## 2 Design and Fabrication of the Device  
A sketch of the device developed to steer and confine cells dispersed in liquid is shown in Figure 1a. The device exploits two piezoelectric actuators placed diametrically opposed on the bottom of a 25 mm in diameter circular alumina (Al2O3) substrate to generate standing acoustic waves, as schematically represented in Figure 1b. The distance between pressure nodes, in which the cells are expected to be confined, corresponds to half the wavelength of the generated acoustic waves.  
The two piezoelectric actuators have been fabricated by the screen-printing technique [23]. Figure 2a,b show a section and a bottom view of the device, respectively, while Figure 2c shows an enlarged view of a cross-section of the piezoelectric actuator.  
The fabrication process of the actuators starts with the screen-printing deposition of a palladium-silver (PdAg) ink to create the bottom 6 mm \(\times\) 6 mm electrode (connected to pad C) on a 635-mm-thick alumina substrate, as shown in Figure 2d. Then drying at 150 \({}^{\circ}\)C and firing at 850 \({}^{\circ}\)C processes are applied to finalize the bottom electrode. A lead zirconate titanate (PZT) 8 mm \(\times\) 8 mm layer is realized on top of the bottom electrode with multiple screen-printing depositions to obtain the final thickness of about 100 \(\upmu\)m, as shown in Figure 2e. Drying at 150 \({}^{\circ}\)C and firing at 950 \({}^{\circ}\)C peak temperature are carried out to each PZT deposition. Finally, an IDT composed of two interleaved comb-shaped arrays of metallic electrodes (connected to pads A and B) is realized on top of the PZT layer by a deposition of PdAg ink, as shown in Figure 2f. Final drying and firing processes are applied to the IDT electrodes, as for the bottom electrode. The IDT has finger width and spacing of 350 \(\upmu\)m, as shown in Figure 2c,f.  
The PZT layer has been poled at 4 MV/m along the thickness by applying a DC voltage between the two armatures formed by A and B electrically shorted and C, respectively. Thus, a sinusoidal excitation voltage applied between A and B (with C floating), or between A and B electrically shorted and C, generates sinusoidal deformation along the PZT-layer thickness by exploiting mainly the d\({}_{33}\) mode in both cases, since the thickness of the PZT layer is thinner than the spacing between two adjacent IDT fingers.  
Compared to similar devices [12; 13; 14], a larger volume of liquid can be used. This is granted by the thickness of the screen-printed PZT film in the order of hundreds of micrometers and typically higher piezoelectric coefficients compared to piezoelectric crystals, thereby providing comparatively higher effectiveness of the actuators. The screen-printed deposition of PZT can be applied to other non-piezoelectric substrates, such as high-temperature glasses, which could grant optical transparency to the device.  
Figure 1: Sketch (**a**) and cross-section A-A (**b**) of the device developed to steer and confine cells dispersed in liquid.  
Figure 3a,b show top-view and bottom-view pictures of the fabricated device fixed to a printed circuit board (PCB) for electrical connections, respectively. The device can be used by filling the 5-mm-high chamber with liquid in which the cells are dispersed and where the generation of standing acoustic FPWs is expected.  
Figure 3: Top-view (**a**) and bottom-view (**b**) pictures of the fabricated device fixed to a printed circuit board (PCB).  
Figure 2: Section (**a**) and bottom view (**b**) of the device. Enlarged view of a cross-section of the piezoelectric actuator (**c**). Fabrication process: deposition of the bottom electrode (**d**), lead zirconate titanate (PZT) layer (**e**), and interdigital transducer (IDT) electrodes (**f**).