# Biomechanical Characterization of Abdominal Aortic Aneurysm: The Rupture Mechanism
## 3 Results
### Micromechanical Properties of the Mice Artery  
Stiffness maps of the media layer in the AAA group and the control group are shown in Figure 4. Both the AAA and control groups clearly showed two phases, which we assumed to be collagen and elastin, respectively. The AAA group showed discontinuities in the stiffness map, as the yellow dashed lines show the boundaries in Figure 4a,b. As a contrast, the control group showed a strip pattern of collagen and elastin (Figure 4c,d). The fragmented pattern in the AAA group and the striped pattern in the control group are consistent with the observations of inverted microscopy. The orientation of the elastin stripes in the stiffness maps was also consistent with observations made using an inverted microscope. The width of the elastin stripes was 2-5 \(\upmu\)m, as quantified with the inverted microscope.  
Figure 2: Representative force–displacement curve.  
Figure 4: Stiffness maps of the (**a**) AAA tested region (15 \(\times\) 15 \(\upmu\)m\({}^{2}\), 16 \(\times\) 16 indentation points); (**b**) AAA tested region (15 \(\times\) 15 \(\upmu\)m\({}^{2}\), 20 \(\times\) 20 indentation points); (**c**) healthy control aorta tested region (15 \(\times\) 15 \(\upmu\)m\({}^{2}\), 16 \(\times\) 16 indentation points); and (**d**) healthy control aorta tested region (15 \(\times\) 15 \(\upmu\)m\({}^{2}\), 20 \(\times\) 20 indentation points).  
Figure 3: Immunofluorescence staining of (**a**) AAA and (**b**) healthy control aorta (blue: cell nuclei and red: elastin). The green channel is autofluorescence with increased brightness and contrast to visualize the artery. Picrosirius Red staining of (**c**) AAA and (**d**) healthy control aorta. (Scale bar is 100 \(\upmu\)m, black arrows refer to the micro vessels).  
Histograms from AFM scanning with two different probe sizes for the AAA group and control group are shown in Figure 5. Both the AAA group and the control group, tested with the probe tip size of 20 nm, showed a larger range of stiffness than the probe size of 5 \(\upmu\)m. This is because the larger probe size showed relative higher averaged values due to the larger indent area, and thus the relatively low and high stiffnesses of individual fibrils were omitted. The differences in histograms detected by different probe sizes indicated that the aortas are highly heterogeneous, no matter if they were from the AAA group or the control group. The AAA group showed a peak frequency in the interval of 40-50 kPa for both probe sizes, which is not observed in the control group. This peak frequency at a high stiffness interval indicates the increased stiffness of AAA due to the increased density of collagen. Although the inverted microscopy for the control group showed separate elastin and collagen phases, this group showed quite a similar histogram with two different probe sizes.  
Figure 5: \(Cont.\)  
The Young's modulus of the AAA group and the control group, in terms of averaged values of different scanning sites and with different probe sizes, are presented in Figure 6. In total, 1395 indentation curves for AAA groups and 1277 indentation curves for the control group were tested. The AAA group showed a significant higher stiffness than the control group, and the results were consistent with different probe sizes (46.42 \(\pm\) 23.18 kPa vs. 30.58 \(\pm\) 28.01 kPa for a probe size of 5 \(\upmu\)m, 50.77 \(\pm\) 62.4 kPa vs. 40.6 \(\pm\) 51.86 kPa for a probe size of 20 nm). The probe size of 20 nm showed a larger standard deviation for both the AAA group and control groups, as expected, as shown by the larger stiffness ranges observed in the histograms in Figure 5.  
Figure 5: Histogram of stiffness data of (**a**) AAA tested with a 20 nm probe; (**b**) AAA tested with a 5 \(\upmu\)m probe; (**c**) healthy aorta tested with a 20 nm probe; and (**d**) healthy aorta tested with a 5 \(\upmu\)m probe.