Preparation And Formation Mechanism Of Drug Carrier Cavities On Surface Of 316L Stainless Steel

Micro- and sub-micro drug carrier cavities were fabricated on surface of 316L stainless steel by the following two compound technologies: low temperature molten salts electrodeposition/anodization and magnetron sputtering/anodization. The effect of structure and thickness of aluminum films and anodizing process parameters on the structure, density and size distribution of the cavities were studied systematically. The formation mechanisms of different structure cavities were discussed. In addition, the preliminary investigations regarding to the structure of cavities on the blood-compatibility of 316L stainless steel were conducted in this work.The surface morphology, composition and phase structure of aluminum films deposited at different processing parameters were studied by scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction methods. The results showed that the properties of aluminum films fabricated by molten salt electroplating were mainly affected by current density, while the properties of aluminum films obtained by magnetron sputtering were mainly affected by the negative bias. In comparison with molten salt electroplating process, aluminum films prepared by magnetron sputtering had finer grain size and denser structure. Morphologies of the cavities on 316L stainless steel were studied by scanning electron microscopy. The resulets showed that two typical type cavities were fabricatied on surface of 316L stainless steel. The cavities composed with numerous of small pores in the inner walls with the substrate corroded at different degree are defined as complex cavities. The cavities without small pores in the inner wall and distributed uniform on substrate are defined as general cavities.In comparison with sputtering deposition/anodizing process, the anodizing voltage used in the preparation of micro-cavities on 316L stainless steel by low-temperature molten salt electroplating/anodizing technique are lower. The shape of cavities on stainless steel surface is mainly affected by the morphology of aluminum coating. The circle or subcircle cavities corresponded to flaky aluminum layers, while the rectangle cavities corresponded to granular aluminum layers. It was also found that the cavity size increased with increasing the anodizing voltage and time.The shape, size and number of the cavities on 316L stainless steel prepared by the magnetron sputtering deposition/anodization are closely related to the aluminum coating structure. When the aluminum grain size distribution is inhomogeneous (distributed from 1.0 to 2.5μm) and there are some porosity and defects between the grains, a large number of cavities with size distributed from 30μm to 100μm will be easily formed on the surface of stainless steel; when the aluminum grain size distribution is uniform and grain size is about 2μm accompany with the decrease of porosity and defects between the grains, the uniform dense rectangle cavities with obvious crystallographic orientation can be obtained on stainless steel surface; when the aluminum film are uniform and dense with grain size of about 1μm and there are almost no defects between the grains, the regular circle cavities can be easily obtained on stainless steel surface. Cavities with the size ranging from 0.2 to 3μm can be obtained on 316L substrates by adjusting the anodizing process parameters. When aluminum films are loose and porous, a large number of cavities with size distributed from 50μm to 100μm will be easily formed on the surface of stainless steel. The characteristics of cavities on stainless steel surface not only affected by the aluminum coating morphology, but also controlled by the thickness of aluminum coating, anodizing voltage, anodizing temperature, anodizing time and some other factors. The factors are interdependence and mutual restriction. When anodizing temperature/potential nearly constant, the critical highest anodizing potential/temperature relates to the thickness of the aluminum coatings. When the aluminum coating has a certain thickness, the characteristics of the cavities on stainless steel are controlled by the anodizing time. That is to say that the change of type of electrolyte and anodizing parameters have a significant impact on the shape, size and number of the cavities forming on 316L stainless steel surface.Through carefully studying on the morphology and formation process of cavities on 316L stainless steels surface, this paper puts forward the formation mechanisms of two typical type of the cavities. For complex cavities, at the initial stage of anodizing process, the pores are preferentially formatted on the defect areas of aluminum coatings. Takes the defect areas as center, several microporous group are formed in certain scale. The micropores in defects center have the fastest migration rate from surface coating to the substrate and the migration rate of surrounding minished in order. These microporous successively arrived at the stainless steel surface and finally formed complex cavities on the surface of 316L stainless steel. And each big cavity is composed with numerous of small pores. The stages for the formation of general cavities on 316L stainless steel surface including: the uniform formation and stable growth of micropores on aluminum coating; mciropores arriving the substrate and cavities formed on substrate at the relevant position; growth and coalescence process of cavities on substrate and the quick- corrosion of the substrate.The results of the blood compatibility indicate that compared with untreated 316L stainless steel, samples with cavities in the size of 0.5~2.0μm can effectively reduce the number of platelet adhesion, increase the hydrophilic and improve the blood compatibility of 316L stainless steel.