Feature Article

Improved flow control

A wide gap in performance exists, particularly in infusion technology used in intensive care, between syringe pump based systems and inexpensive low precision drop infusion systems. Micropumps offer an attractive third option. The requirements of these applications include a controlled system to maintain the required level of safety and accuracy in a range of environmental conditions and the possibility of monitoring the pumping process or adding defined bolus rates. These requirements can be fulfilled by closed-loop micropumps. This article looks at two different types of sensor pump systems. The first is designed for lower accuracy applications in the order of 10% that involve mainly large quantities of units; currently passive solutions such as drop infusion dominate this application area. This lower accuracy level can be achieved using a micropump with an intrinsic sensor function for the feedback signal. The second type of sensor pump system discussed here is for applications demanding accuracies in the order of 2–5% in which portability plays a more significant role. These higher accuracies can be achieved by hybrid systems in which a thermal flow sensor is combined with a micropump.

Piezo membrane micropump

The active component of the closed-loop system is a micropump. Based on the commonly used piezo membrane actuation, the pump with a double configuration of piezo elements in combination with passive valves is utilised in the controlled-loop setup. In this novel design, the two piezo actuator stages have been combined in a single micropump. As well as providing a pressure of up to 550 mbar, the double actuator principle ensures self-filling of the pump at startup and reliable function. Only one polymeric material is employed, which is certified according to ISO 10993, Biological Evaluation of Medical Devices, and United States Pharmacopeia Class VI in contact with the medium. In addition, the pump is produced by automated assembly, which allows tracking of components and process parameters. All these features serve to make this design suitable for medical use.

The demand for constant flow rates with low deviation in different environmental conditions such as temperature and pressure makes closed-loop control of these pumps a necessity. The flow rates of membrane pumps are generally influenced by the pressure levels at the inlet and outlet and by viscosity changes, for example, because of temperature change. This is observed more in membrane pumps than in syringe pumps because membrane pumps do not exhibit high pressure stability and are highly sensitive to changes in the liquid itself.

Intrinsic sensor function

Figure 1: Flow-sensing schematics of intrinsic sensor function. (click to enlarge)

The double actuator setup of the micropump described above provides a novel intrinsic sensor function. The reversibility of the piezo effect means it can be used for actuation and sensing. As schematically shown in Figure 1, the second actuator is used next to the pumping cycles for sensing. This simplified solution avoids the need for additional sensing elements, which is a typical disadvantage of close-loop control, and allows sensing of the flow pressure to be performed directly in the fluidic path using the same set of electrical connectors for actuation and sensing.

To achieve pumping and feedback for the control circuit, the pump is switched between two modes. In full actuation mode both pump stages are working as actuators, providing full performance of the pump. In sensing mode, the first pump stage is actuated and provides fluid pulses, and the second stage is switched to sensing, which provides feedback on the pump strokes of the first actuator. Because the pump is operated at a frequency in the region of 100 Hz, switching between modes does not result in a significant reduction in pump performance. Figure 2 shows the measured performance of a controlled micropump with deionised water. Although the micropump exhibits the typical behaviour of decreasing flow rates at increasing pressures, the flow remains almost constant while running the pump in controlled mode.

Figure 2: First performance graph of piezo sensor controlled pump. (click to enlarge)

Figure 3: Controllable flow range of piezo sensor. (click to enlarge)

Because the piezos are not optimised for sensing purposes, but for effective actuation, this design has limits in terms of accuracy and addressing low flow rates. Figure 3 shows the flow range that can be controlled by the piezo sensor approach. The accuracy of the systems has been determined as 10% in a range of a maximum flow of 5 mL/min down to 500 μL/min. This controlled-loop solution is based on a proven, mass produced micropump. The additional feature of the control circuit is provided by electronics, but because the signal processing is straightforward, the unit keeps its portability and ability to be powered by batteries. The full potential of this system becomes evident in applications where the micropump is used as a disposable unit in which the electronics are to be reused.

Hybrid sensor function

Figure 4: Micropump with thermal sensor.

Thermal flow sensors are used to monitor gas or fluid flow and can be successfully combined with a micropump to achieve controllability. Compared with the previous design with instrinsic sensor functionality, this system completely separates from each other the functional elements of the pump and the sensing element (Figure 4). Because the flow sensors are mainly fabricated from silicon, integrating these into a polymer micropump means a more complex, hybrid system. However, the higher reproducibility and stronger response of the sensors means better accuracy can be achieved with this approach. Small flow ranges can be covered well, which makes them suitable for highly accurate applications with low dosing volumes. Five per cent accuracy in the range of 80–800 μL/min can be achieved. There is also great potential for optimising the acquisition and processing of measurement data by implementing improvements in the electronics and making minor modifications to the fluid path. Work is currently underway to extend the flow range down to the region of 1–100 μL/min. Further optimisation of the measurement cycle will make it possible to operate the unit at the same performance level as a standard pump. Current limitations are found in the flow peaks during actuation, which are much higher than the net flow of, for example, 800 μL/min. To compensate for these variations, a certain distance between pump and sensor must be maintained. Figure 4 shows the sensor mounted approximately 20 mm behind the pump outlet, which currently limits further miniaturisation.

The designer’s choice widens

Both the approaches described here offer interesting new potential for a variety of medical applications, particularly where small, portable battery powered systems are required. As a novel approach, a micropump with a double actuator, in which the closed-loop control can be implemented without the need for additional sensing elements, provides constant performance especially at higher flow ranges under varying system conditions such as pressure. For applications in the lower flow range with higher accuracy demands, the closed-loop can be achieved by micropumps equipped with thermal flow sensors as sensing element. In both designs, smart system electronics will give the user access to the system status and allow manual administration of additional bolus rates.

Severin Dahms is Product Manager, Jochen Uckelmann is Electrical Engineer and Ulrike Michelsen is Head of Marketing and Sales, Bartels Mikrotechnik GmbH, Emil-Figge-Strasse 76a, D-44227 Dortmund, Germany, tel. +49 231 9742 500, e-mail: microengineering@bartels-mikrotechnik.de, www.bartels-mikrotechnik.de