Fully Aqueous and Printable Photonic Inks with Tunable Pitch and Optical Memory via Hydrogen-Bonded HPC-PVA Networks.

Rewritable and structurally colored biopolymer coatings demand fully aqueous processing, optical tunability, and dry-state color retention, yet these requirements remain difficult to reconcile without chemical crosslinking. Competitive hydrogen bonding between hydroxypropyl cellulose (HPC) and poly(vinyl alcohol) (PVA) is leveraged to achieve dynamic pitch modulation and kinetic trapping of cholesteric order, thereby overcoming the intrinsic limitations of HPC-based inks. In this study, a compositionally programmable, fully water-based photonic ink is realized by blending HPC with PVA additives of varied molecular weight and hydrolysis degree. The resulting formulations exhibit continuously adjustable structural colors (λmax = 466-633 nm), high yield stress (>100 Pa), and shear-thinning behavior compatible with direct ink writing. Thermal annealing kinetically arrests the cholesteric structure without covalent fixation, yielding vibrant dry-state color with robust mechanical integrity. The printed films further display humidity-responsive reversible color shifts (Δλmax up to 240 nm) and rewritable optical memory, retained even in complex 3D architectures. This non-covalent design paradigm integrates pitch programmability, environmental responsiveness, and printability in a single biopolymer platform, providing a scalable route toward sustainable photonic coatings and rewritable optical devices.