Feature Article

Made for medical

The mp6 micropump developed by Bartels Mikrotechnik operates safely and accurately under varying conditions.

Infusion therapy demands accurate flow rates. Key to achieving that accuracy are the starting behaviour and stable flow during infusion and minimal short time fluctuations. The mp6 micropump is a micro-membrane pump actuated by a dual configuration of piezo elements in combination with passive valves.

For the first time, two actuator stages have been combined into a single pump. Besides providing reliable function and pressure up to 500 mbar, the double actuator principle ensures self-filling of the pump at startup. But two features, in particular, make the pump suitable for medical applications: a USP Class VI polymer, certified to ISO 10993, is the only material that comes into contact with the medium, and the device is produced by automated assembly.

Characteristics of membrane pumps
The performance of a membrane pump is dependent on the pressure levels at the input and outlet. Viscosity changes, which may be caused by temperature fluctuations, will result in varying flow rates. This effect is stronger in membrane pumps than in syringe pumps, since the former do not exhibit high-pressure stability and are more affected by changes to the liquid itself. The demand in medical applications for constant flow rates with minimal deviation under different environmental conditions (such as temperature and pressure) leads to the need for closed-loop control of these pumps.

The mp6 micropump with a double actuator provides an intrinsic sensor function (Figure 1). The reversible piezo effect can be used in actuation (pumping) and sensing mode.

Figure 1: The intrinsic sensor function helps ensure accuracy in the micropump.

By continually operating between pump and feedback mode, no significant decrease of pump performance is observed. About 10% accuracy is needed for passive solutions such as drop infusion systems. This can be achieved for flow rates in the 0.5 to 5 ml/min range, based on the intrinsic sensor function of the feedback signal. Particularly at higher flow rates under varying system conditions, the piezo sensor principle provides consistent performance.

Use-oriented validation of flow performance
The mp6 has been tested for use in infusion therapy applications. A clear and simple picture of general flow-rate stability over time is demonstrated by a trumpet curve analysis (Figure 2). The trumpet curve shows variations of mean flow accuracy over specific observation periods. The variations are presented only as maximum and minimum deviations from the overall mean flow within the observation window. The pump's flow performance is measured over a 25-hr time period after a 24-hr stabilising phase or emptying of half the reservoir, as specified by the standard DIN EN 60601-2-24.

Figure 2: Flow deviation according to IEC 60601-2-24. The trumpet curve shows the flow performance of the mp6 over specific observation periods.

Since the mp6 provides dynamic behaviour, stable flow conditions can be achieved after 10 minutes. Therefore, the stabilisation phase can be considerably shortened. The trumpet curve analysis over defined time intervals shows the maximum positive and negative errors occurring in the time interval. Achievable flow rates of the mp6 are 0.5 to 5 ml/min, with 10% accuracy. The mp6 trumpet curve for a 125 ml/hr flow rate is shown below. In short observation windows, the error of flow is already less than 5%. In increased intervals, it drops below 1%. Absolute error is less than 2 %.

Looking at the overall complexity from the pump side, this solution is based on a proven, mass-produced component. Additional effort is required for the driving electronics, but because signal processing is straightforward, the unit maintains its portability and capability to be powered by batteries. In applications where the micropump is used as a disposable unit whilst the electronics are reused, the technology’s full potential becomes apparent.

Infusion therapy with flow rates below 0.5 ml/min can be achieved by quasi-continuous pumping, dosing small volumes in defined time intervals. Or, a flow-controlled micropump with an integrated flow sensor can be used, which can achieve more-accurate and lower flow rates. These systems achieve flow ranges from 60 µl/ min to 5 ml/min with 5% accuracy.

Dipl.-Ing. Severin Dahms
is Product Manager, Micropumps,
Dr. Ulrike Michelsen
is Marketing and Sales and
Dipl.-Ing. Jochen Uckelmann
is Project Manager, Electronics,
at Bartels Mikrotechnik GmbH, Emil-Figge-Strasse 76a, 44227 Dortmund, Germany
tel. +49 2319 742 500
e-mail: michelsen@bartels-mikrotechnik.de
www.bartels-mikrotechnik.de