An MD&DI April 1999 Column
A reduction in device size is among the advantages of shrink tubing, which has a wide range of applications.
The demand for less-invasive medical procedures is a major driving force in today's
medical device industry. Smaller and thinner are betterespecially in catheters, endoscopes, and other devices that are inserted into the body. Designers are looking for new ways to downsize existing devices and to develop new minimally invasive devices. The industry is also under pressure to build more features into devices without increasing their profile (size). Thin-wall heat-shrink tubing is one product that can help designers meet this demand by reducing diameters and improving production processes. Applications in which this tubing offers advantages include:
- Variable-stiffness catheters.
- Electrical insulation.
- Encapsulation and protective coverings.
- Bundling of components.
- Tube joining and transitioning.
- Marking and printing.
- Catheter tip forming.
- Micro hose clamps.
- Masking for coatings.
This article compares the key properties of thermoplastic materials used in the manufacture of high-end medical shrink tubingpolyolefin, fluoropolymers (PTFE), polyvinyl chloride (PVC), and polyester, specifically polyethylene terephthalate (PET)and focuses on some of the more interesting product design applications, especially those employing PET.
Table I compares the properties of the primary materials used in the manufacture of thin-wall heat-shrink tubing. PET polyester is the clear leader in terms of thin walls and high tensile strength. It is 10 to 100 times thinner than any other heat-shrink tubing and more than 10 times as strong. Tube walls of 0.00015 to 0.004 in. can be produced from PET while still maintaining high hoop strength, compared with walls of >0.002 in. for PTFE and >0.005 in. for polyolefin and PVC. Polyester also has superior flex-fatigue properties and the lowest shrink temperature (185°F/85°C) of the commonly used materials, which enables it to be used without being concerned about heat degradation to delicate substrates.
|Material||Wall Thickness||Shrink Temp.||Shrink Ratio||Sterilization||Strength||Cost|
|Polyolefin||>0.005 in.||Medium||2:1 to 3:1||Most||Low||Low/medium|
|PTFE||>0.002 in.||Very high||1.3:1 to 4:1||No gamma||Low||High|
|PET||0.000150.004 in.||Low/medium||1.1:1 to 3:1a||Most||High||High|
|aShrinkage over 20% can be accomplished by drawing while shrinking.|
Table I. Comparison of materials used to produce thin-wall heat-shrink tubing.
Although PTFE offers outstanding lubricity, a significant drawback is its very high shrink temperature of about 600°F, which precludes its use with plastic catheters and other plastic components. PTFE cannot be sterilized via gamma irradiation, which is a handicap in some market sectors that are moving away from ethylene oxide sterilization. Neither can the walls be made as thin as with polyester tubing and still retain useful strength, and wall-thickness tolerances are generally quite high. PTFE shrink tubing is typically used as a liner inside devices and as a covering for metal components and devices.
PET SHRINK-TUBING PROPERTIES
The key properties of polyester heat-shrink tubing are listed in Table II. With polyester tubing, shrinkage is a function of temperature: the higher the temperature, the higher the shrinkage. PET shrink temperature ranges from approximately 185° to 374°F (85° to 190°C). Unrestricted, the tubing will shrink both radially and axially, and the best overall performance is achieved with minimal shrinkage (less than 1520%). Whenever a very high radial shrinkage is required (up to 70%), the tubing can be drawn while it is being heated. The ability to draw or stretch the tubing to achieve very high radial shrinkage while maintaining thin walls is unique to polyester.
|Wall thickness||Ultrathin wall thickness can be achieved (0.000150.004 in.)|
|Tensile strength||Very high tensile strength can be realized (>20,000 psi).|
|Electrical insulation||Has one of the highest dielectric strength ratings of any thermoplastic material
Dielectric strength: >4,000 V/mil (60 Hz)
Dielectric constant: 3.3
Dissipation factor: 0.0025
Volume resistivity: 1018 -cm
Surface resistivity: 1014 /square
|Shrink temperature||185° to 374°F (85° to 190°C)|
|Melt point||High, 455°F (235°C)|
|Shrink ratio||515% typicalup to 70% if drawn or stretched during application|
|Surface finish||Ultrasmooth, hard, glossy finish|
|Color/clarity||Optically clear or can be pigmented|
|Bondability||Can be bonded using a wide range of adhesives (surface treatment recommended)|
|Flex fatigue||Very high flex fatigue properties|
|Biocompatibility||Meets USP Class VI requirements|
Variable-Stiffness Catheters. Because of its ultrathin walls, polyester heat-shrink tubing can be used to add stiffness to catheters without significantly adding to the size of the device. By using different thicknesses of tubing along the length of the catheter, varying degrees of flexibility can be created for improved control of the device. This quick and easy tubing application eliminates the need for joining dissimilar materials or adding braid to sections of a catheter in order to achieve multiple zones of stiffness. For example, some manufacturers use shrink tubing with a wall measuring 1-mil thick at the back end of a catheter, ½-mil in the middle, ¼-mil near the end, and no tubing at all on the tip end. This provides varying degrees of stiffness along the length and the flexibility that is required at the tip.
Electrical Insulation. Virtually every type of heat-shrink tubing is used in electrical insulation. Materials are typically chosen based on temperature, dielectric strength, cost, and wall thickness. High dielectric and resistivity properties make polyester heat-shrink tubing an effective electrical insulation material that adds little dimension because of its ultrathin walls. It can be used over needles, for example, to protect the surface of the skin from being burned during electrical stimulation and has also been employed effectively to cover electrical components or to insulate wiring on catheters and other devices (Figure 1). Some manufacturers are using PET tubing over metal shafts for electrical insulation, replacing a coating process. Application of the polyester greatly reduces the likelihood of the kind of pinhole formation that sometimes may develop in coated surfaces.
Figure 1. Black polyester shrink tubing covers a needle (right), leaving only the tip exposed. Electrical wires (left and middle) are covered with clear polyester tubing for insulation.
Protective Covering, Encapsulation, and Bundling. Polyester heat-shrink tubing is often used to cover braided catheter shafts, spring coils, radiopaque marker bands, and other parts that require a thin but tough protective covering. The tubing allows for smooth transitions over sharp edges and can be sealed against fluid leakage. For instance, it has been placed over a rotary spring cutter to keep debris from clogging the coils and to act as a bearing surface inside the device. The tubing provides a fluid seal, yet the cutter remains flexible.
A wide range of heat-shrink tubing compositions are used in various strain relief applications. Many applications call for thick, flexible materials while others call for thinner, stiffer tubes. Polyester heat-shrink tubing can be used to provide strain relief on catheters and other tubes to prevent kinking. A braided catheter will tend to kink at the point where the braid ends, but encapsulation with heat-shrink tubing provides a quick, easily applied reinforcement and a smooth transition over the two surfaces. Repeatable and consistent, shrink tubing is again an efficient alternative to coatings, eliminating solvents and other chemicals and offering an inherently uniform surface.
Figure 2. Clear polyester shrink tubing (top) holds a thermocouple against a molded plastic probe and covers a coil spring (middle). Three tubes and two wires (the wires are not visible in the photo) are bundled with clear polyester tubing (bottom).
Endoscopes and other devices can be downsized or have more features added without increasing overall instrument dimensions by using shrink tubing to bundle various components (other tubes, wires, optical fibers, etc.) into the smallest possible space (Figure 2). Connecting tubes at the ends of a device can also be made of thin-wall polyester to save valuable space. Often, enough space can be freed up to add another working channel inside an endoscope, or to enable a designer to reduce the size of the device by a whole french catheter size.
Figure 3. In tube joining, heat-shrink tubing is used to hold a low-durometer clear tube and a high-durometer white tube tightly together for fusing. The shrink tube is shown partially (middle) and completely removed (bottom) from the joint.
Tube Joining. Both polyester and fluoropolymer heat-shrink tubing (typically FEP) are used in fusing tubes together. Typically, tubes of dissimilar propertiesone stiff and one flexibleare joined (Figure 3). An easy way to accomplish this is to insert a wire mandrel in the tube ends to keep them from collapsing, butt the two ends together, and shrink a piece of tubing over them. Since polyester tubing has a low shrink temperature, the parts do not distort during this initial application when the shrinking process squeezes the tubes and holds them together tightly during fusing. And given polyester's high melt temperature, the high heat applied to fuse the tube ends does not melt the shrink tubing. After the tubes are joined, the shrink tubing can be left on or peeled off to leave an ultrasmooth surface finish; nicking the shrink tubing at an end before shrinking facilitates removal. Because the tubing is clear, the operator can see when the tubes are fused. This ability to monitor the process is very useful during product development and production to avoid applying too much or too little heat.
Figure 4. A clear tube (top) is marked with bands of shrink tubing. Preprinted shrink tubing (middle two images) is shown before and after shrinkage in a labeling application. At bottom, alternating white and black bands of tubing are shrunk onto a clear catheter tube.
Tube Marking and Printing. Nearly all types of heat-shrink tubing can be used in tube marking and printing with the exception of PTFE (because it is extremely difficult to get any type of ink to bond to PTFE). Depth marks and printing can easily be added to catheters and metal shafts with heat-shrink tubing (Figure 4). Typically, thin bands of colored shrink tubing can be accurately positioned and used for marking. Labeling information can be added by preprinting on the shrink tubing, then applying it to the product, avoiding the need to send the devices themselves to a printer for labeling or to bring printing inks and solvents into the manufacturing facility for in-house printing. Some products, such as catheters made from high-density polyethylene, cannot be readily printed without surface treatment, adding more complexity. Manufacturers who do print on their products can position clear heat-shrink tubing on top of the printed surface for protection without adding substantially to the diameter of the product.
Figure 5. In catheter tip forming, clear polyester shrink tubing is first attached to the end of a catheter tube. Heat is applied with a hot-air torch (extending from background) while a hemostat is used to pull and draw the shrink tube, forming a smooth, tapered tip. A wire is used to prevent the tip ID from collapsing during the process.
Catheter Tip Forming. The low shrink temperature and high melt temperature of polyester heat-shrink tubing enables it to be used effectively to form smoothly tapered tips on the ends of catheters (Figure 5). In the initial operation, a section of heat-shrink tubing is slid onto the end of a catheter tube, leaving a tail off the end. A rod is then inserted in the catheter to maintain the tip inner diameter, and heat is applied to shrink the tubing to the substrate. Once the tubing is attached, the heat is increased to cause the substrate to melt and flow. Pulling on the shrink tube draws the catheter tube to a very thin, smooth tip. Once again, because the shrink tubing is clear, an operator can easily monitor the process. Finally, the shrink tubing is peeled off to complete the job.
Figure 6. Clear, 1-mm bands of polyester shrink tubing are shrunk over both ends of a latex balloon (shown uninflated and partially inflated). The shrink tubing acts like hose clamps to reinforce the bonds and prevent the inflated balloon from peeling away from the catheter tube and leaking.
Micro Hose Clamps. Bands of polyester heat-shrink tubing can function as micro hose clamps on balloon catheters to reinforce bonds and help prevent failure under pressure (Figure 6). A narrow band of tubing is applied over the end of the balloon. With its high hoop strength, the polyester grips the part much like a hose clamp, reinforcing the bond and keeping it from lifting off. It also provides a smooth transition without adding significantly to the bond diameter. The tubing can also be used to terminate braiding, spring coils, and other parts to provide a smooth transition.
Figure 7. Clear angioplasty balloons are shown after a white coating has been applied to one end. Polyester shrink tubing was used to mask the small-diameter ends to prevent them from being coated. This enables the manufacturer to use a UV-curable adhesive. The shrink tubing has been removed from the end of the top balloon and partially removed from the bottom balloon.
Masking Procedures. A simple but very effective application of polyester heat-shrink tubing involves masking areas during coating operations. For example, one manufacturer might require a white coating over a clear balloon, but the neck must remain uncoated so that a UV-curable adhesive can be used to bond it to the catheter. A piece of heat-shrink tubing is applied to the neck, and the balloon is then dipped in the coating. After it dries, the tubing is peeled away, leaving the neck uncoated (Figure 7). The key to this application is the thinness of the polyester tubing; a thicker shrink tubing would leave a prominent ridge of coating material on the balloon.
In another recent application, a manufacturer needed to apply a slippery coating to a length of wire that has a fine spring coil at the end. Polyester heat-shrink tubing was used to mask the spring to keep it from being coated during the dipping process. The low shrink temperature permitted the masking operation to be carried out without heat distortion to the coil, and the tight shrink fit prevented the coating from flowing into the spring. At the end of the process, the polyester tubing was simply peeled away.
While some of the applications and many of the specific uses discussed in this article are only possible with polyester heat-shrink tubing, other heat-shrink tubing materials are available including polyolefin, fluoropolymers, and PVC. All of these materials are used in medical device manufacturing. PET heat-shrink tubing is particularly useful because of its ability to be produced with ultrathin walls. Other tubes are often too thick: designers do not have enough space to incorporate shrink tubing with walls thicker than 0.001 in. The ultrathin walls and other properties of PET tubing make it an extremely valuable tool for designers attempting to rethink the way that they build medical devices.
The author would like to thank Mike Barbere and Ilidia Porto, both of Advanced Polymers Inc., for their assistance.
Mark Saab is president of Advanced Polymers Inc. (Salem, NH).