Medical Catheters and Plastics – Part VIII
In our ongoing exploration of the manufacturing process for medical catheters, we previously analyzed the function of the extrusion die, the parts related to the extrusion tubing die, the calculations derived from the die's design focusing on material flow, and the significance of the die's design on the material's output. This article investigates the procedures the polymer undergoes once it exits the die, as well as the transformation of the polymer melt into a catheter.
Polymer melts show a rise in cross-sectional area when they exit extrusion dies. This occurrence is referred to as die swell, or more precisely, extrudate swell. Extrudate swell occurs due to the extensional rheology of the polymer melt and is linked to the memory effect experienced by the polymer melt as it flows. The connection between the die size and the ultimate catheter size is referred to as the draw down ratio. The draw down ratio (DDR) can be described as the ratio between the area at the die exit and the cross-sectional area of the resultant annular extruded product. The exit of the die is shaped by the pin's outer diameter and the die's inner diameter. The DDR calculation ignores the melt die swell effect. In most catheter operations, the value of the DDR is maintained in the range 1.5 to 5.0.
The DDR also results in molecular orientation and also residual stresses in the tubing. The orientation can result in an increase in tensile properties of the material in the machine direction but a corresponding strength decrease in the transverse direction which could result in a decrease of burst pressure of the finished tubing. Stresses can cause problems in the subsequent thermal processing such as sterilization; stresses can also be detrimental to the stability of the material in biological environments and lead to decreased biostability, a concern for longer term implants. The built-up stress is addressed by the use of an annealing process at the end of the extrusion line.
The molten polymer exiting the extrusion die is cooled using a water bath. The cooling process can be critical in the determination of the dimensions, physical properties and the morphology of the catheter. Many polymers are either semi-crystalline or have significant molecular order and the rate of cooling from a melt, i.e. an amorphous state, can have a significant effect on the morphology. Rapid cooling can slow down or even eliminate crystallinity and order, slow cooling, on the other hand, can result in large crystal formation. In some applications, such as balloon manufacturing, the extruded tubing must be amorphous prior to the balloon-forming process. In other applications, increasing the amount of crystallinity may be more desirable as it can improve stiffness and lubricity. Therefore, it is important to verify cooling parameters used for the process.
From the cooling tank, the tube is cut to the appropriate length and subjected to an annealing cycle. The annealing happens in a temperature-controlled oven and is designed to relieve the stresses that are stored in the tubing as a result of the manufacturing process. The annealing cycle, time and temperature, is dependent on the material that is used for the manufacture of the catheter.
Once the catheter is extruded, it has to undergo several secondary operations in order to be ready for its final application. The kinds of secondary operations are dictated by the nature of the application.
Machining holes and other profiles into the catheter form an important part of the catheter manufacturing step as these holes perform an important role in the function of the catheter as either a diagnostic or interventionist tool. The catheters are very tiny, requiring specialized machining tools and techniques for effective drilling or punching. Certain precision micro-machining methods can accomplish this to some degree. Laser technology has significantly contributed to the advancement of these machining tools. Lasers have proven effective in machining small holes with a remarkable level of precision.
Thermal damage is one area that needs careful attention especially when working with parts as small as catheters. A growing trend with laser beam machining (LBM) is the shift towards shorter wavelengths and delivering short laser pulses. Shorter wavelengths are better absorbed by the material and shorter pulses keep the temperature rise of the material in check avoiding instances of thermal damage. Thermal damage is also avoided in standard micro-machining technique by using the appropriate coolants and well-designed machining fixtures. Any damage caused to the part has a big impact on the functionality of the part.
A catheter may be also subjected to reflow operations to join different parts of the catheter together. Reflow, in general, refers to a catheter construction process whereby the inner and outer jacket materials are “reflowed” (i.e. melted) to build a composite catheter shaft. Plastic reflow involves using heating processes where plastics change state from solid to liquid. One use of the reflow process is press fitting a metal part into a plastic part. The metal part, in the process, is heated to a temperature greater than that of the plastic melting point. Reflow can be done with the polymer material and a metallic braid, braided reflow catheters add strength, kink resistance, steerability, and torsion control to medical device tubing. A typical reflow catheter construction involves a lubricious inner liner (usually PTFE), braid reinforcement, and a polymer outer jacket. The materials are reflowed together on a mandrel using heat shrink tubing to build the desired catheter shaft properties.
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Author
Mr. Ajay D Padsalgikar (Ph.D. - California, USA)
Trainer at Polymerupdate Academy
