Matrix stiffness and stress relaxation regulate matrix-bound nanovesicle release from alginate hydrogels

Matrix-bound nanovesicles (MBVs) are a recently discovered subclass of small extracellular vesicles (EVs) that reside within the extracellular matrix of non-mineralized tissues throughout the body. Functionally, MBVs exhibit unique immunomodulatory properties that have been leveraged therapeutically to treat various tissue pathologies, including periprosthetic osteolysis, rheumatoid arthritis, and skeletal muscle injury. However, like other EVs, the therapeutic efficacy of MBV applications is limited by delivery methods, namely bolus injections, that offer poor control of EV persistence and bioavailability at the site of administration. We hypothesized that a superior MBV delivery platform could be developed by entrapping MBVs in a tunable, engineered alginate matrix to control retention and release of MBVs on therapeutically relevant timescales. To this end, we encapsulated dermal fibroblast MBVs in bioinert alginate hydrogels of varying stiffness and stress relaxation rates to determine the impact of matrix mechanical properties on MBV release and retention over a 14-day period. We found that stiffer matrices increased MBV release compared to their softer counterparts. Additionally, fast-relaxing matrices exhibited release of MBVs in the first four days of release experiments, in contrast with slow-relaxing matrices, which promoted long-term sequestration of nearly all encapsulated MBVs regardless of differences in matrix stiffness. Our results offer promise that alginate hydrogels can be utilized for more precise control of MBV delivery in the body and may overcome limitations associated with current EV administration methods.