Dynamic response of micropipettes during piezo-assisted intracytoplasmic sperm injection

In the intracytoplasmic sperm injection (ICSI) process, a piezoelectric actuator is commonly used to assist the piercing of cell membrane. The longitudinal pulses that are performed by the piezo actuator, however, cause undesired lateral vibrations at the drawn tip of the injection micropipette. This mechanism is not well understood, despite its critical role in piezo-assisted cellular microinjection. We provide an analytical model to characterize the micropipette tip vibrations under assumed base excitation arising from the piezoelectric pulses. The resulting dynamic response is determined by using the Duhamel integral method. This study quantifies the effect of fluid damping, embedded mercury, and the apparent cell membrane elasticity. We found that, in practice, a small mercury droplet filled in pipette essentially creates higher shear forces at the membrane-pipette interface. The increased shear due to underdamped eigenmodes is conceived to assist the piercing of the cell membrane.