Abstract
In this thesis, we investigate the influence of reorganization mechanisms on the mechanical properties of supramolecular hydrogels. Specifically, we focus on the characterization of dissociative and associative exchange mechanisms on DNA hydrogels. We try to rationalize how stress applied at the macroscopic scale spreads to the molecular units of the networks, how it deforms the structure of the hydrogel, and how it breaks it in the case of dynamic networks.We designed DNA hydrogels in which we programmed a toehold-mediated strand displacement reaction that allows for an associative exchange at low temperatures. We study dynamic DNA hydrogels through rheological experiments coupled with kinetic analysis by fluorescence spectroscopy and multiple fluorescence imagery/microscopy methods.We developed an efficient method of DNA synthesis and assembly to produce enough material to perform macroscopic rheology experiments on DNA hydrogels. We exploited Rolling Circle Amplification, enzymatic chemistry, and microfiltration to synthesize pure sequence-controlled single-strand concatemers of DNA. We optimized the rheology experiments to obtain accurate measurements of the storage and loss modulus on small samples, with precise temperature control, and reproducibility between tests. This allowed us to perform an extensive mechanical characterization of the key properties of DNA hydrogels in the linear and non-linear regimes.We characterized the control of the relaxation time of DNA hydrogels as a function of the reorganization mechanism. We investigated the respective impact of the dissociative and associative mechanisms on the dynamics and mechanical properties of dynamic DNA hydrogels by rheology experiments by varying temperature. Eventually, we showed that toehold-mediated strand displacement allows for efficient control of the relaxation time of DNA hydrogels over 4 orders of magnitude without impacting their toughness, which is impossible with conventional dissociative DNA hydrogels. To characterize the thermodynamics of the exchange mechanisms occurring in the gel we performed Time Temperature Superposition experiments on gels in a rheometer and kinetic analysis on diluted strands in fluorescence spectroscopy. We proposed energy landscapes for the exchange mechanism occurring in the DNA hydrogels to rationalize the different enthalpic regimes observed depending on temperature. We also showed key features of the system we designed allowing in situ control of the relaxation time with blocking strands or significant strength recovery in a day.At last, we developed a quantitative setup to observe stress at the molecular scale using fluorescent force probes. The tool presented allows us to perform real-time stress and strain measurements simultaneously with fluorescence imagery in a transparent shear cell. We calibrated the stress and strain measures allowing accurate rheometry. We also established a calibration relation between stress and fluorescence allowing for local stress estimation by fluorescence measurements. This work paves the way to real-time characterization of mechanical probing in fluorescence suited for stresses involved in cell mechanics.