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. 2021 Jul 6;10:e65930. doi: 10.7554/eLife.65930

Appendix 1—figure 7. Estimation of the force exerted by radiation pressure onto a hair bundle of known stiffness.

Appendix 1—figure 7.

(A) When a hair bundle from the frog’s sacculus was deflected by a 50 ms laser pulse at 12.5 mW output power, the bundle moved positively by 20 nm. The trace shown is the average of 25 repetitions. (B) The kinocilium of the same hair bundle was connected to the tip of a flexible glass probe of known stiffness kp, whose other end connected to a stiff probe driven by a piezoelectric actuator. To ensure adhesion between the glass and the kinocilium, concanavalin A was used to coat the flexible fiber. The motion of the fiber’s tip Δtip and the hair bundle was tracked with a dual photodiode. A movement of the stiff probe by Δprobe translates into a force Fp=kp(Δprobe-Δtip) delivered by the flexible probe onto the hair bundle, whose reaction force depends on its stiffness kHB times the bundle’s displacement Δtip. The balance of forces provides a way to estimate the hair-bundle stiffness: kHB=kp(Δprobe/Δtip-1). (C) The stiffness of the hair bundle stimulated was measured by varying Δprobe between –10 nm and 90 nm. The estimated stiffness of the hair bundle was kHB=1.8±0.2mNm1 (mean ± SEM). As a result we can estimate the force exerted by radiation pressure that elicited the hair-bundle response shown in panel A to be approximately 40 pN. The hair bundle was bathed in perilymph for the whole duration of the experiment. The data point marked with a cross was not included in the fit. The stiffness of the flexible fiber kp=0.46mNm1 was measured by recording the Brownian motion of the fiber’s tip in water.