The_Scientist_-_December_2018

12.2018 | THE SCIENTIST 27

MODUS OPERANDI

© GEORGE RETSECK

AT A GLANCE

J

ust as music pulls some folks to the dance floor while others

remain stuck to their chairs, sound can now be used to pull some

cells loose from their substrate while other cells stay put. Cells

attach and release from the extracellular matrix and from each other

for a variety of physiological reasons. But existing methods to measure

the intensity and kinetics of these interactions either lack precision or

are exceptionally laborious, explains molecular biophysicist Gijs Wuite

of Vrije Universiteit in Amsterdam.

While thinking of ways to achieve both accuracy and high through-

put, Wuite says, he saw a movie showing microscopic organisms being

manipulated with sound waves. He wondered whether such acoustic

forces could also be used to explore molecular and cellular interactions.

So Wuite and colleagues developed single-cell acoustic force spec-

troscopy (scAFS), which uses acoustic waves to test whether cells are

adhered to a given substrate (cells or molecules) and if so, how firmly.

Cells are introduced into a buffer-filled, microscope-mounted micro-

fluidic chamber, the bottom of which is coated with a substrate and

is in the focal plane of the microscope. Once the cells have settled,

an amplifier creates an acoustic standing wave in the entire chamber.

Cells that are not bound to the substrate rise up to the sound wave’s

nodal plane—where the wave’s force is at its minimum—and appear

blurry as they leave the focal plane. Increasing the amplitude of the

acoustic wave exerts greater force, detaching loosely bound cells first

and thereby revealing the strength of the different cells’ interactions

with the substrate.

Wuite and colleagues have used scAFS to analyze interactions

between T cells and fibronectin, an extracellular matrix protein. Such

interactions, when they occur along blood vessel walls, allow T cells

to slow, stop, and exit the vessel into neighboring tissue. In their

proof-of-principle experiments, the researchers showed that the

T cells’ activation led to faster fibronectin binding, while the cells’

adhesive strength, once they were attached to the protein, remained

the same after activation.

“This device is particularly nice as it can make these measurements

on many cells simultaneously,” says Bruce Drinkwater, an ultrasonic

engineer at Bristol University in the UK who was not involved in the

study. It’s also “quite a general-purpose platform that I think can be

used in a multitude of ways.” (Cell Rep, 24, P3008–16, 2018) g

A blast of sound differentiates cells that are untethered

from those that are stuck in place.

BY RUTH WILLIAMS

Dancing Cells

ADHESION

ASSESSMENT

Atomic force microscopy

Single-cell acoustic force

spectroscopy (scAFS)

APPROACH

A microscopic cantilever is attached to a cell that rests

on top of a substrate layer (cells or molecules). As the

cantilever is moved upward, adhesion between the cell

and the substrate causes the cantilever to bend, the

extent of which can be measured.

Within a microscope-mounted chamber, an acoustic

standing wave forces loose cells upward while those

bound to a substrate stay put.

THROUGHPUT

One cell at a time

Hundreds of cells

simultaneously

POSSIBLE FORCE

APPLIED

Typically less than 10

nanonewtons

1 nanonewton

COST OF

SETUP

Approximately

$150,000

Approximately

$100,000

GET UP, GET ON UP: A piezo-produced acoustic standing wave fills the single

cell acoustic force spectroscopy (scAFS) chamber, forcing non-bound cells to

move up to the wave’s nodal plane and out of the microscope’s focal plane.

Microscope

Glass

Fluid

Glass

Sensor

scAFS

chip

LED

Objective Substrate

Piezo

Focal plane