Fluoropolymers for Cryoablation Catheters

Specialist producer of extruded tubing used for minimally invasive devices Putnam Plastics, based in the USA, has written about its capabilities in manufacturing tubing that may be used for cryoablation devices.

High temperature ablation devices use radio frequency (RF), ultrasound, microwave or laser technology to destroy unwanted cells within a temperature range of approximately 45-50°C (113-122°F). These approaches have been used by surgeons to destroy unwanted tumours in surgical and minimally invasive procedures for years. Today, vascular catheters using RF technology are commonly used to ablate heart muscle cells to control irregular heart rhythms associated with atrial fibrillation.

Cryoablation uses low temperatures to destroy cells at temperatures between -40°C and -20°C (-40°F and -4°F). Previously, cryoablation devices were relegated to surgical procedures due to the large probes necessary to deliver cooling gas to the site. In recently years, percutaneous cryoablation devices, primarily using argon gas, have been used in conjunction with magnetic resonance frequency (MRI) and computed topography (CT) image guidance to destroy malignant cells in the liver, kidney and prostate. As recently as December 2012 vascular catheters used for cryogenic ablation were approved by the FDA for treatment of atrial fibrillation.

For catheter device engineers, low temperatures pose new design considerations. As with many interventional vascular catheters, the shaft must have sufficient mechanical properties to navigate vascular pathways to the designated site. The proximal end must have sufficient rigidity for pushability and torque by the physician, while the distal end must be flexible and minimise trauma. Additionally, the tip of the catheter must not become brittle and crack at sub-zero temperatures associated with cryogenics.

Tips for these catheters will likely use distinct polymers which are more suitable to low temperature performance. Fluoropolymers, such as polytetrafluoroethylene (PTFE), are well recognised for property retention at low temperatures; however, PTFE is not melt processible in conventional extrusion. Fluorinated ethylene propylene (FEP) and polyvinylidene fluoride (PVDF) are melt processible fluoropolymers suitable for extrusion into complex catheter designs.

These complex requirements will likely demand shafts with varying performance characteristics throughout the cross section and along the length, using varying materials of construction. Composite shafts have traditionally been assembled manually from multiple extrusions over a mandrel and then fused together by heat (known as “hand layup”). However, the cost and complexity of such assemblies pose economic and quality challenges for production components. Continuous extrusion technologies that incorporate fluoropolymers or other low temperature resistant materials in necessary sections of traditional catheter designs for cryogenic catheters are expected to be in increased demand.