Vortices, not Vortexing: Replacing the Centrifuge with a Lab-on-a-Chip

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In case you didn’t get the first part of my title, let me tell you a little about centrifugation. Centrifugation is a very common research technique. A solution is centrifuged to isolate suspended particles by spinning it around at high speeds. Depending on the weight of the particles and the force of the centrifuge, the heavier particles will form a pellet at the bottom of the container. The rest will still be suspended in fluid. Depending on which particles you’re after, you can continue doing this by removing the fluid and changing the forces in order to manipulate which particles form the next pellet. But in between each step the pellet must be resuspended and is vortexed (with a vortex mixer) to break up the pellet. Now, keep the definition of vortexing and the process of centrifugation in mind.

Researchers from the University of California, Los Angeles have proposed a ‘Centrifuge-on-a-Chip’ that would replace its bench top counterpart. The article, “Automated cellular sample preparation using a Centrifuge-on-a-Chip” by Di Carlo et al. was featured on the cover of the 2011 Issue 17 of Lab on a Chip. The article proposes a streamlined system that requires no external forces which, like I said before, makes it much more powerful. In order to evaluate the competency of new technology we need to compare it to a golden standard that accomplishes its task the best, no matter how long it may take. In this case, it is the centrifuge and we’ll monitor its three main jobs throughout this post:

  • Separation of cells by size or density
  • Concentration of cells
  • Labeling of cells via solution exchange
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The authors’ forceless Centrifuge-on-a-Chip is defined by its structure. It features a simple channel that suddenly widens. It is this widening that creates vortexes, enabling filtration. When the particles were flowing through the plain channel, they were held in place by a force that pushes them towards the wall and a force by the wall that pushes them away. When the channel widens (the wall effectively disappears) they succumb to the force pushing them away from the center of the channel. The rate that particles move out of the channel area, into the vortex region, is proportional to their size (greater than the square of the diameter). Therefore only particles above a certain threshold can be captured. The particles are able to be selectively released from their orbits by decreasing the flow rate.

Beyond simply organizing different beads, the authors validate the filtration process by separating and concentrating circulating tumor cells (CTCs) from dilute blood. Given that CTCs can be 33% to 900% greater than size than blood cells, they should be able to be separated using the vortices. This was confirmed by processing 10 ml samples with ~500 CTCs and 2.5 billion blood cells. The end result was a volume of 200 µl that recovered about 20% of the CTCs, which constituted 40% of the volume.

Now, so far we’ve been able to see the device filter cells by size and concentrate the cells, both things on our list. The only thing left to do is see if this is able to label the cells like a normal centrifugation process can. In order to traditionally label cells, they must first be incubated with the labels and centrifuged into a pellet. The fluid with extra labels must be removed and then the cells are resuspended. With the Centrifuge-On-A-Chip, the cells merely have to be caught in a vortex, exposed to the labels and rinsed. The traditional and new techniques were used to label intracellular proteins, cell surface proteins, enzyme substrates and DNA. I’m not a labeling expert, but the comparison between the two techniques seems pretty similar. There seems to be more of a difference for the labeling of the cell surface proteins, but I’m not sure if it is significant. They also demonstrated the ability to tag with primary and secondary antibodies, as well as microbeads. After 5 minutes the vortexed cells were labeled with the same number of microbeads as the traditional cells after 30 min (the same amount of labels was used for both techniques in all experiments). Further, after 30 minutes, the vortexed cells had twice the number of microbeads as the traditional cells. Safe to say, this technique can definitely label cells at least as well as the traditional method.

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The Centrifuge-On-A-Chip is clearly a viable contender against the traditional bench top centrifuge and its techniques. While there is no given price for each device, or how many devices would be necessary to provide the same output as a centrifuge, it will certainly cost less than thousands of dollars. Additionally, it is portable and can process the tasks in less time. When compared to the Centrifuge-On-A-Chip’s competitors, it still comes off fairly well. Some others are only able to filter cells and not concentrate them. Some immobilize the cells on membranes, which may prevent them from performing assays, or may clog the membranes. There was also no change in viability before and after the vortexing. However, it is necessary to dilute the blood before it can be processed, but I doubt that this would be negative enough to prevent someone from using this entirely. I suppose that we’ll just have to wait and see how quickly this can enter the market, and how cheaply it can be produced.

Reference:

ResearchBlogging.org

Mach, A., Kim, J., Arshi, A., Hur, S., & Di Carlo, D. (2011). Automated cellular sample preparation using a Centrifuge-on-a-Chip Lab on a Chip, 11 (17), 2827-2834 DOI: 10.1039/c1lc20330d