Notre Dame Researchers Can Now 3D Print Blood Capillary Networks Smaller Than a Hair

The capillary problem has been holding organ printing back

More than 100,000 people in the United States are waiting for an organ transplant, and a new name joins the list every ten minutes. Lab-grown tissues built from a patient's own cells could solve the shortage and remove the risk of rejection, but one obstacle keeps getting in the way: capillaries. These tiny vessels carry oxygen and nutrients to every cell in the body, and they are astonishingly fine. Without them, printed tissues look like organs but cannot function like them.

Researchers at the University of Notre Dame think they have found a way around the problem. A team led by Yanliang Zhang has developed a hybrid bioprinting technique that builds vascular networks with capillaries narrower than ten micrometers. That is smaller than the finest human hair. The work was featured on the cover of Nature Chemical Engineering.

Two printers, one tissue

The new method combines two different printing technologies. First, an extrusion printer lays down the soft gel matrix that acts as artificial tissue. Then an aerosol jet printer deposits fine threads of gelatin inside the matrix. Those threads are later washed away, leaving hollow channels behind. Because aerosol jet printing uses a sheath flow to focus the material, the team can tune the channel width from hundreds of micrometers down to just a few.

The tricky part is calibration. Tiny changes in ink flow or sheath gas flow produce big changes in channel size. Rather than tune every parameter by hand, the researchers trained a machine learning framework to find the right settings for each design. Zhang said the autonomous approach is far faster than the usual trial-and-error routine and gives better, more repeatable channels.

Living cells confirmed the channels work

The real test came when the team seeded the printed channels with endothelial cells, the same cells that line human blood vessels. The cells attached to the channel walls and spread out, forming single-cell layers similar to natural capillaries. The channels did not leak, which is a critical milestone. A vascular network that cannot hold fluid is just a decorative lattice.

Zhang called the result a major breakthrough for bioprinting. His take is direct: achieving capillary-scale resolution is a necessary step toward fully functional tissues and whole organs. The near-term applications are less dramatic but still useful. The networks could power organ-on-a-chip models for drug testing and toxicity screening, letting researchers study how medicines move through living tissue without using animals or human trial subjects.

What comes next

Zhang's group has already secured new funding from the National Institutes of Health to build a more powerful version of the hybrid printer. The long-term goal is an autonomous, intelligent bioprinter capable of producing complete tissues and organs such as hearts, kidneys, and livers. That remains years away, but the capillary hurdle has looked almost impossible for decades. Printing vessels smaller than a hair is the kind of progress that makes the rest of the roadmap look a little more real.