Effect of chitin nanowhiskers on mechanical and swelling properties of Gum Arabic hydrogels nanocomposites

Hydrogels based on biopolymers like Gum Arabic (GA) usually show low applicability due to weak mechanical properties. To overcome this issue, (nano)fillers are utilized as reinforcing agents. Here, GA hydrogels were reinforced by chitin nanowhiskers (CtNWs, aspect ratio of 14) isolated from the biopolymer chitin through acid hydrolysis.

Carbohydrate Polymers 266 (2021) 118116 Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol Effect of chitin nanowhiskers on mechanical and swelling properties of Gum Arabic hydrogels nanocomposites ´tia S Nunes a, Adley F Rubira a, Edvani C Muniz a, d, e, Andr´e Antonio G.B Pereira a, b, *, Ca c, ** R Fajardo a Grupo de Materiais Polim´ericos e Comp´ ositos (GMPC), Maring´ a State University, Av Colombo 5790, 87020-900 Maring´ a, PR, Brazil Laborat´ orio de Biopolímeros, Coordenaỗ ao de Engenharia de Bioprocessos e Biotecnologia, Universidade Tecnol ogica Federal Paran´ a (UTFPR- DV), Estrada para Boa Esperanỗa, 85660-000 Dois Vizinhos, PR, Brazil c Laborat orio de Tecnologia e Desenvolvimento de Comp´ ositos e Materiais Polim´ericos (LaCoPol), Federal University of Pelotas, Campus Cap˜ ao Le˜ ao s/n, 96010-900 Pelotas, RS, Brazil d Departamento de Química, Universidade Federal Piauí, 64049-550 Teresina, PI, Brazil e Programa de P os-graduaỗ ao em Ciencia e Engenharia de Materiais, Universidade Tecnol´ ogica Federal Paran´ a (UTFPR- LD), Avenida dos Pioneiros, 3131, 86036370 Londrina, PR, Brazil b A R T I C L E I N F O A B S T R A C T Keywords: Chitin Whiskers Gum Arabic Hydrogel Composite Swelling Hydrogels based on biopolymers like Gum Arabic (GA) usually show low applicability due to weak mechanical properties To overcome this issue, (nano)fillers are utilized as reinforcing agents Here, GA hydrogels were reinforced by chitin nanowhiskers (CtNWs, aspect ratio of 14) isolated from the biopolymer chitin through acid hydrolysis Firstly, GA was chemically modified with glycidyl methacrylate (GMA), which allowed its cross­ linking by free radical reactions Next, hydrogel samples containing different concentrations of CtNWs (0–10 wt %) were prepared and fully characterized Mechanical characterization revealed that 10 wt% of CtNWs promoted an increase of 44% in the Young’s modulus and 96% the rupture force values compared to the pristine hydrogel Overall, all nanocomposites were stiffer and more resistant to elastic deformation Due to this feature, the swelling capacity of the nanocomposites decreased GA hydrogel without CtNWs exhibited a swelling degree of 975%, whereas nanocomposites containing CtNWs exhibited swelling degrees under 725% Introduction Polysaccharide nanoparticles have been extensively studied in the last few years and present a huge potential of applications in different areas including reinforcing phases in polymer composites (Eichhorn, 2011; Rodrigues et al., 2014; Tian et al., 2015) The second most abundant polysaccharide, chitin or β(1 ⟶ 4)-linked N-acetyl-Dglucosamine polymer, is present in many organisms (e.g., arthropods, nematodes, fungi, etc.) as a structural component in their exoskeletons and cell walls and its highly crystalline structure is suitable for the preparation of rod-like crystalline nanocrystals, or nanowhiskers referred as CtNWs (Fan et al., 2012) In general, CtNWs present high aspect ratio (length to width ratio), low density, high Young’s modulus (~150 GPa and 15 GPa, for longitudinal and transversal, respectively) (Zeng et al., 2012), and high dispersibility in acidified aqueous media Interesting biological properties attributed to CtNWs include biode­ gradability, biocompatibility, and antibacterial activity The functional groups at the surfaces of CtNWs and the high specific surface allow dipolar interactions with other components as well as further chemical modification (Ou et al., 2020) Therefore, because of such interesting features, many potential applications of CtNWs are being unveiled The preparation of CtNWs was first reported by Marchessault et al in 1959 and is well established nowadays (MARCHESSAULT et al., 1959) Today, the methodology for preparing CtNWs is mainly based on the acid hydrolysis, in which the differential hydrolysis of amorphous and crystalline phases are kinetically controlled to produce whiskers with diameters of 5–25 nm and lengths ranging from 150 nm up to μm (Pereira et al., 2014) * Correspondence to: A G B Pereira, Grupo de Materiais Polim´ericos e Comp´ ositos (GMPC), Maring´ a State University, Av Colombo 5790, 87020-900 Maring´ a, PR, Brazil ** Corresponding author E-mail addresses: antoniog@utfpr.edu.br (A.G.B Pereira), andre.fajardo@pq.cnpq.br (A.R Fajardo) https://doi.org/10.1016/j.carbpol.2021.118116 Received 14 October 2020; Received in revised form April 2021; Accepted 18 April 2021 Available online 24 April 2021 0144-8617/© 2021 Elsevier Ltd This article is made available under the Elsevier license (http://www.elsevier.com/open-access/userlicense/1.0/) A.G.B Pereira et al Carbohydrate Polymers 266 (2021) 118116 The earliest report of CtNWs as reinforcement phase in the prepa­ ration of polymer nanocomposites was done by Paillet & Dufresne (2001) The incorporation of CtNWs, at concentration as low as wt%, in polypropylene matrix allowed re-processed PP to recover its original mechanical properties (de Sousa Mol & Or´ efice, 2016) The Young’s modulus of poly(vinyl alcohol) films significantly increased from 28 to 50 GPa at 30 wt% CtNWs (Uddin et al., 2012) CtNWs also improved the mechanical properties of maize films from 1.64 MPa to 3.69 MPa at wt % CtNWs, decreased the water vapor permeability to half of its original value at wt%, and induced antibacterial activity against Listeria mon­ ocytogenes (Qin et al., 2016) Poly(D,L-lactide) films were deep-coated with CtNWs suspension to produce materials with improved tensile properties and better cellular adhesion, proliferation, and osteogenic differentiation of MC3T3-E1 strain (Liu et al., 2019) CtNWs and chi­ tosan whiskers (CsNWs) were used to modify the surfaces of cellulose acetate electrospun nanofibers inducing antibacterial activity in the prepared mats, which have the potential to be applied in the biomedical field (Pereira et al., 2020) Over the past few years, numerous studies focused on the develop­ ment of hydrogels have been reported in the literature (Curvello et al., 2019; Du et al., 2020; Mohammadinejad et al., 2019) The success of these soft materials can be credited to their several attractive properties, which have stimulated their use in a wide range of applications (bio­ materials, delivery systems, soil conditioners, environmental remedia­ tion, among others) Hydrogels are characterized by three-dimensional networks formed by crosslinked hydrophilic macromolecules Such characteristics endow hydrogels the ability to absorb and retain large amounts of aqueous liquids without dissolving Three-dimensional net­ works can be synthesized using different approaches, which depend on the crosslinking process and the starting materials (Ahmed, 2015) Therefore, hydrogels can be elaborated with specific features maxi­ mizing their action in a target application Furthermore, the incorpo­ ration of other types of materials within the hydrogel network (resulting in a composite) has been frequently reported as an efficient approach to obtain hydrogels with superior properties (Feng et al., 2019; Thoniyot et al., 2015) More recently, attention has been paid to filler materials derived from polysaccharides, as CtNWs, mainly due to their reinforcing action when associated with a hydrogel network Hydrogel composites synthesized with these fillers are an alternative to overcome the poor mechanical properties reported to the hydrogels synthesized solely with natural polymers Many studies state that polysaccharide-based hydro­ gels have limited stretchability and are often brittle materials (Wang et al., 2018) The presence of CtNWs in injectable chitosan-based hydrogels improved the mechanical properties of the gel, promoted fast gelation speed as it worked as a crosslinker, and favored biological compatibility according to the MTT method (Wang et al., 2017) Simi­ larly, CtNWs have significant reinforcement effect on hydrogels of gelatin (Ge et al., 2018), chitosan (Sun et al., 2018), chitosan/dextran (Pang et al., 2020), and methylcellulose (Jung et al., 2019) Gum Arabic (GA) is a dried exudate obtained from the stems and branches of Acacia Senegal or Acacia seal consisting of complex and branched polysaccharides structures The hydrolysis of such carbohy­ drates yields mainly arabinose, galactose, rhamnose, and glucuronic acid GA is a water-soluble gum widely used in the food industry and, currently, due to its attractive biological properties (antioxidant, he­ mostatic, nonhemolytic, antibacterial, among others) it has been used to elaborate pharmaceutical and biomedical devices, as hydrogels (Gerola et al., 2015; Li et al., 2017) GA-based hydrogels are often associated with many benign and eco-friendly features (e.g., biocompatibility, biodegradability, non-toxicity, among others) At the same time, this kind of hydrogel is also disgraced due to their poor mechanical prop­ erties, which seems to be a huge shortcoming for their use in various high-end applications To overcome this obstacle, the introduction of filler materials (from organic and/or inorganic sources) into the hydrogel network seems to be a successful strategy Most recently, the use of nano-sized fillers has gained great attention, mainly because of their astonishing ability to dissipate energy under mechanical stress (S.N Li et al., 2020) Up to date, few studies are devoted to synthesizing and characterize hydrogel composites based on GA To the best of our knowledge, this is the first study devoted to evaluating the effect of different amounts of CtNWs on the mechanical and swelling properties of hydrogels prepared with GA We hypothesize that CtNWs can act as an efficient reinforcing agent for GA hydrogel Materials and methods 2.1 Materials Chitin from shrimp shells (practical grade, high molecular weight > 300 kDa, CAS 1398-61-4), Gum Arabic (GA) from acacia trees (molec­ ular weight 250 kDa, high viscosity, CAS 9000-01-5), glycidyl methac­ rylate (GMA, molecular weight 142.15 g/mol, CAS 106–91-2), and sodium persulfate (SPS, CAS 7775-27-1) were purchased from SigmaAldrich (USA) N,N′ -methylenebisacrylamide (MBA, CAS 110-26-9) was purchased from Biorad (USA) Potassium hydroxide (KOH, 85%, CAS 1310-58-3), sodium hydroxide (NaOH, 97%, CAS 1310-73-2), hy­ drochloric acid (HCl, 36.5%, CAS 7647-01-0), and pH buffer acetate (15% sodium acetate and 48% acetic acid) were purchased from Synth (Brazil) Sodium chlorite (NaClO2, 80%, CAS 7758-19-2) was purchased from Alfa Aesar (USA) Ethanol (99.5%, CAS 64-17-5) was purchased from Nuclear (Brazil) Except for chitin, all chemicals were used without further purification 2.2 Isolation of chitin nanowhiskers Chitin nanowhiskers (CtNWs) were isolated from chitin according to previously reported protocols (Paillet & Dufresne, 2001; Pereira et al., 2014) (Fig 1a) Chitin (practical grade) was firstly purified by removing residual proteins followed by bleaching Proteins were removed by heating g of chitin in 150 mL of KOH solution (5 wt/v%) at boil under vigorous stirring for h The suspension was kept under stirring at room temperature for another 12 h, filtered, and washed with water Next, the collected solid was bleached in 150 mL of 1.7% NaClO2 in pH buffer acetate at 80 ◦ C for h, then filtered and washed with water The bleaching reaction was performed twice Finally, the bleached solid was re-suspended in 150 mL of KOH solution (5 wt/v%) for 48 h, then centrifuged to collect the purified chitin at 75% yield (~3.75 g) CtNWs were obtained by hydrolyzing the purified chitin in mol/L HCl at boil for 90 under stirring The ratio chitin/volume of HCl solution (g/mL) was fixed at 1:30 (Pereira et al., 2014) The reaction was stopped by adding 50 mL of cold water and centrifuged (3400 rpm for 15 min) The precipitate was re-suspended in 200 mL of distilled water followed by centrifugation This procedure was repeated three times Next, the precipitate was re-suspended in distilled water and dialyzed (molecular weight cut-off 12–14 kDa) against water up to neutral pH The suspension was sonicated (40% maximum amplitude) for a total of 20 with of interval between every of sonication cycle, followed by centrifugation (3000 rpm, 10 min) for removing any remaining precipitate Finally, the CtNWs suspension was stored at ◦ C The yield of CtNWs (~65% or 2.44 g) was determined by gravimetric analysis, in triplicate For this, aliquots of the CtNWs suspension (1000 ± μL) were collected and the liquid phase was evaporated at 50 ◦ C Then, the residue was weighed and correlated to the total volume of the nanowhiskers suspension The CtNWs concentration in the suspension was adjusted to be wt% or 50 mg/mL CtNWs were kept in the neverdried state prior to hydrogel preparation 2.3 Characterization of CtNWs CtNWs were characterized by zeta potential, Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA), A.G.B Pereira et al Carbohydrate Polymers 266 (2021) 118116 Fig (a) Isolation of CtNWs from raw chitin and (b) synthesis of GA-GMA followed by the synthesis of the hydrogel nanocomposite Differential Scanning Calorimetry (DSC), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Micro­ scopy (TEM), 1H, 13C, and 2D HSQC (Heteronuclear Single Quantum Coherence) Nuclear Magnetic Resonance (NMR) A complete descrip­ tion of these analyses has already been reported in previous papers (Pereira et al., 2014, 2015) added to the reaction system under stirring The resulting solution was heated to 70 ◦ C and this temperature was kept up to the hydrogel for­ mation (Fig 1b) The as-synthesized hydrogel was recovered, soaked in distilled water, and oven-dried (40 ◦ C, 24 h) A set of hydrogel com­ posites containing CtNWs were synthesized using a similar procedure; however, specific volumes of CtNWs suspensions were added to the GAGMA solution prior to the hydrogel formation The compositions of each hydrogel sample as well as their respective labels are detailed in Table It is important to inform that the limiting concentration of CtNWs was 2.4 Synthesis of the hydrogel composite containing CtNWs Before hydrogel synthesis, raw GA was chemically modified with glycidyl methacrylate (GMA) adapting the methodology reported pre­ viously by Paulino et al (2012) (Fig 1b) Briefly, 30 g of GA were sol­ ubilized in L of distilled water at 65 ◦ C under magnetic stirring Thereafter, the pH of the solution was adjusted to 3.5 by adding a few drops of concentrated HCl (36.5 wt/v%) Next, mL of GMA were added, and the reaction system was kept under stirring at 65 ◦ C for 24 h The chemically modified GA was recovered by precipitation using ethanol The precipitate (denoted as GA-GMA) was rinsed with abun­ dant ethanol to eliminate the unreacted chemicals, centrifuged and oven-dried (40 ◦ C) for 24 h The synthesis of the GA-GMA hydrogels was performed as follows; g of GA-GMA were solubilized in 25 mL of HCl acidified water (pH ~3) Next, 0.1 g of MBA (crosslinker) and 0.1 g of SPS (radical initiator) were Table Hydrogels composition Sample GAGMA (g) CtNWs (wt/wt %) GA-GMA/ CtNWs0 GA-GMA/ CtNWs1 GA-GMA/ CtNWs5 GA-GMA/ CtNWs10 5 CtNWs suspension (mL) MBA (g) SPS (g) Final volume (mL) 0.1 0.1 25 1 0.1 0.1 25 5 0.1 0.1 25 10 10 0.1 0.1 25 A.G.B Pereira et al Carbohydrate Polymers 266 (2021) 118116 fixed at 10 wt/wt% because higher concentrations showed a poor dispersion into the polymer solution before hydrogel formation 2.7 Mechanical properties The mechanical properties of as-synthesized hydrogels were exam­ ined by compressive tests performed in a Texturometer analyzer (TA XTplusC - Stable Micro Systems, UK) equipped with a 50 N load cell Swollen samples were cut in cubic shapes (10 × 10 × 10 mm) prior to the tests Experiments were performed at 25 ◦ C under controlled hu­ midity conditions (55%) Other texturometer parameters: 5-mm inden­ tation depth, mm/s downward probe velocity, and initial crosssectional area of 126 mm2 The Young’s modulus was calculated per Eq (2): 2.5 Characterization techniques All reactants used for the following characterizations were ACS grade (purity ≥ 99%), suitable for NMR and FTIR analysis 2.5.1 1H NMR H NMR spectra were recorded on Varian 300 MHz spectrometer (model 300, UK) For this, 10 wt/wt% solutions in D2O (raw GA and GAGMA) and CDCl3 (for GMA) were prepared in mm tubes Tetrame­ thylsilane (TMS) was used as an internal standard and it was suppressed from the spectra Data were collected under the following conditions: 12 K data points; 30 s relaxation delay; angle pulse of 90◦ ; acquisition time of s; temperature of 298 K; 32 scans ′ Young s modulus = F.L1 A.(L2 − L1 ) (2) where F is the necessary force (N) to compress the sample, A is the sample transversal area (m2), L1, and L2 are the initial and compressed sample thicknesses (mm), respectively For each sample, measure­ ments were done 2.5.2 FTIR FTIR spectra were recorded on a spectrophotometer (model MB-100 spectrometer, Bomen, Quebec, Canada) operating in the region from 4000 to 500 cm− with a resolution of cm− and 64 scan acquisitions Before the spectra acquisition, the dried samples were mixed with KBr powder and pressed into pellets Results and discussion 3.1 Isolation of CtNWs TEM images of CtNWs obtained from casting of dilute aqueous sus­ pension are presented in Fig The acid hydrolysis of purified chitin yielded chitin nanorods or nanowhiskers (CtNWs) with averages length and width of 219 ± 42 nm and 16 ± nm, giving an aspect ratio of 14 The CtNWs yield was approximately 65% of the mass of the purified chitin This 35% initial mass loss was attributed to hydrolysis of chitin and dissolution of smaller nano-chitins into soluble molecules (oligomer and monomer) Full characterization of CtNWs has been published elsewhere by our group (Pereira et al., 2014, 2015, 2020), and only the main features are discussed in the present paper Although CtNWs have its core composed mostly of α-chitin, the surface is not completely acetylated, as indicated by the positive charges (+30 mV) noticed from the zeta potential measurements performed at pH Under the acidic condition, the deacetylated amino groups available on the CtNWs surfaces become protonated (R–NH2 + H3O+ ⇋ R–NH+ ), which explains this high positive charge density Indeed, H NMR analysis indicated that only 56% of CtNWs surface remains acet­ ylated, while 44% of the amino groups are deacetylated corroborating the zeta potential data (Pereira et al., 2014) It is worthy to mention that this high value of zeta potential measured under acidic conditions is appreciated since it ensures the stability of CtNWs suspension 2.5.3 XRD XRD patterns were recorded from powder samples using a Shimadzu diffractometer (model XRD 6000, Japan) using Ni-filtered Cu-Kα radi­ ation (λ = 1.5406 Å) at a 30 kV anode voltage and a 20 mA current The scanning angle (2θ) was ranged from 5◦ to 40◦ at a scan rate of 2◦ /min 2.5.4 Thermogravimetric analysis (TGA) TGA analyses were performed from 30 to 600 ◦ C at a heating rate of 10 ◦ C/min and dynamic N2 atmosphere (50 mL/min), in a thermogra­ vimetric analyzer (Netzch, model STA 409 PG/4/G Luxx, USA) 2.5.5 TEM images TEM images were recorded using a JEOL microscope (JEM-1200EX, Japan) using an acceleration voltage of 80 kV Before the TEM imaging, the selected hydrogel sample was embedded in epoxy resin, fully dried and then, cut into 80–100 nm thick transverse sections using an ultra­ microtome The sections were placed onto carbon-coated copper grids and doubly stained with (3 wt/v%) uranyl acetate before imaging The widths and lengths of CtNWs were collected from TEM images and the averages were calculated from measuring over 100 individual samples using ImageJ software (https://imagej.nih.gov/ij) 2.6 Swelling experiments 3.2 Chemical modification of GA with GMA The swelling behavior of the synthesized hydrogels was investigated in distilled water at room temperature using a gravimetric method For this, known amounts of dried samples (particle size 16–18 mesh or 0.18–1.00 mm) were placed in 30 mL filter crucibles (porosity no 0) premoistened with a dry outer wall This set (crucible + hydrogel sample) was immersed in distilled water allowing the hydrogel to be completely submerged At specific time intervals, the system was removed from the water, the external wall dried and weighed The swelling rate at a spe­ cific time interval was calculated per Eq (1): The 1H NMR spectrum of modified GA (GA-GMA) exhibited the typical resonance signals of GA accompanied by the appearance of three new signals (Fig 3) Two of them, at δ 6.20 and δ 6.17 ppm (denoted as a and a′), are ascribed to the hydrogen atoms bonded to the vinyl group of GMA, while the signal at δ 2.21 ppm (denoted as b) is due to hydrogen atoms of the methyl group All the resonance signals observed in the GAGMA spectrum agree with previous reports (Reis et al., 2006) Overall, the chemical modification of polysaccharides with GMA may occur by a distinct reaction mechanism according to the solvent used (Reis et al., 2009) In other words, low-rate and irreversible epoxide ring-opening occur in protic solvents, such as water, while transesterification hap­ pens in an aprotic solvent (e.g., dimethyl sulfoxide, DMSO) yielding the methacrylated polysaccharide and glycidol as a by-product This last mechanism is faster, however, reversible (Reis et al., 2009) Therefore, as the water was used as the solvent, epoxide ring-opening is the most likely modification mechanism of GA The 1H NMR spectrum of GA-GMA was also used to quantify the degree of substitution (DS) on GA after reacting with GMA The DS was Swelling (%) = (wt − wo ) × 100 wo (1) where wt is the swollen hydrogel weight at a specific time (t) and wo is its dry weight A.G.B Pereira et al Carbohydrate Polymers 266 (2021) 118116 Fig TEM images (left - bright field mode; right - dark field mode) of CtNWs isolated from chitin by acid hydrolysis (conditions: mol/L HCl, g/30 mL chitin/HCl solution, at boil, 90 min) Fig 1H NMR spectra of GMA, raw GA, and GA-GMA calculated per Eq (3) that relates the total areas of the two resonance signals ascribed to the vinyl hydrogens (δ 6.20 and δ 6.16 ppm) observed in the GA-GMA spectrum and the resonance signal at δ 5.41 ppm in the GA spectrum This signal can be associated with the hydrogen atom bond to the anomeric carbon in the glucose unit of GA (Fan et al., 2013) [ ] (Iδ6.20 + Iδ6.16 ) × 0.5 DS (%) = × 100 (3) Iδ5.41 (Gerola et al., 2016), as a result of the experimental conditions chosen, it is important to highlight that polymers highly functionalized with GMA moieties (i.e., high DS values) usually result in hydrogels with high crosslinking density In general, the vinylic groups (–C=CH) from GMA react with MBA (the crosslinker) through radical reactions resulting in covalent bonds that hold the polymer chains together So, high DS values imply that the concentration of vinylic groups on the polymer backbone is also high, which in certain aspects can boost the crosslinking process by reacting either with MB or with other vinylic groups from neigh­ boring molecules Hydrogels with high crosslinking densities may show accentuated stiffness and low swelling ability, which impairs their use in some application fields (e.g., biomaterials, adsorption, among others) The chemical nature of the GMA, raw GA, and GA-GMA was also After a careful baseline treatment, the areas of the resonance signals were measured and the values found were Iδ6.20 ≈ 1.0, Iδ6.16 ≈ 1.21, and Iδ5.41 ≈ 9.05, respectively Using these values, the DS calculated for GAGMA was 12.21% Although the DS was slightly lower than other reports A.G.B Pereira et al Carbohydrate Polymers 266 (2021) 118116 – O stretching of GMA moiety), and bands in the range 1718 cm− (C– 1200–1000 cm− are due to the motions of the glycosidic skeleton (Espinosa-Andrews et al., 2010; Stefanovic et al., 2013) Additionally, these spectra also exhibited a shifting of the band ascribed to the – O stretching of carboxylic groups of GA-GMA from 1607 asymmetric C– cm− to 1659 cm− 1, which could be caused by the crosslinking reaction Similarly, the increase of intensity noticed to the bands associated with the N–H bending (around 1550 cm− 1) and C–N stretching of amine bonds (around 1230 cm− 1) can be attributed to MBA, used as the crosslinker agent (Huang et al., 2013) These particularities in the GAGMA and GA-GMA/CtNWs10 spectra confirm the success of the cross­ linking process Furthermore, the spectrum of GA-GMA/CtNWs10 also exhibited two new bands at 1663 cm− and 1556 cm− 1, which are assigned to the amide I and II vibrational modes of CtNWs The shouldertype band at 3264 cm− (N–H stretching) also can be attributed to the presence of CtNWs into the hydrogel matrix Although these bands are discreet due to low CtNWs concentration, their presence confirms the nanocomposite formation Herein, the filler remained embedded into the hydrogel matrix without chemical interactions (i.e., there is no ev­ idence of a covalent bond between the CtNWs and GA-GMA chains) On the other hand, comparing the spectra of GA-GMA/CtNWs0, and GAGMA/CtNWs10, a slight shift of the band attributed to the O–H stretching (hydroxyl groups) from 3437 cm− from 3442 cm− could be observed The interactions among the CtNWs and GA-GMA chains by Hbond explain such behavior Also, this finding indicates compatibility between the hydrogel matrix and CtNWs (the filler material) The XRD pattern recorded for CtNWs exhibited the typical diffrac­ tions peaks of α-chitin in the 2θ range of 5◦ to 40◦ (Goodrich & Winter, 2007) The most important diffraction peaks are at 2θ ≈ 9.3◦ ; 19.2◦ ; 20.8◦ ; 23.3◦ , and 26.3◦ and they are consistent with the crystallographic planes (020), (110), (101), (130) and (013) attributed to the α allo­ morph of chitin (Fig 5b) The data corroborates with FTIR analysis demonstrating that the controlled acid hydrolysis, based on the differ­ ential kinetics of amorphous and crystalline phases, did not affect the crystal structure of chitin, but generated highly crystalline CtNWs (86%) (Liu et al., 2015; Pereira et al., 2014) In contrast, the GA-GMA/CtNWs0 XRD exhibited a halo-shaped pattern (with a maximum of around 2θ ≈ 19.7◦ ) evidencing the amorphous nature of this hydrogel sample (Pau­ lino et al., 2010) On the other hand, the GA-GMA/CtNWs10 XRD exhibited the characteristic diffraction signals proceeding from CtNWs at 2θ ≈ 9.3◦ , 19.2◦ , and 26.3◦ , indicating that the GA-GMA hydrogel was successfully embedded with CtNWs Worth of mention, the peaks attributed to CtNWs in the composite did not shift from CtNWs, indi­ cating the crystalline domains were maintained Thermogravimetry (TGA/DTG) was performed to investigate the thermal stability of the synthesized hydrogels and the effect of CtNWs on the matrix, as shown in Fig 5c The thermal profile of the isolated CtNWs exhibited two main weight loss stages at the temperature range of 30 ◦ C to 600 ◦ C The first weight loss stage (25–120 ◦ C) related to the evaporation of water was minimal showing the hydrophobic nature of CtNWs The second stage (220–440 ◦ C), with a maximum temperature at 379 ◦ C, was due to the thermal degradation of the chitin backbone (Salaberria et al., 2017) Similarly, the TGA/DTG curves of GA-GMA/ CtNWs0 also exhibited two weight loss stages The first (25–180 ◦ C) was due to the evaporation of water adsorbed on the hydrogel (~10% of weight loss) and the second (190–600 ◦ C) was associated with the thermal decomposition of the hydrogel matrix (~70% of weight loss) At 600 ◦ C, the residue of GA-GMA/CtNWs0 was 21% of its initial weight Overall, the GA-GMA/CtNWs10 nanocomposite exhibited a thermal behavior comparable to that presented by the pristine hydrogel; how­ ever, some differences can be pointed out For example, the first stage due to the water evaporation resulted in a lower weight loss percentage (~8%), which may suggest that the introduction of CtNWs affected the hydrophilicity of the hydrogels nanocomposites Moreover, a third weight-loss stage can be noticed in the TGA/DTG curves of GA-GMA/ CtNWs0 This additional weight loss stage with a maximum investigated by FTIR, and the obtained spectra were shown in Fig The spectrum of raw GA exhibited a broad band centered at 3424 cm− (O–H stretching of hydroxyl groups) and other typical bands at 2928 cm− (C–H stretching of CHx groups), at 1607 and 1424 cm− (asym­ – O stretching of carboxylic groups), and 1288 metric and symmetric C– cm− (C–OH stretching) The bands observed in the range 1180–1000 cm− are due to the C–O–C and C–O stretching of glycosidic bonds (Espinosa-Andrews et al., 2010) GMA exhibited typical bands in the range 3065–2930 cm− (C–H stretching of – – CH and –CHx groups) and – O and C– – C stretching of ester bands at 1718 cm− and 1634 cm− (C– conjugated system), at 908 cm− (C–O–C stretching of epoxide ring) (Pereira et al., 2013) After modification with GMA, the GA-GMA spectrum exhibited the typical bands of GA accompanied by some wavenumber shifting and changes in intensity Also, the appearance of new bands was noticed in the GA-GMA spectrum The band associated with the OH stretching of hydroxyl groups was broadened and shifted to 3385 cm− 1, while bands in region 1180–1000 cm− exhibited an in­ crease of intensity caused by the introduction of C–O bonds in the GA backbone Similarly, the increment in the intensity of the bands that occur in the range 1450–1350 cm− can be ascribed to additional CH2 and CH3 groups proceeding form GMA Finally, the appearance of a –O small band at 1718 cm− and a shoulder-type band at 1634 cm− (C– – C groups of GMA) confirm the modification of GA, as previously and C– reported by studies on the modification of polysaccharides with GMA (Gerola et al., 2016; Pereira et al., 2013) 3.3 Characterization of the synthesized hydrogels Vibrational spectroscopy was also used to characterize the chemical nature of the isolated CtNWs and synthesized hydrogels and composites (with CtNWs), as shown in Fig 5a The main absorption bands of CtNWs were observed at 3446 cm− (O–H stretching of hydroxyl groups), at 3264 cm− and 3105 cm− (N–H stretching), at 1663 cm− and 1627 – O stretching, amide I band), at 1560 cm− (a combination of cm− (C– C–N–H stretching and N–H bending, amide II band), and at 1028 cm− (C–O stretching of chitin skeletons) (Goodrich & Winter, 2007; –O Pereira et al., 2014, 2020) The absorbance ratio at 1663 cm− (C– stretching) and 3446 cm− (O–H stretching) (A1663/A3446 × 115) was proposed by Baxter et al to indicate the degree of N-acetylation of CtNWs (Baxter et al., 1992) Herein, the overall N-acetylation degree (in the bulk phase) was calculated to be around 81% The FTIR spectra recorded from the pristine hydrogel (GA-GMA/ CtNWs0) and hydrogel nanocomposite containing 10 wt% of CtNWs (GA-GMA/CtNWs10) showed to be very similar and exhibited the characteristic bands of GA-GMA at 2932 cm− (C–H stretching), at Fig FTIR spectra obtained for GMA, raw GA, and GA-GMA A.G.B Pereira et al Carbohydrate Polymers 266 (2021) 118116 Fig (a) FTIR spectra (b) XRD diffraction patterns and (c) TGA (solid lines)/DTG (dash lines) curves of CtNWs, GA-GMA/CtNWs0, and GA-GMA/CtNWs10 temperature at 374 ◦ C is due to the thermal decomposition of the CtNWs embedded into the hydrogel matrix Finally, at 600 ◦ C there are more residues for GA-GMA/CtNWs10 (~28%) than GA-GMA/CtNWs0, likely due to the presence of CtNWs into the nanocomposite It should be noted that the weight loss stage associated with the thermal decomposition of the hydrogel matrix (at 294 ◦ C) did not varied, suggesting that the introduction of CtNWs did not affect the stability of the GA-GMA hydrogel Although the FTIR analysis indicated the physical interac­ tion between CtNWs and GA-GMA matrix (H-bonds), this physical interaction was unable to increase the temperature of GA-GMA/ CtNWs10 thermal decomposition The lack of chemical bonds between CtNWs and the hydrogel, the low concentration of CtNWs (≤10 wt%) and the similar polysaccharide backbone (with similar thermal stability) could explain the observed behavior For TGA analysis, the aqueous CtNWs suspension was dried prior to analysis Upon drying, CtNWs self-assemble into tens of micron thick sheet-like layers with much smaller sub-micron fibrillated network structures in between, as we have showed (Pereira et al., 2014) In the Fig Schematic illustration of the GA-GMA hydrogel nanocomposite containing CtNWs and TEM image of the GA-GMA/CtNWs10 The white arrows in the TEM image indicate the CtNWs dispersed through the hydrogel matrix A.G.B Pereira et al Carbohydrate Polymers 266 (2021) 118116 900% before h, indicating a superabsorbent ability that can be explained by the great hydrophilicity of GA chains (Khan et al., 2020) After this, the liquid uptake slows down, and the swelling equilibrium (976%) was eventually reached after 10 h CtNWs introduced into the hydrogel matrices exerted a significant effect on their swelling profile for long-term liquid uptake Overall, after the first hour of the experiment, the swelling rate for the nano­ composites slow down and the absorption depended on the concentra­ tion of CtNWs, Fig 7a, however, there was no linear relationship between the concentration of CtNWs in the nanocomposite and water uptake behavior At equilibrium, the swelling rates of GA-GMA/ CtNWs1, GA-GMA/CtNWs5, and GA-GMA/CtNWs10 were calculated to be around 601%, 802%, and 725%, respectively These results reveal that the presence of CtNWs reduces the liquid uptake capacity of the nanocomposites as compared to the pristine hydrogel This trend can be explained by two main reasons: (i) the abundant hydroxyl and amino groups at the CtNWs surfaces can interact (via H-bond and/or dipoledipole interactions) with the functional groups of GA causing an addi­ tional physical crosslinking of the hydrogel matrix; and (ii) these in­ teractions limit the interaction between the hydrogel matrix and water molecules Higher density of crosslinking means higher rigidity of the polymeric matrix, preventing the expansion of the hydrogel volume as well as the water adsorption Additionally, the reduction of hydrophilic groups due to the filler-matrix interaction impairs the anchorage of water molecules This discussion corroborates the TGA analysis, which demonstrated that GA-GMA/CtNWs10 is less hydrophilic than GAGMA/CtNWs0 Furthermore, the interaction between CtNWs and the GA-GMA matrix also aids to explain the non-linear behavior observed for the nanocomposite samples regarding their swelling capacity Earlier studies demonstrated that hydrogel nanocomposites experienced an increment of their swelling capacity with the increase of filler concen­ tration (Spagnol et al., 2012) However, this increment is limited to a composites, the whiskers were well dispersed and immobilized in the GA hydrogel, and did not aggregate or self-assemble upon drying to form a compact structure as in the case of pure CtNWs Therefore, although CtNWs presented higher thermal stability than GA-GMA/CtNWs0, its presence in GA-GMA/CtNWs10 as individual well dispersed whiskers and at low concentration explain the similar thermal behavior of hydrogels with and without CtNWs Hence, the thermal stability of GAGMA/CtNWs10 is little influenced by CtNWs at 10 wt% TEM images recorded of the GA-GMA/CtNWs10 nanocomposite revealed that the nanowhiskers are randomly and uniformly dispersed through the hydrogel matrix, Fig This well dispersed system cor­ roborates the FTIR data indicating compatibility between CtNWs and the GA chains (due to the H-bonds), which could improve the hydrogel mechanical properties as well as the water uptake kinetics, for instance 3.4 Swelling behavior The ability of absorb and retain large amounts of aqueous fluids is the most notorious feature of hydrogels Overall, this ability has assured a wide spectrum of potential applications for hydrogels, such as the adsorption of pollutants, soil conditioning, drug release, and scaffolding in tissue engineering, among others (Curvello et al., 2019; Du et al., 2020; Mohammadinejad et al., 2019) The amount of liquid absorbed by a dry hydrogel mass is generally computed by a parameter known as swelling capacity or swelling ratio As reported, the swelling capacity depends on several factors, including the nanocomposite formation and kind of filler used (Ebrahimi, 2019) Swelling experiments were per­ formed to investigate the effect of different CtNWs concentrations on GA-GMA/CtNWs nanocomposites The hydrogels presented fast initial swelling rate, but the pristine hydrogel (GA-GMA/CtNWs0) displayed greater swelling rate and higher water uptake at equilibrium than the composites The swelling rate computed to GA-GMA/CtNWs0 achieved Fig (a) Swelling kinetics of nanocomposites synthesized with different CtNWs concentrations, (b) Diffused water rate, (c) Power-law plot, and (d) values of exponent n (diffusion coefficient) calculated from the power-law model as function of CtNWs concentration A.G.B Pereira et al Carbohydrate Polymers 266 (2021) 118116 maximum filler concentration, that when exceeded causes the impair­ ment of the swelling The excess of filler lead to higher filler-filler and filler-matrix interactions, negatively affecting the liquid uptake and the swelling of the nanocomposite (Cˆ andido et al., 2012; Carsi et al., 2019) Indeed, the increase of the filler-filler interactions at CtNWs concen­ trations higher than wt/wt% can trigger a phase separation process reducing the surface area and the hydrophilicity of the nanocomposite Previous work has shown that polysaccharide rod-like nanoparticles (e g., cellulose nanowhiskers) undergo a concentration dependent phase transition under aqueous media (Khandelwal & Windle, 2013) To better understand the mechanism that drives the liquid uptake, the power-law model was applied to the swelling data (Brannon-Peppas & Peppas, 1990) This widely known mathematical model is valid for liquid uptakes below 60% of the equilibrium, and it is given by the following equation: / Wt = ktn (4) Weq depicted in Fig 7a, the values of n can be correlated to different physical mechanisms, such as Fickian diffusion, non-Fickian diffusion (known as anomalous), or Case II (relaxation-controlled) transport, that controls liquid uptake by a swellable polymer matrix (Carbinatto et al., 2014; Ganji et al., 2010) Considering the geometry of the hydrogel nano­ composite synthesized in this work, n = 0.45 indicates Fickian diffusion, 0.45 < n < 0.89 indicates non-Fickian diffusion, and n > 0.89 implies Case II transport The values of n calculated using the power-law model are displayed in Fig 7d and are around 0.45 indicating that the liquid uptake mechanism for these hydrogels is consistent with a Fickian diffusional process In this case, the liquid diffusion rate is much lower than the polymer relaxation rate, which means that the liquid molecules diffuse easily through the hydrogel matrices because of the high flexi­ bility of the polymer chains Interestingly, the introduction of CtNWs even at different concentrations did not impede the access of water in­ ward the hydrogel matrices; however, the filler affected their liquid uptake capacities These observations are consistent with earlier studies that investigated the swelling properties of hydrogel (nano)composites (Toledo et al., 2018) It is important to mention that the power-law model was able to fit adequately the experimental swelling data since the coefficients of determination (R2) were higher than 0.965 where Wt and Weq are the absorbed water weights in the nanocomposites at a specific time (t) and at equilibrium, respectively k is a swelling constant associated with the hydrogel network The exponent n is known as the diffusion coefficient and its value is used to describe the liquid uptake mechanism The swelling profile of hydrogels and hydrogels nanocomposite has been assessed by plotting the water uptake (Wt/Weq) as a function of time (t) (Fig 7b), while the values of n are calculated from the plot log (Wt/Weq) versus log t using simple linear regression (Fig 7c) The parameters computed from the experimental swelling data and equations from the power-low model are: log (Wt/Weq) = − 0.175 + 0.376 log t (R2 = 0.970), log (Wt/Weq) = − 0.153 + 0.434 log t (R2 = 0.983), log (Wt/Weq) = − 0.218 + 0.348 log t (R2 = 0.965) and log (Wt/ Weq) = − 0.245 + 0.415 log t (R2 = 0.988) for GA-GMA/CtNWs0, GAGMA/CtNWs1, GA-GMA/CtNWs5, and GA-GMA/CtNWs10, respectively According to the empirical power-law model used to fit the data 3.5 Mechanical properties Hydrogel nanocomposites were synthesized using different concen­ trations of CtNWs (0–10 wt%) and their mechanical properties were examined through compressive tests The computed mechanical prop­ erties are shown in Fig According to the data displayed in Fig 8a, a remarkable increase of Young’s modulus (from 6.23 to 8.95 KPa) was observed as the CtNWs concentration increased from to 10 wt%, which represents an improvement of 45% compared to the pristine hydrogel In general, this mechanical parameter denotes the stiffness or resistance of the hydrogel Fig Mechanical properties of nanocomposite hydrogels embedded with CtNWs (a) Young’s modulus, (b) rupture force, and (c) maximum compressive strain at rupture A.G.B Pereira et al Carbohydrate Polymers 266 (2021) 118116 towards elastic deformation under load Therefore, the introduction of 10 wt% CtNWs allows increasing the ability of the GA-GMA/CtNWs10 to resist compressive deformation better than GA-GMA/CtNWs0 Simi­ larly, the force requested to crack the nanocomposites samples (or rupture force) increased from 1.07 to 2.10 N (96% of increment) when the GA-GMA/CtNWs0 and GA-GMA/CtNWs10 samples are compared Notoriously, these mechanical enhancements were only noticed to the samples embedded with at least wt% CtNWs As assessed, the nano­ composites with the lowest concentration of CtNWs did not show a significant increase in Young’s modulus or rupture force value Araki et al stated that for reinforcing the hydrogel network, the CtNWs must connect (or interact) to at least more than two crosslinking points per single nanocrystal, as otherwise, CtNWs not bear mechanical stress nor strength the hydrogel (Araki et al., 2012) According to the swelling data, the introduction of wt% CtNWs in the hydrogel matrix affected considerably the liquid uptake capacity of GA-GMA/CtNWs1 as compared to GA-GMA/CtNWs0, however, this effect did not reflect an enhancement of GA-GMA/CtNWs1 mechanical properties Analyzing the swelling and mechanical data, which are convergent properties of hydrogels, it can be assumed that the introduction of a low concentra­ tion of CtNWs only causes a reduction of hydrophilic groups within the hydrogel matrix or an increase of hydrophobicity, whereas the cross­ linking was not affected Therefore, the results depicted in Fig 8a and b suggest that an additional crosslinking effect is only noticed to the nanocomposite samples synthesized with at least wt% CtNWs Indeed, these finds corroborate with other studies that demonstrated that hydrogel matrices are reinforced by CtNWs only when concentrations higher than wt% are used (Araki et al., 2012) Finally, although the introduction of CtNWs in the hydrogel matrix enhanced their mechan­ ical properties, it does not have a significant effect on the maximum strain computed at the rupture point for the nanocomposite samples (Fig 8c) This is an interesting result since it stands out that the ability to strain of the nanocomposites is preserved even after the reinforcement with CtNWs, which can be an advantage from practical and handling viewpoints A comparative table presenting swelling and mechanical data of this study and others reported for GA-based hydrogels (composites or not) is presented in Table In general lines, the composite hydrogels show an antagonistic behavior between swelling capacity and mechanical prop­ erties Although the introduction of filler materials within the hydrogel network improves mechanical properties, the swelling capacity ends up being affected Depending on the target application, the impairment of the swelling capacity is a limiting factor As noticed, the hydrogels synthesized in this study exhibited maximum swelling capacity comparable to or even superior to other hydrogels based on GA In contrast, the simple numeric comparison of the mechanical properties data revealed that GA-GMA/CtNWs0 and GA-GMA/CtNWs10 are slightly inferior to the other examples Herein, one must be aware of the comparison must be done with care since these hydrogels have different formulations and/or were prepared using different protocols These two variables (i.e., composition and preparation) exert a straight effect on the morphology and mechanical properties of the resulting hydrogels Moreover, due to inconsistencies in the data reported in the literature, it is difficult to compare the mechanical properties of this kind of hydrogel as they are estimated at different analytical conditions Beyond these constraints, few studies focused on the synthesis of GA-based hydrogels have devoted time to investigate the mechanical properties of such materials Due to this, a trustable data comparison is not a simple task In summary, it was demonstrated here for the first time that GA-GMA hydrogels can be mechanically reinforced by CtNWs Due to the large aspect ratio, high mechanical strength, and unique rod-like shape, these nanowhiskers were easily dispersed through the hydrogel matrix resulting in a noticeable enhancement on the mechanical properties of the conventional GA-GMA hydrogel Surely, these novel nano­ composites can broaden the range of applications of this kind of hydrogel, which include biomedical (e.g., tissue engineering, wound healing, and delivery systems) and environmental (e.g., wastewater remediation and soil conditioning) applications Conclusion Nanocomposites were efficiently synthesized by introducing different amounts of CtNWs in a GA-GMA hydrogel matrix CtNWs were efficiently isolated from raw chitin and GA was modified with cross­ linkable functional groups using GMA Although chemical bond be­ tween GA-GMA and CtNWs were not evidenced by FTIR analysis, the physical interaction through H-bond was enough to provide well dispersed and distributed CtNWs throughout the hydrogel matrix Overall, the nanocomposites showed mechanic and swelling properties dependent on the amount of CtNWs; however, no linear correlation between these properties and the amount of CtNWs was observed CtNWs increased the rigidity and reduced the swelling capacity of hydrogel that could be modulated by adjusting the amount of CtNWs This is an attractive advantage since it widens the range of functionality and applicability of these nanocomposites based on abundant and nat­ ural polymers It is expected that further studies investigate the appli­ cability of these soft materials in the biomedical field (biomaterials or biomedical devices), for example Table Comparison of maximum swelling and mechanical properties of various GA-based hydrogels and composited hydrogels Hydrogela GA-GMA/CtNWs0 GA-GMA/CtNWs10 GA-g-PAAc/pinus residue GA-g-PAAc/eucaliptus residue Oxidized GA-g-PVA GA-GMA-g-PAAc GA-GMA-g-PAAc/graphene GA-GMA-g-PAAc/MOF-UIO66 GA-gelatin-AAM GA/O-carboxymethyl chitosan GA‑sodium alginate-chitosan GA-g-PAAm GA-GMA/magnetite GA/starch nanocrystals GA-gelatin/cellulose whiskers a Filler (wt/wt%) Maximum swelling (%) – 10.0 10.0 10.0 – – – 0.1 – – – – 5.5 33.3 5.0 976 725 98 102 210 1000 280 800 865 750 208 1428 148 200 500 Mechanical properties Ref Young’s modulus (KPa) Rupture force (N) Strain at rupture (%) 6.23 8.94 – – – 26.60 66.90 183.40 40.0 – – – – – – 1.07 2.10 – – – – – – – – – – – – – 58 60 – – 75 – – – 1050 – – – – – – Abbreviations: PAAc - poly(acrylic acid); PVA - poly(vinyl alcohol); MOF - metal organic framework; PAAm - poly(acrylamide) 10 This study This study (de Souza et al., 2019) (de Souza et al., 2019) (Pandit et al., 2019) (Fan et al., 2013) (Fan et al., 2013) (Ribeiro et al., 2019) (Wang et al., 2019) (Huang et al., 2016) (Younis et al., 2018) (Kaith & Ranjta, 2010) (Paulino et al., 2010) (Alwaan et al., 2019) (Favatela et al., 2021) A.G.B Pereira et al Carbohydrate Polymers 266 (2021) 118116 CRediT authorship contribution statement in dental treatments: Study of lidocaine as model case Journal of Drug Delivery Science and Technology, 61, 101886 https://doi.org/10.1016/j.jddst.2020.101886 Feng, K., Hung, G.-Y., Yang, X., & Liu, M (2019) High-strength and physical cross-linked nanocomposite hydrogel with clay nanotubes for strain sensor and dye adsorption application Composites Science and Technology, 181, 107701 https://doi.org/ 10.1016/j.compscitech.2019.107701 Ganji, F., Vasheghani Farahani, S., & Vasheghani-Farahani, E (2010) Theoretical description of hydrogel swelling: A review Iranian Polymer Journal, 19, 375–398 Ge, S., Liu, Q., Li, M., Liu, J., Lu, H., Li, F., Zhang, S., Sun, Q., & Xiong, L (2018) Enhanced mechanical properties and gelling ability of gelatin hydrogels reinforced with chitin whiskers Food Hydrocolloids, 75, 1–12 https://doi.org/10.1016/j foodhyd.2017.09.023 Gerola, A P., Silva, D C., Jesus, S., Carvalho, R A., Rubira, A F., Muniz, E C., … Valente, A J M (2015) Synthesis and controlled curcumin supramolecular complex release from pH-sensitive modified gum-arabic-based hydrogels RSC Advances, (115), 94519–94533 https://doi.org/10.1039/C5RA14331D Gerola, A P., Silva, D C., Matsushita, A F Y., Borges, O., Rubira, A F., Muniz, E C., & Valente, A J M (2016) The effect of methacrylation on the behavior of Gum Arabic as pH-responsive matrix for colon-specific drug delivery European Polymer Journal, 78, 326–339 https://doi.org/10.1016/j.eurpolymj.2016.03.041 Goodrich, J D., & Winter, W T (2007) α-Chitin nanocrystals prepared from shrimp shells and their specific surface area measurement Biomacromolecules, 8(1), 252–257 https://doi.org/10.1021/bm0603589 Huang, C.-H., Wang, C.-F., Don, T.-M., & Chiu, W.-Y (2013) Preparation of pH- and thermo-sensitive chitosan-PNIPAAm core–shell nanoparticles and evaluation as drug carriers Cellulose, 20(4), 1791–1805 https://doi.org/10.1007/s10570-013-9951-1 Huang, G.-Q., Cheng, L.-Y., Xiao, J.-X., Wang, S.-Q., & Han, X.-N (2016) Genipincrosslinked O-carboxymethyl chitosan–gum Arabic coacervate as a pH-sensitive delivery system and microstructure characterization Journal of Biomaterials Applications, 31(2), 193–204 https://doi.org/10.1177/0885328216651393 Jung, H.-S., Kim, H C., & Ho Park, W (2019) Robust methylcellulose hydrogels reinforced with chitin nanocrystals Carbohydrate Polymers, 213, 311–319 https:// doi.org/10.1016/j.carbpol.2019.03.009 Kaith, B S., & Ranjta, S (2010) Synthesis of pH — Thermosensitive gum arabic based hydrogel and study of its salt-resistant swelling behavior for saline water treatment Desalination and Water Treatment, 24(1–3), 28–37 https://doi.org/10.5004/ dwt.2010.1145 Khan, M., Shah, L A., Rehman, T., Khan, A., Iqbal, A., Ullah, M., & Alam, S (2020) Synthesis of physically cross-linked gum Arabic-based polymer hydrogels with enhanced mechanical, load bearing and shape memory behavior Iranian Polymer Journal, 29(4), 351–360 https://doi.org/10.1007/s13726-020-00801-z Khandelwal, M., & Windle, A H (2013) Self-assembly of bacterial and tunicate cellulose nanowhiskers Polymer, 54(19), 5199–5206 https://doi.org/10.1016/j polymer.2013.07.033 Li, M., Li, H., Li, X., Zhu, H., Xu, Z., Liu, L., … Zhang, M (2017) A bioinspired alginateGum Arabic hydrogel with micro-/nanoscale structures for controlled drug release in chronic wound healing ACS Applied Materials & Interfaces, 9(27), 22160–22175 https://doi.org/10.1021/acsami.7b04428 Li, S.-N., Li, B., Yu, Z.-R., Li, Y., Guo, K.-Y., Gong, L.-X., Feng, Y., Jia, D., Zhou, Y., & Tang, L.-C (2020) Constructing dual ionically cross-linked poly(acrylamide-coacrylic acid) /chitosan hydrogel materials embedded with chitosan decorated halloysite nanotubes for exceptional mechanical performance Composites Part B: Engineering, 194, 108046 https://doi.org/10.1016/j.compositesb.2020.108046 Liu, M., Huang, J., Luo, B., & Zhou, C (2015) Tough and highly stretchable polyacrylamide nanocomposite hydrogels with chitin nanocrystals International Journal of Biological Macromolecules, 78, 23–31 https://doi.org/10.1016/j ijbiomac.2015.03.059 Liu, W., Zhu, L., Ma, Y., Ai, L., Wen, W., Zhou, C., & Luo, B (2019) Well-ordered chitin whiskers layer with high stability on the surface of poly(d,l-lactide) film for enhancing mechanical and osteogenic properties Carbohydrate Polymers, 212, 277–288 https://doi.org/10.1016/j.carbpol.2019.02.060 Marchessault, R H., Morehead, F F., & Walter, N M (1959) Liquid crystal systems from fibrillar polysaccharides Nature, 184(4686), 632–633 https://doi.org/10.1038/ 184632a0 Mohammadinejad, R., Maleki, H., Larra˜ neta, E., Fajardo, A R., Nik, A B., Shavandi, A., … Thakur, V K (2019) Status and future scope of plant-based green hydrogels in biomedical engineering Applied Materials Today, 16, 213–246 https://doi.org/ 10.1016/j.apmt.2019.04.010 Ou, X., Cai, J., Tian, J., Luo, B., & Liu, M (2020) Superamphiphobic surfaces with selfcleaning and antifouling properties by functionalized chitin nanocrystals ACS Sustainable Chemistry & Engineering, 8(17), 6690–6699 https://doi.org/10.1021/ acssuschemeng.0c00340 Paillet, M., & Dufresne, A (2001) Chitin whisker reinforced thermoplastic nanocomposites Macromolecules, 34(19), 6527–6530 https://doi.org/10.1021/ ma002049v Pandit, A H., Mazumdar, N., Imtiyaz, K., Rizvi, M M A., & Ahmad, S (2019) Periodatemodified Gum Arabic cross-linked PVA hydrogels: A promising approach toward photoprotection and sustained delivery of folic acid ACS Omega, 4(14), 16026–16036 https://doi.org/10.1021/acsomega.9b02137 Pang, J., Bi, S., Kong, T., Luo, X., Zhou, Z., Qiu, K., Huang, L., Chen, X., & Kong, M (2020) Mechanically and functionally strengthened tissue adhesive of chitin whisker complexed chitosan/dextran derivatives based hydrogel Carbohydrate Polymers, 237, 116138 https://doi.org/10.1016/j.carbpol.2020.116138 Paulino, A T., Guilherme, M R., Mattoso, L H C., & Tambourgi, E B (2010) Smart hydrogels based on modified Gum Arabic as a potential device for magnetic Antonio G.B Pereira: Conceptualization, Formal analysis, Meth­ ´tia S Nunes: Formal analysis odology, Writing – original draft Ca ´ R Adley F Rubira: Supervision Edvani C Muniz: Supervision Andre Fajardo: Methodology, Writing – original draft Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Acknowledgments The authors thank CNPq for its financial support and for the PQ fellowship to A.R.F (Grant number 303873/2019-5) This study was financed in part by the Coordenaỗ ao de Aperfeiỗoamento de Pessoal de Nớvel Superior, Brazil (CAPES), Finance Code 001 The authors are thankful to INOMAT (Chemistry Institute, UNICAMP, SP, Brazil) for its technical support regarding TEM microscopy References Ahmed, E M (2015) Hydrogel: Preparation, characterization, and applications: A review Journal of Advanced Research, 6(2), 105–121 https://doi.org/10.1016/j jare.2013.07.006 Alwaan, I M., Jafar, M M R M., & Allebban, Z S M (2019) Development of biodegradable starch nanocrystals/gum Arabic hydrogels for controlled drug delivery and cancer therapy Biomedical Physics & Engineering Express, 5(2), 25021 https://doi.org/10.1088/2057-1976/aafc14 Araki, J., Yamanaka, Y., & Ohkawa, K (2012) Chitin-chitosan nanocomposite gels: Reinforcement of chitosan hydrogels with rod-like chitin nanowhiskers Polymer Journal, 44(7), 713–717 https://doi.org/10.1038/pj.2012.11 Baxter, A., Dillon, M., Anthony Taylor, K D., & Roberts, G A F (1992) Improved method for i.r determination of the degree of N-acetylation of chitosan International Journal of Biological Macromolecules, 14(3), 166–169 https://doi.org/10.1016/ S0141-8130(05)80007-8 Brannon-Peppas, L., & Peppas, N A (1990) Dynamic and equilibrium swelling behaviour of pH-sensitive hydrogels containing 2-hydroxyethyl methacrylate Biomaterials, 11(9), 635–644 https://doi.org/10.1016/0142-9612(90)90021-H Cˆ andido, J.d S., Leit˜ ao, R C F., Ricardo, N M P S., Feitosa, J P A., Muniz, E C., & Rodrigues, F H A (2012) Hydrogels composite of poly(acrylamide-co-acrylate) and rice husk ash I Synthesis and characterization Journal of Applied Polymer Science, 123(2), 879–887 https://doi.org/10.1002/app.34528 Carbinatto, F M., de Castro, A D., Evangelista, R C., & Cury, B S F (2014) Insights into the swelling process and drug release mechanisms from cross-linked pectin/high amylose starch matrices Asian Journal of Pharmaceutical Sciences, 9(1), 27–34 https://doi.org/10.1016/j.ajps.2013.12.002 Carsi, M., Sanchis, M J., G´ omez, C M., Rodriguez, S., & G Torres, F (2019) Effect of chitin whiskers on the molecular dynamics of carrageenan-based nanocomposites In , Vol 11, Issue Polymers https://doi.org/10.3390/polym11061083 Curvello, R., Raghuwanshi, V S., & Garnier, G (2019) Engineering nanocellulose hydrogels for biomedical applications Advances in Colloid and Interface Science, 267, 47–61 https://doi.org/10.1016/j.cis.2019.03.002 Du, H., Shi, S., Liu, W., Teng, H., & Piao, M (2020) Processing and modification of hydrogel and its application in emerging contaminant adsorption and in catalyst immobilization: A review Environmental Science and Pollution Research, 27(12), 12967–12994 https://doi.org/10.1007/s11356-020-08096-6 Ebrahimi, R (2019) The study of factors affecting the swelling of ultrasound-prepared hydrogel Polymer Bulletin, 76(2), 1023–1039 https://doi.org/10.1007/s00289-0182423-x Eichhorn, S J (2011) Cellulose nanowhiskers: Promising materials for advanced applications Soft Matter, 7(2), 303–315 https://doi.org/10.1039/C0SM00142B Espinosa-Andrews, H., Sandoval-Castilla, O., V´ azquez-Torres, H., Vernon-Carter, E J., & Lobato-Calleros, C (2010) Determination of the gum Arabic–chitosan interactions by Fourier Transform Infrared Spectroscopy and characterization of the microstructure and rheological features of their coacervates Carbohydrate Polymers, 79(3), 541–546 https://doi.org/10.1016/j.carbpol.2009.08.040 Fan, J., Shi, Z., Wang, J., & Yin, J (2013) Glycidyl methacrylate-modified gum arabic mediated graphene exfoliation and its use for enhancing mechanical performance of hydrogel Polymer, 54(15), 3921–3930 https://doi.org/10.1016/j polymer.2013.05.057 Fan, Y., Fukuzumi, H., Saito, T., & Isogai, A (2012) Comparative characterization of aqueous dispersions and cast films of different chitin nanowhiskers/nanofibers International Journal of Biological Macromolecules, 50(1), 69–76 https://doi.org/ 10.1016/j.ijbiomac.2011.09.026 Favatela, F., Horst, M F., Bracone, M., Gonzalez, J., Alvarez, V., & Lassalle, V (2021) Gelatin/cellulose nanowhiskers hydrogels intended for the administration of drugs 11 A.G.B Pereira et al Carbohydrate Polymers 266 (2021) 118116 de Souza, A G., Cesco, C T., de Lima, G F., Artifon, S E S., Rosa, D.d S., & Paulino, A T (2019) Arabic gum-based composite hydrogels reinforced with eucalyptus and pinus residues for controlled phosphorus release International Journal of Biological Macromolecules, 140, 33–42 https://doi.org/10.1016/j ijbiomac.2019.08.106 Spagnol, C., Rodrigues, F H A., Pereira, A G B., Fajardo, A R., Rubira, A F., & Muniz, E C (2012) Superabsorbent hydrogel nanocomposites based on starch-gpoly(sodium acrylate) matrix filled with cellulose nanowhiskers Cellulose, 19(4), 1225–1237 https://doi.org/10.1007/s10570-012-9711-7 Stefanovic, J., Jakovljevic, D., Gojgic-Cvijovic, G., Lazic, M., & Vrvic, M (2013) Synthesis, characterization, and antifungal activity of nystatin—Gum arabic conjugates Journal of Applied Polymer Science, 127(6), 4736–4743 https://doi.org/ 10.1002/app.38084 Sun, G., Zhang, X., Bao, Z., Lang, X., Zhou, Z., Li, Y., Feng, C., & Chen, X (2018) Reinforcement of thermoplastic chitosan hydrogel using chitin whiskers optimized with response surface methodology Carbohydrate Polymers, 189, 280–288 https:// doi.org/10.1016/j.carbpol.2018.01.083 Thoniyot, P., Tan, M J., Karim, A A., Young, D J., & Loh, X J (2015) Nanoparticle–hydrogel composites: Concept, design, and applications of these promising, multi-functional materials Advanced Science, 2(1–2), 1400010 https:// doi.org/10.1002/advs.201400010 Tian, D., Maiti, S., Liu, D., Ma, Z., & Cao, X (2015) Structure and morphology of fractions separated from mechanical-assisted enzyme hydrolyzed chitin microfibrils Cellulose, 22(1), 1–8 https://doi.org/10.1007/s10570-014-0492-z Toledo, L., Racine, L., P´erez, V., Henríquez, J P., Auzely-Velty, R., & Urbano, B F (2018) Physical nanocomposite hydrogels filled with low concentrations of TiO2 nanoparticles: Swelling, networks parameters and cell retention studies Materials Science and Engineering: C, 92, 769–778 https://doi.org/10.1016/j msec.2018.07.024 Uddin, A J., Fujie, M., Sembo, S., & Gotoh, Y (2012) Outstanding reinforcing effect of highly oriented chitin whiskers in PVA nanocomposites Carbohydrate Polymers, 87 (1), 799–805 https://doi.org/10.1016/j.carbpol.2011.08.071 Wang, H., Xu, Z., Wu, Y., Li, H., & Liu, W (2018) A high strength semi-degradable polysaccharide-based hybrid hydrogel for promoting cell adhesion and proliferation Journal of Materials Science, 53(9), 6302–6312 https://doi.org/10.1007/s10853018-2019-8 Wang, Q., Chen, S., & Chen, D (2017) Preparation and characterization of chitosan based injectable hydrogels enhanced by chitin nano-whiskers Journal of the Mechanical Behavior of Biomedical Materials, 65, 466–477 https://doi.org/10.1016/j jmbbm.2016.09.009 Wang, Y., Tong, L., Zheng, Y., Pang, S., Sha, J., Li, L., & Zhao, G (2019) Hydrogels with self-healing ability, excellent mechanical properties and biocompatibility prepared from oxidized gum arabic European Polymer Journal, 117, 363–371 https://doi.org/ 10.1016/j.eurpolymj.2019.05.033 Younis, M K., Tareq, A Z., & Kamal, I M (2018) Optimization of swelling, drug loading and release from natural polymer hydrogels IOP Conference Series: Materials Science and Engineering, 454, 12017 https://doi.org/10.1088/1757-899x/454/1/012017 Zeng, J.-B., He, Y.-S., Li, S.-L., & Wang, Y.-Z (2012) Chitin whiskers: An overview Biomacromolecules, 13(1), 1–11 https://doi.org/10.1021/bm201564a biomaterial Macromolecular Chemistry and Physics, 211(11), 1196–1205 https://doi org/10.1002/macp.200900657 Paulino, A T., Pereira, A G B., Fajardo, A R., Erickson, K., Kipper, M J., Muniz, E C., … Tambourgi, E B (2012) Natural polymer-based magnetic hydrogels: Potential vectors for remote-controlled drug release Carbohydrate Polymers, 90(3), 1216–1225 https://doi.org/10.1016/j.carbpol.2012.06.051 Pereira, A G B., Fajardo, A R., Gerola, A P., Rodrigues, J H S., Nakamura, C V., Muniz, E C., & Hsieh, Y.-L (2020) First report of electrospun cellulose acetate nanofibers mats with chitin and chitosan nanowhiskers: Fabrication, characterization, and antibacterial activity Carbohydrate Polymers, 250, 116954 https://doi.org/10.1016/j.carbpol.2020.116954 Pereira, A G B., Fajardo, A R., Nocchi, S., Nakamura, C V., Rubira, A F., & Muniz, E C (2013) Starch-based microspheres for sustained-release of curcumin: Preparation and cytotoxic effect on tumor cells Carbohydrate Polymers, 98(1), 711–720 https:// doi.org/10.1016/j.carbpol.2013.06.013 Pereira, A G B., Muniz, E C., & Hsieh, Y.-L (2014) Chitosan-sheath and chitin-core nanowhiskers Carbohydrate Polymers, 107, 158–166 https://doi.org/10.1016/j carbpol.2014.02.046 Pereira, A G B., Muniz, E C., & Hsieh, Y.-L (2015) 1H NMR and 1H–13C HSQC surface characterization of chitosan–chitin sheath-core nanowhiskers Carbohydrate Polymers, 123, 46–52 https://doi.org/10.1016/J.CARBPOL.2015.01.017 Qin, Y., Zhang, S., Yu, J., Yang, J., Xiong, L., & Sun, Q (2016) Effects of chitin nanowhiskers on the antibacterial and physicochemical properties of maize starch films Carbohydrate Polymers, 147, 372–378 https://doi.org/10.1016/j carbpol.2016.03.095 Reis, A V., Fajardo, A R., Schuquel, I T A., Guilherme, M R., Vidotti, G J., Rubira, A F., & Muniz, E C (2009) Reaction of glycidyl methacrylate at the hydroxyl and carboxylic groups of poly(vinyl alcohol) and poly(acrylic acid): Is this reaction mechanism still unclear? The Journal of Organic Chemistry, 74(10), 3750–3757 https://doi.org/10.1021/jo900033c Reis, A V., Guilherme, M R., Cavalcanti, O A., Rubira, A F., & Muniz, E C (2006) Synthesis and characterization of pH-responsive hydrogels based on chemically modified Arabic gum polysaccharide Polymer, 47(6), 2023–2029 https://doi.org/ 10.1016/j.polymer.2006.01.058 Ribeiro, S C., de Lima, H H C., Kupfer, V L., da Silva, C T P., Veregue, F R., Radovanovic, E., … Rinaldi, A W (2019) Synthesis of a superabsorbent hybrid hydrogel with excellent mechanical properties: Water transport and methylene blue absorption profiles Journal of Molecular Liquids, 294, 111553 https://doi.org/ 10.1016/j.molliq.2019.111553 Rodrigues, F H A., Spagnol, C., Pereira, A G B., Martins, A F., Fajardo, A R., Rubira, A F., & Muniz, E C (2014) Superabsorbent hydrogel composites with a focus on hydrogels containing nanofibers or nanowhiskers of cellulose and chitin Journal of Applied Polymer Science, 131(2) https://doi.org/10.1002/app.39725 Salaberria, A M., H Diaz, R., Andr´ es, M A., Fernandes, S C M., & Labidi, J (2017) The antifungal activity of functionalized chitin nanocrystals in poly (lactid acid) films In , Vol 10, Issue Materials https://doi.org/10.3390/ma10050546 de Sousa Mol, A., & Or´ efice, R L (2016) Preparation of chitin nanofibers (whiskers) and their application as property-recovery agents in re-processed polypropylene Polymer Bulletin, 73(3), 661–675 https://doi.org/10.1007/s00289-015-1512-3 12 ... appearance of a –O small band at 1718 cm− and a shoulder-type band at 1634 cm− (C– – C groups of GMA) confirm the modification of GA, as previously and C– reported by studies on the modification of polysaccharides... and mechanical data, which are convergent properties of hydrogels, it can be assumed that the introduction of a low concentra­ tion of CtNWs only causes a reduction of hydrophilic groups within... (2020) Processing and modification of hydrogel and its application in emerging contaminant adsorption and in catalyst immobilization: A review Environmental Science and Pollution Research, 27(12),