Cell Alignment in Aqueous Solution Employing a Flexural Plate Wave Piezoelectric MEMS Transducer

The possibility to steer floating cells dispersed in water by means of flexural plate waves (FPWs) generated by a 9 mm
$\times9$
mm piezoelectric MEMS transducer has been explored. The MEMS transducer has a squared cavity etched out in a silicon substrate formed by a 6 mm
$\times6$
mm composite diaphragm made of a piezoelectric aluminum nitride (AlN) layer on top of a doped silicon plate. The piezoelectric layer can be electrically actuated by means of metal interdigital transducers (IDTs) placed over the AlN film at the diaphragm edges. Cell alignment has been sought for by inducing standing FPWs in the diaphragm and in the contacting water layer in the cavity by the one-dimensional (1D) acoustic field pattern obtained by exciting two IDTs located symmetrically with respect to the diaphragm centre. The working principle has been validated by means of 2D finite element modelling and simulations. The MEMS transducer has been fabricated using the PiezoMUMPs process and experimentally tested by exploiting a tailored front-end electronic circuit. Inactive fibroblast cells with an approximate diameter of
$15~\mu \text{m}$
have been dispersed in demineralized water within the cavity at a concentration in the order of 105 cells/ml. By applying sinusoidal excitation signals to faced IDTs with zero phase shift and peak amplitude of 10 V, lines of cells spaced by half wavelength
$\lambda $
/2 =
$56~\mu \text{m}$
have been achieved at 12.5 MHz, in good agreement with theoretical predictions and simulation results.