Coating wires and fibers by means of thermoplastic extrusion
There is a wide range of medical applications for plastic-coated wires, braids and fibers. They can be used as guides in minimally-invasive surgery or as signal-transmitting electrodes for neuro-stimulation. The process of wire coating is in demand for medical engineering purposes as well. For instance, it is needed when adding very thin-walled inner layers with excellent sliding properties to small-scale catheter systems. In this case, the internal wire serves as an assembly aid to manufacture components like PTFE liners.
A variety of applications and manufacturing methods
In a nutshell: Plastic-coated wires, braids and fibers can fulfill very different functions in medical and medical-technology applications:
- to transmit electric signals
- as insulation or corrosion-protection for the substrate inside
- to produce a very thin polymer layer with excellent sliding properties
Almost as diverse as the fields of application are the methods that can be used to coat the metal or synthetic substrate. The polymer coating is often applied in multiple process steps and adhered to the wire, braid or fiber under high temperatures. In the final stage, the coating must have the required stability and bond strength. Multiple drying and curing cycles are necessary to achieve this, which prolong the manufacturing time. In addition to the lengthier time factor, multi-stage processes also have the risk that particles will adhere to the coating after every step.
Importance of preventing particle detachment
“Often, it isn’t until there is some mechanical strain during later operational use that the coating quality is evident,” says Daniel Riechelmann, applications engineer at RAUMEDIC, and he points out: “When the wire is bent, for instance, the coating can separate from the substrate if it has not been optimally applied.” Microscopic particles can also chip off the polymer layer. “We call this material flaking,” Riechelmann says about the undesired effect.
For medical applications, these consequences must be unconditionally precluded. This is because the coated wires sometimes remain in the body as long-term implants. On the other hand, liquids used in intracorporal treatments could penetrate porous coatings and allow plastic particles to circulate in the patient’s bloodstream.
Rethinking thermoplastic micro extrusion
This is why the Helmbrechts-based company relies on micro extrusion. “With VariCoat®, we have developed a single-step method that enables a robust and uniform coating with a wide variety of high-temperature polymers, technical polymers and standard polymers,” says Daniel Riechelmann in describing the thermoplastic extrusion process. Instead of being applied layer by layer, polyamides (PA) and materials like PTFE, FEP, amorphous PEEK and PUR are directly extruded onto the substrate in a single processing step. Inline measuring systems check the coating for any imperfections and measure eccentricity and outer diameter.
“We can produce layer thicknesses of 0.01 to 1.0 mm (0.0004″ to 0.039″) and use only biocompatible, medical grade plastics. Our fluoropolymers are 100 percent PFOA-free and have a homogeneous, closed surface structure,” Riechelmann explains. These favorable sliding properties make an additional coating obsolete.
Variety with optimal production conditions
The selection of substrates is virtually unlimited. With the help of the VariCoat® process, wires and braids from conducting materials like stainless steel, copper and platinum alloys as well as glass fibers and synthetic Kevlar can be wrapped in a layer of polymer. The core to be covered can have a diameter of 0.025 mm to 1.5 mm (0.001″ to 0.059″). So there are many variable parameters which the customer can influence. “We don’t have a standard portfolio,” Daniel Riechelmann sums up, and continues: “Whether layer thickness, coating, substrate or the degree of bonding strength of both components – VariCoat® enables customized solutions.”
RAUMEDIC produces the coated wires and fibers exclusively in an ISO Class 7 clean room. As a result, contamination by germs and particles is kept extremely small from the very beginning. In addition, the products can be further processed immediately afterward because no subsequent cleaning procedure is required.
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Product Manager Application Technology,