materials Article Controlling the Porosity and Biocidal Properties of the Chitosan-Hyaluronate Matrix Hydrogel Nanocomposites by the Addition of 2D Ti 3 C 2 T x MXene Anita Rozmyslowska-Wojciechowska 1, *, Ewa Karwowska 2 , Michal Gloc 1 , Jaroslaw Wo´ zniak 1 , Mateusz Petrus 1 , Bartlomiej Przybyszewski 1 , Tomasz Wojciechowski 3 and Agnieszka M. Jastrz ˛ ebska 1 1 Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland; Michal.Gloc.WIM@pw.edu.pl (M.G.); jaroslaw.wozniak@pw.edu.pl (J.W.); mateusz.petrus.dokt@pw.edu.pl (M.P.); bartlomiej.przybyszewski.dokt@pw.edu.pl (B.P.); agsolgala@gmail.com (A.M.J.) 2 Faculty of Building Services, Hydro and Environmental Engineering, Warsaw University of Technology, Nowowiejska 20, 00-653 Warsaw, Poland; ewa.karwowska@pw.edu.pl 3 Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland; twojciechowski@ch.pw.edu.pl * Correspondence: anita.wojciechowska.dokt@pw.edu.pl Received: 1 August 2020; Accepted: 24 September 2020; Published: 15 October 2020   Abstract: A recent discovery of the unique biological properties of two-dimensional transition metal carbides (MXenes) resulted in intensive research on their application in various biotechnological areas, including polymeric nanocomposite systems. However, the true potential of MXene as an additive to bioactive natural porous composite structures has yet to be fully explored. Here, we report that the addition of 2D Ti 3 C 2 T x MXene by reducing the porosity of the chitosan-hyaluronate matrix nanocomposite structures, stabilized by vitamin C, maintains their desired antibacterial properties. This was confirmed by micro computed tomography (micro-CT) visualization which enables insight into the porous structure of nanocomposites. It was also found that given large porosity of the nanocomposite a small amount of MXene (1–5 wt.%) was effective against gram-negative Escherichia coli, gram-positive Staphylococcus aureus, and Bacillus sp. bacteria in a hydrogel system. Such an approach unequivocally advances the future design approaches of modern wound healing dressing materials with the addition of MXenes. Keywords: MXenes; chitosan; hyaluronate; nanocomposite hydrogel; antibacterial; porosity; computed tomography 1. Introduction Chitosan is a linear copolymer of β-(1–4) linked 2-acetamido-2-deoxy-β-D-glucopyranose and 2-amino-2-deoxy-β-D-glycopyranose. It one of the most common natural polymers obtained by alkaline deacetylation of chitin, which occurs in the skeletons of various invertebrates, such as mollusks and arthropods. It is also present in the cell wall of algae and fungi [1]. Chitosan is well-soluble and stable in an acidic environment, but insoluble in a neutral environment [2]. It is cationic which makes it unique among other polysaccharides and specific in the context of biological properties [3]. Thanks to its biocompatibility, it can be used medicinally in the area of implantation [4], wound healing promotion [5], and exhibits bacteriostatic effects [2,6,7]. Due to the positive charge at physiological Materials 2020, 13, 4587; doi:10.3390/ma13204587 www.mdpi.com/journal/materials