**6. Thermophoresis**

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 alter-

Acoustophoretic manipulation of model organism has been demonstrated by Sundvik et al. [39]. They study levitation of zebrafish embryos using acoustic radiation force in a noncontact wall-less platform for a period of less than 2000 s, while the embryos still in development at 2–14 hours postfertilization, and they found that the levitation does not interfere the development, though it might influence mortality rate. Urbansky et al. [40] perform the manipulation of peripheral blood progenitor cells (PBPCs) with the focus on sorting out CD8 lymphocytes (target cells) from the mixture that contains CD4, CD19, CD34, and CD56 lymphocytes as well. They label the CD8 with affinity beads, forming bead-cell complex, to modify the acoustic mobility of the target cells. Furthermore, they modify the medium properties of central buffer, using Ficoll wash buffer, to adjust the acoustic force on different particles, so that bead-CD8 complexes are pushed into central buffer under acoustophoretic force exerted by piezoceramic transducer, while other unbounded cells remain flowing at the side due to lower acoustic mobility. Urbansky et al. [41] further perform separation of mononuclear cells (MNCs) from diluted whole blood using acoustophoretic microfluidic device. They managed to overcome the behavior similarity of MNCs and RBCs in acoustic standing wave by optimizing the buffer conditions to modify the acoustophoretic mobility of the cells. Antfolk et al. [42] accomplish separation of spiked prostate cancer cells (DU145) from whole blood using ACT-DEP-integrated platform consisting of acoustophoretic pre-alignment, separation, and concentration of targeted DU145 cells, prior to single-cell array trapping using DEP microwell. Bacteria manipulation has been accomplished by Ohlsson et al. [38], who developed a microsystem for bacteria separation, enrichment, and detection from blood, as demonstrated in **Figure 3b**. The system is integrated with acoustic separation to remove RBCs from blood sample, with subsequent enrichment of bacteria from plasma by acoustic trapping to polystyrene seed particles, and polymerase chain reaction (PCR) for detection and identification of the bacteria at the final stage. They demonstrate the system using whole blood samples, which, respectively, spiked with *Pseudomonas putida* and *E. coli*. While for virus manipulation, Ness et al. [43] demonstrate extraction and enrichment of MS2 virus from human nasopharyngeal samples by integration of acoustic force to remove host cells, debris, and pollen from the sample and later with electric field force to attract the virus, which is a negatively charged species, and to migrate from sample to co-flowing buffer. Further, Park et al. [44] perform washing and screening of prostate-specific antigen (PSA)-binding aptamer, a single-stranded DNA (ssDNA), using an acoustophoretic separation. The ssDNA pool comprising

nating between maximum and minimum pressure values.

**5.2. Acoustophoretic manipulation of bioparticles**

92 Microfluidics and Nanofluidics

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 or lower temperature regions [10].
