Genetic switches guide 3D-printed spheroids to grow vascularized bone tissue

An interdisciplinary team at Pennsylvania State University has developed a method to 3D-print living cell clusters, called spheroids, that can regenerate bone tissue complete with blood vessels. The researchers started with commercially sourced, undifferentiated stem cells and introduced two specific strands of microRNA — miR-148b to promote bone formation and miR-210 to encourage vascularization — effectively programming the cells before they matured.

After allowing the transfected cells to culture into spheroids, the team used aspiration-assisted bioprinting, a technique that picks up and precisely places individual spheroids within a hydrogel scaffold. This precision enables uniform spacing, which the researchers say is critical for consistent tissue regeneration. They printed scaffolds containing spheroids with only one microRNA type as well as scaffolds with an alternating pattern of both types.

In laboratory culture over 28 days, the spheroids differentiated along the intended pathways. The team then tested the constructs in immunodeficient mice with critical-size bone defects over six weeks. Untreated defects healed to about 35% coverage, while a scaffold-only control reached 93%, confirming the bioprinting procedure itself aids regeneration.

Scaffolds containing the mixed population of spheroids — those programmed for bone and those programmed for vessels — showed greater overall bone coverage and higher expression of CD31, a protein that marks the inner lining of blood vessels, compared with scaffolds containing only one spheroid type. The researchers suggest the different cell populations may cooperate to drive both bone and vascular growth, though the exact mechanism remains under investigation.

The work is aimed at patients who have lost substantial bone due to trauma, cancer, or infection, not routine fractures. Co-corresponding authors Daniel Hayes and Ibrahim Ozbolat emphasized that clinical translation will require deeper understanding of how vascularization and bone formation interact in larger animal models. The materials used are commercially available and scalable, which the team sees as an opportunity to build that foundational knowledge.

The study was published in Chemical Engineering Journal (2026) under the title "Bioprinting of miRNA-induced spheroids for vascularized, heterocellular bone regeneration."