(426h) The Unique Mechanism of Covalently Adaptable Hydrogel Degradation Characterized with Passive Microrheology

Covalently adaptable hydrogels (CAHs) mimic aspects of the native extracellular matrix enabling cells to shear, degrade and apply traction similar to in vivo motility. Understanding the degradation of this material is key to designing it for applications such as drug delivery and 3D culture platforms. In this work, we use multiple particle tracking microrheology (MPT) to measure the dynamic scaffold properties during CAH degradation due to a push out of equilibrium. The CAH is pushed out of equilibrium by changes in the incubation pH. MPT measures the movement of particles embedded in the CAH and the resulting mean-squared displacement describes the network state and connectivity. The CAH is formed by self-assembly of 8-arm polyethylene glycol (PEG)-aldehyde and 8-arm PEG-hydrazine to form covalently adaptable hydrazone bonds. CAHs are formed at both acidic and physiological pHs. Degradation at acidic pH is measured for both CAHs formed in acidic and physiological buffers. MPT measures that the scaffold undergoes at least two cycles of near complete degradation and gelation over several hours. The last degradation event results in irreversible degradation. An acidic CAH incubated in a physiological buffer degrades over a longer time period, approximately a week. During degradation, this scaffold undergoes bond breakage and reformation several times before irreversible degradation occurs. The extent of bond breakage and reformation is much smaller than measured at acidic conditions and, instead, the number of bonds oscillates over the period of several days, with bond hysteresis each cycle. Time-cure superposition (TCS) is used to determine the critical degradation time of each cycle, t_c, and the critical relaxation exponent, n. TCS is the superposition of viscoelastic functions at different extents of reaction. Using TCS, we determine that the value of n for degradation in acidic buffer is 0.48±0.11 and in physiological buffer 0.86±0.04. This indicates that during degradation at pH 4.3 the scaffold is a tightly cross-linked network that can store energy and during degradation at pH 7.4 the scaffold has a more open porous structure and dissipates energy. Additionally, the degradation is modeled using first and second order reaction kinetics. The unique rheological properties and mechanism of degradation of this CAH makes it an ideal candidate for biological applications such as sustained release of active molecules within the body.