The trend to decrease volumes of liquids required for sample testing has led fluidics device OEMs to focus on new engineering solutions for better handling precision dispensing.
This is a complex problem: a small volume of liquid limits the velocity of the liquid being moved by stepper motor driven syringe pumps and does not create enough momentum and deceleration to overcome the surface tension that retains the droplet at the end of the probe.
An improvement over stepper motor driven pumps
To solve this problem, the Precision Fluidics Division at Parker developed the Smart Syringe Pump, capable of higher acceleration, running speed and deceleration than stepper motor driven pumps.
OEMs rely on two methods of precision dispensing --
- air displacement
- positive displacement
However, air displacement devices lack the precision of their positive displacement counterparts. They also generally require disposable tips, which add to the cost per sample and increase the amount of waste for disposal. Positive displacement devices utilize syringe and piston pumps, providing the best accuracy and precision. Typically they deliver larger volumes well with good ejection of liquid from the probe tip. However, in ranges around 10µL and lower stepper motor driven syringe and piston pumps begin to struggle because of the difficulty of completely clearing droplets off the probe.
Improving small volume performance
The strategy to improve small volume performance with these positive displacement systems has been to use a small volume syringe barrel or liquid head, the portion of the pump that is actually pumping liquid. Unfortunately, this leads to issues with successfully priming the pump, as bubbles tend to adhere to the small passageways inside the liquid head. Using small volumes of liquid limits the velocity of the liquid being moved and prevents enough momentum and deceleration to overcome the surface tension that adheres the droplet to the end of the probe, therefore, the accuracy and precision of the measurement is compromised as the reproducibility of the droplet clearing is compromised.
The reliable solution Parker developed, the Smart Syringe Pump, calls for using a larger volume syringe barrel or liquid head, along with changing the equipment design from using a stepper motor to a servo motor. Servo motors typically are capable of much higher acceleration, running speed and deceleration. This makes it possible to obtain sufficient flow velocity behind the dispense to reliably eject the droplet, while still having enough resolution to precisely and accurately deliver the right amount of low volume fluid to the desired site.
While the servo motor design is an important element behind a good solution, liquid volumes play a role as well. Typically a minimum 500µL or larger volume is required; volumes of 1000µL are preferred. These larger volumes allow for enough fluid velocity behind the dispense, assuming the acceleration, running speed and deceleration of the motor is rapid enough to clear the droplet with high levels of precision.
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Article contributed by Donald S. McNeil, Sr. Product Manager, Parker Hannifin Corporation, Precision Fluidics Division
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