Efforts on quantitative measurements of the interactive forces of red blood cells (RBC) have been pursued for many years in hopes of a better understanding of hemodynamics and blood rheology. In this paper, we report an approach based on an ultra-high frequency (410 MHz) single beam acoustic tweezer (SBAT) for quantitative measurements of inter-RBC forces at a single cell level. The trapping forces produced by this ultra-high frequency (UHF) SBAT can be quantitatively estimated with a micropipette. Since the focal beam diameter of the 410 MHz ultrasonic transducer used in this SBAT was only 6.5 micrometer (μm), which was smaller than that of a RBC (~7.5 μm), it was made possible to directly apply the beam to a single RBC and measure inter-RBC forces against the pre-calibrated acoustic trapping forces as another example of potential cellular applications of the SBAT. The magnitude of these forces was found to be 391.0 ± 86.4 pN. Finally, it is worth noting that unlike several other methods, this method does not require the measuring device to be in contact with the cells.

Extracellular matrix proteins such as fibronectin (FNT) play crucial roles in cell proliferation, adhesion, and migration. For better understanding of these associated cellular activities, various microscopic manipulation tools have been used to study their intracellular signaling pathways. Recently, it has appeared that acoustic tweezers may possess similar capabilities in the study. Therefore, we here demonstrate that our newly developed acoustic tweezers with a high-frequency lithium niobate ultrasonic transducer have potentials to study intracellular calcium signaling by FNT-binding to human breast cancer cells (SKBR-3). It is found that intracellular calcium elevations in SKBR-3 cells, initially occurring on the microbead-contacted spot and then eventually spreading over the entire cell, are elicited by attaching an acoustically trapped FNT-coated microbead. Interestingly, they are suppressed by either extracellular calcium elimination or phospholipase C (PLC) inhibition. Hence, this suggests that our acoustic tweezers may serve as an alternative tool in the study of intracellular signaling by FNT-binding activities.