Field of the Invention

The present invention relates to a method and apparatus for inspecting fluid flow in a microfluidic system.

Background of the Invention

Microfluidic systems are used for decreasing the size required for performing analysis or transport or distribution of liquids. Examples are so-called lab-on-a-chip devices for performing analyses in a confined "miniaturized" area where reactants may be ready for use and only a sample to be analyzed need to be injected. These devices may be easily made portable for field analysis and may be disposable. Medical tests are one important application of these devices. Devices of this type may also operate according to a predetermined sequence for injecting reactants, analytes and possible rinsing solutions to the microchannels of the device, and may be reused several times. Microfluidic systems that operate automatically are gaining importance in analytic work that requires processing a large number of samples, such as in DNA research. Examples of microfluidic systems are shown by EP 1813348, US 6192939, US 6448090, US 2003/0092172, US 2005/0220629, US 2006/0228259 and US 2007/0166199.

Various liquids may be injected and extracted by a pneumatic device, which may be programmed to follow the sequence. A pneumatic system that is well-suited for this purpose is shown in WO 2006/117436.

DE10125126 shows an image processing system for detection of a reaction product on a detector surface or in a detector volume of a miniature laboratory where several parallel reaction processes take place. The system is arranged to detect a change in an optically detectable property which is a response to the reaction product. Thus, the system is used for analysis and for collecting reaction process data which could be processed further. The flow behaviour of liquids in microchannels of a microfluidic device is a key factor for proper operation of the system. The microchannels where the flow of various liquids takes place have small dimensions and contain bends and sometimes complicated circuitry for directing different flows to desired spots and for mixing different flows. It is therefore of importance that the dynamic properties of the liquid and characteristics related to the flow of the liquid can be monitored to ensure that the device is working without disturbances. There is also need for testing microfluidic systems, for example as response to various parameters, for example to changes in a pneumatic system that controls the propagation of different streams through the system.

Up to date, there has not been a feasible method and apparatus for inspecting the behaviour of microfluidic systems that could give precise quantitative data of the dynamic behaviour or flow-related characteristics of a liquid that is passed through the system.

Description of the Invention

It is the purpose of the invention to provide a method and apparatus which make it possible to inspect fluid flow through microfluidic systems. The method is mainly characterized in that the flow is inspected by means of machine vision. Machine vision involves taking several consecutive images of a flow in a microchannel or any other parts of the system (passive valves, constrictions, enlargements, connecting points etc.) and performing automatic analysis on the basis of differences between consecutive images. For example the movement of a flow front of a liquid in a microchannel (filling of the microchannel) or the movement of the tail of the liquid (evacuation of the microchannel) can be monitored by machine vision.

The novel system is particularly suitable for characterization of liquid plug flows, and it may be used for improving the research and development work of academic and industrial research teams working with liquid flows in microchannels. According to an advantageous embodiment, the method may have one or several following functions:

1. Flow rate, flow velocity, displacement and location measurements of liquid plugs in transparent microchannels using machine vision technology

2. Measurement of static and dynamic contact angles (both front and rear menisci) of liquid plug flows in transparent microchannels using machine vision technology

3. Characterization of mixing of two or more liquids in transparent microchannels using machine vision technology

4. Characterization of the number and size of air bubbles in transparent microchannels and chambers using machine vision technology

5. Measurement of other quantities using additional sensor, such as a pressure drop over a liquid plug in a microchannel using a pressure sensor

All the measured data and images are recorded for further analysis.

The system is modular such that the aforementioned functions can be included in the system in any combination.

The developed system can be used for enhancing phases of la-on-chip products. The inspection of liquid flow is important in design and characterization of microfluidic chips, in production of chips and in the use of the chips. The developed method and apparatus enhances the R&D work especially in microfluidics but also in chemical microsensors, biosensors and various other detection methods by the data acquisition of fluid flows in microchannels. It can be used when designing new transparent microfluidic cartridges and chips for lab-on-chip, μTAS and point-of-care applications for example. In chip production, the system can be used for quality control and in chip use for chip behavior control. The developed system provides versatile quantitative data about the behavior of various sample liquids (whole blood, serum, plasma, saliva, food and beverage samples, process samples, environment and waste water samples, etc), buffers, reagents, washing liquids and gas bubbles in microchannels fabricated on different materials, having different cross-sectional shapes, dimensions and surface roughness, consisting of passive valves with various geometries, and having various functional coatings e.g. dried chemistries or hydrophobic / hydrophilic valves. In addition to the development of diagnosis, assay and analysis chips and cartridges, the system can be used for the development and verification of various analytical, numerical and data- based liquid flow and fluid behavior models in microchannels.

The invention will be described in more detail below by means of a general setup of the apparatus and a specific example of performing the inspection, which shall not be regarded as restrictive.

Fig. 1 shows a general principle of the system. The system comprises imaging means (digital camera) arranged to take consecutive images of the microfluidic device (miniaturized channel structure), a data line form the imaging means to an image and data processing unit (measurement PC) that contains an image processing algorithm and a calculation algorithm for determining a characteristic of the flow so that it can be represented in numerical form, and means of displaying the results (illustrated by a square in the figure). The image and data processing unit can contain several image processing algorithms and several calculation algorithms for determining various characteristics of the flow on the basis of the image data from the same flow. The system may also comprise a pressure sensor for measuring a driving pressure (positive or negative) of a pneumatic control device that is arranged to give the necessary movement energy for the liquid in the microfluidic device and to control the supply and extraction of liquids to and from the microfluidic device, respectively. The pneumatic control device may have the structure according to the above-mentioned WO 2006/1 17436. The pressure sensor is connected to data line to the means of displaying the results, for displaying the pressure value along with the results obtained from image processing.

One embodiment of the measurement system exploiting machine vision includes the following parts:

• Digital camera + optics

• Illumination • Image processing algorithms

The imaging in the system can be done using e.g. FireWire camera and macro video zoom lens. As a light source, and in order to create uniform illumination for the target a LED based ring light is used (16 LEDs, 8 white light, 8 red light). For controlling flows, a syringe pump or an accurate pressure generation unit can be used. Image data from the camera is transferred to a measurement PC. In the measurement PC, the image data is processed using image processing algorithms. Using the algorithms, the measurement quantities such as flow rate, velocity, displacement and dynamic contact angle are calculated. A pressure measurement with a proper sensor can also be included in the system. The characteristics of various parts are only exemplary and do not restrict the scope of the invention.

Instead of a FireWire camera, any camera with a large enough image size and fast enough frame rate can be used. If channel structures are in micro-/nanoscale, the camera should be attached to a microscope. Optics with a constant focal length is recommended for the camera because of the better calibration possibilities and more accurate measurements. Image processing can be also done using various programming languages. As illumination, any light source with ability to create uniform and accurate illumination is good. Standard ring lights available in the market are acceptable, also back lights can be considered.

The microchannels in a microfluidic system have typically cross- sectional areas less than 2 square millimeters. The walls limiting the interior of the channel have great influence on the flow. Therefore, it is important to follow the behaviour of the liquid and its interaction with channel walls with great accuracy.

The image processing methods in the algorithms consist of arithmetic subtraction of consecutive image frames and thresholding the subtraction result to a binary image. The result of the subtraction represents the moved liquid column between two image frames. From the binary image, the area and length of a blob can be calculated easily and by these, the flow rate (assuming that channel height is known), velocity and displacement can be determined. Coordinates of the front meniscus of the liquid column can also be located and by applying a circle fitting to the coordinates, dynamic contact angle can be determined. These methods are depicted in Fig. 2, where the image processing steps are illustrated on the left hand side, showing original image frame (top), result of subtraction (middle) and binary image (bottom) with front meniscus coordinated located. The dynamic contact angle measurement method using the circle fitting method is shown on the right hand side.

The measurement system gives possibility to automatically measure quantities which have been so far difficult to measure such as dynamic contact angle and instantaneous displacement or total displacement (wetted region in a channel). However, the invention is not restricted to only measurement of these flow characterics but it can be use for the inspection of all flow phenomena mentioned in the present description or covered by the enclosed claims.