Chitosan nanoparticles have emerged as a promising platform for mucosal drug delivery due to their unique physicochemical properties, biocompatibility, and biodegradability. Derived from chitin, a natural polysaccharide, chitosan exhibits mucoadhesive characteristics, enabling prolonged residence time at mucosal surfaces. This property is critical for enhancing drug absorption across oral, nasal, and pulmonary routes. The cationic nature of chitosan, attributed to protonated amine groups at acidic pH, facilitates electrostatic interactions with negatively charged mucosal surfaces and macromolecules such as proteins or nucleic acids. These interactions underpin its utility in vaccine and protein delivery, where stability and controlled release are paramount.

Synthesis of chitosan nanoparticles primarily involves ionic gelation, a mild and scalable technique that avoids organic solvents. In this method, chitosan is dissolved in an acidic aqueous solution, and a polyanionic crosslinker, such as tripolyphosphate (TPP), is added under stirring. The electrostatic attraction between chitosan’s amine groups and TPP’s anions leads to the formation of nanoparticles with sizes typically ranging from 100 to 300 nm. Parameters such as chitosan molecular weight, degree of deacetylation, chitosan-to-TPP ratio, and pH influence particle size, zeta potential, and drug loading efficiency. For instance, higher degrees of deacetylation enhance mucoadhesion due to increased amine content, while lower molecular weight chitosan may improve nanoparticle stability.

Characterization of chitosan nanoparticles involves multiple techniques to confirm structural integrity and functionality. Fourier-transform infrared spectroscopy (FTIR) is employed to verify the presence of amine groups, with peaks around 1650 cm−1 (amide I) and 1590 cm−1 (amine N-H bending) serving as key indicators. Dynamic light scattering (DLS) provides hydrodynamic diameter and polydispersity index, while zeta potential measurements indicate colloidal stability, with values above +30 mV suggesting strong electrostatic repulsion and reduced aggregation. Transmission electron microscopy (TEM) reveals morphology, often showing spherical or near-spherical particles. Encapsulation efficiency and drug release profiles are assessed using UV-Vis spectroscopy or high-performance liquid chromatography (HPLC), with release kinetics dependent on chitosan degradation and diffusion mechanisms.

Mucoadhesion is a defining feature of chitosan nanoparticles, mediated by hydrogen bonding and electrostatic interactions between chitosan’s amine groups and sialic acid residues in mucosal glycoproteins. This property prolongs contact time at absorption sites, enhancing drug bioavailability. For oral delivery, chitosan nanoparticles resist gastric degradation due to chitosan’s insolubility at neutral pH, enabling targeted intestinal release where pH-sensitive swelling occurs. Nasal delivery leverages the nasal mucosa’s high vascularity and avoidance of first-pass metabolism, with chitosan enhancing paracellular transport by transiently opening tight junctions. Pulmonary delivery benefits from chitosan’s ability to adhere to alveolar surfaces, promoting sustained release and reducing clearance by mucociliary action.

Biocompatibility and enzymatic degradation are critical for clinical translation. Chitosan is degraded by lysozyme and bacterial enzymes in the colon, producing non-toxic oligosaccharides that are excreted or metabolized. In vitro cytotoxicity assays, such as MTT or LDH release, demonstrate minimal toxicity in epithelial and immune cells at concentrations below 1 mg/mL. In vivo studies in rodents confirm biocompatibility, with no significant inflammatory responses observed following mucosal administration. However, the degree of deacetylation impacts degradation rates; highly deacetylated chitosan resists enzymatic breakdown, enabling tunable release profiles.

Applications in vaccine and protein delivery highlight chitosan nanoparticles’ versatility. For vaccines, chitosan’s adjuvant properties stimulate mucosal and systemic immune responses by promoting antigen uptake by dendritic cells and activating Th1/Th2 pathways. Studies show that chitosan-based nasal vaccines induce both IgA and IgG antibodies, crucial for combating respiratory pathogens. Protein delivery exploits chitosan’s ability to stabilize labile molecules like insulin or growth factors, shielding them from proteolytic degradation. For example, insulin-loaded chitosan nanoparticles administered orally exhibit hypoglycemic effects due to enhanced intestinal absorption and controlled release.

Despite these advantages, challenges remain. Batch-to-batch variability in chitosan properties necessitates rigorous quality control. Scalability of ionic gelation must address reproducibility in particle size and drug loading. Regulatory approval requires comprehensive toxicological profiling, though chitosan’s GRAS (Generally Recognized As Safe) status by the FDA facilitates progression. Future directions may explore functionalization with targeting ligands or tuning degradation kinetics for specific therapeutic windows.

In summary, chitosan nanoparticles offer a robust platform for mucosal drug delivery, combining mucoadhesion, biocompatibility, and controlled release. Their synthesis via ionic gelation is scalable and environmentally benign, while characterization techniques ensure reproducible quality. Applications in vaccine and protein delivery underscore their potential to address unmet clinical needs, provided challenges in manufacturing and regulation are met. As research advances, chitosan-based systems are poised to play a pivotal role in next-generation mucosal therapeutics.