Immobilization of Anticoagulant Heparin on HMDI-activated 316L Stainless Steel for the Application of Coronary Artery Disease

Poor compatibility between blood and metallic coronary artery stents is one reason for arterial restenosis. Immobilization of anticoagulant heparin on the stent’s surface is feasible for improving compatibility. Prior to heparin immobilization, we examined possible surface-coupling agents for heparin immobilization. Hexamethylene diisocyanate (HMDI) and 3-aminopropyl-triethoxysilane (APTS) were examined as surface-coupling agents to activate 316L stainless steel (e.g., stent material). The activated surface was characterized by Fourier transformation infrared spectroscopy (FTIR), atomic force microscope (AFM), surface plasmon resonance (SPR), and trinitrobenzene sulfonic acid (TNBS) assay. In the FTIR analysis, HMDI and APTS were both covalently linked to 316L stainless steel. In the AFM analysis, it was found that the HMDI-activated surface was smoother than the APTS-activated one. In the SPR test, the shift of the SPR angle for the APTS-activated surface was much higher than that for the HMDI-activated surface after being challenged with acidic solution. The TNBS assay was utilized to determine the amount of immobilized primary amine groups. The HMDI-activated surface was found to consist of about 1.32 μmole/cm2 amine group, whereas the APTS-activated surface consisted of only 0.89μmole/cm2 amine group. We conclude that the HMDI-activated surface has more desirable surface characteristics than the APTS-activated surface, such as surface roughness, chemical stability, and the amount of active amine groups. The HMDI-activated 316L stainless steel (SS) was then utilized for heparin linking. The compound 1-ethyl-3-(3-dimethyl-aminopropyl) carbodiimide (EDAC) has often been utilized for the immobilization of heparin, but the critical carboxyl groups of heparin (with regards to heparin’s anticoagulant activity) will be reduced by this method. We were trying to examined possible methods of heparin immobilization without consuming these carboxyl groups. Sodium periodate (NaIO4; SP) was then used to oxidize heparin to form aldehyde groups and then coupled with bis-amine-terminated poly(ethylene glycol) (Bis-amine PEG) so as to form heparin-PEG complexes. The complexe could then be grafted onto the activated surface of the test material without losing its carboxyl groups. The heparin-PEG complex formed by EDAC method was used as comparison group. Effective surface modification of the HMDI-activated and heparin-PEG grafted 316L SS surface was confirmed using Fourier Transform Infrared Spectroscopy (FTIR), Electron Spectroscopy for Chemical Analysis (ESCA) and a water contact angle test. After the heparin grafted by SP, the surface showed an improvement in antithrombrin (AT) binding ability, its anticoagulant property, and hemocompatibility in comparison to heparin grafted by EDAC.