I guess you have experienced a few times a sound that is so strong, particularly the "bass" part of a sound, that it feels like it is pushing you (usually the word is "pumping").
So it should not come as a surprise that researchers have thought to use sound for moving particles. The difficult part, of course, is to make sure that you move the particle in the desired direction to the desired place.
A team of researchers spread out in three universities, Carnegie Mellon, Penn State and MIT, have created a sort of acoustic-tweezer that can be used to manipulate object at cellular size.
They are using ultrasounds to create tiny pockets (see figure) to pick up and move a single cell (or clusters of cells). The resulting system is very precise and can deliver the cell, without disturbing it, to the desired location creating 2D and 3D structures. This looks promising for printing tissues and organs where the exact location of cells matter.
Tissue and organ structure is of fundamental importance. As an example, the researchers mention the cardiac tissues. A human heart contains some 2 billion muscle cells and they need to be organised in very specific ways to ensure a smooth functionality. Alteration in their structure, as it may be the result of a stroke, can hamper the functionality. Hence, if a doctor wants to repair the damage it is not enough to provide new working cells, they need to be organised and included in very precise positions.
There are a few methods that allows researchers (we are still in the lab side...) to manipulate cells to create tissues, but so far it has been impossibile to achieve the desired precision without damaging the cell (a cell don't like too much to be pushed around). The sound, on the other hand, is a much more gentle way of moving a cell.
The device created by the researchers can focus 3 sound waves to create a sort of pocket capturing the cell and then this pocket can be moved to the desired location placing the cell where it should be.
In their paper they present the result of one experiment where they have been able to create complex cellular structures by using this method.