Melanin Nanoparticle-Incorporated Silk Fibroin Hydrogels for the Enhancement of Printing Resolution in 3D-Projection Stereolithography of Poly(ethylene glycol)-Tetraacrylate Bio-ink

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It is not easy to design structures with transparent solutions, especially in light projection three-dimensional (3D) printing, since the penetration of light in solution is limitless. Here, silk fibroin incorporated with melanin nanoparticles (SFM) is used as a transparency modifier of poly(ethylene glycol)-tetraacrylate (PEG4A) solution. The incorporation of melanin into the SF hydrogel is performed in the range of 0.05-0.2% (w/v), and the SFM was added to the PEG4A precursor solution at 0.25-1.0% (w/v). The printing accuracy was examined by comparing the printed and designed feature sizes. The addition of 1.0% (w/v) SFM to a 4% (w/v) PEG4A (PEG4A/SFM) precursor solution effectively reduces the transparency of the solution and improves the printing resolution by confining the light beam to a designed region. This enables the fabrication of hard-to-express features such as hollow blood vessels or vacant tubes. Furthermore, the elastic modulus of the printed PEG4A/SFM composite hydrogel increases 2.5-fold higher than the PEG4A hydrogel without SFM. For the bio-ink, PEG4A/SFM-containing cells show non-cytotoxicity and improve the proliferation rate of embedded cells, confirming the high biocompatibility of PEG4A/SFM hydrogels.