From R&D in the laboratory to production process control, from disposable component assembly to OEM diagnostic instrumentation, a broad array of medical and medical support applications rely on precise dispensing or metering of fluids. But just as diverse as the applications are the varieties of fluids, their chemical and physical characteristics, and the specific application parameters. It could then be concluded that fluid control components, including pumps, need to have the flexibility to handle many different fluids and applications.
In general, the trend for fluidic systems used for medical applications is to deliver smaller and smaller volumes at higher and higher accuracy. Manufacturers of diagnostic instrumentation, for example, continue to develop clinical analyzers that can perform more and more tests using smaller and smaller samples. Very often, samples, reagents, and buffers used in diagnostic instrumentation are controlled within a range of a few microliters. Equally important is to have a fluid control system that will maintain drift-free precision thus eliminating the need for recalibration, long term. Dispensers and metering pumps from Fluid Metering, Inc. which utilize FMI’s patented CeramPump® technology, provide solutions for medical fluid control applications that require low-volume, drift-free accuracy.
The CeramPump® is a low-volume piston pump that uses one moving part to accomplish both the pumping and valving functions, thereby eliminating the check valves that are present in all other reciprocating (syringe, diaphragm, bellows, piston) designs. The CeramPump® uses a unique rotating and reciprocating ceramic piston, moving within a precision mated ceramic liner to accurately pump fluid in one direction without allowing any backflow. The reciprocation action of the piston is similar to a standard piston pump. As the piston moves back, it draws fluid into the pump chamber. As it moves forward, fluid is pushed out of the pump.
In addition to reciprocating, the piston also simultaneously and continuously rotates in one direction. The piston is designed with a flat cut into the end closest to the inlet and outlet port (see Fig. 1). As the piston rotates, the flat is alternately aligned with the inlet and outlet port, essentially functioning as a valve. At no time are the inlet and outlet ports interconnected; therefore, the need for check valves is eliminated. One complete synchronous rotation and reciprocation is required for each suction and discharge cycle as shown in Fig. 1. An animation of continuous metering using the CeramPump® principle can be found on Fluid Metering’s Web site.
Adjustment of Dispense Volume
The piston displacement (or volume pumped per stroke) is variable and controlled by the angle of the pump head to the drive. When the pump angle is zero, the pump head is in straight alignment with the drive, and the flow is zero. In this situation, there is no reciprocation and the piston is only rotating. As the angle of the pump head increases above zero in either direction with respect to the drive, the piston reciprocates, and fluid is moved through the pump (see Fig. 2). The greater the angle, the greater the displacement per cycle (also know as stroke). Adjustment is infinite between zero and 100%, and a flow rate indicator provides for accurate and simple linear calibration. The pump is designed so that at any angle and flow rate, the piston always bottoms for maximum bubble clearance. This is especially important for very small dispenses and flow rates, as the presence of even a minute bubble will significantly affect accuracy.
The valveless feature of the Ceram-Pump® design is its most significant feature. There are typically four check valves present in diaphragm, bellows, and traditional piston pumps. Even during normal operation, these will wear over time and not seal properly allowing backflow. As a result, accuracy drifts and minimally the pumps need recalibration. Eventually, the check valves need to be serviced.
Syringe pumps, also very common for low-volume fluid control in medical instrumentation, use electronically actuated valves. Typically, for uninterrupted dispensing, syringes are paired so that while one syringe is dispensing the other is refilling. For this two-syringe system to operate, there are 2–4 valves per dispensing unit and 2–4 electronic valve actuators with control boards.
The CeramPump® uses sapphire-hard ceramics for both the piston and mated liner. These components are dimensionally stable in that they will not change shape or dimension over time. Therefore, the pumping chamber remains stable for millions of dispenses without downtime or recalibration.
Accuracy & Precision
Consistency in dispensing can be measured by monitoring both the accuracy and precision of the dispenses. Accuracy is a comparison of the average value of the dispense volume compared to the desired or target value. Precision is the range or degree of variation from dispense to dispense. See Figures 3 & 4.
Typical Medical Applications
This valveless piston technology is useful in numerous applications, ranging from micro-volume reagent fluid control in OEM clinical diagnostic instrumentation, hemodialysis machines, and automated immunoassay processing, to precision production dispensing of cyclohexanone solvent in the manufacture of disposable medical component kits (IV tubing sets), dispensing monomers for contact lens manufacturing, coating of drug eluting stents, dispensing electrolytes used to made button cell batteries for hearing aids, and much more.
- Micro-volume fluid control of samples, reagents, and buffers in clinical chemistry instrumentation.
- Dialysate recirculation pumps for hemodialysis machines
- Fluid control for automated microplate washing and preparation
- Coating of drug-eluting stents with slow-release drugs to prevent cell proliferation
- Dispensing of cyclohexanone for solvent welding for IV tubing kit assembly
- Dispensing of UV curable and two part epoxy adhesives for medical component assembly
- Dispensing of lubricants for assembly of catheters
- Dispensing of monomers in the manufacture of contact lenses
- Dispensing of electrolytes used in the manufacture of button cell batteries used in hearing aids
- Dispensing of marking inks used for packaging of medical components.
- Dissolution Systems for Pharmaceutical Testing
In each application, the key features of the pump include one moving part, ceramic internals, and valveless design. The pump technology provides what medical manufacturers are looking for: long term, drift-free accuracy and virtually no maintenance or downtime.
This article was written by Herb Werner, Marketing Manager for Fluid Metering Inc., Syosset, NY. For more information visit http://info.hotims.com/34459-196.