Medical tube is becoming ever smaller and thinner while adding new features like high-tech material combinations, more wire braiding/wrapping, and heat-shrink sheathing for strength and kink resistance. Smaller diameter, thinner walls, and more complex structures are the latest in medical tubing. This micro-fine product is more apt to incorporate high-performance engineering polymer, wire braiding or wrapping, and shrink tubes to hold increasingly complex structures together. The market is exploring more complex composite tubes, using more materials in a given tube with more wire wrapping and braiding.
The new composite tube is driven by growing demand for minimally invasive surgery, for wires to stimulate nerves and regulate heartbeats, for vascular access and urology, and smaller catheters including that for surgery on infants.
With smaller micro tubing comes demand for new or improved characteristics. Tube has to thread more easily into the body, requiring more lubricated outer surface; and wire has to thread more easily through the tube with smaller clearances inside, requiring lower coefficients of friction on interior surfaces. These requirements often dictate using a composite of PTFE inside and a more conventional thermoplastic outside. In-house braiding with stainless-steel micro wire is growing rapidly, as finer tubing requires braiding to add burst strength or prevent kinking.
High-heat, high-priced PEEK resin from Victrex has been used in small pharmaceutical tubes because of its chemical resistance. A medically implantable grade has been available since 2001 from Invibio, a unit of Victrex.
Zeus Inc., USA, launched in 2005, the world’s only heat-shrinkable PEEK tubing. Wall thickness is as small as 0.002 in. before shrinking and 0.01 to 0.25 mm afterward. The ratio of shrunk to preshrunk wall length (circumferential) is 1.3:1.
PEEK tube insulates wires to protect medical devices like the pacemaker against short circuits. The tube has high dielectric strength (>500 v/mil), chemical inertness, kink resistance and low coefficient of friction (0.35 to 0.5).
However, PEEK is corrosive to extruders, so the barrel and screw require special metallurgy. The machine has to withstand very high temperature (600 degree C). The big difficulty is controlling micro-tube dimension while PEEK cools. The resin needs to achieve maximum crystallinity to achieve top physical properties, so cooling has to be slow and controlled.
Both thermoplastic and silicone rubber tubes are achieving new thinness records, especially the latest heat-shrink tubes. Heat-shrink polyester (usually PET) and PE tubes replace spiral wrapping with thin film and adhesive to cover tiny high-pressure catheters, micro-wire devices and joints between tubes. Shrink tubes can also insulate electrically and compress the OD of a micro wire bundle. Medical tube is also finding ways to cram in more and more lumens.
Coextrusion is growing in popularity as a way to make micro tubes more slippery inside and more lubricated outside. Liner materials can be PTFE with a coefficient of friction (COF) of 0.1, PTFE mixed with polyimide (COF of 0.3), or polyimide (COF of 0.5). Solutions to higher lubricity on the outside are PTFE, nylon, Pebax, and surface-treated silicones. Other tubing surface modifications use pharmaceutically functional molecules such as the anticoagulant Heprin.