Piezo-Driven MicroDispensers (PDMD) rely on ultra-fast Volume Displacement (VD) and can dispense a minimum of 20-30 pL. PDMDs can dispense > 1000 droplets/s, meaning larger volumes can easily be transferred too.
The core elements of a PDMD are a thin borosilicate glass capillary with a piezo ceramic actuator attached to it. The glass capillary is tapered toward the lower end, forming the dispense nozzle. Most of the glass capillary and the piezo ceramic actuator are protected by an enclosure consisting of an upper and lower part and a transparent plexiglass tube (Figure 2) in between, which serves as an observation window. 13 mm of the capillary is not protected and used for sample aspiration. A restriction of the capillary above the piezo actuator provides the high backflow resistance needed during operation.
- 1000 Hz dispensing frequency
- Single droplet volume 30 pL to 400 pL
- Volume accuracy cv < 2 %
- Different orifice diameters available
- Highly inert borosilicate glass
- Not coated or treated otherwise
Basic Operation Principle
The piezo ceramic actuator contracts by 100-250 nm when voltage is applied. This causes a fast displacement of the liquid inside the glass capillary. Most of it is ejected out of the nozzle and only a small fraction escapes upward through the restriction. The degree of contraction correlates linearly with the voltage applied and, therefore, is an essential dispense parameter.
The duration for which the voltage is applied (pulse width) is an equally important second dispense parameter. Both influence the volume and velocity of the ejected small droplet. The two parameters are complementary, as the voltage mostly affects the velocity, and the pulse duration mostly affects the volume.
- Stop-to-spot with Z-moves: The dispenser stops for each droplet deposition and moves up and down between consecutive positions to bypass possible obstacles. These movements cost time but can be necessary when dispensing onto specific locations of a non-planar, 3-dimensional target.
- Stop-to-spot, no Z-moves: Without the need to move the head up between consecutive positions, dispensing is much faster. This is always the case when dispensing onto planar targets.
- On-the-fly: The dispenser “flies” over the target and releases a droplet on defined positions during the flight. That way, up to 1000 droplets can be deposited, each at its own specific coordinates.
Digital Liquid Handling
Larger volumes are not dispensed as larger droplets but, instead, as a fast series of small droplets with a well-defined volume.
Advantage: Only the number of droplets varies, making individual and time-consuming calibration of larger droplet volumes unnecessary. This makes the dispensing of different volumes in one experiment simple and efficient.
Disadvantage: Larger volumes require more time, e.g. 1000 droplets require 1 s.
The video shows dispensing of 1, 5, 10, 50, 100, 500, and 1000 droplets each with a calibrated volume of 100 pL and a frequency of 1000 Hz. The result is consecutive deposition of 0.1, 0.5, 5, 10, and 100 nL.
PDMDs enable efficient use of precious samples. As little as 2 µL are sufficient to dispense 10,000 100 pL-aliquots of it at different locations on one target, either for different experiments or on 10,000 targets during the production of diagnostic chips.
The video shows the aspiration of 2 µL of red, aqueous solution from a 5 µL droplet, followed by the dispensing of 150 pL-aliquots of it at different positions. The mixing of the coloured solution with the water inside the dispense capillary is clearly visible. The consequence is that not all the aspirated sample can be dispensed, without dilution toward the end.
If the mixing is not enhanced by osmosis or aspiration that is too fast, then a minimum of 1 µL of the original 2 µL can be dispensed with no dilution, and 1.5 µL with very little dilution. How much is acceptable determines the sample loss.