Pumps, Valves & Liquid Handling
The Increasing use of Syringe Pumps Professor Dr Knut Ohls
Syringe pumps represent a very special case in the broad family of metering pumps. They are the result of highly developed, modern mechanical engineering, that now makes it possible to manufacture exceptionally high-precision gears, as well as of control technology based on specifi c computer software. Their task is to transport fl uids of differing viscosities continuously and in precise volumes. It all started 65 years ago with a simple infusion device designed by Dr Hess which possessed only a primitive gearing (Figure 1). Modern syringe pumps bear very little relation to this original device. The transportation of fl uids, which in the vast majority of cases is continuous in nature, is still accomplished using peristaltic infusion pumps equipped with slide members. Although these make it possible to achieve a constant fl ow with low pulsation, the actual volume transported is only as precise as the volume of the tubing that is compressed by the rotation of the shaft. By contrast, in syringe pumps, the continuous fl ow rate is limited by the fl ow rate of the syringe itself (1-50 mL). The main task of such pumps is the precise, constant metering of small volumes.
Structure of Syringe Pumps
Although modern syringe pumps may have differing designs depending on their area of application, they all nevertheless contain the same basic components: the syringe holder, the threaded spindle that drives the syringe plunger, and a control program. In most cases, the spindle drive makes it possible to transport a continuous volume at a tolerance that is often given as + 2%. The fl uid volume that is actually transported depends on the predefi ned drive speed as well as on the volume of the syringe. The decisive factor is therefore the distance the syringe plunger has to travel in a given time in order to achieve the desired fl uid fl ow with the employed syringe volume. Absolute volume control is achieved through the principle of direct displacement. It is not necessary to calibrate the pump because the precision of the fl ow speed is always equally high.
supply, it may sometimes be necessary to operate pumps using a rechargeable battery if the entire apparatus has to be mobile.
Recently, low-pressure syringe pumps that permit the high-precision transportation of even extremely small volumes of 0.005 µL/h have become available.
Application Fields for Syringe Pumps
Syringe pumps are now used in many other applications and in particular wherever precise metering is required.
Modern syringe pumps are also used in the fi eld of medical and pharmaceutical research. They are of particular value whenever volumes in the µL range have to be precisely dispensed, for example in order to add tiny concentrations of an active ingredient or mix very small quantities of reagents during experiments.
As examples of typical applications, manufacturers of syringe pumps list, for example, the intravenous administration of products in animal experiments, the dispensing of adhesives, use in fi lling systems or, more generally, all types of mixing process. For biological research purposes, the Ruhr University in Bochum has developed an electrochemical robotic system for the implementation of electroanalytical methods on microliter plates and biological assays and, more generally, for the investigation of biological questions that also involve the use of syringe pumps.
It is also known that syringe pumps, and in particular those that operate at high pressure, are also used to transport chemicals, for feeding reactors or in the fi eld of catalysis. The requirements are always precision and the provision of a stable, pulse-free fl ow. High-
Figure 1. A simple infusion device designed by Dr Hess possessing primitive gearing
In modern pumps, it is possible to select and adjust numerous parameters. In principle, syringe pumps can also be used in suction mode. It is important to note that the transported media are never in contact with the mechanical pump components. Instead they are located in the closed syringe system (Figure 2 Adjustable syringe holder in the SYMAX pump). The differently sized syringes are easy to exchange, with the result that the syringe pump can be used for a variety of applications. There are also devices that permit multiple arrangements of up to 10 syringe pumps. Alongside the high-precision pumps, it is also necessary to mention the high-pressure syringe pumps that can be used in all technical applications where pulsation-free, precise metering of specifi c transported media is required, including, for example, high-viscosity solutions. A distinction is made between medium-pressure pumps that are used to introduce viscous fl uids into systems at the appropriate pressure (up to 200 bar), high-pressure pumps (up to 510 bar) and, fi nally, ultra-high-pressure pumps (up to 890 bar).
However, syringe pumps may not be used for human medical applications without appropriate authorisation. Infusion syringe pumps in which the syringe is adapted to the pump design and which operate at infusion rates of, generally, between 0.01 mL/h and 200 mL/h, are authorised for such applications. The output options (e.g. mL/h) can be chosen freely. It is also possible to make use of various alarm functions, for example for the detection of air bubbles in the case of infusion pumps. Due to the need for a power
Figure 2. Adjustable syringe holder in the SYMAX pump
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