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Examining the Impact of Squaric Acid As a Crosslinking Agent on the Properties of Chitosan-Based Films

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Examining the Impact of Squaric Acid As a Crosslinking Agent on the Properties of Chitosan-Based Films International Journal of Molecular Sciences Article Examining the Impact of Squaric Acid as a Crosslinking Agent on the Properties of Chitosan-Based Films Ewa Olewnik-Kruszkowska 1,* , Magdalena Gierszewska 1,* , Sylwia Grabska-Zieli ´nska 1 , Joanna Skopi ´nska-Wi´sniewska 2 and Ewelina Jakubowska 1 1 Department of Physical Chemistry and Physicochemistry of Polymers, Faculty of Chemistry, Nicolaus Copernicus University in Toru´n,Gagarina 7, 87-100 Toru´n,Poland; [email protected] (S.G.-Z.); [email protected] (E.J.) 2 Department of Chemistry of Biomaterials and Cosmetics, Faculty of Chemistry, Nicolaus Copernicus University in Toru´n,Gagarina 7, 87-100 Toru´n,Poland; [email protected] * Correspondence: [email protected] (E.O.-K.); [email protected] (M.G.); Tel.: +48-56-611-2210 (E.O.-K.); +48-56-611-4524 (M.G.) Abstract: Hydrogels based on chitosan are very versatile materials which can be used for tissue engineering as well as in controlled drug delivery systems. One of the methods for obtaining a chitosan-based hydrogel is crosslinking by applying different components. The objective of the present study was to obtain a series of new crosslinked chitosan-based films by means of solvent cast- ing method. Squaric acid—3,4-dihydroxy-3-cyclobutene-1,2-dione—was used as a safe crosslinking agent. The effect of the squaric acid on the structural, mechanical, thermal, and swelling properties of the formed films was determined. It was established that the addition of the squaric acid significantly improved Young’s modulus, tensile strength, and thermal stability of the obtained materials. More- Citation: Olewnik-Kruszkowska, E.; over, it should be stressed that the samples consisting of chitosan and squaric acid were characterized Gierszewska, M.; Grabska-Zieli´nska, by a higher swelling than pure chitosan. The detailed characterization proved that squaric acid could S.; Skopi´nska-Wi´sniewska,J.; be used as a new effective crosslinking agent. Jakubowska, E. Examining the Impact of Squaric Acid as a Crosslinking Keywords: chitosan; squaric acid; crosslinking; swelling; mechanical properties Agent on the Properties of Chitosan-Based Films. Int. J. Mol. Sci. 2021, 22, 3329. https://doi.org/ 10.3390/ijms22073329 1. Introduction Chitosan is a natural polymer consisting of randomly distributed β-(1-4)-linked D- Academic Editor: Iolanda Francolini glucosamine and N-acetyl-D-glucosamine units. The increasing interest in this polymer is related to its low toxicity and biocompatibility, as well as its susceptibility to enzymatic Received: 24 February 2021 degradation. Moreover, contrary to a synthetic polymer, chitosan does not present im- Accepted: 20 March 2021 Published: 24 March 2021 munogenicity. For this reason, chitosan is one of the polymers used for the formation of hydrogels, which have to meet a significant number of requirements set forth by the Publisher’s Note: MDPI stays neutral medical, pharmaceutical, and food packaging sectors. It is well known that hydrogels with regard to jurisdictional claims in have to be characterized by improved mechanical properties, and that they have to ensure published maps and institutional affil- effective stability in different pH levels simultaneously, exhibiting pH-responsive swelling. iations. For this reason, in an aim to achieve all of the properties mentioned above, the process of crosslinking started to be viewed as a viable solution. It is well known that the crosslinking of chitosan significantly affects the swelling and sorption potentials, as well as its transport and mechanical properties. In order to obtain crosslinked chitosan, different types of methods and agents can Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. be applied. For this purpose, primarily chemical and physical modifications are used. This article is an open access article In the case of physical methods, where ionic interactions between the polymer chains distributed under the terms and occur, among different inorganic ionic compounds, calcium chloride and pentasodium conditions of the Creative Commons tripolyphosphate are applied as crosslinking agents [1–3]. Attribution (CC BY) license (https:// In the case of chemically crosslinked materials, the most popular crosslinking molecules creativecommons.org/licenses/by/ include: glutaraldehyde [4,5], formaldehyde [6], boric acid [7], epichlorohydrin [8], and ethy- 4.0/). lene glycol diglycidyl ether [9]. These crosslinking agents, however, are characterized by toxic Int. J. Mol. Sci. 2021, 22, 3329. https://doi.org/10.3390/ijms22073329 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, x 2 of 18 In the case of chemically crosslinked materials, the most popular crosslinking mole- cules include: glutaraldehyde [4,5], formaldehyde [6], boric acid [7], epichlorohydrin [8], and ethylene glycol diglycidyl ether [9]. These crosslinking agents, however, are charac- terized by toxic properties in their unreacted forms. For this reason, although a high de- gree of crosslinking can efficiently be achieved in their case, further application is signifi- Int. J. Mol. Sci. 2021, 22, 3329 cantly limited. A solution to the toxicity dilemma can be attained by application of a2 new of 18 crosslinking agent; for example, one of the aldehyde-functionalized compounds such as cellulose [10], β-cyclodextrin [11], starch [12], dialdehyde starch [13], or dextran [14]. Moreover, instead of toxic crosslinking agents, enzymatic crosslinking by means of properties in their unreacted forms. For this reason, although a high degree of crosslinking can transglutaminase can be applied [14–17]. Despite the considerable number of components efficiently be achieved in their case, further application is significantly limited. A solution to used as crosslinking agents, new reactants are still being sought and tested. One of the the toxicity dilemma can be attained by application of a new crosslinking agent; for example, components which can be used as a chitosan crosslinking agent is 3,4-dihydroxy-3-cyclo- one of the aldehyde-functionalized compounds such as cellulose [10], β-cyclodextrin [11], butene-1,2-dione, known as squaric acid (H2SQ). It should be stressed that squaric acid is starch [12], dialdehyde starch [13], or dextran [14]. Moreover, instead of toxic crosslinking a cyclic organic acid without any carboxyl groups (Figure 1). Its name refers to the square agents, enzymatic crosslinking by means of transglutaminase can be applied [14–17]. Despite shapethe considerable of the molecule number created of components by four usedinterconnected as crosslinking carbon agents, atoms. new The reactants reactivity are still of squaricbeing sought acid with and tested.amine Onegroups of the was components presented whichin the canwork be of used Wurm as a chitosanand Klock crosslinking [18]. The formulation of magnetic nanoparticles coated with chitosan and crosslinked with squaric agent is 3,4-dihydroxy-3-cyclobutene-1,2-dione, known as squaric acid (H2SQ). It should be acidstressed was that described squaric by acid Ziegler-Borowska is a cyclic organic et acid al. [19]. without Moreover, any carboxyl squaric groups acid was (Figure described1). Its byname Skopi refersńska-Wi to theś squareniewska shape as an of effective the molecule crosslinking created byagent four of interconnected the collagen/elastin carbon hydro- atoms. gelsThe reactivity[20]. It is worth of squaric noting acid that with Skopi amineńska-Wi groupsśniewska was presented et al. [20] in theanalyzed work of also Wurm the andbio- logicalKlock [ 18properties]. The formulation of collagen/elastin of magnetic materials nanoparticles crosslinked coated by with H2SQ. chitosan The in and vitro crosslinked test per- formedwith squaric with acidthe 3T3 was cell described line revealed by Ziegler-Borowska that the addition et al. of [19the]. crosslinking Moreover, squaric agent acidresulted was indescribed only a byslight, Skopi´nska-Wi´sniewska not statistically significant, as an effective decrease crosslinking in the cellular agent of response. the collagen/elastin Based on thesehydrogels findings, [20]. it It can is worthbe suggested noting that HSkopi´nska-Wi´sniewska2SQ can be regarded as et a al. safe [20 compound] analyzed alsoin com- the parisonbiological to propertiesother, toxic of crosslin collagen/elastinking agents materials indicated crosslinked above. by H2SQ. The in vitro test performedObtained with results the 3T3 allowed cell line us revealed to establish that thethat addition squaric of acid the readily crosslinking reacts agent with resultedboth of thein only protein a slight, materials not statistically mentioned significant, above. To decreasethe best of in our the knowledge cellular response. there is Based no indication on these offindings, any paper it can reporting be suggested on the that effect H2SQ of can squaric be regarded acid as as the a safecrosslinking compound agent in comparison of chitosan- to basedother, toxicfilms. crosslinking agents indicated above. Figure 1. Chemical structure of squaric acid (H 2SQ).SQ). ForObtained this reason, results allowedin the present us to establish work, the that in squaricfluence acid the readilyintroduction reacts withof squaric both of acid the intoprotein thematerials chitosan mentionedmatrix hasabove. on its properties To the best ofhas our been knowledge examined. there is no indication of any paperIn reporting order to onestablish the effect the of extent squaric of acidchanges as the occurring crosslinking in the agent structure of chitosan-based
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