Unprecedented mechanical wave energy absorption observed in multifunctional bioinspired architected metamaterials

The study addresses the need for lightweight materials that can simultaneously absorb sound (acoustic waves) and stress (impact) wave energy, a pressing requirement in applications such as aerospace where liners must both attenuate noise and resist local impacts. Traditional approaches often combine separate materials, increasing weight and complexity. Bioinspired and additively manufactured metamaterials offer a route to integrate multiple functions. Prior work on cuttlebone-inspired lattices emphasized sinusoidal undulating walls to enhance strength, but largely overlooked two key natural features: (1) the multilayered wall–septa architecture with porous plates that could enable acoustic dissipation and hybrid resonance; and (2) the asymmetric, vertically growing camber of cell walls that may enhance mechanical damage tolerance. The authors propose a weakly-coupled design that decouples acoustic resonators (governing sound absorption) from mechanical load-bearing cambered walls (governing stress wave energy absorption), aiming to overcome the typical trade-off between broadband absorption and high strength.

The paper surveys advances in sound-absorbing materials and metamaterials, including perforated panels, foams, fabrics, sound-absorbing metamaterials, aerogels, graphene foams, and microlattices. While these approaches can offer strong acoustic performance, achieving both broad-spectrum absorption and high mechanical strength remains difficult. Recent microlattice designs struggle to balance these properties. Prior cuttlebone-inspired designs primarily exploited horizontally undulating walls for strength, neglecting the multilayered wall–septa structure that could serve as cascaded resonant plates with dissipative pores for acoustic loss, and the asymmetric cambered wall morphology that can improve mechanical response. The authors position their work to leverage these overlooked biological features to achieve simultaneous acoustic and mechanical energy absorption.

Design principle: A weakly-coupled acousto-mechanical architecture is inspired by cuttlebone. Acoustic functionality is provided by multilayered resonant plates with dissipative micro-pores; mechanical functionality is provided by asymmetric cambered cell walls. The two subsystems are designed to minimally interfere: pore geometries chiefly impact acoustic impedance with negligible effect on axial mechanical response; wall shapes define air-phase geometry with minimal effect on acoustic performance under isovolumetric constraints. Acoustic subsystem: Three cascaded resonant panels per unit cell are designed, each panel divided into two heterogeneous parallel parts (Part 1 and Part 2) with consistent pore size and arrangement per part across layers, yielding six acoustic segments. This creates a multimodal hybrid resonance system equivalent to two parallel branches of three series resonators. Pore diameter dp for each part/layer is optimized via a genetic algorithm using an impedance model. Other key geometric parameters include pore thickness t, cavity depth D (equal to unit height H0), and surface porosity. The sound absorption coefficient α is computed from the surface impedance Zs using α = 1 − |(Zs/Z0 − 1)|^2 / |(Zs/Z0 + 1)|^2, where Z0 = ρ0 c0. The air phase is governed by sinusoidal wall patterns in-plane and cambered profiles through thickness. Mechanical subsystem: Cell walls follow a horizontally sinusoidal pattern y = A0 sin(2πx/3) (with a period T = 3 mm in x) and a vertically cambered profile derived from a cuttlebone-inspired nonlinear relation between wall profile length P and height H: P = (0.998 + 0.002 e^{6.55(-0.11)}) P0, leading to gradual-to-sharp increases in curvature at higher H/H0. Four camber levels are studied: straight wall and A0 = 0.5, 1.0, 1.5. Wall thicknesses are adjusted across camber levels to keep relative density at 33%. The overall sample thickness ttotal is 21 mm; samples comprise 3×3×3 unit cells in x, y, z. Fabrication: Samples are additively manufactured by selective laser melting (SLM) of Ti6Al4V on an XDM 750 machine, in vacuum, powder size <53 µm. Process parameters: 500 W laser power, 60 µm layer thickness, 1200 mm/s scan speed. Post-processing includes wire-cutting from the substrate and powder removal. Material properties from tensile tests: Es = 104 GPa, σ0 = 864 MPa, ν = 0.3, ρs = 4.43 g/cm³. Acoustic measurements: Two-microphone impedance tube (SKC ZT13) per ISO 10534-2. Samples rigidly backed in a 30 mm diameter holder; cubic samples adapted with rigid polymeric adapters to fit the circular tube (adapter effects neglected). Effective test frequency range 0.8–6.3 kHz. Absorption coefficients averaged over three datasets. Mechanical testing: Quasi-static uniaxial compression on Shimadzu AG25-TB at strain rate 0.002 s⁻¹ along the SLM build direction, until densification. Deformation recorded by digital camera. Results reported as means with standard deviations across triplicates. Modeling: Finite element (FE) simulations in COMSOL Multiphysics. Acoustics modeled with Thermo-Viscous Acoustics Module using boundary layer theory, incident waves via pressure-acoustics, and PML for nonreflecting boundaries. Mechanical large-strain plastic deformation simulated on the multi-cell geometries; validation via comparison of stress–strain curves (details in Supplementary).

• Broadband, efficient acoustic absorption: Average absorption coefficient α = 0.80 over 1.0–6.0 kHz with a compact total thickness of 21 mm; 77% of measured data points exceed α = 0.75.
• Absorptance–thickness performance: A benchmark map indicates the MBAMs surpass state-of-the-art structured aerogels, flexible aerogels, phononic crystals, and various lattice-based absorbers in achieving high average α at low thickness.
• Mechanistic understanding: High-fidelity microstructure-based modeling identifies air friction (thermo-viscous) damping, with broadband response arising from multimodal hybrid resonance enabled by heterogeneous, cascaded resonators.
• Mechanical performance: Average elastic modulus ≈ 4.93 GPa; compressive strength ≈ 211 MPa; density ≈ 1.53 g/cm³; specific energy absorption (SEA) ≈ 50.7 J/g.
• Deformation mode control: Cambered cell walls shift failure from catastrophic to progressive deformation with stable stress plateaus, yielding ultrahigh SEA. The SEA represents a 558.4% increase compared to a straight-wall design at similar relative density.
• Weakly-coupled multifunctionality: Acoustic pore geometries minimally affect axial mechanical properties; cambered wall geometry governs mechanical energy absorption while maintaining isovolumetric air-phase shapes that minimally affect acoustics, enabling near-independent optimization.

The results demonstrate that a weakly-coupled bioinspired architecture can concurrently optimize acoustic and mechanical wave energy absorption, addressing the longstanding trade-off between absorptance and structural performance. Heterogeneous cascaded resonators create multiple hybrid resonance modes and leverage thermo-viscous losses to achieve high, broadband α within a thin 21 mm profile. Meanwhile, asymmetric cambered walls inspired by cuttlebone transform compressive response from brittle or catastrophic failure to progressive collapse with extended stress plateaus, markedly enhancing specific energy absorption. The minimal cross-influence between the acoustic and mechanical subsystems validates the design premise, allowing tailored pore patterns for acoustics and tailored camber for mechanics. Collectively, the findings position MBAMs as promising lightweight multifunctional materials for environments where both noise attenuation and impact mitigation are critical, such as aerospace liners and protective structures.

This work introduces multifunctional bioinspired architected metamaterials (MBAMs) that combine heterogeneous multilayered acoustic resonators with asymmetric cambered mechanical cell walls via a weakly-coupled design. The metamaterials achieve ultrahigh, broadband sound absorption (average α ≈ 0.80 from 1–6 kHz at 21 mm thickness) and exceptional mechanical energy absorption (SEA ≈ 50.7 J/g with progressive deformation), outperforming many existing thin absorbers on absorptance–thickness metrics. Modeling reveals air friction damping and multimodal hybrid resonance as the keys to broadband acoustics, while cambered walls drive damage-tolerant mechanics. The study proposes a research framework for acousto-mechanical metamaterials and broadens design strategies for multifunctional materials. Potential future directions include: extending the weakly-coupled design to other materials and scales; optimizing camber and pore distributions for targeted frequency bands and loading scenarios; exploring active or tunable elements to adapt acoustic/mechanical responses; evaluating durability under environmental extremes (temperature, humidity, corrosion) and high-rate impacts; and integrating design-for-manufacture and repair for large-scale applications.

• Measurement constraints: Acoustic tests were limited to 0.8–6.3 kHz by the small-diameter impedance tube; results outside this band were not assessed. The effects of rigid polymeric adapters used to fit cubic samples into the circular tube were neglected, potentially introducing minor uncertainty.
• Coupling not fully independent: Although designed as weakly coupled, the acoustic and mechanical subsystems are not completely independent; for example, excessively thick resonant plates could influence mechanical response.
• Sample scope: Demonstrations used Ti6Al4V SLM-fabricated samples of fixed total thickness (21 mm) and 3×3×3 unit cells; scalability, manufacturing variability, and behavior under different thicknesses or unit counts were not fully explored.
• Loading regime: Mechanical tests were quasi-static; high strain-rate impact behavior and fatigue/durability under cyclic or environmental loading were not reported.