For more than 30 years, polyimides have been used in a wide variety of micro-diameter tubing applications. However, some of the innovative ways this exceptionally strong and durable polymer can be used-particularly in catheters and other medical devices used in minimally invasive procedures-may actually surprise medical device engineers. This article will explore 6 innovative uses for micro-diameter polyimide tubing in medical devices, all of which can be made possible through the film casting manufacturing process.
Common Applications for Polyimide Tubing
· Retention sheaths for self-expanding stents
· Balloon inflation lumens
· Aspiration lumens for atherectomy devices
· Lining materials for lumens containing catheter guidewires
· Small diameter guidewire designs or guidewire outer sheaths
· High temperature or gamma-ray resistant tube applications
· Applications requiring high tensile strength, torque transfer, or column stiffness
The film casting process begins by applying a thin layer of liquid polyimide resin to a solid mandrel.High temperature polymerization is used to cure the resin to a solid state.The process is then repeated, adding layers of material until the desired tube wall thickness is achieved.
This casting process makes it very easy to customize the tube.Different polymers, filament reinforcements, and particulate material additives can be layered onto the cross-section of the tube.The film casting process also allows for the manufacture of thinner walled tubes with smaller outside diameters (OD) (down to 0.0003" wall and 0.005" OD) and greater dimensional stability than tubes made via extrusion.
1. As Discrete Lengths of Polyimide-Coated Tubing
For applications that require discrete lengths of tubular polyimide coating, specially designed polyimide tubing offers a possible solution. This incompletely polymerized tubing can be reduced in size by heating.
For example, consider a hypotube that needs to be partially coated with polyimide. A length of thermoset polyimide tubing can be designed to fit over the portion of the hypotube to be coated. This thermoformable polyimide can then be slid over the hypotube like a sleeve. When exposed to a varying temperature profile, the tube cures and shrinks slightly, effectively covering that portion of the hypotube with polyimide.
The polyimide-coated sleeve-which shrinks by 0.003 to 0.005 inches in both ID and OD-is ideal for applications such as covering exposed braid or covering a joint between two pieces. It can also be used to add a tip to a product that has been machined to have an inconsistent profile. However, it is important to note that there is a key limitation: the wall must be at least 0.002 inches thick.
2. In micro-diameter hydraulic fluid or liquid delivery lines
Polyimide is a thermosetting polymer. This means that once formed, it is solid-it will not remelt or reflow when exposed to high-temperature environments. It is also very stable and can withstand corrosive and acidic chemicals that would normally harden or degrade other thermoplastics. In addition, polyimide tubing reinforced with stainless steel braid can withstand high pressures.
These desirable qualities make polyimide tubing a suitable choice for use in lines that transport hydraulic fluids or other liquids. Materials such as mineral oils, aromatic hydrocarbons, and polyalkylene glycols can be safely transported through tubing made of polyimide without failing in high-temperature or even nuclear environments. For example, many micro-robotic medical tools rely on hydraulic systems.
3. As conduits for heat exchange or cooling systems
Thermal conductivity is critical for tubing materials used in heat exchange or cooling systems. As a dense material that conducts heat at a rate of 0.471 W/m·K, polyimide is an ideal choice for such applications. It is also very suitable for compounding with particles and fibers, which means that the thermal conductivity of the tubing can be extended beyond the standard capacity of polyimide.
When polyimide is in a liquid, viscous state, it is easy to add particulate materials to enhance functionality. In this case, materials such as ceramic or graphite powder can be added to the polymer matrix to increase the thermal conductivity level of the tube by 70% to 90%.
For example, this polyimide composite is well suited for surgical tools used in ablation procedures, where heat must be conducted away from tissue that does not need to be treated. Because polyimide can be manufactured with thin walls and small inner diameters, it can also be used to create micro-diameter tubing that can be bundled together in specialized cooling systems.
4. In electromagnetic shielded coaxial cables
Catheter-based ultrasound imaging applications often rely on micro-diameter coaxial cables. Because polyimide is easy to compound, these cables can benefit from using polyimide to provide electromagnetic shielding.
In much the same way that graphite powder is added to a polymer matrix to improve thermal conductivity, a material that provides electromagnetic shielding, such as silver or copper powder, can be mixed with polyimide in a liquid, viscous state. Once cured, the composite tube will act as an electromagnetic shield for the internal conductor.
5. Creating Multilumen Shafts
Combining multiple micro-diameter tubes in a bundled multi-lumen shaft can take advantage of a variety of polyimide capabilities. For example, a multi-lumen shaft can include an electromagnetically shielded tube and a tube that has been composited with metallic tungsten to become radiopaque for viewing by X-ray or fluoroscopy, making the shaft dual-functional.
Several micro-diameter tubes made by the film casting process can also be bundled together to create a single shaft with multiple lumens.
Nylon or Pebax® outer coatings can be applied to the tubes being joined; these outer coatings are then fused together to bond the tubes to each other. In many catheter applications, multi-lumen shafts that easily combine multiple polyimide tubes can be used in place of typical extruded multi-lumen shafts.
6. As a Polymeric Needle or Piercing Tool
With a tensile strength of 20 to 40 Kpsi and an elastic modulus of 300 to 500 Kpsi, polyimide is so strong and stiff that it can actually be used to create tubes that can be used as blade-like cutting or piercing tools. Tubes with wall thicknesses of 0.002 to 0.004 inches can cut through different types of tissue, serving as a polymer alternative to conNordsonal metal blades.
Summary
Polyimide is a versatile polymer with a wide range of desirable properties that work well in many different micro-diameter tubing applications. Thanks to a novel film cast tubing process, medical device engineers can take advantage of this stable, durable polymer in ways that may not be obvious at first glance.
