In this new method for studying the shape recovery of deformed red blood cells, three optical traps ("optical tweezers") induce a parachute-shaped red cell deformation, which is comparable to the deformation in small capillaries. The shape recovery is recorded, and a relaxation time is obtained for each individual red blood cell. The sensitivity of this technique for the detection of differences in relaxation times is demonstrated on subpopulations of density-separated red blood cells: "young" cells have shorter (162 ms) and "old" cells have longer (353 ms) relaxation times compared with the total population (271 ms). The relaxation time is remarkably shorter (114 ms) when the plasma surrounding the cells is replaced by a phosphate-buffered saline solution. The main advantages of this technique are the relatively short measuring and preparation time and the physiological type of deformation and shape recovery in which all relevant cell properties play a role. Therefore, especially when automated further, the technique may be a powerful tool for the study of (sub)populations of pathological red blood cells.

In this study we present experimental data on the inhomogeneous distribution of platelets in polyethylene tubes (200 microns diam) based on the inverse Fåhraeus effect for platelets. It is shown that platelets are expelled toward the red blood cell-depleted marginal layer near the tube wall by mutual interaction with erythrocytes. By means of a straightforward model, the near-wall concentration of platelets could be estimated from measurements on the average tubular platelet concentration. The marginal layer originates from the hydrodynamic interaction of the deformable erythrocytes with the tube wall. If the tube diameter is large compared with the size of the erythrocytes, the lateral migration effects can effectively be scaled on the absolute distance between the erythrocytes and the tube wall. This results in the main conclusion that the near-wall concentration of platelets is significantly enhanced up to about seven times the average concentration, practically irrespective of the tube diameter in the range of 100-500 microns. Where comparable, the results of this study are in accordance with experimental data of other authors.

Summary. We used multiple optical trapping to study the mechanism of red cell (dis)aggregation. Two sets of optical 'tweezers' were used to bring two red blood cells together to form a two-cell aggregate and then to pull them apart, to study the interaction between the cells.We found that cross-bridging occurred in normal reversible aggregation as we observed binding and the occurrence of small tethers between opposite cell membranes. Furthermore, the cells could only be parted by sliding them side by side with a maximum velocity in the order of mm/s indicating accumulation of the cross-bridges.

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