Fig. 2

Optical trapping of circulating RBCs, image analysis and photodiode measurements. (A) Scheme of the experiment where the optical trap is located in the caudal plexus (DA) of the ZF embryo. The trapped RBC is subjected to the blood flow–driven dragging force as well as to the restoring force from the optical trap. (B) Thanks to the flow pulsatility occurring in arterial vessels, the time trace of the optically trapped RBC is followed on the quadrant diode. The time trace is further treated to access the power spectrum (black curve), revealing peaks of pulsatility as well as a corner frequency f_c around 250 Hz. This is highlighted with the theoretical curve for steady optical trapping experiments (blue curve). The cutoff frequency, obtained from a Lorentzian approximation, provides a first approximation of the trap stiffness, which is ∼4.7*10^–5 N/m. (C) A kymographic analysis of a single trapped RBC imaged at 200 fps is performed. Kymographs allow fine tracking of RBC displacement within the optical trap, thereby providing a real time track in micrometers. (D) A mirror force track over time is obtained and results from the use of the theoretical trap stiffness derived from the cutoff frequency (theoretical curve). (E) Any given position in the ZF vasculature can be probed, providing an accurate measure of hemodynamic forces. Here a few trapping RBCs in a cross-section of the DA permit appreciation of the behavior of a Poiseuille-type of flow.