Three-Dimensional Stiffness Control of Double-Network Hydrogels by Bi-Ag Redox Reaction

Connecting soft and rigid materials is crucial for numerous applications, including synthetic tendons and stretchable electronics. However, large gradients in stiffness can delaminate interfaces under strain. Gradual stiffness gradients allow stress to be distributed evenly to maintain adhesion. Previous work on stiffness gradients has been limited to stiffness control in only one dimension and often lacked precision. This work utilizes redox reactions in double network hydrogels to modulate the stiffness in three dimensions and form gradual stiffness gradients. Polyacrylamide-alginate hydrogels containing bismuth particles were reacted with silver nitrate to form silver particles and bismuth ions, which crosslinked the carboxylic groups of the alginate. Stiffness gradients were created by changing the exposure time to silver nitrate solution. This gradient in dynamic modulus was mapped by cutting the hydrogels into pieces and measuring each individually with a rheometer. Digital image correlation (DIC) analysis was used to create a 2-D map of strain level, confirming the gradient in stiffness and revealing that stiffness only increased at the locations of the bismuth particles. Therefore, stiffness gradients were additionally created by changing the particle distribution. This was done by altering the viscosity of the precursor solution and curing time to allow the particles to partially settle during curing. The resulting hydrogels actuated during the redox reaction due to localized stress on one side of the hydrogel. Lastly, the hydrogels exhibited excellent antibacterial properties due to the presence of bismuth and silver, both potent antibacterial agents in their nanoparticle and ionic form. Zone of inhibition testing and a colony-forming unit evaluation was performed to analyze these properties. In summary, this system allows for stiffness to be controlled in three dimensions, down to the size of individual bismuth particles. Therefore, these Bi-Ag double network hydrogels offer a solution for the fluid integration of rigid and soft materials, and their antibacterial properties could be helpful in many biomedical applications.