Filtration is a simple technique used to separate solid particles from suspension in a liquid solution. There are many filtration methods available, but all are based on the same general principle: a heterogenous mixture is poured over a filter membrane. The filter membrane has pores of a particular size. Particles larger than the pores will be unable to pass through the membrane, while particles smaller than the pores will pass through unhindered. Additionally, all liquids will pass through. The final result of a filtration process is a collection of residue on the filtration membrane. This residue is therefore effectively separated from the rest of the mixture that passed through the membrane.
The filtration process can be mediated by the force of gravity. This is the simplest way to achieve a separation. A common example is the filter paper used in drip coffee makers. The coffee grounds are larger than the pores of the coffee filter so they stay in place while the hot water can pick up the coffee oils, flavors, and caffeine molecules and travel through to the pot below.
In the laboratory it is often impractical to wait for gravity to separate a mixture. In these cases we can use a filtration process that employs a vacuum to pull the liquid and small particles through the pores of the filtration membrane. The suction of the vacuum greatly improves the speed of the filtration process. Similarly, a filtration centrifuge can be used to rapidly separate a mixture. The centripetal/centrifugal force of the centrifuge pushes the liquid and small particles through the filter membrane while the large particles remain.
The most important step of the filtration process is determining the size of the particle you are trying to separate. Then you can choose a filter membrane with an appropriate pore size. Filter membranes can be extremely small, on the order of micrometers, which is around the size of an individual cell. The filtration process should be chosen with the goal of maintaining the viability of separated products. For example, the use of vacuum filtration is perfectly fine for chemical products, but may not be optimal for cell isolation due to high force that could damage the cell membrane
There is an alternative to conventional filtration; it is called tangential flow filtration, where the flow of liquid is parallel to the filter membrane pores. The benefit of this type of filtration is that the build-up of material at the pore barrier is greatly reduced. The laminar liquid flow sweeps larger cells, protein, or molecules away from the pores throughout the filtration process, which leaves the pores open for the smaller particles to be filtered out. This is a self-cleaning filtration technique, and it can be more gentle for cell filtration.
Both filtration techniques are being incorporated into lab-on-a-chip devices in which cells need to be separated out of whole blood prior to a protein assay or chemical analysis of the serum. Nanometer-sized structures are patterned onto such devices in a way that separates the blood without the need for a centrifuge, and greatly simplifies the device for at-home use or field deployment.
Another potential isolation method that can be used in lieu of a filtration process is biomagnetic separation. The benefits of biomagnetic separation include specificity, speed, and product viability. The superparamagnetic nanoparticles used in biomagnetic separation are easily surface-functionalized to specifically target a specific cell, substrate, or molecule. Another great benefit of biomagnetic separation is speed of separation and cell viability. It is important to note that these benefits are only seen with well-engineered separation racks that produce a homogeneous force throughout the working volume. A properly engineered biomagnetic separation rack won't cause cell lysis or bursting.
The optimal filtration process always depends on the experimental parameters and goals. These can vary across experimental and industrial scenarios, but the basics of the filtration process remain the same.