(159g) Microrheological Characterization of Covalent Adaptable Hydrogel Degradation in Response to Change in pH That Mimic the Gastrointestinal Tract

Dynamic covalent chemistry has recently emerged as a powerful tool to design dynamic, responsive materials. This chemistry creates reversible covalent bonds, which can be implements in hydrogel design to introduce structural dynamic features into a scaffold. A type of these scaffolds are covalent adaptable hydrogels (CAHs). CAHs can reversibly break and reform their network bonds in a controllable manner. This dynamic network arrangement can be controlled by a wide range of external stimuli, from mechanical force to environmental conditions, such as chemical and biological cues. These stimuli-responsive structural changes make CAHs a strong candidate for biological applications, including as a delivery vehicle for controlled and targeted oral delivery. We are specifically interested in designing a CAH as a pH-responsive delivery vehicle. This would require the scaffold to degrade in response to pH changes that mimic in the native gastrointestinal (GI) tract to release active molecules. Since this is a requirement, this degradation must be first characterized. The CAH we characterize is composed of 8-arm poly(ethylene glycol) (PEG)-hydrazine that self-assembles with 8-arm star PEG-aldehyde, forming covalent adaptable hydrazone bonds. To characterize CAH degradation in response to pH changes that mimic the pH through the GI track, we use μ²rheology. μ²rheology is multiple particle tracking microrheology (MPT) in a microfluidic device. In MPT, fluorescent probes particles are embedded into the sample and material rheological properties are calculated from particle Brownian motion. To mimic changes in the pH environment in each part of GI tract, we use a two-layer microfluidic device. This device enables consecutive fluid exchange around a single sample with minimal sample loss, regardless of the state of the sample. Using μ²rheology, we characterize CAH degradation at a single pH (pH 4.3, 5.5 and 7.4), with a single pH exchange (pH 4.3 to 7.4 and pH 7.4 to 4.3) and during temporal pH changes that mimic the pH in the entire GI tract. To characterize the gel-sol transition, the critical relaxation exponent, n, is calculated from single pH and single pH exchange experiments. The n value quantitatively identifies the state of the material and pinpoints when the phase transition occurs. n also identifies the material microstructure at the phase transition. We determine that the value of n for this scaffold is independent of incubation pH, indicating that the material microstructure and phase transition are independent of the degradation pH. From measurements of single pH exchange and temporal pH changes through the whole GI tract, we determine that degradation history has no influence on scaffold degradation kinetics and material property evolution. However, the initial cross-link density of the scaffold at each pH exchange can be reduced by previous degradations. This decreases the degradation time for the scaffold to transition from a gel to a sol, which will change molecular release from this CAH. These results indicate degradation can be tuned by changing scaffold cross-link density, which can be done by changing the ratio of backbone to cross-linker functional groups. To design this CAH as an oral delivery vehicle, especially for chemically conjugated therapeutic release, CAH degradation will directly drive drug release. Our ongoing efforts focus on investigation of molecular release initiated by pH-depend CAH degradation. To do this, fluorescent molecules are covalently tethered into our hydrazone matrix. The fluorescent molecules we are using are linear fluorescein PEG aldehyde (1kDa) as a mimic of small molecule therapeutics. We characterize the accumulated molecular release in response to temporal pH changes that mimic the pH in the whole GI tract using fluorescent spectroscopy. The CAH is degraded in our microfluidic device and the fluorescence released in the incubation fluid is measured. This work will link previous rheological characterization to molecular release profiles to better design this CAH for controlled, targeted oral drug or nutrient delivery.