In the United States, the waiting list for organ transplants kills as many as 20 people every day. While over 30,000 transplants are being carried out every year, there are more than 113,000 patients who are on organ waitlists.
Many people perceive artificially grown human organs as the “holy grail” to overcome the shortage of organs, and developments in 3D printing have made this method so popular that it is used for building living tissue constructs similar to the shape of human organs.
But, to date, all 3D-printed human tissues do not have the required organ-level functions and cellular density and, hence, they could not be used for repairing and replacing organs.
A novel method known as sacrificial writing into functional tissue (SWIFT) developed by scientists from Harvard’s Wyss Institute for Biologically Inspired Engineering and John A. Paulson School of Engineering and Applied Sciences (SEAS) overcomes that significant barrier.
To achieve this, vascular channels were 3D printed into living matrices made up of stem-cell-derived organ building blocks (OBBs), producing viable and organ-specific tissues that have excellent function and cell density. The study has been published in Science Advances.
This is an entirely new paradigm for tissue fabrication. Rather than trying to 3D-print an entire organ’s worth of cells, SWIFT focuses on only printing the vessels necessary to support a living tissue construct that contains large quantities of OBBs, which may ultimately be used therapeutically to repair and replace human organs with lab-grown versions containing patients’ own cells.
Mark Skylar-Scott, Study Co-First Author, PhD, and Research Associate, Wyss Institute, Harvard University
The SWIFT technique is a two-step process. It starts with creating an unlimited number of stem-cell-derived aggregates into a thick, living matrix of OBBs containing around 200 million cells for each milliliter.
Then, a vascular network via which various nutrients, including oxygen, can be sent to the cells is integrated inside the matrix by writing and eliminating a sacrificial ink.
Forming a dense matrix from these OBBs kills two birds with one stone: not only does it achieve a high cellular density akin to that of human organs, but the matrix’s viscosity also enables printing of a pervasive network of perfusable channels within it to mimic the blood vessels that support human organs.
Sébastien Uzel, Study Co-First Author, PhD, and Research Associate, Wyss Institute and SEAS, Harvard University
Within the SWIFT technique, cellular aggregates are obtained from adult-induced pluripotent stem cells. These cells, in turn, are combined with a customized extracellular matrix (ECM) solution to create a living matrix that is compacted through centrifugation.
At cold temperatures, that is, 0 °C to 4 °C, the dense living matrix has the consistency of mayonnaise—meaning it is soft enough to control without causing damage to the cells but dense enough to retain its shape. These aspects make it the best medium for sacrificial 3D printing.
In this method, a thin nozzle travels through the dense matrix and deposits a gelatin “ink” strand that forces the cells out of the way without causing damage to them.
When heated to 37 °C, the cold matrix turns stiff and becomes more solid (similar to an omelet being cooked), and at the same time, the gelatin ink melts, which can be cleaned out. This creates a network of channels that are integrated into the tissue construct, and this, in turn, can be perfused with oxygenated media to provide nourishment to the cells.
The team was able to alter the channels’ diameter from 400 μm to 1 mm and flawlessly linked them to create branching vascular networks inside the tissues.
Organ-specific tissues printed with integrated vascular channels using the SWIFT method and perfused in this fashion continued to be viable. However, tissues that were cultured without these channels underwent cell death in their cores within a period of 12 hours.
In order to observe whether the tissues exhibited organ-specific functions, the researchers first printed, then evacuated, and finally perfused a branching channel design into a matrix containing heart-derived cells. Next, they allowed the media to flow via the channels for more than a week.
At that time, the cardiac OBBs combined together to create a relatively solid cardiac tissue and the contractions of this tissue turned out to be more synchronous and more than 20-fold stronger, imitating important traits of a human heart.
Our SWIFT biomanufacturing method is highly effective at creating organ-specific tissues at scale from OBBs ranging from aggregates of primary cells to stem-cell-derived organoids. By integrating recent advances from stem-cell researchers with the bioprinting methods developed by my lab, we believe SWIFT will greatly advance the field of organ engineering around the world.
Jennifer Lewis, Sc.D., Study Corresponding Author, Core Faculty Member, Wyss Institute, Harvard University
Lewis is also the Hansjörg Wyss Professor of Biologically Inspired Engineering at SEAS.
Collaborations are ongoing with Wyss Institute faculty members Chris Chen, MD, PhD at Boston University and Sangeeta Bhatia, MD, PhD, at MIT in order to embed these tissues within the animal models and examine their host integration. This would be done as part of the 3D Organ Engineering Initiative, co-headed by Lewis and Chris Chen.
“The ability to support living human tissues with vascular channels is a huge step toward the goal of creating functional human organs outside of the body,” stated Donald Ingber, MD, PhD, and Founding Director of Wyss Institute.
Ingber continued, “We continue to be impressed by the achievements in Jennifer’s lab including this research, which ultimately has the potential to dramatically improve both organ engineering and the lifespans of patients whose own organs are failing.”
Ingber is also the Judah Folkman Professor of Vascular Biology at HMS, the Vascular Biology Program at Boston Children’s Hospital, and Professor of Bioengineering at SEAS.
The paper’s additional authors include John Ahrens, a present graduate student at the Wyss Institute at Harvard University and Harvard SEAS, and also former Wyss Institute and Harvard SEAS members Lucy Nam, Ryan Truby, PhD, and Sarita Damaraju.
The Office of Naval Research Vannevar Bush Faculty Fellowship, the National Institutes of Health, GETTYLAB, and the Wyss Institute for Biologically Inspired Engineering at Harvard University supported the study.
This video shows the SWIFT bioprinting process, including forming dense organ building blocks of living cells, printing and evacuating of sacrificial gelatin ink, and creating cardiac tissue that successfully beats like a living heart over a seven-day period. (Video credit: Wyss Institute at Harvard University)