3.2.2. The Effect of the Exposure Time

Subsequent studies were focused on the measurement of the zeta potential of red blood cell and platelet membranes as a function of pH depending on the exposure time of the membranes to different polymers. Experimental research was performed immediately after suspending the blood components in a 0.155 M NaCl solution containing the polymer at a concentration of 0.1 mg/mL (at which changes in relation to the control sample were observed) and after 1 h and 3 h exposure of erythrocytes and thrombocytes to the polymer.

The dependence of the zeta potential as a function of pH for RBCs is shown in Figure 5. Statistically significant changes were observed at the lowest and the highest pH values. Moreover, changes were observed in the case of (1) PS-100 at pH 4–5 (Figure 5a), (2) PS-NH2-100 in a pH range from 4 to 6 (Figure 5c), and (3) PS-NH2-200 at pH = 4 (Figure 5d). *Membranes* **2022**, *12*, x 13 of 17

**Figure 5.** Zeta potential of the erythrocytes' cell membranes as a function of the pH of the electrolyte solution. The parameter was measured immediately (●), after 1 h (♦), and 3 h (■) of exposing the sample to the polymer: (**a**) PS–100, (**b**) PS–200, (**c**) PS–NH2–100, and (**d**) PS–NH2–200. **Figure 5.** Zeta potential of the erythrocytes' cell membranes as a function of the pH of the electrolyte solution. The parameter was measured immediately (•), after 1 h (), and 3 h () of exposing the sample to the polymer: (**a**) PS–100, (**b**) PS–200, (**c**) PS–NH2–100, and (**d**) PS–NH2–200.

All particles used in this study were associated with erythrocytes—attached to or internalized by cells, depending on the size and surface chemistry. From the data shown in Figure 5, it can be seen that both processes occurred quickly and efficiently within one hour of the cells and particles coming into contact. Any coincidental discrepancies between the zeta values obtained for different durations of cell exposure to the polymer are likely due to the possibility of particles detaching from erythrocytes, as a result of shear forces, cell–cell interactions, and cell–vessel wall interactions [49]. *Membranes* **2022**, *12*, x 14 of 17

> In the case of studies on thrombocyte membranes (Figure 6), significant changes in the *ζ* potential were observed while exposing these cells to PS-100 and PS-NH2-200. In the case of studies on thrombocyte membranes (Figure 6), significant changes in the *ζ* potential were observed while exposing these cells to PS-100 and PS-NH2-200.

**Figure 6.** Zeta potential of the platelets' cell membranes as a function of the pH of the electrolyte solution. The parameter was measured immediately (•), after 1 h (), and 3 h () of exposing the sample to the polymer: (**a**) PS–100, (**b**) PS–200, (**c**) PS–NH2–100, and (**d**) PS–NH2–200.

From the data depicted in Figure 6, it can be noted that PS-200 and PS-NH2-100 quickly and efficiently attached to platelet membranes within an hour. For PS-100 and PS-NH2-200 particles, the association of polystyrene with cells was slower. This can be seen in the obtained dependencies, in which the zeta potential values measured after 3 h of thrombocyte exposure to the polymer are more similar to those measured directly than to those determined after 1 h of cell exposure. In the subject literature, studies have been presented in which internalization was noticeable as early as 1 min after incubation [46], and in which, as the duration of cell exposure to polystyrene increased, a greater probability of polystyrene association with cells was reported [37].
