Engineering for life sciences: Sound waves a 'forcefield' to corral single cells

Using ultrasound, Monash researchers create a regular grid of wells
like an egg carton. The cells sit neatly in the wells, like marbles.
Image: katherinemaries.com

Monash researchers in a cross-disciplinary collaboration have developed an ingenious technique for corralling single cells using sound waves.

Single cell analysis is a staple method of investigation in life sciences as populations can be very diverse. Averages from samples of many cells do not explain the range of behaviour from individual cells and existing systems for cell capture have  drawbacks.

Associate Professor Adrian Neild explained, “We’ve developed a system that holds cells in a regular pattern, which is necessary to understand behaviour over time.

“If you have a solution of cells, and want to know how they respond to certain treatment, you can get a snapshot of the number of cells which have responded in a certain way. In order to get time history information, you need to know how a certain cell responded in one way to a stimulus, then responds to a subsequent stimulus, ensuring you are still looking at the same cell.

“Our system allows us to make sure we are looking at the same cell, as each cell is held in one location. In addition, it allows us to look at and track many individual cells, because of that very fact - each cell is held in its own unique location. Hence, information can not only be gathered about each cell individually, but there are enough cells to give us a population of individual cells which can yield statistical data.”

The team established a technique for inducing a force field using ultrasound which pushes cells to the force’s potential minima. Associate Professor Neild said, “The way to visualise this ‘potential minima’ is to imagine an undulating landscape. Effectively we have a series of round valleys, and the cells roll to the bottom of each.

“We do two things in designing these valleys. Firstly we make them align in two directions, so we get a regular grid of hills separated by a regular grid of wells (like a tray for holding eggs). We also make the valleys very small, so that there is only enough space in each for a single cell, so that we can hold the cells individually.

“To make these valleys, we excite our fluid containing the cells with ultrasound which causes ultrasonic standing waves to form. The best analogy is that of a piece of string being held at one end and shaken at the other - at the right frequency you will get a series of points which do not move, separated by a series of points which move strongly. In an ultrasonic field you get a series of locations in which there is no pressure variation separated by locations with strong pressure variations. The cells move to the locations of no pressure variation. This technique also means our cells don’t get damaged, as they do in optical, magnetic and electrical capture, or adhere to the container as they do in mechanical capture."

Professor Magdalena Plebanski, Department of Immunology and Pathology, said, "The research arose from an inter Faculty collaboration (Medicine and Engineering), and is a testament to the synergy of cross-disciplinary team work."

The team used two different types of cells, lymphocytes and red blood cells infected by the malarial parasite, in order to assess whether the technique could be more widely used.

Reference:
David J. Collins, Belinda Morahan, Jose Garcia-Bustos, Christian Doerig, Magdalena Plebanski & Adrian Neild. Two-dimensional single-cell patterning with one Q1 cell per well driven by surface acoustic waves. Nature Communications, 6:8686. DOI: 10.1038/ncomms9686