System and method for dispensing fluid droplets of known...

Dispensing – Drop formers

Reexamination Certificate

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Details

C222S014000, C222S055000

Reexamination Certificate

active

06213354

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is concerned with systems and methods of dispensing precise volumes of fluid in droplet form and of generating very low fluid flow rates.
2. Description of the Related Art
The need to dispense very small volumes of fluid, or to control very low fluid flow rates, routinely arises in chemical and biomedical laboratories, among other places. For example, a type of blood test may require that 5 &mgr;L of a blood sample be precisely dispensed onto a slide or plate, or an intravenous medication may need to be provided at a rate expressed in terms of nanoliters or picoliters per second.
Conventionally, such low flow rates and/or dispensed volumes have been provided using a syringe pump to displace a known volume of liquid, and then to provide tubing and/or a needle to route the displaced fluid to its intended destination. This method assumes that the volume displaced by the pump is accurately delivered to the dispensing end of the system. Unfortunately, as the volumes to be controlled become smaller and smaller, significant errors are produced by, for example, the compliance of the tubing, fluids which cling to the needle tip, temperature variations, and inaccuracies in the syringe itself. When the volume needed for a particular purpose can be contained in a single fluid droplet, the accuracy obtainable with such a conventional system is often inadequate.
SUMMARY OF THE INVENTION
A system and method for dispensing fluid droplets of a known volume and for generating very low fluid flow rates is presented which overcomes the problems noted above.
A precisely-controlled fluid flow rate is created using a closed-loop control system. A pump pressurizes a fluid within a piece of tubing against an aperture having a known size, causing fluid to pass through the aperture and out a dispensing tip at a flow rate that varies with the applied pressure. The pressure of the fluid upstream of the aperture is measured and fed to a controller, which also receives a setpoint input representative of desired flow rate. The controller is arranged to control the operation of the pump to maintain the fluid pressure against the aperture as needed to obtain the desired flow rate.
To dispense a fluid droplet having a particular volume, the flow rate loop described above is used as an inner control loop. An outer control loop is formed using a droplet volume measuring system capable of determining the volume of the droplet that forms at the outlet of the dispensing tip as a result of the controlled flow rate. The droplet volume measurement is fed to the controller, along with the measurement of pressure against the aperture and a setpoint representative of the desired dispensed volume. The controller is arranged to maintain the pressure against the aperture (using the inner control loop) as needed to produce a flow rate which will quickly produce a droplet of the desired volume (as determined using the outer control loop), at which time the pressure is reduced to zero. Because the droplet volume is measured at the dispensing tip outlet, all errors introduced along the dispensing path are compensated for. Furthermore, using a controlled flow rate to produce a desired dispensed volume eliminates the need to provide a pump that can control volumetric displacement with great accuracy.
Two methods of measuring droplet volume are described. Under one approach, an imaging system is trained on the forming droplet, with its output provided to a processor. With the focal length of the imaging system known, the processor determines the volume of the nearly symmetric droplet based on its two-dimensional image. A second approach to measuring droplet volume employs a dispensing tip that bends as the forming droplet grows. A laser beam is reflected off the bending tip and onto a position sensitive detector (PSD). The spring rate of the tip and the density of the dispensed fluid are determined in advance, such that the position of the reflected beam provides a measure of the mass of the droplet. The volume of the droplet is then calculated based on its mass.
The invention further contemplates a complete system in which the dispensing tip is moved into a container such as a blood vial from which a sample of fluid is withdrawn, the tip removed from the container and moved into position over a receptacle, and a precise volume of the withdrawn fluid dispensed into the receptacle.


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