Ultrasonic transducers for acoustic vortex tweezing application

Abstract

Similar to optical tweezers, acoustic tweezers have been extensively studied for non-contact and non-invasive micro-particles, microbubbles (MBs) and cells manipulation. To enhance the potential of in vivo applications, low-frequency (<20 MHz) acoustic vortex-based transducers, offering flexible manipulation with relatively large penetration depth and strong trapping force, have been studied. Finite element modeling (FEM) simulation of vortex-based transducers has been attempted using COMSOL Multiphysics software and FOCUS software to understand the mechanism of acoustic-vortex generation and compare the difference between the plane and focused 4-element vortex-based transducers. 4-element transducers with the frequency of 5 - 12 MHz based on PZT-5H 1-3 piezoelectric/epoxy composite materials have been designed, fabricated and characterized. Electrical impedance measurement illustrated that the active elements of the transducers exhibited uniform electrical properties. The pulse-echo response showed good sensitivity of developed acoustic vortex-based transducers, indicating their great potential for acoustic tweezing applications. Their acoustic characteristics presented that the acoustic pressure decreased with the increasing transducer frequency, and the characterized beam patterns agreed well with the FEM simulation results using FOCUS software. Microbubbles (MBs) manipulation performance has been evaluated with different driving parameters and experimental configurations. 7-MHz, 9-MHz and 12-MHz vortex-based transducers with a f-number of 1.0 have been employed to explore the effect of driving frequency while two 12-MHz vortex-based transducers with different f-numbers (1.0 and 2.0) have been used to study the effect of f-number. It was found that the size of trapped MBs cluster reduced with the increasing driving frequency while the trapping region of high f-number vortex-based transducer is larger than that of low f-number one. After a series of investigation, the experimental results showed that higher excitation peak pressure and number of duty cycles are beneficial to aggregate the cluster of MBs promptly. Artificial thrombus has been prepared for in vitro thrombolysis experiments using a 5-MHz acoustic vortex-based transducer. The experimental results of thrombolysis using 3 different treatments showed that the vortex-based ultrasound combined with the recombinant tissue plasminogen activator (r-tPA) solution circulation is the optimal combination for having the performance that is better than that of the focus-mode ultrasound assisted thrombolysis with the r-tPA control group. In short, the development of low-frequency acoustic vortex-based transducers for biomedical applications is reported. The experimental findings showed that the developed vortex-based transducers exhibited the satisfied performance, which aligned well with the simulation results. Moreover, both the MBs manipulation and in vitro thrombolysis performance confirmed the promising potential of vortex-based transducers for various biomedical applications.