Lab on a Chip
PAPER
Cite this: Lab Chip, 2014, 14, 2791
Received 14th February 2014,
Accepted 20th May 2014
DOI: 10.1039/c4lc00202d
www.rsc.org/loc
Focusing of sub-micrometer particles and bacteria
enabled by two-dimensional acoustophoresis†
M. Antfolk,*a P. B. Muller,b P. Augustsson,ac H. Bruusb and T. Laurell*a
Handling of sub-micrometer bioparticles such as bacteria are becoming increasingly important in the
biomedical field and in environmental and food analysis. As a result, there is an increased need for less
labor-intensive and time-consuming handling methods. Here, an acoustophoresis-based microfluidic
chip that uses ultrasound to focus sub-micrometer particles and bacteria, is presented. The ability to
focus sub-micrometer bioparticles in a standing one-dimensional acoustic wave is generally limited by
the acoustic-streaming-induced drag force, which becomes increasingly significant the smaller the particles
are. By using two-dimensional acoustic focusing, i.e. focusing of the sub-micrometer particles both
horizontally and vertically in the cross section of a microchannel, the acoustic streaming velocity field
can be altered to allow focusing. Here, the focusability of E. coli and polystyrene particles as small as
0.5 μm in diameter in microchannels of square or rectangular cross sections, is demonstrated. Numerical
analysis was used to determine generic transverse particle trajectories in the channels, which revealed
spiral-shaped trajectories of the sub-micrometer particles towards the center of the microchannel;
this was also confirmed by experimental observations. The ability to focus and enrich bacteria and other
sub-micrometer bioparticles using acoustophoresis opens the research field to new microbiological
applications.
Introduction
The ability to control and process sub-micrometer bioparticles,
e.g. bacteria and subcellular organelles, is becoming
increasingly important in biomedicine and in environmental
and food analysis.1,2 Methods such as blood culture of
bacteria1 and subcellular fractionation3 are, however, laborintensive,
complicated, and time-consuming, and new technologies
are being sought to redress these shortcomings.
Microfluidics offers a means of automated handling and
analysis of sub-micrometer bioparticles with the associated
advantage of a continuous mode of sample handling.
Thus, considerations such as initial sample volume or batch
volume are no longer relevant. Previously used methods for
handling of sub-micrometer particles included filters,4,5
dielectrophoresis,6–8 inertia in combination with hydrodynamic
forces,9 magnetophoresis,10,11 deterministic lateral
displacement,12 and surface acoustic waves (SAW).13 These
methods have been mainly used for handling of bacteria and
particles of around 1 μm in diameter. Recently, SAW were
used to separate 0.5 μm polystyrene particles from 0.3 μm
particles,14 Stoneley waves were used to focus 0.5 μm polystyrene
particles at flow rates of 200 nL min−1,15 and acoustic
trapping has been used to successfully trap 0.1 μm particles
using seeding particles.16 Although acoustic seed trapping
gives good recovery of sub-micrometer particles and bacteria,
the system operates in batch mode, which is limited by the
capacity of the acoustic trap. In spite of these developments,
one common need is the ability to process sub-micrometer
particles in continuous-flow mode together with the possibility
of handling rare species in crude samples with high recovery
rates without previous sample preparation.
In this regard, the use of acoustophoresis in microfluidic
systems has attracted much attention in recent years as a
continuous-flow and non-contact mode method of separating
or enriching microparticles or cells while offering a reasonable
degree of throughput. The method involves the use of
ultrasound standing waves to focus cells or particles in the
nodal (or anti-nodal) plane of the standing wave according to
their intrinsic properties: size, density, and compressibility.17
Furthermore, this label-free and gentle18,19 method—which
operates independently of the biochemical and electrical
properties of the suspension medium—has been extensively
explored to separate, wash, or concentrate various biological
a Department of Biomedical Engineering, Lund University, Box 118, SE-221 00
Lund, Sweden. E-mail: thomas.laurell@bme.lth.se, maria.antfolk@bme.lth.se
b Department of Physics, Technical University of Denmark, DTU Physics Bldg 309,
DK-2800 Kongens Lyngby, Denmark
c Department of Electrical Engineering and Computer Science, Massachusetts
Institute of Technology, Cambridge, MA, USA
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4lc00202d
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