Programmable Ion-Protein Networks from Sodium Caseinate: A Sustainable Platform for Soft Functional Materials

Abstract

Ion–protein coordination represents an underexploited design principle to assemble multiresponsive, soft, sustainable materials. Here, we harness sodium caseinate to construct programmable ionically crosslinked hydrogels in which multivalent cation selection governs the network architecture and enables a broad tunability of mechanical and other functional properties. A systematic study of cation types, including Ca2+, Sr2+, Ba2+, Mn2+, Cu2+, Zn2+, Fe3+, Al3+, and Zr4+, revealed pronounced ion-specific control over mechanical stiffness (1.5 kPa to 1.8 MPa), thermal stability, and hierarchical architecture. Multimodal characterization of their compositional, structural, morphological, thermal, spectroscopic, and mechanical properties enabled the tailoring of an empirical packing hierarchy for MX+-caseinate networks (MCas). Leveraging this tunability, we demonstrate proof-of-concept piezoresistive soft sensors in which ionic crosslinking of caseinate modulates the mechanical properties of caseinate–gelatin organohydrogel matrices. These matrices, crosslinked with Sr2+ and Zn2+, exhibit linear ΔR/R0 responses with gauge factors (1.84–2.20) competitive with state-of-the-art organohydrogel sensors. Real-time measurements further demonstrate their ability to detect bending angles and encode dynamic inputs, such as Morse-code signals. These results position MCas as a sustainable, ion-tunable platform for the rational design of mechanically programmable protein hydrogels, opening opportunities in bioinspired materials for soft electronics and wearable sensing.