Citation: Flinˇ cec Grgac, S.; Tesla, T.; ˇ Corak, I.; Žuvela Bošnjak, F. Hydrothermal Synthesis of Chitosan and Tea Tree Oil on Plain and Satin Weave Cotton Fabrics. Materials 2022, 15, 5034. https://doi.org/10.3390/ ma15145034 Academic Editor: Halina Kaczmarek Received: 18 June 2022 Accepted: 14 July 2022 Published: 20 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Hydrothermal Synthesis of Chitosan and Tea Tree Oil on Plain and Satin Weave Cotton Fabrics Sandra Flinˇ cec Grgac * , Tea Tesla, Ivana ˇ Corak and Franka Žuvela Bošnjak Faculty of Textile Technology, University of Zagreb, Prilaz baruna Filipovi´ca 28a, 10000 Zagreb, Croatia; tea.tesla97@gmail.com (T.T.); ivana.corak@ttf.unizg.hr (I. ˇ C.); franka.zuvela.bosnjak@ttf.unizg.hr (F.Ž.B.) * Correspondence: sflincec@ttf.unizg.hr Abstract: The paper aimed at enhancing the antimicrobial activity of chitosan by using tea tree essential oil with the purpose of durably finishing cotton fabrics for use in a hospital environment. The influence of crosslinkers and catalysts on the possibility of obtaining stable bonds using hydrothermal in situ synthesis between cellulosic material and chitosan with and without tea tree essential oil was investigated in detail. The morphology of the sample surface before and after the treatment and textile care cycle was investigated using a field emission scanning electron microscopy (FE-SEM) and indicated the presence of chitosan and a thin film on all treated samples, which showed durability of the treatment. The FTIR spectra obtained by Fourier transform infrared spectroscopy (FTIR) using attenuated total reflection measurement technique (ATR) analysis, showed that all the samples tested recorded physicochemical changes in the structure. The analysis of the samples on the goniometer proved the hydrophilicity of the materials, with a film forming on the surface of the treated samples, which is extremely beneficial given the end use of dressing samples to promote wound healing. The presence of a significant amount of bound chitosan with tea tree oil was confirmed by measuring the mass per unit area of the samples after the treatment and textile care cycles. The results of antimicrobial efficacy show that the materials treated with chitosan were resistant to bacteria and fungi in most cases, but only the samples treated in Bath I showed a zone of inhibition against the fungus Candida albicans, indicating the positive effect of tea tree essential oil. Keywords: cotton; chitosan; tea tree oil; in situ hydrothermal synthesis; hydrophilicity; antimicrobial effect 1. Introduction We live in a time where the requirements for maintaining a healthy, safe, and pleasant environment, as well as for protecting against infections caused by pathogenic microorgan- isms are becoming increasingly stringent [1–3]. Therefore, there is always a high demand for medical textiles with antimicrobial properties. Numerous chemicals are used to achieve antimicrobial properties on textiles, including organometals, inorganic salts, phenols and thiophenols, heterocyclic compounds with anionic groups, antibiotics, urea, and similar urea-based compounds, nitro compounds, amines, etc. However, many of these substances are toxic to human health and cannot be easily degraded in nature [1,2]. In the last decade, research on the possibility of using chitosan in textiles intensified. Chitin is the second most important biopolymer after cellulose, with the production of over 100 million tons per year. Chitosan, a linear biopolymer of glucosamine monomers and small amounts of N-acetyl-glucosamine monomers, is obtained by alkaline N-deacetylation of chitin and is considered to be of natural origin. Due to its excellent biocompatibil- ity, biodegradability, and non-toxicity, it is used in the pharmaceutical industry, e.g., for drug delivery and wound healing, in agriculture, genetic engineering, food industry, en- vironmental control, water purification, paper production, photography, textile industry, etc. [1,4–9]. In textile finishing processes, chitosan is most commonly applied to cotton, Materials 2022, 15, 5034. https://doi.org/10.3390/ma15145034 https://www.mdpi.com/journal/materials