Originally published March 1997
Over the last few years, medical tubing has evolved into one of the most critical medical device components, placing ever more difficult demands on the manufacturing process. Tubing dimensions must now be held to tolerances that were virtually unachievable until recently. Outside diameters (ODs) have become smaller, resulting in diminished wall thicknesses. Many new medical procedures call for multiple lumens, often with these same reduced--or even smaller--ODs. To provide access to the lumens, it may be necessary to profile, or taper, the tubing. When finished, the tubing must pass rigorous tests for pressure, flow, and leak integrity. Given the high cost of some raw materials, it is important to meet these requirements with a minimum of waste: the production process, once developed, should be controllable and repeatable.
Medical tubing extruder with electronic pressure control system. Photo: Davis-Standard Corp.
To satisfy these increasingly stringent demands, extrusion equipment has evolved into computer-driven systems that rely heavily on electronic sensors and controls. Temperatures are routinely controlled electronically within tight limits. Dimensional-measurement systems based on lasers or ultrasonic sensing maintain tubing dimensions by controlling haul-off speed.
One of the most critical parameters to control is the lumen inner diameter (ID) air pressure, which keeps the lumen open during cooling of the material. With multilumen tubing, each lumen requires its own controlled ID air supply. Traditionally, mechanical pressure regulators have been used to control ID air pressure. For bump or tapered profile tubing, however, the ID air pressure cannot simply be held constant. Rather, pressure must be ramped up and down in a controlled manner as the OD changes, so as to maintain the relative dimensions between the various lumens. This new requirement to vary the air pressure mandates some form of pressure control that can be programmed to accurately follow these changes. Since haul-off speed is typically used to generate the profile bump, the pressure controller must be fast enough to follow rapid haul-off speed variations.
Electronic pressure control (EPC), a technology used in other industries for many years, is a promising solution to many of the current demands of medical tubing production. Figure 1 shows a simple electronic pressure control system. First, a pressure transducer senses the pressure to be controlled. Its signal is fed to a controller, where it is compared with the desired pressure set point. The controller then generates a drive signal to a fast-acting proportional control valve, which opens or closes to admit the precise amount of air or gas needed to adjust pressure to the desired value. Such a system is capable of response times on the order of tens of milliseconds, which is sufficient for most current medical tubing extrusion applications. An analog set point signal from a computer or system controller--such as the haul-off speed controller--can provide the programmed ramps needed to follow the variations in the OD.
Figure 1. Schematic of a basic pressure control system including flow measurement components.
In addition to the basic EPC system, Figure 1 also shows a flowmeter installed in the air-supply line. The flowmeter is not part of the closed-loop feedback system, but provides only a measurement of how much air is being supplied. This information offers assurance of continued flow of air through the lumen, identifying any blockages or restrictions. (The system could be configured with the flowmeter as the feedback sensor and the pressure transducer serving only as an indicator, in which case air would be regulated at a constant flow, instead of pressure. This is not normally done in extrusion control, however.)
EPC CLOSED-LOOP FEEDBACK
Elements of a typical EPC closed-loop feedback system include a pressure transducer, mass flowmeter, control valve, and pressure controller.
Pressure Transducer. In tubing extrusion, the ID air pressure is generally considered the most important variable to control. Pressure between the inside lumen and the outside of the tube typically ranges from a few inches of H2O to several tens of inches. One side of a differential pressure transducer is connected to the ID air supply near the entry to the extrusion head. The other side of the transducer is connected to the pressure surrounding the tubing as it exits the head--often the cooling-tank pressure.
Figure 2. Cutaway view and schematic of a variable-capacitance pressure transducer.
Figure 2 illustrates a typical pressure transducer used in extrusion pressure control. Based on the variable-capacitance principle, the sensor consists of a taut metal diaphragm positioned close to a pair of capacitance electrodes. As pressure deflects the diaphragm, the capacitance values change by different amounts, unbalancing an electronic bridge circuit, which results in a change in the electronic signal output. The types of transducers used in extrusion provide fast response to pressure changes, on the order of a few tens of milliseconds.
Figure 3. Cutaway view and principle of operation of a thermal mass flowmeter.
Mass Flowmeter. Figure 3 shows a cutaway of a thermal mass flowmeter (MFM). Air or gas enters the MFM at one end and is immediately split into two flow paths. One path contains a laminar-flow bypass element, the other houses the thermal sensor assembly. The flow sensor is designed for a maximum flow of 10 sccm (or 100 sccm in some models). The bypass element creates a fixed ratio between the flow paths, dependent on the MFM's full-scale flow range. For example, a 5000-sccm flowmeter would pass 10 sccm through its sensor, while the remaining 4990 sccm would go through the bypass.
The thermal sensor consists of a small, U-shaped length of tube surrounded by a heater in the middle and temperature sensors at either end. With zero flow, the two end temperatures are equal and the electronic bridge circuit is balanced, giving zero output. As air or gas flows through the tube, heat is drawn away from the inlet temperature sensor and toward the exit sensor. The higher the flow rate, the greater the temperature difference, which causes an increase in the electronic output.
Figure 4. Cutaway view and flow curve for a control valve.
Control Valve. Variations of the control-valve design shown in Figure 4 are used in both pressure and flow control systems. Air or gas enters on one side of the valve, travels through a precision orifice, and exits on the other side. A Viton nosepiece above the orifice is attached to one end of a magnetic core, which travels up and down inside a nonmagnetic, stainless-steel housing surrounded by an electromagnetic coil. The core/nosepiece assembly is spring-loaded to hold the nosepiece down against the orifice, closing off flow. When an electrical signal is applied to the electromagnetic coil, the core/nosepiece pulls up, away from the orifice, allowing some air or gas to flow through the valve.
Operation of the valve is similar to that of common solenoid valves, except that its opening is proportional to the signal applied to the coil instead of being merely on/off or open/ closed. Gas control valves of this type are capable of responding to signal commands within a few tens of milliseconds.
Pressure Controller. The main purpose of the control electronics is to compare actual pressure with the pressure set point and then to adjust the air-inlet control valve as necessary to reduce the error to zero. Controllers may also provide additional functions, such as supplying power and display capabilities for the pressure transducer. Different tubing dimensions may require different air-pressure and flow-rate settings. To accurately track fast-changing pressure set point variations, the controller provides adjustable phase lead and gain so that the control loop can be tuned for optimum response for each tubing application. One important feature of electronic controllers is that they can be interfaced to a system controller for full remote control of pressure. For bump or profile tubing, a simple analog signal from the extrusion system controller can be used to generate the required pressure ramping.
Pressure control electronics are available in several different configurations depending on the application, though all variations are functionally the same. One common model, designed for use in the R&D or development lab, incorporates all the necessary controls on the front panel of a rack-mountable instrument. Another model--electronically identical but packaged as a small box with no front panel--is intended for use in production systems and is operated by electronic commands only. A third variation incorporates the control electronics, pressure transducer, and control valve in one small package. A power supply and display are required for this last version, but the model's compact size simplifies the mechanical installation of the control valve and transducer.
EXTRUSION PRESSURE CONTROL
Figure 5 illustrates a typical tubing extrusion system with electronic pressure control. A single-channel EPC is installed on the ID air supply; for multilumen tubing, one EPC system would be needed for each lumen. The pressure transducer is shown connected to the cooling tank, which is assumed to be controlled at some pressure slightly below atmosphere. (If the cooling tank is at atmosphere, the reference side of the transducer can be left open to ambient.) The EPC pressure set point is shown connected to the extruder system controller, for generation of a pressure ramp required for bump or profile tubing.
Figure 5. Tubing extrusion system with pressure control and flow measurement on the ID air control and pressure control of the cooling tank.
The system presented in Figure 5 also includes a flowmeter and readout in order to provide a measurement of how much air is being used to maintain the constant ID air pressure. This measurement is an indicator of continued airflow through each lumen, and can warn of a blockage or other restriction in the tubing. Pressure control alone does not give any indication of lumen blockage, since the pressure control loop could achieve its pressure set point in a blocked line by simply shutting off the inlet air. Given the speed of today's production extruders, the absence of some kind of airflow indicator means that a good deal of defective tubing could be produced before a blockage is detected.
A second EPC system is shown installed on the cooling tank. This system maintains pressure above the cooling water at some pressure just below ambient, typically a few inches of water. Pressure control of the cooling tank for vacuum sizing has been found to be very effective in maintaining critical tubing dimensions. Although response time is not critical here--pressure is generally held constant--the same EPC equipment as that employed on the ID air line can be used.
Electronic pressure control offers significant advantages for medical tubing extrusion. Readily programmable for full computer control of the extruder system, EPC provides fast, accurate control of ID air and cooling-tank pressures. For multilumen and bump or profile tubing, the ability to generate pressure ramps and hold critical shapes and dimensions is invaluable, opening up possibilities for new tubing designs that previously required the welding or gluing together of separate segments. Finally, by adding flow measurement to EPC, blockages or restrictions can be detected before significant material is processed and lost.
A version of this paper was presented at a medical tubing technology clinic sponsored by the Society of Manufacturing Engineers (SME). The author would like to thank the following for their input and advice: Bob Bessemer (Conair-Gatto), Mikel Messick (RDN Manufacturing), Rip Palladini (MetriLogic), and Peter Rinaudo (DavisStandard).
J. Grant Armstrong is a market specialist with MKS Instruments, Inc. (Andover, MA), a manufacturer of instrumentation for the measurement and control of gases. He has worked at MKS for more than 20 years in various positions, and now concentrates on the manufacture of medical devices and instruments. He is a member of a number of professional societies, including the Society of Manufacturing Engineers, the Association for the Advancement of Medical Instrumentation, the Parenteral Drug Association, and the International Society of Pharmaceutical Engineers.