Introduction: Absorbable metals have been widely tested in various in vitro environments to evaluate their biodegradation as vascular stent materials[1]. However, there exists a gap between in vivo and in vitro test results. A key step is to identify the relevant biochemical and biophysical microenvironments in test-systems. The purpose of this study was to establish an appropriate methodology for accurate and standardized determinations of degradation parameters in the pre- and post- endothelialization stage after stent implantation, which plays a vital part in predicting the fate of magnesium (Mg)-based stent.
Materials and Methods: As-drawn Mg wires of 99.9% purity with a length of 10 mm and a diameter of 250 μm (Goodfellow, USA) were used. In the standardized immersion test, Mg wires were immersed in DMEM solution according to the standard protocol ASTM-D1141-98[2]. The LumeGen bioreactor (TGT DynaGen® Series, USA) as a vascular bioreactor was chosen to stimulate physiological aortal conditions (Fig. 1a). Each porcine abdominal aorta with a diameter of ~8 mm and a length of 5 cm was mounted into a chamber with a flow rate of 100 ml/min and a pulse pressure of 80-120 mmHg (Fig. 1b). In aortal in vivo test, rat abdominal aortas were used for implantation. Two Mg segments were symmetrically implanted into the lumen and wall of each aorta in the bioreactor or in vivo tests (Fig. 1c). Samples were analysed by X-ray computed tomography (CT).
Fig. 1.(a) Photograph of bioreactor. Mg wires in the wall and lumen of a porcine aorta in the chamber (b) and rat aorta in vivo (c).
Results and Discussion: One of two Mg wires exposed on the lumen and contacted the circulating medium to simulate a pre-endothelialization stage. The other Mg wire was embedded under the intima to simulate a post-endothelialization stage. The degradation product volumes, residual Mg volumes and average degradation rates were calculated utilizing the CT data. In Fig. 2, in vivo degradation was slower than in vitro degradation both in the standardized static and flow bioreactor conditions. In terms of the aortal bioreactor, flow convection on the lumen surface severely accelerated Mg degradation due to the increase of mass transfer, fluid shear stress and pulsatile stress, compared with low Mg degradation rate in the wall. In the aortal in vivo model, the degradation rate of Mg wire in the wall was faster than that in the lumen, and calcification was observed around Mg surrounding tissue. The degradation of Mg wire in the wall (diffusion limited region) was accelerated, which may result in the decrease of local pH from lysosome.
Fig. 2. Volume ratios of residual Mg and corrosion products in the static no aorta, and the aortal lumen and wall of the bioreactor and in vivo conditions for 3-day.
Conclusions: First-time investigation of degradation testing in the aortal bioreactor revealed that hydrodynamics plays a dominant role on the degradation. The established porcine aortal bioreactor and rat aortal in vivo model are expected to provide a better understanding of degradation mechanism of absorbable metallic stents.
NIH grant (1SC3GM113728-01); NSF-ERC for revolutionizing metallic biomaterials (NSF-0812348); National Natural Science Foundation of China under Grant (81330031); Thanks Prof. Deling Kong, Dr. Zhihong Wang and Ms. Yifan Wu from Nankai University for in vivo test.
References:
[1] ASTM-D1141-98: Standard Practice for the Preparation of Substitute Ocean Water Annual Book of ASTM, Philadelphia, Pennsylvania, USA (2004)
[2] Zheng Y F, Gu X N, Witte F. Biodegradable metals[J]. Materials Science and Engineering: R: Reports, 2014, 77: 1-34.