Hyaluronic Acid Microgels as Sequential Drug Delivery Systems

Precise delivery of drugs is essential for improving medical treatment outcomes and minimizing side effects. While there have been advancements in preparing drug delivery systems that release their payload in response to specific stimuli, challenges remain in creating carriers, capable of taking up multiple substances and releasing them in sequence. However, this approach could facilitate synergistic effects and reduce the dosing frequency. In this work, we synthesized hyaluronic acid microgels and investigated their suitability for sequential release of multiple compounds. We modified hyaluronic acid with methacrylate groups (6/15/30/60 mol % of HA repeating units), which allow for cross-linking. These precursors were dissolved in water at a concentration of 50 mg mL^-1, and microgels within a size range of 114-250 μm were formed via droplet-based microfluidics. After detailed characterization of the polyanionic microgels, we examined the loading of three cationic (model) drugs: doxorubicin, methylene blue, and besifloxacin. Release was investigated under varying conditions (pH and ionic strength), revealing a rapid burst release within a few hours in a physiological environment. We then examined the loading of larger, fluorescently labeled biomolecules, which remained trapped within the polymeric network under similar conditions. Sustained release over several days was achieved through enzymatic degradation of the microgels using hyaluronidase. Afterward, we loaded both types of payload, drug and biomolecule, into the microgels and released them in sequence. We show that release profiles can be controlled by varying enzyme concentration or the microgels' cross-linking density. Our approach facilitates the sequential delivery of two substances and paves the way toward more effective treatment strategies.