# Mueller Matrix-Based Approach for the Ex Vivo Detection of Riboflavin-Treated Transparent Biotissue
## 1 Introduction  
The fast and non-invasive detection of tissue properties can be relevant to investigations in the life sciences or medical diagnostics and could provide interesting alternatives to invasive approaches. Optical techniques have already proven their impressive capabilities for the detection of subtle effects in biotissues, in many cases also for in vivo measurements. However, techniques such as Brillouin microscopy [1], optical coherence tomography (OCT) [2], optical coherence elastography [3, 4], laser scanning microscopy [5], hyperspectral imaging [6], multi-photon imaging [7] or diffuse reflectance methods [8, 9] rely on relatively sophisticated equipment and might not be applicable to certain use cases. With regard to achievable contrast or sensitivity, all approaches have their individual strengths and weaknesses. In particular, the visualization of effects in transparent biological media with reasonable sensitivity and speed, ideally in vivo and under real conditions asclinical settings represent an important challenge to be addressed. Additionally, as laser light is employed as the coherent radiation in most cases, safety aspects must be considered. The highly ordered structure of some bio-tissues [10; 11] provides measurable depolarization values in polarimetry setups [12]. The human cornea is one such bio-tissue consisting of successive layers of collagen fibrils immersed in an amorphous ground substance containing water, proteins and salts [13]. Such a formation is optically anisotropic and is therefore a suitable sample for polarimetry measurements [14]. Corneal collagen cross-linking (CXL) is a promising treatment that increases the mechanical stiffness of the cornea. This is particularly useful for several diseases in ophthalmology such as keratocous in which the cornea thins over time. The treatment combines the application of ultraviolet-A light and a chromophore-like riboflavin following the Dresden protocol to induce polymerization [15]. The quantitative assessment of the treatment was performed via corneal stroma depth and biomechanical stiffness measurements. To date, optical methods such as OCT [16], confocal microscopy [17] and non-linear microscopy [18] were also applied for this purpose. Mueller matrix (MM) polarimetry is promising for the evaluation of CXL treatment because the degree of orientation in the samples can in principle be detected by polarization-sensitive measurements [19; 20]. Compared to the optical methods mentioned above, it provides a relatively simple and straightforward approach that delivers information about the polarization changing properties of a sample [9; 21; 22; 23]. This relies on the measurement of the laser intensity reflected from or transmitted through a sample for different polarization states of the radiation before and after the interaction with the biotissue. Among many others, Mueller matrix polarimetry was utilized for the examination of skin tissue properties in dermatology [24], the assessment of the quality of tissue implants [20] or the identification of cancerous tissue sections from excised samples [25]. In addition, the technique was applied for the evaluation of \(\beta\)-amyloid progression in [PERSON]'s disease [21]. Depending on the application, measurements in reflection or transmission mode are required and can be realized. As the Mueller matrix contains the full polarization information of a sample, it is advantageous over other biotissue imaging techniques such as non-contact dermoscopy which uses cross-polarization configurations for imaging [19; 26; 27]. In this work, we employed Mueller matrix polarimetry for the fast and non-invasive analysis of transparent biological media. As an example, we investigated porcine cornea samples ex vivo to demonstrate that small effects originating from photo-induced collagen cross-linking processes in the tissue can be detected.