Biomaterial scaffolds provide platforms for biological study and support tissue engineering and regenerative medicine applications by acting as 1) physical scaffolds which can mimic biological properties to interact with cells or tissues and 2) localized reservoirs for controlled release of therapeutic biomolecules and drugs. These scaffolds have been found to benefit from incorporating macroporous networks which facilitate cell infiltration, proliferation, and delivery of therapeutic factors including gene therapy vectors. While several techniques have been established to create non-injectable scaffolds with cell-scale macroporosities to improve integration with host tissues or facilitate gene delivery, only recently have biomaterial scaffolds been developed that are both macroporous and injectable. Macroporous injectable scaffolds would enable minimally invasive delivery to irregularly shaped defects and thus increases their potential for clinical translation.

We describe development and characterization of several such scaffolds based on hyaluronic acid and their respective abilities to improve efficacy of transgene delivery to the mouse mammary fat pad and spinal cord and tissue regeneration after spinal cord injury (SCI). We compare nanoporous scaffolds to two general techniques to achieve macroporous injectable scaffolds: 1) hydrogels encapsulating degradable microparticle porogens and 2) scaffolds formed from crosslinked microparticles. Our findings indicate that while both injectable, macroporous, scaffold strategies are capable of in vivo administration, crosslinked microparticle scaffolds are significantly more effective at improving cell infiltration and transgene expression both in the mouse mammary fat pad model and the mouse clip-compression SCI model. We further investigated regeneration after SCI when utilizing crosslinked microparticle scaffolds in conjunction with viral vectors encoding for brain derived neurotrophic factor (BDNF) and neurotrophin 3 (NT3). We found significantly greater axon density, myelination, and functional recovery when scaffolds were delivered in conjunction with BDNF-over expressing virus but not NT3. This work establishes significant potential in the strategy of crosslinked microparticle scaffolds for developing injectable tissue engineering therapies for SCI and in broad regenerative medicine applications.