Medical Device Link.

CATHETER DESIGN

(click to enlarge) Parallel design: single ends, each insulated bare conductors, twisted together (a). Twisted design: two single ends, each individually insulated, twisted together (b). For details on these designs, see the extended version of this article online.

Read Part 1 of this article.

Insulated microdiameter wire designs can be manufactured reliably down to sizes of around 0.001 in. (0.026 mm). Most applications currently use diameters within the range of 0.0025–0.0075 in. (0.064–0.192 mm). The pressure to reduce these sizes, as with most catheter-based products, continues to be problematic to manufacturers.

The production of medical wire products occurs through a series of different manufacturing steps or processes. Part 1 of this article, published in the June 2008 issue, described these processes as they relate to the manufacturing of wire products for use in medical catheter designs. Part 2 focuses on the design process to incorporate microwires into catheter applications. It also describes how these electrical pathways open the way for a variety of sensors to be used within catheters and invasive medical devices.

Dimension

The dimension of any conductor or wire design depends on the pathway that the conductor will need to follow from proximal to distal locations within the body of the finished catheter or device. It also depends on the maximum allowable space within this pathway. Envisioning the pathway first enables designers to understand how much room the conductors can take in terms of their dimension. Generally, the more dimensional room there is for a conductor, the less problematic the conductor will be, both for creating a working design and for manufacturing. Reducing the available space should be based on design refinement and necessity. It should not be the target at the onset of design development.

Dielectric Strength

Table I. (click to enlarge) Usage guide for insulating materials. (Source: Tubing and Bare Insulation Materials, Phelps Dodge High Performance Conductors SpecSheet.pdf on CD, p. 14-22).

Dielectric strength defines the average voltage released if the wire’s insulation breaks down and energy shorts through failed insulation. Defining the maximum voltage applied to the conductor material helps engineers define the minimum thickness of insulation material needed around a wire conductor. Very few extruded polymers have the capacity to provide adequate dielectric strength from 0.001 in. of insulation wall thickness (see Table I).

Dielectric testing involves twisting two wires together and applying a direct current (dc) voltage potential to each leg until a short occurs. The dielectric strength is usually listed in units of volts per mil (0.001 in.) of insulation thickness and is calculated from the breakdown voltage divided by twice the insulation’s wall thickness. If using alternating current (ac), reduce the given dielectric strengths to half the given value.

Keep in mind that this value is tested in a dry condition. Introduction of moisture of any kind, especially ion-laden moisture such as saline, results in 30–50% less dielectric strength. If a conductor’s insulation is exposed to any moisture or humidity for an extended length of time, consider using a fluoropolymer such as FEP or PTFE because these materials possess strong hydrophobic properties. With the exception of fluoropolymers, many insulations are hydroscopic due to the nature of their molecular bonding. As such, they will begin to absorb water after an hour’s exposure to any moisture.

Dielectric strength is also affected by high temperatures. Dielectric strength of a polymer insulation begins to diminish exponentially at temperatures that climb about 10% beyond the operating temperatures of that particular plastic material. For high-temperature thermoset polymer insulations, such as polyimide and polyamide-imide, once temperatures move past the operating temperature, dielectric degradation or breakdown can be even faster than an exponential process.

Tensile Strength and Elongation

When evaluating a wire design, always perform a number of tensile and elongation tests with the purpose of establishing a stress-strain diagram. The diagram should represent the entire wire construction. Catheter devices are primary dynamic systems reacting to applied forces, and stress-strain diagrams show wire design under dynamic influences. This goes for both single-wire and multiconductor designs. The stress-strain diagram goes a long way in demonstrating how a design will react during use. Such diagrams help identify potential problems before a product goes into use. The most important elements of a stress-strain diagram for catheter design are the elastic limit and the slope of the line up to the elastic limit value.

The elastic limit is the stress or force at which the conductor transitions from elastic elongation to plastic elongation (at which point, the wire product permanently elongates).¹ This point is the maximum force limit that can be applied during the assembly process of the catheter, as well as the catheter’s use during its application. A wire conductor that experiences a force beyond the elastic limit results in permanent property changes that negatively change the metal’s conductivity and break-strength characteristics.

The slope of the line up to the elastic limit value (also called Young’s modulus or elastic modulus calculation) indicates the wire construction’s flexibility. The elastic modulus calculation is the slope of the linear portion of the stress-strain diagram. It is a ratio of change of stress over change of strain, or, in other words, the amount of stress produced by one unit of strain. For the wire construction being tested, a smaller number indicates a higher level of flexibility.

Figure 1. (click to enlarge) Examples of stress-strain diagrams. The y-axis represents the tensile force or tensile stress applied to the wire construction. The x-axis represents the change in the length of the test sample as this tensile force is applied.

Figures 1a and 1b show examples of stress-strain diagrams. The y-axis represents the tensile force or tensile stress applied to the wire construction. The x-axis represents the change in the length of the test sample as tensile force is applied. The x-axis is also referred to as the strain component of the plotted curve in the diagram. As already stated, the stress-strain diagram allows a designer to make predictions and observations concerning the wire component to be used in the catheter design. Below are examples of these observations associated with the presented samples of stress-strain diagrams A and B.

Diagram A. This is a good, high-tensile conductor that does not start plastically elongating until it reaches its maximum tensile strength. Its tensile strength is well balanced with a significant amount of elongation. An observer can conclude that this is not a brittle wire design. It is strong as well as tough and will resist damage during assembly. The slope of the elastic linear line is very steep, meaning it has a high elastic modulus value indicating a high degree of bend stiffness. Although the wire adds resistance to bending, it also helps in transmitting torque if needed.

Diagram B. This design is very elastic. The line of the diagram almost stays linear or elastic up to two-thirds of its maximum tensile strength. The slope calculation of the linear portion of the diagram line produces a relatively small number, therefore indicating good bend flexibility. Care must be taken in manufacturing not to expose the wire conductor to tensile forces over the elastic limit. The type of material indicated by this curve suggests the wire component’s conductivity should be checked after assembly and after usage to verify that there are no changes to the material’s resistance.

Conductivity

Conductivity and resistance are products of a metal’s inherit conductive properties and the cross-sectional area of the metallic portion of the conductor. The catheter’s application and use usually determine the required resistance of the conductive pathway. Understanding the maximum resistance for your electrical wire pathways is one of the first steps in determining a wire’s material type and size. Because maximum resistance is an application hardware–dependent characteristic, it is important to consider this value early in the design phase. If a conductor has excessive conductivity, consider using a material with less conductivity, but with greater tensile strength. This will make the assembly less susceptible to conductor damage during manufacturing. In cases for which conductivity is plentiful, it is better to exchange conductivity for stronger material properties or a smaller cross-sectional dimension.

Conductivity of a wire is also dependent on the cross-sectional area of that conductor. Note the equation for resistance is R = pl/A, where p is the material resistivity, l is the length of the wire, and A is the cross-sectional area of the wire. Therefore, a given length of wire is only as conductive as its smallest cross-sectional area. For this reason, wire diameter control becomes very important during the wire-drawing process. Also, designers must consider manufacturing steps that could potentially damage and therefore reduce the wire’s cross section at one particular location.

Conductive wires are susceptible to work hardening and, depending on the severity of that hardening, can increase the resistance of a wire. Work hardening is a product of physical force in some way being applied to the metal wire material. Bending, crushing, elongating, and twisting are some of the ways in which wire can become work hardened. Work hardening causes disorder and stress at the grain boundaries, which is what causes increased resistance or reduced conductivity.

Flexibility

Flexibility, or a wire’s resistance to bending, greatly affects the catheter’s maneuverability and torque transmission. Strong materials with high tensile strength and a high modulus of elasticity value (steep slopes in the elastic linear range of the stress-strain diagram) mean a stiff, less flexible material. Wire conductors with less strength, great elongation, and low modulus of elasticity values generally have more flexibility.

When dealing with wire conductor systems, a catheter designer’s greatest control over flexibility is how the wire is deployed within the catheter shaft. When deploying wires down a lumen in the catheter shaft, greater flexibility is achieved by leaving open room in the lumen’s cross section. Generally, 20% of the available cross-sectional area should be left open or unfilled. In addition to extra room in the passage lumen, multiple wires should be twisted or coiled into a helical geometry, not loaded straight in. Using a low-friction top coating or lubricant on the outer surface of the wire can improve the flexibility of a wire-filled catheter shaft.

Free movement is critical. An individual wire should not be bonded or secured to other individual wire members or to the body of the catheter. Free movement enables the wire material to adjust to the compression and elongation associated with mechanical bending, thereby producing less resistance to the bending motion. Keep in mind that the wire elements do not need to be coiled around anything; simply twisting multiple wire elements together achieves the same coiling effect that will benefit the catheter shaft’s flexibility.

Termination of Wire Product in Finished Assembly

The insulation materials used in the described microdiameter wire constructions are strong, often thermoset, polymers that provide a high level of mechanical strength in relatively small dimensions. Although the mechanical strength of these insulations is usually considered a positive attribute, when it comes to stripping away the insulation to expose the metal, such strength can pose a serious problem.

(click to enlarge) Twisted designs: four single ends, each individually insulated, twisted together (a). Flat-wire design: an insulated ribbon (b). See the extended version of this article online for more about wire designs.

Various techniques are employed to remove these insulation materials to access the metallic conductor as part of the catheter’s assembly. Some techniques are very simple, and others are more complex. One simple method is to mechanically scrape the insulation away. Sandpaper, razor blades, or anything with a hard sharp edge is often used in conjunction with a microscope to systematically remove insulation from a specific location along the length of an insulated wire.

Such time-consuming methods create low-quality results and are difficult to repeat consistently in a manufacturing environment. As previously mentioned, medical devices have become dimensionally smaller; therefore, the cross-sectional size of the wire used has become smaller as well. The move toward smaller conductor sizes has compounded the problem of stripping insulation materials. Many means of mechanically stripping small diameter wires can result in damage to the metallic conductor. Such damage can produce stress concentration points that can cause tensile failures in the catheter during or after assembly. The damage can also create variations in the wire’s conductivity or resistance.

Mechanical stripping has shown some improvements with the use of insulated microconductors. One such improvement is bead blasting or particle abrasion. This method is very similar to sandblasting, but works on a microscale. Specialized equipment is used to blow particulate material (such as microsized plastic pellets or even baking soda). Chemicals can also be employed to chemically break down the polymer insulation, but this method is not popular because of the severe corrosiveness of the chemicals used and the possibility that chemical residues could remain with the wire.

The cleanest and safest means of stripping insulations is the use of laser technology. Laser energy can effectively ablate the insulation material without causing harm to the metallic conductor. In addition, a laser is well suited to perform this task on a microscale. Although many laser systems have been used in this way, a good platform that has a history of success at stripping wire products is a CO₂ laser with a cold process.

A cold process is designed to apply its laser energy in extremely short pulses. The pulses last only for a few nanoseconds, which cause the insulation material to ablate without excessively heating up and potentially damaging the metallic conductor. One benefit of a CO₂ laser is that the frequency of laser light produced is such that it is reflected by most metallic surfaces. This means that the laser energy is reflected and not absorbed and therefore does not heat the wire’s metallic aspects. Heating the metallic component of a wire product could result in localized annealing at the stripped area, which creates a stress concentration point, not to mention variation to material properties along the length of wire.

CO₂ lasers do have a downside. Because the laser light reflects off the metal surface, it can leave an extremely thin film of insulation behind after the vast majority of the insulation has been ablated away. This film is usually invisible to the naked eye, but can be seen with magnification up to 10–15×. Many CO₂ laser systems can address this issue by employing a laser gas that causes a brief plasma cloud or explosion to remove this film without affecting the metallic material.

Multilayer Constructions

One aspect of wire design most often overlooked in the development process is the use of insulating materials for characteristics other than dielectric strength. The liquid dip–coating technology lends itself to using different insulations, at different layers, to enhance the functionality of the overall conductor design.

Figure 2. (click to enlarge) Cross section of insulated wire.

It is best to think of the insulated wire from the perspective of its cross section (see Figure 2). From this perspective, the insulation as different layers is visible and enables designers to consider how using different materials for the layers can produce varying functionality in the finished design.

Because the liquid dip–coating process can produce wall thicknesses as thin as 0.00025 in., a wire with a 0.00075-in. total wall thickness can contain up to three different materials. Although three different materials are possible, most often hybrid-insulation designs use only two different materials. The inner layer material provides dielectric strength, and the outer layer can be used to benefit the outer surface’s interaction with the outside environment. The key to using this design is to understand the properties of available insulating materials that can be dip coated and incorporated into the insulating process. Table II shows examples of hybrid wire insulation designs.

Multiend Constructions

Table II. (click to enlarge) A comparison of copper and copper alloys.

Often medical device designs require more than one wire. If multiple wires are used for a design, they can be assembled into a single group by either physically bonding the neighboring wires together via an adhesive process or by twisting together the individual wires. Multiend wire constructions take two basic forms: twist and parallel. Within these two design forms, the individual single ends can be insulated, uninsulated, or a mix of both.

Generally speaking, insulated elements are used for electrical conduction in some manner, and uninsulated elements serve as structural reinforcements, such as stainless steel. Also, single-end conductive wires are usually insulated with different colors to aid in proximal and distal termination. Structural wires or filaments do not need to be conductive or metallic at all. Polymeric filament materials like Kevlar, Vectran, and Kynar have been used successfully in such applications. When using polymeric filament materials amid conductive wires in a single group, it is generally best to use filaments with diameters that are the same or close to the diameter of the conductive wires.

Twist Multiend Constructions. Twisted designs use mechanical twisting to secure the single individual elements together in one continuous grouping. The individual conductors or filaments are not bonded together but remain separate, forming multiple loose helical coils. The pros of twist design include good flexibility, good torque transmission, and good break strength. Flexibility improves as the twist concentration increases and can increase the number of twists per linear length of the construction. Good torque transmission also improves by increases to twist concentration.

Also, the break strength is greater than the sum of all individual wire or filament break strengths. In addition, bending stress is not focused on specific points along length of material during repeated bend cycles. The cons of twist design are that electrical resistance is increased because of the added length from wires following coiled pathways. In addition, twist constructions of multiple wires tend to require more space within a catheter construction.

Parallel Multiend Construction. Parallel constructions use a bonding material to join the single conductors side by side. As with the twisted designs, these individual ends can be composed of different metallic or nonmetallic materials, insulated or uninsulated. Unlike the twisted designs, every individual element in a parallel design needs to be coated with a bonding material. The bonding material is separate and distinct from the insulation applied to the single end to produce dielectric isolation. In addition, the material is always located at the top of the overall multiend construction. Parallel multiend designs create a ribbon cable that can include up to 24 individual wires. It may be possible to have more than 24 wires, but I personally have not seen such designs.

Although the parallel multiend design option tends to be less flexible than the twist option, it is often preferred because of its flat profile. Even large numbers of conductor ends can be run through a catheter device with little relative increase in overall diameter. The conductors can be assembled into the device in a straight longitudinal manner, or as a coil wrapped within the device’s wall.

Flat and Nonround Wires

To maximize conductivity of a wire conductor in a catheter, a designer may wish to form the conductor’s shape to the available space within the catheter’s profile or cross-sectional area. This means constructing wire in a shape other than the standard round cross section. Shaped wires can be manufactured, but they are usually expensive. Even small prototype orders can cost up to $20,000. Manufacturers often require the customer to pay the cost of the specialized tooling needed to run these types of products. Also, insulating such shapes can be difficult and can cause a lot of variability, which is undesirable from a quality perspective. Insulation’s dielectric strength and damage resistance is difficult to maintain at the sharp edges of a nonround shaped profile.

The wire-flattening process is an economic and versatile means of creating a wire shape that more efficiently occupies the available space within a catheter’s cross section. Flattened wire is created from preinsulated round wire. It creates a product that is cost-effective and ensures consistent thickness of insulation around the conductor. Flattening also produces no sharp edges—all edges are rounded. By varying the degree of flattening, wire can be manufactured with cross sections that are flat ribbon, square, or rectangular.

The main limitation of this product and process is that the flattening is performed after the insulation is applied; therefore, the insulation must be a robust type that allows for flattening without damaging the insulation. The process of flattening is a cold-working process, meaning that the resistance of a flattened wire length will increase as a result of flattening. Even so, the increase is not as large as one might guess. In comparing the width-to-thickness ratio of 1.0:5.0, we see an increase of 0.30–0.45 Ω/ft of electrical resistance. Regardless, this type of process greatly limits the choice of materials, essentially creating a need for materials from opposite sides of the strength and toughness spectrum. The metallic portion of the wire needs to be made from the softest and most ductile metals available, and the insulation needs to be made from the strongest and toughest polymers available.

Into the Future

Using sensors and micromachinery for medically invasive procedures requires the use of electricity or electrial impulses. Electrical impulses can be applied creating a reaction, or they can be generated as a result of a reaction that is then to be read. In both cases, a device, more specifically a medical device, is either applying or reading electrical impulses. Therefore, a conductive pathway is required. Microdiameter wire constructions provide an effective and efficient means of creating these pathways. Once these pathways are in place, a broad range of new sensor and micromachine applications open up to a catheter designer.

Here are some possible applications for use in the design of an invasive medical catheter device that would require conductive microwires to transmit electrical energy or impulses:

• Micropumps: Use the electrical energy to drive micropumps on catheters within the body to suction or dispense materials for temporary use within the body.
• Nanosystems: Use of electical energy to drive or change functional nanosystems in the body, such as nanorobotics.
• Nervous system: Because the nervous system produces and uses electrical impulses to create reactions in the body, artificial application of electrical impulses could be used to augment functions in muscle, tisssue, and pain receptors.
• Heart kinetic information: Real-time feedback could be collected from sensors deployed within a cardiovascular catheter to evaluate a condition or an outcome of a procedure.
• Micromachines: Certain techniques exist to build microsize machines that are so small they could be delivered into the body with a catheter-based system. Electrical energy would then need to be transmitted to these micromachines to control their functions.

Conclusion

The placement of an invasive medical catheter is still, for the most part, a very manual procedure that is based on feel, touch, and experience. Although the abilities of an experienced physician can never be substituted, the use of microwires as small electrical pathways opens the way for a variety of sensors to be used within catheters and invasive medical devices. With the means of transmitting a sensor’s signal from the distal end to the proximal end, such sensors could be used to aid in the delivery of the catheter to the treatment area, as well as to provide diagnostic evaluation within the body.

Use of these sensor technologies can and will allow for objective feedback during catheter placement, catheter treatment, and posttreatment evaluation of a procedure. During catheter placement, sensors can provide feedback to computer systems that can monitor outputs and ensure that such outputs stay within allowed parameters. Then, if a limit is exceeded, alarms can notify the physician to prevent permanent damage. For example, a pressure sensor could be placed on the tip of a guidewire or catheter, then pressure could be monitored during initial placement, greatly reducing the risk of vascular damage. Another example would be in the area of posttreatment evaluation of a catheter procedure. Flow rate downstream of a stent or angioplasty site could be sensed to determine the success level of the treatment and then determine whether more treatment is needed. The ideas are limited only by the number of sensor applications.

New materials and newly refined processes have allowed this simple wire concept to evolve into a very sophisticated product line. Microwire’s historical predecessors found use in great applications of the 19th century to telecommunications technology. Wire has found its place again, enabling yet another technology of the current time.

Brett Steen is U.S. sales manager for Degania Silicone (Smithfield, RI). He can be reached at bsteen@deganiasilicone.com.