Detection of HOCl-driven degradation of the pericardium scaffolds by label-free multiphoton fluorescence lifetime imaging

Artificial biomaterials can significantly increase the rate of tissue regeneration. However, implantation of scaffolds leads not only to accelerated tissue healing but also to an immune response of the organism, which results in the degradation of the biomaterial. The synergy of the immune response and scaffold degradation processes largely determines the efficiency of tissue regeneration. Still, methods suitable for fast, accurate and non-invasive characterization of the degradation degree of biomaterial are highly demandable. Here we show the possibility of monitoring the degradation of decellularized bovine pericardium scaffolds under conditions mimicking the immune response and oxidation processes using multiphoton tomography combined with fluorescence lifetime imaging (MPT-FLIM). We found that the fluorescence lifetimes of genipin-induced cross-links in collagen and oxidation products of collagen are prominent markers of oxidative degradation of scaffolds. This was verified in model experiments, where the oxidation was induced with hypochlorous acid or by exposure to activated neutrophils. The fluorescence decay parameters also correlated with the changes of micromechanical properties of the scaffolds as assessed using atomic force microscopy (AFM). Our results suggest that FLIM can be used for quantitative assessments of the properties and degradation of the scaffolds essential for the wound healing processes in vivo.

Blind source separation of molecular components of the human skin in vivo: non-negative matrix factorization of Raman microspectroscopy data

Determination of the molecular composition of the skin is crucial for numerous tasks in medicine, pharmacology, dermatology and cosmetology. Confocal Raman microspectroscopy is a sensitive method for the evaluation of molecular depth profiles in the skin in vivo. Since the Raman spectra of most of the skin constituents significantly superimpose, a spectral decomposition by a set of predefined library components is usually performed to disentangle their contributions. However, the incorrect choice of the number and type of components or differences between the spectra of the basic components measured in vitro and in vivo can lead to incorrect results of the decomposition procedure. Here, we investigate an alternative data-driven approach based on a non-negative matrix factorization (NNMF) algorithm of depth-resolved Raman spectra of skin that does not require a priori information of spectral data for the analysis. Using the model and experimentally measured depth-resolved Raman spectra of the upper epidermis in vivo, we show that NNMF provides depth profiles of endogenous molecular components and exogenous agents penetrating through the upper epidermis for the spectra and concentration. Moreover, we demonstrate that this approach is capable of providing new information on the molecular profiles of the skin.

Melanin distribution from the dermal-epidermal junction to the stratum corneum: non-invasive in vivo assessment by fluorescence and Raman microspectroscopy

The fate of melanin in the epidermis is of great interest due to its involvement in numerous physiological and pathological processes in the skin. Melanin localization can be assessed ex vivo and in vivo using its distinctive optical properties. Melanin exhibits a characteristic Raman spectrum band shape and discernible near-infrared excited (NIR) fluorescence. However, a detailed analysis of the capabilities of depth-resolved confocal Raman and fluorescence microspectroscopy in the evaluation of melanin distribution in the human skin is lacking. Here we demonstrate how the fraction of melanin at different depths in the human skin in vivo can be estimated from its Raman spectra (bands at 1,380 and 1,570 cm-1) using several procedures including a simple ratiometric approach, spectral decomposition and non-negative matrix factorization. The depth profiles of matrix factorization components specific to melanin, collagen and natural moisturizing factor provide information about their localization in the skin. The depth profile of the collagen-related matrix factorization component allows for precise determination of the dermal-epidermal junction, i.e. the epidermal thickness. Spectral features of fluorescence background originating from melanin were found to correlate with relative intensities of the melanin Raman bands. We also hypothesized that NIR fluorescence in the skin is not originated solely from melanin, and the possible impact of oxidized species should be taken into account. The ratio of melanin-related Raman bands at 1,380 and 1,570 cm-1 could be related to melanin molecular organization. The proposed combined analysis of the Raman scattering signal and NIR fluorescence could be a useful tool for rapid non-invasive in vivo diagnostics of melanin-related processes in the human skin.

Label-Free Multiphoton Microscopy: The Origin of Fluorophores and Capabilities for Analyzing Biochemical Processes

Multiphoton microscopy (MPM) is a method of molecular imaging and specifically of intravital imaging that is characterized by high spatial resolution in combination with a greater depth of penetration into the tissue. MPM is a multimodal method based on detection of nonlinear optical signals - multiphoton fluorescence and optical harmonics - and also allows imaging with the use of the parameters of fluorescence decay kinetics. This review describes and discusses photophysical processes within major reporter molecules used in MPM with endogenous contrasts and summarizes several modern experiments that illustrate the capabilities of label-free MPM for molecular imaging of biochemical processes in connective tissue and cells.