Surface acoustic wave actuated cell sorting (SAWACS)
Oftentimes, in both research and in clinical settings, it is important to sort cells- that is, to separate some group of cells into two or more groups. There are a few methods for accomplishing this out there already. In the big picture, there are two types of sorting techniques: droplet sorting and continuous flow sorting. In droplet sorting, the cells are somehow encapsulated into a droplet, and the droplets are sorted. The sorting can be accomplished by using liquid for the droplets that has very different polarizability, charge, miscibility, etc. than the surrounding liquid. However, droplet sorting has its disadvantages, as the encapsulation requires an extra step in the sorting process, and sometimes one may not want the cells to be encapsulated in a droplet for whatever it is one wants to do with the cells after sorting. Continuous cell sorting, on the contrary, does not require the cells to be encapsulated in the droplet but are separated continuously. One common way in which this is done is by selectively attaching magnetic beads to the cells, and then using a magnetic force to sort the cells. However, most continuous cell sorting schemes are slow as compared with the droplet sorting schemes. This paper presents a new method for continuous cell sorting that uses surface acoustic waves to actuate the cells.
A PDMS device is created with a micro-channel in which the cells can flow. By flowing another liquid into micro-channel through two side channels, the cells can be hydrodynamically focused. There are two outlets at the end of the channel: one to the 'waste' bin and one to the 'collect' bin. When the device is 'off', the cells flow to the waste bin.
The PDMS device is put into contact with a piezoelectric substrate. Piezoelectric materials allow electric energy to be transformed into mechanical energy (and vice versa). A surface acoustic wave (SAW) can be generated in a piezoelectric material by sending an electric frequency applied to an interdigitated transducer (IDT). The IDT is just two gold electrodes sitting on the substrate. The IDT's are comb-like in structure, having finger protrusions, and the spacing of the fingers determines what SAW frequency it can produce. An effective SAW can be produced when the frequency determined by finger spacing is set such as to match the resonance frequency of the piezoelectric crystal. Then a strong acoustic wave passes along the surface of the material. When it encounters a liquid on the surface, momentum is transferred to the liquid and the liquid can be actuated.
When a signal is sent to the IDT, a SAW is sent through the piezo substrate. When the wave encounters the fluid, it pushes it over (in the picture, to the right) as the liquid absorbs some of the wave's momentum. Thus the stream is diverted into the 'collect' bin for as long as the IDT is on. When the signal is turned off, the stream reverts back to its original position and continues flowing to the 'waste' bin. Thus the cells can be sorted left and right.
The group was able to achieve effective cell sorting for frequencies up to a few kHz. The response time of the method, that which sets the frequency for sorting, is limited not by the speed of the SAW (which is quite fast) but rather of the time required for the liquid stream to respond once it has been hit by the wave. This depends on the speed of the liquid and the length the liquid has to travel. In any case, kHz sorting is up there with some of the fastest methods available and is a solid achievement.
The group tests the 'gentleness' of their method by doing a cell viability test. A small amount of fluorophore is added to the cells which only labels cells which are alive. They found that 93% of their cells survive the sorting process, compared with 97% control (no sorting). This shows that the SAWACS method would be an effective tool when high cell viability is needed.
The SAWACS approach developed by the group seems to be an effective continuous cell sorting technique. It should be an important tool where small volumes and high viability of cells is needed.