"Transient Soft Matter Fueled by Chemical Reactions"
Conventional approaches for reconfiguring soft matter utilize external stimuli to produce steady state changes to the shape or microstructure of self-assembled colloids and soft materials. These materials exhibit singular steady state responses to stimulation, but lack temporal control seen in biological systems. In recent years, synthetic chemical reaction cycles have been discovered that enable transient soft materials that mimic the dynamic nature of biological systems, enabling active soft materials that exhibit complex, transient responses to chemical stimuli. This seminar will describe recent work in Professor Woehl’s lab that utilize chemical reactions to drive transient and autonomous responses in colloids and hydrogels. In the first of three vignettes, I will discuss the colloidal interactions occurring during chemically fueled assembly of micron sized colloids. Here, colloids are exposed to a carbodiimide chemical fuel that temporarily renders their surface hydrophobic. We systematically identify ranges of fuel concentration that lead to no assembly, transient assembly, and irreversible assembly of colloids. Experimental measurements of colloid surface charge and hydrophobicity inform a colloidal interaction model that corroborates the experimental observations. The second part of the seminar will highlight ongoing work in the Woehl lab utilizing electrochemical redox reactions to drive transient assembly and disassembly of colloidal crystals in oscillatory electric fields. We demonstrate the use of electroactive molecules to induce transient pH changes, which mediate the strength of competing electrokinetic fluid flows. By tuning attractive and repulsive hydrodynamic interactions between neighboring colloids, this approach enables transient assembly and disassembly of 2D colloidal crystals over time scales of seconds to tens of minutes. Reaction-diffusion modeling of the electrochemical reaction network uncovers the origin of the pH transients. Finally, I will discuss the use of redox reaction cycles to generate transient protein hydrogels. Here, a strong oxidizer and weak reducing agent are concurrently added to denatured serum albumin, which causes formation of a transient disulfide crosslinked hydrogel. We found that increasing the denaturant concentration decreased the lifetime of the hydrogel. Biophysical measurements revealed this to be due to changes in the unfolding state of the protein, which mediated the concentration of free cysteines available to react with the oxidizer. This work uncovered a novel approach to tune the lifetime of transient protein hydrogels and established a connection between molecular protein structure and macroscopic hydrogel properties.