     SUMMARY 
     Embodiments described herein relate generally to cannula assemblies and methods of using cannula assemblies for cannulating a fistula vein of a patient. In particular, embodiments described herein relate generally to cannula assemblies that include cannulas including a higher flexibility portion that is configured to be inserted into the fistula vein, and to cannulas including a fluid receiving channel and a fluid delivery channel integrated into a single cannula, and methods of using such cannula assemblies. 
     In some embodiments, a cannula assembly comprises a cannula comprising: a first portion, and a second portion extending from a distal end of the first portion, the second portion being more flexible than the first portion, and configured to be inserted into a fistula vein of a patient; and a needle removably disposed in the cannula and configured to be displaced axially within the cannula. 
     In some embodiments, a cannula assembly comprises a cannula comprising: a first channel, a second channel disposed adjacent to the first channel and fluidly separated from the first channel by a wall, and a tip portion located at a distal end of the first channel and/or the second channel and configured to be inserted into a fistula vein of a patient, the first channel and the second channel merging into a single channel at the tip portion; and a needle removably disposed in the first channel and configured to be displaced axially through the first channel so as to be selectively extendable through the tip portion beyond the distal end of the cannula, wherein one of the first channel or the second channel is configured to deliver a fluid into the fistula vein of a patient, and the other of the first channel or the second channel is configured to receive a fluid from the fistula vein. 
     In some embodiments, a method for cannulating a fistula vein of a patient comprises: providing a cannula assembly comprising: a cannula comprising: a first portion, and a second portion extending from a distal end of the first portion, the second portion being more flexible than the first portion, and configured to be inserted into the fistula vein; and a needle removably disposed in the cannula and configured to be displaced axially within the cannula; pushing the needle through the first portion and the second portion until a tip of the needle extends beyond a distal end of the second portion; inserting the needle into the fistula vein until at least the distal end of the second portion of the cannula is inserted into the fistula vein; retracting the needle from the second portion and out of the first portion; inserting the second portion into the fistula vein up to a desired length; and delivering a fluid into, or drawing a fluid out of the fistula vein via the cannula. 
     In some embodiments, a method for cannulating a fistula vein of a patient comprises providing a cannula assembly, comprising: a cannula comprising: a first channel, a second channel disposed adjacent to the first channel and fluidly separated from the first channel by a wall, and a tip portion located at a distal end of the first channel and/or the second channel and configured to be inserted into the fistula vein, the first channel and the second channel merging into a single channel at the tip portion; and a needle removably disposed in the first channel and configured to be displaced axially through the first channel; pushing the needle through the first channel until a tip of the needle extends beyond the tip portion through the distal end of the cannula; inserting the needle into the fistula vein until at least the tip portion is inserted into the fistula vein; retracting the needle from the first channel and out of the cannula; inserting the cannula into the fistula vein up to a desired length; delivering a fluid into the fistula vein via one of the first channel or the second channel; and drawing a fluid out of the fistula vein via the other of the first channel or the second channel. 
     It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the inventive subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the inventive subject matter disclosed herein. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The foregoing and other features of the present disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only several implementations in accordance with the disclosure and are, therefore, not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through use of the accompanying drawings. 
         FIG. 1  is a schematic cross-sectional side view of a cannula assembly, according to a first embodiment. 
         FIG. 2  is a schematic cross-sectional side view of a cannula assembly, according to a second embodiment. 
         FIG. 3  is a schematic cross-sectional side view of a cannula assembly, according to a third embodiment. 
         FIG. 4  is a schematic cross-sectional side view of a cannula assembly, according to a fourth embodiment. 
         FIG. 5  is a flow chart for a method for cannulating a fistula vein of a patient using the cannula assembly of  FIG. 1 , according to an embodiment. 
         FIG. 6  is a flow chart for a method for cannulating a fistula vein of a patient using the cannula assembly of  FIG. 2, 3 , or  4  according to another embodiment. 
     
    
    
     Reference is made to the accompanying drawings throughout the following detailed description. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative implementations described in the detailed description, drawings, and claims are not meant to be limiting. Other implementations may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and made part of this disclosure. 
     DETAILED DESCRIPTION 
     Embodiments described herein relate generally to cannula assemblies and methods of using cannula assemblies for cannulating a fistula vein of a patient. In particular, embodiments described herein relate generally to cannula assemblies that include cannulas including a higher flexibility portion that is configured to be inserted into the fistula vein, and to cannulas including a fluid receiving channel and a fluid delivery channel integrated into a single cannula, and methods of using such cannula assemblies. 
     Embodiments of the cannula assemblies described herein may provide one or more benefits including, for example: (1) providing a cannula including a flexible portion that can be easily manipulated for inserting into complex shaped fistula veins and traversing the tortious path of such cannulas; (2) reducing backpressure and increasing flow rate without increasing a luminal diameter of the cannula; (3) providing ease of use which allows a minimally trained patient to self-cannulate the patient&#39;s fistula vein in a home setting without requiring an expert medical professional; (4) enabling fluid (e.g., blood) to be delivered to the fistula vein and received from the fistula vein via a single cannula which allows the patient to only cannulate him/herself once for performing a hemodialysis session; and (5) reducing hospital visits, which reduces medical costs. 
       FIG. 1  is a schematic cross-sectional side view of a cannula assembly  100 , according to an embodiment. The cannula assembly  100  includes a cannula  110 , a hub  130 , and a needle  150 . The cannula  110  may be configured to be inserted into a fistula vein of a patient to perform hemodialysis, as described herein. It should be appreciated that while various embodiments of the cannula assemblies described herein are described in the context of hemodialysis for receiving blood from and/or delivering filtered blood to a fistula, in other embodiments, the cannula assemblies described herein can be used for cannulating a fistula vein, vein, or artery of a patient for delivering any fluid (e.g., blood, plasma, saline, drugs, platelets, etc.) or drawing/draining any fluid (e.g., blood, cerebrospinal fluid, pus, etc.) from a vein, artery, muscle, or skin of a patient. 
     The cannula  110  may have any suitable outer cross-sectional width (e.g., diameter). For example, the cannula  110  may be a 14 gauge cannula (2.1 mm outer diameter), a 16 gauge cannula (1.8 mm outer diameter), an 18 gauge cannula (1.3 mm outer diameter), a 20 gauge cannula (1.1 mm outer diameter), a 22 gauge cannula (0.9 mm outer diameter), a 24 gauge cannula (0.7 mm outer diameter), a 26 gauge cannula (0.5 mm outer diameter), or any other suitable outer diameter cannula. 
     The cannula  110  comprises a first portion  112 , and a second portion  114  extending from a distal end of the first portion. The second portion  114  is more flexible than the first portion  112  and is configured to be inserted into the fistula vein of a patient. The higher flexibility of the second portion  114  allows the second portion  114  to easily flex which facilitates insertion of the second portion  114  into complex shaped fistula veins defining tortious flow paths. In some embodiments, the first portion  112  and the second portion  114  may be formed monolithically. In other embodiments, the second portion  114  may be formed separately from the first portion  112  and a proximal end of the second portion  114  may be coupled to the distal end of the first portion  112  (e.g., via a clamp, an adhesive, or fusion bonded thereto). 
     As used herein, the term “distal” when used in conjunction with a location of the cannula  110  or any other cannula described herein refers to location that is distant from a user and proximate to the fistula vein. Similarly, the term “proximal” when used in conjunction with a location of the cannula  110  or any other cannula described herein refers to location that is located proximate to a user and distant from the fistula. 
     In some embodiments as shown in  FIG. 1 , a portion of a wall of the cannula  110  forming the first portion  112  has a first thickness t 1  (e.g., in a range of about 0.05 mm to about 0.5 mm, inclusive), and a portion of the wall of the cannula  110  forming the second portion  114  may have a second thickness t 2  that is less than the first thickness. For example, the second thickness may be 0.75 times or less than the first thickness, or 0.5 times or less than the first thickness, or 0.25 times or less than the first thickness. The smaller thickness t 2 , for example, in a range of about 0.01 mm to about 0.4 mm (e.g., 0.01 mm to 0.05 mm, 0.05 mm to 0.1 mm, 0.1 mm to 0.2 mm, 0.2 mm to 0.3 mm, and 0.3 mm to 0.4 mm, inclusive) of the second portion  114  relative to the first portion  112  allows the second portion  114  to have a higher flexibility (e.g., bendability) than the first portion. In such embodiments, the first portion  112  and the second portion  114  may be formed from the same material such as, for example, polytetrafluoroethylene (PTFE), plastics (e.g., polyethylene, low density polyethylene, high density polyethylene), polyurethane, silicone, or any other suitable material. The cannula  110  may have a uniform outer diameter, such that the smaller second wall thickness t 2  of the causes the second portion  114  to have a larger inner diameter than the first portion  112 , which may facilitate fluid (e.g., blood) communication into or out of the fistula. 
     In other embodiments, the first portion  112  may be formed from a first material and the second portion  114  may be formed from a second material that is more flexible than the first material. For example, the first material may have a first elastic modulus (e.g., in a range of 0.38 GPa to 2.25 GPa) and the second material may have a second elastic modulus that is smaller than the first elastic modulus which causes the second portion  114  to be more flexible than the first portion  112 . For example, the first portion  112  may be formed from harder and less flexible plastic, polytetrafluoroethylene (PTFE), polyurethane, silicone, or any other material having a first elastic modulus and the second portion  114  may be formed from a softer and more flexible plastic, PTFE, polyurethane, silicone, or any other material that has a second elastic modulus less than the first elastic modulus. In this embodiment, the second thickness t 2  may be substantially equal to the first thickness t 1  (e.g., within +10% thereof). 
     In some embodiments, one or more apertures  117  extend radially through a wall of the second portion  114  at a distal end of the second portion  114 , which is configured to be inserted into the fistula vein. The one or more apertures  117  provide additional locations for the fluid (e.g., blood) to exit the distal end of the cannula  110 , or be drawn into the cannula  110 . The one or more apertures  117  may beneficially reduce back pressure and/or increase flow rate through the cannula  110 . 
     The cannula assembly  100  may also include a hub  130  coupled to a proximal end of the first portion  112  of the cannula  110  opposite the second portion  114 . The hub  130  defines a fluid conduit  132  to which a tube  140  configured to deliver fluid (e.g., blood) into, or receive fluid (e.g., blood) from the cannula  110  is coupled. A securing member  134 , for example, a nut, clamp, or a luer lock, may be used to couple the tube  140  to the fluid conduit  132 . The needle  150  extends through the hub  130  and is axially displaceable through the hub  130 . For example, the hub  130  may define a channel  149  through which the needle  150  is disposed. In some embodiments, the hub  130  may be transparent or translucent, for example, to allow the patient or caregiver to observe the flow of the fluid through the hub  130 . 
     In some embodiments, a sealing valve  147  (e.g., a septum) may be disposed in the hub  130  at a proximal end of the channel  149 . The needle  150  may be inserted through the sealing valve  147  into the channel  149  and therethrough into the cannula  110 . The sealing valve  147  is configured to reseal once the needle  150  is removed from the cannula  110  and the hub  130  to prevent fluid leakage. 
     The needle  150  is removably disposed in the cannula  110  and configured to be displaced axially within the cannula  110 . A needle hub  152  is coupled to a proximal end of needle  150 , and may be configured to be engaged by a user (e.g., the patient or caregiver) to displace the needle  150  within the cannula  110 . In some embodiments, a fluid conduit  154  may be coupled to the needle  150 , and configured to provide a fluid through the needle  150 . The needle  150  may be used to pierce the fistula vein to allow insertion of the second portion  114  of the cannula  110  therewith into the fistula vein. For example, the needle  150  may be pushed through the cannula  110  until a tip of the needle  150  extends beyond the distal end of the cannula  110  and the needle  150  cannot be pushed any further through the cannula  110  due to the needle hub  152  contacting the hub  130 . The patient or caregiver may then insert the tip of the needle  150  into the fistula vein and continue insertion until at least the distal end of the second portion  114  is inserted into the opening formed in the fistula vein by the tip of the needle  150 . The patient or caregiver may then retract the needle  150  out of the cannula  110 . The patient or caregiver may further insert the second portion  114  into the fistula vein up to a desired length and then secure the cannula  110  in place, for example, via medical tape. 
       FIG. 2  is a schematic cross-sectional side view of a cannula assembly  200 , according to another embodiment. The cannula assembly  200  comprises a cannula  210 , a hub  230 , and a needle  250 . The cannula  210  may be configured to be inserted into a fistula vein of a patient to perform hemodialysis or any other fluid transport. The cannula assembly  200  is configured to receive a fluid (e.g., blood) from the fistula vein as well as deliver a fluid (e.g., blood) to the fistula vein. 
     The cannula  210  includes a first channel  214 , and a second channel  218  disposed adjacent to the first channel  214  and fluidly separated from the first channel  214  by a wall  216 . As shown in  FIG. 2 , the first channel  214  is formed between an outer wall  212  of the cannula  210  and the wall  216 , and the second channel  218  is also formed between the outer wall  212  and the wall  216  such that the wall  216  fluidly separates the first channel  214  from the second channel  218 . 
     A tip portion  228  is located at a distal end of the cannula  210  and is configured to be inserted into the fistula vein. The first channel  214  and the second channel  218  merge into a single channel at the tip portion  228 . As shown in  FIG. 2 , the wall  216  extends into the cannula  210  up to the tip portion  228  and ends before the tip portion  228  such that first and second channels  214  and  218  merge together into a single channel at the tip portion  228 . 
     The cannula  210  has a first cross-sectional width d 1  (e.g., in a range of about 0.5 mm to about 2.2 mm) at locations where the first channel  214  is fluidly separated from the second channel  218 . The tip portion  228  has a second cross-sectional width d 2  at distal end  229  thereof that is smaller than the first cross-sectional width d 1  of the cannula  210  (e.g., 0.75 times, 0.5 times, or 0.25 times the first cross-sectional width d 1 , inclusive). For example, the outer wall  212  of the cannula  210  tapers towards the first channel  214  at the tip portion  228  such that that the second cross-sectional width d 2  of the tip portion  228  at a distal end  229  of the cannula  210  is smaller than the first cross-sectional width d 1 . In some embodiments, the second cross-sectional width d 2  is substantially the same as a cross-sectional width of the first channel  214  (e.g., within +10% of the cross-sectional width of the first channel  214 ). 
     The cannula  210  may have any suitable first cross-sectional width d 1 . For example, the cannula  210  may be a 14 gauge cannula (2.1 mm outer diameter), a 16 gauge cannula (1.8 mm outer diameter), an 18 gauge cannula (1.3 mm outer diameter), a 20 gauge cannula (1.1 mm outer diameter), a 22 gauge cannula (0.9 mm outer diameter), a 24 gauge cannula (0.7 mm outer diameter), a 26 gauge cannula (0.5 mm outer diameter), or any other suitable outer diameter cannula. The cannula  210  may be formed from any suitable material, for example, polytetrafluoroethylene (PTFE), plastics (e.g., polyethylene, low density polyethylene, high density polyethylene), polyurethane, silicone, etc. 
     The tip portion  228  is configured to be inserted into the fistula. In some embodiments, one or more apertures  227  may extend radially through the outer wall  212  of the cannula  210  at the distal end  229 , for example, to increase flow rate and/or reduce back pressure. In some embodiments, only one aperture  227  may extend through the outer wall  212  at the tip portion  228  at a location of the outer wall  212  that is distal to the second channel  218  through which fluid is delivered into the fistula vein. This may overcome back pressure due to fluid entering from the fistula vein into the cannula  210  through the tip portion  228  as well as the aperture  227 . 
     One of the first channel  214  or the second channel  218  is configured to deliver fluid to the fistula vein, and the other of the first channel  214  or the second channel  218  is configured to receive fluid from the fistula vein. For example, the first channel  214  may be configured to receive fluid (e.g., blood) from the fistula vein, and the second channel  218  may be configured to deliver fluid (e.g., filtered blood) to the fistula vein, as shown in  FIG. 2 . 
     The hub  230  is coupled to a proximal end of the cannula  210  opposite the tip portion  228 . The hub  230  defines a first fluid conduit  232  that is in fluid communication with the first channel  214 , and a second fluid conduit  234  that is in fluid communication with the second channel  218 . The first fluid conduit  232  and the second fluid conduit  234  may be fluidly isolated from each other via a hub wall  231 . One of the first fluid conduit  232  or the second fluid conduit  234  may serve as a fluid inlet for delivering fluid to the corresponding one of the first channel or the second channel, and the other of the first fluid conduit  232  or the second fluid conduit  234  may serve as fluid outlet for receiving a fluid from the other of the first channel  214  or the second channel  218 . For example, as shown in  FIG. 2 , the first fluid conduit  232  receives fluid (e.g., blood) from the first channel  214 , and the second fluid conduit  234  delivers fluid (e.g., filtered blood) to the second channel  218 . Alternatively, the first fluid conduit  232  may receive fluid (e.g., blood) from the first channel  214 , and the second fluid conduit  234  may deliver fluid (e.g., filtered blood) to the second channel  218 . In some embodiments, the hub  230  may be transparent or translucent, for example, to allow the patient or caregiver to observe the flow of the fluid through the hub  230 . 
     Securing members  236  and  238 , for example, nuts, clamps, or luer locks, may be used to couple tubes  240  and  242  to the first fluid conduit  232  and the second fluid conduit  234 , respectively. The needle  250  is disposed through the hub  230  and axially displaceable therethrough. For example, the hub  230  may define a channel  249  that is axially aligned with the first channel  214 , through which the needle  250  is disposed. In some embodiments, a sealing valve  247  (e.g., a septum) may be disposed in the hub  230  at a proximal end of the channel  249 . The needle  250  may be inserted through the sealing valve  247  into the channel  249  and therethrough into the first channel  214 . The sealing valve  247  is configured to reseal once the needle  250  is removed from the cannula  210  and the hub  230  to prevent fluid leakage. 
     The needle  250  is removably disposed in the first channel  214  and configured to be displaced axially through the first channel  214  so as to be selectively extendable through the tip portion  228  beyond the distal end of the cannula  210 . A needle hub  252  is coupled to a proximal end of needle  250 , and may be configured to be engaged by a user (e.g., the patient or caregiver) to displace the needle  250  within the first channel  214  of cannula  210 . In some embodiments, a fluid conduit  254  may be coupled to the needle  250 , and configured to provide a fluid through the needle  250 . The needle  250  may be substantially similar to the needle  150  and therefore, not described in further detail herein. 
     The user may push the needle  250  through the first channel  214  of the cannula  210  until a tip of the needle  250  extends beyond the distal end of the cannula  210  and the needle  250  cannot be pushed any further through the cannula  210  due to the needle hub  252  contacting the hub  230 . The patient or caregiver may insert the needle  250  into the fistula vein until the tip portion  228  is also inserted along with the needle  250  into the fistula vein. The patient or caregiver can then retract the needle  250  from the first channel  214  and out of the cannula  210 , and then insert the cannula  210  up to desired length into the fistula vein. The cannula  210  can be used to deliver fluid to or draw fluid from the fistula vein, for example, draw blood from the fistula vein and deliver filtered blood to the fistula vein to perform hemodialysis without the use of two cannulas. In some embodiments, the fluid (e.g., blood) may be received from the fistula vein and delivered to the fistula vein simultaneously. In other embodiments, the fluid (e.g., unfiltered blood) may first be drawn from the fistula vein via the first channel  214  for a first time period and thereafter the fluid (e.g., filtered blood) may be delivered to the fistula vein for a second time period, and the process repeated. In other words, fluid drawing and fluid delivery pulses may be applied to sequentially draw fluid and deliver fluid to the fistula vein. 
       FIG. 3  is a schematic cross-sectional side view of a cannula assembly  300 , according to another embodiment. The cannula assembly  300  comprises a cannula  310 , the hub  230 , and the needle  250 . The cannula  310  may be configured to be inserted into a fistula vein of a patient to perform hemodialysis or any other fluid transport. The cannula assembly  300  is configured to receive a fluid (e.g., blood) from the fistula vein, and deliver fluid (e.g., filtered blood) to the fistula vein. 
     The cannula  310  includes a first channel  314 , and a second channel  318  disposed adjacent to the first channel  314  and fluidly separated from the first channel  314  by a wall  316 . As shown in  FIG. 3 , the first channel  314  is formed between on outer wall  312  of the cannula  310  and the wall  316 , and the second channel  318  is also formed between the outer wall  312  and the all 316 such that the wall  316  fluidly separates the first and second channels  314  and  318 . A tip portion  328  is located at a distal end  329  of the cannula  310  and is configured to be inserted into the fistula vein. The first channel  314  and the second channel  318  merge into a single channel at the tip portion  328 . 
     The tip portion  328  is configured to be inserted into the fistula vein. In some embodiments, one or more apertures  327  may extend radially through the outer wall  212  of the cannula  310  at the distal end  329 , for example, to increase flow rate and/or reduce back pressure. One of the first channel  314  or the second channel  318  is configured to deliver fluid to the fistula vein, and the other of the first channel  314  or the second channel  318  is configured to receive fluid from the fistula vein. For example, the first channel  314  may be configured to receive fluid (e.g., blood) from the fistula vein, and the second channel  318  may be configured to deliver fluid (e.g., filtered blood) to the fistula vein, as shown in  FIG. 3 . 
     Different from the cannula  210 , the cannula  310  includes a first portion  311  including a first segment of the first channel  314  and the second channel  318 , and a second portion  321  extending from a distal end of the first portion  311 . The second portion  321  includes a second segment of the first channel  314  and the second channel  318 , as well as the tip portion  328  which is located proximate to the fistula vein. The second portion  321  is more flexible than the first portion  311  and is configured to be inserted into the fistula vein. 
     In some embodiments as shown in  FIG. 3 , a portion of the outer wall  312  of the cannula  310  and, in some embodiments, the wall  316  forming the first portion  311  has a first thickness t 1  (e.g., in a range of about 0.05 mm to about 0.5 mm, inclusive). Moreover, a portion of the outer wall  312  of the cannula  310  and, in some embodiments, the wall  316 , forming the second portion  321  may have a second thickness t 2  that is less than the first thickness e.g., 0.75 times or less than the first thickness, or 0.5 times or less than the first thickness, or 0.25 times or less than the first thickness. The smaller thickness t 2 , for example, in a range of about 0.01 mm to about 0.4 mm (e.g., 0.01 mm to 0.05 mm, 0.05 mm to 0.1 mm, 0.1 mm to 0.2 mm, 0.2 mm to 0.3 mm, and 0.3 mm to 0.4 mm, inclusive) of the second portion  321  relative to the first portion  311  allows the second portion  321  to have a higher flexibility (e.g., bendability) than the first portion  311 . In such embodiments, the first portion  311  and the second portion  321  may be formed from the same material such as, for example, polytetrafluoroethylene (PTFE), plastics (e.g., polyethylene, low density polyethylene, high density polyethylene), polyurethane, silicone, or any other suitable material. The cannula  310  may have a uniform outer diameter, such that the smaller second wall thickness t 2  of the second portion  321  causes the second portion  321  to have a larger inner diameter than the first portion  311 , which may facilitate fluid (e.g., blood) communication into and out of the fistula. 
     In other embodiments, the first portion  311  may be formed from a first material and the second portion  321  may be formed from a second material that is more flexible than the first material. For example, the first material may have a first elastic modulus (e.g., in a range of 0.38 GPa to 2.25 GPa), and the second material may have a second elastic modulus that is smaller than the first elastic modulus causing the second portion  321  to be more flexible than the first portion  311 . For example, the first portion  311  may be formed from harder and less flexible plastic, polytetrafluoroethylene (PTFE), polyurethane, silicone, or any other material having a first elastic modulus and the second portion  114  may be formed from a softer and more flexible plastic, PTFE, polyurethane, silicone, or any other material that has a second elastic modulus less than the first elastic modulus. In such embodiment, the second thickness t 2  may be substantially equal to the first thickness t 1  (e.g., within +10% thereof). 
     The needle  250  is removably disposed in the first channel  314  and configured to be displaced axially through the first channel  314  so as to be selectively extendable through the tip portion  328  beyond the distal end of the cannula  310 , as described with respect to the cannula assembly  200 . The hub  230  is coupled to a proximal end of the cannula  210  opposite the tip portion  228 , as already described in detail with respect to the cannula assembly  200 . 
       FIG. 4  is a schematic cross-sectional side view of a cannula assembly  400 , according to yet another embodiment. The cannula assembly  400  comprises a cannula  410 , the hub  230 , and the needle  250 . The cannula  410  may be configured to be inserted into a fistula vein of a patient to perform hemodialysis or any other fluid transport. The cannula assembly  200  is configured to receive a fluid (e.g., blood) from the fistula vein as well as deliver a fluid (e.g., blood) to the fistula vein. 
     The cannula  210  includes a first tube  412  defining a first channel  414 . In some embodiments, the first channel  414  may be configured to receive a fluid (e.g., unfiltered blood) from the fistula vein. The cannula  210  also includes a second tube  416  disposed adjacent to the second tube such that an outer wall of the second tube  416  is coupled to an outer wall of the first tube  412 . In some embodiments, the first tube  412  and the second tube  416  may be monolithically formed. In some embodiments, a thin separating layer  411 , for example, a septum may be disposed between the first tube  412  and the second tube  416  to fluidly separate the first tube  412  and the second tube  416 , as shown in  FIG. 4 . The second tube  416  defines a second channel  418 . In some embodiments, the second channel  418  may be configured to deliver fluid (e.g., filtered blood) to the fistula vein, as shown in  FIG. 4 . In other embodiments, the first channel  414  may be configured to deliver fluid to the fistula vein and the second channel  418  may be configured to receive fluid from the filtered vein. 
     The second channel  418  is longer than the first channel  414  and extends beyond a tip of the first channel  414 . The second channel  418  includes a tip portion  428  located a distal end of the second channel  418  and configured to be inserted into the fistula vein. Expanding further, a distal end of the second channel  418  extends beyond the first channel  414  and curves towards a longitudinal axis A L  defined by the first channel  414  such that at least a segment of the tip portion  428  is axially aligned with the longitudinal axis A L . In some embodiments, a cross-sectional width (e.g., diameter) of a distal end of the tip portion  428  may be substantially equal to (e.g., within +10%) of a cross-sectional width (e.g., diameter) of the first channel  414 . 
     The second channel  418  defines a second channel first outlet  419  that is located proximate to and is axially aligned with a first channel inlet  415 , and a second channel second outlet  420  axially aligned with and located distal from the first channel inlet  415 . The hub  230  is coupled to a proximal end of the cannula  410 . The needle  250  is removably disposed in the first channel  414  and configured to be displaced axially through the first channel  414 . A tip of the needle  250  extends through the first channel inlet  415 , and through the tip portion  428  via the second channel first outlet  419  and the second channel second outlet  420 . In some embodiments, a distal end  429  of the tip portion  428  may be tapered so as to form a sharp end that may facilitate insertion of the tip portion  428  into the fistula vein. In various embodiments, a distal end of any of the cannulas described herein (e.g., the cannula  110 ,  210 ,  310 ) may be tapered to form a sharp tip. 
     For example, the patient or caregiver may engage the needle hub  252  to displace the needle  250  through the first channel  414  and the tip portion  428  defined by the second channel  418 . The user may push the needle  250  through the first channel  414  and the tip portion  428  until the tip of the needle  250  extends beyond a distal end of the tip portion  428  and the needle  250  cannot be pushed any further through the first channel  414  due to the needle hub  252  contacting the hub  230 . 
     The patient or caregiver may insert the needle  250  into the fistula vein until the tip portion  428  is also inserted along with the needle  250  into the fistula vein. The patient or caregiver can then retract the needle  250  from the tip portion  428 , the first channel  414  and out of the first channel  414 . The cannula  410  is then inserted up to desired length into the fistula vein. The cannula  210  can be used to deliver fluid to or draw fluid from the fistula vein, for example, draw blood from the fistula vein via the first channel inlet  414 , and deliver filtered blood to the fistula vein via the second channel first outlet  419  and the second channel second outlet  420 . 
       FIG. 5  is a flow chart for a method  500  for cannulating a fistula vein of a patient using the cannula assembly  100 , according to an embodiment. The fistula vein may be cannulated, for example, to perform hemodialysis on the patient. The method  500  includes providing a cannula assembly (e.g., the cannula assembly  100 ), at  502 . The cannula assembly comprises a cannula (e.g., the cannula  110 ), including a first portion (e.g., the first portion  112 ) and a second portion (e.g., the second portion  114 ) extending from the first portion. The second portion is more flexible than the first portion and is configured to be inserted into the fistula vein. 
     In some embodiments, a wall of the cannula forming the first portion has a first thickness, and a wall of the cannula forming the second portion has a second thickness that is less than the first thickness, as previously described herein. In some embodiments, the first portion of the cannula is formed from a first material, and the second portion of the cannula is formed from a second material that is more flexible than the first material. In some embodiments, at least one aperture (e.g., the aperture  117 ) extends radially through a wall of the second portion proximate to a distal end of the second portion, which is configured to be inserted into the fistula vein. A needle (e.g., the needle  150 ) is removably disposed in the cannula and configured to be displaced axially within the cannula. In some embodiments, a hub (e.g., the hub  130 ) is coupled to a proximal end of the cannula, as previously described herein. 
     At  504 , the needle is pushed through the first portion and the second portion until a tip of the needle extends beyond a distal end of the second portion. At  506 , the needle is retracted from the second portion and out of the first portion. At  508 , the second portion of the cannula is inserted into the fistula vein up to a desired length. At  510 , a fluid is delivered into, or drawn out of the fistula vein via the cannula depending on whether the user intends to use the cannula as a fluid drawing cannula or a fluid delivery cannula. For example, blood may be drawn out of the fistula vein via the cannula, and a separate cannula may be used to deliver filtered blood into the fistula vein or vice versa. 
       FIG. 6  is a flow chart for a method  600  for cannulating a fistula vein of a patient using the cannula assembly  200 ,  300 , or  400 , according to an embodiment. The fistula vein may be cannulated, for example, to perform hemodialysis on the patient. The method  600  includes providing a cannula assembly (e.g., the cannula assembly  200 ,  300 ,  400 ), at  602 . The cannula assembly includes a cannula (e.g., the cannula  210 ,  310 ,  410 ) including a first channel (e.g., the first channel  214 ,  314 ,  414 ), and a second channel (e.g., the second channel  218 ,  318 ,  418 ) disposed adjacent to the first channel, for example, fluidly separated from the first channel by a wall (e.g., the wall  216 ,  316 ). A tip portion (e.g., the tip portion  228 ,  328 ,  428 ) is located at a distal end of the cannula and is configured to be inserted into the fistula. The first channel and the second channel may be fluidly coupled into a single channel at the tip portion, or the tip portion (e.g., the tip portion  428 ) may be defined by one of the first channel or the second channel (e.g., the second channel  418 ), as previously described herein. 
     The tip portion of the cannula has a cross-sectional width that is smaller than a cross-sectional width of the cannula at locations of the cannula where the first channel is separated from the second channel. For example, an outer wall of the cannula tapers towards the first channel at the tip portion such that a cross-sectional width of the tip portion at a distal end thereof is substantially the same as a cross-sectional width of the first channel. In some embodiments, at least one aperture (e.g., the aperture  217 ,  317 ) extends radially through a wall of the tip portion, as previously described herein. In some embodiments, the cannula includes a first portion (e.g., the first portion  311 ) including a first segment of the first channel and the second channel, and a second portion (e.g., the second portion  321 ) extending from a distal end of the first portion. The second portion includes a second segment of the first channel and the second channel. The second portion is more flexible than the first portion and is configured to be inserted into the fistula vein, as previously described herein. A needle (e.g., the needle  250 ) is removably disposed in the first channel and configured to be displaced axially through the first channel. A hub (e.g., the hub  230 ) may be coupled to a proximal end of the cannula. In other embodiments, the tip portion (e.g., the tip portion  428 ) may be defined by a distal end of a second channel (e.g., the second channel  418 ) that is coupled to a first channel (e.g., the first channel  414 ). In such embodiments, the needle is removably displaced in the first channel and the tip portion defined by the second channel. 
     At  604 , the needle is pushed through the second channel until a tip of the needle extends beyond the tip portion through the distal end of the cannula. At  606 , the needle is inserted into the fistula vein until at least the tip portion is inserted into the fistula vein. At  608 , the needle is retracted from the first channel and out of the cannula. At  610 , the cannula is inserted into the fistula vein up to a desired length. At  612 , a fluid (e.g., blood) is drawn out of the fistula vein via one of the first channel or the second channel of the cannula (e.g., via the first channel  214 ,  314 ,  414 ). At  614 , a fluid (e.g., blood) is delivered to the fistula vein via the other of the first channel or the second channel (e.g., via the second channel  218 ,  318 ,  418 ). For example, the cannula may be fluidly coupled to a hemodialysis machine (e.g., via the hub  230 ) for filtering blood of the patient. The drawing and delivery of the fluid may be performed simultaneously or sequentially as previously described herein. 
     As used herein, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, the term “a member” is intended to mean a single member or a combination of members, “a material” is intended to mean one or more materials, or a combination thereof. 
     As used herein, the terms “about” and “approximately” generally mean plus or minus 10% of the stated value. For example, about 0.5 would include 0.45 and 0.55, about 10 would include 9 to 11, about 1000 would include 900 to 1100. 
     It should be noted that the term “exemplary” as used herein to describe various embodiments is intended to indicate that such embodiments are possible examples, representations, and/or illustrations of possible embodiments (and such term is not intended to connote that such embodiments are necessarily extraordinary or superlative examples). 
     The terms “coupled,” “connected,” and the like as used herein mean the joining of two members directly or indirectly to one another. Such joining may be stationary (e.g., permanent) or moveable (e.g., removable or releasable). Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate members being attached to one another. 
     It is important to note that the construction and arrangement of the various exemplary embodiments are illustrative only. Although only a few embodiments have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter described herein. Other substitutions, modifications, changes and omissions may also be made in the design, operating conditions and arrangement of the various exemplary embodiments without departing from the scope of the present invention.