Fig. 1.

Videomicrograph (A) of the BFP (labeled by the “spring” on the left) juxtaposed with a 4-μm target microsphere; force-time plot (B) from a steady ramp test of a P-selectin–PSGL-1 bond; force time plot (C) from a jump/ramp test of a P-selectin–PSGL-1 bond; comparison of the perceived frequencies (D) of BFP tip-target attachments in jump/ramp and steady ramp tests of P-selectin–PSGL-1 bonds. (A) Pressurized by pipette suction, a PEG-biotinylated red blood cell acts as the elastic transducer for force in the BFP. To form an active tip, a 2-μm glass bead was bound with P-selectin (“green pins”) as well as PEG biotin and then attached to the transducer with streptavidin. With the PSGL-1 ligands (“red wedges”) linked covalently to a target glass bead (on the right) in the same way, bonds to P-selectin were formed and broken by moving the target into and out of contact with the BFP tip by using a piezo-mounted pipette. (B) A steady ramp test shows the BFP response during target approach “soft” touch, then retraction at fixed speed. Formed at touch, a PSGL-1–P-selectin bond failed at ≈75 pN under the force ramp of ≈700 pN/sec. (C) A jump/ramp test shows the BFP response during target approach, touch, then a rapid retraction abruptly switched after 0.004 sec to slow retraction. Again formed at touch and surviving the jump in force (at ≈5,000 pN/sec), a PSGL-1–P-selectin bond failed at ≈50 pN under the final force ramp of ≈140 pN/sec. (D) Frequencies of tip-surface attachments in steady ramp tests appeared to increase with ramp rate (filled red triangles), whereas frequencies in the jump/ramp tests were essentially independent of ramp rate (open red triangles). The dotted red curve estimates the apparent frequency that would be perceived if some attachments went undetected under steady ramps within the first force bin (≈10–12 pN), assuming rapid dissociation (≈10 pN/sec) along a separate pathway that could be blocked by a 20- to 30-pN jump in force.