**5. Acoustophoresis**

Acoustophoresis (ACT) is the motion of particles when experiencing a surface acoustic wave (SAW) radiation pressure, either by standing surface acoustic wave or traveling surface acoustic wave [36, 37].

#### **5.1. Fundamentals of ACT**

et al. [30]. They exploit the paramagnetic property of deoxygenated hemoglobin of maturing erythrocytes compared to reticulocyte which is diamagnetic for the MAG-based separation. Jack et al. [31] demonstrate immunomagnetic isolation of diverse groups of magnetic bead-labeled tumor cells (with different levels of labeling), based on the surface protein expressions. The sample is introduced to multiple levels of magnetic sorter according to the adjustment of external permanent magnet distance to flowing magnetic-labeled cells, resulting in separation of the cells according to their epithelial cell adhesion molecule (EpCAM) levels into low, moderate, and high expression. Bacteria manipulation using MAG has been achieved by Wang et al. [32], who demonstrate the trapping of *Bacillus megaterium*, which are nonmagnetic bioparticles in a ferrofluid suspension while experiencing uniform external magnetic field. The sample continuously flows in a microfluidic channel with nonmagnetic island located in the middle of the channel. Due to the magnetic susceptibility difference between the island and the surrounding ferrofluid, the nonmagnetic bacteria experience magnetophoretic force and, thus attract the bacteria to the island, while the ferrimagnetic particle which comprises the ferrofluid is also attracted to the island; however, the bacteria and the magnetic particles are accumulated at different regions. Wang et al. [33] perform magnetophoretic concentration of avian influenza virus (H5N1) in continuous flow microfluidic system. The H5N1 magnetic nanoparticle complexes are formed using aptamerbiotin-streptavidin binding and injected to three-dimensionally printed magnetophoretic platform which experiences magnetic field from external neodymium magnets, to migrate the complexes from the original sample flow to phosphate-buffered saline carrier flow. Shim et al. [34] demonstrate the magnetophoretic capture of DNA-conjugated magnetic particles in microfluidic device by short-range magnetic field gradient exerted by micro-patterned nickel array on the bottom surface of the separation channel, as well as enhancement with oppositely oriented array of external permanent magnet for a long-range magnetic field gradient at the interfaces between magnets. The DNA is then collected by performing detachment from the captured DNA magnetic particle conjugates by enzymatic reaction with uracil-specific excision reagent enzyme. Magnetophoretic manipulation of protein has been achieved by Lim et al. [35] in which manipulation of Atto-520 biotin-streptavidin-magnetic particle conjugates is demonstrated. The magnetic force is applied by external rotating magnetic field, while soft permalloy (Ni80Fe20) magnetic tracks composed of radii and spiral tracks, known as spider web network, are patterned underneath the manipulation plane. The structure facilitates the directional transportation of magnetic particles, which is of potential application as biomolecule cargo, to the desired path, either converging to the spider web center or dispersing away, according to the external rotating magnetic field, i.e.,

Acoustophoresis (ACT) is the motion of particles when experiencing a surface acoustic wave (SAW) radiation pressure, either by standing surface acoustic wave or traveling surface acous-

clockwise or counterclockwise.

**5. Acoustophoresis**

90 Microfluidics and Nanofluidics

tic wave [36, 37].

Acoustophoretic force, *FACT*, experienced by a particle is determined by.

$$F\_{\rm ACT} = 4\pi a^3 E\_{\rm ACT} k \sin(2k\alpha) \phi \tag{8}$$

where *FACT* is the acoustic radiation force, *EACT* is the acoustic energy density, *a* is the particle radius, *x* is the distance from pressure antinode in the wave propagation axis, *k* is the wave number (2*πflc*<sup>0</sup> ), and *ϕ* is the acoustic contrast factor [9].

The direction of the particle motion, either toward the pressure node or the antinode, is governed by the sign of the acoustic contrast factor, *ϕ*, given by

$$\phi = \frac{\rho\_p + \frac{2}{3}(\rho\_p - \rho\_m)}{2\rho\_p + \rho\_m} - \frac{1}{3}\frac{\rho\_m c\_m^2}{\rho\_p c\_p^2} \tag{9}$$

where *ρ<sup>p</sup>* and *ρm* are the density of the particle and medium, respectively, while *cp* and *c*<sup>0</sup> are the speed of sound within the particle and medium, respectively. As shown in **Figure 3a**, for a condition with positive acoustic contrast factor (*ϕ*>0), the particles are pushed toward the pressure node, and the phenomenon is known as positive ACT (pACT). In contrast, for a negative acoustic contrast factor condition (*ϕ*<0), the particles are pushed toward the pressure antinode, and it is called as negative ACT (nACT).

**Figure 3.** (a) Acoustophoresis. (Left) Bioparticles are randomly dispersed in the suspending medium without the presence of acoustic radiation pressure. (Right) Bioparticles experience acoustic radiation pressure when SAW transducer is turned on. Bioparticles A (purple) are pushed toward the pressure node due to positive acoustic pressure contrast (*φ<sup>A</sup>* > 0). Bioparticles B (yellow) which experience the opposite (*φ<sup>B</sup>* < 0) are pushed toward the pressure antinodes (Reprinted with permission from Md Ali et al. [13]. Copyright 2016 Royal Society of Chemistry). (b) Manipulation of bacteria in blood by acoustophoresis. Separation of bacteria from RBCs is performed by acoustic separation chip. Bacteria enrichment from blood plasma is performed subsequently at acoustic trapping capillary before the release for detection and identification by dry-reagent PCR chips (Reprinted with permission from Ohlsson et al. [38]. Copyright 2016 American Chemical Society).

Mainstream applications of ACT in particle manipulation employ either (1) traveling surface acoustic wave (TSAW) [37] or standing surface acoustic wave (SSAW) [36]. A TSAW is a condition when a surface acoustic wave (SAW) is propagating from interdigitated transducer (IDT) electrodes, while SSAW occurs when two TSAWs constructively interfere and form a standing or stationary SAW. TSAW can be generated by a single IDT electrode, while the SSAW can be generated either by a pair of IDT electrodes or a combination of a single IDT and wave reflectors. In TSAW acoustophoresis, bioparticles move together with the wave propagation, while in SSAW acoustophoresis, they are pushed toward the SAW pressure node or the antinode. Pressure node is the region of constant pressure, while pressure antinodes are regions alternating between maximum and minimum pressure values.

of ssDNA library is incubated with microbeads which modified with target molecule, before it is introduced to acoustophoretic separation device, where the microbeads with target-bound DNA fragments are focused on central buffer due to acoustophoretic force, while unbound protein and ssDNA are remained in the original buffer at the side flow. The target-bound DNA on the microbeads is collected and amplified by PCR for subsequent round of washing and screening. Recently, Kennedy et al. [45] communicate the process of purifying target biomolecules utilizing acoustic standing wave in a fluidic chamber to partition and maintain solid-phase bead in an acoustically fluidized bed format, for capturing, washing, and elution of target biomolecules, including monoclonal antibody by protein A beads from a crude cell culture system and recom-

Biological Particle Control and Separation using Active Forces in Microfluidic Environments

http://dx.doi.org/10.5772/intechopen.75714

93

Thermophoresis (THM) is the motion of particles driven by thermal gradients in the suspending medium. Thermal gradients are commonly generated by local absorption of infrared (IR) laser. The thermal gradients induce diffusional motion of the particles, either toward higher

Thermophilic particles diffused to the region with higher temperature, while thermophobic

*J* = *D*[∇*C* + *ST C*(1 − *C*)∇*T*] (10)

where *D* is the diffusion coefficient, *C* is the concentration, *T* is the temperature, and *ST* is the Soret coefficient, defined as the ratio of thermal diffusion coefficient, *DT*, over diffusion coef-

Steady-state concentration changes for a given spatial temperature difference, *ΔT*, which is

Studies prove that a temperature difference between 2 and 8 K in the beam center with a

(*e* = 2.71828) indicates the beam diameter where intensity drops to 13.5% of the maximum

*Ccold*

where *Chot* is the molecule concentration in the hot area, while *Ccold* is in the cold area.

diameter of 25 μm managed to induce thermophoretic motion, while the "1/*e*<sup>2</sup>

*DTD* (11)

= *exp*(−*ST* Δ*T*) (12)

diameter"

binant green fluorescent protein (GFP) by anion exchange of a crude cell lysate.

particles move to the opposite direction, as shown in **Figure 4a**.

*ST* <sup>=</sup> \_\_\_

*<sup>C</sup>*\_\_\_\_*hot*

Liquid flow density, *J*, driven by thermophoretic field, is governed by

**6. Thermophoresis**

or lower temperature regions [10].

**6.1. Fundamentals of THM**

ficient, *D*, which is given by

given by

1/*e*<sup>2</sup>
