Skin consists of a lamellar structure with diverse cell types (e.g., immune cells, melanocytes, and basal cells) that periodically detach from the basement membrane, move to the surface, and die for self-renewal. [3] Melanocytes are a critical cell type that generate melanin to absorb UV light (290-400 nm), which is a major risk for skin diseases (e.g., melanoma) due to DNA damage. [4][5][6][7] Here, melanin-containing organelles called melanosomes are transferred to the surrounding keratinocytes. This increase in melanosome concentration leads to darker skin phototypes, and darker phototypes can be a function of racial background or previous sun exposure, that is, tanning. [8] Indeed, skin pigmentation depends on variations in the size, number, clustering phase, and the proportions between melanin species (e.g., eumelanin and pheomelanin). [9] Skin pigmentation has been quantified using melanosome volume fraction (M f ) parameter: 1.3-6.3% for lightly pigmented adults, 11-16% for moderately pigmented adults, and 18-43% for darkly pigmented adults. [10] Variations in skin phototypes can complicate biomedical optics. Melanin absorption increases linearly from 800 to 600 nm and exponentially from 600 to 300 nm. [11,12] Darker skin phototypes can absorb and scatter more photons: as a result, incident light is attenuated before it reaches the target of interest, and signal transmission can be impeded back to the sensor. Therefore, variations in skin phototypes have negatively affected many forms of medical optic technology including pulse oximetry, [13,14] cerebral tissue oximeters, [15] optical coherence tomography, [16] wearable electronics, [17][18][19] photoacoustic (PA) imaging, [20] fluorescence imaging, [21] and photothermal therapy. [22] One recent study compared 48 097 pairs of oxygen saturation levels measured by pulse oximetry and arterial blood gas test obtained from 8675 white patients and 1326 black patients. [13] The results found that pulse oximetry had trouble in diagnosing hypoxemia in 11% Black patients and 3% white patients due to light absorption by melanin. [13,14] Furthermore, wearable electronics (e.g., smartwatches) have reported inaccuracies in heart rate readings occurring more often in users with dark skin than light skin. [17,18] Clearly, the impact of differences in skin phototypes underscore the ongoing need to understand and correct racial bias in optical technologies. While larger 3D-bioprinted skin-mimicking phantoms with skin colors ranging across the Fitzpatrick scale are reported. These tools can help understand the impact of skin phototypes on biomedical optics. Synthetic melanin nanoparticles of different sizes (70-500 nm) and clusters are fabricated to mimic the optical behavior of melanosome. The absorption coefficient and reduced scattering coefficient of the phantoms are comparable to real human skin. Further the melanin content and distribution in the phantoms versus real human skins are validated via photoacoustic (PA) imaging. The PA signal of the phantom can be improved ...

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Inorganic nanomaterials hold immense potential in theranostics, but their translation is limited by the toxicity resulting from non-degradability. Bulk phosphate-based glasses offer great biodegradability, biocompatibility, and ease in incorporating imaging dyes and drugs. However, the facile and mild solution-based synthesis of micro-/nanoparticles of this material is yet to be explored. Inspired by the biosilicification process in the diatom, we created polyethylenimine (PEI)/phosphate aggregates via the hydrogen bonding between amine (PEI) and hydroxyl (phosphate) groups, which was proved by dynamic light scattering and 1 H solution nuclear magnetic resonance. The sol−gel reaction between the calcium precursor and PEI/phosphate aggregates yielded degradable PEI/ calcium phosphate (PEI/CP) micro-/nano-composites with versatile sizes (396 ± 128 nm to 63 ± 8 μm) and morphologies (hexagonal micro-disc, micro-flower, micro-leaf, nano-butterfly, and nanoribbon). The PEI/CP composition and chemical structure were examined. PEI/CP has negligible cell cytotoxicity and degrades within 24 h. In vitro studies showed the promise for PEI/CP in transient ultrasound and photoluminescence imaging.