NEW ENAMEL-DENTIN ADHESIVES BASED ON CHEMICALLY MODIFIED NATURAL POLYSACCHARIDES

FIELD OF THE INVENTION The invention relates to a chitosan chemically modified with the introduction of covalently bonded acrylic groups, the use thereof for preparation of adhesive compositions, in particular for enamel-dentin adhesive compositions. STATE OF THE ART The development of appropriate enamel-dentin adhesive systems in the field of dental restorations is a very important and broadly developed application sector. Dental caries are historically considered the main disease among the diseases of the oral cavity, representing the most common disease in most high-income countries, where this disease affects approximately 60-90% of school children and almost 100% of the adult population in most countries. The consequences of caries can be pain, tooth and oral function loss. Caries, affecting the aesthetics of the smile, can also have psychological and behavioral effects on the patient, thus resulting in a reduced quality of life. In addition, some of its effects, such as dental abscesses, are risk factors or cause of many systemic diseases, even potentially fatal. The modern caries therapy consists in its complete removal and subsequent application of a restoration to restore the correct morphology and function of the dental element. The creation of effective and long-lasting dental restorations is essential since this allows reducing the long-term cost of the dental treatment and improving the quality of life of patients. Currently, the adhesive systems and composite resins, in particular acrylic-based, are the most used materials for dental restorations. The main reasons for which conservative treatments are carried out using acrylic-based composite materials and adhesive systems are: primary caries, secondary caries, tooth fracture and other causes, including fractures/infiltration of the margin or loss of previous restoration or loss of tooth substance related to the endodontic cavity accession. The longevity of the dental restoration depends on many factors, including the characteristics of the materials used, the level of oral hygiene and the patient's type of diet and the manual skills of the dentist. The adhesive systems interact with the enamel-dentin tissue, at the level of the outermost layer of such tissues and on the cutting debris constituting the smear layer produced by the mechanical instruments (mechanical or manual) used for the removal of caries, through two adhesion strategies: "etch & rinse" and "self-etch" (Breschi L. et al., Dental Mater., 2008, 24, 90-101 ). Currently, the classification of different adhesive systems is made according to the number of steps required for proper application (Van Meerbeek B. et al., Operating Dent, 2003, 28, 215-235). The impregnation by the adhesive systems of the surface of the tooth tissue causes the formation of the hybrid layer, due to the interaction between the adhesive system and the substrate demineralized by the etching treatment. The stability over time of the restoration depends on the formation of a stable and homogeneous hybrid layer. Typically, the average length of a restoration carried out using composite resins is about six years (Bohaty B.S. et al., Clin. Cosmet. Investig. Dent, 2013, 5, 33-42). The possibility of increasing the average length of a dental restoration would certainly have a positive impact on the treatment of secondary dental caries and the subsequent reconstruction of the restoration. This last statement is supported by the fact that both researchers and dental material industries are constantly looking for more effective and lasting, more easily manipulated and applicable materials and for this reason, often the manufacturers tend to vary the composition of adhesive systems and composite resins. For many applications in the biomedical field, polysaccharide-based systems are particularly interesting. Generally, these polymers are highly biocompatible; as such, they are ideal for applications involving the direct contact with biological tissues. Among the polysaccharides of natural origin, chitosan is one of the most studied and commercially used. This is a basic polysaccharide, of molecular weight between 20 and 1 ,500 kDa, consisting of a chain of residues of D-glucosamine (GlcNH2) bonded by β1→ 4 bonds with interspersed units of N-acetyl-glucosamine (GlcNAc) and usually the "degree of acetylation" or "residual acetylation" (i.e., the percentage of GlcNAc units on the sum of GlcNh^ and GlcNAc units) determines the features of chitosan together with the molecular weight. It is a cationic polysaccharide normally insoluble in neutral or basic aqueous solutions; in acidic solutions with a pH< 6.5, the free amino group is protonated making the polymer soluble. This polymer is already widely used in the biomedical field as it shows a low immune, pathological or infectious response (Suh Francis J.K., Matthew H.W.T. Biomaterials, 2000, 21, 2589-2598; Miyazaki S. et al. Chem. Pharm. Bull., 1981 , 29, 3067-3069). In particular, it is not cytotoxic, thus it is highly biocompatible with cells and biological tissues. Chitosan further has chemical-physical properties ideal for use as a biomaterial, such as the high density of cationic charge in acidic solution and the high processability, thanks to which it is possible to obtain porous structures on which cell cultures can be easily cultivated. Furthermore, the high hydrophilicity of the polysaccharide allows the interaction thereof with other biological macromolecular structures present in tissues, such as collagen and glycosaminoglycans. Many recent studies have focused on the development of methodologies to enhance the biological effects of chitosan. In particular, most of the efforts have been aimed at increasing the cationic charge of the polymer or at modifying the chemical and bioavailability features thereof through (bio)chemical modifications. It is actually in its derivatized forms that chitosan takes the necessary properties for use as a biomaterial. The use of chitosan as such or its derivatives in compositions usable as filler materials for orthopedic and dental applications has long been known. Patents EP0323632 and EP0329098 disclose the use of acidic solutions of chitosan in combination with hydroxyapatite and with other metal oxides, such as zinc and/or magnesium oxide, as reinforcement in the production of hardening filler materials for applications in the orthopedic and dental fields. Likewise, patent application US2005/0074415 describes the use of chitosan and derivatives thereof, such as non-toxic gelling agents in the preparation of a formulation based on calcium phosphate for the dental restoration and for implants and bone restoration. The role of gelling agents, including chitosan and its derivatives, is to increase the cohesiveness of the restoration paste. There are, however, no specific references to the type of chitosan derivatives that are proposed. Patent application US2007/0092580 describes the use of chitosan and biocompatible derivatives thereof, in particular malate and lactate chitosan, as compounds forming hydrogels usable in the preparation of a dual-phase cement precursor. In this specific use, the formation of the chitosan hydrogel and of biocompatible derivatives thereof occurs following the treatment of the system with a strong base, e.g. sodium hydroxide. Patent application WO2009/029049 describes the use of unsaturated derivatives of chitin and chitosan, and in particular of chitin-methacrylate and chitosan itaconate, as tixotropic agents in a composition usable as a bone and/or dental cement and as bone filler. Patent application WO2009/029734 discloses the use of chitin and chitosan as a filler in a cement for the treatment of bone defects in dental or orthopedic applications composed of a polymerizable resin comprising ethylene unsaturated bonds, a glycidyl ether and/or isocyanate group and primary, secondary, tertiary or quaternary amines. Patent application US201 1 /0182995 describes calcium-based cements usable as dental or orthopedic fillers, which includes the use of synthetic and natural polymers, including chitosan, as biodegradable and absorbable materials. Patent application US2012/0178684 describes the use of chitosan as a biocompatible filler material for surgical bone cements for applications in the orthopedics, dental, maxillofacial surgery fields. Patent application WO2012/009555 describes the use of chitosan as an additional filler in the preparation of a polymeric composite for dental restoration containing a polymeric matrix and a reinforcing filler in the form of nanofibers, nanoplates and submicrometric fibers or plates. The use of chitosan in dentistry is booming even as adhesive. Recently, Elsaka et al. (Elsaka S.E. Quintessence Int., 2012, 43, 603-613) have proposed the use of non-modified chitosan as an additive to the "self-etch" adhesive system RealSeal®. However, despite the antibacterial properties due to the presence of chitosan, the adhesive system containing the polysaccharide showed no significant difference in the strength of the bond with the restoration as compared to the same system without non-modified chitosan. SUMMARY A first purpose of the present invention is the preparation of a covalently derivatized chitosan, so as to obtain a compound which is able to interact with both the organic component of the demineralized dentin (mainly collagen) and with the monomers composing the restoration resin. A second purpose of the present invention is the use of a derivatized chitosan having suitable features (molecular weight of the polymer, degree of derivatization) within an enamel-dentine adhesive composition in order to significantly increase the average lifetime of the dental restoration. The increase of the adhesive strength of adhesive systems, which include the chemically modified chitosan, may find useful application, besides the dental field, also in the orthopedic and ophthalmic fields. A further purpose of the present invention therefore is the use of a derivatized chitosan for the development of adhesives also usable in the orthopedic and ophthalmic fields. In order to achieve the above purposes, the inventors have developed appropriate water soluble polysaccharide systems based on a modified chitosan able to produce effective adhesive compositions and to guarantee a longer duration of the dental restoration. Such adhesive compositions are characterized in that the usual resins used for the purpose are admixed with a chitosan modified by covalent derivatization with acrylic units. Therefore, in a first aspect the object of the invention is a chitosan modified by derivatization of residues -NH2 of D-Glucosamine units, which can be schematically represented by the general formula I - the chitosan has a degree of total substitution of D-Glucosamine units with CO-C(CH2)Ri acrylic and acetyl residues comprised from 10% to 50% with a ratio between acrylic and acetyl residues comprised from 0.1 to 9; and - Ri is selected from H, a halogen, a linear or branched C1-C3 alkyl chain, optionally substituted with an OH or a halogen, and NHCOR2 with R2 equal to a linear or branched C1-C3 alkyl chain. In another aspect, the object of the invention is the use of the chitosan chemically modified by derivatization of -NH2 residues of the D-glucosamine of general formula I as a conditioner for the preparation of adhesive systems usable in the dentistry, orthopedic and ophthalmic fields. Yet in a further aspect, the object of the invention is a composition with adhesive properties comprising at least one chitosan chemically modified by derivatization of the -NH2 residue of D-glucosamine according to the general formula I in combination with appropriate additives and excipients. The objects and the advantages of the use of the chitosan modified with acrylic groups within adhesive compositions object of the present invention will be better understood in the course of the following detailed description where, by way of a non-limiting example of the invention, some examples of preparation of chitosan derivatives with acrylic groups, their use in combination with adhesive systems, their physical-chemical characterization as well as the mechanical tests for the evaluation of their properties will be described.