Upscaling Osteoclast Generation by Enhancing Macrophage Aggregation Using Hollow Microgels

Osteoclasts, the bone resorbing cells of hematopoietic origin formed by macrophage fusion, are essential in bone health and disease. However, in vitro research on osteoclasts remains challenging due to heterogeneous cultures that only contain a few multinucleated osteoclasts. Indeed, a strategy to generate homogeneous populations of multinucleated osteoclasts in a scalable manner has remained elusive. Here, the investigation focuses on whether microencapsulation of human macrophages in microfluidically generated hollow, sacrificial tyramine-conjugated dextran (Dex-TA) microgels could facilitate macrophage precursor aggregation and formation of multinucleated osteoclasts. Therefore, human mononuclear cells are isolated from buffy coats and differentiated toward macrophages. Macrophages are encapsulated in microgels using flow focus microfluidics and outside-in enzymatic oxidative phenolic crosslinking, and differentiated toward osteoclasts. Morphology, viability, and osteoclast fusion of microencapsulated cells are assessed. Furthermore, microgels are degraded to allow cell sorting of released cells based on osteoclastic marker expression. The successful encapsulation and osteoclast formation of human macrophages in Dex-TA microgels are reported for the first time using high-throughput droplet microfluidics. Intriguingly, osteoclast formation within these 3D microenvironments occurs at a significantly higher level compared to the conventional 2D culture system. Furthermore, the feasibility of establishing a pure osteoclast culture from cell transfer and release from degradable microgels is demonstrated.