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Description
Acoustic boundary conditions strongly influence pressure fields and biological responses in in-vitro ultrasound experiments, yet their mechanistic role remains poorly quantified. This study investigates the effects of standing waves on acoustic pressure and cell viability. A multiphysics finite-element model integrating pressure acoustics, solid mechanics, and electrostatics was developed to simulate wave propagation and represent the piezoelectric transducer as both transmitter and receiver. A 1 MHz transducer delivered pulsed ultrasound into a water-filled chamber with cells suspended in RPMI medium. The upper boundary was configured with either air or an acoustic absorber to compare reflective and absorptive interfaces. Simulated pressure fields and pulse-echo responses agreed with experimental measurements. The air interface produced strong reflections and standing waves with minimal heating, whereas the absorber reduced reflections but increased medium temperature. Simulations showed constructive and destructive pulse interference depending on pulse duration and duty cycle, producing localized pressure amplification. Standing waves impaired cell viability more than thermal effects at comparable acoustic pressure. Cells exposed to 1.8 W and 3.2 W showed similar viability reductions without an absorber, while pronounced apoptosis occurred at 6.15 W. These results highlight the importance of controlling reflections for mechanistically interpretable LIPUS studies.
