Lab on a chip devices are used to create laboratory results with one single device. As input one needs a sample; for the device to be able to deliver accurate and quantitative data accurate volume metering is required. Most sensor principles can count molecules well, but if the original sample volume is unknown it is impossible to calculate concentrations. For most assays multiple dilution and or washing steps are required in a timely matter. This requires accurate control of liquids with regard to fluid movement and timing. In the following article the use of passive microfluidics based on capillary action will be demonstrated to provide all the above requirements.
Capillary action provides a convenient means for moving liquid through microfluidic devices, especially when external actuation using pumps is undesirable. Once the meniscus of the liquid has reached the end of the microfluidic channels the capillary force will reduce to zero and the fluid flow stops. In order to make a fully functional lab on chip product one needs a mechanism to control the flow of liquids. In pressure driven flow mostly some kind of mechanical valve structure is used. For capillary action an alternative method is provided that allows valving behavior without the need of any moving parts. Basic flow control is accomplished by adjusting the cross-sectional area and the length of the microfluidic channels.
For many applications the filling of a microfluidic channel only is insufficient and more control over the flow is required, for example in order to deliver a number of different liquids in a specific timed sequence. In order to accomplish this capillary burst valves are used. Geometrically defined burst valves rely on an abrupt increase in the cross-section of a microfluidic channel causing the capillary filling to stop at the transition.
Now that the fluid can be stopped inside a microfluidic device, the next step is to implement a suitable method to actuate the valve such that the fluid can enter the channel downstream. Several options are possible but here we look at two methods specifically that can be used to generate predefined sequences or a method allowing external control with a minimum of hardware. Burst valves can be triggered as shown below using the arrival of a second fluid. This second fluid can be derived for example from the first fluid by adding a microfluidic channel with specific dimensions to create a certain delay.
Capillary Valve Actuation
Liquid triggered capillary valves work well, but delay times are fixed and are affected by the properties of the fluids which can change for example with temperature. Furthermore it is unavoidable that a small amount of the trigger fluid gets mixed with the main fluid. In order to obtain more control over the capillary valves some form of external actuation is needed. This can be accomplished using for example electrostatically triggered capillary valves. These operate using an electrode in contact with the fluid and a second electrode at a short distance from the capillary burst valve such that it is not in contact with the fluid. By applying a voltage over the two electrodes an electrostatic force develops attracting the meniscus of the liquid towards the second electrode thereby triggering the valve. This type of valve uses very little power and is therefore very suitable for use in handheld instruments.
Finally, metering specific volumes is required for certain applications. Microfluidic channels provide an excellent means for this owing to the tight tolerances that can be achieved using micro fabrication techniques. By placing a capillary burst valve at the end of the channel and a splitter at the start, the channel will fill with a precisely defined volume. The relative strength of the capillary force at different places inside the device is used to cleanly shear off the liquid at the junction. The isolated volume of liquid can then be manipulated using e.g. an electrostatic or liquid triggering of the burst valve.
The above demonstrates all the elements that are required to implement a multistep assays on a single microfluidic device that does not have any moving parts and can be controlled in an easy way with electrical signals.