# Mueller Matrix-Based Approach for the Ex Vivo Detection of Riboflavin-Treated Transparent Biotissue
## 4 Summary and Outlook  
In this work, we reported MM measurements on non-planar collagen containing animal tissue, i.e., ex vivo porcine corneas. We developed a setup which allowed for the investigation of such samples in transmission and reflection mode capable of spatially resolving the inhomogeneities of the sample or the effect of photo-induced processes. The application of riboflavin followed by UV exposure along the Dresden protocol leads to noticeable changes in the MM entries indicating that these can be correlated with the photo-induced cross-linking. Furthermore, we found that the careful choice of the region-of-interest segmentation, i.e., removing overexposed areas as well as bright patches on the surface originating from impurities, leads to an improved analysis. The polarization decomposition metrics point towards changes induced by the CXL. This indicates that the results of subtle processes occurring in such media can be observed using non-invasive MM polarimetry in reflection mode. One potential application of such polarimetric measurements may address the detection of the photo-induced cross-linking of collagen fibers in the human corneas in vivo [46]. This procedure, applied to increase the mechanical stability of the cornea, was employed as therapy for various diseases in ophthalmology and demands for better monitoring technologies. Furthermore, the studies of the change of collagen texture in scars to monitor wound healing appear feasible [47]. However, better systematic quantification is required in the future steps to improve the insight into photo-induced processes as well as the dynamics of cross-linking. For the measurement setup and procedure, several improvements are reasonable to overcome the current limitations. For example, an improved mechanical holder can be developed including a spherically shaped quartz glass form to induce tension in the cornea sample tissue [48]. In the future, the analysis of the statistical moments of the distributions of individual MM elements may reveal static and dynamic changes for the cornea samples and enhance accuracy [49]. In addition, exposure bracketing can be applied to reduce overexposed areas. Also, the visualization of the MMs within the Poincare sphere remains a promising approach for the quantitative manifestation of the physical changes of sample targets. Experimentally, the polarimetric measurements could be compared with data from Brillouin spectroscopy to better correlate the optical and mechanical properties of biotissue for further validation [50].  
Conceptualization, D.F. and B.R.; data curation, G.S. and L.J.; data/formal analysis, L.J., G.S. and D.F.; funding acquisition, B.R.; experimentation, D.F. and M.F.; methodology, D.F., M.W. and B.R.; software, D.F., L.J. and G.S.; supervision, M.W. and B.R.; visualization, L.J., D.F. and G.S.; writing--original draft, D.F. and B.R.; draft editing, G.S., L.J., B.R. and M.F.; writing--review and editing, all authors. All authors have read and agreed to the published version of the manuscript.  
This project was funded by the Lower Saxony Ministry for Culture and Science (MWK) through the program Tailored Light and the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy within the Cluster of Excellence PhoenixD (EXC 2122, Project ID 390833453).  
Ethical review and approval were waived for this study, due to the fact that the samples can be considered slaughterhouse waste.  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the size of the dataset.  
The authors thank [PERSON] for his experimental support.  
The authors declare no conflict of interest.