Pushability

Pain point

Pushability-the ability to transmit longitudinal force from the hub to the distal tip-is fundamental to guidewire performance. Without adequate pushability, a guidewire cannot support the advancement of catheters, stents, or other interventional devices across tight lesions or through tortuous vessels. The pain point arises when the hypotube is either too flexible, causing it to buckle under compression, or too stiff, making it difficult to navigate complex anatomy. Many laser-cut hypotubes suffer from a loss of pushability because the cutting patterns remove too much of the axial load-bearing cross-section. This is especially problematic in percutaneous coronary interventions (PCI) where the guidewire must maintain column strength while crossing calcified or chronically occluded lesions. The challenge is to design a hypotube that is pushable enough to support device delivery but flexible enough to track without causing vessel trauma.

Principle

Pushability is a function of column strength, which depends on the material's modulus of elasticity (E), the cross-sectional area (A), and the moment of inertia (I). In a laser-cut hypotube, the axial lands between the cuts act as the primary load paths. The more material removed, the lower the column strength. However, by strategically leaving uncut sections or using interrupted spiral patterns, engineers can preserve axial stiffness while allowing for flexibility. The principle is to create a balance: the proximal section should have minimal cuts to maximize pushability, while the distal section can have more aggressive cuts for trackability. The transition between these zones must be smooth to avoid stress concentrations. Additionally, the wall thickness plays a crucial role; thinner walls are more flexible but buckle easier, while thicker walls provide more push but may be too stiff for delicate vessels.

Equipment / classification

• Laser Cutting Machines: High-precision fiber lasers with rotary indexing to create consistent patterns along the tube length.
• Tube Drawing and Grinding Equipment: Used to achieve precise wall thicknesses and diameters before laser cutting.
• Tensile/Compression Testers: To measure the column strength and buckling force of the hypotube.
• Materials: 304 stainless steel for general use, 316L for enhanced corrosion resistance, 17-7PH for high strength-to-weight ratio, and Nitinol for its unique combination of flexibility and strength.
• Cut Patterns: Continuous spiral for uniform flexibility, interrupted spiral for balanced push and flexibility, radial cuts for localized bending, and bespoke patterns tailored to specific procedural needs.

Practical guide

To optimize pushability:

• Zone-Based Design: Divide the hypotube into three zones: proximal (high push), transition (moderate push/flex), and distal (high flex). Use FEA to determine the optimal cut pattern for each zone.
• Axial Land Width: Ensure that the uncut sections between spiral cuts are wide enough to carry compressive loads without buckling. A general rule is to keep the land width at least equal to the wall thickness.
• Wall Thickness Selection: Choose a wall thickness that provides sufficient column strength for the proximal section. For example, a 0.35 mm OD tube might use a 0.05 mm wall for pushability, while a 0.20 mm OD tube might use 0.03 mm.
• Pattern Transition: Gradually change the cut density between zones. Avoid abrupt transitions that can cause stress risers and local buckling.
• Testing: Perform pushability tests using a simulated vascular model with tight curves and resistance. Measure the force required to advance the wire and the degree of buckling.
• Hybrid Approaches: Consider combining a laser-cut hypotube with a solid core wire or a coiled tip to enhance pushability where needed.

Real-world experience

A peripheral intervention guidewire was designed with a continuous spiral cut along its entire length to maximize flexibility. However, when used to cross a tight iliac stenosis, the wire buckled and could not support the catheter. The solution was to redesign the proximal 50 cm with an interrupted spiral pattern, leaving wider axial lands. This simple change increased pushability by 40% without significantly affecting trackability in the distal vessels. Another example involved a neuro guidewire where the transition zone between the cut hypotube and a solid core wire was too abrupt, leading to kinking. By tapering the core wire and using a gradual reduction in cut density, the pushability improved and the kink risk was eliminated.

Summary

Pushability is not just about being stiff; it's about intelligent design that places material where it's needed for load transmission. A well-designed laser-cut hypotube can provide the necessary column strength while still offering the flexibility to navigate challenging anatomies.

Outlook

Future guidewires will incorporate smart materials that change stiffness in response to temperature or electrical signals, allowing for dynamic adjustment of pushability during a procedure. Additionally, advances in additive manufacturing may enable the creation of hypotubes with internal structures that optimize pushability without compromising flexibility.