Kevin Grimley, managing director at PI, discusses the increasing use of piezo actuators for microdosing applications, focusing on the need for high precision and durability.
Automated dispensing of liquids in the nanolitre to microlitre range has become increasingly common, with high-precision microdispensers now commonplace in a variety of medical engineering and biotechnology applications.
Contactless pipetting requires careful selection of a drive mechanism with a stroke and energy compatible with the intended use; generating perfect droplets while taking into account the dispensing speed, viscosity and surface tension of the media to prevent misting, satellite formation on impact and subsequent dripping of liquid.
Already widely used in nano-positioning applications, multilayer piezo stack actuators from PI allow high electric fields to be reached with relatively low voltages (less than 150V), 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 PI Ceramic PICMA actuators make them ideally suited to the requirements of contactless microdispensing, promoting reproducible droplet formation and breakaway.
Another benefit of direct drive piezo actuators is that they are virtually maintenance free, as 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 and means piezo actuators have become commonplace in applications requiring continuous operation, such as in production plants.
Industry, in particular, places high demands on the robustness of piezo actuators.
PICMA actuators have a demonstrated endurance of several billion cycles without measurable changes in their behaviour, fulfilling all the requirements necessary for dosing or pumping applications.
Resolutions in the sub-nanometer 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.
The biggest drawback of piezo actuator drives has been the high cost of replacement.
Although high-quality actuators 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.
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 (Pipejet from Biofluidix).
These dispensers use a removable elastic polymer tube with a well-defined internal diameter, allowing easy, cheap replacement of any parts that become contaminated by fluid, while retaining the valuable actuator.
Using a PICMA stack actuator with the aid of a piston, this set-up is able to provide sufficient power reserves for reliable dosing, even for difficult media.
Dosing volumes are controlled by the amplitude of the piezo actuator and are virtually independent of the viscosity and surface tension of the fluid to be dosed.
This design, coupled with simple fluidic geometry, means that even particle containing fluids (such as paints, bead or cell suspensions) can easily be dosed in precise droplet form.
This technology has already proved beneficial in the clinical diagnostics area, where it has been successfully employed for lateral flow assays.
Overall, piezo actuators have made an important contribution towards improving the efficiency, accuracy and durability of microdispensing devices for dosing and printing applications in various fields, including medical engineering, diagnostics and biotechnology.
Their excellent reproducibility and minimal maintenance requirements make them an attractive option for numerous low-volume dispensing applications and recent advances in dispenser design have significantly expanded the potential market for this compact, high-precision technology.
