Colorful Pigments for Hair Dyeing Based on Enzymatic Oxidation of Tyrosine Derivatives

Bioinspired Three-Component System to Prepare Full-Color Functional Biomimetic Pigments

Nature provides a great source of inspiration for the development of sustainable materials with excellent properties, among which melanin with optical, electronic, and radiation protection properties are considered to be promising coloring materials. However, compared to chemical pigments, the single color, complex oxidation process, and poor solubility of natural melanin strongly limit their further applications. Here, we introduce a series of melanin-like polymeric pigments with amino acid-encoded physicochemical properties by a simple three-component reaction system. Our protocol enables artificial control of the chromophore structures through the rational design of the substrates and dopants, thereby combining the safety and functionality of biopigments with the color richness of chemical dyes. Similar to the photoprotective effect of natural melanin, the polymeric pigments showed excellent antioxidant activity in reducing free radicals and have the advantages of iridescent color, strong tinting strength, stability, and affordability. Furthermore, due to their ability to dye substrates, these biomimetic are expected to become new low-cost bioactive chromophores and find various biochemical applications such as in clothing and hair dyeing, food addition, and anticounterfeiting detection.

Melanin for Photoprotection and Hair Coloration in the Emerging Era of Nanocosmetics

Nanotechnology is revolutionizing fields of high social and economic impact. such as human health preservation, energy conversion and storage, environmental decontamination, and art restoration. However, the possible global-scale application of nanomaterials is raising increasing concerns, mostly related to the possible toxicity of materials at the nanoscale. The possibility of using nanomaterials in cosmetics, and hence in products aimed to be applied directly to the human body, even just externally, is strongly debated. Preoccupation arises especially from the consideration that nanomaterials are mostly of synthetic origin, and hence are often seen as "artificial" and their effects as unpredictable. Melanin, in this framework, is a unique material since in nature it plays important roles that specific cosmetics are aimed to cover, such as photoprotection and hair and skin coloration. Moreover, melanin is mostly present in nature in the form of nanoparticles, as is clearly observable in the ink of some animals, like cuttlefish. Moreover, artificial melanin nanoparticles share the same high biocompatibility of the natural ones and the same unique chemical and photochemical properties. Melanin is hence a natural nanocosmetic agent, but its actual application in cosmetics is still under development, also because of regulatory issues. Here, we critically discuss the most recent examples of the application of natural and biomimetic melanin to cosmetics and highlight the requirements and future steps that would improve melanin-based cosmetics in the view of future applications in the everyday market.

Melanin-Inspired Composite Materials: From Nanoarchitectonics to Applications

Synthetic melanin is a mimic of natural melanin analogue with intriguing properties such as metal-ion chelation, redox activity, adhesion, and broadband absorption. Melanin-inspired composite materials are formulated by assembly of melanin with other types of inorganic and organic components to target, combine, and build up the functionality, far beyond their natural capabilities. Developing efficient and universal methodologies to prepare melanin-based composite materials with unique functionality is vital for their further applications. In this review, we summarize three types of synthetic approaches, predoping, surface engineering, and physical blending, to access various melanin-inspired composite materials with distinctive structure and properties. The applications of melanin-inspired composite materials in free radical scavenging, bioimaging, antifouling, and catalytic applications are also reviewed. This review also concludes current challenges that must be addressed and research opportunities in future studies.

D-A Structured High-Performance Photothermal/Photodynamic Thionin-Synthetic Melanin Nanoparticles for Rapid Bactericidal and Wound Healing Effects

Synthesized melanin nanoparticles (SMNPs) are used as advanced photothermal materials. However, their internal structures are complex and disordered, and tuning the photothermal performance of nanoparticles is still a hot spot of concern. This article presents thionin (Th)-doped SMNPs, namely Th-SMNPs, which are the first SMNPs formed using the one-pot polymerization of Th with Levodopa. Th can undergo Michael addition and Schiff base reaction between indole dihydroxy/indolequinone and their oligomers to form donor-acceptor pairs in the structure to modulate the photothermal performance of SMNPs. Structural and spectroscopic analyses and density functional theory simulations further confirm the existence of the donor-acceptor structure. Th-SMNPs exhibit excellent total photothermal efficiency (34.49%) in the near-infrared region (808 nm), which is a 60% improvement compared to SMNPs. This allows Th-SMNPs to exhibit excellent photothermal performance at low power 808 nm laser irradiation. Meanwhile, Th not only enhances the photothermal properties of SMNPs, but also imparts photodynamic effects to SMNPs. Th-SMNPs can produce 1 O2 under 660 nm laser irradiation. A dual-function photothermal and photodynamic textile named Th-SMNPs@cotton is constructed based on Th-SMNPs, which can act as a rapid photothermal/photodynamic sterilization and is promising for wound healing treatment of bacterial infections under low-power dual laser irradiation.

Functionalization of and through Melanin: Strategies and Bio-Applications

A unique feature of nanoparticles for bio-application is the ease of achieving multi-functionality through covalent and non-covalent functionalization. In this way, multiple therapeutic actions, including chemical, photothermal and photodynamic activity, can be combined with different bio-imaging modalities, such as magnetic resonance, photoacoustic, and fluorescence imaging, in a theragnostic approach. In this context, melanin-related nanomaterials possess unique features since they are intrinsically biocompatible and, due to their optical and electronic properties, are themselves very efficient photothermal agents, efficient antioxidants, and photoacoustic contrast agents. Moreover, these materials present a unique versatility of functionalization, which makes them ideal for the design of multifunctional platforms for nanomedicine integrating new functions such as drug delivery and controlled release, gene therapy, or contrast ability in magnetic resonance and fluorescence imaging. In this review, the most relevant and recent examples of melanin-based multi-functionalized nanosystems are discussed, highlighting the different methods of functionalization and, in particular, distinguishing pre-functionalization and post-functionalization. In the meantime, the properties of melanin coatings employable for the functionalization of a variety of material substrates are also briefly introduced, especially in order to explain the origin of the versatility of melanin functionalization. In the final part, the most relevant critical issues related to melanin functionalization that may arise during the design of multifunctional melanin-like nanoplatforms for nanomedicine and bio-application are listed and discussed.

Enzymatic polymerization of enantiomeric L-3,4-dihydroxyphenylalanine into films with enhanced rigidity and stability

_L-3,4-dihydroxyphenylalanine is an important molecule in the adhesion of mussels, and as an oxidative precursor of natural melanin, it plays an important role in living system. Here, we investigate the effect of the molecular chirality of 3,4-dihydroxyphenylalanine on the properties of the self-assembled films by tyrosinase-induced oxidative polymerization. The kinetics and morphology of pure enantiomers are completely altered upon their co-assembly, allowing the fabrication of layer-to-layer stacked nanostructures and films with improved structural and thermal stability. The different molecular arrangements and self-assembly mechanisms of the _L+D-racemic mixtures, whose oxidation products have increased binding energy, resulting in stronger intermolecular forces, which significantly increases the elastic modulus. This study provides a simple pathway for the fabrication of biomimetic polymeric materials with enhanced physicochemical properties by controlling the chirality of monomers.

Permanent Low-Toxicity Hair Dye Based on Pregrafting Melanin with Cystine

A pregrafting strategy is presented to construct covalent bridges between synthetic melanin, i.e., polydopamine (PDA), and hair for permanent hair dyeing. As a result, PDA is more uniformly distributed throughout the hair surface, and the dyed hair shows higher color intensity and better durability to washing than the control samples dyed with PDA directly at the same conditions.