Chitin nanoparticles as innovative delivery system

1–4 It is still a challenge, in fact, to develop new carriers and techniques that allow the delivery of active compounds through the skin layers and hair follicles, in a non-invasive way at sufficient concentration. Naturally, obvious benefits can arise from ingredients that, because of their natural structure, are not only biologically safe, but can also positively enhance the acceptability of human and environmental systems. Thus, there has been renewed interest for chitin, chitosan and chitin nanofibrils (CN) to be used in different industrial fields such as pharmaceutical, cosmetic, biotextile and food packaging.5,6 At this point, it is important to underline the easy reparability in great quantities of chitin as a raw material, being a waste derivative of the fisheries industry produced at a level of one trillion tonnes per year.7–9 Thus, pure chitin is a naturally occurring biopolymer, i.e. a linear polysaccharide in which each residue is the fully acetylated N-acetyl-glucosamine. In contrast, its derivative, called chitosan, which also has a glucosamine residue, is deacetylated; that is, without acetyl groups attached to the residues. In any way, chitin is a copolymer between the fully acetylated and fully deacetylated polymers. Naturally, each of the sources of this natural product has different properties according to the proportion of these two polymer components, especially relating to their solubility and chemicophysical characteristics. Moreover, its purest form, named chitin nanofibrils (CN) (Fig. 1), being nanostructured as a crystal of 240 x 7 x 5 nm, has recently revealed to be more efficacious for cosmetic delivery, because of its cationic nature.10,11 Its relatively easy process of combination with other natural negatively charged ingredients, such as hyaluronic acid (HA) and alginate families, has led to the development of new delivery systems based on the production of CN-block copolymers’ nanoparticles. In these innovative systems, pharmaceutically or cosmetically active molecules can be easily entrapped and then subsequently released within the skin and mucous membranes, at appropriate sites or under appropriate conditions, without any harmful side effects (Fig. 2). The easy degradability of these nanoparticles and the ability to be fabricated in a variety of forms, such as gels and fibres or porous matrices, gave rise to research projects to produce biomedical tissue-non-tissue or nanocomposite films for medical purposes and food packaging. Thus the high biocompatibility of CN as nanoparticles resulted in positive effects on wound healing12,13 and also the similarity between the molecular structure of CN and hyaluronic acid (HA) (Fig. 3) has led to the production of the co-polymer CN-CN, entrapping water and different active ingredients, to be used as a moisturising and skin barrier-repairing compound14,15 or anti-ageing agent.16,17 In any case, it appears that the combination of intrinsic biological activity with a high degradability and the ability to manufacture objects in a variety of different physical forms suggest that CN is attractive for a number of innovative medical technology applications and delivery systems.

Delivery systems

In a typical procedure we fabricated skincompatible nano particles entrapping lutein as an active ingredient by using the in situ precipitation and encapsulation procedure, as reported elsewhere.10,11 Basically, the process consists of the complexation that takes place between oppositely charged biopolymers, as CN is positively charged and HA is negatively charged. HA is, in fact, a polyanion, which can interact with the polycationic CN by electrostatic forces.18 Through this method the CN acidic suspension is added drop wise under constant stirring to polyanionic HA suspension and vice versa. Due to the complexation between oppositely charged species, CN undergoes ionic gelation and precipitate to form micro and nano lamellae or spherical particles positively or negatively charged on their surface, depending on the method of preparation (Fig. 4). Thus, it is also possible to entrap within the nanoparticles different active ingredients, hydro or liposoluble, via the use of pre-selected tensides. The active ingredients and the tensides are solubilised into the water suspension of CN or HA or in both, before going on with the ionic gelation. The obtained medium size of nanoparticles was between 250 nm and 400 nm. However the size of the particles can be controlled by controlling the size of aqueous droplets. The particle size of the final product depends upon the nature and molecular weight of the active ingredient and the tenside, speed of stirring, pH of environment, as well as concentration of the anionic and cationic polymer and the stabilising agent (i.e. tenside). The obtained nanolamellae or nanospheres have been separated by filtration followed by successive washing with water. Soon after, they were also purified by centrifugation, re-suspended in demineralised water and atomised in a stream of hot air by spray-drying technique. The obtained CN nano particles entrapping lutein have been controlled for their load and entrapping capacity in comparison with chitosan and amorphous chitin, as shown on Table 1. Moreover, also controlled was the releasing capacity of lutein from the nanoparticles together with the obtained medium size dimension. The highest and most regular release of lutein was obtained from the CN nanoparticles in which the dimensions were the lowest compared to chitosan and amorphous chitin. It is useful to underline that the superficial electrical charges of the nanoparticles greatly influence their penetrability through the stratum corneum. As shown in Figure 5, and reported elsewhere by in vitro and in vivo studies,10,11 positively charged nanocapsules (i.e. CN-HA nanoparticles) had the ability to easily load active lutein, controllably releasing it at a level of different skin layers, and as a function of storage time. It has been shown that positively charged nanoparticles are able to increase skin penetration of lutein or other active ingredients, overcoming the stratum corneum barrier function, while negatively charged nanoparticles (i.e. CN-HA nanoparticles) remain at the level of the outermost corneocytes. What is the supposed mechanism of action? Probably the positive electrical charges, covering the nanoparticles, have the possibility to change the physical properties of the stratum corneum, temporally increasing the diffusivity of the entrapped active ingredients. Also, electropopration, sonophoresis or iontophoresis are able to create spaces of penetration among corneocytes and lipid lamellae by their mechanical energy. However, the major advantage of these nanocapsules is their 100% natural composition and the possibility to manipulate their properties, such as surface charges and size, so that the relative release of the entrapped active ingredients may arrive at the level of the designed skin layers. The release of active ingredients from CN particulate systems generally involves three different mechanisms: