## 2 Keratin-Based Nanoparticles
### Chicken feathers Keratin-Based Nanoparticles  
Chicken feathers represent the major processed waste product from the poultry industry with around six billion tons of feathers produced annually. A detailed chemical characterization of chicken feathers has been conducted by [PERSON] et al., aimed at ascertaining the possible valorization strategies of these waste products [66]. The proximate analysis indicated the following composition: crude lipid (0.83%), crude fiber (2.15%), crude protein (82.36%), ash (1.49%), nitrogen free extract (1.02%), and moisture content (12.33%), whereas the ultimate analysis showed: carbon (64.47%), nitrogen (10.41%), oxygen (22.34%), and sulfur (2.64%). Thanks to keratin's durability and self-extinguishing properties, chicken feathers represent a valuable raw material in several applications, spanning from textile, plastic, cosmetics, pharmaceuticals, biomedical, and bioenergy industries (Figure 11). Furthermore, the keratin fibers could be used as moisture adsorbent, preventing skin from rashes and infections, and as bio-sorbents for treating waste water [67]. Feather KNPs have been obtained through a plethora of processes, showing tunable and different properties, and making them suitable systems for tissue engineering, wound dressing, and drug carriers. [PERSON] et al. obtained a keratin nano-powder by electro-spraying the trifluoracetic acid solution of recovered feathers protein. This procedure allowed to obtain nanoparticles with tunable diameters, depending on different parameters. The authors observed that the particle size follows a keratin-dependent concentration trend, yielding smaller KNPs at lower polymer concentration (from 24 to 53 nm), as well as increasing the voltage and the nozzle-collector distance, while a higher injection rate led to bigger KNPs. The optimization study allowed for the establishment of the following parameters for obtaining more stable KNPs with an average diameter of 53 nm: keratin concentration = 0.2% (\(w/v\)), voltage = 15 kV, nozzle-collector distance = 15 cm, feed rate = 0.047 mL/h [68]. Chicken feather KNPs displayed extraordinary biocompatibility in vitro, as demonstrated by [PERSON] and collaborators on fibroblasts, showing improved cells attachment and proliferation [69]. As reported, fibroblasts incubated with a medium containing KNPs showed a gradual and continued proliferation for five days, while cells in KNPs-free medium reached a maximum population at days two and three, followed by a steady decrease. These KNPs, prepared by phase separation and ultrasonication methods, showed an average diameter around 70 nm, and, after injection into mice, were found prevalently in kidneys, where 18% of cells were penetrated by nanoparticles. In 2013, [PERSON] et al. developed a biodegradable and biocompatible keratin/chitosan-based scaffold for bone tissue engineering applications. In detail, spherical keratin nanoparticles (nKer) with a mean diameter of 150 nm were firstly prepared by the desolvation technique, and then incorporated into a chitosan matrix (CS/nKer). CS/nKer showed a pore size ranging from 17 to 30 \(\upmu\)m, which decreases with increasing nKer content; the addition of keratin nanoparticles to the chitosan scaffold produced a slower degradation of the matrix, making it less accessible to lysozyme, despite the fact that no alteration of the swelling behavior was observed. Remarkably, CS/nKer scaffolds exhibited no significant cytotoxicity towards human osteoblastic cells (MG-63), and the addition of nKer significantly improved proteins absorption on the matrix, resulting in a porous architecture that allowed cells and nutrients penetration and tissue ingrowth [70]. Furthermore, keratin-based carriers displayed other promising biological activities, including hemostatic [71], antioxidant, and anticancer efficacies [72]. Figure 11: Possible valorization route of waste chicken feather based on chemical properties [66].