Mobile drug pumps have become indispensable in outpatient therapy for many diseases, such as pain therapy, oncology, parenteral nutrition or metabolic diseases. They give the patient mobility and thus a better quality of life. At the same time, the therapy costs are reduced, as the patients do not have to be cared for in the clinic as an inpatient or outpatient. There is, however, room for improvement in the mobile drug pumps commonly used today. Piezo ultrasonic motors from PI (Physik Instrumente) as pump drives can open up interesting perspectives here.

For mobile drug pumps, one first thinks of balloon pumps because they are lightweight, handy and easy to use by the patient. But they work with large runtime deviations and the injection quantities are difficult to document. However, as the "mobile" patient is usually outside the clinic, precise tracking of the course of therapy is extremely important. The doctor needs accurate information for successful treatment. What is needed is a pump system that also allows accurate documentation outside the clinic.

Currently, in such cases, peristaltic pumps are the drug of choice, but due to their high weight and considerable size, they severely restrict the patient and are therefore not really mobile. However, they allow precise metering and programmable delivery volumes. Depending on the therapy, larger quantities, smaller amounts or even longer periods can not be administered. Large volume infusions are also possible, as you can connect to the pumps drug reservoirs with different capacities. The peristaltic pumps are suitable for various types of medication because they provide a continuous flow (basal rate). Equally, however, special injections (bolus rates) can be dosed, which can be placed at any time. In addition, all processes can be documented in order to evaluate the volume of medication dispensed and the course of therapy and, for example, to prove it against insurance companies.

Mobility and documentation

Apart from mobility, peristaltic pumps combine all desired requirements. They should therefore have the smallest possible dimensions, be light and quiet, in order to restrict the user as little as possible in his freedom of movement. Equally important is low energy consumption to keep the batteries long and reliable, low-wear operation for long service intervals. In this context it pays to take a closer look at the drives commonly used in peristaltic pumps:

The often used small, gear-translated electric motors generate the necessary for the pumping principle high forces or torques. But they are also slow. Your mechanical micro gear rarely work without play and are on top of that very susceptible to wear. Therefore, such a solution can not be described as really reliable, because short maintenance intervals cause effort and costs.

Even those who therefore use gearless direct drives, unfortunately, must usually take disadvantages. Depending on the technical design of the peristaltic pumps, comparatively little force or torque is available. You then have to choose the drive larger, which in turn can affect the size of the pump. In addition, the direct drives are responsive, but not self-locking due to the lack of gear mechanism. In order to keep the position stable when the pump is not operating, it must therefore be energized. With short operating cycles, ie when the duration of the medication is short and the "non-operating times" long, that would have a devastating effect on the battery life. The motor must therefore be mechanically blocked, which is also associated with effort. So there are plenty of reasons to look around for an alternative drive solution for mobile drug pumps:

Compact drive for more mobility

Piezo-based ultrasonic drives, for example, can open up new perspectives here. They are very compact, wear-resistant and self-locking. They therefore keep their position even when the machine is switched off, even in falls and blows. Thanks to their functional principle, their flat design and their individual adaptation options, they can be easily integrated into mobile drug pumps and work virtually noiselessly.

How the compact piezo drives work is easy to understand: oscillations with ultrasonic frequencies of a piezoceramic actuator are converted into linear motion along a rotor and thus drive the moving part of a mechanical structure. The core of the drive is a monolithic piezoceramic (the stator), which is segmented on one side by two electrodes. Optionally, the left or the right electrode is excited to the high-frequency natural oscillations of the piezoceramic element in the range of a few hundred kilohertz and thereby determines the direction of movement. A coupling element on the piezoceramic is thus excited to rapid linear movements. In contact with the rotor arise micro-pulses that move the moving part of the dosing mechanism such as slides or turntable forward or backward. Each oscillation cycle moves the mechanics by a few nanometers. Overall, this results in a uniform movement with theoretically unlimited range.

Different constructions can be realized

The drives are very compact, as they consist practically only of the piezo actuator and a mounted moving disk or linear axis. The driver electronics can be integrated as a chip and thus takes up little space. Since there are few mechanical components such as gears, the solution is also very low-wear and reliable. Dosing is possible over a wide range of speeds, from a few degrees to several revolutions per second, depending on the model, or for linear solutions from less than 1 to several 100 mm / s. The position feedback for the control is available for precise documentation.

The structure of such a driven mobile drug pump is as follows: The motor consists of a piezoelectric ring. This actuator is so excited that a so-called traveling wave is generated. The thin aluminum oxide rings on top and bottom of the piezo ring absorb the vibrations. With the help of the three coupling elements used in the rotor, the recorded vibration is transmitted to the prestressed rotor and converted into a rotary motion. The ring-shaped piezoelectric ultrasonic drives move a disk here. This is designed in such a way that thanks to the special geometry different amounts of drugs can be dosed.

With piezo ultrasonic drives but other constructions are feasible. The piezoelectric actuator can also be arranged laterally, for example, and turn a ring, in the large aperture of which further components can then be arranged. The drug pumps designed in this way are particularly small, light and quiet. In addition, they offer a high degree of flexibility with regard to different therapies and dosages. Even more patients will be able to benefit from non-inpatient or outpatient treatment in the future.

Piezo drives for medical technology