Among the most difficult and exacting medical device tubing applications is the extrusion of fine-wire jacketing. Although a highly specialized niche process, jacketing fine wires with polymer-based materials is crucial for manufacturing miniaturized medical devices.
Insulating Fine Wires
In several medical device applications, polymer material is jacketed over fine wires in order to insulate them. For example, in neurostimulation devices used for electrically stimulating the spinal cord, brain, or bladder to treat chronic pain, thin lead wires are jacketed to protect the body from undirected electrical pulses. Similarly, in cochlear implants, polymer material is used to insulate the wires between the implant body and the electrode contact. In both types of devices, very fine lead wires are jacketed using an ultrafine layer of such fluoropolymers as ETFE, FEP, PFA, and PTFE.
However, extruding a polymer layer around a wire core is easier said than done. Made from titanium, 300-series stainless steel, silver-plated copper, platinum composites, or such super alloys as MP35N, the solid wires used in neurostimulation and cochlear implant applications typically range in diameter from 0.0015 to 0.008 in. (0.038 to 0.203 mm). Depending on the diameter of the wire, the jacket’s minimum wall thickness can measure as low as 0.0005 in. (0.013 mm)–smaller than the diameter of a human hair. A common wire used in neurostimulation devices and cochlear implants is made from titanium alloy, has an outer diameter of 0.004 in. (0.102 mm), and is jacketed by an ultrathin fluoropolymer layer with a thickness up to 0.001 in. (0.025 mm). Such wires are so small that they are difficult to see with the naked eye.