Label-free cell sorting

Using intrinsic cell characteristics

Cell sorting is an essential technique in cell biology research and in many diagnostic and therapeutic applications. Especially sorting methods that refrain from the biochemical labeling of cells are in high demand. Several microfluidic techniques provide solutions to this by making use of the intrinsic characteristics of cells like size, shape, electrical polarisability or hydrodynamic properties. Further advantages: microfluidic technologies operate on the micrometer scale and require small sample volumes.

An asymmetrical array of pillars

One of the microfluidic label-free sorting techniques is deterministic lateral displacement (DLD). In short, this is a way to separate particles in fluids by driving them through an array of regularly placed pillars. By positioning the pillars at determined angles, particles of different sizes are forced into different streamlines and are this way separated. The pattern of the array of pillars is determining the streamlines of the particles.

Blood tests in the battle against sleeping sickness

One of the projects Micronit was involved in over the past few years, was the Lapaso project, initiated by Professor Jonas Tegenfeldt of Lund University. In this project, DLD was used to detect a certain parasite in human blood that causes sleeping sickness. Sleeping sickness is a disease found in rural areas of sub-Saharan Africa. It is spread by the tsetse fly and infected patients suffer from painful symptoms. If not treated, the disease is fatal. Quick examination of the blood is therefore of the essence. The DLD sorting method, based on particle morphology, proved to be very useful.

To get a good impression of the DLD-process, take a look at this video, published on the Lapaso website, and made by Stefan Holm, one of the project’s participants.

 

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Polymer

For this project, a thermoplastic polymer chip was developed that contains an array of pillars as small as 20 µm in diameter. Developing the first version of this chip in polymer was a cost-driven choice. When the chips need to be replicated in larger volumes, the process can be easily switched into injection moulding at a reasonable cost and without losing much time.

Silicon/glass

At Micronit GmbH, a DLD-chip was developed in silicon and glass. Both silicon and glass are known for their chemical and biological inertness. Besides, both materials can be cleaned from biological deposits, and are therefore very suitable for reusable products. The hybrid combination of silicon and glass makes a sustainable and reliable chip. To test the functionality of the chip, fluorescent microparticles were used.

Sorting by size

DLD is a process that sorts microparticles of different sizes. During the testing phase of the chip, several images were made of the DLD process. Observations show that larger particles follow a different trajectory than smaller particles. Large particles move in line with the row of pillars, thus laterally steering away from the streamline because of the diagonal placement of the pillars. Smaller particles on the other hand, do not divert from the streamline. Instead, they continue to follow the direction of the streamline and therefore change between the rows of pillars.

 

The following image shows the trajectory of a large particle. You can see how it stays in its row and therefore diverts from the streamline.

The image below shows the trajectory of a small particle. You can see that it continues to follow the streamline and therefore changes between the rows.

(For both pictures, keep in mind that the image is made up of several recordings in order to show the whole trajectory. The particles were coloured red afterwards. The arrow shows the direction of the streamline.)

Various applications

The goal of this project was to make a microfluidic DLD-chip that can be used for as many different applications as possible. This is a novelty, for up until now, DLD-chips were often developed for a dedicated use. In these cases, the connection system for example could only be used with specific devices. By choosing durable materials and making the chip suitable for general use, a multi-purpose, sustainable microfluidic DLD-chip was developed.