Two common criteria for catheter shafts are pushability and navigation. Pushability refers the force applied by a physician to advance the catheter to the designated therapeutic site. Navigation refers to the ability of the catheter shaft to move freely through the non-linear vascular pathway.
To advance a vascular catheter, sufficient push force must be exerted by the physician to overcome the friction forces between the outer surface of the catheter shaft and the interior vascular wall. As the catheter advances and vascular surface contact increases, the push force must also increase to continue advancement. As push force increases, the catheter shaft portion between the physician’s hand (proximal end) and the patient’s vascular access is prone to buckle and kink.  Increasing the shaft diameter, wall thickness, and flexural modulus of the shaft material can improve pushability and resist buckling.
Navigation requires that the catheter shaft remain flexible in order to easily bend to accommodate curvatures of the blood vessels without causing trauma to the patient.  Reducing shaft diameter, wall thickness, and flexural modulus of the shaft material can improve flexibility and navigational properties of the catheter.
Modifications to outer and inner catheter shaft diameters, and thus wall thickness, are often constrained. Blood vessel diameters limit the maximum shaft size. Diagnostic and therapeutic requirements of the catheter determine the minimum lumen size. Thus, shaft material properties are of considerable interest for optimal pushability and navigation.
Selection of a material with the precise flexural modulus is critical for the right balance between resistance to buckling during catheter advancement and flexibility for navigation. Small changes in modulus can substantially affect the performance of a catheter. For this reason, it is common for competitive catheters to use slightly different polymers. Examples include Arkema Pebax® 6333 (flexural modulus: 285 MPa, 41 ksi), Pebax® 7033 (390 MPa, 57 ksi), and Pebax® 7233 (513 MPa, 74 ksi).
Reducing vascular friction reduces the push force require for catheter advancement. This allows for use of lower modulus shaft materials, which improves navigation. Hydrophilic coatings, which reduce surface friction when exposed to aqueous solutions, are particularly advantageous for vascular catheters. These coatings are not slippery in the dry hand of a physician, yet move easily in the fluidic vascular pathway.
Segmented catheter shafts, using polymers with varying moduli, is also common for optimizing pushability and navigation. High modulus materials, such as Pebax® 7233, can be used on the proximal end (i.e., close to the physician’s hand) for improved pushability. Low modulus materials, such as Pebax® 4033 (flexural modulus: 77 MPa, 11 ksi) or Pebax® 3533 (21 MPa, 3 ksi), can be used on the distal end for flexibility and navigation. Intermediate modulus materials can be used between the proximal and distal ends. These multiple segments can be assembled over a mandrel and joined by heat (i.e., hand-layup method), or extruded in sequence (i.e., continuous extrusion method).