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In this work, pH-sensitive and chromiumloaded mineralized nanoparticles (Cr-PPA NPs) were developed by self-assembly of poly (poly(ethylene glycol) methyl ether acrylate-co-acrylic acid) (poly(PEG-co-AA)) copolymers templated Cr(OH) 3 mineralization. The Cr-PPA NPs exhibit excellent colloidal stability in water with high salt concentration, protein, and pH above 3.0. As a novel chrome tanning agent, such Cr-PPA NPs could effectively protect and deliver chromium into pickled pelt without reacting with collagen fibers during the penetration process, due to its antifouling PEG surface and nanosize. When the interior pH of the pickled pelt was lower than 2.5, the dissolution of the mineralized Cr(OH) 3 core would cause disassembly of Cr-PPA NPs and rapid release of Cr 3+ ions, leading to uniform chromium distribution in leather. After basifying, the released Cr 3+ ions would cross-link the carboxyl groups both on collagen fibers and poly(PEG-co-AA) copolymers, resulting in chromium adsorption efficiency in leather higher than 90% and less chromium discharge in wastewater. Simultaneously, the resultant leather was given enhanced hydrothermal stability and physical properties based on above efficient chromium cross-linking. Compared with traditional chrome tanning agent, this pHsensitive Cr-PPA NPs can really break through the existing balance limitation between chromium penetration and combination in the conventional tanning process and become a promising strategy for cleaner leather production.

Foot plantar pressure monitoring and gait analysis are of great significant in footwear design, sport biomechanics, injury prevention, and early warning of disease. Flexible and wearable smart insoles pave a feasible way for these application scenarios. However, the majority of the currently developed smart insoles are composed of synthetic polymers (e.g., plastics, rubbers, etc.), leading to inevitable problems associated with air permeability, hygiene condition, biocompatibility, and wearing comfort. Here, a new paradigm of a natural collagen fiber network (CFN) with soft and breathable features, which can be obtained by facilely treating animal hides via conventional leather pretreatment process, is selected as substrate material for constructing smart insoles due to its high permeability and porosity. Further, biocompatible zwitterionic silver nanoparticles (AgNPs) with both carboxybetaine and catechol groups on the interface were designed for firmly and uniformly immobilization onto the hierarchical micro-/nanoscale fibers of CFN through mussel-inspired catechol/amino chemistry, giving rise to both good antibacterial property and pressure sensing capability of the resultant material. The finally developed smart insole by using the AgNPs decorated CFN exhibits good capability for plantar pressure mapping and gait feature analysis. Especially, the smart insole will be very suitable for pressure monitoring and gait analysis of a diabetic foot with sensitive skin that requires a high biocompatible and antibacterial environment.

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