- TPU main layer with active inner layer
- Layer thickness measurement to resolution of 0.0001 inch (0.00254 mm)
- Puller/cutter with scrap/QC mode
At the upcoming MD&M West 2018 trade show, the companies Conair Group, Davis Standard and Zumbach Electronics will co-extrude a thermoplastic polyurethane (TPU) catheter tube, demonstrating how active pharmaceutical ingredients (APIs) can be incorporated in a thin layer on the outside of the plastic tube. Typical applications could include the addition of an antimicrobial outer layer to prevent formation of biofilm that can cause infection, or an anticoagulant to avoid catheter thrombosis.
TPU main layer with active inner layer
The main body of the catheter tube, a single-lumen adult PICC line with an outside diameter of just 0.067 inch (1.7mm), is made of clear TPU delivered by a 3/4-inch main extruder of Davis Standard. A 3/4-inch co-extruder delivers the blue resin representing the active layer. The inner/outer layer ratio is approximately 80/20. Each extruder feeds a positive displacement melt pump, which controls polymer flow to a cross-head co-extrusion die supplied by Guill Tool & Engineering.
“The same technology that can add color to the surface layer of medical tubing also can be used to apply a pharmaceutically active ingredient like silver sulfadiazine,” explains Bob Bessemer, Sales Manager, Medical Extrusion for Conair. “For the demonstration, we are using color instead of an API, but the process is exactly the same.”
Layer thickness measurement to resolution of 0.0001 inch (0.00254 mm)
As the two-layer tube exits the die, it enters a Conair vacuum-sizing/cooling tank. The vacuum creates pressure inside that tube that helps stabilize dimensions and prevent cooling water drooling out of the tank’s feed opening from affecting the critical surface finish of the tube. An ultrasonic gauge inside the tank monitors the wall thickness and the thickness of each of the two layers (to 0.0001 inch resolution), while a Zumbach Electronics 3-axis OD laser gauge, positioned downstream from the tank provides not only closed-loop dimensional control, but also displays the tube for concentricity adjustments. The data from these gauges is used to control melt-pump output, as well as puller speed and vacuum to maintain layer thicknesses and other critical dimensions. A vision system is also incorporated into the line to detect imperfections, including color inconsistencies, gels, surface scratches and more. The system communicates in real time with quality controls to automatically separate good vs bad tubes in-line.
Multi-layer catheter tube with active pharmaceutical ingredients (source: Conair)
Puller/cutter with scrap/QC mode
Next, the tubing passes through a Conair puller/cutter. The puller portion of the unit consistently pulls the tubing down the line during sizing and cooling. The rotary-knife cutter slices the tube precisely to the specified length. Cut lengths are then carried away on a medical takeaway belt conveyer. Once on the conveyor, cut pieces that do not meet quality standards, based on the OD/ID gauge readings and the inline machine-vision, can be removed from the production stream with a puff of compressed air. In fact, the MedLine puller/cutter has a scrap/QC mode that accepts all three inputs (from the wall-thickness gauge, the laser OD gauge and the cameras) and automatically sets a length offset so the cutter ‘knows’ when the defective piece will reach it. Then the cutter can be programmed either to cut faulty tubing differently from good product or not cut at all, or it can change the way defective section is handled by the conveyor.