After 4 billion years of evolution, nature has developed a wide variety of amazing structures and functions , therefore learning from nature can pave the way for designing and preparing new materials. It has been proved in most cases that the structure especially micro/nano structure of the natural biomaterials often determine its function in the actual situation [27–30]. The new type of vascular graft we designed has similar submicron longitudinally aligned topography with the native blood vessel, and it also showed higher patency rate and less thrombus formation.
The adhesion of platelets to blood-contacting surface directly influenced the blood compatibility of vascular graft. Once platelets adhered, they become activated and aggregate to form platelet thrombus, and then gradually diminish blood flow, consequently lead to vascular occlusion . A commonly accepted fact is that increase the surface roughness can lead to more platelet adhesion, because extra surface roughness usually means larger area exposed to the platelets . However, with the development of nanotechnology, the definition of roughness is further refined on the basis of roughness dimensions, and the roughness dimensions can impact the relationship between roughness and platelet adhesion. It has been proved that the materials with micrometer-scale topography exhibit more platelet adhesion at early blood contact times (2 to 5 min) [33, 34]; however, the ones with submicron-scale topography can decrease the number of the adhered platelets [35, 36]. In our study, we chose the vascular graft with smooth topography as the less surface roughness model, and evaluated the influence of surface topography on the blood compatibility through the platelet adhesion, patency rates and thrombosis. As the dimension of topography on the bionic vascular graft (267 ± 40 nm) is smaller than the diameter of individual platelet (2–4 μm), the effective contact area between platelets and actual surface is reduced, which decreases the number of the adhered platelets.
Under flow conditions, the collision frequency of platelets with the surface can also influence the interaction between material surface and platelets, which is negatively correlated with the velocity of the boundary layer [37, 38]. Previous research has indicated that if a hydrophobic surface is covered with micro/nanostructure such as posts, grooves, or others, the interstitial pores of patterned structure will not be filled with liquid due to the effect of surface tension. This non-wetting state will reduce solid/liquid contact area and friction of the fluid passing the boundaries, resulting in increased boundary velocity [39–41]. In our study, the PU vascular graft with biomimetic topography had a hydrophobic surface (contact angle 132.5 ± 3.5°) and submicron longitudinally groove-like topography. They concertedly remodeled the boundary conditions of the blood flow, which in turn largely accelerated the velocity of the boundary layer and decreased the collision frequencies of platelets with the surface, consequently leading to less platelet adhesion.
It is interesting that vascular grafts with smooth topography also had few activated platelets adhered on the surface, but both the lower patency rate and more thrombosis indicated that there were different reasons for that. We speculated that when blood flowed through the graft, the platelets were activated, aggregated and thrombus formation was triggered. Then the thrombus was detached from the grafts by the blood flow of abdominal aorta, and embolized the distal portion of the vessels. Actually, the in vitro platelet adhesion experiments under flow condition had confirmed that the vascular graft with smooth topography had more platelet adhesion in highly activated form . Our results, once again, indicated the importance of minor differences in the surface topography in directing a desired platelet response.