Decorated bacteria-cellulose ultrasonic metasurface

Abstract

Cellulose, as a component of green plants, becomes attractive for fabricating biocompatible flexible functional devices but is plagued by hydrophilic properties, which make it easily break down in water by poor mechanical stability. Here we report a class of SiO₂-nanoparticle-decorated bacteria-cellulose meta-skin with superior stability in water, excellent machining property, ultrathin thickness, and active bacteria-repairing capacity. We further develop functional ultrasonic metasurfaces based on meta-skin paper-cutting that can generate intricate patterns of ~10 μm precision. Benefited from the perfect ultrasound insulation of surface Cassie-Baxter states, we utilize meta-skin paper-cutting to design and fabricate ultrathin (~20 μm) and super-light (<20 mg) chip-scale devices, such as nonlocal holographic meta-lens and the 3D imaging meta-lens, realizing complicated acoustic holograms and high-resolution 3D ultrasound imaging in far fields. The decorated bacteria-cellulose ultrasonic metasurface opens the way for exploiting flexible and biologically degradable metamaterial devices with functionality customization and key applications in advanced biomedical engineering technologies.

Fig. 1. Fabrication and characterization of decorated BC meta-skin.

a Illustration of the fabrication process of the superhydrophobic BC meta-skin. b Illustration of the hydrophilicity of BC nanofibers, where the hydrophobic SiO₂ nanoparticles can hardly enter into BC hydrogel. c Illustration of the ethanol-assisted method that allows SiO₂ nanoparticles to easily diffuse into the BC alcogel and bond with BC fibers through intermolecular forces. d, e SEM images of the surface and sectional morphologies of the decorated BC meta-skin. f An optical image showing a BC meta-skin sample (4 × 4 cm²) resting on dandelion’s petals. The inset shows a strong ‘lotus effect’ on the sample surface, with the CA of a water droplet being 170.2°.

Fig. 2. Mechanical, acoustic, bacteria-repairing properties of the BC meta-skin.

a Illustration of BC meta-skin paper-cutting, via which we can realize the ultrathin meta-lenses with different functionalities. The inset shows that the BC meta-skin has a surprising bacteria-repairing property. b Three stages (I, II, III) in the bacteria repairing of a broken meta-skin. c Complex pattern comprising ‘building’, ‘flag’, ‘HUST’, and other elements. d Laser-cut BC meta-skin with the narrowest line width of ~200 µm, showing excellent processability. e Optical image of the patterned meta-skin immersed in water, where the silvery surface indicates the existence of a very stable air-filled interface (or the Cassie-Baxter states) on the meta-skin. f Illustration of perfect mirror reflection for ultrasound on the nanosurface due to the stark acoustic impedance mismatch. g The time-domain measurement in reflection (with $\pi$-phase compensation) and transmission for a short ultrasound pulse incident on the meta-skin. h Spectral analyses of incident, transmitted, and reflected signals, indicating broadband ultrasound reflection.

Fig. 3. A holographic meta-lens via the BC meta-skin paper-cutting.

a Illustration of the ultrasonic hologram via BC meta-skin paper-cutting. The distance between the holographic meta-lens and the hologram plane is 35 mm. b An optical image showing the ultrathin and ultralight holographic meta-lens fabricated via BC meta-skin paper-cutting. c Intensity fields on the hologram planes from the calculated, simulated and experimentally measured results. d Intensity distribution on section lines I, II and III, as marked by the white dashed lines in (c) ($x=0.75\mathrm{cm}$, $0.0\mathrm{cm}$, $-0.75\mathrm{cm}$, respectively) for quantitative comparison.

Fig. 4. A 3D imaging meta-lens via BC meta-skin paper-cutting.

a Graphite rods for 3D ultrasonic imaging, for which the diameter is 200 μm. The graphite rods are set to position in three planes (1, 2, 3) with different orientations for testing the longitudinal resolution, and the projection forms a hexagram pattern. Inset shows an imaging meta-lens based on BC meta-skin paper-cutting. b The ultrasonic images of graphite rods with different depths (b1, b2, b3). Here the 3D ultrasonic images are reconstructed by extracting and arranging the echo signal data. c 3D reconstruction image of graphite rods. d 3D-printed metal patterns ‘HUST’ and ‘SIAT’ for 3D ultrasonic imaging. The distance between the two patterns is 900 μm. The horizontal shift between letters ‘T’ is 200 μm for testing the transverse resolution. e Reconstructed images of ‘HUST’ and ‘SIAT’ at the depths of e1, e2. f Intensity profiles extracted on the dashed lines (e1, e2) in (e). We also show the superimposed result of e1+e2.