Fig. 2.

Measurement of frequency shifts to determine apical epithelium mechanical properties. a Schematic diagram of the noncontact acoustic frequency modulation method to measure epithelial apical surface mechanical properties. As the acoustically vibrating cantilever with a microsphere approaches the epithelium, a hydrodynamic interaction emerges and increases monotonically as the sphere gets closer. This interaction can be measured by the AFM as an increase in frequency shift. f is the cantilever drive frequency, ∆f is the frequency shift, A is the oscillation amplitude, R is the microsphere radius, h is the distance between the microsphere and the epithelium apical surface, and A _p is the effective probed area. b Phase-frequency response curves showing frequency shifts acquired by keeping π/2 phase when the acoustically vibrating microsphere is moved closer to the apical epithelium surface. Dotted line represents π/2 phase. c Representative example of frequency shift measurements performed on a MDCK II polarized monolayer at different distances. Reconstructed frequency shift-gap curves were then fitted using Eq. (1) to determine the apical epithelial surface tension. The red line depicts the fitted curve