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Cerebral oximetry based on near-infrared spectroscopy (NIRS) has seen increasing clinical use for monitoring of premature infants as well as during neonatal, pediatric and adult cardiac surgery. One key confounding factor and a likely contributor to observed inconsistency amongst commercial NIRS oximeters is skin pigmentation. Clinical studies have shown negative bias in oxygen saturation (StO2) with increasing melanin content. In prior work, we developed a cerebral oximetry phantom comprised of a 3D-printed channel array module representing brain tissue and molded silicone layers simulating extracranial regions. The purpose of the current study was to develop and test epidermis-simulating layers that exhibit realistic human pigmentation properties. Initially, we performed spectroscopic characterization of potential melanin simulating agents – including coffee, India ink, synthetic melanin, and water-soluble nigrosin – in a polydimethylsiloxane (PDMS) substrate. We determined that the NIR absorption spectrum of water-soluble nigrosin most accurately matched human melanin. Layers of 0.1 mm thickness were fabricated with different nigrosin concentrations to simulate epidermis with light, moderate, and dark pigmentation. The brain module channels were filled with bovine blood in the 30-100% oxygenation range and measurements performed with neonatal/pediatric probes from commercially available cerebral oximeters. We found that StO2 reported by the oximeters decreased monotonically with increasing pigmentation level. The magnitude of this impact increased with decreasing StO2, producing a maximum change in saturation of approximately 8%. The consistency of our results with prior clinical findings provides preliminary evidence of the utility of our approach for assessing the impact of epidermal melanin in phantom-based performance testing.
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