A theoretical time-course model of acoustic tweezers: Pulse-wave mode

The optical tweezers has been a very useful tool in biological research. However, due to finite penetration depth in optics, it is only utilized for transparent particle. This limitation reduces its potential of in-vivo manipulation. The idea of acoustic tweezers with better penetration ability than optics was recently proposed. In this paper, a novel pulse-wave mode is theoretically demonstrated to implement this idea to reduce the risk of damage from thermal effect. The simulations are based on a time-varying acoustic field with a 100-MHz, 50% bandwidth, and 1.5 MPa transmitting Gaussian pulse. For simplicity, all calculations are simply done for the axis-symmetric case. The data of instantaneous intensity can be obtained from time tables of the simulated field. After applying it into the theory of acoustic tweezers, and integrating it over the spherical surface of a particle, net radiation forces at specific axis point can be obtained. Then, average forces at pulse durations along the beam axis are calculated, showing a transducer with F#/1, acoustic impedance of 1.38 MRayls, and particle size ranging from 120 μm to 210 μm are suitable for the existence of negative force induced by pulse waves. Furthermore, track of a particle versus time are simulated with 10 MPa pulse waves with PRF of 100 kHz, the results showing the average speed of the particle to be stably trapped is about 350 μm per second. This reveals the mobility of transducer should keep low to prevent particle escaping from the beam axis, but higher with both higher PRF and sound pressure. In conclusion, these results certainly show the feasibility of the acoustic tweezers in the pulse wave mode, indicating significant acoustic trapping induced in the pulse-wave mode depends on not only acoustic impedance mismatch and degree of focusing, but also adjustments of PRF and sound pressure.