Bamboo-culm-inspired graded axial channel cellulose cryogels for acoustic absorption and acoustoelectric conversion

Lightweight, bio-based absorbers are needed for noise mitigation without bulky cavities. Inspired by graded natural porous media, we prepared regenerated cellulose (RC) cryogels by dissolving and regenerating cellulose, followed by directional or isotropic freezing and subsequent freeze-drying. Directional freezing produced channels with an axial alignment of gradients. In directionally frozen RC cryogels, larger pores were near the hot end, whereas finer pores were near the cold end. Increasing the cellulose content refined the pores. In contrast, epichlorohydrin (ECH) cross-linking stabilized the pore walls while decreasing open porosity. Compression tests showed that compressive strength and volumetric energy absorption increased with solid content, and directionally frozen cryogels were superior at the same cellulose content. The optimal directional RC cryogel exhibited a noise reduction coefficient (NRC) of 0.56, exceeding those of the isotropic and cross-linked RC cryogels (0.54). The directional RC cryogel enabled acoustoelectric energy conversion. The device achieved a voltage output of 3.2 V and a current output of 0.38 μA at 100 Hz. These results demonstrate a scalable, bio-based route to lightweight cryogels that couple centimeter-scale sound absorption with acoustoelectric functionality, offering a sustainable platform for buildings, transportation, and personal noise mitigation.