Optimizing White Blood Cell Contrast In Graded-Field Capillaroscopy Using Capillary Tissue Phantoms

Capillaroscopy is a simple microscopy technique able to measure important clinical biomarkers non-invasively. For example, optical absorption gaps between red blood cells in capillary vessels of the nailfold have been shown to correlate with severity of neutropenia. The direct visualization of individual white blood cells with capillaroscopic techniques is elusive because it is challenging to generate epi-illumination phase contrast in thick turbid media. Here, we evaluate white blood cell visibility with graded-field capillaroscopy in a flow phantom. We fabricate capillary phantoms with soft photolithography using PDMS doped with TiO2 and India ink to emulate skin optical properties. These glass-free phantoms feature channels embedded in scattering media at controlled depths (70-470 mu m), as narrow as 15 x 15 mu m, and permit blood flow up to 6 mm/s. We optimize the contrast of the graded-field capillaroscope in these tissue-realistic phantoms and demonstrate high speed imaging (200 Hz) of blood cells flowing through scattering media.