Biofabrication Platforms for Increasing the Dimensionality of Electrospun Hydrogels

The extracellular matrix (ECM) is highly complex and dynamic, regulating cell functions through the presentation of various biophysical and biochemical signals. ECM composition is tissue system-dependent, but it generally consists of fibrous proteins that dictate tissue structure with additional amorphous interstitial material contributing to function and soft-tissue mechanics. Historically, there has been considerable focus on using hydrogels to model the ECM; however, these materials are often static and isotropic and therefore unable to recapitulate some of the complexities of natural tissue. Work to close this gap in biomaterials research includes a focus on hydrogel advances that aim to capture the highly structured and dynamic nature of the ECM. Processing hydrogel-forming biomaterials via electrospinning enables these advancements in dynamic hydrogels to be translated into a fibrous form, which offers opportunities to model some of the biochemical and biophysical attributes found in endogenous tissue environments. While it is possible to generate dynamic, fibrous hydrogel architectures in this way, they are often still limited in their dimensionality – both in 3D space and in dynamics across time. In light of this, the goal of this thesis was to develop new classes of biofabrication tools that address these limitations in electrospun systems based on hydrogels.

We leveraged hyaluronic acid (HA) and polyethylene glycol (PEG) as the base materials for this work and installed reactive groups (e.g., norbornene, methacrylate, vinyl sulfone) that enable spatially-controllable, photomediated crosslinking and biochemical functionalization of the resultant fibers. The flexibility offered by these reaction mechanisms, along with the geometry of the electrospun hydrogel fibers, are exploited herein to develop the demonstrated biofabrication platforms. First, to address challenges in the time dimension, a peptide-based platform that enabled dynamic presentation of bioactive molecules on hydrogel (both isotropic and fibrous) substrates was demonstrated. User-defined, reversible presentation of these biomolecules was achieved through the formation and disruption of coiled coil complexes through toehold-mediated strand displacement. Next, towards the translation of electrospun fibers into 3D space, a novel granular hydrogel medium comprised of segmented electrospun hydrogel fibers was developed. These granular hydrogel materials exhibited unique mechanics with tunable viscoelasticity and stress relaxation properties – enabling not only injection/extrusion, but also serving as 3D, permissive culture environments for cell encapsulation. Finally, a foundational layer-by-layer biofabrication platform based on spatially-patternable electrospun substrates was investigated to enable the localization of cellular and material content at high resolution in 3D, macroscale constructs. Taken together, the electrospun hydrogel-based systems developed throughout this thesis offer new opportunities in designing functional biomaterials and address broad challenges in recapitulating complex biochemical and biophysical architectures in engineered tissues.