Ultrastrong and tough paper structure from densified hybrids of multiscale cellulose fibers

Domain: biology
Published: 2026-03-13
Editorial outcome: major_revision

Abstract
Replacing petrochemical plastics with sustainable materials requires strategies that combine high mechanical performance with low environmental impact. Here, we report an ultrastrong and tough cellulose paper created by integrating multiscale cellulose materials to overcome the weak fiber-to-fiber adhesion found in conventional paper. Bacterial cellulose microgels fill microscale voids between pulp fibers, while bacterial cellulose nanofibers reinforce nanoscale gaps, forming densely bonded interfaces through capillary-driven assembly during dehydration. This hybrid structure significantly enhances interfacial interactions and hydrogen bonding, producing cellulose paper with isotropic tensile strength of 811 MPa and robust wet-state performance. The process is highly energy-efficient due to rapid dewatering, consuming far less energy than nanocellulose-paper manufacturing. The resulting material also exhibits strong adhesion to hydrophilic substrates, expanding its application potential. By bridging the performance gap between natural cellulose fibers and traditional paper, this work provides a biodegradable, energy-efficient alternative to plastics for sustainable structural and packaging applications.

Introduction
Driven by increased concerns about the long-term degradation of petrochemical plastics and the uncontrolled spread of microplastics, various paper-based packaging solutions with significantly improved mechanical performances have been developed as sustainable alternatives to non-degradable plastics.1, 2, 3, 4 It is important to note that individual wood pulp fibers, the fundamental elements for paper, possess remarkable yet often overlooked mechanical strength. Single wood fibers with low cellulose microfibril angle (MFA) have ultimate strength as high as 1000 MPa,5 while the ultimate tensile strength of a single wood pulp fiber with low MFA can reach 1700 MPa.6 However, the tensile strength of paper fabricated from pulp fibers only reached 50 MPa, due to weak interfacial adhesion between the fibers.7, 8, 9 To overcome this limitation, various strengthening agents have been introduced to enhance the interfacial interactions between the pulp fibers through chemical bonding such as hydrogen bonds,10 electrostatic interactions,11 or covalent bonds.12, 13 Another approach was to increase the interfacial bonding area through the introduction of cellulose microfibrils (CMF) or cellulose nanofibers (CNF).14, 15 Although these approaches could improve the strength of paper or board to some extent, a substantial gap remains when compared to the mechanical properties of individual pulp fibers.

Benefit from parallel packed load bearing wood cells and naturally aligned cellulose microfibrils in wood cell walls, a two-step top-down process has been previously developed to fabricate strong wood cellulose films showing tensile strength of 449 MPa in the longitudinal direction. This process involves in-situ removal of lignin and hemicellulose, followed by a complete densification, facilitating the formation of a substantial interfacial area and ultimately stronger adhesion between naturally aligned wood cellulose fibers.16, 17 However, this strategy is not suitable for individualized and random-oriented wood pulp fibers used in paper structures, as they are typically micron-scale in width (10 to 50 µm) and millimeter-scale in length (0.7 to 3 mm), and the voids within their network exhibit microscale dimensions. Nano-scale CMF and CNF materials could strengthen the joints between wood pulp fibers but could not adequately fill the microscale voids between them. The hydrogen bonding, van der Waals force, and electrostatic interactions are short-range forces that only operate effectively when the spacing of material interfaces is below 300 Å.18 The capillary forces, which are generated during the drying process in papermaking, are crucial for drawing the adjacent cellulosic fibers into close contact.19, 20 To achieve optimal mechanical properties, the microscale voids between the fibers must be filled to achieve a substantial densified interfacial surface and ensure effective interfacial interactions.21, 22

In this work, we utilized square-shaped bacterial cellulose microgels to fill the voids between wood pulp fibers. The 3D gel structure effectively interlocked with surrounding particles during the dehydration process, primarily driven by capillary forces facilitated by its porous architecture.23 To enhance this effect, cellulose nanofibers were incorporated, generating stronger capillary forces due to the nanoscale distances between them, as predicted by the Young-Laplace equation.24 These nanoscale cellulose fibers further filled the nano-voids in the paper structure. The combined use of micro- and nanoscale cellulose fibers effectively filled the gaps between wood pulp fibers, forming a tightly bonded interface driven by capillary forces. This configuration promoted the formation of additional cellulose-cellulose and cellulose-bound water hydrogen bonds, ultimately resulting in paper structure of superior mechanical properties at both dry and wet states. Furthermore, the adhesive performance of the hybrid of multiscale cellulose fibers for substrates of hydrophilic nature was demonstrated.

Results
Hybrid of cellulose microfibers and nanofibers on paper strength

Hybridization of fibers with different sizes such as carbon fibers, nanofibers, and nanotubes has been widely applied in fiber reinforced cementitious and polymer composites to achieve higher strength, modulus, and fracture toughness.34, 35, 36, 37 Fibers of smaller dimensions effectively bridge micro-cracks, whereas larger fibers are designed to inhibit the propagation of macro-cracks within the composite material. Herein, the preparation, structure and properties of the hybrid paper structures of cellulose fibers, microfibers, and nanofibers are explored. As illustrated in Fig. 1a, millimeter long spruce wood pulp fibers (Fig. 1b), microscale cellulose microgel (average size of 72.9 μm) obtained from bacterial cellulose (Fig. 1c and Supplementary Fig. 1), and TEMPO-oxidized bacterial cellulose nanofibers (B-CNF, Fig. 1d) were mixed in various ratios to prepare the hybrid cellulose papers (HCPs) using a standard papermaking procedure. The impact of the compositions of pulp fibers, microgel, and B-CNF on the mechanical properties of the resulting HCPs were studied. Notably, the HCP composed of pulp fibers (65° SR), microgel, and B-CNF in a 1:1:1 ratio exhibited high mechanical properties, particularly a synergistic enhancement in modulus, tensile strength, and strain-to-failure compared to the neat fibers (Fig. 1e). The HCP showed an exceptionally high ultimate tensile strength of 811 MPa and a high modulus of 36.1 GPa, surpassing all random-in-the-plane oriented (isotropic) cellulose papers or films reported to date (Fig. 1f). These values, particularly a specific tensile strength of 7.37 ×10-1 MPa m3/kg, a specific Young’s modulus of 3.28 ×10-2 GPa m3/kg, a toughness of 56.5 MJ/m3 are comparable and even partially higher than those of strong cellulose films with cellulose microfibrils or wood fibers aligned-in-the-plane,17 super aligned bacterial cellulose (BC) films,26 and of anisotropic nanofibers-structured regenerated cellulose films25 (Supplementary Table 1). Interestingly, the HCP showed a strain hardening behavior during tensile testing (insert picture in Fig. 1e and Supplementary Movie 1), similar to the plastic deformation of elastomeric polymers. Notably, upon unloading after the onset of strain hardening but prior to fracture, the sample retained its deformed (homogeneous necking) shape, confirming the presence of plastic deformation (Supplementary Fig. 2). This phenomenon was previously reported in cellulose nanopaper prepared from microfibrillated cellulose (MFC), which showed a yield stress of 91 MPa and a slope of 1.82 GPa in the plastic region. In comparison, the HCP sample demonstrated remarkably enhanced mechanical performance, with a yield stress of 305 MPa, a plastic deformation slope of 4.76 GPa, and a high strain-to-failure of 11.4%. These values suggest significantly increased frictional resistance to fiber slippage within the network and superior toughness resulting from the hybridization of multiscale cellulose fibers. To elucidate the failure mechanisms underlying this behavior, the fracture surface of the HCP was examined using scanning electron microscopy (SEM, Supplementary Fig. 3). Two dominant fracture modes were identified: fiber pull-out and fiber rupture. The presence of ruptured fibers confirms that load transfer through the network reached a threshold sufficient to cause fiber breakage, enabling greater energy absorption compared to pull-out alone. This mixed-mode fracture behavior is a key contributor to the enhanced mechanical properties observed in the HCP.

We thoroughly investigated the mechanical properties of papers prepared from ternary hybrids of pulp fibers, microgel, and B-CNF at different ratios (Fig. 1g). The results revealed that small adjustments in composition (e.g. 1.2:1:1 or 0.8:1:1) could not lead to further improvements compared to the HCP at 1:1:1. The hybrid papers with higher content of pulp fibers (2:1:1) or microgel (1:2:1) showed higher strain-to-failure but significantly lower modulus and tensile strength. The HCP with a composition of 1:1:1 remarkably outperformed paper with other compositions in terms of Young’s modulus, tensile strength, and toughness (Supplementary Table 2). In addition, the beating degree of the pulp fibers has a strong impact on the mechanical properties of HCPs, which initially improved and then declined with increasing beating degree of the pulp fibers (Fig. 1h), as determined by the Schopper-Riegler (°SR) scale. HCPs fabricated with pulp fibers at beating degrees of 45 °SR, 65 °SR, and 75 °SR exhibited tensile strengths of 586 MPa, 811 MPa, and 538 MPa, respectively. This trend mirrors the effect of beating degree on mechanical properties of traditional paper,38 highlighting the dominant role of pulp fiber condition in determining the mechanical performance of HCPs. The initial increase in beating degree promoted fiber fibrillation, enhancing interfacial interactions between the pulp fibers and improving mechanical strength. However, excessive beating caused excessive fiber shortening and fines generation, ultimately reducing mechanical performance.

The respective roles of microgel and B-CNF in enhancing paper strength were evaluated using composite papers prepared from binary combinations of B-CNF and pulp fiber, microgel and pulp fiber, and microgel and B-CNF (Supplementary Fig. 4). When combined with pulp fibers, B-CNF led to a modest increase in tensile strength and stiffness, attributable to its nanofibrillar structure and reinforcing effect. In contrast, the incorporation of microgel into B-CNF networks resulted in increased strain-to-failure, albeit at the expense of strength and stiffness. Notably, when microgel was combined with pulp fibers, it enhanced interfacial adhesion and contributed to a synergistic improvement in both tensile strength and toughness.

The influence of microgel average particle size on the mechanical properties of the ternary hybrids was further investigated. A reduction in particle size from 72.9 to 52.1 and 13.5 µm did not alter the overall shape of the stress-strain curve, but it led to a decrease in both strain-to-failure and ultimate tensile strength (Supplementary Fig. 5). This trend is analogous to the mechanical performance deterioration observed with decreasing average molar mass of cellulose.29 Furthermore, mechanically homogenized MFC, derived from bleached softwood kraft pulp and characterized by widths of 10–20 nm and lengths of 5–10 µm, was used in place of B-CNF, which has a carboxylate content (1.18 mmol/g) and dimensions of 5–12.5 nm in width and 1–2.3 μm in length. Although the microgel/MFC combination showed moderate synergistic effect compared to individual components (Supplementary Fig. 6), the substitution resulted in HCP samples with lower yield stress (100 MPa) and ultimate tensile strength (210 MPa). These findings emphasize the critical role and synergistic interaction between thin nanoscale cellulose fibrils (B-CNF) and microscale cellulose microgel particles in governing the mechanical performance of the HCP.

Structure of hybrid cellulose paper

The structure of the HCP with remarkable mechanical properties was thoroughly investigated. The drying process of the hybrid multiscale fibers is illustrated in Fig. 2a and characterized by rheological characterizations and SEM analysis. The pulp fibers, microgel, and B-CNFs initially assembled into a gel-like dispersion (Supplementary Fig. 7 and 8). After vacuum filtration and drying at 105 ºC, the wet hybrid fiber web formed a densified paper structure with a bulk density of 1.10 g/cm³ and a porosity of 27% (Supplementary Table 3). The cross-sectional SEM images of the papers fabricated from the neat pulp fibers (Fig. 2b), a 1:1 mixture of the pulp fibers and B-CNF (Fig. 2c), and a 1:1 mixture of the pulp fibers and microgel (Fig. 2d) showed a more porous, typical multilayered paper structure with distinct micrometer scale gaps between layers and voids of between pulp fibers. In contrast, the cross-sectional SEM images of the HCP (Fig. 2e) revealed a compact, continuous, multilayered structure formed by the hybridization of the pulp fibers, microgel, and B-CNF, with the gaps and voids filled by the microgel and B-CNF. As shown in Supplementary Table 3, the HCP paper exhibited porosity comparable to that of neat B-CNF and microgel papers yet demonstrated markedly superior mechanical performance (Fig. 1e). This enhancement suggests strong interfacial adhesion and improved interfacial toughness arising from the synergistic hybridization of pulp fibers with B-CNF and microgel.

The crystalline structures of the HCP and papers prepared from neat pulp fiber, microgel, and B-CNF were characterized using Wide-angle X-ray diffraction (WAXS). When the X-ray beam was perpendicular to the paper surface as the X-ray diffractograms shown in Supplementary Fig. 9, all papers showed typical ring patterns of cellulose I crystals in the paper surface plane, indicating random-in-the-plane distribution of cellulose fibrils in all papers. The structure in the cross-sectional plane was revealed with the X-ray beam parallel to the paper surface (Fig. 2f). In contrast, all papers showed a pair of bright arcs corresponding to the (11̅0), (110), and (200) diffractions of cellulose I crystal, indicating cellulose fibrils were aligned parallel to the paper surface as a result of vacuum filtration in the papermaking process. The cellulose crystal orientation was quantified using the (200) reflection to evaluate the degree of parallel packing of the multiscale cellulose fibers. The HCP sample had an orientation factor (fc) value of 0.83 higher than those of papers from neat pulp fibers (fc of 0.54), microgel (fc of 0.57), and B-CNF(fc of 0.63), suggesting improved dense parallel packing of cellulosic fibers as also observed by SEM analysis (Fig. 2e). The crystallinity indices of the pulp fiber, microgel, B-CNF, and the HCP were measured to be 55.9%, 65.5%, 67.6%, and 63.5%, respectively (Supplementary Fig. 10). These results indicate that the hybridization of multiscale cellulose fibers did not significantly alter the overall crystallinity of the composite.

Influence of bound water on mechanical performance of the hybrid fibers

Capillary force is a long-range force that can draw multiscale cellulose fibers together over larger distances, while hydrogen bonding becomes effective at distances below 300 Å. It can be inferred that when the distance between the surfaces of cellulose fibers is greater than 300 Å, capillary force plays a dominant role, pulling the fibers closer. Once the distance is less than 300 Å, hydrogen bonding, combined with capillary force, further pulls the fibers together, forming more hydrogen bonds and resulting in a dense structure. As referenced, there are two types of hydrogen bonds in cellulose materials: those between cellulose molecules and those between bound water and cellulose. To determine which type of hydrogen bonding primarily influences mechanical properties, the HCP was completely dried under vacuum at 110 °C for approximately 2 to 3 hours to remove bound water. The NIR spectra (Fig. 3a) revealed a significant difference between HCP subjected to complete drying and natural drying, marked by the absence of the peak at 5178 cm⁻¹, which corresponds to bound water.39 To determine the hydration state of the bound water in the HCP sample, low field nuclear magnetic resonance (LF-NMR) measurements were performed at room temperature (Fig. 3b). The natural dried HCP sample had a moisture content of 5.8% under 50% relative humidity (RH) and showed only a strong peak at T2 < 1 ms, corresponding to tightly bound water that was mostly located in the first hydration layer, where water binds to the hydroxy groups of cellulose. When the HCP samples were conditioned under 70% and 90% RH, their moisture contents increased to 10.8% and 18.5%, respectively. Correspondingly, the T2 relaxation times of the bound water shifted to values above 1 ms and exhibited broader distributions, indicating a greater contribution from more loosely bound water under high humidity conditions. Additionally, small relaxation peaks appeared at 70 ms and 500 ms, which suggests the presence of a minor fraction of free or bulk-like water at high RH levels. After complete drying, the HCP sample had a moisture content of 7.4% under 50% RH and showed a significantly decreased peak for tightly bound water (T2 < 1 ms) and 3 additional peaks with T2 time of 5 ms, 40 ms, and 500 ms, respectively. These peaks correspond to loosely bound water that is located not far away from hydroxy groups, as well as free water confined in the pores due to capillary condensation and bulk-like water resulting from surface condensation, respectively. These results indicate the irreversible loss of tightly bound water in HCP after completely drying. The mechanical properties of HCP declined drastically after complete drying, with tensile strength decreasing from 811 MPa to 167 MPa (Fig. 3c). Furthermore, when the RH was increased to 70% and 90%, the ultimate tensile strength of the natural dried HCP decreased significantly while the strain-to-failure increased (Fig. 3d), consistent with the higher moisture content and the greater proportion of loosely bound water (Fig. 3b). Notably, the tensile strength of the natural dried HCP was largely restored after reconditioning the samples back to 50% RH (Fig. 3d), indicating that the humidity-induced changes were reversible. These observations indicate that tightly bound water, which is hydrogen-bonded to the cellulose chains,40, 41 plays a key structural role in HCP. This tightly bound water contributes to the dense packing and load transfer within the network of multiscale cellulose fibers and is therefore a primary factor underlying the material’s ultrahigh strength and toughness.

On the other hand, we introduced free water into the HCP and papers prepared from neat individual components, as well as binary and ternary hybrids with different compositions. The coefficient of water absorption for the neat pulp fibers, microgel, and their hybrids are at least 5-fold lower than that of neat B-CNF and hybrids containing B-CNF (Supplementary Fig. 11). This is attributed to the carboxyl groups generated by TEMPO-mediated oxidation in B-CNF, which ionize in water, leading to electrostatic interactions that disassemble the dense stacking of cellulose materials. Correspondingly, the mechanical properties of the HCP and the paper prepared from neat B-CNFs and pulp fibers significantly decreased after immersed in water for 24 hours (Fig. 3e). The wet tensile strength of the HCP was 1.3 MPa, while the cellulose microgel exhibited higher wet strength of 62 MPa due to lower water absorption, suggesting the interlocking structure of the cellulose microgel is robust enough to resist disassembly and water infiltration.

The high energy consumption typically associated with vacuum filtration during the fabrication of nano-sized or micro-sized cellulose materials has often limited their practical applications. The energy consumptions for the fabrication of HCP and papers from the three neat component cellulose fibers were evaluated. The vacuum filtration time for the HCP was 1.417 h, significantly lower than the 7.79 h required for B-CNF, highlighting its great potential for practical applications (Supplementary Fig. 12). Moreover, the HCP exhibited superior mechanical properties to cellulose films prepared from TEMPO-CNF,31, 32 Holo-CNF,31 and Enz-CNF,29, 42 while also achieving a shorter filtration time and better energy savings, as shown in Fig. 3f and Supplementary Table 4.

Adhesive performance of the hybrid cellulose fibers

To further understand the underlying mechanism responsible for the formation of dense structure from the pulp fiber (65° SR), microgel, and B-CNF, the adhesive properties of the hybrid multiscale cellulose fibers were investigated. We hypothesize that the driving forces for formation of the wet paper web and the natural drying process are primarily attributed to capillary forces and hydrogen bonding between bound water and cellulose fibers. Both driving forces are related to interfacial interactions, which effectively pull the multiscale cellulose fibers together. We demonstrated the difference of adhesion mechanism between B-CNF and microgel using glass slides as the substrate ss shown in Fig. 4a. The B-CNF formed scattered cellulose nanofibrils aggregates between the glass substrates due to fractures occurred in the gelled B-CNF suspension upon drying similar to cellulose nanocrystals,43 while cellulose microgel formed a homogeneous nanofibril network without fractures owing to the high internal cohesion in the cellulose microgel upon drying. We further investigated the interfacial adhesion of dispersions of the pulp fiber (65° SR), microgel, and B-CNF, along with their binary mixtures and ternary mixture, on glass (Supplementary Fig. 13) and paper (Supplementary Fig. 14) substrates. Remarkably, the cellulose hybrid containing pulp fibers (65° SR), microgel, and B-CNF in a 1:1:1 ratio exhibited strong adhesion with a lap-shear strength of 6.6 ± 0.5 MPa and 27.0 ± 1.1 MPa for the glass and paper substrates, respectively, significantly higher than those of the individual cellulose fiber components (Fig. 4b and 4c). The lap-shear strength of the hybrid cellulose fibers adhered to the glass sheet also surpasses that of other polysaccharide-based materials (Fig. 4d), including chitin-amyloid,44 cellulose nanocrystal,43 dihydroxypropyl cellulose,45 DOPA-cellulose,46 and dialdehyde cellulose.47 Furthermore, the adhesive behavior of the hybrid cellulose fibers for paper sheets closely resembled its behavior in tensile tests (Fig. 1e), exhibiting synergistic higher modulus, strength, and strain compared to its individual components (Fig. 4c). These findings confirm that the enhanced toughness of the HCP primarily arises from the interfacial interactions among the three cellulose fibers of different sizes. The resulting densely packed microstructure not only governs the mechanical response but also imparts high gas-barrier performance with an oxygen transmission rate of 110.2 cm3/m2dayPa (Fig. 4e).

Methods
Materials
Spruce wood pulp fibers were obtained from Jiulong pulp & paper Co. Ltd. (China). Sodium chlorite (NaClO2, 80%), sodium hypochlorite (NaClO), sodium phosphate, and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) were purchased from Sigma-Aldrich (Germany). Bacterial cellulose (BC) cubes (1 × 1 × 1 cm3) were sourced from Yeguo Co. Ltd. (China). The BC cubes were homogenized to fabricate the microgel using a high-speed blender (HR3865, Philips, China) at 42000 rpm for 20 minutes. The mean particle size of the obtained microgel was 72.9 μm. To reduce the particle size, the microgel dispersion was further homogenized for 5 and 20 min using a colloid grinder (MX-JTM-G25, mixpower, China). The microgel suspension had a solid concentration in the range of 0.40–0.65 wt.%.

Preparation of B-CNF
BC cubes (1 g) were suspended in 100 mL water containing TEMPO (0.016 g, 0.1 mmol) and sodium bromide (0.1 g, 1 mmol). The TEMPO-mediated oxidation was initiated by adding the desired amount of the NaClO solution (equivalent to 10 mmol NaClO per gram of cellulose) and was carried out at room temperature with continuous stirring at 500 rpm. The pH of the reaction was maintained at 10 by adding 0.5 M NaOH solution. After NaClO was consumed, the TEMPO-oxidized BC fibers were washed thoroughly and disintegrated using a high-speed blender (HR3865, Philips, China) at 42000 rpm for 10 minutes to obtain B-CNF. The resulting B-CNF water suspension had a solid concentration in the range of 0.2–0.3 wt.%.

Preparation of hybrid cellulose paper
Multiscale cellulose fibers including spruce wood pulp fibers, microgel, and B-CNF were combined in various ratios and mixed at 500 rpm for 10 minutes. The pulp fiber suspension in water had a solid concentration of 10 wt.%. After mixing pulp fiber, microgel, and B-CNF, the final hybrid suspension was adjusted to a solid concentration of 0.17 wt.%. The hybrid cellulose papers were then prepared using a standard procedure, which involved vacuum filtration followed by drying in a sheet former (Model IMT-GZ01) at 105 °C for 60 min.

Lap-shear adhesion
Various dispersions of multiscale cellulose fibers hybrid were applied to glass or paper substrates (dimensions: length × width = 12 cm × 2.5 cm) with a coating area of 2.5 cm x 2.5 cm and a constant solid content of 10 mg. The dispersions were applied at a solid concentration of approximately 1.5–2.0 wt.%, achieved by concentrating the hybrid suspension by rotary evaporation. The substrates were then transferred to a sheet former and dried at 105 °C for 60 minutes. The paper substrate was prepared from the spruce wood pulp fibers (65°SR) following the same procedure as the hybrid cellulose paper with a thickness of 0.1 mm. The thickness of the glass slides was 4 mm.

Characterizations
The particle size distribution of the cellulose microgel dispersions was measured by a laser diffraction particle size analyzer (S3500, Microtrac, USA). The tensile mechanical properties of the samples were measured using an Instron 5944 tensile tester equipped with a 500 N load cell. The lap-shear adhesion tests were performed using a universal testing machine (UTM-2502) at a tensile speed of 2 mm/min. The sample morphology was characterized using scanning electron microscopy (SEM) with a GeminiSEM 460 instrument operating at an accelerating voltage of 5.0 kV. Wide-angle X-ray scattering (WAXS) data were collected using a Xeuss 2.0 instrument equipped with a curved detector (Xenocs Corp.) and CuKα radiation (λ = 0.1541 nm) at an operating voltage of 40 kV and a current of 30 mA. Two-dimensional diffraction patterns were recorded by mounting the samples either perpendicular or parallel to the incident beam. Near-infrared (NIR) spectra of the HCP films were collected using a FT-NIR spectrometer (MPA Ⅱ, Bruker, Germany) equipped with an InGaAs detector and a diffuse reflectance integrating sphere (60 mm diameter) at 25 oC and 50 % RH. Spectra were acquired in the wavelength range of 8000–4000 cm⁻¹ with 1024 scans at a spectral resolution of 8 cm⁻¹. All spectra were normalized using the min–max normalization method, implemented in the OPUS software. Gas barrier properties of the samples were evaluated according to the GB/T 1038.1-2022 standard method using a Basic201 differential pressure gas permeameter. Circular samples (diameter: 9 cm) were cut using a mold and placed in the test cell and then degassed under vacuum prior to testing.

Nuclear magnetic resonance (NMR) relaxation measurements were conducted using a low-field pulsed NMR imager and analyzer (NMI20, Niumag Corporation, Shanghai, China) operating at a resonance frequency of 18 MHz. One gram of pretreated cellulose membrane powder was placed in a glass tube (15 mm inner diameter) and inserted into the NMR probe, which was maintained at a constant temperature of 32 °C. The transverse spin-spin relaxation time (T2) was measured using the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence. The 90° and 180° pulse durations were 14 μs and 27 μs, respectively, with an inter-pulse delay of 200 μs. A total of 18,000 echoes (data points) were collected per measurement. To improve the signal-to-noise ratio, eight scans were averaged using a repetition time of 4 s. All measurements were performed in triplicate. The relaxation data were analyzed using MultiExp Inv Analysis software, which employs a modified simultaneous iterative reconstruction technique (SIRT) algorithm for accurate relaxation time distribution under low signal-to-noise conditions.48

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, Liqiong Liao et. al. reported an innovative approach to enhancing the mechanical properties of cellulose-based materials by hybridizing multiscale cellulose fibers. The study effectively addresses a long-standing limitation in paper-based materials, weak interfacial adhesion between fibers, by utilizing cellulose microgels and nanofibers to reinforce both microscale and nanoscale voids. The unprecedented tensile strength (811 MPa) represents the highest reported value for isotropic cellulose films to date. The innovative hybridization strategy, superior mechanical performance, energy efficiency and practical implications, adhesive performance and excellent wet strength position this material as a strong contender for replacing petrochemical-based plastics in structural and packaging applications. This is a novel and timely contribution to sustainable materials research. However, before publication in Nature Communications, the following improvements are recommended:\n1. The “necking” behavior in Figure 1E suggests significant plastic deformation, which is unusual for cellulose-based materials. More discussion on whether this behavior results from fiber alignment, hydrogen bonding dynamics, or interfacial slippage would be valuable.\n2. A fracture mechanics analysis (e.g., crack propagation studies, work of fracture measurements) could provide deeper insight into the material’s failure mechanisms.\n3. Since bound water is a key factor in maintaining high strength, how does prolonged exposure to humid environments affect performance?\n4. Previous studies focused on CNF-pulp composites. What distinguishes cellulose microgel’s role in enhancing HCP mechanics?\n5. For mechanical properties, error bars or standard deviation bands should be included to show variability across samples.\n6. Request for higher-resolution images of Figures 2B, 2C, and 2D to clarify the differences between the neat pulp fiber structure and the hybridized paper.\n7. Why does HCP exhibit low wet strength?\n8. Clarify theoretical water absorption calculation in Figure S8.\n9. The study finds that a 1:1:1 ratio of pulp fibers, microgel, and B-CNF gives the best mechanical properties, but it is unclear if this is an absolute optimum. Could small adjustments in composition (e.g., 1.2:1:1 or 0.8:1:1) lead to further improvements?", "reviewer_2": "The manuscript by Zhou and coworkers reports on how a strong paper can be made from cellulose by mixing cellulose fibers/fibrils of different dimensions. The study builds on a large volume of work on nanopaper and various strengthening additives to paper from cellulose pulp. The paper in the current study is produced by vacuum filtration of aqueous mixtures of pulp fibres with TEMPO oxidized cellulose nanofibers and cellulose microgels. The produced papers/films displayed a high tensile strength and strain to fracture and also a good adhesion strength. The mechanical measurements are supported by SEM and WAXS to characterize the paper structure and fiber alignment. There is essentially no data supporting the discussion on the bonding mechanism and the investigated parameter space is limited.\nSpecific points to address:\n1. Are the reported comparisons with literature data on the tensile strength and Youngs modulus comparable, i.e. are the temperature and the relative humidity the same in all cases? Also, how does the density of the different materials compare? The density of the new paper presented in the manuscript needs to be determined and reported.\n2. The bonding mechanisms should be investigated in detail, using e.g. spectroscopy (ssNMR, neutron spectroscopy) and MD simulations.\n3. The parameter space of the films/paper needs to be expanded to better understand the critical parameters. Pulp fibers and cellulose nanofibers of different dimensions and surface chemistry should be investigated in different combinations and compositions.\n4. The authors argue that the presented material could be a biodegradable alternative to plastics. What plastic should this material replace? The biodegradability of the paper needs to be determined.\n5. The amount of bound water and its contribution to the interfibrillar bonding needs to be assessed and discussed.\n6. Figure 4 is mixed up.", "reviewer_3": "The study shows a concept by using hybrid components (BC or other H-bond mediating components and fibers for cellulose-based composites) to enhance the mechanical properties which is widely used and well known. Quite often, the mechanism for promoting the mechanical properties of final products is unclear because of the use of many different components, which is exactly the case in this study. E.g. the distribution of these different components to excert their contribution to mechanical properties is already unclear. As well, more data about microgel for clear characterization is required. Moreover, to realize the adhesive properties between the hydrophilic substrates via H bonds should be common.\n1. The author repeatedly mentioned in the Abstract that the material preparation is energy-efficient. Authors should quantitatively compare the energy inputs of your paper with those in other studies to support the claim of energy efficiency.\n2. In Figure 1E, what do the 1, 2, 3 and three curves mean? Also, the necking indicates the decrease in local cross-sectional area, and authors can better display this decrease in the insert photograph.\n3. More advanced data should be provided to support the results shown in Figure 2A. Also In Figure 3A, the explanation for the super tensile strength of cellulose paper is somewhat unclear and unsatisfactory. The explanation from the perspective of free water and hydrogen bonding of cellulose also lacks data support.\n4. In Figure 2, it seems that the thickness of different samples varies greatly. Did the authors use the same method to prepare the samples (especially for the neat fibers)? Or can the authors give some discussions on the thickness results as well as related to mechanical properties?\n5. In Figure 3C, the strength and other specific properties of the paper sheet are also important for this test, authors should give more information about this substrate.\n6. The size of microgels and nanofibers, as well as the types and quantities of their surface functional groups, have a significant impact on capillary forces and hydrogen bonding. Such interfacial interactions represent one of the core innovative aspects of this work. I suggest the authors to provide a more detailed discussion on this topic.\n7. Figure 4A: Did the author perform normalization when using NIR to compare hydrogen bond content? How is it done?\n8. There are many errors in the text that need to be corrected. For example: Line 153, Figure caption of Figure 4."}, "author_response": "We thank the reviewers for their valuable time, insightful comments and suggestions which helped us to improve our manuscript. We have made all possible efforts to address the comments and revised the manuscript accordingly. The changes made in the manuscript are highlighted in red. A point-by-point response to the comments by the reviewers is provided below.\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nIn this manuscript, Liqiong Liao et. al. reported an innovative approach to enhancing the mechanical properties of cellulose-based materials by hybridizing multiscale cellulose fibers. The study effectively addresses a long-standing limitation in paper-based materials, weak interfacial adhesion between fibers, by utilizing cellulose microgels and nanofibers to reinforce both microscale and nanoscale voids. The unprecedented tensile strength (811 MPa) represents the highest reported value for isotropic cellulose films to date. The innovative hybridization strategy, superior mechanical performance, energy efficiency and practical implications, adhesive performance and excellent wet strength position this material as a strong contender for replacing petrochemical-based plastics in structural and packaging applications. This is a novel and timely contribution to sustainable materials research. However, before publication in Nature Communications, the following improvements are recommended:\nResponse: We appreciate the encouraging comments provided by the reviewer and opinion about innovativeness and quality. We have carefully considered each comment and have revised our manuscript, doing our maximum to comprehensively address all the points raised.\n1. The “necking” behavior in Figure 1E suggests significant plastic deformation, which is unusual for cellulose-based materials. More discussion on whether this behavior results from fiber alignment, hydrogen bonding dynamics, or interfacial slippage would be valuable.\nResponse: Thanks for pointing this behavior out. Indeed, upon unloading after the onset of strain hardening but prior to fracture, the sample retained its deformed (homogeneous necking) shape, confirming the presence of plastic deformation (Supplementary Fig. 2). We have corrected this behavior as “strain hardening” in Fig. 1e. We have updated the insert pictures in Fig. 1e and provided a movie recording tensile test of an HCP sample. We have revised the discussion as below.\nInterestingly, the HCP showed a strain hardening behavior during tensile testing (insert picture in Fig. 1e and Supplementary Movie 1), similar to the plastic deformation of elastomeric polymers. Notably, upon unloading after the onset of strain hardening but prior to fracture, the sample retained its deformed (homogeneous necking) shape, confirming the presence of plastic deformation (Supplementary Fig. 2). This phenomenon was previously reported in cellulose nanopaper prepared from microfibrillated cellulose (MFC), which showed a yield stress of 91 MPa and a slope of 1.82 GPa in the plastic region. In comparison, the HCP sample demonstrated remarkably enhanced mechanical performance, with a yield stress of 305 MPa, a plastic deformation slope of 4.76 GPa, and a high strain-to-failure of 11.4%. These values suggest significantly increased frictional resistance to fiber slippage within the network and superior toughness resulting from the hybridization of multiscale cellulose fibers.\nFig. 1e Tensile stress–strain curves of the HCP prepared from the pulp fiber, microgel, and B-CNF with a composite of 1:1:1 by weight as compared to the papers prepared from the individual components; the inset photograph shows strain hardening of the HCP under tensile loading.\nSupplementary Fig. 2 Photograph of the HCP sample when the stress was removed after strain hardening and before breakage, showing the plastic deformation remained.\n2. A fracture mechanics analysis (e.g., crack propagation studies, work of fracture measurements) could provide deeper insight into the material’s failure mechanisms.\nResponse: We have performed the morphological analysis of tensile fractured surface of HCP using SEM and included the discussion of fracture mechanics in the revised manuscript as below.\nTo elucidate the failure mechanisms underlying this behavior, the fracture surface of the HCP was examined using scanning electron microscopy (SEM; Supplementary Fig. 3). Two dominant fracture modes were identified: fiber pull-out and fiber rupture. The presence of ruptured fibers confirms that load transfer through the network reached a threshold sufficient to cause fiber breakage, enabling greater energy absorption compared to pull-out alone. This mixed-mode fracture behavior is a key contributor to the enhanced mechanical properties observed in the HCP.\nSupplementary Fig. 3 SEM images of the tensile fracture surface of the HCP sample.\n3. Since bound water is a key factor in maintaining high strength, how does prolonged exposure to humid environments affect performance?\nResponse: We have performed tensile test at higher relative humidity and added the result and discussion in the revised manuscript as below.\nIn addition, as RH increased to 70% and 90%, the ultimate tensile strength of the HCP decreased significantly while the strain-to-failure increased (Fig. 3d), due to higher moisture content and increased loosely bound water. These observations confirm that hydrogen bonds between cellulose and tightly bound water play a more critical role in the formation of HCP and are primarily responsible for its ultrahigh strength and toughness.\nFig. 3d Typical stressstrain curves of the HCP at different relative humidity (RH) levels.\n4. Previous studies focused on CNF-pulp composites. What distinguishes cellulose microgel’s role in enhancing HCP mechanics?\nResponse: We have revised our manuscript to clarify the roles of B-CNF and microgel as below.\nThe respective roles of microgel and B-CNF in enhancing paper strength were evaluated using composite papers prepared from binary combinations of B-CNF and pulp fiber, microgel and pulp fiber, and microgel and B-CNF (Supplementary Fig. 4). When combined with pulp fibers, B-CNF led to a modest increase in tensile strength and stiffness, attributable to its nanofibrillar structure and reinforcing effect. In contrast, the incorporation of microgel into B-CNF networks resulted in increased strain-to-failure, albeit at the expense of strength and stiffness. Notably, when microgel was combined with pulp fibers, it enhanced interfacial adhesion and contributed to a synergistic improvement in both tensile strength and toughness.\nSupplementary Fig. 4 Stress-strain curves of the hybrid papers prepared from binary hybrids of (a) B-CNF and pulp fiber, (b) microgel and pulp fiber, and (c) microgel and B-CNF.\nFurthermore, we have also evaluated the impact of microgel average particle size on the mechanical properties of the HCP and integrated the results in the revised manuscript as below.\nThe influence of microgel average particle size on the mechanical properties of the HCP was investigated. A reduction in particle size from 72.9 to 52.1 and 13.5 µm did not alter the overall shape of the stress-strain curve, but it led to a decrease in both strain-to-failure and ultimate tensile strength (Supplementary Fig. 5). This trend is analogous to the mechanical performance deterioration observed with decreasing average molar mass of cellulose.29\nSupplementary Fig. 5 a Size distribution of the microgels measured by Mastersizer. b Effect of the microgel size on the mechanical properties of the HCP.\n5. For mechanical properties, error bars or standard deviation bands should be included to show variability across samples.\nResponse: We have summarized the mechanical properties data with standard deviations in Supplementary Table 2.\nSupplementary Table 2 Tensile mechancial properties of the different hybrid papers presented in Fig. 1e, 1g, and 1h, Fig. 3c and 3d.\nPulp fiber Tensile Strength Strain at failure Young’s Modulus Toughness\nSamples a (SR) (MPa) (%) (GPa) (MJ/m3)\nPulp fiber 65° 17.9±1.5 5.9±1.0 1.3±0.3 0.6±0.2\nMicrogel - 102.3±7.2 3.6±0.4 5.5±0.7 2.7±1.1\nB-CNF - 327.4±15.9 2.2±0.4 20.6±2.9 4.7±0.7\nHCP (1:1:1) 65° 811.0±17.1 11.4±1.0 36.1±1.8 56.5±6.1\nHCP (1:1:1.2) 65° 691.2±4.0 6.6±1.1 23.7±1.0 25.5±1.6\nHCP (1.2:1:1) 65° 665.2±5.0 6.2±0.7 21.3±1.0 29.1±1.3\nHCP (0.8:1:1) 65° 457.2±8.0 4.8±0.4 19.9±0.6 14.6±1.0\nHCP (1:1:0.8) 65° 440.6±8.9 6.2±0.5 15.6±0.9 17.6±1.0\nHCP (1:1:2) 65° 38.5±3.1 9.0±1.3 3.1±0.7 2.2±0.4\nHCP (1:2:1) 65° 77.8±9.1 13.2±1.3 2.9±0.2 6.6±1.1\nHCP (2:1:1) 65° 55.2±3.7 12.9±3.4 1.3±0.2 4.7±0.6\nHCP (1:1:1) 15° 344.2±26.1 4.0±0.5 17.0±2.0 9.3±1.2\nHCP (1:1:1) 25° 437.4±27.0 4.5±1.0 22.2±2.8 13.8±1.6\nHCP (1:1:1) 35° 541.4±9.0 5.5±0.5 26.0±2.6 19.0±1.8\nHCP (1:1:1) 45° 585.8±14.2 5.8±0.8 29.7±0.7 22.4±1.3\nHCP (1:1:1) 55° 623.7±20.5 7.5±0.5 30.5±3.3 30.7±2.0\nHCP (1:1:1) 65° 811.0±17.1 11.4±1.0 36.1±1.8 56.5±6.1\nHCP (1:1:1) 75° 531.8±27.2 12.0±1.0 12.5±2.2 34.4±3.8\nHCP (1:1:1)\nNatural drying 65° 811.0±17.1 11.4±1.0 36.1±1.8 56.5±6.1\nCompletely\n65° 157.3±15.9 5.3±1.1 8.5±0.5 4.4±0.8\ndrying\nRH 70% 65° 370.7±4.8 16.0±0.6 17.6±0.5 7.0±0.06\nRH 90% 65° 168.0±6.7 19.9±0.8 9.4±0.7 9.2±0.3\na The riatio in the parenthesis for each sample indictate the weight ratio of Pulp fiber, Microgel, and B-CNF.\n6. Request for higher-resolution images of Figures 2B, 2C, and 2D to clarify the differences between the neat pulp fiber structure and the hybridized paper.\nResponse: The higher-resolution images of Figures 2b, 2c, and 2d were added in Fig. 2 in the revised manuscript as below.\n7. Why does HCP exhibit low wet strength?\nResponse: This is attributed to the high water absorption (550 wt%) of HCP, while the water absorption of microgel is 145 wt% (Supplementary Fig. 11). As we discussed in the manuscript, the wet tensile strength of the HCP was 1.3 MPa, while the cellulose microgel exhibited higher wet strength of 62 MPa due to lower water absorption, suggesting the interlocking structure of the cellulose microgel is robust enough to resist disassembly and water infiltration.\n8. Clarify theoretical water absorption calculation in Figure S8.\nResponse: The theoretical water absorption of the composite film was calculated based on the weighted average of the individual components’ measured water absorption values, specifically, those of pulp fiber, microgel, and B-CNF. For each component, the actual water absorption was first experimentally determined. Subsequently, the theoretical value for the composite was derived by applying the respective mass fractions of the components. We have added this clarification to the revised supplementary information.\n9. The study finds that a 1:1:1 ratio of pulp fibers, microgel, and B-CNF gives the best mechanical properties, but it is unclear if this is an absolute optimum. Could small adjustments in composition (e.g., 1.2:1:1 or 0.8:1:1) lead to further improvements?\nResponse: Thanks for your suggestion.\nWe thoroughly investigated the mechanical properties of papers prepared from ternary hybrids of pulp fibers, microgel, and B-CNF at different ratios (Fig. 1g). The results revealed that small adjustments in composition (e.g. 1.2:1:1 or 0.8:1:1) could not lead to further improvements compared to the HCP at 1:1:1. The hybrid papers with higher content of pulp fibers (2:1:1) or microgel (1:2:1) showed higher strain-to-failure but significantly lower modulus and tensile strength. The HCP with a composition of 1:1:1 remarkably outperformed paper with other compositions in terms of Young’s modulus, tensile strength, and toughness (Supplementary Table 2).\nFig. 1g Tensile stressstrain curves of HCPs fabricated from different weight ratios of pulp fibers, microgel, and B-CNF.\nReviewer #2 (Remarks to the Author):\nThe manuscript by Zhou and coworkers reports on how a strong paper can be made from cellulose by mixing cellulose fibers/fibrils of different dimensions. The study builds on a large volume of work on nanopaper and various strengthening additives to paper from cellulose pulp. The paper in the current study is produced by vacuum filtration of aqueous mixtures of pulp fibres with TEMPO oxidized cellulose nanofibers and cellulose microgels. The produced papers/films displayed a high tensile strength and strain to fracture and also a good adhesion strength. The mechanical measurements are supported by SEM and WAXS to characterize the paper structure and fiber alignment. There is essentially no data supporting the discussion on the bonding mechanism and the investigated parameter space is limited.\nResponse: Many thanks for your comments. To support the bonding mechanism, we have performed Low-Field Nuclear Magnetic Resonance (LF-NMR) measurements to study the spin-spin relaxation time (T ) of water proton in the hybrid paper samples. We demonstrated the difference of adhesion mechanism between B-CNF and microgel using glass substrates. We have also expanded the parameters including adjustments in composition of HCP, the beating degree of pulp fibers, the average particle size of microgel, mechanically homogenized MFC instead of B-CNF.\nSpecific points to address:\n1. Are the reported comparisons with literature data on the tensile strength and Youngs modulus comparable, i.e. are the temperature and the relative humidity the same in all cases? Also, how does the density of the different materials compare? The density of the new paper presented in the manuscript needs to be determined and reported.\nResponse: Yes, they are comparable in terms of density, temperature, and relative humidity. We have added Supplementary Table 1, summarizing the mechanical properties of the hybrid cellulose paper (HCP) in our work and different studies correlated with the Ashby plot in Fig. 1f, as shown below. The densities and porosities of the papers shown in Fig. 2 were measured and summarized in Supplementary Table 3.\nSupplementary Table 1 Mechanical properties of the hybrid cellulose paper (HCP) developed in this work and different studies correlated with the Ashby plot in Fig. 1f.\nSpecific Spcific\nTensile Young’s Strain at\nDensity tensile Young’s Toughness\nSample Strength Modulus failure\n(g/cm3) strength Modulus (MJ/m3)\n(MPa) (GPa) (%)\n(MPa cm3/g) (GPa cm3/g)\nRandom oriented (isotropic) cellulose papers or films\nRegenerated cellulose1 106 a 4.3 12.4 1.245 85.1 3.45 9.8\nBC film2, 3 194 b 11.4 3.4 0.64 303 17.8 3.8\nTEMPO-oxidized BC4 102 a 18.5 0.9 1.19 85.7 15.5 0.6\nEnz-CNF5 214 c 13.2 10.1 1.08 198 12.2 15\nLignin-rich MFC6 283 c 15.1 7.4 1.39 204 10.9 - e\nHolo-CNF7 320 c 21.0 4.8 1.47 218 14.3 9.8\nTEMPO-CNF8 321 d 17 9 1.41 228 12.1 16\nMFC/wood fiber9 160 c 10.1 4.2 0.97 165 10.4 4.4\nHCP (this work) 811 d 36.1 11.4 1.10 737 32.8 56.5\nOriented or stretched (anisotropic) cellulose papers and films\nDenlignified wood film10 449 c 51.1 1.6 1.32 340 38.7 - e\nRegenerated cellulose1 253 a 14.6 16.5 1.245 203 11.7 41.1\nBC film2 1005 b 48.1 4.4 1.18 852 40.8 24.7\na Tensile test condition: data not reported.\nb Tensile test condition: relative humidity (RH) of 40% and Temperature (T) of 23 oC.\nc Tensile test condition: RH of 50% and T of 23 oC.\nd Tensile test condition: RH of 50% and T of 25 oC.\ne Data not reported.\nSupplementary Table 3 Densities of the papers of the HCP paper as compared to the papers prepared from neat pulp fiber (65° SR), neat microgel, and neat B-CNF.\nPaper samples Buck density (g/cm³) Porosity (%)\nPulp fibers 0.66 56\nB-CNF 1.11 26\nMicrogel 1.15 23\nPulp fiber/B-CNF=1:1 0.84 44\nPulp fiber/Microgel=1:1 0.79 47\nHCP - Pulp fiber/Microgel/B-CNF=1:1:1 1.10 27\na Porosity is calculated according to the following equation, where ρ corresponds to the bulk density of the\nbulk\npaper and ρ corresponds to the density for the cellulose, which is assumed to be 1.5 g/cm3.\ncellulose\n𝜌\nbulk\n𝑃𝑜𝑟𝑜𝑠𝑖𝑡𝑦(%)=(1− )×100\n𝜌\ncellulose\n2. The bonding mechanisms should be investigated in detail, using e.g. spectroscopy (ssNMR, neutron spectroscopy) and MD simulations.\nResponse: Thanks for your suggestion. To study the effect of bound water on mechanical performance of the hybrid fibers, in addition to Near-infrared (NIR) spectroscopy, we have employed LF-NMR method to test the bonding water and free water in the HCP samples. The results, discussion and method are integrated in the revised manuscript as below.\nTo determine the hydration state of the bound water in the HCP sample, low field nuclear magnetic resonance (LF-NMR) measurements were performed at room temperature under relative humidity (RH) of 50% (Fig. 3b). The natural dried HCP sample had a moisture content of 5.8% and showed only a strong peak at T < 1 ms, corresponding to tightly bound water that was mostly located in the first hydration layer, where water binds to the hydroxy groups of cellulose. After complete drying, the HCP sample had a moisture content of 7.4% and showed a significantly decreased peak for tightly bound water (T < 1 ms) and 3 additional peaks with T time of 5 ms, 40 ms, and 500 ms, respectively. These new peaks correspond to loosely bound water that is located not far away from hydroxy groups, water confined in the pores due to capillary condensation, and bulk-like water resulting from surface condensation, respectively. These results indicate the irreversible loss of tightly bound water in HCP after completely drying.\nFig. 3b T inversion spectra for the HCP prepared by natural and complete drying.\nRegarding the adhesive bonding mechanism, we included the following demonstration in the revised manuscript.\nWe demonstrated the difference of adhesion mechanism between B-CNF and microgel using glass slides as the substrate ss shown in Fig. 4a. The B-CNF formed scattered cellulose nanofibrils aggregates between the glass substrates due to fractures occurred in the gelled B-CNF suspension upon drying similar to cellulose nanocrystals,41 while cellulose microgel formed a homogeneous nanofibril network without fractures owing to the high internal cohesion in the cellulose microgel upon drying.\nFig. 4a Comparison of adhesive mechanism between B-CNF and microgel for hydrophilic substrates (glass slides). 10 mg of B-CNC or microgel suspended in 1 mL of water were left to assemble between two glass plates and heated at 105 °C for 12 h. The bonded area between two glass slides were observed in bright field (scale bar: 200 µm).\n3. The parameter space of the films/paper needs to be expanded to better understand the critical parameters. Pulp fibers and cellulose nanofibers of different dimensions and surface chemistry should be investigated in different combinations and compositions.\nResponse: We thoroughly investigated the mechanical properties of papers prepared from ternary hybrids of pulp fibers, microgel, and B-CNF at different ratios (Fig. 1g). In addition, the beating degree of the pulp fibers has a strong impact on the mechanical properties of HCPs, which initially improved and then declined with increasing beating degree of the pulp fibers (Fig. 1h), as determined by the Schopper-Riegler (°SR) scale.\nThe influence of microgel average particle size on the mechanical properties of the HCP was investigated. A reduction in particle size from 72.9 to 52.1 and 13.5 µm did not alter the overall shape of the stress-strain curve, but it led to a decrease in both strain-to-failure and ultimate tensile strength (Supplementary Fig. 5). This trend is analogous to the mechanical performance deterioration observed with decreasing average molar mass of cellulose.29\nSupplementary Fig. 5 a Size distribution of the microgels measured by Mastersizer. b Effect of the microgel size on the mechanical properties of the HCP.\nFurthermore, mechanically homogenized MFC, derived from bleached softwood kraft pulp and characterized by widths of 10–20 nm and lengths of 5–10 µm, was used in place of B-CNF, which has a carboxylate content (1.18 mmol/g) and dimensions of 5–12.5 nm in width and 1–2.3 μm in length. Although the microgel/MFC combination showed moderate synergistic effect compared to individual components (Supplementary Fig. 6), the substitution resulted in HCP samples with lower yield stress (100 MPa) and ultimate tensile strength (210 MPa). These findings emphasize the critical role and synergistic interaction between thin nanoscale cellulose fibrils (B-CNF) and microscale cellulose microgel particles in governing the mechanical performance of the HCP.\nSupplementary Fig. 6 Tensile stressstrain curves of the HCP papers prepared from the pulp fiber, microgel, and MFC with various weight ratios as compared to the papers prepared from the individual components.\n4. The authors argue that the presented material could be a biodegradable alternative to plastics. What plastic should this material replace? The biodegradability of the paper needs to be determined.\nResponse: The high-performance HCP, with its superior mechanical properties, has the potential to serve as a sustainable alternative to conventional plastic packing straps typically made from PVC or PET.\nDegradation of the HCP paper during soil burial was carried out in nature soil under ambient temperatures of 18–25 °C with soil moisture level of approximately 30–45 % in May in Kunming, Yunnan, China. The samples were buried in the soil at a depth of 5cm with gently hand-compacting. The HCP paper was biodegradable, and it showed a weight loss of 70% at 23 days as shown below.\nResponse Fig. 1 a The appearance of the HCP paper sample during soil burial degradation experiments. b The effect of soil burial on sample weight.\n5. The amount of bound water and its contribution to the interfibrillar bonding needs to be assessed and discussed.\nResponse: Please refer to our answer to comment 2.\n6. Figure 4 is mixed up.\nResponse: Thanks. The legend of Figure 4 was corrected, and it is now Fig. 3 in revised manuscript.\nReviewer #3 (Remarks to the Author):\nThe study shows a concept by using hybrid components (BC or other H-bond mediating components and fibers for cellulose-based composites) to enhance the mechanical properties which is widely used and well known. Quite often, the mechanism for promoting the mechanical properties of final products is unclear because of the use of many different components, which is exactly the case in this study. E.g. the distribution of these different components to excert their contribution to mechanical properties is already unclear. As well, more data about microgel for clear characterization is required. Moreover, to realize the adhesive properties between the hydrophilic substrates via H bonds should be common.\n1. The author repeatedly mentioned in the Abstract that the material preparation is energy-efficient. Authors should quantitatively compare the energy inputs of your paper with those in other studies to support the claim of energy efficiency.\nResponse: We appreciate the reviewer’s insightful comment. Our claim of energy efficiency is based primarily on the rapid dewatering capability of our HCP system, which significantly reduces the energy consumption typically associated with drying in conventional papermaking processes. To address this point more quantitatively, we compared the filtration time of our method for HCP to the processes reported in the literature for cellulose films prepared from TEMPO-CNF, Holo-CNF, Enz-CNF, and MFC, and summarized in Supplementary Table 4.\nWe have added a quantitative comparison and brief discussion of this point in the revised manuscript and modified the Abstract to reflect this more precisely.\nThe fabrication process demonstrated notable energy efficiency, primarily due to its rapid dewatering capability, which led to significantly lower energy consumption compared to the preparation of nanocellulose-based paper.\nThe vacuum filtration time for the HCP was 1.417 h, significantly lower than the 7.79 h required for B-CNF, highlighting its great potential for practical applications (Supplementary Fig. 12).\nMoreover, the HCP exhibited superior mechanical properties to cellulose films29,31,32,40 prepared from TEMPO-CNF, Holo-CNF, and Enz-CNF, while also achieving a shorter filtration time and better energy savings, as shown in Fig. 3f and Supplementary Table 4.\nSupplementary Table 4 Filtration time and mechanical properties of the HCP paper from this work as compared to the cellulose films prepared from TEMPO-CNF,8 Holo-CNF,7 and Enz-CNF5 as reported in literature.\nHCP TEMPO-CNF8 Holo-CNF7 Enz-CNF5\nFiltration time (h) < 1.5 ∼ 24 < 2.5 < 2.5\nThickness (µm) 70 30–40 35 60–80\nTensile strength (MPa) 811 321 320 214\nTensile modulus (GPa) 36.1 17 21 13.2\nStrain at break (%) 11.4 9 4.8 10.1\nToughness (MJ/m3) 56.5 16 9.8 15\nSpecific strength (MPa cm3/g) 737 228 218 198\nSpecific modulus (GPa cm3/g) 32.8 12.1 14.3 12.2\n2. In Figure 1E, what do the 1, 2, 3 and three curves mean? Also, the necking indicates the decrease in local cross-sectional area, and authors can better display this decrease in the insert photograph.\nResponse: Thank you for raising this question. The three curves labeled 1, 2, and 3 in Fig. 1e represent the tensile stressstrain profiles of three independent specimens of the HCP with a component ratio of 1:1:1. These curves illustrate the reproducibility and variability of the mechanical behavior under standardized tensile testing conditions. Upon unloading after the onset of strain hardening but prior to fracture, the sample retained its deformed (homogeneous necking) shape, confirming the presence of plastic deformation (Supplementary Fig. 2). We have corrected this behavior as “strain hardening” in Fig. 1e. We have updated the insert pictures in Fig. 1e and provided a movie recording tensile test of an HCP sample. We have revised the discussion as below.\nInterestingly, the HCP showed a strain hardening behavior during tensile testing (insert picture in Fig. 1e and Supplementary Movie 1), similar to the plastic deformation of elastomeric polymers. Notably, upon unloading after the onset of strain hardening but prior to fracture, the sample retained its deformed (homogeneous necking) shape, confirming the presence of plastic deformation (Supplementary Fig. 2). This phenomenon was previously reported in cellulose nanopaper prepared from microfibrillated cellulose (MFC), which showed a yield stress of 91 MPa and a slope of 1.82 GPa in the plastic region. In comparison, the HCP sample demonstrated remarkably enhanced mechanical performance, with a yield stress of 305 MPa, a plastic deformation slope of 4.76 GPa, and a high strain-to-failure of 11.4%. These values suggest significantly increased frictional resistance to fiber slippage within the network and superior toughness resulting from the hybridization of multiscale cellulose fibers.\nFig. 1e Tensile stressstrain curves of the HCP prepared from the pulp fiber, microgel, and B-CNF with a composite of 1:1:1 by weight as compared to the papers prepared from the individual components; the inset photograph shows strain hardening of the HCP under tensile loading.\nSupplementary Fig. 2 Photograph of the HCP sample when the stress was removed after strain hardening and before breakage, showing the plastic deformation remained.\n3. More advanced data should be provided to support the results shown in Figure 2A. Also In Figure 3A, the explanation for the super tensile strength of cellulose paper is somewhat unclear and unsatisfactory. The explanation from the perspective of free water and hydrogen bonding of cellulose also lacks data support.\nResponse: To study the effect of bound water on mechanical performance of the hybrid fibers, in addition to Near-infrared (NIR) spectroscopy, we have employed LF-NMR method to test the bonding water and free water in the HCP samples.\nTo determine which type of hydrogen bonding primarily influences mechanical properties, the HCP was completely dried under vacuum at 110 °C for approximately 2 to 3 hours to remove bound water. The NIR spectra (Fig. 3a) revealed a significant difference between HCP subjected to complete drying and natural drying, marked by the absence of the peak at 5178 cm⁻¹, which corresponds to bound water.39 To determine the hydration state of the bound water in the HCP sample, low field nuclear magnetic resonance (LF-NMR) measurements were performed at room temperature under relative humidity (RH) of 50% (Fig. 3b). The natural dried HCP sample had a moisture content of 5.8% and showed only a strong peak at T < 1 ms, corresponding to tightly bound water that was mostly located in the first hydration layer, where water binds to the hydroxy groups of cellulose. After complete drying, the HCP sample had a moisture content of 7.4% and showed a significantly decreased peak for tightly bound water (T < 1 ms) and 3 additional peaks with T time of 5 ms, 40 ms, and 500 ms, respectively. These new peaks correspond to loosely bound water that is located not far away from hydroxy groups, water confined in the pores due to capillary condensation, and bulk-like water resulting from surface condensation, respectively. These results indicate the irreversible loss of tightly bound water in HCP after completely drying. The mechanical properties of HCP declined drastically after complete drying, with tensile strength decreasing from 811 MPa to 167 MPa (Fig. 3c). In addition, as RH increased to 70% and 90%, the ultimate tensile strength of the HCP decreased significantly while the strain-to-failure increased (Fig. 3d), due to higher moisture content and increased loosely bound water. These observations confirm that hydrogen bonds between cellulose and tightly bound water play a more critical role in the formation of HCP and are primarily responsible for its ultrahigh strength and toughness.\nFig. 3: Influence of bound water on the performance of hybrid fibers. a Near-infrared (NIR) spectra of the HCP paper after complete and natural drying. b T inversion spectra for the HCP prepared by natural and complete drying. c Representative stressstrain curves of the HCP under both drying conditions. d Typical stressstrain curves of the HCP at different relative humidity (RH) levels.\n4. In Figure 2, it seems that the thickness of different samples varies greatly. Did the authors use the same method to prepare the samples (especially for the neat fibers)? Or can the authors give some discussions on the thickness results as well as related to mechanical properties?\nResponse: Indeed, the thickness of different samples varies as they have different densities when we used the same method to prepare different samples including neat fibers, binary fibers, and ternary fibers. Their densities and porosities are summarized in supplementary Table 3, and we discussed the porosity and mechanical properties as below.\nAs shown in Supplementary Table 3, the HCP paper exhibited porosity comparable to that of neat B-CNF and aerogel papers yet demonstrated markedly superior mechanical performance. This enhancement suggests strong interfacial adhesion and improved interfacial toughness arising from the synergistic hybridization of pulp fibers with B-CNF and microgel.\nSupplementary Table 3 Densities of the papers of the HCP paper as compared to the papers prepared from neat pulp fiber (65° SR), neat microgel, and neat B-CNF.\nPaper samples Buck density (g/cm³) Porosity (%)\nPulp fibers 0.66 56\nB-CNF 1.11 26\nMicrogel 1.15 23\nPulp fiber/B-CNF=1:1 0.84 44\nPulp fiber/Microgel=1:1 0.79 47\nHCP - Pulp fiber/Microgel/B-CNF=1:1:1 1.10 27\na Porosity is calculated according to the following equation, where ρ corresponds to the bulk density of the\nbulk\npaper and ρ corresponds to the density for the cellulose, which is assumed to be 1.5 g/cm3.\ncellulose\n𝜌\nbulk\n𝑃𝑜𝑟𝑜𝑠𝑖𝑡𝑦(%)=(1− )×100\n𝜌\ncellulose\nWe also prepared HCP samples with different thickness, 40, 70, and 100 µm. The mechanical properties of samples with thickness of 40 and 70 µm were similar, while the sample with even higher thickness (100 µm) showed slightly higher plastic deformation slope and lower strain-to-failure, as shown below. In general, the HCP samples are homogeneous and defects free in these different thicknesses.\nResponse Fig. 2 Stress-strain curves of HCP with different thickness.\n5. In Figure 3C, the strength and other specific properties of the paper sheet are also important for this test, authors should give more information about this substrate.\nResponse: Thanks for raising this point. The paper sheets for the adhesive test were the control paper prepared from the neat pulp fibers (65°SR).\n6. The size of microgels and nanofibers, as well as the types and quantities of their surface functional groups, have a significant impact on capillary forces and hydrogen bonding. Such interfacial interactions represent one of the core innovative aspects of this work. I suggest the authors to provide a more detailed discussion on this topic.\nResponse: Thanks for your suggestion. We have provided further detailed discussion on the topic in the revised manuscript as below.\nThe influence of microgel average particle size on the mechanical properties of the ternary hybrids was further investigated. A reduction in particle size from 72.9 to 52.1 and 13.5 µm did not alter the overall shape of the stress-strain curve, but it led to a decrease in both strain-to-failure and ultimate tensile strength (Supplementary Fig. 5). This trend is analogous to the mechanical performance deterioration observed with decreasing average molar mass of cellulose.29 Furthermore, mechanically homogenized MFC, derived from bleached softwood kraft pulp and characterized by widths of 10–20 nm and lengths of 5–10 µm, was used in place of B-CNF, which has a carboxylate content (1.18 mmol/g) and dimensions of 5–12.5 nm in width and 1–2.3 μm in length. Although the microgel/MFC combination showed moderate synergistic effect compared to individual components (Supplementary Fig. 6), the substitution resulted in HCP samples with lower yield stress (100 MPa) and ultimate tensile strength (210 MPa). These findings emphasize the critical role and synergistic interaction between thin nanoscale cellulose fibrils (B-CNF) and microscale cellulose microgel particles in governing the mechanical performance of the HCP.\nSupplementary Fig. 5 a Size distribution of the microgels measured by Mastersizer. b Effect of the microgel size on the mechanical properties of the HCP.\nSupplementary Fig. 6 Tensile stressstrain curves of the HCP papers prepared from the pulp fiber, microgel, and MFC with various weight ratios as compared to the papers prepared from the individual components.\n7. Figure 4A: Did the author perform normalization when using NIR to compare hydrogen bond content? How is it done?\nResponse: All near-infrared (NIR) spectra were normalized using the min–max normalization method, implemented in the OPUS software. The normalization was carried out according to the following formula:\nOriginal spectrum−Minimum value\nNormalized spectrum =\nMaximum value−Minimum value\nThis approach ensures that the spectral data are scaled uniformly, facilitating accurate comparison and analysis.\n8. There are many errors in the text that need to be corrected. For example: Line 153, Figure caption of Figure 4.\nResponse: We carefully revised the manuscript and corrected the errors in the text including the caption of Figure 4 (currently Fig. 3 in the revised manuscript)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The issues have been addressed.", "reviewer_2": "The authors have responded well to several points that was raised. However, the NMR study and the interpretation of the data is confusing and incomplete. The ratio of bound and free water should be determined at 70 and 90% RH, i.e. higher moisture contents. The arguments for \"hydrogen bonding\" are vague and needs to be supported by modeling.", "reviewer_3": "Authors could successfully address my concerns, which make the manuscript acceptable."}, "author_response": "Response to the comments\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nThe issues have been addressed.\nResponse: Thank you.\nReviewer #2 (Remarks to the Author):\nThe authors have responded well to several points that was raised. However, the NMR study and the interpretation of the data is confusing and incomplete. The ratio of bound and free water should be determined at 70 and 90% RH, i.e. higher moisture contents. The arguments for \"hydrogen bonding\" are vague and needs to be supported by modeling.\nResponse: Thank you very much for your valuable comments on the NMR analysis in “influence of bound water on mechanical performance of the hybrid fibers”. In response, we performed additional low-field NMR (LF-NMR) measurements on naturally dried HCP samples conditioned at 70% and 90% relative humidity (RH) and further reconditioned the samples back to 50% RH to evaluate the recovery of mechanical properties.\nThese new results confirm that tightly bound water, which is hydrogen-bonded to cellulose chains, plays a key structural role in the fiber network. This interpretation is supported by previously reported simulation studies, and we have now added the relevant citations. The new LF-NMR results are presented in Fig. 3b and the corresponding mechanical property recovery results in Fig. 3d. The Discussion section has been revised accordingly in the updated manuscript.\nFor clarity, the revised text has also been included below.\nFig. 3: Influence of bound water on mechanical performance of the hybrid fibers. b T inversion spectra for the HCP prepared by natural and complete drying and conditioned at different relative humidity (RH) levels. d Typical stressstrain curves of the natural dried HCP at different RH levels.\nTo determine the hydration state of the bound water in the HCP sample, low field nuclear magnetic resonance (LF-NMR) measurements were performed at room temperature (Fig. 3b). The natural dried HCP sample had a moisture content of 5.8% under 50% relative humidity (RH) and showed only a strong peak at T < 1 ms, corresponding to tightly bound water that was mostly located in the first hydration layer, where water binds to the hydroxy groups of cellulose. When the HCP samples were conditioned under 70% and 90% RH, their moisture contents increased to 10.8% and 18.5%, respectively. Correspondingly, the T relaxation times of the bound water shifted to values above 1 ms and exhibited broader distributions, indicating a greater contribution from more loosely bound water under high humidity conditions. Additionally, small relaxation peaks appeared at 70 ms and 500 ms, which suggests the presence of a minor fraction of free or bulk-like water at high RH levels. After complete drying, the HCP sample had a moisture content of 7.4% under 50% RH and showed a significantly decreased peak for tightly bound water (T < 1 ms) and 3 additional peaks with T time of 5 ms, 40 ms, and 500 ms, respectively. These new peaks correspond to loosely bound water that is located not far away from hydroxy groups, as well as free water confined in the pores due to capillary condensation and bulk-like water resulting from surface condensation, respectively. These results indicate the irreversible loss of tightly bound water in HCP after completely drying. The mechanical properties of HCP declined drastically after complete drying, with tensile strength decreasing from 811 MPa to 167 MPa (Fig. 3c). Furthermore, when the RH was increased to 70% and 90%, the ultimate tensile strength of the natural dried HCP decreased significantly while the strain-to-failure increased (Fig. 3d), consistent with the higher moisture content and the greater proportion of loosely bound water (Fig. 3b). Notably, the tensile strength of the natural dried HCP was largely restored after reconditioning the samples back to 50% RH, indicating that the humidity-induced changes were reversible. These observations indicate that tightly bound water, which is hydrogen-bonded to the cellulose chains,40,41 plays a key structural role in HCP. This tightly bound water contributes to the dense packing and load transfer within the network of multiscale cellulose fibers and is therefore a primary factor underlying the material’s ultrahigh strength and toughness.\nReviewer #3 (Remarks to the Author):\nAuthors could successfully address my concerns, which make the manuscript acceptable.\nResponse: Thank you"}, {"round": 3, "reviewer_comments": {"reviewer_2": "The addition of NMR data has addressed the remaining concerns sufficiently."}, "author_response": ""}]

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