(415b) Human Mesenchymal Stem Cell Response to Hydrogel Viscoelasticity

Covalent adaptable networks (CANs) are self-healing materials that can be used for cell and drug delivery. These applications require network degradation at physiological conditions and timescales with microstructures that: (1) support, protect and deliver encapsulated cells or molecules and (2) provide structure to the surrounding tissue. Due to this, the evolving microstructure and rheological properties during scaffold degradation must be characterized. In this work, we characterize degradation of photo-polymerized networks composed of poly(ethylene glycol) (PEG)-thiol and PEG-thioester norbornene, which undergoes a thioester exchange reaction. We first measure degradation of PEG-thioester networks with varying amounts of excess thiol, which defines the degree of adaptability, by incubation in L-cysteine using multiple particle tracking microrheology (MPT). MPT measures the Brownian motion of probe particles embedded in a material and relates this to rheological properties using the Generalized Stokes-Einstein Relation. We measure scaffold rearrangement during degradation and network elasticity increases non-monotonically with the amount of excess thiol. We then 3D encapsulate hMSCs in these networks and use MPT to measure cell-mediated degradation in the pericellular region. hMSCs degrade these CANs ~4 days post-encapsulation. We hypothesize that scaffold degradation is mainly due to cytoskeletal tension applied to the network triggering the exchange reaction. To test this, we measure degradation of thioester networks by hMSCs with cytoskeletal tension inhibited using blebbistatin. We continue to measure degradation by treated hMSCs, but to at a lower extent. This could be the result of the cell-secreted esterases that hydrolyze thioester bonds and initiate the exchange reaction. This work can inform thioester network design with properties that can instruct basic cellular processes for applications including scaffolds that promote cell delivery and tissue engineering. These scaffolds enable the material to be delivered by injection and protect the cells at the wound site and the properties cells engineer into the scaffold can be designed into the microenvironment to instruct cellular processes after injection.