title: Targeted cell immobilization by ultrasound microbeam.
abstract: Various techniques exerting mechanical stress on cells have been developed to investigate cellular responses to externally controlled stimuli. Fundamental mechanotransduction processes about how applied physical forces are converted into biochemical signals have often been examined by transmitting such forces through cells and probing its pathway at cellular levels. In fact, many cellular biomechanics studies have been performed by trapping (or immobilizing) individual cells, either attached to solid substrates or suspended in liquid media. In that context, we demonstrated two-dimensional acoustic trapping, where a lipid droplet of 125 µm in diameter was directed transversely toward the focus (or the trap center) similar to that of optical tweezers. Under the influence of restoring forces created by a 30 MHz focused ultrasound beam, the trapped droplet behaved as if tethered to the focus by a linear spring. In order to apply this method to cellular manipulation in the Mie regime (cell diameter > wavelength), the availability of sound beams with its beamwidth approaching cell size is crucial. This can only be achieved at a frequency higher than 100 MHz. We define ultrasound beams in the frequency range from 100 MHz to a few GHz as ultrasound microbeams because the lateral beamwidth at the focus would be in the micron range. Hence a zinc oxide (ZnO) transducer that was designed and fabricated to transmit a 200 MHz focused sound beam was employed to immobilize a 10 µm human leukemia cell (K-562) within the trap. The cell was laterally displaced with respect to the trap center by mechanically translating the transducer over the focal plane. Both lateral displacement and position trajectory of the trapped cell were probed in a two-dimensional space, indicating that the retracting motion of these cells was similar to that of the lipid droplets at 30 MHz. The potential of this tool for studying cellular adhesion between white blood cells and endothelial cells was discussed, suggesting its capability as a single cell manipulator.
authors.affiliation: Department of Biomedical Engineering, NIH Resource Center for Medical Ultrasonic Transducer Technology, University of Southern California, Los Angeles, California 90089, USA. jungwool@usc.edu
authors.forename: Jungwoo
authors.initials: J
authors.lastname: Lee
authors.forename: Changyang
authors.initials: C
authors.lastname: Lee
authors.forename: Hyung Ham
authors.initials: HH
authors.lastname: Kim
authors.forename: Anette
authors.initials: A
authors.lastname: Jakob
authors.forename: Robert
authors.initials: R
authors.lastname: Lemor
authors.forename: Shia-Yen
authors.initials: SY
authors.lastname: Teh
authors.forename: Abraham
authors.initials: A
authors.lastname: Lee
authors.forename: Koping Kirk
authors.initials: KK
authors.lastname: Shung
mesh_terms: D002448:Cell Adhesion; D045744:Cell Line, Tumor; D018914:Cells, Immobilized; D003584:Cytological Techniques / Q000379:methods*; D042783:Endothelial Cells / Q000502:physiology; D006801:Humans; D007962:Leukocytes / Q000502:physiology; D014465:Ultrasonics / Q000379:methods*