Non-contact observation of cultured cells by acoustic impedance microscope

We are proposing the acoustic microscope for imaging cross sectional acoustic impedance of biological tissues. A focused acoustic beam is transmitted to the object placed on the ldquorear surfacerdquo of a plastic substrate. By scanning the focal point on the surface, a 2-D reflection intensity profile is obtained. A reference material is observed under the same condition. The reflection is interpreted into characteristic acoustic impedance. This paper deals with a new system with a high resolution for observing cultured cells. A pulse voltage was applied to a ZnO type transducer with a sapphire rod. The frequency range of the acoustic wave employed was 200 - 400 MHz. A plastic film of 50 - 70 mum in thickness was used as the substrate. Rat-derived glial (astrocyte) cells were cultured on the film. The reflection was converted into local acoustic impedance at the focal spot. The distance between adjacent pixels was 2 mum, however, the diameter of the focal spot would be as large as 3 - 5 mum. The image reveals the morphological features of astrocyte. Calibration and compensation of acoustic impedance by considering oblique incidence is demonstrated. The acoustic impedance of the cells was spreading between 1.5 - 1.7 MN s/m³. We believe that this quantitative method is useful for observing cultured cells in vivo.