Synthesis, Characterization, and Biological Studies of Biopolyurethane-Chitosan Composites Based on Diphenylmethane Diisocyanate and Polyol Derived from Castor Oil for the Development of Biomaterials for Topical Use

Polyurethanes (PUs) are polymers that have aroused considerable interest in the medical and tissue engineering fields due to their physicochemical properties, such as mechanical and thermal stability, elasticity, and biocompatibility. PUs have been used in the manufacturing of medical devices since the 1960s, such as catheters, artificial hearts, and blood bags. This class of polymers is synthesized from the polyaddition reaction of a polyol (soft segment) with a diisocyanate (hard segment). In view of the tendency to replace polyols derived from fossil resources with those derived from renewable resources, vegetable oils and byproducts of hydroxylated biomass are seen as emerging raw materials for the synthesis of PUs. Raw materials derived from renewable sources include polysaccharides (starch, cellulose, and chitosan) as well as fats and oils of a vegetable and animal origin and have been employed in the creation of biomaterials. Castor oil extracted from the seeds of the plant Ricinus communis is a natural polyol made up of 89% of ricinoleic acid, with an 18-carbon chain and two groups subject to reaction: an unsaturation on carbon 9 and a hydroxyl on carbon 12. Chitosan (CTS) is a polysaccharide in the form of a copolymer formed from 2-amino-2-deoxy-d-glucopyranose and 2-acetamido-2-deoxy-d-glucopyranose units randomly linked by β (1 → 4) glycosidic bonds. Its physicochemical and biological properties make CTS attractive for various applications due to its biocompatibility, biodegradability, mucoadhesivity, and absence of toxicity, along with antimicrobial activity, the ability to coordinate metals, and the ability to serve as a matrix for the loading and controlled release of substances. In the present study, biopolyurethane-chitosan (PUCTS) composites were prepared using the "one-shot method", in which a polyol derived from castor oil was mixed with CTS and then with methylene diphenyl diisocyanate (MDI) in a reaction flask at room temperature with constant stirring. The mixture was degassed and poured into a silicone mold for curing at room temperature. The polymers were characterized by FTIR, ¹³C NMR, X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermoanalytical methods (thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA)), followed by the investigation of cytotoxicity and cell adhesion on the surface of the composite. The hydroxyl number determined for the polyol was 304 mgKOH/g, and the isocyanate content was 36%. FTIR spectroscopy revealed changes in the profiles of the OH bands of the polyol in the 3500-3200 cm^-1 region, the disappearance of the NCO band of MDI at 2189 cm^-1 and the increased intensity of the CO and N-H bands in the 1750-1500 cm^-1 region due to the formation of the urethane bond. ¹³C NMR demonstrated the presence of CTS in the PU matrix, and the XRD graphs illustrated the amorphous, crystalline region of the polymers. SEM revealed roughness on the surface and inside the PU as well as circular spots with diameters smaller than 200 μm, which are characteristic of the outflow of gases during polymerization. The TGA curves of PUCTS showed the loss of mass, with thermal stability ranging from around 170-200 °C. Based on the DMA curves, the glass transition was between 16 and 20 °C. The biological test revealed that PUCTS exhibited mild cytotoxicity, and cell adhesion tests revealed that PU90CTS10 and PU50CTS50 composites promoted cell adhesion in the fibroblast cell line (L929). The results demonstrated the potential of the PUCTS composite for application as a biomaterial for topical use (bandage), with the ability to insert drugs to accelerate the healing process.