Feature Article

By: S. Arnold, Physik Instrumente (PI) GmbH & Co. KG, Karlsruhe, Germany, and Peter Koltay, Managing Director, BioFluidix GmbH, Freiburg, Germany

 

Already widely used in nanopositioning applications, multilayer piezo actuators allow high electric fields to be reached with relatively low voltages (<150 V), offering a straightforward method for achieving high-speed linear motion with nanometre precision. Through direct attachment to a fluid-filled dosing tube, piezo actuators offer flexible dispensing of liquids at speeds ranging from a few nanolitres to several microlitres per second. The direct-drive characteristics of piezo actuators make them ideally suited to the requirements of noncontact microdispensing, promoting reproducible droplet formation and breakaway.

 

Another benefit of direct-drive piezo actuators is that they are virtually maintenance-free. They have no moving parts in the conventional sense, relying instead on motion-based crystalline solid-state effects. The lack of rotating or friction-producing mechanical components ensures ultra-high reliability, which has made piezo actuators commonplace in applications requiring continuous operation. Industry, in particular, places high demands on the robustness of piezo actuators. Actuators developed by PI Ceramic (Figure 1), for example, have a demonstrated endurance of several billion cycles without measurable changes in behaviour (Figure 2), fulfilling all the requirements necessary for dosing or pumping applications. Resolutions in the subnanometer range at high dynamics and frequencies of up to several thousand hertz make it possible to implement short dosing cycles, with variable strokes precisely controlling the dosing process.

 

Figure 3: The PipeJet dosing principle developed by BioFluidix uses a valveless method based on piezo-driven direct displacement of liquid from an elastic polymer tube with a well-defined internal diameter.

The biggest drawback of piezo actuator drives has been the high cost of replacement. Although the actuators themselves have an almost limitless working life, the fluid-filled dosing tube, which is permanently attached to the actuator and is normally made of glass or steel capillaries, does not. Depending on the nature and properties of the liquid, as well as the operation environment of the microdispenser, this can make traditional piezo actuators economically nonviable for some applications, despite their performance advantages.

 

Recent advances in piezo-driven dispenser design and polymer technology have allowed development of a new generation of dispensers in which the dosing tube is not permanently fixed to the piezo actuator. These dispensers use a removable elastic polymer tube with a well-defined internal diameter, allowing easy, inexpensive replacement of parts that become contaminated by fluid, while retaining the valuable actuator (Figure 3). With the aid of a piston, a stack actuator provides sufficient power reserves for reliable dosing, even when difficult media are being dispensed. Dosing volumes are controlled by the amplitude of the piezo actuator and are virtually independent of the viscosity and surface tension of the fluid. This design, coupled with simple fluidic geometry, means that even particle-containing fluids (such as paint, bead or cell suspensions) can easily be dosed in precise droplet form (Figure 4). This technology has already proved beneficial in the clinical diagnostics area, where it has been successfully employed for lateral flow assays.

 

Piezo actuators are very compact, offering high efficiency at a small installed size. This is a key characteristic for multichannel dispensing applications with only small separations between the dispensing points, and offers the additional advantage of allowing every channel to be controlled individually. Piezo technology has been successfully used for the printing of microarrays or biochips (Figure 5), where reproducible, noncontact printing is vital to the reliability of the resulting assay chips. For these applications, media flow through capillaries in a print head, usually with either 24 or 96 fluid reservoirs arranged in a standard microplate format, to micro nozzles that simultaneously print onto the substrate in a grid. Driven by a stack piezo actuator, a piston is forced into a pressure chamber, producing a homogenous pressure pulse in the nozzles. This set-up can achieve droplets as small as one nanolitre, depending on liquid viscosity, making several thousand doses possible from a single filled print head.

 

Piezo actuators have made an important contribution towards improving the efficiency, accuracy and durability of microdispensing devices for dosing and printing applications. Their reproducibility and minimal maintenance requirements make them an attractive option for numerous low-volume dispensing applications. Recent advances in dispenser design have significantly expanded the potential market for this compact, high-precision technology. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

is Head of Marketing and Products, Physik Instrumente (PI) GmbH & Co. KG, Auf der Römerstr. 1, D-76228 Karlsruhe, Germany

tel. +49 7214 8460

e-mail: s.arnold@pi.ws

 

is Managing Director at BioFluidix GmbH, Georges-Köhler-Allee 106, D-79110 Freiburg, Germany

tel. +49 7612 037 282

e-mail: peter.koltay@biofluidix.com

 

*to whom all correspondence should be addressed