Jolanta
Liesiene
a,
Sandra
Kiselioviene
b,
Audrius S.
Maruška
c and
Odeta
Baniukaitiene
*a
aKaunas University of Technology, Department of Polymer Chemistry and Technology, Radvilenu pl. 19, LT-50254 Kaunas, Lithuania. E-mail: odeta.baniukaitiene@ktu.lt; Tel: +370 676 04891
bKaunas University of Technology, Food Institute, Radvilenu pl. 19, LT-50254 Kaunas, Lithuania
cVytautas Magnus University, Instrumental Analysis Open Access Centre, Universiteto g. 10, LT-53361 Kaunas, Lithuania
First published on 28th March 2025
This study introduces a novel approach for preparing rigid, porous cellulose hydrogels using cellulose acetate as the starting material. The method relies on the slow hydrolysis of acetyl groups directly in an acetone/aqueous ammonia solution. The gradual pace of the process creates conditions favourable for reconstructing of inter- and intramolecular hydrogen bonding networks between the newly formed hydroxyl groups in the cellulose, resulting in a rigid three-dimensional structure. The hydrogels demonstrated excellent mechanical properties, with a compressive (Young's) modulus of up to 43 MPa and an elastic modulus of up to 0.23 MPa. X-ray analysis indicated that the cellulose hydrogels are semi-crystalline, with a crystallinity index of 43–45% and an average crystallite size of 4.3–4.5 nm. Wide-angle X-ray diffraction, along with FT-IR and Raman spectroscopy, confirmed that the gels belong to the cellulose II structural modification. The porous structure of the hydrogels was characterized using inverse size exclusion chromatography, revealing exclusion limits for linear polymers of up to 4 × 106 Da. Thanks to their enhanced mechanical properties and high porosity, crushed hydrogels show potential applications in column technologies for protein chromatography and heterogeneous biocatalysis processes with immobilized enzymes. In film form, the gels' elasticity makes them promising candidates for biomedical applications, such as wound dressings or artificial skin. Furthermore, the lyophilized gels create porous structures suitable for vascularization and bone tissue ingrowth, positioning them as ideal scaffolds for bone tissue engineering.
Cellulose hydrogels can be fabricated in various sizes and morphologies, from nano- to macroparticles, and in diverse physical forms such as films, membranes, fibers, and rods, depending on the desired application. Thus, spherical cellulose nanogels, due to their versatile properties, are ideal for a broad range of applications, including drug delivery and biomedical purposes such as disease detection and diagnosis.3,6–8 Cellulose microgels can be used as emulsion stabilizers, with their preparation and application broadly described in reviews.9 Cellulose hydrogels in the form of films, membranes, or fibers have potential as wound dressings and artificial skin due to their specific benefits, such as biocompatibility and controlled drug delivery.10–12 Additionally, they offer potential applications in electronic devices.4,13,14
Cellulose hydrogels, particularly those fabricated as micro- to macroparticles, have gained significant attention in column technologies like liquid chromatography and immobilized heterogeneous biocatalysis. The preparation of cellulose hydrogels in spherical or irregularly shaped macroparticles received intense research interest in the eighties, driven by developments in biochemistry and contributing to progress in protein purification.15 Subsequently, cellulosic sorbents were improved and applied to protein chromatography in various types of processes.15–17 The structure of the sorbents and the amount of functional groups were optimized for the specific purified protein.18 Currently, cellulose-based chromatographic media are integral to nearly all technological schemes for the separation and purification of proteins and other biomaterials.17,19
Cellulose carriers, thanks to their porosity, biocompatibility, and simple functionalization, are highly suitable for enzyme immobilization and subsequent biocatalysis processes.20–23
Nowadays, the development of three-dimensional (3D) scaffolds for tissue regeneration is one of the current challenges in tissue engineering. Various materials are proposed for the fabrication of 3D scaffolds, with significant attention focused on natural polymers. Cellulose hydrogels, for instance, serve as a valuable starting material in developing bone tissue engineering aerogels.24–28 To enhance osteoconductive properties, the scaffolds are prepared by combining cellulose with mineral components such as hydroxyapatite, tricalcium phosphate, cuttlebone, or others.29–31 Aerogel preparation methods are based on drying cellulose hydrogels using lyophilization, supercritical CO2 treatment, or other techniques.32 A notable advantage of cellulose aerogels is that their preparation typically does not involve toxic substances, enhancing their suitability for use in pharmaceuticals, biomedicine, food, and cosmetics. Detailed reviews on the structure, properties, and applications of cellulose aerogels are extensively covered in the literature.33–35
There are different approaches to obtaining cellulose hydrogels, but the main principle is based on dissolving cellulose or its derivatives and subsequently crosslinking the polymer through various interactions. Different crosslinking techniques, such as chemical, physical, and polymerization methods, are used. The structural and mechanical characteristics of the gels depend on the concentration of cellulosic material, precipitation conditions, and the degree of crosslinking.36
One of the primary challenges in processing native cellulose is its limited solubility in conventional organic or inorganic solvents due to the extensive network of hydrogen bonds. Efforts to develop more effective methods for addressing this limitation are continually advancing. Consequently, in recent years, research has increasingly focused on using hydrolyzed low-molecular cellulose dissolved in NaOH solutions,35 various complex organic solvents, or ionic liquids. These methods are extensively discussed in detailed reviews4,37–39 and articles.40–42
Mechanical properties, such as mechanical stability and flexibility, are among the most important factors limiting the practical application of gels. Different application areas require specific mechanical characteristics of the gels. For example, in column technologies such as protein chromatography or enzymatic biocatalysis, the packing materials must withstand high pressure and changes in pH and ionic strength. Therefore, these materials must demonstrate substantial mechanical strength despite their high porosity. The gels in film or membrane form should be flexible and strong. Unfortunately, cellulose gels described in the literature are often characterized as soft gels.37,40,43 To improve their mechanical properties, cellulose hydrogels are crosslinked or reinforced by incorporating other polymers44–46 or mineral materials.47,48
The aim of these studies is to develop a novel method for producing rigid cellulose gels with a tunable porous structure and high mechanical properties. Cellulose diacetate, an industrial raw material used in the production of cellulose acetate fibers, served as the initial material. The method involves the gradual hydrolysis of acetyl groups directly within the solution. The kinetics of acetyl group hydrolysis and the correlation between gel contraction and the parameters of the hydrolysis process were investigated. The porous structure of the gels in the wet state was investigated by means of inverse size exclusion chromatography, while in the dry state by microcomputed tomography. The morphological structure of the cellulose gels was evaluated using wide-angle X-ray diffraction, Raman, and FT-IR spectral analysis. It was demonstrated that the porous structure of the gels, as well as their mechanical properties, can be regulated by varying the initial cellulose acetate concentration. Prerequisites for obtaining nanoporous structured gels with customizable porosity and high mechanical strength have been established.
Degree of substitution (DS) was calculated using the formula:
CI(FTIR) = 1376/2890 (%) |
Additionally, utilizing the intensity ratio of spectral peaks at 3440 and 2890 cm−1, the relative amount of hydrogen bonds (RAHB) in cellulose derivatives was calculated:
RAHB = 3440/2890 (%) |
The gel hardness was determined using tablet-shaped samples. The sample was placed on a measuring table, and the cone tip was gently pressed against the sample. The initial readings h0 of the micrometer indicator were recorded. Subsequently, a weight of predetermined size was hung on the lever, creating a force F of 7 N acting on undried samples. After 30 seconds, the lever with the weight was lifted, and the indicator reading h1 was recorded. Using the formulas below, Δh (mm), the contact surface area S (mm2) between the cone and the sample, and the hardness HH (N mm−2) of the sample were calculated:
Δh = h1 − ho + 0.2 |
The hardness dimension is force per unit area, N mm−2, and the index next to H indicates the force value and duration. The final result was calculated as the arithmetic average of 10 measurements.
The isotropic compression bulk modulus K (elasticity parameter) was calculated using the formula:
Young's modulus E (compression modulus) is a physical quantity that characterizes a material's resistance to crushing or stretching. It is defined as the ratio of stress to the relative elongation (deformation) using the formula:
The product of the modulus compression and the cross-sectional area is called the tensile-compressive strip stiffness ξ. It quantitatively assesses the strip's ability to resist the effects of deformable load.
ξ = S·E (m2 Pa) |
The final result was calculated as the arithmetic average of 5 measurements.
The final result was the arithmetic average of 5 measurements.
The standard deviation (SD) was calculated using the following formula:
During the hydrolysis of the acetyl groups, the solubility of the cellulose derivative gradually decreased in the reaction mixture. This led to the slow precipitation of cellulose macromolecules and the formation of new bonds between the newly formed hydroxyl groups. A sol–gel transition occurred through the reconstruction of inter- and intramolecular hydrogen bonding networks. The gel obtained through this process maintained the volume of the initial solution with minimal shrinkage. Inter- and intramolecular hydrogen bonding between the hydroxyl groups of cellulose molecules contributes to the formation of a rigid gel structure.
The cellulose diacetate used, with a degree of substitution (DS) of 2.4, is soluble in acetone. Cellulose acetates with a lower DS are soluble in solvents of higher polarity, with cellulose monoacetate being soluble even in water. However, as the DS decreases further, cellulose acetates lose their solubility both in water and common organic solvents. The gel formation method we used exploited this phenomenon of solubility of acetylated cellulose derivatives. The CDA was dissolved in acetone, followed by the gradual addition of an aqueous ammonia solution, avoiding the precipitation of the cellulose from the solution. After thorough mixing, the polymer solution was left in a sealed container at room temperature until the hydrolysis reaction was completed. The released ammonium acetate was washed from the gel with water.
The gel produced in this study constitutes regenerated cellulose, which is of higher chemical activity than native cellulose. The hydroxyl groups of regenerated cellulose are more accessible to reactants. This property is particularly significant in applications requiring the attachment of functional groups, such as the preparation of chromatographic sorbents or carriers for enzyme immobilization. During the formation of the porous structure of the regenerated cellulose gel, it was found that the properties of the obtained product depend on many factors, especially on the composition of the reaction mixture. One of the main factors is the concentration of the polymer in the solution used for gel formation. Lower concentrations of CDA in the solution lead to larger pores in the gel structure, as the distance between macromolecules increases. Additionally, the concentration of ammonia in the solution directly influences the gel structure, impacting the speed of the hydrolysis reaction and the resulting gel contraction.
To assess the impact of ammonia concentration in the solution on the reaction rate, the kinetics of hydrolysis of acetyl groups were investigated (Fig. 3).
The results indicate that the initial hydrolysis of acetyl groups occurs rapidly. The higher the concentration of ammonia, the more intense the hydrolysis is. For instance, at an NH4OH/acetone ratio of 0.59 v/v, a sol–gel transition occurs within just 5 hours, while at a ratio of 0.23 v/v, gel formation takes place only after 7 days. However, to obtain a gel with the largest possible pores, hydrolysis must be carried out slowly, as rapid hydrolysis is accompanied by gel contraction.
The sol–gel transition point is observed before the complete hydrolysis of the ester bonds. Depending on the amount of ammonia in the mixture, gel formation occurs within a range of 15–39% bound acetic acid. This is due to differences in the solubility of the cellulose derivative in reaction mixtures of different polarity. After the gel is formed, the hydrolysis of the acetyl groups continues, but at a slower rate. As the hydrolysis of acetyl groups progresses, the rigidity of the gel gradually increases.
After evaluating the time required for gel formation, the hydrolysis rate of acetyl groups, and the impact of ammonia concentration on gel characteristics such as rigidity and contraction, three samples of cellulose gel were prepared: Granocel-500, Granocel-2000, and Granocel-4000 with a water content that ranged from 92% to 97% (Table 1).
Hydrogel | CDA, w/v,% | Acetone, v,% | NH4OH, v,% |
---|---|---|---|
Granocel-500 | 9.4 | 70 | 30 |
Granocel-2000 | 8.4 | 64.5 | 35.5 |
Granocel-4000 | 6.5 | 62.5 | 37.5 |
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Fig. 4 X-ray diffraction patterns of regenerated cellulose gels: 1 – Granocel-4000, 2 – Granocel-2000. |
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Fig. 5 X-ray diffraction patterns of different cellulosic materials: 1 – Granocel-4000, 2 – CDA, 3 – native cotton cellulose. |
The crystallinity index CI(XRD) of the investigated materials and the size of the crystallites were calculated from the X-ray diffraction (XRD) graphs according to the most popular peak height method proposed by Jayme and Knolle:51,52
The results are presented in Table 2. The calculated CI from XRD of the prepared Granocel gels is approximately 43–45%. However, it is important to note that the peak height method used produces values that are significantly higher than those obtained by other methods.52
Cellulose | CI(XRD) (%) | Crystallite size (nm) |
---|---|---|
Granocel-500 | 45 | 4.3 |
Granocel-2000 | 43 | 4.5 |
Granocel-4000 | 43 | 4.4 |
Native cotton cellulose | 72 | 5.4 |
The size of the crystallites in the 002 plane characterizes the crystallites in a horizontal position, i.e. along the axis of the cellulose macromolecule. The crystallite size of cellulose derivatives for crystallographic plane 002 was calculated according to the Scherrer formula:
From the obtained results, it is evident that the Granocel gels of regenerated cellulose have significantly lower crystallinity compared to cotton cellulose. Moreover, the crystallite sizes of the regenerated cellulose are smaller compared to those of cellulose modification I.
The results confirm that the gelation process was accompanied by precipitation and crystallization, characterized by the gradual arrangement of cellulose macromolecules into regular crystallite structures, as observed by Pereira et al.54
FT-IR spectral analysis. FT-IR spectral analysis allows to identify the structure of the compound as well as to determine the predominant bonds.
At 3440 cm−1 a broad spectral peak of high intensity indicates the stretching vibrations of –OH groups (Fig. 6). These groups are connected by hydrogen bonds, which is why the absorption band is wide. At 2890 cm−1, vibrations of –CH group valence bonds are observed. The relative amount of hydrogen bonds was calculated from the ratio of absorption bonds 1376 to 2890 cm−1. While the amount of –OH groups is variable, the amount of –CH groups in cellulose gels remains constant.
The peak at 2943 cm−1 in the FT-IR spectrum of CDA indicates asymmetric valence vibrations of the –CH3 (–CH2–) group. Meanwhile, a prominent peak at 1747 cm−1 in the FT-IR spectrum of CDA corresponds to the asymmetric valence vibrations of the carbonyl group. In Fig. 6, this carbonyl peak is also present in the spectra of regenerated cellulose but is less intense than in CDA. This may be due to the oxidation of hydroxyl groups or incomplete hydrolysis of acetyl groups. The peak at 1227 cm−1 in the spectrum of CDA represents asymmetric valence vibrations of the C–O–C bond, while the peak at 1050 cm−1 corresponds to symmetric valence vibrations of the C–O–C bond.
The crystallinity index CI(FTIR) of cellulose derivatives was also calculated from the FT-IR spectrum (Table 3).
Sample | CI(FTIR) (%) | Relative amount of hydrogen bonds |
---|---|---|
Granocel-500 | 38 | 2.1 |
Granocel-2000 | 33 | 2.0 |
Granocel-4000 | 32 | 1.8 |
Cotton cellulose | 71 | 2.3 |
CDA | — | 1.4 |
It is known that the crystallinity index can vary significantly depending on the measurement method used. Determining the crystallinity index with FT-IR spectroscopy provides only relative values because the FT-IR spectrum always includes contributions from both crystalline and amorphous regions.52 Therefore, crystallinity estimated from FT-IR spectral analysis differs from that estimated from X-ray diffraction analysis. The largest discrepancy in the calculated crystallinity index is observed in regenerated cellulose gels, whereas the difference is less pronounced in native cotton cellulose. In both cases, crystallinity index calculations from both FT-IR spectra and X-ray diffraction patterns show that the crystallinity of regenerated cellulose gels is significantly lower than that of cotton cellulose.
The calculated relative amount of hydrogen bonds (Table 3) indicates that after hydrolysis of acetyl groups, a dense network of hydrogen bonds is formed between the newly generated OH groups.
FT-IR spectra are useful not only for identifying which bonds or groups are prevalent in materials but also for distinguishing between different types of cellulose formation. In this case, based on the shifts of the corresponding absorption bands, we can determine that the obtained regenerated cellulose is of cellulose type II. For cellulose I, the spectral peak at 1430 cm−1 identifies the deformational vibrations of the –CH2 group, whereas for cellulose II, this peak shifts to 1420 cm−1. Additionally, the peak identifying the vibrations of the C-1 group is observed at 896 cm−1 for cellulose I and at 893 cm−1 for cellulose II (Fig. 7).
Raman spectroscopy. Raman spectroscopy is of particular importance with compared to IR and NMR spectroscopy in determining the conformation of molecules and the formation of hydrogen bonds in cellulose. Fig. 8 and 9 show the Raman spectra of regenerated cellulose gels Granocel-500, -2000, -4000 and cotton cellulose.
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Fig. 8 Raman spectra of cellulose gels and cotton cellulose: 1 – Granocel-500, 2 – Granocel-2000, 3 – Granocel-4000, 4 – cotton cellulose. |
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Fig. 9 Raman spectra at high (a) and low (b) frequency region: 1 – cotton cellulose, 2 – Granocel-4000. |
The peaks at 896 and 1096 cm−1 in the Raman spectra reflect mixed vibrations of C–C–C, C–H–O bonds, and C–O bonds in the glucose ring, respectively (Fig. 8 and 9a). In cotton cellulose, the C–C–C and C–H–O bond oscillations at 896 cm−1 are less intense compared to those in regenerated cellulose gels containing cellulose II. In cotton cellulose, the peak at 2896 cm−1 is attributed to the vibrations of the methylene group. In cellulose II, this peak shifts to 2886 cm−1, indicating a shift toward shorter wavelengths. In the fingerprint region, for cellulose II, the peak at 1462 cm−1 is attributed to the C–O–H group, while peaks at 1376 and 1262 cm−1 correspond to the in-plane and out-of-plane vibrations of the methylene group, respectively. Peaks at 1117 and 1096 cm−1 are assigned to symmetric and asymmetric C–O–C vibrations of glycosidic groups. Vibrations of C–C, C–O, and C–O–C bonds in anhydrous glucopyranose (which forms the cellulose skeleton), as well as vibrations of H–C–C, H–C–O, C–O–H, and –CH2 groups, are observed in the region beyond 600 cm−1. The assignments of these groups are not unambiguous because the vibrations of most groups are not purely distinct.55,56 In cellulose II, the system of hydrogen bonds is more complex compared to cellulose I.50 Differences in hydrogen bond formation between cellulose I and cellulose II result in corresponding shifts or changes in signal intensity in Raman spectroscopy, which help distinguish one modification from another. The signal attributed to the rotation of methylene groups is useful for distinguishing cellulose I from cellulose II: the peak at 1294 cm−1 in cellulose I shifts to 1265 cm−1 in regenerated cellulose (Fig. 9a). In the Raman spectrum of cellulose I, the peaks at 1412 and 1480 cm−1 indicate two stereochemically nonequivalent CH2OH groups rotating around the C(5) and C(6) atoms of the cellulose chain. In cellulose II, which contains only one type of CH2OH group, these two scissor vibrations of methylene groups merge into a single signal at 1462 cm−1.
Changes in modifications of the glucopyranose ring are particularly visible in the low-frequency region (Fig. 9b). In the cellulose II spectrum, the intensity of the signal at 378 cm−1 decreases, while the intensity of the signal at 350 cm−1 increases compared to the cellulose I spectrum. This change is attributed to the conformational transition of the glucopyranose ring from cellulose I to cellulose II.55
Fig. 10 illustrates representative 2D and 3D images of Granocel, obtained by micro-computed tomography, emphasizing its porous structure. The structural parameters were identified via 3D analysis, revealing an interconnected porous structure in lyophilized Granocel. The analysis indicated a mean pore diameter of 750 μm, a framework volume percentage of 25%, a porosity of 75%, a specific surface area of 15 mm−1, and an average framework thickness of 210 μm.
The obtained results suggest that Granocel gels have the potential to be used in bone tissue engineering due to their morphological characteristics, which are suitable for successful vascularization and bone tissue formation. Additionally, the technology adheres to the principles of Green Biomaterials59 by producing 3D scaffolds that are non-toxic, biocompatible, and biodegradable.
Sample | Relative deformation (%) | Bulk modulus 10−6 (N m−2) | Young's modulus (MPa) | Stiffness (m2 Pa) |
---|---|---|---|---|
Granocel-500 | 25.7 ± 0.3 | 2.8 ± 0.3 | 43 | 30![]() |
Granocel-2000 | 34.5 ± 0.5 | 1.8 ± 0.2 | 41 | 25![]() |
Granocel-4000 | 43.6 ± 0.6 | 1.3 ± 0.3 | 41 | 23![]() |
The compression test and the calculated parameters indicate that the Granocel-500 gel exhibits the highest compression resistance, with a relative deformation of approximately 26%. The parameters characterizing material strength and resistance to compression, such as the bulk modulus, Young's modulus and material stiffness, are highest for this sample. Mechanical properties of the hydrogels depend on the concentration of cellulosic material. Lower concentration results in softer gel. The softest one is Granocel-4000, obtained using CDA concentration equal to 6.5 w/v%. This corresponds to approx. 5.4% of cellulose in the gel after CDA hydrolysis.
The cellulose hydrogels, particularly Granocel-500 and Granocel-2000, exhibit exceptional flexibility. Unlike typical hydrogels, which have an elastic modulus within the 100–102 kPa range and can easily break or slump under their own weight,60 these cellulose hydrogels show significantly improved characteristics. They possess an elastic modulus approximately twice that of conventional gels, indicating enhanced structural integrity and flexibility. This flexibility is also evident when the hydrogels are in thin film form, as demonstrated in Fig. 11, which showcases their remarkable ability to maintain elasticity when bent between the fingers.
The mechanical properties of crosslinked cellulose hydrogels are influenced by network density.37,61 Single-crosslinked cellulose hydrogels generally have limited mechanical strength. To reinforce these gels, additional crosslinking is performed. Double-crosslinked hydrogels consist of cellulose connected by multiple crosslinking mechanisms, such as a combination of covalent and physical interactions, two distinct chemical crosslinking methods, or two different types of physical interactions.
Zhao et al. developed double-crosslinked cellulose hydrogels using sequential chemical and physical crosslinking and compared their mechanical properties with those of single-crosslinked hydrogels.61 Their findings showed that double-crosslinked cellulose hydrogels were mechanically superior. The compression modulus increased from 0.01 MPa for single covalently crosslinked gels to 0.61 MPa for double-crosslinked gels, while the tensile modulus increased from 0.03 MPa to 4.9 MPa.
Isobe et al. prepared cellulose hydrogels from an aqueous lithium bromide solution with tunable mechanical properties.43 Compression tests demonstrated that these cellulose hydrogels covered a wide range of mechanical properties, with the compressive Young's modulus adjustable from 30 kPa to 1.3 MPa by varying the initial cellulose concentration.
Cellulosic hydrogels can also be reinforced by combining them with other polymers through grafting or physical interpenetration. The mechanical properties of grafted or interpenetrating hydrogels strongly depend on the polymer composition. Yang et al. evaluated the mechanical properties of single and multiple-network hydrogels.62 They found that cellulose hydrogels crosslinked with epichlorohydrin had a compressive modulus of up to 107 kPa, whereas cellulose-polyacrylamide interpenetrating hydrogels reached 460 kPa. The authors also reported a significant improvement in the elastic modulus, which was determined to be 54 kPa for cellulose hydrogels and over 121 kPa for the interpenetrating material.
A comparison of cellulose-based hydrogels found in the literature (Table 5) reveals that Granocel hydrogels exhibit superior mechanical characteristics. The compression modulus of Granocel gels reaches up to 43 MPa - approximately 30 to 70 times greater than those prepared by other researchers, even using more complex methods. The elastic modulus of Granocel gels reaches up to 0.23 MPa in bending tests, whereas the elastic modulus of typical hydrogels is only up to 0.1 MPa.60 Zhao et al.'s double-crosslinked hydrogels exhibit very high elasticity, but their compression modulus remains low (Table 5).61
Hydrogel | Compression modulus, MPa | Elastic modulus, MPa |
---|---|---|
a Cellulose/polyacrylamide interpenetrating gel.62 | ||
Single crosslinked61 | 0.01 | 0.03 |
Double crosslinked61 | 0.61 | 4.9 |
Crosslinked with EClH62 | 0.107 | |
From LiBr solution43 | 1.3 | |
Cellulose/PAAa | 0.46 | 0.12 |
Granocel-500 | 43 | 0.23 |
Granocel-2000 | 41 | 0.22 |
Granocel-4000 | 41 | 0.10 |
The superior mechanical properties of Granocel gels result from a fundamentally different cellulose hydrogel preparation strategy, which is based on the reconstruction of cellulose's own inter- and intramolecular network. As is well known, native cellulose possesses a strong network of inter- and intramolecular bonds, which prevents its dissolution in water despite its hydrophilic nature (due to the abundance of hydroxyl groups). In our method, the gradual hydrolysis of acetyl groups of cellulose acetate in solution promotes the reconstruction of inter- and intramolecular hydrogen bonding networks between the newly formed hydroxyl groups in the cellulose, leading to the formation of a dense network and a rigid three-dimensional gel structure.
Structural analyses of the obtained gels confirmed that gelation is accompanied by precipitation and crystallization, characterized by the gradual arrangement of cellulose macromolecules into regular structural crystallites. Wide-angle X-ray diffraction, FT-IR, and Raman spectroscopy identified the gels as semi-crystalline materials with a cellulose II structure, and crystallite sizes of 4.3 to 4.5 nm. The gels exhibited exclusion limits for linear polymers of up to 4 × 106 Da.
The mechanical properties of the gels, such as hardness and resistance to bending and compression, classify them as rigid porous materials derived from biopolymers. They demonstrated a compressive (Young's) modulus of up to 43 MPa and an elastic modulus of up to 0.23 MPa in bending tests.
Thanks to their enhanced mechanical properties and high porosity, crushed gels show promise for applications in column technologies for protein chromatography and in heterogeneous biocatalysis processes with immobilized enzymes. Additionally in film form, the elasticity of the gels makes them suitable for biomedical applications such as wound dressings or artificial skin. Moreover, the cellulose gels are excellent foundational materials for bone tissue engineering, as they can create porous structures after lyophilization that support vascularization and bone tissue ingrowth.
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