FIGURE 1. The spring in the biomembrane force probe BFP is a pressurized membrane capsule¹².

Membrane tension sets the force constant k_f (force/capsule extension) and is controlled by micropipette suction P and radius R_p, k_f PR _p. Using a red blood cell as the transducer, the BFP stiffness was tuned between 0.1 and 3 pN nm^-1 to measure forces from 0.5 to 1,000 pN. As the BFP tip, a glass microbead of 1–2 m diameter was chemically glued to the membrane (see Methods). a, Operated on the stage of an inverted microscope, the BFP (on the left) in the horizontal mode was kept stationary and the microbead test surface (on the right) was translated to/from contact with the BFP tip by precision piezo control. With fast video (1,000 frames per s) processing, a simulated cursor was required to track the image of the bead as shown, which yielded a resolution of 8–10 nm for transducer deflection. b, Reflection interference contrast image of the BFP tip translated along the optical axis by piezo control to/from a coverglass test surface in the vertical mode. Standard video (30 frames per s) processing of the circular interference pattern was used to track elevation of the tip at a resolution of 2–5 nm. Transducer deflection was obtained from the difference between piezo translation and bead displacement.