BACKGROUND

The present invention relates to the precise control of fluids in small volumes, and in particular relates to the precise control of fluid mixing in sophisticated chemical reactions in such small volumes.

In many circumstances mixing is a relatively straightforward and well understood process. Physical agitation is applied to the components until they reach a desired condition, often a solution, dispersion, suspension, or emulsion, but sometimes a form such as a slurry.

In larger volumes, gravity, friction, viscosity, turbulence, container geometry, utensil shape, and available time may tend to dominate the mixing process.

In the context of small volumes, however, the factors that affect mixing change proportionally. In particular, forces such as viscosity, surface tension, and laminar flow become more dominant in small volumes and must be accounted for.

Mixing is important in many fields, but precise mixing of liquids in small volumes is particularly important in a number of areas of biology and chemistry such as analysis, screening, genetics, diagnostics, synthesis, and environmental monitoring.

The capability to efficiently mix small volumes enables multiple reactions to be carried out more rapidly and thus more cost-effectively. In other circumstances, biological and chemical reactions and testing (“assays”) require multiple steps, many or all of which require mixing small volumes.

In some cases, the small volumes may be driven by the cost of materials, while in others, the small volume is determined by the desire to carry out a large number of almost similar but not identical reactions (e.g., combinatorial libraries) and in yet other circumstances the small volume is determined because the amount of (for example) a biological or chemical sample to be tested is quite limited.

As one example, PCR (polymerase chain reaction) amplification of DNA is often carried out in picogram (pg) or nanogram (ng) quantities in microliter (ul) volumes.

Within the field of small-volume reactions, the “microtiter plate” or “microplate” has become a fairly standard tool in many circumstances. Although the term “standard” does not represent every possibility, relevant organizations e.g., the American National Standards Institute (ANSI) and the Society for Laboratory Automation and Screening (SLAS)—provide helpful guidelines and standard dimensions and definitions (www.slas.org/resources/information/industry-standards/; accessed Nov. 22, 2014).

Such standards allow plate manufacturers, manufacturers of automated plate handling equipment (particularly robotics), and end users, to make and use differently-sourced items conveniently and predictably. Thus, a microtiter plate that can conform to ANSI/SLAS standards is (for simple descriptive purposes) about 5 inches (128 mm) long, about 3.4 inches (85 mm) wide, and about 0.5 inches (14 mm) high. The plate contains 96 wells that have their positions and pitches (slope sides) defined by the ANSI/SLAS standards. The wells can hold between 100 and 200 μL of fluid.

Other standard plates include 384 wells or 1536 wells, in which case each well is proportionally quite smaller than the 96 well plate.

Because microtiter plates are used for reactions, in many circumstances reactant compositions must be (or should be) physically mixed in the wells in the small volumes used or available. Typical mixing devices for microtiter plates tend to use a rotational movement in which the plate is positioned on a horizontal platform and the platform is mechanically rotated or oscillated in the horizontal plane (i.e., parallel to the microtiter plate).

Such rotational mixing tends to be time-consuming and often requires adding a plate cover to prevent spilling and cross-contamination between and among wells. In turn, the plate covering step is typically carried out manually, thus defeating some of the automation advantages offered by a standardized plate. To the extent that plate-covering (and therefore cover-removing) steps can be carried out automatically, they require added structure with resultant associated inefficiencies and higher costs. At a minimum, because physical space is often at a premium in the laboratory context, additional mechanical devices compete for such premium space.

Published international application WO2006027602 explores the possibility of mixing within small droplets by generating a vibrational motion rather than a rotational one, and other disclosures (e.g., U.S. Pat. Nos. 7,980,752 and 6,578,659) attempt to use ultrasonic energy to mix small volumes in microtiter plates. U.S. Pat. No. 8,662,739 uses a side-to-side movement to attempt mixing.

Among end users, frequent concerns about microplate mixing include the invasive nature of some mechanical stirrers; the necessity of sealing plates in some circumstances; the heating effect of mixing which can activate or inactivate certain components in an undesired manner; problems with automation compatibility; the need for a large dead volume in the well; slow mixing times; overly aggressive or uncontrolled mixing; incompatibility with various microplate formats and materials; and large instrument footprints.

Therefore, a need exists for better mixing capabilities for small liquid volumes that are compatible with the requirements and advantages of microtiter plates in an automated context.

SUMMARY

In one aspect the invention is a microtiter plate mixing control system that includes a frame, a spring or other suspension system (i.e., capable of storing potential energy) attached to the frame, and a magnetically responsive horizontal platform supported for reciprocating vertical motion on the frame by the spring. A solenoid is adjacent the platform for moving the platform in the vertical axis on the spring, and a proximity sensor is adjacent the platform for determining the vertical position of the platform with respect to the frame. A controller is in communication with the proximity sensor and the solenoid for driving the solenoid and moving the platform in response to the proximity sensor.

In another aspect the invention is a mixing control system that includes an optical source and an optical sensor responsive to the source that together define an optical path that includes at least one plate well when a microtiter plate is on the magnetically responsive horizontal platform. The optical detector is in communication with the controller to provide the controller with information based upon the well's effect on optical transmissions between the optical source and the optical detector along the defined optical path.

In another aspect the invention is a method of mixing compositions in small volumes of liquids by positioning a microtiter plate containing compositions to be mixed on a suspension mounted horizontal platform, initiating resonant motion of the horizontal platform in the vertical axis on the suspension system, measuring the vertical position of the platform as the platform moves, and driving the vertical motion of the plate on the suspension system in response to the measured vertical position.

In another aspect the invention is a method of controlled mixing of compositions in small volumes of liquids by driving the resonant motion in the vertical axis of a horizontally oriented, microtiter plate in response to the inductively measured vertical position of the plate.

In another aspect the invention is a microtiter plate mixing control system comprising a microtiter plate on a moveable metal platform that can be magnetically driven, a proximity sensor adjacent the platform to identify the position of the platform, a solenoid responsive to the sensor to drive the platform, the plate, and any contents of the plate at a desired resonant frequency, and a microcontroller that receives input from the sensor and provides output to the solenoid.

In another aspect the invention is a method of mixing compositions in small volumes of liquids that includes the steps of positioning a microtiter plate containing compositions to be mixed on a suspension mounted horizontal platform, initiating resonant motion in the vertical axis of the horizontal platform on the suspension system, measuring the optical turbulence of the composition in at least one well in the microtiter plate, and driving the vertical motion of the plate on the suspension system in response to the optical turbulence.

The foregoing and other objects and advantages of the invention and the manner in which the same are accomplished will become clearer based on the followed detailed description taken in conjunction with the accompanying drawings.

