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83 Dr. rer. nat. Michaela Müller Phone +49 711 970-4140 Dr. rer. nat. Christian Oehr Phone +49 711 970-4137 Contact Funding We would like to thank the German Federal Ministry of Educa- tion and Research (BMBF) for funding the project “PeTrA”, promotional reference 13N11457. Project partners Evonik Industries AG, Darmstadt, Germany | Merck KGaA, Darmstadt, Germany | EMC microcollections GmbH, Tübingen, Germany | Helmholtz Centre for Infection Research (HZI), Braun- schweig, Germany Specific infrared bands Infrared spectra of the tissue model and the particle formula- tion show that the particles have a specific absorption band at 1760 cm–1 and can therefore be distinguished from the surrounding tissue matrix. The matrix is composed mainly of collagen and exhibits typical absorption bands at approx. 1650 cm–1 and 1550 cm–1 (amide bands). The infrared images were then analyzed such that the intensity distribution of the relevant integrated absorption bands is illustrated in the ranges of 1800–1700 cm–1 and 1700–1500 cm–1 . A comparison between the optical IR microscope image (Fig. 3) and the two images that were analyzed (Figs. 4 and 5) clearly shows the regions with larger accumulations of particles. The formulation investigated here accumulated in the apical portion of the tissue model Results were confirmed using alternative methods. Based on this example, we were able to demonstrate that detection of accumulations of non- labelled active ingredient particles in cross-sections of the intestinal tissue model is possible using IR microscopy. Perspectives Infrared microscopy on biological samples is an option when- ever spatially resolved chemical information in the range of approx. > 3 µm is required. For example, it is also suitable for the identification of particles in tissue from implants that have been produced by abrasion or for the identification of optically visible deposits in tissue as a result of protein dena- turation or mineralization. 1 Light microscopy of a tissue model with dyed Caco-2 cells. 2 Infrared spectrum in the range of 2000–1000 cm–1 . 3 Optical image of the sample site sub- jected to analysis. 4 FPA imaging in the range of 1700–1500 cm–1 ; blue/green: tissue. 5 FPA imaging in the range of 1800–1700 cm–1 ; blue/green: particles. 3 4 5 Phone +49711970-4140 Phone +49711970-4137 345