6. The Application of Melanin
6.1. Applications in Agriculture and Industry
In industry, melanin can be used as an adsorbent for heavy metal pollution control. Heavy metals in various chemical forms and states are highly migratory, enriched, latent, and biotoxic and accumulate when they enter the environment and human body, threatening ecological environment safety and human health
[194][101]. The prevention and control of heavy metal pollution are important to solve environmental problems. Compared with other adsorbents, nanoparticles have a higher reaction rate and adsorption capacity due to their small size, large specific surface area, high surface energy, and chemical activity. As a typical natural, non-toxic nanoparticle
[195][102], melanin has a large specific surface area, and the structure surface has many functional groups and active sites that can bind to heavy metal ions. Secondly, melanin itself can be uniformly dispersed but insoluble in water, which facilitates subsequent operations such as solid-liquid separation. This excellent property of melanin can be effectively utilized and applied in various industries.
It has been proved that melanin has an ideal effect as an adsorbent, which can effectively remove Hg, Cr, Pb, Cu, and other heavy metals in a short period of time (3 h). In addition, melanin also shows good adsorption capacity when the concentration of heavy metals in the solution is lower than 10 mg/L. The adsorption capacity even surpasses some commercial adsorbents currently available on the market
[185,196][103][104]. Melanin can also adsorb some organic drug molecules because of its diverse and heterogeneous structure. Melanin can bind widely used drugs in high concentrations, such as chloroquine, amlodipine, atorvastatin, and telmisartan. The binding amount is positively correlated with the drug concentration
[197,198][105][106]. In agriculture, melanin can also be used as a pesticide photoprotective agent, prolonging the action of drugs because of the good light absorption effect. Melanin can reduce the influence of light and climate conditions on the biopesticides of Bacillus thuringiensis and is an ideal photo-protectant of biopesticides
[199][107].
6.2. The Application in the Food Industry
Food packaging is an important part of the food industry. Its main role is to protect food from various external factors, such as temperature, ultraviolet light, humidity, oxygen, pressure, microorganisms, etc., in order to maintain the quality of food and extend its shelf life. Melanin has effective antioxidant, anti-radiation, and antibacterial activities, which can be used in food packaging to extend the shelf life of food products. The addition of melanin to the PLA films can improve the antioxidant and antibacterial activities and enhance mechanical strength and air-tightness
[200][108]. The addition of melanin to PVA films can enhance the UV shielding and antioxidant properties
[201][109]. When melanin isolated from squid ink is added to a nanocomposite film based on carrageenan, the thermal stability, UV radiation resistance, and the ability to resist foodborne pathogens are enhanced
[202][110]. Low-density polyethylene films mixed with melanin can significantly enhance thermal stability and UV shielding, as well as water and oil resistance
[203][111]. As discussed, melanin has a promising future in food packaging.
6.3. Biomedical Applications
Melanin can significantly increase the levels of specific and non-specific (including carbon scavenging) immunity, humoral immunity, and cellular immunity
[29][112]. Calf RPE melanin can be involved in the regulation of retinal immune response, as evidenced by a significant increase in IL-6 secretion and expression in retinal epithelial cells
[204][113]. Melanin extracted from the surface of Aspergillus fumigatus conidia combined with surfactant protein D exerts a PAMP effect, stimulates the secretion of proinflammatory cytokines, activates the host immune response, and promotes the spores to be more effectively phagocytic by macrophages
[205][114]. This unique function could have promising applications for the modulatory treatment of immunocompromised patients and the development of new immune-enhancing nutraceuticals or drugs.
Melanin has the anti-tumor activity of promoting apoptosis and inhibiting angiogenesis. It can effectively improve the body weight, liver, spleen, and thymus index of mice with cancer and effectively regulate inflammation-induced high levels of various proteases, creatinine (CRE), blood urea nitrogen (BUN), and uric acid (UA) without causing toxic and side effects on other normal cells or tissues of the body
[16][115]. Due to its good light absorption effect, melanin can absorb light from the ultraviolet region to the near-infrared region and has good photothermal conversion efficiency, which can be used as a photothermal agent for multimodal imaging-guided photothermal therapy (PTT) to selectively destroy tumor cells or tissues
[40][116]. It has been reported that a transdermal microneedle patch containing melanin generates heat and promotes the uptake of tumor antigens by dendritic cells under near-infrared light. It also enhances the activity of anti-tumor vaccines and promotes anti-tumor immune responses
[41][117].
Melanin or melanoidin themselves have good metal ion chelation properties and can bind various paramagnetic metal ions, such as gadolinium, iron, manganese, etc., without causing toxicity to humans. They have ideal biocompatibility and biosafety and are potential materials for T1 contrast agents in the MRI
[36][118]. The longitudinal relaxation rate of MNP-Mn particles is significantly higher than that of Omniscan, which is about three times higher than that of gadolinium diamide. The intensity of the MRI signal reaches a maximum of 3 h after injection. Moreover, analysis with the cck-8 kit showed that MNP-Mn particles are virtually non-cytotoxic
[39][119]. In addition to binding metal ions, melanin can also enhance the signals of MRI by binding liposomes. After intravenous injection of LIP-Mel in living mice for a certain time, the MRI images of the tumor site become bright, and the MRI signals are gradually enhanced
[206][120].
6.4. Application in the Cosmetics Industry
The development and application of natural active ingredients are favored in the cosmetic industry. Melanin is a natural nanocomponent with a variety of active ingredients. Firstly, melanin, as a non-toxic natural pigment, can be used as a natural colorant in cosmetics. Secondly, the antioxidant and anti-radiation activity of melanin can be added to cosmetics as a photoprotective agent to prolong the aging time of cosmetics. In addition, it has a certain opsonizing effect on the skin, acting as an anti-aging agent, similar to vitamin C and vitamin E
[159,207][92][121].