Bioaerogels for Ultra-Broadband Perfect Acoustic Absorption

doi:10.21203/rs.3.rs-3468655/v1

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Bioaerogels for Ultra-Broadband Perfect Acoustic Absorption

https://doi.org/10.21203/rs.3.rs-3468655/v1

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Noise has become the world's second-largest environmental risk factor^1-3. The simultaneous achievement of ultra-broadband and perfect acoustic absorption is a quite vital yet long-standing challenge^4-11. Herein, we propose a new "gradient pore circulation (GPC)" strategy for building hierarchical ordered architecture of bioaerogels, by using highly active microfibers precisely dissociated from the wood S₂ sublayer as basic units. The bioaerogels comprise anisotropic parallelly-layered microchannels, enriched with intricate multilevel pores within each layer, alongside abundant spring-shaped strips bridging these adjacent layers. Under the “GPC” strategy, the soundwaves effortlessly enter the parallelly-layered microchannels possessing moderate flow resistance, while the synergy of long microchannels, multilevel pores, and abundant interlamellar strips create plentiful closed loops, fostering a repetitive cyclic reflection‒friction‒dissipation of soundwaves. The bioaerogels achieve near-perfect acoustic absorption abilities, with a sound-absorption-coefficient of 0.95~1 across an ultra-broad frequency range of 520‒6300 Hz and a superb noise-reduction-coefficient of 0.82, the highest recorded to date. More importantly, the sound absorption properties retain almost unchanged at extreme temperatures (‒60~+60 ºC) over three months. The revolutionary bioaerogels hold great promise as sustainable sound absorption materials for diverse applications.

Physical sciences/Materials science/Soft materials/Gels and hydrogels

Biological sciences/Biochemistry/Carbohydrates

Physical sciences/Materials science/Theory and computation/Computational methods

According to the WHO, noise pollution has emerged as the second-largest environmental risk factor, following closely behind air pollution¹. Globally, over 250 million people suffer from noise-induced hearing loss greater than 25 dB², which is clinically significant. Also, noise pollution has been linked to cardiovascular diseases³. Daily noise covers a wide range of frequencies, with the most usual frequencies falling between 300 and 5000 Hz in everyday situations (Supplementary Figure S1). However, most common sound absorption materials like ceramics, polymer foams, and polyvinylidene fluoride films, have limited effectiveness in absorbing low-frequency sounds (<1000 Hz) due to their monotonous microstructure^4-6. Acoustic metamaterials have attracted attention for their unique properties achieving good absorption of low-frequency sounds^7,8, but they only perform well in specific narrow ranges. Recently, some researchers reported broadband sound absorption materials to some degree owing to well-designed microstructure, e.g., dual-3D structure of graphene/polydimethylsiloxane⁹, wood-like cyclodextrin aerogel¹⁰, and wavy-type graphene-polyurethane foam¹¹. However, these materials still have three main limitations in practical applications: (1) far from ultra-broadband perfect acoustic absorption. These materials are adept at broadband acoustic absorption when compared with traditional options, but maintaining a high SAC across all frequencies remains a challenge; (2) poor flexibility and durability. Their strong structural stiffness makes them prone to cracking. Also, certain materials age and become brittle at extreme temperatures, diminishing their sound absorption abilities; (3) some materials are not environmental-friendly and contain hazardous chemicals. Overall, developing green flexible materials with ultra-broadband perfect acoustic absorption properties remains a crucial challenge.

Wood microfibers (MFs) are abundant biomass resources that offer great potential for producing sustainable sound absorption products^12,13. Moreover, compared to popular nanocellulose, micro-scaled fibers require lower energy consumption to dissociate from wood cell walls and avoid the wastage of non-cellulosic substances¹⁴, rendering them advantageous for scaling up uses. Nonetheless, existing methods to assemble wood MFs often gains randomly oriented fibers and underdeveloped pore structure with a narrow pore diameter distribution^15,16. This hinders achieving ultra-broadband sound absorption. Furthermore, the assembled products typically exhibit poor mechanical properties^17,18. Considering these above issues, herein, we propose a new strategy called "gradient pore circulation (GPC)" to build hierarchical ordered architecture of bioaerogels, for gaining ultra-broadband perfect sound absorption. Higher activity of MFs, precisely dissociated from the S₂ sublayer of wood cell walls that contain higher proportion of cellulose than traditional MFs from middle lamella (ML), act as basic units to assemble the 3D aerogels. The bioaerogels possess anisotropic parallelly-layered microchannels, multilevel pores within each layer, and abundant interlamellar spring-shaped strips, synergistically contributing to create plentiful closed loops for cyclic reflection-friction-dissipation of soundwaves. The lightweight bioaerogels with a small thickness of 30 mm achieve near-perfect sound absorption properties, and the sound absorption properties almost retain intact even under extreme temperatures for three months. Besides, the bioaerogels show superelasticity with high compressibility up to 80% strain and strong compression fatigue resistance over 1000 cycles.

Fabrication of highly active wood MFs

Wood is a nature hierarchical composite. At the cellular level, besides ML and ultrathin primary cell wall, the thick secondary cell wall includes sublayers S₁, S₂, and S₃ (Figure 1a). At the molecular level, cellulose is incorporated into these layers by crosslinking with hemicellulose and lignin. Due to cellulose's higher activity and easier modification compared with hemicellulose and lignin, MFs dissociated from the S₂ sublayer with the highest cellulose proportion are believed to have higher activity and accessibility than traditional MFs dissociated from the ML^19,20.

To identify highly active wood MFs, three types of MFs are dissociated from various cell wall layers of P. massoniana (ML, S₂, and random layer), and are coded as MFs-ML, -S₂, and -RL, respectively. Using the conventional way (high-temperature and -pressure refining and alkaline sodium sulfite treatment), a single fiber is dissociated from the ML layer but seriously damaged (Figure 1b and Supplementary Video S1). The superficial randomly-oriented fibrils confirm the dissociation occurring at ML (Figure 1c)²¹. Under the synergy of heat-moisture, alkaline H₂O₂ and mechanical refining, the thickest S₂ sublayer undergoes softening and partial rupture, ultimately exposing the highly active S₂ sublayer (Figures 1d-e and Supplementary Video S2). The microfibril angle (MFA) between the fibrils and the longitudinal cell axis falls within the range of 10º‒30º, which proves the dissociation of S₂ sublayer (Figure S2 and the Supplementary Discussion). For MFs-RL, the primary wall and S₁ sublayer are partially detached but still adhered to the S₂ surface (Figures 1f-g), reducing the accessibility of cellulose on the S₂ sublayer.

Confocal Raman spectrograms expose cellulose and lignin distributions in MFs-ML, -S₂, and -RL. The bands at 1599 cm⁻¹ (aromatic skeletal vibrations of lignin), 381 cm⁻¹ (glucoside bond of cellulose) and 2900 cm⁻¹ (C–H/C–H₂ stretchings of cellulose)²², are revealed (Figure 1k). Figures 1h-j illustrate the distribution of cellulose/lignin (I₂₉₀₀/I₁₅₉₉), reflecting that MFs-S₂ has the highest cellulose concentration, while MFs-ML has the highest lignin concentration. The variations in cellulose/lignin proportions across various wall layers align with those observed in individual cellulose or lignin (Figures 1l and Supplementary S3-S4). The higher cellulose concentration of MFs-S₂ contributes to the subsequent crosslinking and reassembling processes.

Figure 1m presents the X-ray diffraction (XRD) patterns of MFs-ML, -S₂, and -RL. The Segal crystallinity index for MFs-S₂ is 55.16%, higher than that of MFs-ML (43.71%) and MFs-RL (46.22%) (Supplementary Figure S5, Table S1 and the Supplementary Discussion), revealing a smaller content of amorphous phase (primarily lignin) in MFs-S₂. The weak lignin's Fourier transform infrared (FTIR) bands (1661, 1600, 1506, 1260, 1232, 1162 and 808 cm^–1) of MFs-S₂ provide additional evidence (Supplementary Figure S6 and Table S2). The d-spacing for the cellulose (200) crystallographic plane calculated by Bragg's equation is 3.978 Å for MFs-S₂, larger than that for MFs-ML (3.897 Å) and MFs-RL (3.859 Å) owing to the swelling effect caused by the alkaline H₂O₂ treatment. Moreover, the crystalline size, calculated by the Scherrer equation²³, shows an opposite trend (Supplementary Table S1). The larger d-spacing and smaller crystalline size are favorable for the penetration and crosslinking of PVA chains, and promote a decrease in structure stiffness and an improvement in fatigue life²⁴. This makes it possible to build elastic materials that can withstand repeated stress and strain.

3D Chemical crosslinking of wood MFs

Cellulose materials are usually strong and stiff, but lack flexibility and shape variability. To create superelastic bioaerogels for engineering acoustics, we use MFs-S₂ (which has high cellulose content and hydroxyls) to crosslink with flexible PVA at various contents (Figure 2a), with glutaraldehyde as a crosslinker. The as-prepared bioaerogels are coded as MFs-S₂-GA-PVA. Upon integration with PVA, the O−H stretching of MFs-S₂ at 3398 cm⁻¹ weakens (Figure 2b), possibly because the associating hydroxyls are destroyed and chemically crosslinked with PVA. A prominent strengthening of the stretching vibration band of O−C−O/C−O−C of MFs-S₂ at 1106 cm⁻¹ is found upon integration with PVA²⁵, while the C–OH vibration bands of PVA (1330 and 1142 cm⁻¹) weaken remarkably. The result demonstrates the occurrence of an aldolization reaction between glutaraldehyde and either cellulose or PVA in the bioaerogels. A new band appearing at 1244 cm⁻¹ for the MFs-S₂-GA-PVA bioaerogels is assigned to the C–O of CH₂OH groups of cellulose forming a hydrogen bond with the OH of PVA (Supplementary Table S3)²⁶, indicating that there are both chemical and physical interactions between MFs and PVA.

The characteristic XRD peaks of PVA at 19.82º and 23.26º, representing the (101) and (200) planes, noticeably decrease and shift to lower values after integration with MFs-S₂ (Figure 2c). This shift reveals a strong interaction between them²⁷, aligning with the FTIR analysis findings. The disappearance of characteristic cellulose peaks in the XRD pattern of MFs-S₂-GA-PVA reveals that the interaction disrupts cellulose crystallization and increases the amorphous region, beneficial to boost the material's toughness²⁸. The C 1s X-ray photoelectron spectrum (XPS) of MFs-S₂ are deconvoluted into three peaks (Figure 2d): C1 (C−C, 285.0 eV), C2 (C−OH, 286.7 eV), and C3 (O−C−O, 288.0 eV)²⁹. The aldolization reaction causes the notable increase of C3 (from 13.09% to 40.74%) and decrease of C2 (from 54.06% to 19.10%) (Figure 2e and Supplementary Table S4). Similarly, the O 1s XPS spectrum of MFs-S₂ demonstrates that the proportion of the O1 fitting peak (−OH, 532.9 eV) decreases upon integration with PVA, while the proportion of the O2 fitting peak (O−C−O, 534.5 eV) increases (Figures 2f-g), confirming that the aldolization reaction promotes the conversion of −OH to O−C−O^30,31. Figures 2h and i illustrate the thermogravimetric (TG) and differential TG (DTG) plots of the MFs-S₂-GA-PVA bioaerogels. The exothermic peak at 300 ºC comes from the partial dehydration of PVA chains (Supplementary Figure S7)³². The broad peak at 420 ºC is a result of the combined pyrolysis signals of cellulose and PVA (cleavage of C–C backbone)³³. Besides, the green bioaerogels have low residual amount of 2.6% at 800 ºC, making them suitable for disposal through simple direct burning.

Directional freezing and hierarchical ordered architecture

To achieve ultra-broadband perfect sound absorption, we propose a new “GPC” strategy to construct hierarchical ordered architecture of bioaerogels with three key elements:

(1) anisotropic, parallelly-layered and long microchannels that enable most soundwaves to enter and offer ample paths for sound energy transport; (2) plentiful multilevel pores within each layer for effective friction dissipation of various wavelengths of soundwaves; (3) a vast number of strips bridging adjacent layers to create closed loops that facilitate repetitive and cyclic reflection-friction-dissipation of soundwaves.

Moreover, the designed anisotropic architectures are anticipated to possess deformability resembling that of an accordion-like model^34,35, enabling the simultaneous development of superelastic bioaerogels. The post-crosslinked MFs-S₂-GA-PVA hydrogels undergo directional-freezing by placing them on a pre-cooled Cu plate in liquid nitrogen. Under the forces of ice crystal extrusion and traction, the vertical growth of ice crystals causes the MFs to assemble into a parallel-layered structure (Figure 3a). Figures 3b-e, f-i, and j-m present SEM images of the anisotropic structure of MFs-S₂-GA-PVA bioaerogels with different MFs contents (1%, 3%, and 5% referred to as MFs-S₂-GA-PVA-1, 3, and 5). All three materials have an anisotropic lamellar structure with long microchannels parallel to the growth direction of ice crystals, with apparent variations observed within the side viewport (X-Z plane). The small angle X-ray scattering (SAXS) analysis further proves MFs orientation through directional-freezing, showing symmetric luminous arcs, brighter spots at the center, and prominent integration peaks (Supplementary Figure S8 and the Supplementary Discussion). The Herman’s orientation factor (f_c) increases from 0.092 to 0.834 after the directional-freezing, further affirming a long-range aligned fiber structure along the direction of ice growth³⁶.

The bioaerogels with a low proportion of MFs (1%) display a macropore-dominant pore structure (X-Z plane, Figures 3b-c). A rise in the content of MFs from 1% to 3% develops a hierarchical multilevel architecture, comprising macropores (>50 nm), mesopores (2‒50 nm) and micropores (<2 nm), in the bioaerogels (Figures 3f-g). The stiffness of MFs and good crosslinking with PVA help to prevent smaller pores from collapsing/merging during freeze-drying³⁷. However, incorporating a higher content of MFs (5%) into the bioaerogels lead to an inadequate number of pores (Figures 3j-k). In the front view (Y-Z plane), the MFs-S₂-GA-PVA-3 biohydrogels exhibit spring-shaped strips (1.0‒2.7 μm wide and 10‒24 μm long) connecting adjacent layers (Figures 3h-i). These strips, formed through generating a crosslinking network of covalent bonds between PVA and MFs, establish a solid scaffold for superelasticity (discussed later). Also, these strips between the layers are expected to create closed loops for cyclic reflection-friction-dissipation of soundwaves. At 1% and 5% MFs contents, strips don't adequately connect adjacent layers (Figures 3d-e and l-m), demonstrating that 3% MFs content is better for efficient crosslinking with PVA, leading to a better-connected lamellar architecture.

Flexibility and superelasticity of the bioaerogels

By combining the simplicity of directional freezing with the large-scale applicability of chemically-crosslinked MFs, we have developed a straightforward and efficient method that guarantees rapid consistent production of large meter-sized bioaerogels like panels, felts, and even prefabricated pipes (Figure 4a). Moreover, the bioaerogels can be formed in any shape desired, e.g., a cylinder, cuboid, triangular prism, or even the premolded shapes of “C”, “S”, “U”, “F” and “T” (Figure 4b). These bioaerogels can be immediately used as common engineering materials without additional processing. Figure 4c shows a small bioaerogel (ρ=0.030 g cm⁻³) that can easily balance on the tip of dandelion seed villus, showcasing its interesting ultralight feature. The MFs-S₂-GA-PVA-3 bioaerogels can undergo substantial compression beyond 80% strain and complete recovery to their initial shape without any visible dimensional alterations upon pressure release (Figure 4b). By comparison, both the pure MFs-S₂ bioaerogels and isotropic MFs-S₂-GA-PVA-3 bioaerogels lacking directional freezing have inadequate compressibility, with residual deformation unable to fully revert (Supplementary Figure S10). These limitations may stem from the high stiffness of MFs and the absence of effective bridging between these MFs aggregations (Supplementary Figure S11). Figure 4e shows a quantitative analysis. MFs-S₂-GA-PVA-3 bioaerogels have excellent elasticity, reverting to their initial value after unloading. MFs-S₂-GA-PVA-1 bioaerogels also recover their initial value but with a significant decrease of 54.9% in maximum stress compared with MFs-S₂-GA-PVA-3. Other bioaerogels including MFs-S₂-GA-PVA-5, pure MFs, and isotropic counterpart lack organized cellular geometry and stable bonding, leading to an inability to withstand large deformation (Supplementary Figure S12 and Table S5).

Figure 4f plots the compressive stress–strain curves of MFs-S₂-GA-PVA-3 bioaerogels with different maximum strains. The maximum strain can reach 80% under an applied stress of 94.6 kPa, suggesting that the bioaerogels can support over 17,975 times their own weight without fracture, an ability that has rarely been observed in other porous bioaerogels. The maximum stress surpasses the values of many bioaerogel products like hemp bast fiber aerogels (0.45 kPa)¹⁷, cellulose nanofibrils aerogels (0.542 kPa)³⁸, and wood aerogel (4.9 kPa)³⁹ (Supplementary Table S6). What's more, the bioaerogels showcase terrific compression fatigue resistance, with stress-strain curves showing no distinct change after 1000 cycles. The energy loss coefficient (ΔU/U) decreases initially to 93.4% over the first 100 cycles and stabilizes at 91.5% after 900 cycles (Figure 4h). Moreover, the bioaerogels maintain 90.5% of their maximum stress and 90.1% of their elastic modulus even after 1000 compression cycles. This proves their high compression resistance and superelastic structure, attributed to the distinctive spring-shaped strips (Figure 4i).

Multilevel pore structure and ultra-broadband perfect acoustic absorption

Compared to the Type II adsorption isotherms of the feedstock MFs-S₂, which is feature of macroporous solids⁴⁰, the bioaerogels exhibit Type IV adsorption isotherms, featured with three stages: (1) an initial fast N₂ uptake from micropores; (2) a distinct hysteresis loop resulting from mesopore capillary condensation; (3) the absence of a plateau near P/P₀=1.0 due to macropores. The Type H4 hysteresis loop originates from the compact stacking of assembled fiber sheets during the directional freezing. The well-crosslinked MFs-S₂-GA-PVA-3 bioaerogels achieve the largest BET surface area of 605.3 m² g^‒1, 49.0, 0.76, and 0.08 times higher than those of MFs-S₂, MFs-S₂-GA-PVA-1, and -5, respectively. The BJH average pore diameter distinctly decreases from 28.1 nm (MFs-S₂) to 3.4~4.3 nm (bioaerogels) (Figure 5b and Supplementary Table S7), indicating the improved proportion of micropores and mesopores in the bioaerogels.

Mercury porosimetry is used to study the macroporosity. There are distinct compression stages in the uptake curves except for MF-S₂ (Figure 5c), revealing the elastic structure of the bioaerogels⁴¹. The bioaerogels inherits widespread macropores from the MF-S₂ and gain a cumulative pore diameter reaching 400 μm, with a porosity of up to 97.84% (Supplementary Figure S13). Additionally, the bioaerogels achieve 7.2‒9.9 times greater macropore volumes. The bioaerogels feature a hierarchical pore structure characterized by an extremely broad pore diameter distribution ranging from 4.6×10^‒4 to 400 μm (Six orders of magnitude). This unique pore structure is expected to facilitate the achievement of ultra-broadband perfect acoustic absorption.

Given their ordered microstructure, outstanding mechanical characters, and sustainable components, the MFs-S₂-GA-PVA bioaerogels hold immense promise in noise absorption. For all bioaerogels, the SAC shows a direct correlation with thickness (Figures 5d-f), attributed to the extended path of sound propagation⁴². Particularly, there is a prominent increase in SAC at low frequency range (63‒1000 Hz), for the 30 mm thick MFs-S₂-GA-PVA-3 bioaerogels. In this case, the SAC rapidly reaches 0.998 from 63 to 600 Hz and consistently retains between 0.95 and 1 across the 520‒6300 Hz range. This result proves the attainment of approximate ultra-broadband perfect sound absorption abilities. This value is more superior to that of its isotropic counterpart and a commercial sound absorbing cotton (SAC<0.76, Supplementary Figures S15-S16). The sound absorption properties are further studied by the noise reduction coefficient (NRC), the average of SAC data at 250, 500, 1000, and 2000 Hz, which reflects the average level across the entire frequency range. The MFs-S₂-GA-PVA-3 bioaerogels (thickness: 30 mm), reach the pinnacle with a maximum NRC of 0.82 (Figure 5g). This remarkable NRC value surpasses that of the majority of sound absorption materials with comparable thickness, setting a new record in the field. These sound absorbing materials are divided into: (1) commercial products, (2) conventional biomass materials, (3) aerogel materials and (4) other novel materials, all of which show NRC ranging from 0.14 to 0.77 (Figure 5h and Supplementary Table S8). More importantly, upon subjected to extreme temperatures (–60 or 60 ºC) for three months, the SAC plots of the bioaerogels change slightly (Figure 5i). The exceptional stability of the bioaerogels' sound absorption property at extreme temperatures may expand their serviceable range in tropical or cold regions.

To assess the practical feasibility of the bioaerogels, we study the use of MFs-S₂-GA-PVA-3 bioaerogels as sound absorption walls in wooden houses to reduce traffic noise (Figure 5j and Videos S3). Using wooden walls only slightly decreased the noise from 75 dB to 71 dB (Figure 5k). Conversely, using bioaerogel walls significantly reduced the noise level to 32 dB (Figure 5l), meeting the IOS standards for an optimal rest and sleep environment. Besides, bioaerogels as indoor wall materials demonstrate excellent biocompatibility, with a high relative growth rate of 98.2% for HaCaT cells⁴³ (Figures S17-S18 and the Supplementary Discussion). We also investigate the use of bioaerogels for military purposes, specifically reducing loud noise generated by submarines. Using bioaerogels as sound absorption walls in the vessel effectively reduces underwater noise levels by 53.6% to a low level of 52 dB (Figures 5m-o and Videos S4-S5), showcasing its potential as viable sound-absorbing walls for underwater crafts.

A simulation study of material-structure-property relationship is undertaken through the Johnson-Champoux-Allard (JCA) semi-phenomenological model^44,45. The JCA model explains that noise absorption is due to viscous and temperature effects, represented by equivalent density ρ(ω) and equivalent bulk modulus K(ω), respectively. Determining ρ(ω) and K(ω) requires thickness (t) and five indispensable non-acoustical parameters: tortuosity (α_∞), airflow resistivity (σ), porosity (ϕ), viscous characteristic length (Λ), and thermal characteristic length (Λ′). Experimental measurements of these parameters and their mathematical relations with ρ(ω) and K(ω) are detailed in Supplementary Table S9 and the Supplementary Discussion.

The surface impedance (Z_s) is calculated as:

$${Z_{\text{s}}}=\frac{{ - {\text{i}}\sqrt {\rho (\omega )K(\omega )} \cot (\omega \sqrt {\frac{{\rho (\omega )}}{{K(\omega )}}} t)}}{\phi }$$

where ω = 2πf is the angle frequency and f is the frequency of soundwaves entering the bioaerogel. Finally, the SAC (α) is calculated as:

$$\alpha =1 - {\left| {\frac{{{Z_{\text{s}}} - {\rho _0}{c_0}}}{{{Z_{\text{s}}}+{\rho _0}{c_0}}}} \right|^2}$$

where c₀ is the speed of sound in air and ρ₀ is the density of air. Therefore, the function α = f(frequency) can be derived from these calculations.

As depicted in Fig. 6a, the JCA model reveals remarkable accuracy in simulating the SAC data of the bioaerogels across the entire frequency range. The MFs-S₂-GA-PVA-3 bioaerogels gain the maximum ρ(ω) among the three bioaerogels due their largest σ, α_∞, and shortest Λ (Fig. 6b and Table S9) according to the Supplementary equation S17, indicating greater viscous effects between air and pore walls (Fig. 6b and Table S9). The highest K(ω) of MFs-S₂-GA-PVA-3 bioaerogels is mainly due to the longest Λ′ (Fig. 6c) based on the Supplementary equation S20. Additionally, in terms of Z_s in the low frequency range, the MFs-S₂-GA-PVA-3 bioaerogels outperform the other two samples, demonstrating their superior ability to absorb low-frequency noise (Fig. 6d).

The SAC (α) can be expressed in relation to Z_s⁴⁶:

$$\alpha {\text{=}}\frac{{{\text{4}}\operatorname{Re} ({Z_{\text{s}}}/{\rho _{\text{0}}}{c_{\text{0}}})}}{{{{[\operatorname{Re} ({Z_{\text{s}}}/{\rho _{\text{0}}}{c_{\text{0}}})+1]}^2}+{{[\operatorname{Im} ({Z_{\text{s}}}/{\rho _{\text{0}}}{c_{\text{0}}})]}^2}}}$$

Notably, the α attains its maximum value (α = 1) when Re(Z_s/ρ₀c₀) = 1 and Im(Z_s/ρ₀c₀) = 0. Generally, the magnitudes of the SAC of bioaerogels are closely linked to their proximity to the point (1, 0)⁴⁸. Specifically, the MFs-S₂-GA-PVA-3 bioaerogels, with its smaller real part of Z_s, exhibits a plot that is situated closest to the (1, 0) point and thus yields highest SAC compared to the MFs-S₂-GA-PVA-1 and − 5 bioaerogels (Fig. 6e) and the isotropic bioaerogels (Supplementary Figure S19).

Based on the JCA model analysis, three crucial factors are identified as contributing to the ultra-broadband perfect acoustic absorption for MFs-S₂-GA-PVA-3 bioaerogels:

(1) The interpenetrating 3D webs in the isotropic bioaerogels, characterized by higher σ (17830 Pa s m^‒2) and a shorter Λ′ (Table S9), causes certain soundwaves to either reflect or escape (Figure S20)⁴⁷. Conversely, the anisotropic bioaerogels comprise of parallelly-layered micron-channels with a large length-to-height ratio and abundant interlamellar spring-shaped strips. This distinctive structure allows for more efficient propagation of sound waves along the channels and aids in creating plenty of closed loops to weaken sound energy via cyclic reflection‒friction‒dissipation (Fig. 6f), leading to the superb sound absorption. The effectiveness of the “GPC” strategy is validated by the moderate σ (1537 N s m^− 4) and long Λ′ (459 µm) for MFs-S₂-GA-PVA-3 bioaerogels (Table S9).

(2) The "GPC" strategy allows soundwaves to repeatedly reflect between the layers. The layers have a multilevel pore structure (including large-sized air bubble pores, middle-sized open window pores, and small-sized inter-wall bubbles and ice-templated pores), encompassing pore diameters spanning six orders of magnitude. This sophisticated pore arrangement effectively reduces soundwave leakage, increases frictional surface areas in contact with air, and synergistically captures different wavelengths of noises^11,48.

(3) The abundant presence of strips within the layers of MFs-S₂-GA-PVA-3 bioaerogels modifies the wave propagation path, leading to an elevated tortuosity from 1.01 and 1.08 (MFs-S₂-GA-PVA-1 and − 5) to 1.33 (MFs-S₂-GA-PVA-3). This modification elevates the σ from 1024 to 1537 N s m^− 4 and, thus increases the friction between the soundwaves and the fibers, causing more consumption of sound energy⁴⁹.

In summary, we innovatively propose a "GPC" strategy to construct novel bioaerogels, which display nearly perfect sound absorption properties, with a SAC of 0.95 ~ 1 across an ultra-broad frequency range of 520 ~ 6300 Hz, and an exceptional NRC of 0.82 (the highest recorded to date). The synergy of hierarchical ordered architecture, featuring anisotropic parallel-layered microchannels, sophisticated multilevel pores within each layer, and abundant strips bridging these adjacent layers, plays a crucial role in forming plentiful closed loops, fostering a repetitive cyclic reflection-friction-dissipation of soundwaves. More significantly, the sound absorption abilities keep virtually unaltered even when subjected to extreme temperatures (‒60 ~ 60 ºC) over 90 days. Moreover, the superelastic bioaerogels withstand strains of up to 80% and achieve a 90.5% retention of strength even after undergoing 1000 cycles. With their scalability and exceptional properties, the bioaerogels will hold huge promise as next-generation sustainable sound absorption materials in various applications, like traffic, industrial, and underwater vehicle noise mitigation.

Materials

The Pinus massoniana was collected in Hunan province, P.R. China. Sodium hydroxide (NaOH), hydrogen peroxide (H₂O₂), sodium metasilicate nonahydrate (Na₂SiO₃·9H₂O), magnesium sulfate (MgSO₄), diethylenetriaminepentaacetic acid (DTPA), polyethylene oxide (PEO, dispersant, M_w = 600,000), polyvinyl alcohol (PVA, enhancer, M_w ≈ 95,000 g mol^‒1), glutaraldehyde (GA, crosslinker, 25 wt.% in H₂O), and concentrated sulfuric acid (H₂SO₄, 98%) were purchased from Aladdin Reagent Co. Ltd., P.R. China. These chemicals were all directly used without further purification. Deionized water (DI) was used throughout the experiment.

Dissociation of MFs from the S ₂ sublayer of secondary wall of Pinus massoniana

The dissociation of MFs from the S₂ sublayer of P. massoniana's secondary wall was accomplished via a combined approach encompassing heat-moisture treatment, chemical impregnation, and mechanical refining, as detailed in our recent study⁵⁰. Briefly, after being cut and chopped into smaller pieces, the wood sample underwent a steam treatment at 105 ℃ for 15 minutes. The resulting small wood pieces were then fed into a TSMP twin-screw extruder (Jiangsu Jinwo Machinery Co., Ltd., China) operating at 70 ℃ and 1500 r/min for a defibrillation period of 15 min. Next, the alkaline hydrogen peroxide (AHP) solution was prepared by mixing NaOH, H₂O₂, Na₂SiO₃ 9H₂O, and DTPA in DI water at dosages of 3%, 5%, 2%, and 0.2%, respectively. The resulting solution was then mixed with the coarse-milled wood fibers at a solid-liquid ratio of 1:4 and stirred uniformly. The mixture was then placed into a KRK-2500 disc refiner (Kumagaya Riki Industry Co. Ltd., Japan) with a 0.1 ~ 0.5 mm gap at 3000 r/min and refined at 105 ℃ to enhance heat transfer and promote chemical permeation in the sample. Afterwards, the sample was incubated in a 95 ℃ oven for 1 hour and subjected to a second round of mechanical refining using the same disc refiner. The second refining was repeated four times. After the completion of the process, the product underwent a DI water wash to neutralize it and was then passed through a vibrating screen to eliminate any remaining undissociated fiber shives. The final outcome yielded dissociated wood MFs specifically obtained from the S₂ sublayer of the secondary wall, named as wood MFs-S₂.

To dissociate MFs from the intercellular layer of P. massoniana, the coarsely ground wood fibers were combined with an alkaline sodium sulfite solution at a solid-to-liquid ratio of 1:4. The alkaline sodium sulfite solution was prepared by mixing Na₂SO₃ and DTPA in DI water, in the amounts of 5% and 0.2%, respectively. The mixture was stirred evenly, and cooked in a 15-L rotating digester cooker at 70°C for 1 h. The mixture was then refined at 135 ℃ under high pressure in the disc refiner with a gap of 0.1 ~ 0.5 mm at 3000 r/min. Finally, the refined wood fibers were washed with DI water until neutral and subsequently passed through a vibrating screen to remove any undissociated fiber shives, and finally the wood MFs dissociated from the intercellular layer were obtained and labeled as wood MFs-ML.

As a control, we also dissociated MFs from the random cell wall layers of P. massoniana. The only difference in the dissociation process is the use of water instead of the alkaline sodium sulfite solution that was used to prepare the wood MFs-ML. The remaining steps of the process were kept the same. The obtained sample was labeled as wood MFs-RL.

Fabrication of wood MFs-GA-PVA hydrogels

The as-prepared wood MFs were blended with a 0.08 wt.% PEO solution in specific proportions (1 wt.%, 3 wt.%, and 5 wt.%) using a high-speed homogenizer (RCD-1A, Jiangsu Dongpeng Instrument Manufacturing Co. Ltd., China) at a rotational speed of 8000‒10000 rpm for 2.5 hours. The PVA solution was prepared by dissolving 5.0 g of PVA in 100 mL of DI water at 95°C for 4 hours until the PVA was completely dissolved.

Next, 1 g of MFs dispersion was mixed with 3 mL of the prepared PVA solution. The pH of the mixture was adjusted to 4‒6 by adding 0.8 mL of 1 V% H₂SO₄. A crosslinking agent GA solution (0.8 mL, 25 wt.%) was then added to the mixture, which was stirred for 6 hours at room temperature. The solution was then placed in a vacuum oven for 30 min to remove any residual air bubbles. The resulting mixture was transferred to a square polytetrafluoroethylene (PTFE) mold and crosslinked at 85 ℃ for 3 hours in an oven. After that, the wood MFs-GA-PVA hydrogels were obtained.

Fabrication of hierarchical ordered architecture of superelastic bioaerogels

The prepared wood MFs-GA-PVA hydrogel precursor underwent a directional-freezing process by placing it onto a Cu-plate directional-freezing table that had been pre-cooled in liquid nitrogen. The microstructure of the hydrogel was precisely tuned by regulating the proportion of MFs. Freeze-drying at − 60°C (with pressure < 10 Pa) was adopted to obtain the superelastic bioaerogels. It is worth noting that the intact wood MFs aerogels cannot be effectively prepared by using the same experimental steps because of their significant dimensional shrinkage and weak mechanical properties.

Material characterization

The micromorphology was studied by SEM (Zeiss Sigma). XRD patterns were acquired with a Bruker D8 Advance TXS X-ray diffractometer at 40 kV and 90 mA with Cu Kα (target) radiation (λ = 1.5418 Å) at a scan rate (2θ) of 4° min^‒1 in the scan range of 5 to 40°. SAXS experiments were conducted on a Xeuss 2.0 small angle X-ray scatterometer equipped with MetalJet-D2 X-ray source. The wavelength of incident X-rays was λ = 0.134 nm, and the sample-to-detector distance was 2521.81 mm. FTIR spectra were recorded on a Thermo Scientific Nicolet iS20 spectrometer (Thermo Scientific) at a resolution of 4 cm^‒1. XPS analysis was used to study the surface chemical compositions by using a Thermo Scientific K-Alpha X-ray photoelectron spectrometer. The thermal stability was determined by using a TG analyzer (TA, Q600) from room temperature to 800°C at the heating rate of 10°C min^‒1 under N₂ atmosphere. The macropore volume and porosity of samples were measured by mercury injection method (Micromeritics Auto Pore IV-9500). Brunauer–Emmett–Teller (BET) adsorption-desorption isotherms were determined through N₂ adsorption tests at 77 K using a Quantachrome Autosorb-IQ gas adsorption analyzer. Mechanical compression tests were conducted by using a CMT6103 universal testing machine (Jinan Chenxin Testing Machine Manufacturing Co. Ltd., China) with a 50 N load cell. The loading-unloading rate was set to 5 mm min^− 1 during stress–strain tests and 20 mm min^− 1 during cyclic compression fatigue tests. Raman spectra were acquired with a Confocal Raman Imaging system (WITec alpha 300) equipped with a confocal microscope and motorized scan stage. Chemical images were formed using a sum filter by integrating over defined wavenumber layers in the spectrum. Average spectra from these layers of interest were extracted and baseline-corrected at 1800 cm^− 1 and 2200 cm^− 1 for detailed analysis. The sound absorption properties were measured by transfer function method using impedance tubes (SW477, SW422, BSWA Technology Co. Ltd.) in the frequency range of 63‒6300 Hz based on the test criteria of ISO 10534-2:1998 (Supplementary Figure S14 and the Supplementary Discussion). Three samples were tested in each group; each sample was rotated twice. Six replicate measurements were conducted, and the average values were reported.

Data Availability

Data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

This work is supported by the National Natural Science Foundation of China (Grant No. 31890771), the Hunan Provincial Technical Innovation Platform and Talent Program in Science and Technology (Grant No. 2020RC3041), the Hunan Provincial Natural Science Foundation of China (Grant No. 2022JJ30079), the Training Program for Excellent Young Innovators of Changsha (Grant No. kq2106056), the Young Elite Scientists Sponsorship Program by CAST (Grant No. 2019QNRC001), and the China Postdoctoral Science Foundation (Grant Nos. 2020M672846 and 2022T150556).

Author Information

Contributions

C.W. and Y.W. initiated and designed the research. C.W., H.C., and K.S. designed and carried out the experiments; H.C., C.W., X.L., Y.Y., J.Y., W.C., Z.Z, Y.X., J.S., and S.W. analyzed data. C.W., Z.Y., H.C., J.Y., and Y.Y. provided the theory framework; C.W. and H.C. wrote the manuscript. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Caichao Wan or Yiqiang Wu.

Ethics Declarations

Competing interests

The authors declare no competing financial interests.

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Graphicalabstract.tif
Graphical abstract
VideoS1Schematicdiagramforthedissociationofmicrofibersfromthemiddlelamellaofwoodcellwall.mp4
SUPPLEMENTARY VIDEO S1
VideoS2SchematicdiagramforthedissociationofmicrofibersfromtheS2sublayerofwoodcellwall.mp4
SUPPLEMENTARY VIDEO S2
VideoS3Trafficnoisereductionexperiment.mp4
SUPPLEMENTARY VIDEO S3
VideoS4Underwaternoiseexperimentacomparativestudywithoutbioaerogelwalls.mp4
SUPPLEMENTARY VIDEO S4
VideoS5Underwaternoiseexperimentacomparativestudywithbioaerogelwalls.mp4
SUPPLEMENTARY VIDEO S5
SupplementaryInformationforNature.docx
SUPPLEMENTARY INFORMATION Supplementary Figures S1-S20; Tables S1-S9; Videos S1-S5; Supplementary Discussion.

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Bioaerogels for Ultra-Broadband Perfect Acoustic Absorption

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Fabrication of wood MFs-GA-PVA hydrogels

Fabrication of hierarchical ordered architecture of superelastic bioaerogels

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