This catheter demonstrates some of the inner workings of a simple deflectable catheter that would not be obvious to one who has not seen
Balloon Tip Deflection Luer Hub
Balloon Tip Deflection Luer Hub
Figure 4.1 Parts of the relay catheter.
one put together before. The distal catheter shaft is a soft 30 Shore D Pebax material, and the proximal shaft is a stiffer 72D Pebax (Figure 4.1).
One of the things about catheter building, especially when seeing it for the first time, is how much touch and art are involved. Some of the features of the device are rather small and tricky to get right the first time. In an R&D environment, it can be very helpful to have the assistance of an experienced medical device assembler. This technician can be a valuable resource and partner with the designer to build the device for assembly from the beginning. It is one thing to hand-build a one-off device; however, it is another matter to build five or ten devices that are reliable and consistent, and yet another to scale up to make devices in the hundreds or thousands. Getting the input of a skilled and knowledgeable assembler will help the engineer design devices that are of higher quality, reliable, and consistent, with good yields and without unnecessary labor content. They often know efficient ways to put a device together that the engineer may not know.
This catheter consists of a proximal luer fitting for inflating the balloon and an integrated screw mechanism for actuating a pull wire to deflect the tip section. In this demo unit this feature is insert molded to the catheter shaft, meaning that the catheter shaft is placed into an injection mold, and the hub is injection molded around it. This allows the hub to be fused to the catheter shaft without adhesives. This is a useful method for higher production numbers; however, an off-the-shelf proximal hub may just as easily be bonded to the catheter shaft by either thermal bonding, cyanoacrylate, or UV cure adhesive.
Note that the joint between the luer hub and the catheter shaft is covered with heat shrink tubing. This is to provide a strain relief between the hub and the shaft, to prevent the shaft from kinking.
This deflectable catheter operates by having a proximal shaft that is relatively stiff, and a softer distal tip. A small-gauge stainless steel pull wire runs the length of the shaft to provide pulling force to the tip and deflect the catheter. The catheter shafts are a standard two-lumen design, with a small lumen for the pull wire and a larger lumen to pass air to inflate the balloon. Both the softer and stiffer shafts have the same extrusion profile. The catheter diameter is 8F (2.7 mm, or 0.105 inches).
The method shown in the example makes a catheter tip that deflects in one direction. Other ways to make a steerable catheter are as follows. If the catheter is to steer in two or more axes, the extrusion profile will have two wire lumens 180° apart, with the larger lumen in the center. These wires are anchored in the tip, and to get bidirectional steering, the wires are connected to a bell-crank mechanism in the handle (Figure 4.2). A lever bends the tip in its two directions of deflection. This may be expanded to allow four axes of deflection if wires are placed at the 12:00, 3:00, 6:00, and 9:00 positions in the catheter shaft, and connected to two bell-crank actuators at 90° to one another. Gastroscopes and sigmoido-scopes have this type of four-way steering. This type of mechanism makes the catheter more versatile; however, it also makes it larger, and more complex and expensive to build. This may be justified for a reusable endoscope costing thousands of dollars, but is difficult to justify in a single-use device. It is often just as simple to torque a device to turn the catheter tip as to make a more complex four-way steering device.
Another way to make a flexible tip on a catheter is by means of a vertebrated section. As the name implies, a tube of metal or plastic is notched to produce a series of rings, leaving a spine of material. This vertebrated tube is then covered with a flexible elastomeric sheath. The spine may be made of the remaining material in the tube, or it may be a piece of flat metal or wire spot welded to a series of rings.
Figure 4.3 A vertebrated tube for flexibility.
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