All but the simplest microfluidic devices are part of a system that includes the device, a base station, and reagents. The base station usually includes some clamping/alignment, sensors, and fluidic interfaces whether it is detection, pumping, or actuation including valving that regulate the flow of samples and reagents.

The device itself must meet many demands simultaneously. It must be able to accept the sample to be analyzed, and sometimes the device must take a precisely measured aliquot from an unmeasured sample. Removing the need to precisely measure the aliquot away from the user and automating this step helps reduce the level of expertise and training needed by the user. This simplification in turn increases the likelihood that the system would qualify for a Clinical Laboratory Improvement Act (CLIA) waiver which greatly expands the potential market for the system.

It makes economic sense to put expensive computer-controlled elements in the base station so that these costs can be amortized over many devices. This helps to keep the cost of the consumable device as low as possible. When the reagent volumes are large, common reagents such as assay buffers, wash buffers, and diluents can be stored on the base station for use by the device. This frees up space on the device and can simplify its design.

A microfluidic system must have the ability to move fluids (including the sample) in the device. In the most basic system, this may be done with capillary action and “passive” flow. However, most systems have some sort of pump. Air pumps have been used to push fluids through devices, but a common alternative is to use an onboard peristaltic pump to move fluids (such as assay buffers) around. Digital microfluidics use electronic “switches” to change the surface tension or magnetic properties of one region within a defined grid; by turning neighboring switches on and off, fluids can be moved as needed.

Chambers
Chambers are cartridge features that have particular designs critical to the specific assay process step. This could include mixing, aliquoting, extraction/lysis, amplification, detection, dried reagent storage, and so on. These designs are dependent on many factors such as fluid properties, process parameters, and performance specifications. As such, there are many ways to solve the same assay process problem.

For instance, various hormones are tightly bound to their carrier proteins in serum or plasma, and specific chemical agents are needed to release these analytes so they can be measured. Because the release agents vary from hormone to hormone, it makes sense to put them on individual devices. Chambers are created on the device to hold these reagents. Some are in liquid form, but a freeze-dried pellet of reagent can also be placed in the chamber for later reconstitution.

Liquid Reagent Storage
Liquid reagent storage containers have their own requirements due to potential water loss. Liquid reagents are often placed into containers (foil blister or lidded plastic) that are welded to the device. Foil blisters are highly desirable due to shelf life and concentration variability requirements, and some thermoformed plastics have great water vapor barrier properties.

Channels
Fluids generally move through the microfluidic device between chambers and storage containers via channels. Channel sizes must be carefully designed to make the device functional. Channels that are too small will create back pressure that requires larger, more expensive pumps. These pumps may compromise the integrity of seals in the device. If the channels are too large, voids and bubbles may form that can interfere with the analytical reactions and the signal readouts.

A good starting point for determining channel size is the sample size needed for the analysis. Some low concentration analytes can require analytical samples of 100 uL or more. Add in the other reagents in the reaction, and the result is the size of slug of liquid that is to be manipulated. Volumes can be converted into length, width, and height dimensions with simple geometry.

For liquids to flow, the channels and chambers (see below) need to be covered. This is done by adding a cover film over the top of the device after the onboard reagents have been added. Sometimes the film is sealed to the device with an adhesive, but ultrasonic welding, laser welding, or heat bonding are also commonly used.

Valves
One of the real advantages of microfluidic devices is the ability to control the flow of liquids with valves. With valves, reagent streams can be directed where and when they are needed.

For example, consider a microfluidic device that uses an immunoassay to measure Troponin (a heart attack marker) and uses glow chemiluminescence as the readout chemistry. The whole-blood sample is added to the device, plasma filtered, and pumped over the capture antibody which has been immobilized within the device during manufacturing. Sensitivity is improved by washing the captured analyte by continuing to pump assay or wash buffer past the capture zone. At the appropriate time, valves actuate to connect a previously closed chamber containing the detector reagent. The flow resumes and carries the detector reagent to the captured analyte, and unreacted detector reagent is washed away minimizing background noise and improving sensitivity.

The only way that this sequence of reactions can take place is with careful control of the flows of the various reagents. If all the reagents were mixed with no sequencing, the result would be a tremendous amount of light being generated independent of the amount of Troponin in the sample with no differentiation between patients having heart attacks and healthy patients with indigestion.

There are multiple types of valves that can be employed in microfluidic devices. The exact type of valve best suited for a particular application should be discussed with experienced professionals.

Optical Properties
If the readout is light based, the device must meet is its optical properties. This means the material must meet the wavelength transparency for the methods needed. Fluorescence and chemiluminescence are two common detector chemistries that are used in microfluidic devices.

For fluorescence or chemiluminescence to work properly, the light paths must be controlled. If optical properties differ from device to device, it will be nearly impossible to tell if an altered signal is due to variations in the analyte concentration or simply due to light loss.

Fluid Waste Storage
Finally, there is the requirement for onboard storage of waste for all in-process reagents. Since human samples always carry the risk of contamination with pathogens, it requires a sequestration and containment of this waste for proper disposal. The best way to do that is to keep all the fluids that encountered the potentially infectious sample fully contained within the device, which is then discarded by the user. For additional protection, an absorbable sponge or a sealed chamber can be the best way to ensure nothing escapes.

By now, it should be clear that building a microfluidic device is a team effort. Input is needed from the reagent standpoint, the device design experts, the device fabrication manufacturer, the base station engineers, and the software team to make a manufacturable device.

If your organization already has such a team in place, they may still benefit from discussions with us. Discussions between groups of experienced professionals frequently result in cross fertilization and better outcomes. If your organization does not have such a team, we are a great place to start.