Air Displacement Pipetting

TTP Ventus has developed a concept for an air displacement pipetting module enabled by our high-performance piezoelectric micropump, Disc Pump. The new system—which we call the Volume Control Module (VCM)—leverages the exceptional pressure regulation of Disc Pump to deliver high precision fluidic dosing. This concept can enable devices that are smaller, lighter and lower cost than conventional air displacement systems.

This application page describes a simple system architecture that enables the pump to be used in an air displacement regime, exploiting the high-precision pressure control and pulsation-free output of the pump to deliver exceptional performance. This scheme also enables aspiration and dispensing without the need for additional valving to reverse the flow.

Contact TTP Ventus to speak to one of our experts about implementing this scheme in your next product.

Application Note AN049: Pipetting with Disc Pump pdf1.16 MB

*Above: the system architecture includes the pump, an orifice (flow restrictor), a pressure sensor (Pr) and a reservoir volume (Vr), connected to a pipette tip.*

Above: A plot showing CV against aspirated volume, from ~15uL to ~400uL.

How it works

Step 1: With the tip above the fluid, the pump is turned on. The flow restriction created by the orifice causes the pressure in Vr to rise. The pump drive power is controlled so that a target pressure of P1 is reached in Vr. This elevated pressure provides an excess of stored air to ensure full and complete dispensing in step 5.

Step 2: The pipette tip is lowered into the fluid.

Step 3: The pump drive power is increased so that a higher target pressure, P2, is reached in Vr. An additional discrete quantity of air, proportionate to (P2-P1), is displaced from the tip into the system. The air pressure in the tip reduces, drawing fluid into the tip. The exceptional pressure regulation offered by Disc Pump in turn provides high-precision control over the quantity of fluid aspirated, Va.

Step 4: The pipette tip is withdrawn from the fluid.

Step 5: The pump power is reduced. The pressure in Vr discharges through the orifice into the tip. This results in fluid dispense from the tip. Dispense speed is controlled by the rate that the pump power (and therefore the pressure in Vr) is reduced - turning the pump off completely will result in the fastest dispense speed.

Key Design Considerations

There are some key design considerations that impact pipetting performance depending on the desired fluid dispense volume and speed:

  • Ambient Temperature - Ambient temperature has very little effect on pipetting performance—providing that the temperature of the air in the pipette tip is similar to the temperature of the air in the VCM.
  • Heat - It is important to manage the amount of heat generated by the pump for consistent pipetting, particularly where the fluid must be held in the pipette tip for extended periods of time.
  • Component Selection - Care must be taken when selecting key components such as the pump, reservoir, orifice and pressure sensor. Different configurations will suit different application requirements.
  • Fluid-tip Interaction - Where reusing pipette tips, we found that inconsistences in fluid-tip interaction (such as fluid pinning and/or droplets remaining in the tip) can cause significant variation in the amount of fluid aspirated/dispensed; we had to explore alternative experimental setups and measurement techniques to demonstrate the performance limit of the system.
  • Additional Components - The VCM can be extended with additional components, including valving to enhance the pipette tip “blow-off” function, and temperature sensors to enable temperature compensation.
  • Equations - We derived a number of useful equations that can help when designing VCM systems.

Our Pipetting Application Note provides provides a detailed technical overview of the system, together with design guidance for customers interested in building their own systems: Application Note AN049: Pipetting with Disc Pump pdf1.16 MB.

If you have any questions, please get in touch with us at: