2.1. Fabrication of the Microchip and Acoustic Field

A microfluidic channel was fabricated using the standard replica molding technique [20,29,30]. The geometry of the device is shown in Figure 1. The device was mainly composed of two parts: a PDMS channel and a piezoelectric ceramic transducer (PZT). The fabrication process of the PDMS channel: After removing the residual impurities on the surface of the silicon wafer with alcohol, pickling and water, the silicon wafer was spin-coated by photoresist. Then, negative photoresist (SU-8 50, Microlithography Chemical Corp., Newton, MA, USA) was put onto silicon wafers to form a thin film at 550 rpm for 2 min. The adhesion between the silicon wafer and photoresist could be enhanced by placing the silicon wafer onto a horizontal heating plate at 60 °C for 4 min and 90 °C for 5 min. The photoresist was then exposed to an ultraviolet light source at a dose of 600 cJ/cm² for a duration of 20 s and developed in the photoresist developer. The PDMS prepolymer (Glue A) and curing agent (Glue B, Sylgard 184, Dow Corning, Midland, MI, USA) were mixed at a ratio of 10:1. The mixture was poured onto the silicon template, vacuum degassing, and baked at 80 °C for 30 min. The cured PDMS was stripped from the silicon template and a puncher (Harris, Uni-Core, Jed Pella, Inc., Redding, CA, USA) was used to create the entry and exit of the microchannel. Finally, the microchannel was bonded to the glass substrate by the plasma treatment. The PZT transducer was placed adjacent to the PDMS microchannel on the same substrate with ultrasound coupling agent (Guang Gong, Guang Dong, China), to ensure that the acoustic energy could be coupled to the substrate efficiently. The rectangular PDMS channels array with the uniform size was fabricated at the sidewall. The PDMS channels could generate an array of monodisperse microbubbles with the same oscillation by ultrasound, ensuring cell lysis with a high efficiency. To ensure the free oscillation, the depth of the microchannel was deigned to be 50 μm. The width of the rectangular channel was approximately 40.8 μm, respectively, measured by a step profiler (XP1, MTS, San Jose, CA, USA). The transducer was composed of PZT-4 ceramic with a diameter of 26 mm and, in our experiment, was operated in the thickness vibration mode.

Schematic of experiment setup. The polydimethylsiloxane (PDMS) channel and piezoelectric ceramic transducer (PZT) were placed on the surface of the glass substrate. The PZT was employed to excite microbubble oscillation.

Surface tension is a key property of liquid–air interfaces. As an important factor in the formation of bubbles at the gas–liquid phase interface, surface tension can be defined as the contraction force acting on any part of the liquid surface on a straight line per unit length. Due to the hydrophobic treatment of PDMS channel in advance, the presence of surface tension makes the bubble formation easy in the PDMS channel when the liquid flows through. According to the ordinary Young–Laplace equation for a spherical interface, we know that surface tension is related to liquid–gas pressure difference and PDMS channel size [31]. Moreover, from Kumar and Kuloor (1970) study, we obtain that orifice size and material-dependent surface wettability as equipment variables affect bubble formation [32]. The resonant frequency of the microbubble was approximately 107 kHz and a more detailed calculation can be found in previous work [20]. The signal amplified by a power amplifier (100A400A, Amplifier Research, Souderton, PA, USA) was applied to PZT to excite the microbubble vibration.

We applied a laser Doppler vibrometer (LDV, UHF-120, Polytec, Berlin, Germany) to measure the acoustic pressure within the microchannel. The LDV was the propagation direction of acoustic waves and was utilized to measure the vibration of glass substrate in a non-contact manner. The LDV’s laser beam was positioned perpendicular to the surface of the glass substrate. As the surface of the glass substrate moved, the vibration amplitude could be obtained from the Doppler shift of the reflected laser beam frequency.