Fig. 1: Production of nickel-doped chitosan.

a Conceptual schematic of regional circular production of chitin-derived polymers. Chitin and chitosan, typically byproducts of the shrimp and crab processing industry, are structural components in most heterotrophs used for the bioconversion of organic waste and the local production of nutrients. As part of any ecological cycle, chitin can be reincorporated into circular production cycles through waste management or, if unmanaged, into natural ecological cycles. b Photographs showing the color of vitrified chitosan films with increasing trapped Ni (quantified in Supplementary Fig. 1). Scale bar is 1 cm. c Schematic illustration of plausible Ni ion locations relative to a chitosan chain; the distribution depends on local degree of deacetylation, water content, and crystallinity, and is expected to comprise a mixture of these possibilities. d FTIR spectra of Ni-doped films at different Ni contents, showing that spectral changes associated with added water dominate over Ni-specific contributions. e FTIR region dominated by water’s O-H bond vibrations and normalized to the carbohydrate skeletal vibrations (800–500 cm⁻¹, inherent to the chitosan structure), showing increased Ni-associated intermolecular water content. f X-ray diffraction patterns: pristine vitrified chitosan shows peaks at 9.5° and 20° and a broad amorphous contribution (15–30°); at low Ni content, the peaks’ shift is consistent with a doping process where small molecules take the interstitial space between the organized ones, whereas at higher Ni and water contents the amorphous regions come to dominate the structure. g Representative tensile stress–strain curves of dry Ni-doped films; Ni concentrations below 0.8 M have a limited effect on tensile strength, while at higher concentrations, they increase elasticity without loss of strength, simultaneously achieving strength and toughness, a characteristic functional versatility of structural biomaterials. h Tensile strength before (open bars) and after (filled bars) immersion in water for films prepared from initial Ni concentrations of 0.6–1.4 M; Cs (black) denotes pristine chitosan (no Ni). Elastic moduli and toughness are reported in Supplementary Tables 2 and 3, with statistical tests in Supplementary Fig. 2. Measurements include at least three points; data are presented as mean values +/- SD.