Microfluidics: FAQ on basics, production and application

Microfluidics deals with the targeted handling and analysis of the smallest quantities of fluid in structures with micrometer-sized dimensions. The technology uses the physical properties of fluids, which exhibit different behavior on a micrometer scale than on a macroscopic level, for example, through dominant surface forces and laminar flow behavior. "Laminar" means that a fluid moves in ordered layers without turbulent mixing.

This allows the systems to work passively without the need for external pressure, as through a pump.

Typical applications can be found in medical technology, biotechnology, chemistry and environmental analysis. The controlled movement of very small fluid volumes allows, for example, precise diagnoses from minimal amounts of blood or the automated execution of reactions with high repeatability.

Microfluidic systems consist of a network of micro channels, chambers and functional units on substrates such as glass, silicon or plastic. These structures are often only a few micrometers wide. The devices are designed to perform specific tasks such as mixing, separating or dosing a fluid.

Micro elements such as valves, filters, sensors or integrated electrodes can take on additional functions. In combination with digital control, highly complex processes can be implemented in a single device, for example the analysis of blood samples or the production of microemulsions from different fluids.

Various microfabrication technologies are used to produce microfluidic systems. These include

• photolithography (especially for glass or silicon substrates),
• hot stamping or injection molding (for plastic chips),
• soft lithography with PDMS (polydimethylsiloxane),
• laser micromachining and CNC micromilling,
• 3D printing for complex geometries.

The choice of process depends on the application, the material required and the desired production quantity.

A microfluidic channel guides a fluid through the system. Due to the small dimensions, different physical conditions exist on the micro level than in macroscopic structures. Flows are usually laminar.

They are controlled by pressure differences, electric fields or capillary forces. The small cross-sectional area of the micro channels allows precise dosing and a high reproducibility of chemical reactions.

Micro pumps can be important for the movement of fluid in microfluidic systems. Various principles are used here:

• peristaltic pumps for continuous flow control,
• pneumatic or piezoelectric microactuators,
• electrokinetic methods such as electrophoresis or electroosmosis,
• passive mechanisms such as capillary forces or gravity.

The choice of pump influences flow rate, pulsation and the complexity of system integration. Passive mechanisms are often used in disposable chips for diagnostics.

In medicine, microfluidics enables the construction of compact analytical devices with high sensitivity and low sample consumption. Applications include

• point-of-care diagnostics (e.g. for infections or metabolic diseases),
• blood analysis, including cell analysis and plasma separation,
• production and characterization of lipid nanoparticles for mRNA vaccines,
• organ-on-a-chip systems for drug testing,
• cell cultivation and single cell analysis.

These technologies help to speed up diagnoses, relieve laboratory capacity and develop the basis for personalized therapies.

A lab-on-a-chip (LoC) is a miniaturized laboratory on a chip made of polymer materials. It combines various laboratory functions, such as sample preparation, reaction, separation and detection on just a few square centimeters. A device like this works automatically using external drives or passive mechanisms and requires only minimal amounts of reagents and sample material, i.e. a fluid to be analyzed.

In a lab-on-a-chip, biological or chemical processes take place in microfluidic structures. Liquids are passed through the system via micro channels. There they are mixed with reagents, filtered or tested for certain analytes. Sensors, usually optical or electrochemical, detect the results and provide analyzable signals. One example of this is rapid viral tests that make the presence of viral proteins visible through color reactions.