## 3 Results and Discussion
### In Vivo Study
#### 3.4.2 Histology study  
Half of the explants containing skin and stent samples were embedded in paraffin, sectioned and stained using a standard histological stain (H&E, Figure 18a,b). For both metals, the ribbon material left cavities in the tissue sections because it either degraded during tissue processing (Mg) or, if hard material remained, it almost always rinsed off the sectioned slide. As shown in Figure 18a, the presence of a Mg sample stimulated a very mild inflammatory response, with no significant capsule (black open arrowheads) formation. As shown in Figure 18b, where two sections of Zn segments can be seen, there was slightly more inflammation (black solid arrow points to a region with an accumulation of inflammatory cells). There was a more thickened capsule region around the Zn stent compared to the Mg stent in Figure 18a (black open arrowheads), however, this capsule was still not complex nor dense compared to reactions to other implanted biomaterials. No evidence of necrotic tissue was observed. The thicker capsule with the Zn stent (versus Mg) might be explained by the more intact nature of Zn. A denser material could be somewhat more irritating to the tissues. Other ribbon samples were processed as whole mounts, which were visualized via confocal microscopy (Figure 18c), and paraffin sections were immunost and viewed via transmission microscopy (Figure 18d,e). In Figure 18c, blood  
Figure 16: Optical images of ribbon segments after 15 days of subcutaneous implantation: (**a**) uncoated Zn segment. The red circle shows that the strut outlines were still similar to the original after 15 days; (**b**) AZ31 Mg alloy segments showed gaps and irregular surfaces (white arrowheads point to a few strikingly corroded areas).  
Figure 17: Optical images (taken on a light microscope) of ribbon segments after in vivo implantation for 15 days, removal with skin, tissue fixation, and tissue clearing. (**a**) An uncoated Zn segment was basically intact. The red circle directs attention to segments of the stents that were intact after 15 days; (**b**) an AZ31 Mg segment was degraded to a greater extent, as shown by gaps and irregular surfaces (white arrowheads point to a few extensively corroded areas).  
vessels (non-specifically labeled in this view due to the autofluorescence of red blood cells) appeared as light green (arrows). The blood vessels clustered closely around the stent material. This suggests that the Zn ribbon was not irritating to the vessels and there was no dense capsule that prevented blood vessels from approaching the metal. Sections from the paraffin-embedded stents were also immunostined for the presence of axons (with an antibody against a neurofilament protein, red in Figure 18d,e) and blood vessels (with an antibody against smooth muscle actin, green in Figure 18d,e). Figure 18d and e show the immunostaining of sections from a Mg (Figure 18d) and Zn (Figure 18e) explant. In both, nerves (open white arrows) and blood vessels (solid white arrows) were seen in tissues close to the explant (outline of explant is shown in white). Nuclei are labeled blue in 18d and e. Taken together, the histological staining suggested no toxicity of either ribbon material and excellent biocompatibility with normal tissues.