Life‐Like Ferrous Oxide Microspheres from Kombucha

ABSTRACT We report the preparation and electrochemical characterization of iron oxide–alginate microspheres derived from kombucha bacterial cellulose. Microspheres were formed by ionic gelation of three simple components: a kombucha bacterial cellulose mat, ferrous nitrate, and sodium alginate, dispensed dropwise into calcium nitrate solution. Scanning electron microscopy revealed that each sphere was enclosed by a continuous, elastic polysaccharide membrane. The membrane exhibited wrinkling, sub‐micron spherical surface protrusions connected by narrow necks, and multi‐sphere assemblies sharing a common outer membrane consistent with minimal‐surface geometry. These features arose spontaneously from ionic cross‐linking, osmotic stress, and bending of the polysaccharide gel. Optical analysis gave an absorption onset of 2.41 eV. The material absorbed light across the visible spectrum, peaking near 540 nm and extending to 900 nm. Over 24 h, electrochemical recordings revealed spontaneous, self‐sustained voltage oscillations at a frequency of 0.01–0.014 Hz (period 70–100 s) with a power spectral tail. Electrochemical impedance spectroscopy showed that the alginate membrane acts as an active electrochemical interface exhibiting a low‐frequency Nyquist loop inconsistent with passive RC behavior, attributed to redox cycling of the iron couple at the membrane interface. Under theta‐band stimulation (4–8 Hz), the system behaved as a linear low‐pass filter, transforming square‐wave inputs into smooth outputs with a stable transfer coefficient of and zero measurable lag. Collectively, these results demonstrate that iron–alginate–bacterial cellulose composites are electrochemically active soft materials supporting self‐sustained low‐frequency voltage oscillations and passive frequency‐selective signal transfer.