Investigation of human platelet adhesion under low shear conditions in a rotational flow chamber

Shear-dependent platelet aggregation size

Nonsurgical bleeding is the most frequent complication of left ventricular assist device (LVAD) support. Supraphysiologic shear rates generated in LVAD causes impaired platelet aggregation, which increases the risk of bleeding. The effect of shear rate on the formation size of platelet aggregates has never been reported experimentally, although platelet aggregation size can be considered to be directly relevant to bleeding complications. Therefore, this study investigated the impact of shear rate and exposure time on the formation size of platelet aggregates, which is vital in predicting bleeding in patients with an LVAD. Human platelet‐poor plasma (containing von Willebrand factor, vWF) and fluorochrome‐labeled platelets were subjected to a range of shear rates (0‐10 000 s⁻¹) for 0, 5, 10, and 15 minutes using a custom‐built blood‐shearing device. Formed sizes of platelet aggregates under a range of shear‐controlled environment were visualized and measured using microscopy. The loss of high molecular weight (HMW) vWF multimers was quantified using gel electrophoresis and immunoblotting. An inhibition study was also performed to investigate the reduction in platelet aggregation size and HMW vWF multimers caused by either mechanical shear or enzymatic (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13—ADAMTS13, the von Willebrand factor protease) mechanism under low and high shear conditions (360 and 10 000 s⁻¹). We found that the average size of platelet aggregates formed under physiological shear rates of 360‐3000 s⁻¹ (200‐300 μm²) was significantly larger compared to those sheared at >6000 s⁻¹ (50‐100 μm²). Furthermore, HMW vWF multimers were reduced with increased shear rates. The inhibition study revealed that the reduction in platelet aggregation size and HWM vWF multimers were mainly associated with ADAMTS13. In conclusion, the threshold of shear rate must not exceed >6000 s⁻¹ in order to maintain the optimal size of platelet aggregates to “plug off” the injury site and stop bleeding.

In Vitro Thrombogenicity Testing of Biomaterials

The short- and long-term thrombogenicity of implant materials is still unpredictable, which is a significant challenge for the treatment of cardiovascular diseases. A knowledge-based approach for implementing biofunctions in materials requires a detailed understanding of the medical device in the biological system. In particular, the interplay between material and blood components/cells as well as standardized and commonly acknowledged in vitro test methods allowing a reproducible categorization of the material thrombogenicity requires further attention. Here, the status of in vitro thrombogenicity testing methods for biomaterials is reviewed, particularly taking in view the preparation of test materials and references, the selection and characterization of donors and blood samples, the prerequisites for reproducible approaches and applied test systems. Recent joint approaches in finding common standards for a reproducible testing are summarized and perspectives for a more disease oriented in vitro thrombogenicity testing are discussed.

Analysis of the effect of the size of three-dimensional micro-geometric structures on physical adhesion phenomena using microprint technique

Thrombus formation on biomaterial surfaces with microstructures is complex and not fully understood. We have studied the micro-secondary flow around microstructures that causes components of blood to adhere physically in a low Reynolds number region. The purpose of this study was to investigate the effect of micro-column size on the adhesion phenomena and show a quantitative relationship between the micro-secondary flow and physical adhesion phenomena, considering microstructures of various sizes. The flow simulation and quantitative assessment of adhesion rates around micro-columns was conducted using four sizes of micro-columns. This study also calculated the vectors of micro-secondary flow and average shear rate around a micro-column using a computational fluid dynamics analysis. The simulation showed the micro-secondary flow toward the bottom surface at upstream side and low shear rate distribution generated around a micro-column. Furthermore, physical adhesion tests were conducted using microbeads and a perfusion circuit to examine the size effect of the micro-columns on the physical adhesion. The results showed that the average adhesion rate around the micro-column increases with the associated size increase of the micro-column. Our results indicate that quantification of micro-secondary flow on a material surface with microstructures of several sizes and shapes (such as in a rough surface) is important for the evaluation of the adhesion phenomenon even though the surface roughness value on the material surface is small.

An experimental study of shear-dependent human platelet adhesion and underlying protein-binding mechanisms in a cylindrical Couette system

Undesirable thrombotic reactions count among the most frequent and serious complications for patients who rely on the use of medical devices. To improve the design of medical devices, it is essential to develop a more precise understanding of platelet reactions. Clinical studies and experiments have shown a strong dependence of platelet deposition behavior on the flow. However, today the influence of hemodynamic parameters such as the shear rate on thrombotic reactions is not well understood. For the study of the shear-dependent mechanisms leading to the activation, adhesion and aggregation of platelets, a Couette flow system was used to investigate thrombocyte behavior with regard to well-defined flow conditions at shear-rate values between γ˙=400 $\dot \gamma = {\rm{400}}$ and 1400 1/s. Results were calculated for physiological temperature. It could be shown that the platelet adhesion density increased with increasing shear rates up to γ˙=800 1/s $\dot \gamma = {\rm{800 1/s}}$ and the adhesion pattern was homogeneous. At γ˙=800 1/s, $\dot \gamma = {\rm{800 1/s}},$ a sudden drop in platelet adhesion density occurred and platelets adhered in filaments. Fluorescence microscopy results of von Willebrand factor (vWF) confirm that a shear rate of γ˙=800 1/s $\dot \gamma = {\rm{800 1/s}}$ represents the threshold where a switch of the platelet-binding mechanism from fibrinogen-mediated to vWF-mediated platelet adhesion takes place.

Numerical and experimental evaluation of platelet deposition to collagen coated surface at low shear rates

Platelet deposition to collagen-coated surface under low shear conditions was investigated using an experimental model. The flow chamber was created by combining a stationary and a rotational glass plates spaced 50 μm apart. Blood filled into this space was subjected to a simple Couette flow. Both glass plates were covered with albumin to render them anti-thrombogenic. However, one spot 1×1 mm in size was covered with collagen. This spot was where the platelets deposited. The device was mounted on an inverted microscope and the platelet deposition was recorded. Platelets were dyed to render them fluorescent. The blood used was human blood from healthy volunteers. It was subjected to a range of low shear rates (below 7001/s) to find out how they act on platelet deposition. The results show a characteristic curve with elevated platelet deposition in the range of 1501/s. For the interpretation of these results a numerical model was developed. It applies the Monte Carlo method to model a random walk of platelets. This diffusive motion was superimposed on the convective motion by the Couette flow. A satisfactory match to the experimental data was achieved.