(267b) Covalent Adaptable Hydrogel Networks Measured with ?2rheology to Mimic the pH Environment in the GI Tract

Covalent adaptable hydrogels (CAHs) are attractive biomaterials due to their ability to break and reform bonds in response to external stimuli. The adaptability of these materials are often characterized using bulk rheology and a stress relaxation experiment. This experiment determines whether the scaffold returns to equilibrium properties after bond degradation due to an applied external stress. This information is extraordinarily useful, but does not describe how the material will react to subtle changes in the environment, like changes in pH during digestion. In this work, we characterize dynamic changes in a CAH in response to changes in pH in the incubation environment. The CAH we are characterizing is composed of 8-arm star poly(ethylene glycol) (PEG)-hydrazine that chemically cross-links with an 8-arm star PEG-aldehyde creating a covalent adaptable hydrazone bond. Previous work used multiple particle tracking microrheology (MPT) to characterize this scaffold as a function of the pH of the incubation environment. In MPT, fluorescently labeled probe particles are embedded in the material and their Brownian motion is captured using video microscopy. This particle movement is related to rheological properties, such as the creep compliance, using the Generalized Stokes-Einstein Relation. In our previous work, we determined that scaffold degradation was pH dependent with vastly different rheological evolution. To exploit these dynamic properties, we measure CAH degradation in a microfluidic device, a technique called μ²rheology. This device enables changes in the incubation liquid around a sample even when the sample is a sol. With this device, we can change the pH of the incubating fluid regardless of the state of the material. We mimic the changing pH environment through part of the gastrointestinal tract (pH 4.3 to 7.4 or pH 7.4 to 4.3) in our microfluidic device. We determine that dynamic material property evolution is consistent with degradation at a single pH. However, the time scale of degradation is reduced by the history of degradation. These investigations inform the design of this material as a new vehicle for targeted delivery. This new technique and CAH characterization will be used to mimic the temporal pH changes during digestion to determine the degradation of this CAH and, in future work, the ability to release molecules from the scaffold throughout the entire process.