The effect of ultrasound-related stimuli on cell viability in microfluidic channels

BACKGROUND: In ultrasonic micro-devices, contrast agent micro-bubbles are known to initiate cavitation and streaming local to cells, potentially compromising cell viability. Here we investigate the effects of US alone by omitting contrast agent and monitoring cell viability under moderate-to-extreme ultrasound-related stimuli. RESULTS: Suspended H9c2 cardiac myoblasts were exposed to ultrasonic fields within a glass micro-capillary and their viability monitored under different US-related stimuli. An optimal injection flow rate of 2.6 mL/h was identified in which, high viability was maintained (~95%) and no mechanical stress towards cells was evident. This flow rate also allowed sufficient exposure of cells to US in order to induce bioeffects (~5 sec), whilst providing economical sample collection and processing times. Although the transducer temperature increased from ambient 23°C to 54°C at the maximum experimental voltage (29 V(pp)), computational fluid dynamic simulations and controls (absence of US) revealed that the cell medium temperature did not exceed 34°C in the pressure nodal plane. Cells exposed to US amplitudes ranging from 0–29 V(pp), at a fixed frequency sweep period (t(sw) = 0.05 sec), revealed that viability was minimally affected up to ~15 V(pp). There was a ~17% reduction in viability at 21 V(pp), corresponding to the onset of Rayleigh-like streaming and a ~60% reduction at 29 V(pp), corresponding to increased streaming velocity or the potential onset of cavitation. At a fixed amplitude (29 V(pp)) but with varying frequency sweep period (t(sw) = 0.02-0.50 sec), cell viability remained relatively constant at t(sw) ≥ 0.08 sec, whilst viability reduced at t(sw) < 0.08 sec and minimum viability recorded at t(sw) = 0.05 sec. CONCLUSION: The absence of CA has enabled us to investigate the effect of US alone on cell viability. Moderate-to-extreme US-related stimuli of cells have allowed us to discriminate between stimuli that maintain high viability and stimuli that significantly reduce cell viability. Results from this study may be of potential interest to researchers in the field of US-induced intracellular drug delivery and ultrasonic manipulation of biological cells.