Microfluidic Encapsulation of Human Bone Mesenchymal Stromal Cells (hBMSCs) in Microgels for Cartilage Regeneration

INTRODUCTION: Microgels are defined as microscale particles of conventional bulk hydrogels. With enhanced surface area to volume ratio, microgels provide biocompatible scaffolds for encapsulated cells with increased nutrient/gas exchange and greater cell‐cell interactions to promote the formation of engineered tissues. In cartilage regeneration, microgels laden with human bone marrow‐derived mesenchymal stromal cells (hBMSCs) have achieved promising outcomes in mimic cartilage tissues production. Recent findings revealed that encapsulated cells tend to migrate toward the surface of the microgels after a few days and eventually deposit their cartilaginous matrix in the voids formed between the microgels. However, the mechanism behind this phenomenon has not yet been clear. Microgel size has been hypothesized to be one of the key factors influencing this phenomenon. Our work has focused on elucidating the impact of microgel size on cell migration and cartilage tissue formation.

METHODS: Devices: We previously developed a microfluidic system assembled from laboratory consumables to produce gelatin‐based microgels. This study modified the device nozzle part size and optimised flow rate combinations of the oil fluid versus the hydrogel fluid to produce microgels of three different sizes.
Cells: hBMSCs were encapsulated within microgels and subsequently induced towards chondrogenic differentiation for engineered cartilage production.
Analysis: Cell viability and migration within varying‐sized microgels were evaluated and histological staining was assessed to visualise the newly formed cartilage quality.

RESULTS: Gelatin‐PEG based microgels with high uniformity and controllable size (200 – 600 μm) were fabricated. Three microgel sizes 500 μm, 350 μm, and 180 μm were used to investigate cell viability and migration. Live/dead staining showed high viability in all conditions. At 3 days, more cells reached the microgel surface in the 180 and 350 μm microgels, compared to 500 μm microgels but on day 7 most cells were on the microgel periphery, regardless of size. Cartilage tissue structure was also affected by microgel size, a uniform cartilage structure was present in voids between 350 μm microgels, whereas cells loaded with 500 μm microgels showed sparse cartilage accumulation. The 180 μm microgel structure collapsed, leaving a contracted cartilage aggregate.

DISCUSSION & CONCLUSIONS: This work fabricated a microfluidic system that enabled microgel production at a wide range of sizes (200 – 600 μm). Microgels of varying sizes were used to better understand the connection between cell migration and cartilage formation in microgel constructs, showing an optimum microgel size of 350 μm. These insights can be applied to improve the quality of engineered cartilage for cartilage regeneration.