Bioinspired Dry‐Steam Superinsulation Straw Foam

Abstract

Cellulosic materials offer sustainable advantages for building energy conservation. However, their development has been hindered by reduced thermal performance, often caused by structural collapse during the transition from solution to solid. Inspired by natural goose down, a bio‐based, lightweight insulation foam derived from agricultural waste straw is presented. Through in situ synthesis, bio‐silica fibers with branched structures capable of supporting hollow silica microspheres are fabricated. After steam‐mediated processing, the resulting foam exhibit low density (95 mg cm^{− 3}), high porosity (95.5%), low thermal conductivity (0.03 ± 0.003 W mK⁻¹), and a cyclic compressive strength of 90 kPa at 50% strain. Owing to the synergistic microstructure formed by branched bio‐fibers and hollow silica spheres, the bio‐silica foam exhibit outstanding thermal insulation performance relative to other bio‐based foams prepared by ambient drying. A passivated insulation panel is further developed by incorporating this material as the core component, achieving a thermal conductivity of 0.0275 W mk⁻¹ and flexural strength of 6.85 MPa. The panel demonstrated durability with stable thermal performance throughout a 60‐day outdoor test. Moreover, the bio‐silica foam shows a carbon footprint of 7.50 kgCO₂ kg⁻¹ at 70.2 wt.% silica, highlighting its promise as a sustainable insulation solution for green buildings.

Bioinspired lightweight cellulose–silica foams, featuring a synergistic microstructure formed by branched biofibers and hollow silica spheres, exhibit both carbon‐sequestration and superinsulation capabilities. These multifunctional materials present a scalable and practical solution for energy‐efficient, low‐carbon, and sustainable building applications.

1. Introduction

The growing urgency of climate change and environmental concerns has placed significant pressure on the construction industry to transform. Recent statistics reveal that this sector accounts for nearly 40% of global energy consumption and contributes approximately 36% of worldwide carbon emissions.^{[ 1} , ^{2 ]} In particular, building insulation systems are pivotal to energy efficiency, prompting a fresh look at the sustainability of traditional insulation materials. Synthetic polymer (including polystyrene, polyurethane, and polyethylene) and inorganic fibers (such as mineral and glass) foams dominate more than 95% of the global building insulation market.^{[ 3} , ⁴ , ^{5 ]} Despite their excellent thermal insulation properties and cost‐effectiveness, these materials come with a steep environmental toll.^{[ 6 ]} Their production heavily relies on nonrenewable petroleum resources and generates significant greenhouse gas emissions. Moreover, their poor biodegradability not only leads to environmental pollution but also poses risks as they may release harmful substances during use and disposal.^{[ 7} , ^{8 ]} With low recycling rates, most of these foams end up in landfills, further deepening environmental concerns.

Against this backdrop, bio‐based foams derived from renewable resources such as cellulose,^{[ 9 ]} starch,^{[ 10 ]} and date palm pit ^{[ 11 ]} have attracted increasing attention due to their renewability, biodegradability, and reduced environmental impact. Sourced from renewable resources, cellulosic fibers boast a carbon footprint that is 58–72% lower than that of synthetic fibers, and they naturally biodegrade.^{[ 12 ]} The cellulosic fiber foams form open‐pore structures via 3D fiber networks, achieving a low density (typically below 200 kg m^{− 3}) while maintaining high specific strength).^{[ 13} , ^{14 ]} However, the very open structure that confers many benefits also presents challenges for thermal insulation. On one hand, the free flow of air within the pores enhances convective heat transfer; on the other, the interconnected cellulose fiber network facilitates solid‐state heat conduction. The combination of these factors makes it difficult for cellulose fiber foams to achieve the low thermal conductivity required for efficient insulation.^{[ 15 ]}

Furthermore, the manufacturing process of cellulosic fiber foams markedly differs from that of conventional synthetic polymer foams. While synthetic foams are typically produced through melt in high‐temperature and molded into shape,^{[ 16 ]} cellulosic fiber foams usually undergo a phase transition from a dispersed state to a solid form in an aqueous environment.^{[ 17 ]} To preserve their porous structure and thus their thermal insulation properties, dry methods like freeze‐drying and supercritical drying are commonly employed.^{[ 18 ]} These techniques effectively prevent structural collapse during the phase transition but come with high costs, complex processing requirements, and, in some cases, the need for organic solvents. These drawbacks have significantly hindered their large‐scale adoption in building insulation. In contrast, air drying presents a more economically attractive alternative due to its high production efficiency, low energy consumption, mild operating conditions, ease of use, and scalability.^{[ 19} , ^{20 ]} During ambient drying, the surface tension of water induces capillary forces that cause the fibers to clump together. This aggregation leads to the collapse and densification of the cellulosic fiber network, resulting in a high‐density structure that in turn increases thermal conductivity, significantly limiting the practical development of cellulosic fiber foams.

2. Result and Discussion

2.1. Bioinspired Design of Insulation Structure

Nature provides inspiration for addressing these challenges through materials like goose down, which exhibits exceptional thermal insulation properties through its unique hierarchical structure. The multi‐scale hierarchical structures play a crucial role in suppressing heat transfer. This is achieved through three key mechanisms: controlling air cavity dimensions to inhibit convective heat transfer, minimizing solid‐phase contact points to reduce thermal conduction, and enhancing surface scattering to diminish radiative heat transfer (Figure 1A).^{[ 21 ]} Goose down features a hierarchical structure consisting of a central fiber with numerous branched microfibers, creating an intricate 3D network (Figure 1B). The scanning electron microscopy (SEM) image reveals that each branched fiber possesses multiple branches, which helps create numerous microscopic air pockets. This unique structural organization serves as the foundation for its air‐trapping efficiency while maintaining a lightweight structure, inspiring our design and preparation of synthetic insulation fibers. The branched architecture simultaneously fulfills multiple functional requirements: maximizing static air entrapment, minimizing heat conduction pathways, and maintaining structural integrity. The densely interconnected network of fibers synergistically provides mechanical stability and effectively traps a large volume of static air within its structure. When heat attempts to transfer through the material, it encounters multiple air‐fiber interfaces, where thermal energy is reflected and scattered due to interfacial thermal resistance. This phenomenon, combined with the predominantly air‐filled structure that minimizes solid conduction pathways, results in superior thermal insulation performance, making it an excellent model for lightweight yet thermally efficient designs. The superior thermal insulation of goose down stems from its sophisticated structural design: branched fibers radiating from a central point create numerous air pockets, while the 3D network of these fibers restricts air convection. The trapped air, with its low thermal conductivity (0.026 W mK⁻¹), significantly reduces heat transfer. Additionally, the elastic recovery properties of down fibers maintain the stability of this porous structure.^{[ 22} , ^{23 ]}

Figure 1.

Biomimetic design and fabrication of bio‐silica foam (A) Schematic illustration of thermal insulation principles of the branched goose down (B) Natural goose down structure and its hierarchical morphology. C) Transformation of agricultural waste straw into bio‐silica fiber through alkaline pressure treatment and in situ sol‐gel synthesis. SEM image reveals a branched structure decorated with silica spheres. D) Steam‐mediated foam processing: schematic illustration of branched bio‐silica fiber, compression‐molded foam before steam treatment, and after drying process. E–H) Characterization of bio‐silica foam properties: (E) ultra‐lightweight property, (F) thermal insulation performance verified by thermal imaging, (G) cyclic elasticity and (H) large‐scale panel fabrication (12 × 12 × 1 inch).

Inspired by the hierarchical branched structure of natural goose down, this work proposes an organic‐inorganic hybridization strategy to construct downy bio‐silica fibers by in situ generating hollow silica spheres within branched cellulose scaffolds derived from agricultural straw (Figure 1C). The SEM analysis confirms that these engineered fibers indicated the branched architecture, with silica spheres effectively entangled along the fiber branches. The key innovation lies in steam‐assisted molding, which precisely controls moisture to promote hydrogen bonding and prevent structural collapse during drying (Figure 1D). Through such a low‐emission biomimetic strategy, the resulting bio‐silica foams exhibit ultralight density, superior thermal insulation, and excellent mechanical stability (Figure 1E–G). Furthermore, the successful fabrication of large‐area panels demonstrates their potential for scalable application in sustainable building insulation (Figure 1H).

2.2. Morphological Characterization and Structural Analysis

In this study, we utilized rice straw as the primary raw material for the manufacturing of bio‐silica foam, which represents an abundant renewable natural resource predominantly composed of lignin, cellulose, and hemicellulose. Here, straw foam and biofiber foam serve as control samples, with the former produced from mechanically crushed raw straw fibers and the latter manufactured using alkaline‐treated branched biofibers. The bio‐silica foam is fabricated from biofibers containing hollow silica spheres. All these foams are prepared through a dry‐steaming process. As illustrated in Figure S1A (Supporting Information), the fundamental structural organization of rice straw consists of fiber bundles that are aggregated by hollow fiber cells. The straw foam fabricated from unmodified rice straw exhibits a characteristic brown coloration (Figure 2A). Scanning electron microscopy (SEM) analysis reveals an interconnected skeletal network formed by the interwoven fiber bundles. The fiber cell walls display relatively smooth surfaces with parallel‐aligned cellulose microfibrils visible on their surfaces. These microfibrils are densely encapsulated by lignin, resulting in a compact structural configuration. Following alkaline treatment, lignin undergoes a chemical reaction with the alkali and is subsequently removed from the cell walls. This delignification process substantially alters the morphological and structural characteristics of the natural straw. The originally compact fiber bundles undergo dissociation, where individual fibers separate from the bundle structure to form biofibers with branching structures. Each fiber exhibits a certain degree of flexibility, displaying a naturally curved morphology rather than a rigid linear arrangement (Figure S1B, Supporting Information). This morphological change indicates that the treatment process of the material effectively breaks the tight bond between the fibers in the fiber bundle, allowing single fibers to unfold and rearrange freely. The foam manufactured from these biofibers displays a beige coloration, attributed to the removal of brown lignin components (Figure 2B). SEM images revealed that the material possesses a distinctive 3D network architecture, characterized by flexible fibers interconnected through well‐defined void spaces, forming a hierarchical porous framework. At the fiber intersection points, stable junction nodes are established, significantly enhancing the mechanical integrity and structural stability of the network. Furthermore, cellulose microfibrils are prominently exposed from the cell structure, displaying flexible “tentacle” configurations. These features not only increase the material's specific surface area but also provide additional active sites for subsequent functionalization and modifications. By further introducing biofiber into the silica sol‐gel^{[ 24} , ^{25 ]}system, hollow spherical silica structures are in situ formed and embedded within the biofiber framework. This hybridization process not only reinforces the mechanical integrity of the biofiber network but also facilitates the construction of a hierarchically porous structure. The bio‐silica foam, composed of biofibers with hollow silica, reveals a creamy white appearance (Figure 2C). SEM observations indicate that silica particles occupy the network voids between fibers, enhancing structural stability. The branching microfibrils emanating from the fibers effectively anchor spherical silica particles, which possess a hollow structure (Figure S2, Supporting Information). These hollow silica particles contribute to enhanced porosity and thermal performance of the bio‐silica foam. Figure 2D represents the Energy‐Dispersive X‐ray Spectroscopy (EDS) mapping of the bio‐silica foam, illustrating that the bio‐silica foam contained large amounts of C from the biofiber, and significant amounts of Si and O from the generated silica. The results show that silica particles are evenly distributed in the material network and are well combined with biofibers. The successful removal of lignin and synthesis of hollow nanoparticles within the biofiber were further investigated. The X‐ray diffraction (XRD) (Figure 2E) analysis reveals distinct structural transformations during the Bio‐silica processing. The diffraction pattern of straw exhibits characteristic peaks at 15.6° and 22.1°, corresponding to the (101) and (002) crystalline planes of cellulose. After alkaline treatment, the crystalline structure of cellulose undergoes a transformation, leading to the disappearance of the 22.1° peak in biofiber. In contrast, pure silica displays a broad diffraction peak centered around 23°, characteristic of its amorphous silica (SiO₂) structure, with no discernible crystalline features. Notably, the XRD pattern of bio‐silica closely resembles that of pure silica, with the reappearance of a broad peak near 23°, confirming the successful incorporation of silica into the biofiber matrix and the formation of a biofiber‐SiO₂ composite structure. Figure 2F exhibits the Fourier‐transform infrared spectroscopy (FTIR) spectrum of origin straw fibers, biofiber, and bio‐silica. The FTIR analysis of the biofiber showed the absence of the characteristic peaks at 1597, 1504, and 1460 cm⁻¹, which are associated with aromatic skeletal vibrations, confirming the removal of lignin. From the FTIR spectrum of biofiber, characteristic peaks were observed at 2971, 1272, and 781 cm⁻¹, corresponding to the structural units Si‐CH₃, C‐Si‐O, and O‐Si‐O in bio‐silica formed by the polycondensation of TEOS. Si‐O‐C vibration absorption generally appears as a broad peak at 950–1250 cm⁻¹, the broad peak at 1051 cm⁻¹ could be assigned to the Si‐O‐C vibration in the hollow silica. Additionally, the decreased intensity of the characteristic peak at 3447 cm⁻¹ in bio‐silica, associated with ─OH stretching vibration, may result from the reaction between hydroxyl groups on the cellulose of biofiber and hydroxyl groups generated by the hydrolysis of TEOS, leading to the formation of Si─O─C bonds.^{[ 26 ]}

Figure 2.

Morphological characterization and structural analysis of straw foam, biofiber foam, and bio‐silica foam. A) Optical photograph and SEM images of straw foam at different magnifications showing fiber bundles and fibrils structure. B) Optical photograph and SEM images of biofiber foam demonstrating the network structure and separated fibers at different magnifications. C) Optical photograph and SEM images of bio‐silica foam revealing the hierarchical structure with hollow silica particles at different magnifications. D) SEM and EDS mapping of the bio‐silica foam structure. E) XRD patterns of straw, biofiber, and bio‐silica showing characteristic crystalline peaks. F) FTIR spectra of straw, biofiber, and bio‐silica demonstrating their chemical compositions.

2.3. Thermal Insulation Properties

Thermal insulation is the crucial index for energy‐effective building applications of insulation foam. Figure 3A compares the thermal conductivity of straw foam, biofiber foam, and bio‐silica foam. Experimental results demonstrate that straw foam exhibits the highest thermal conductivity (0.0385 ± 0.0005 W mK⁻¹). Through modification of the straw foam, the thermal conductivity of Biofiber foam decreased to 0.0356 ± 0.0007 W mK⁻¹, indicating that fiber dispersion and optimization of the network structure enhanced thermal performance. The reduction in thermal conductivity of biofiber foam can be attributed to both the removal of high‐conductivity lignin and the extended thermal conduction pathways created by the cell wall roughness. In bio‐silica foam, the thermal conductivity further decreased to ≈0.03 ± 0.003 W mK⁻¹, primarily due to the incorporation of hollow silica spheres. To investigate the effect of silica content on thermal conductivity, we prepared bio‐silica foams with different fiber‐to‐TEOS mass ratios (1:1, 1:3, 1:5, and 1:7), resulting in silica contents of 13.7, 33.4, 52.0, and 70.2 wt.%, respectively. Figure 3B analyzes the influence of silica content on foam thermal conductivity. As silica content increases, thermal conductivity shows a significant decreasing trend, declining from 0.0356 to 0.030 W mK⁻¹. This trend can be attributed to the hollow nature and uniform distribution of silica particles, which create additional air pockets within the material and effectively interrupt heat flow pathways. Figure S3 (Supporting Information) demonstrates the effect of varying silica content on the bio‐silica foam microstructural morphology. As the silica content increases (from 13.7% to 70.2%), significant changes in the surface morphology can be observed. At low silica content (13.7%), silica particles can be observed around the branches of biofibers. With increasing silica content, the agglomerates formed by silica particles gradually grow in the area and progressively fill the pores within the biofiber scaffold. When the silica content reaches 70.2%, the extensive silica agglomerates almost completely fill the scaffold pores, resulting in a more compact overall structure. We further demonstrated the effect of silica content change on pore structure by quantifying porosity. Figure 3C illustrates the effect of silica content on bio‐silica foam porosity. As silica content increases from 0% to 70.2%, porosity rises from approximately 92.5% to over 95.5%. The enhanced porosity further improves the material's thermal insulation properties, as the porous structure effectively restricts both conductive and convective heat transfer. We employed infrared thermal imaging technology to visually assess the thermal insulation of bio‐silica foam. The bio‐silica foam (⌀2 cm × 1.5 cm) and EPS (⌀2 cm × 1.5 cm) samples were placed on a heating plate, and their heat conduction process was monitored in real‐time using a thermal IR image tester. As shown in Figure 3D and Movie S1 (Supporting Information), under the same heating conditions, bio‐silica foam maintains a more pronounced temperature gradient, with its upper region consistently maintaining lower temperatures (blue region), while EPS shows more extensive heat conduction (larger areas of red and purple regions) at the same time points. Particularly during the 30–90 s period, the upper region of bio‐silica foam maintains its low‐temperature state, while EPS exhibits more evident heat diffusion. During the 60–120 s period, bio‐silica foam continues to demonstrate excellent thermal resistance, maintaining a stable temperature gradient. Figure 3E shows the temperature‐time curves comparing the thermal insulation performance between bio‐silica foam and EPS foam. Bio‐silica foam demonstrates superior thermal insulation properties, exhibiting both a significantly lower temperature rise rate at the bottom surface and consistently lower temperatures at the top surface compared to EPS foam. These results indicate that bio‐silica foam more effectively impedes heat transfer, demonstrating exceptional thermal insulation characteristics. As shown in Figure 3F, the exceptional thermal insulation performance of Bio‐silica foam can be attributed to its unique hierarchical architecture, which comprises hollow silica spheres strategically anchored within a branched biofiber network. This sophisticated structure effectively impedes heat transfer through multiple synergistic mechanisms. The branched biofibers create extended and tortuous thermal pathways, while the hollow silica spheres introduce numerous air‐filled cavities, significantly reducing thermal conductivity. Furthermore, the interfaces between biofibers and silica particles create additional thermal resistance points. The hollow silica spheres also play a crucial role in radiation suppression by effectively scattering and reflecting thermal radiation, while their distributed arrangement throughout the network maximizes radiation interference. Simultaneously, the interconnected network structure of branched biofibers, combined with the isolated air pockets created by hollow silica spheres, effectively restricts convective heat transfer by minimizing air movement within the material. This multilevel thermal barrier system, incorporating conductive, radiative, and convective heat transfer impedance, results in the material's superior thermal insulation capabilities ^{[ 27 ]} Above all, we performed a comparative analysis of the thermal insulation properties of bio‐silica foam. As shown in Figure S4 (Supporting Information), compared with other biomass foams prepared by ambient drying, the bio‐silica foam in this work shows the lowest thermal conductivity, indicating superior thermal insulation performance.

Figure 3.

Thermal performance characterization and insulation mechanism of bio‐silica foam. A) Comparison of thermal conductivity among straw foam, biofiber foam, and bio‐silica foam. B) Effect of silica content on the thermal conductivity of bio‐silica foam. C) Relationship between porosity and silica content in bio‐silica foam. D) Digital photographs of bio‐silica foam and EPS foam (left), and infrared thermal images showing the temperature distribution at different time intervals (0, 30, 60, 90, and 120s). E) Temperature‐time curves of the top and bottom surfaces for both bio‐silica foam and EPS foam during the thermal insulation test. F) Schematic illustration of the thermal insulation mechanism in bio‐silica foam, showing the synergistic effects of hollow silica spheres and branched biofibers in impeding heat transfer through convection, conduction, and radiation pathways.

2.4. Mechanical Performance of Bio‐Silica Foam

Retaining the structural stability of thermal insulation materials is critical to endure a variety of external forces without being damaged. Benefiting from the stable physical entanglement structure, the bio‐silica foam exhibits elastic properties. In this study, the mechanical behavior of bio‐silica foam was investigated through cyclic compression testing. As shown in Figure 4A and = Movie S2 (Supporting Information), the mechanical compressive behavior of the bio‐silica foam was recorded through a full loading—unloading cycle. The foam underwent progressive deformation under vertical compression from its original shape (0% strain), to moderate deformation at 15% strain, and significant compression at 30% strain. Upon release of the load, the foam largely recovered its original shape, demonstrating excellent elastic resilience. Notably, the first and fourth images illustrate the foam's structural integrity before and after compression, indicating that the bio‐silica foam can withstand repeated stress without visible breaking. This sequential imaging clearly demonstrates the reversible deformation characteristics during compression and release, confirming the bio‐silica foam's elastic recovery capabilities. The compressive strength‐strain curves (Figure 4B) reveal the significant influence of silica content on the mechanical properties. All samples exhibited notable stress‐hardening behavior at high strains (60–80%). With the increase in silicon content, the maximum compressive strength of the material shows an obvious decreasing trend. In order to further understand the cyclic deformation mechanism of bio‐silica foam. To elucidate the cyclic deformation mechanisms, we examined the stress–strain relationships at various strain levels (10%, 30%, and 50%) with five loading–unloading cycles (Figure 4C). At 10% strain, the material exhibited low compressive stress (0.2 kPa) with quasi‐linear behavior and minimal hysteresis, indicating a predominantly elastic response. As strain increased to 30%, the maximum stress rose to 5 kPa, accompanied by emerging nonlinearity and enlarged hysteresis loops, suggesting enhanced energy dissipation. Under 50% strain, the stress increased substantially to 90 kPa, displaying pronounced nonlinearity and significantly expanded hysteresis loops, while maintaining recoverable deformation capacity. Multiple loading cycles demonstrated excellent repeatability, confirming the material's stable cyclic compression characteristics. Detailed analysis of the compression response within 10% strain (Figure 4D) revealed that while all compositions exhibited nonlinear stress–strain behavior with characteristic hysteresis loops, increasing silica content led to systematic reductions in both loop area and curve slope. This indicates diminishing energy dissipation capacity and reduced stiffness with higher silica content. The consistent results across multiple testing cycles further validate the reliability and stability of the material's cyclic compression properties.

Figure 4.

Mechanical performance of bio‐silica foam (A) Photographs showing the cyclic compression process of bio‐silica foam at different strain levels. B) Compressive strength‐strain curves of bio‐silica foams with different silica contents Figure (C) Cyclic compression stress–strain curves at different strains. D) Cyclic compression stress–strain curves (10% strain) for different silica contents.

2.5. Passivated Insulation Panel (PIP) Design, Outdoor Testing, and Environment Impact for Building Applications

To enhance the feasibility of bio‐silica foam for building thermal insulation, we designed a PIP using bio‐silica foam as the core material. The outdoor performance and environmental impact of the panel were thoroughly evaluated. Figure 5A illustrates the step‐by‐step process of creating a PIP panel (12 × 12 × 1 inch), utilizing bio‐silica foam as the core material and extruded polystyrene (XPS) as the frame. Initially, bio‐silica foam and XPS were assembled as the core insulation materials. These layers were arranged in a square format to combine the thermal insulating properties of both materials. Next, the core assembly was encased in a protective film, which served as a barrier against external elements while maintaining the integrity of the insulation structure. The assembly was then vacuum‐sealed to eliminate air and create a sealed environment. This PIP design effectively integrates the high strength of the XPS framework with the lightweight, soft properties of the core material, resulting in a stable, highly insulating composite panel. Figure 5B showcases a large‐scale PIP with dimensions of 105 mm × 35 mm × 5 mm. Its foldable design enhances flexibility for application and simplifies stacking (Figure S5, Supporting Information). Figure 5C demonstrates that the PIP achieves a lower thermal conductivity of 0.0275 W mk⁻¹) compared to bio‐silica foam (Thermal conductivity of 0.0300 W mk⁻¹), indicating superior thermal insulation. The vacuum sealing likely reduces air gaps, enhancing overall thermal resistance. Moreover, Figure 5D highlights the PIP's enhanced mechanical strength, exhibiting greater flexural strength (6.85 MPa), which improves rigidity. The aluminum foil covering effectively prevents powder leakage during bending (Figure S6, Supporting Information), ensuring the core material remains intact. The bio‐silica foam core is securely embedded within the XPS framework, maintaining its structure even under mechanical deformation. The vacuum‐sealing process further reinforces the panel's durability, making it more resistant to deformation. To evaluate the PIP's outdoor application for building insulation, we constructed a house model and subjected it to thermal insulation stability tests in outdoor conditions. As shown in Figure S7 (Supporting Information), the PIP and an EPS control panel were attached to the house model, and an IR camera was used to monitor thermal performance. The PIP exhibited lower surface temperatures than XPS, confirming its superior insulation properties. Over a 60‐day period, the PIP's R‐value demonstrated stable performance under outdoor conditions. Figure 5F illustrates the relationship between the carbon footprint (kgCO₂ kg⁻¹) and the weight percent of silica in bio‐silica foams. The data reveals a steady increase in carbon footprint with higher silica content, starting at ≈1.58 kgCO₂ kg⁻¹ for 0 wt.% silica and reaching 7.50 kgCO₂ kg⁻¹ at 70.2 wt.% silica. While the increase is gradual up to 33.4 wt.% silica, it becomes steeper between 52.0 and 70.2 wt.%. This suggests that silica integration significantly contributes to the overall carbon footprint, likely due to energy‐intensive production processes. Although silica enhances thermal insulation, balancing its content is critical to optimize performance while minimizing environmental impact.

Figure 5.

Passivated insulation panel (PIP) design, outdoor testing, and environment impact for building applications. A) Assembly process of the PIP, showing the integration of bio‐silica foam as the core material and XPS as the supporting frame. B) A photograph of the constructed large‐scale PIP (105 mm × 35 mm × 5 mm). C) Comparison of the R‐value between bio‐silica foam alone and the PIP. D) Comparison of the between bio‐silica foam with frame and the PIP. E) Stability of the PIP's R‐value over a 60‐day outdoor test, demonstrating consistent thermal insulation performance under environmental conditions. F) Carbon Footprint of the bio‐silica foam at weight percent silica.

3. Conclusion

In this study, we present a bio‐based thermal insulation material inspired by the structure of goose down. By combining chemical modification with in situ synthesis, agricultural waste straw was transformed into branched cellulose fibers supporting hollow silica microspheres, effectively enhancing air entrapment. A steam‐mediated drying strategy was introduced to preserve the porous architecture and prevent structural collapse., while simultaneously avoiding the complex manufacturing steps and prolonged drying times typically associated with aqueous processing. The resulting foam features low density (95 mg cm^{− 3}), high porosity (95.5%), and low thermal conductivity (0.03 W mK⁻¹), along with notable mechanical resilience. To enable practical use, a passivated insulation panel (PIP) was further fabricated, demonstrating improved strength (6.85 MPa), low thermal conductivity (0.0275 W mK⁻¹), and long‐term durability. This work provides a sustainable and scalable strategy for developing high‐performance insulation materials from agricultural waste.

4. Experimental Section

Materials

Straw was obtained from Amazon (unprocessed natural straw). Sodium hydroxide (NaOH, ≥ 98%), cetyltrimethylammonium bromide (CTAB, ≥ 99%), tetraethyl orthosilicate (TEOS, 98%), ammonium hydroxide solution (28–30% NH₃ basis), and mesitylene (≥ 98%) were purchased from Sigma–Aldrich and used as received without further purification.

Preparation of Biofiber

First, 50 g of NaOH was dissolved in 1 L of deionized water. Next, 50 g of natural wheat straw was mixed with NaOH to create a slurry. This slurry put in a pressure cooker to autoclaving for 1 h. Then the slurry was blended using a blender for 5 min. After this, the cellulosic slurry was filtered using a nylon mesh and washed with deionized water until a neutral pH was reached. Finally, the treated straw was in filtration to remove water.

Preparation of Bio‐Silica Fiber

To begin, 9.375 g of CTAB was dissolved in 1.5 L of water at 50 °C and stirred for 2 h until completely dissolved. Following this, 4.95 mL of ammonium hydroxide and 30.9 mL of mesitylene were introduced into the solution and stirred for an additional 30 min. Next, 50 g of treated biofibers were added to the mixture and blended for 30 s. Subsequently, TEOS was gradually added into the foaming slurry, followed by another 30 s of blending. The resulting mixture was then dried in an oven at 60 °C. The dried bio‐silica mixture was mechanically crushed using a blender and obtain uniformly dispersed bio‐silica fibers.

Preparation of Bio‐Silica Foam

The bio‐silica fibers were then packed into a mold, compressed into shape, and subjected to steam treatment in a steamer for 1 hour at atmospheric pressure. The steaming process was employed to adjust the moisture content of the bio‐silica fibers to ≈22 wt.% and to promote the formation of a preliminary hydrogen‐bonding network of bio‐silica foam. After steaming, the wet foams were carefully demolded and subsequently dried in an oven at 60 °C until reaching a constant weight.

Characterization

The microstructural morphology and chemical composition of the silica‐coated straw and composite were examined using Transmission Electron Microscopy (TEM, JEOL JEM 2100 LaB6), Scanning Electron Microscopy (SEM), and energy dispersive X‐ray analysis (EDX) (Hitachi SU‐70 FEG SEM). The bulk density (ρ_m​) of the composites was defined as the ratio of the mass (m) over the volume (V) of the composite, as shown in Equation (1):

$${\rho }_m=\frac{\mathrm{m}}{\mathrm{V}}$$ (1)

The porosity of the composite samples was determined using Equation (2):

$$\mathrm{Porosity}\left(\%\right)=\left(1-\frac{{\rho }_{\mathrm{m}}}{{\rho }_{\mathrm{s}}}\right)\times 100$$ (2)

where ρ_m is the bulk density and ρ_s is the skeletal density obtained using the pycnometer system (Ultrapyc 3000, Anton Paar). For each data point, three samples were tested. The thermal conductivity of the composite was measured using the Thermtest Heat Flow Meter 100 series (HFM‐100) following ASTM C518 standards. Before measurement, calibration was performed using the NIST SRM 1450e reference sample. For packaging the composite, the blended mixture was filled into an Extruded Polystyrene (XPS) frame, and then attached with a hot melt adhesive layer or aluminum foil on both sides. For the fully aluminum‐sealed composite, the heat‐pressed composite was placed in aluminum foil seal bags and sealed with a vacuum sealer. The compressive strength measurements were conducted on specimens (⌀2 cm × 1.5 cm) using a universal testing system (Instron 68SC‐05, United States) at a testing speed of 5 mm min⁻¹.

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

T.M. and S.R. contributed equally to this work. S.R. and T.M conceived the project. T.M. designed the experiment. T.M., L.Z., D.S., performed the experiments and data analysis. J.N.A. calculated the carbon footprint. T.T.M. drafted the manuscript. S.R. and T.M. revised the manuscript. All authors contributed to this work.

Supporting information

Supporting Information

Supplemental Movie 1

Supplemental Movie 2

Meng T., Zhu L., Stone D., Armstrong J. N., Ren S., Bioinspired Dry‐Steam Superinsulation Straw Foam. Small 2025, 21, 2503511. 10.1002/smll.202503511

Data Availability Statement

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