A critical process
Designing new catheters, or enhancing current designs, is a complex process. Especially now that catheters are getting smaller in diameter, designed for multiple functions, and in some cases steerable or implantable. From simple drainage catheters to complicated neurological and cardiovascular catheters, catheter designs must be robust, they must be able to be produced reliably, and their quality must be reproducible. Bonded junctions must meet pull-test strength requirements; formed tips must be smooth and flash-free; the tubing must be flexible yet rigid enough to avoid kinking; and flow rates through the lumens must meet tight requirements. So, why is the process for placing holes in catheter tubing typically an afterthought in its design?
Even today there are production lines staffed by people who are cutting notches in catheter tubing using straight blades. In some cases, this is sufficient. In most cases, however, hole position, size and quality are critical aspects to the form and function of the catheter design. In addition, because hole cutting is typically conducted at or near the end of the production cycle where unit scrap costs are most significant, it makes sense to consider the best type of hole for the application and the best process to make those holes.
Hole making in catheter tubing is accomplished using one of two methods: laser or mechanical processing. Laser processing is recommended when hole diameters range from 0.1 mm down to the hundreds of microns; this is not the subject of this article. For hole diameters greater than 0.1 mm, mechanical hole making is the most widely utilised and least expensive method for placing holes in catheter tubing. However, the science and technology that goes into mechanical hole making should not be underestimated.
The right tool for the job
With a few exceptions, the hollow-core cutter is the tool most utilised to create holes in catheter tubing (Figure 1). This seemingly simple and inexpensive tool is designed to cut through the wall of the tube and removed the “plug” with as little distortion or remaining flash as possible. The plug is then ejected from, or fed through, the cutter in preparation for another cut. In reality, these hollow-core cutters are highly engineered tools and selecting the right style could make the difference between success and failure of the application.
Some of the considerations when selecting the right tool for the application includes the style of the cutter tip, that is, the position of the cutting edge in relation to the outside diameter (O.D.) and inside diameter (I.D.) of the cutter, the grade of cutter material (for example stainless steel) and whether or not coating is beneficial. The style of the cutter tip, in combination with the processing method and tubing polymer, determine the resultant hole geometry and quality of the cut. The type of cutter material and coating has an effect on the cutter life and wear characteristics, and also has an impact on the cost of the cutter.
The next consideration when mechanically cutting holes into catheter tubing is the three basic types of processes: drilling, skiving and punching. Within each of these types of hole-making processes, there are nuances that could have a great impact on the resultant quality of the hole and production cycle times.
Figure 2: Drilling gives good hole quality and geometry in catheters.
Figure 3: The “eye” shaped hole after skiving.
Figure 4: Punching offers the ability to make any shape of hole.
Drilling is the most commonly employed process for catheter hole making (Figure 2). It is performed using a round, rotating, hollow core cutter to remove the plug and gain access to the lumen. Some of the
benefits of drilling include:
Some limitations to drilling include the fact that the ratio of tubing wall thickness to hole diameter needs to be less than 2:1 and, with certain polymers, flash can occur in the tubing when not using an I.D. support mandrel.
Skiving is a form of drilling and the process of side porting that most resembles the result of manually using a razor blade to notch the tubing (Figure 3). The resultant hole when skiving is that of an “eye” shape where the major and minor axis dimensions are controlled through cutter diameter and depth of cut. When skiving, there is no need for an I.D. support mandrel and no chance that the removed portion of the tubing wall or “chip” will remain in the tubing. Another benefit to skiving is the ability to access the complete lumen diameter when maximum fluid flow rate or guidewire access is required. The best quality of cut is obtained using a cutter with an O.D. cutting edge and close-tolerance fixtures that stabilise the tubing to minimise flash.
Punching creates a hole using a round or shaped cutter that is not under rotation (Figure 4). The benefits of punching include:
Punching is preferable for this size tubing for the following reasons. Drilling thick-walled tubing has a higher potential to leave flash because the plug begins to rotate with the cutter before it completes the cut. With braided tubing, the wire braid must be sheared to cut cleanly. Drilling tends to distort the wire (and plastic) before completing the cut resulting in burrs and flash.
A common result when punching, especially in thick-walled or low durometer tubing, is that the resultant hole geometry is typically tapered: the entry is equal to the cutter diameter and tends to be slightly smaller at the exit diameter into the lumen I.D.. To minimise the hole taper and flash in the I.D., punching is usually done with the use of an I.D. support mandrel made from low density polymers such as low density polyethylene. However, when punching without the use of an I.D. support mandrel such as with through-punching, the resultant hole geometry is unique to each application in combination with the cutter tip style selected. For example, in some cases the hole taper will invert with larger hole diameter at the I.D. than at the O.D. of the tubing. In others, the hole through the wall of the tube where the punch entered is conical, but the hole through the exit wall may be cylindrical.
Effective tool selection
Whatever the application may be, whether the tubing is multilumen or single lumen, shaped or round, tipped, formed or braided, with the proper tools and production equipment, catheter hole making can be done efficiently and effectively. It all starts with selecting the right tool for the job.