Beneficial micropipette oscillation in vision-guided piezo-assisted ICSI

Various designs of piezo-assisted Intracytoplasmic sperm injection (ICSI) has been developed, which achieved higher success rate than conventional ICSI. A common issue is that lateral oscillation of the injection pipette tip is always excited by the intended axial actuation. Researchers hold different opinions on which oscillation, axial or lateral, has more dominate effects on the piercing process. In this paper, different functionalities of the axial and lateral oscillations during cell injection are investigated. A novel vibration-assisted injector is developed, which uses a piezoelectric shear actuator with free stroke of 10 μm in two axes. Different from previous designs, axial and lateral oscillations are generated separately. For axial vibration experiment, a short pipette (450 μm in length, 30 μm in diameter) with high first bending and longitudinal natural frequencies (137.6 kHz and 1.08 MHz) is fabricated. While driven in axial direction at frequency below 40 kHz, the pipette can be considered as a rigid body. In lateral case, a 51 mm long micropipette is driven in the transverse direction at the thicker end to excite bending modes. Experimental results obtained using zebrafish embryos reveal that lateral oscillations reduce the cell deformation ratio dramatically during penetration, while axial oscillations show little effects. As less deformation on the cell membrane leads to less pressure change inside the cell, thus lateral oscillation is more beneficial than axial oscillation in piezo-assisted ICSI. These findings help to understand the underlying physics of vibration-assisted injector. Future design vibration-assisted injector should focus on generating lateral oscillations instead of axial ones.