Calcium carbonate precipitation within the tubular sponge reactor. (a) Schematic illustration of the gas-diffusion setup used for CaCO3 precipitation within hollow polymer tubes. (b) Photograph showing the longitudinal cross section of a sponge reactor after mineral deposition. The green dot marks the sponge wall, while the red line indicates the inner wall surface exposed to the reactant solution. (c) Light microscopy image of the polymer fibers within the tube wall (green dot in (b), scale bar = 200 μm). (d) Scanning electron micrograph showing calcium carbonate particles (indicated by white arrows) grown on the inner wall of the reaction channel (red delineation in (b), scale bar = 100 μm). Inset: overview image obtained by light microscopy demonstrating the full coverage of the channel surface with mineral particles (red arrows). (e) Rhombohedral calcium carbonate crystal grown around a polymer fiber (scale bar = 10 μm). (f) Scanning electron micrograph showing the position, where a polymer fiber (white diamond) protrudes from a calcium carbonate crystal. Small nanoscale particles mediate the contact between polymer and mineral within the interfacial region (dashed box, scale bar = 2 μm).