Vertical graphene microstructures break the thickness-performance tradeoff in thermoacoustic speakers, enabling flexible audio devices that stretch to 500% strain without significant sound loss.
(Nanowerk Spotlight) Flexible electronics have come tantalizingly close to merging with the human body. Thin-film displays that bend around wrists, sensors that stretch with skin, and circuits that conform to curves all point toward a future where technology wraps seamlessly around us. Yet one stubborn holdout remains: the loudspeaker.
Conventional speakers rely on vibrating membranes to push air molecules back and forth, creating sound waves. This mechanical approach demands rigid structures and bulky enclosures, making true flexibility impossible. The diaphragm must move freely, which inherently conflicts with the ability to bend, fold, or stretch.
Thermoacoustic speakers offer an elegant alternative. Instead of physical vibration, they generate sound through heat. When an alternating electrical current passes through a thin conductor, rapid Joule heating causes the surrounding air to expand and contract in sync with the electrical signal. This produces pressure waves we perceive as sound, with no moving parts required.
Carbon-based nanomaterials such as graphene and carbon nanotubes have proven especially suited to this task because their extremely low mass allows them to heat and cool almost instantaneously. The thinner and lighter the film, the faster it responds, and the louder the sound.
But this same property creates a fundamental problem. To achieve acceptable volume, thermoacoustic devices typically require nanometer-scale films. A shape-shifting MXene-based speaker reported in our previous Nanowerk Spotlight (“MXene opens way to futuristic shape-shifting speakers“), for instance, relied on a heating element just 78 nm thick to reach 74.5 dB and could stretch to 50% strain. Such films are fragile, difficult to handle, and limited in power capacity. Increase the thickness for durability, and performance plummets. Heat generated at the surface cannot penetrate deep enough into the material quickly enough, leaving interior regions acoustically dead. This thickness-performance tradeoff has kept thermoacoustic technology confined to laboratory demonstrations.
A research team based at the Korea Research Institute of Chemical Technology has now developed a fabrication approach that disrupts this limitation. Published in the journal Advanced Science (“Laser‐Architected Shape‐Configurable Vertical Graphene Thermoacoustic Loudspeakers for 3D Acoustic Emission”), their work describes vertically aligned reduced graphene oxide (VrGO) films that maintain strong acoustic output even at substantially greater thicknesses than conventional designs. The researchers transform flat graphene oxide sheets into three-dimensional microstructures with improved thermal properties by combining two different laser treatments in sequence.
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: Reproduced from DOI:10.1002/advs.202522911, CC BY) (click on image to enlarge)
Fabrication begins with commercially available graphene oxide, a form of graphene heavily decorated with oxygen-containing chemical groups. This material disperses easily in water and forms films of controlled thickness. The researchers prepared films ranging from 20 to 170 μm thick, then subjected them to a continuous-wave CO₂ laser operating at 10.6 μm wavelength.
The laser induces intense, localized heating reaching approximately 2500 °C. This strips away oxygen groups, restoring the electrically conductive graphene structure. More importantly, the rapid heating causes violent outgassing and exfoliation, forcing graphene sheets to stand upright rather than lying flat. The result is a forest of vertical graphene walls rising from a dense base layer, creating a hierarchical architecture with substantially increased surface area exposed to air.
Chemical analysis confirmed the transformation. X-ray photoelectron spectroscopy showed the carbon-to-oxygen ratio jumped from roughly 2.7 in untreated graphene oxide to about 27.6 in the laser-treated material. Raman spectroscopy revealed spectral features characteristic of highly graphitized, multilayered graphene, a structural quality unattainable through conventional thermal reduction alone.
The vertical architecture addresses the thickness problem directly. In a flat film, heat must travel through successive layers before reaching the air interface, a slow process that limits acoustic efficiency. In the VrGO structure, graphene walls project upward into surrounding air, providing numerous parallel pathways for rapid heat dissipation.
Thermal measurements quantified the improvement. The VrGO film showed a specific heat of 0.503 J g⁻¹ K⁻¹, roughly 30% lower than the 0.718 J g⁻¹ K⁻¹ measured for conventionally reduced planar films. This means less energy is required to change the material’s temperature, enabling more efficient thermoacoustic conversion. VrGO films also ran cooler under identical Joule heating conditions and cooled faster once power was removed.
Acoustic tests revealed the practical impact. A VrGO loudspeaker produced 85 dB at 10 kHz, compared to just 78.9 dB for a conventional planar reduced graphene oxide film of similar starting thickness. The difference grows more pronounced as thickness increases.
When starting film thickness reached 170 μm, the planar device’s output collapsed to 66.1 dB, a drop of nearly 13 dB. The vertically structured device dropped only about 2.4 dB, maintaining 82.6 dB output despite being over eight times thicker than the thinnest tested samples.
Beyond acoustic performance, the researchers incorporated a second laser process to enable geometric flexibility. A pulsed fiber laser operating at 1.06 μm wavelength performs high-precision ablation, cutting intricate patterns into the VrGO films. This allows the material to be shaped into kirigami structures, patterns of strategic cuts that transform flat sheets into stretchable, morphable geometries.
Kirigami engineering converts in-plane tension into rotations at cut hinges rather than material strain, enabling stiff films to stretch without fracturing. The researchers demonstrated VrGO loudspeakers that maintained over 75% of their original sound output even when stretched to 500% strain, a tenfold improvement over the 2023 MXene device.
Devices with 2 mm cut spacing endured 1000 stretch-release cycles at 100% strain with no significant performance degradation. Because the acoustic measurements required electrical heating, this testing inherently combined mechanical and thermal cycling. Post-cycling electron microscopy and Raman spectroscopy confirmed the vertical graphene architecture remained intact with only minimal defect accumulation.
The patterning approach extends beyond simple stretchability. Auxetic patterns, designs with a negative effective Poisson’s ratio that expand uniformly in all directions when stretched, allowed VrGO loudspeakers to conform smoothly to curved surfaces while emitting sound evenly across their area.
Pop-up prismoid patterns enabled flat films to fold into three-dimensional shapes such as hexagonal or rectangular boxes. These 3D configurations produced more spatially uniform sound distribution than their flat counterparts, demonstrating omnidirectional acoustic emission capabilities.
The combined benefits of vertical microstructuring and geometric programmability position this technology as a platform for next-generation audio devices. Wearable electronics, spatial sound systems, augmented reality interfaces, and soft robotics all demand sound sources that can bend, stretch, and morph while maintaining consistent output. The VrGO approach achieves these requirements using a scalable process based on widely available graphene oxide and standard laser equipment.
This work establishes that the fundamental tradeoff between thickness and performance in thermoacoustic speakers can be overcome through deliberate microstructural engineering. By forcing heat pathways to align with the direction of acoustic emission, the vertical graphene architecture enables thicker, more robust films without sacrificing the rapid thermal response essential for sound generation. Combined with kirigami patterning, this creates a versatile foundation for shape-adaptive acoustic devices that can integrate into environments where conventional speakers cannot.
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Saewon Kang (Korea Research Institute of Chemical Technology)
, 0000-0001-5932-6636 corresponding author
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