Researchers from the Korea Research Institute of Chemical Technology have demonstrated a new class of shape‑configurable thermoacoustic loudspeakers that defy the traditional tradeoff between film thickness, flexibility, and acoustic output. Instead of relying on vibrating membranes, these devices use vertical graphene microstructures to convert electrical signals into sound through rapid heating and cooling, enabling loud, robust, and stretchable audio elements that can be integrated directly onto curved or deformable surfaces.
Fabrication, morphology, and structural features of patterned VrGO films using dual-laser patterning. (a) Schematic illustration of the dual-laser processing strategy for fabricating patterned VrGO TA loudspeakers and their working mechanism. (b,c) SEM images of CO2 -laser and pulsed-laser-irradiated GO films (scale bar: 500 μm). (d) SEM image showing vertically aligned rGO sheets in the VrGO structure (scale bar: 10 μm). (e) 100% stretched kirigami and (f) 3D-structured VrGO films fabricated via pulsed laser-based patterning. Image from: Advanced Science
A core challenge in thermoacoustic speakers is that high sound pressure levels (SPLs) typically require ultrathin conductive films, which are mechanically fragile, difficult to process at scale, and limited in power handling. Thicker films are more durable and easier to manufacture, but they trap heat within their bulk, suppressing thermoacoustic efficiency and causing SPL to collapse as thickness increases. This thickness–performance tradeoff has restricted most previous flexible TA speakers, such as MXene‑based devices, to tens of nanometers thickness, SPLs below 75 dB, and moderate strains around 50%.
To overcome this issue, the team started from solution‑processed graphene oxide (GO) films with controllable thickness in the 20–170 μm range and subjected them to a continuous‑wave CO₂ laser at 10.6 μm wavelength. The laser drives localized heating up to approximately 2500 °C, stripping oxygen functionalities and inducing graphitization while simultaneously causing violent outgassing that forces graphene layers to stand upright. This single step transforms flat GO into a porous, high‑aspect‑ratio “forest” of vertically aligned reduced graphene oxide (VrGO) walls on a dense base, dramatically increasing the surface area exposed to air and establishing efficient heat pathways along the thickness direction.
Spectroscopic analysis confirms the extent of this transformation: X‑ray photoelectron spectroscopy shows the carbon‑to‑oxygen ratio increasing from about 2.7 in the starting GO to roughly 27.6 after CO₂ laser treatment, indicating extensive deoxygenation and restoration of conjugated graphene domains. Raman spectra point to highly graphitized, multilayer graphene, while thermal measurements show that VrGO films display a specific heat of 0.503 J g⁻¹ K⁻¹, around 30 % lower than planar reduced GO films at 0.718 J g⁻¹ K⁻¹. This means less energy is required to drive the rapid temperature oscillations that underpin thermoacoustic sound generation.
These microstructural gains translate directly into acoustic performance. A VrGO thermoacoustic loudspeaker achieves an SPL of 85 dB at 10 kHz, outperforming a conventional planar reduced GO device of comparable starting thickness, which reaches only 78.9 dB under similar conditions. More striking is the thickness scaling behavior: when the initial GO film thickness increases to 170 μm, the planar device’s output drops to 66.1 dB (a reduction of nearly 13 dB), whereas the VrGO device maintains 82.6 dB, losing only about 2.4 dB despite being more than eight times thicker than the thinnest tested films. The devices also show excellent thermal management, running cooler at a given input power and cooling more rapidly when the drive signal is removed, which supports stable long‑term operation.
Beyond material engineering, the researchers introduced geometric programmability via a second, pulsed laser step at 1.06 μm, which performs fine kirigami‑style cutting directly into the VrGO film. Because this ablation is highly localized, it preserves the underlying vertical graphene architecture and electrical continuity while defining patterns that can transform flat 2D sheets into complex 3D structures such as cylinders, domes, auxetic lattices, and box‑like prismoids. These kirigami architectures convert in‑plane stretching into rotations at hinge regions rather than large material strain, enabling films that are intrinsically stiff and thick to undergo large deformations without cracking or delaminating.
Mechanically, VrGO kirigami loudspeakers exhibit extreme stretchability while retaining functional output. Devices patterned with appropriate cut density sustain up to 500 % tensile strain while preserving more than 75 % of their initial SPL, representing roughly an order‑of‑magnitude improvement in strain tolerance compared with earlier MXene‑based TA speakers that reached 74.5 dB at 15 kHz and about 50 % strain. Samples with a 2 mm cut spacing endure 1000 stretch–release cycles at 100 % strain without significant electrical or acoustic degradation, confirming that the vertical walls and conductive pathways remain intact under coupled thermo‑mechanical loading. Post‑cycling electron microscopy and Raman analysis show only minor defect accumulation, reinforcing the robustness of the hierarchical VrGO network.
The 2D‑to‑3D reconfiguration also opens new routes for spatial sound control. Auxetic patterns with an effective negative Poisson’s ratio allow the loudspeakers to expand laterally when stretched, improving conformal contact to curved substrates and maintaining fairly uniform acoustic emission over the entire active area. Pop‑up prismoid designs fold flat VrGO films into hexagonal or rectangular box‑like emitters that radiate sound more uniformly in three dimensions than planar films, enabling quasi‑omnidirectional sources without bulky enclosures or mechanical actuators. Together, vertical microstructuring and kirigami patterning decouple thickness, flexibility, and directivity, creating a thermoacoustic platform that can be wrapped, stretched, and morphed while retaining high SPL.
This work highlights how laser‑architected 3D microstructures can unlock new high‑value applications in wearable audio, spatial sound, soft robotics, and human–machine interfaces, directly leveraging the low heat capacity and high thermal conductivity of graphene‑based materials. The process builds on solution‑processable graphene oxide and commercially available laser systems, pointing toward scalable manufacturing routes for next‑generation, graphene‑enabled acoustic technologies.
