While medical microcables must ensure the performance, reliability and safety essentials for any medical application, their size is an increasingly important factor. This is because medical device manufacturers are facing a major ongoing challenge to increase the miniaturization of their components for use in minimally invasive surgical and diagnostic procedures. Smaller diameter cables are easier to insert inside catheters, such as ones used for transcatheter applications in cardiovascular electrophysiology and neurology. They are also less traumatic for patients when used for direct insertion in the body.
One of the main limiting factors in the miniaturization of cables is the thickness of the insulating jacket. Until quite recently, it was not possible to go below 30 microns thickness using the extrusion method. Although at this size the insulation is just a little thinner than the finest human hair (75-125 microns), it would still take up approximately 84 percent of the overall cross-section of a 0.1 mm diameter cable.
New developments in microcables have focused on further reduction in the insulation thickness. There are two competing approaches in creating this insulation–wrapping with tape or microextrusion of the insulation over the cable. Microextrusion enables the diameter of the insulation to be maintained within very tight limits, as well as providing a smooth and cosmetically appealing surface finish. An added advantage of microextruded cables is they are much easier for medical device manufacturers to strip in order to reveal a bare conductor for soldering to a PCB, sensor or microconnector.
Microextrusion provides a 15-microninsulating layer
Nexans has refined the microextrusion process to create an insulating layer that is only 15 microns thick (Figure 1). A key element for this has been the computer modelling geometry of the extruder screw and cross head. As well as providing the capability for very fine control of the outer cable diameter, this has also eliminated the possibility of stagnation areas, as well as reduce the dwell time of the raw insulating material within the machine. The less time that the raw material spends in the extruder the better, as this prevents it being degraded by the heat and pressure, preserving its mechanical, chemical and electrical properties.
Fluropolymers such as fluorinated ethylene propylene (FEP) are commonly used as outer jacket material for microcables. It is a low friction material that makes it easier to insert cables inside catheter channels. Transcatheter cables are usually only used for a short period of time, (< 24h) but in some cases can remain inside the body for seven days.
Cables intended for invasive use are manufactured in closely controlled factory conditions and need to be tested extensively according to specific customer specifications. These tests can include: electrical performance; tensile strength, bending and flexibility; salt water tests to simulate behavior in blood environments; neutrality and biocompatibility; sterilizability by various methods (e.g. using ethylene oxide or EtO, autoclave, gamma rays or other); ability to support extreme temperatures and shock during transportation and storage; and resistance to chemicals.
Space-saving design for transcatheter applications
With microextrusion, it is now possible to design microcables with an overall diameter of less than 0.1 mm. The main benefit of this new generation of microcables is that they can accommodate many conductors within a very small diameter, which allows for more electrical functions inside catheters (such as navigation, power feed for ablation and monitoring) without increasing size. In this case, electromagnetic shield of microcables (under outer jacket) is also very important to protect different signals from interferences.
One of the first microcables to adopt this approach was the NEWSENSE® spacesaving cable for transcatheter applications. This design (Figure 2) uses two AWG52 (20-micron copper diameter) insulated wires stranded together as a twisted pair. Shielding against external electromagnetic disturbances is done by 15-micron diameter wires in a silver plated copper alloy. Externally, the cable has a low-friction fluoropolymer jacket extruded with a 0.14 mm final outer diameter, which is fully compliant with the ISO10993 standard for the biocom- patibility of materials in direct contact with human tissue.
Will cables continue to shrink in size?
We have not yet reached the limit in what might be technically achievable with reducing the size of microcables; however, feedback from customers is that, right now, the limit is ease of handling. Currently, medical device manufacturers employ operatives with nimble fingers to assemble cables into place manually, and they simply cannot handle anything smaller. This situation may change in time with the introduction of more automated robotic tools into manufacturing processes of medical cables.