Biology, geometry unite to thwart common cardiovascular diseases

Science Highlights
February 22, 2018
Thomas M Johnson

Implanted patches seeded with specific patterns of endothelial cells produce organized, tissue-saving vascular networks

When cardiovascular disease causes blocked blood vessels, tissues die because the oxygen carried by blood cells cannot reach the tissue. Tissue starved of oxygen is called ischemic. Surgery can remove blockages in large vessels in the heart or legs but is not possible in small vessels; they are just too small to manipulate. To address this problem, researchers funded by the National Institute of Biomedical Imaging and Bioengineering (NIBIB) designed 3D-printed patches seeded with vessel-inducing endothelial cells in various geometric patterns. Using a mouse model of hindlimb ischemia, the researchers identified specific patch patterns that induced growth of organized, tissue-saving blood vessels, demonstrating the potential for the novel technology to address this significant public health problem.

Ischemic cardiovascular disease is the leading cause of death and disability in the United States and is growing worldwide. Impaired blood circulation to tissues and organs causes many serious conditions including peripheral artery disease, heart failure, and stroke. Much of the damage that causes these conditions occurs in small vessels that are difficult to treat. 
Vascular patch placed in removed section of femoral artery in mouse model
In the experimental model of an ischemic limb, a section of the femoral artery in the mouse leg is removed, and a vascular patch is placed in the resected area. The patch contains channels lined with endothelial cells that induce formation of new blood vessels. Credit: Springer Nature: Mirabella, et al.1

Now, an interdisciplinary group of engineers and physicians has teamed up to apply biomedical engineering strategies to address the problem of ischemia caused by small vessel damage. Led by Christopher Chen, M.D., Ph.D., Professor of Biomedical Engineering and Founding Director of the Biological Design Center at Boston University, the results of their work are reported in the June issue of Nature Biomedical Engineering.1

Guiding the growth of functional vascular networks

With surgery not possible in small vessels, researchers have turned to strategies to induce the formation of new vessels. These attempt to mimic the body’s natural repair process where vascular endothelial cells and various vascular growth factors can induce the sprouting of new vessels in response to damage. However, using this approach to develop a successful treatment has proven difficult.

“We know that when growth factors are injected into a tissue, they do induce the sprouting of new blood vessels, but in a disorganized pattern unable to deliver oxygen to ischemic tissues” explained Chen. “Our goal was to use engineering to direct the growth of new vessels into an orderly, functional network.”

To direct organized vessel formation, the researchers designed and constructed 3D-printed vascular patches with different patterns of channels. The channels were lined with the endothelial cells that induce the sprouting of new blood vessels. The patterns included straight, parallel rows of channels, a grid pattern with channels crisscrossing each other, and a “no pattern” control with the endothelial cells scattered randomly over the entire patch.

Testing the 3D-printed patches

The patches were tested in a mouse model of ischemia. In the model, a section of the femoral artery in the back, left leg is removed (about a one-centimeter piece), causing ischemia in the foot. The patches were implanted in that gap to test their ability to induce growth of organized new blood vessels and deliver oxygen to the ischemic foot. The right leg was left undisturbed for comparison to the treated left leg. The team used laser Doppler imaging to follow the progress of any vessel formation in the treated ischemic limb. 
vascular patches tested in mouse leg ischemia model
A section of the mouse femoral artery was removed and replaced with each of three different vascular patches (VP) in one leg of the mouse (white arrows). The lack of blood in the ischemic foot can be seen post-surgery. Five days after surgery the Par VP patch, engineered with parallel channels of endothelial cells, resulted in complete restoration of blood flow to the ischemic foot (yellow arrow). The Grid pattern restored some blood flow, and the random pattern (EP) failed to restore any blood flow. Springer Nature: Mirabella, et al.1

At day five post-surgery the sites with patches bearing straight rows of channels of endothelial cells gave the best result, producing a vigorous network of vessels that oxygenated the foot of the mouse to a level nearly equal to the normal right foot. The “no pattern” patch had the worst result—at day five the foot was as ischemic as directly after surgery, with few blood vessels having been formed in the leg. The grid pattern gave an intermediate result with oxygenation of the foot getting to about 70% of the normal control.

“Although we are still at the very early stages of this project, we are encouraged by the initial results,” concludes Chen. And his group plans to continue refining the architectural design of the patches to optimize their effectiveness. The other plan is to continue the collaboration with biologists and clinicians. “I can’t stress enough how the current and future success of this project is completely dependent on the partnership of engineers, biologists and clinicians,” said Chen. “We are all excited about what can be accomplished when there is so much diverse, yet interdependent expertise focused on beating this major public health problem.”

Adds Rosemarie Hunziker, Ph.D., Director of the NIBIB Program in Tissue Engineering, “The results of this collaboration are an excellent example of how engineers can take what biologists and physicians know about how our bodies work and use the information to create practical, innovative medical treatments.”

The work was a collaborative effort of engineers, biologists, and physicians from the Department of Bioengineering and the Biological Design Center, Boston University; the Wyss Institute for Biologically Inspired Engineering, Harvard University; the Department of Surgery, University of Pennsylvania; the Department of Surgery, Brigham and Women’s Hospital and Harvard Medical School, Boston; the Department of Cardiothoracic Surgery, Stanford University, Palo Alto, California; and Innolign Biomedical, Boston, Massachusetts.

The work was supported by grants EB00262 and EB08396 from the National Institute of Biomedical Imaging and Bioengineering, The National Heart, Lung, and Blood Institute, the Biological Design Center of Boston University and the BU-Coulter Foundation Translational Partnership Program, the National Science Foundation, and the American Heart Association.

1. 3D-printed vascular networks direct therapeutic angiogenesis in ischaemia. T. Mirabella, J. W. MacArthur, D. Cheng, C. K. Ozaki, Y. J. Woo, M. T. Yang and C. S. Chen. Nature Biomedical Engineering, June 13, 2017