An all-natural bioinspired structural material for plastic replacement

The study addresses the urgent need to replace petroleum-based plastics, which raise significant environmental and health concerns from production to end-of-life. Although bio-based structural materials are promising, they often suffer from limited mechanical performance or complex, costly manufacturing that impedes scale-up. Drawing on biomimicry—specifically the brick-and-mortar architecture of nacre—the authors aim to design and manufacture a sustainable, high-performance structural material that reconciles strength and toughness while offering superior thermal stability. They introduce a simple, scalable “directional deforming assembly” approach to create an ordered brick-and-mortar composite from all-natural building blocks: cellulose nanofibers (CNF) as the 1D organic mortar and TiO2-coated mica microplatelets (TiO2-mica) as the 2D inorganic bricks, with interfacial chemistry engineered to enhance bonding and performance.

Biomimetic strategies have successfully improved structural materials by emulating hierarchical architectures found in nature. Nacre’s brick-and-mortar structure is a leading model for combining high strength and toughness under mild, environmentally friendly conditions (refs. 12–22). Prior efforts have demonstrated nacre-like hybrids and bioinspired ceramics with enhanced fracture resistance and stiffness, yet sustainable bio-based materials still often involve complex processing or lack mechanical robustness (refs. 9–11, 18–22). CNF is highlighted as a promising biopolymer matrix due to its high intrinsic strength, extremely low CTE, and rich surface chemistry for bonding (refs. 23–27). Mica-based platelets, including TiO2-coated mica, provide 2D inorganic building blocks with surface nanograin features akin to aragonite platelets in nacre and are commercially available with tunable coloration (ref. 28–29). The literature underpins the choice of building blocks and the nacre-mimetic design, while motivating the need for a simpler, scalable assembly route that yields superior multifunctional performance for plastic replacement.

Material design and assembly: The authors designed a nacre-mimetic, highly ordered brick-and-mortar architecture using 2D TiO2-coated mica microplatelets (bricks) and 1D cellulose nanofibers (CNF; mortar). To realize high orientation and uniform layering, they developed a one-step, scalable “directional deforming assembly” in which a composite hydrogel is pressed to dramatically reduce thickness while maintaining in-plane dimensions, aligning platelets and distributing CNF evenly between them. Interfacial engineering enhances bonding via APTES pretreatment of TiO2-mica and Ca2+ crosslinking of CNF. Fabrication protocol: All reagents were commercially sourced. A typical recipe: disperse 15 g TiO2-mica (KC100W) in 200 mL deionized water, add 10 mL (3-aminopropyl)triethoxysilane (APTES), and stir at room temperature for 1 day. Filter and wash the APTES-treated TiO2-mica three times with DI water. Mix the treated TiO2-mica with 500 mL TEMPO-oxidized CNF (3 wt%) and stir for 10 min to form a mixed hydrogel. Crosslink the hydrogel by spraying CaCl2 solution (150 mL, 0.5 mol L−1). Conduct directional deforming assembly by pressing: initially 1 MPa for ~12 h, followed by 100 MPa at 80 °C for ~1 h, yielding the dense bulk all-natural bioinspired structural material with density ~1.7 g cm−3. Characterization: Morphology and structure were examined via FE-SEM (Carl Zeiss Supra 40, 5 kV), TEM (Hitachi HT7700), AFM (Bruker Dimension FastScan), FT-IR (Bruker Vector-22), XRD (PANalytical X’pert PRO MRD, Cu Kα), and X-ray microtomography (Zeiss Xradia 520) with 3D reconstruction in Dragonfly. Mechanical testing: Three-point bending and single-edge notched bend (SENB) tests were performed on an Instron 5565A at room temperature with a 12.5 mm support span. Bending specimens: ~30 × 2 × 2 mm3, loading rate 1.0 mm min−1. SENB specimens: ~30 × 2 × 2 mm3, notch to ~W/2 using a ~300 µm diamond saw followed by razor sharpening; loading rate 1 µm s−1. Hardness measured by Shore D durometer. At least five replicates per material. Loading direction was perpendicular to platelet planes. Thermal measurements: Coefficient of thermal expansion (CTE) by NETZSCH TMA 402F3 from −130 to 150 °C; CTE measured parallel to platelet planes. Dynamic mechanical analysis (DMA) on TA Q800 in three-point bending, samples ~3 mm thick × 10 mm wide × 20 mm long, preload 0.05 N, from 30 to 190 °C; loading perpendicular to platelets; storage modulus and tan δ recorded. Thermal conductivity/diffusivity by HotDisk 2500s at 25 °C in the direction perpendicular to platelets. Calculations: Density ρ = mass/volume on cuboid samples. CTE = ΔL/(LΔT). Fracture toughness for crack initiation: KIC = (Pic·S)/(B·W3/2)·f(a/W); f(a/W) given by standard SENB geometry function. Maximum fracture toughness (KJC) from J-R curve: KJC = (Ja + Jpt)1/2·E′1/2, with E′ = E/(1 − ν2), Ja = KIC2/E′, and Jpt = 2Ap/[B(W − a)]. Controls and variables: Mechanical performance was compared across four conditions to isolate interfacial and structural effects: untreated (hydrogen bonding only), Ca2+ crosslinking only, APTES pretreatment only, and combined APTES + Ca2+ with directional deforming assembly (final material).

• The directional deforming assembly produced a nacre-like, highly ordered brick-and-mortar composite from all-natural CNF and TiO2-mica with density ~1.7 g cm−3.
• Mechanical performance (flexural tests): untreated composites showed ~83 MPa flexural strength and ~6 GPa modulus; after APTES pretreatment, Ca2+ crosslinking, and directional assembly, strength reached 281 MPa and modulus 20 GPa (>3× improvements).
• Fracture toughness: crack-initiation toughness KIC ≈ 6.7 MPa·m1/2; steady-state toughness KJC ≈ 11.5 MPa·m1/2, surpassing natural nacres (A. woodiana ~5.0 MPa·m1/2; P. margaritifera ~8.5 MPa·m1/2 for KJC).
• Multiscale extrinsic toughening mechanisms observed via 3D micro-CT and fractography: crack deflection, delamination, crack branching, multiple cracking, crack bridging (by CNF), platelet pull-out of TiO2-mica with frictional sliding facilitated by surface TiO2 nanograins, and strengthened platelet–polymer interfaces from APTES/Ca2+.
• Thermal properties: exceptional dimensional stability from −130 to 150 °C; CTE ~7 × 10−6 K−1 at 25 °C (>10× lower than typical plastics). Storage modulus remains ~20 GPa and nearly constant from 25–200 °C, while common plastics soften markedly and fully at ~250 °C.
• Thermal transport: higher thermal diffusion/conductivity than typical polymers, aiding heat dissipation.
• Processability and scalability: demonstrated large panels (310 × 300 × 18 mm3), tunable coloration using different mica pigments (e.g., TiO2-mica, Fe2O3-mica), and fabrication of functional parts (e.g., mobile phone shell).
• Overall, the material combines high strength, stiffness, and toughness with low CTE and high thermal stability, outperforming petroleum-based plastics on both mechanical and thermal metrics for structural applications.

The work demonstrates that a biomimetic, hierarchically ordered brick-and-mortar architecture, combined with targeted interfacial chemistry and an efficient directional deforming assembly process, resolves the typical trade-off between strength and toughness while providing superior thermal stability relative to plastics. The aligned TiO2-mica platelets facilitate efficient load transfer and stiffness; CNF provides a compliant yet strong mortar. APTES strengthens platelet–CNF bonding, while Ca2+ crosslinking reinforces the CNF network. The observed extrinsic toughening mechanisms—crack deflection, branching, bridging, delamination, and platelet pull-out with frictional sliding—redistribute stresses and dissipate energy, elevating KJC. Thermal performance (low CTE, stable modulus up to 200 °C, no softening at 250 °C) overcomes key limitations of plastics in variable or high-temperature environments. The scalable, simple processing route substantiates the feasibility of mass production and practical deployment as a structural plastic replacement, including in thermally demanding applications such as electronic device components.

A simple, scalable directional deforming assembly method enables bulk, all-natural nacre-mimetic composites from CNF and TiO2-mica with a unique combination of high flexural strength (281 MPa), high stiffness (20 GPa), high fracture toughness (KJC ≈ 11.5 MPa·m1/2), low CTE (~7 × 10−6 K−1), and excellent thermal stability. These properties collectively surpass those of common petroleum-based plastics, positioning the material as a sustainable, high-performance structural alternative. The approach supports mass production, color tunability, and good processability for real-world components. Future work can extend the method to other 1D/2D building blocks to tailor functionality and performance across broader application spaces.