Manufactured Human Tissues and Organs Are on the Way

3D bioprinting is a highly sophisticated manufacturing process that is designed to make it possible to print tissue and vital organs from cells. Once perfected, approved, and commercialized, this technology promises to transform the economics of transplantation, directly benefiting patients who need replacement organs as well as society as a whole.

Instead of waiting for a suitable donor and having the risk of their bodies rejecting a transplanted organ, this technology will enable patients to have customized organs fabricated specifically to replace their faulty ones.

The Trends editors have been tracking the development of bio-printing since stem cell research began in the 90s. But despite the headway that 3D bioprinting has made over the last two-plus decades, it still lacks the ability to produce complex 3D tissue constructs that function like a patient’s own.

At this point, the biggest hurdle involves tissue culture techniques able to address the bottleneck of “maturing” bio-printed multi-cellular 3D tissue constructs into functional tissues.

In February, a team of researchers in Singapore published an in-depth review of: 

- Recent improvements in bioprinting techniques

- Progress in bio-ink development, and 

- Implementation of new strategies for “tissue maturation” in bioprinting

They paid special attention to the role of polymer science in creating human organs and how it complemented 3D bioprinting by overcoming some of the remaining impediments to the field of organ printing, such as achieving biomimicry, vascularization, and forming anatomically correct 3D biological structures.

Specifically, it is now possible to fabricate human-scale tissues or organs that can potentially mature into vascularized and partially functional tissues. But the industry is still unable to reliably bio-print transplantable human tissues or organs due to the complexities in tissue-specific extra-cellular matrices (or ECMs), as well as the tissue maturation process. That’s because of the lack of a suitable “co-culture medium” to support multiple types of cells and the need for further tissue conditioning prior to implantation.

In short, while 3D bio-printed organs are still far from commercialization, the remarkable leaps this technology has made in recent years point to the inevitability of lab-grown, functional organs. To realize this potential, researchers must overcome the remaining technical challenges in creating tissue-specific bio-inks and optimizing the tissue maturation process.

To get an idea of where we stand, consider just seven breakthrough research programs now underway to address the specific roadblocks to reliably and economically manufacturing transplantable human organs:

Breakthrough #1: Today’s most popular 3D printing approach uses a solution of biological material called a bio-ink loaded into a syringe pump extruder and deposited in a layer-by-layer fashion to build the 3D object. Gravity, however, can distort the soft and liquid bio-ink. This distortion of bio-ink results in a loss of fidelity and so presents a challenge to fabricating functional adult-sized tissues and organs. A team at Carnegie Mellon has addressed this with a new technique called Freeform Reversible Embedding of Suspended Hydrogels (or FRESH) which embodies several unique concepts. First, a support bath enables the printing of cells and bio-inks that maintain their position as they cure, while still allowing for the movement of the extrusion needle. Second, the FRESH support bath also provides an environment during the printing process that maintains high cell viability. And finally, FRESH provides the ability to work with the widest range of bio-inks of any 3D-bioprinting method. So far, FRESH has been adopted by several other research institutions.

Breakthrough #2: Today’s bioprinting technology is not only too imprecise to create large organs, it is also too slow. But another recent breakthrough enables bioprinting 10-to-50 times faster than the industry standard. It combines a 3D printing method called stereolithography with jelly-like hydrogels which are already used to make many products including contact lenses and scaffolds used in tissue engineering. According to its developers, this method allows rapid printing of centimeter-sized hydrogel models, while reducing both “parts deformation” and cellular injuries caused by prolonged exposure to environmental stresses.

Breakthrough #3: Most tissues and organs that have been grown in the lab have trouble maintaining the 3D positions of the living cells in the bio-ink once printed. Silk nanofibers embedded in the bio-ink seem to serve this purpose without creating other problems. When printed with bio-ink containing silk fibers, the new technique developed by researchers at the University of Osaka helps printed configurations retain their shape.  

Breakthrough #4:  Today tissues and organs are typically grown with a scaffolding approach where cells are seeded onto a biodegradable supportive structure that provides the underlying architecture for the organ or tissue desired. However, scaffolds can be problematic. First, getting them to degrade and disappear at the right time to coincide with the maturation of the organ is tricky.  Second, degradation byproducts can sometimes be toxic.  And third, scaffolds can also interfere with the development of cell-to-cell connections, which are important for the formation of functional tissues. Fortunately, the University of Illinois at Chicago scientists recently developed a process that enables 3D printing of biological tissues without scaffolds, using "ink" made up only of stem cells. This breakthrough allows for the 3D printing of cells without a classical scaffold support by using a temporary “hydrogel bead bath” in which printing takes place. The gel beads support the living cells as they are printed, keep them in place, and preserves their shape.  Once the cells are printed into the hydrogel bead matrix, it is exposed to UV light, which cross-links the beads together, in effect freezing them in place. This lets the printed cells connect with each other, mature, and grow within a stable structure. Then, simply controlling the degradation of the hydrogel beads, leaves only the intact tissue behind.

Breakthrough #5: Another alternative to today’s conventional tissue scaffolds was recently developed by biomedical engineers at Rutgers University. Hyaluronic acid, a natural molecule found in many tissues throughout the body, has many properties ideal for creating customized scaffolds, but it lacks the required durability. The Rutgers engineers combined modified hyaluronic acid with polyethylene glycol to form a gel that is strengthened via chemical reactions to serve as a scaffold. The mixture is then fine-tuned to have properties that are right for specific cells to multiply, differentiate and remodel the scaffold into the appropriate tissue. The focus is on the stiffness of the gel and “scaffold binding sites” that cells can latch onto. Groups of cells in the body generally make their own support structures, or scaffolds, but scientists can build them from proteins, plastics, and other sources. The researchers envision a system where hyaluronic acid and polyethylene glycol serve as the basic "ink cartridges" for 3D printing. The system would also have other ink cartridges featuring different cells and ligands, which serve as binding sites for cells. The system would print gel scaffolds with the right stiffness, types of living cells, and ligands, based on the type of tissue desired. Both the stiffness and the presence of binding sites provide important signals to the living cells which they need to mature into functioning organs.

Breakthrough #6: One of the biggest obstacles to bioprinting complex tissues and organs is integrating a fully functional vascular system into the tissue. A natural place to start is human skin which is simple compared to most organs, but it has all of the elements of a complex tissue structure. Recently, researchers at Rensselaer Polytechnic Institute demonstrated, for the first time, a way to 3D print living skin, complete with blood vessels. In order to make these skin grafts usable at a clinical level, researchers still need to be able to edit the donor cells using something like CRISPR technology, so that the vessels can integrate and be accepted by the patient's body. The research is just now beginning to address that step. This significant development highlights the vast potential of 3D bioprinting in precision medicine, where solutions can be tailored to specific situations and eventually to individuals. And,

Breakthrough #7: Perhaps the most provocative recent breakthrough comes from Tel Aviv University where, for the first time anywhere, researchers successfully engineered and printed an entire heart replete with cardiac cells, blood vessels, ventricles, and chambers. This heart is made from human cells and patient-specific biological materials. In this process, these materials serve as the bio-inks for 3D printing of complex tissue models. For this proof of concept, the 3D heart is only the size of a rabbit's heart. But larger human hearts can be produced with the same technology. For the research, a biopsy of fatty tissue was taken from a patient. The cellular and non-cellular materials in the tissue were then separated. While the cells were reprogrammed to become pluripotent stem cells, a three-dimensional network of extracellular macromolecules including collagen and glycoproteins, called the extracellular matrix (or ECM), was processed into a personalized hydrogel that served as the printing "ink." After being mixed with the hydrogel, the pluripotent stem cells were efficiently differentiated into both cardiac and endothelial cells to create patient-specific, immune-compatible cardiac patches with blood vessels. And these patches were subsequently integrated to form an entire heart. Importantly, the project addressed the biocompatibility of engineered materials which is crucial to eliminating the risk of implant rejection, which can jeopardize the success of such treatments. As described in the journal Advanced Science, the resulting cardiac tissue completely matched the immunological, cellular, biochemical, and anatomical properties of the patient. The researchers at Tel Aviv University are now planning on culturing printed hearts in the lab and "teaching them to perform" like natural hearts. Then, they plan to transplant the 3D-printed hearts into animal models.

What’s the bottom line?

There is both good and bad news.  

The bad news is that bioprinting transplantable human organs have turned out to be far more difficult than was originally anticipated. After twenty years of research and development, there are still many hurdles yet to be overcome.

The good news is that the remaining hurdles have been identified and are being addressed by researchers around the world.  Slowly, but surely, progress is being made.

Given this trend, we offer the following forecasts for your consideration.

First, by 2030, there will be organ printers in the finest hospitals around the world, and manufactured organs will be routinely transplanted. While many hurdles remain, these are increasingly well understood. Progress will accelerate as both the solutions and the rewards become clearer.

And, Second, by 2040, millions of lives will be saved each year by transplantation of manufactured organs. The net impact on quality of life and human productivity will be enormous. However, the biggest impact may be in reducing long-term care costs for patients waiting to receive transplants or otherwise ineligible to receive them.

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