–Pulling Back the Curtain
All of the preceding articles in this series have concentrated on current challenges to 3D bioprinting as well as ones on the horizon that are now coming into focus. By identifying key barriers to the field, we gated the technology into four major stages which are:
Stage 1: Lacks the ability to print using cells and/or proteins with high fidelity.
Stage 2: Prints cells and/or proteins at high fidelity, but no significant tissue function upon print completion.
Stage 3: Combines an immature 3D printed tissue with a coordinated maturation system, resulting in a functional tissue for use or study.
Stage 4: Produces a tissue or organ that is functional immediately upon print completion.
As we learn to print soft extracellular matrix (ECM) proteins with higher precision (Stage 1 to 2), we will begin to print structures that better recapitulate blood vessels that aid in keeping tissues alive with the assistance of more complicated bioreactors to mature them for implantation (Stage 2 to 3). Stage 4, however, is the furthest level of advancement possible to the point of being science fiction. Much of the difficulty in Stage 3 lies in combining siloed areas of research together into a coherent pipeline. To use a historical example, successfully launching a rocket led to controlling its trajectory which led to returning it to earth intact. These gated steps were each realistic challenges and had all been done in separate lower risk scenarios, but the successful lunar landing in 1969 represented a monumental achievement. Stage 4, however, represents something of jumping to light speed to continue using space exploration as an analogy. In Part 1 of this series, we discussed lifting the veil off the headlines surrounding the field to gain a sober view of its future. While the first three parts of this series focused mainly on technical challenges, this final piece is meant to follow through on dispelling some of the more fantastical claims on the potential future of bioprinting a full organ as frequently seen in headlines (Fig. 1).
Simply put, the challenges of bioprinting fall into two major categories:
- Addressable: a discrete challenge that is not currently solved, but a path to a solution can be hypothesized.
- Fundamental: a core assumption upholding the field that has yet to be answered.
Addressable problems: Referring to space exploration as an archetypal example, an addressable challenge is creating a zero-waste environment where all resources like water and food are replenishable with the aid of solar energy. This challenge is not trivial, but already being tackled in the International Space Station. Some of the large, but addressable, challenges to printing an organ are listed below.
- Patient-specific cell culture: Using patient-derived cells reduces a large risk factor of the immune response, but the challenge here is analogous to the clothing industry. Off-the-rack clothes are affordable but come in limited sizes while bespoke garments fit perfectly at a high labor cost. The difficulty in culturing numerous patient-derived cells is the labor and logistical tracking required. Although one cell type might make up a substantial portion of an organ (cardiomyocytes in the heart), that does not mean the minority components (pacemaker, endothelial, vascular smooth muscle cells, etc.) can be done away with. Obtaining and culturing all these various cell lines in separate media prior to printing will be a nightmare in coordination.
- Highthroughput cell culture: A major bottleneck to tissue engineering research is simply the rate at which human cells can be cultured. Expanding culture by increasing surface area scales only as a squared function, while volume scales cubically. The current practice of culturing in more 2D flasks will eventually reach a breaking point of inefficiency when billions of cells are needed per round of an experiment. Fortunately, there are several areas of research trying to grow cells efficiently in 3D such as carrier beads and tissue scaffolds.
- Multimaterial printing: If each cell type can be made into its own bioink, then printing with a litany of inks accurately becomes crucial. The difficulty here can become that print time increases drastically with each new bioink added. The good news is that this challenge is mechanical (for syringe-based printers), and more of a matter of implementation than creating from scratch. Many machine systems can move with extreme precision in 3D, they have just yet to be combined on a bioprinter system.
- Regulations: Without getting drawn into a discussion about what size of a role regulatory agencies should play in medicine, it is important to acknowledge that it will be a significant gatekeeper. The time and money required to administer animal and human trials in pharmaceuticals take roughly a decade and a billion dollars for one drug. To attempt to shift this over to a bespoke, patient-specific, cell-based medical device where the recipe changes for literally every patient is a quality control challenge for both manufacturers and regulators.
Fundamental problems: Reverting back to space exploration, a fundamental problem with travel between the stars lies in achieving speeds even 10% of the speed of light. For reference, Voyager (the farthest man-made object from earth) is travelling at 1/18,000th the speed of light. In the same sense, there are many far-off, fundamental challenges to bioprinting an organ that have yet to be answered, of which some are listed below.
- Assuming function will follow form: A core assumption in the field of tissue engineering is that if we use a native tissue as a cheat sheet and organize cells in the same general manner, we will develop an equivalent tissue. A mature tissue is a result of an immensely complex system of developmental biology where form and function complement one another from the microscopic scale upward. Simply placing cells in the same general spatial arrangement and thinking it equivalent to an organ could turn out to be analogous to throwing lumber, tubing, insulation, and wiring together in the shape of a building and thinking it will work just like the neighbor’s house. Current engineered tissues are fortunate if they produce even 10% of native tissue function. The belief that a printed organ will function equivalent to a native organ immediately after being printed assumes a lot about the value (or lack thereof) in allowing tissues to mature over long periods of time.
- Creating a tumor instead of a tissue: Tissue engineering is heavily focused on keeping large numbers of cells alive by trying to vascularize 3D tissues. There is a likelihood that placing such an emphasis on vascularizing a tissue that we will then realize what we have created is more akin to a tumor than healthy tissue. Aside from the microvascular organizational differences serving as a potential canary in the coalmine, the next question becomes, “When this engineered tissue is implanted, will it display contact inhibition?”. Contact inhibition is a regulatory phenomenon in healthy cells in which cells slow their proliferation rate when they begin to contact one another. Tumor cells, on the other hand, continue to grow unregulated, resulting in tumor progression and metastasis. A potential risk to tissue engineering is focusing on maximizing cell growth and angiogenesis to speed up tissue maturation while increasing the risk of unintentionally building tumor-like tissues that grow out of control when implanted.
- Developing better alternatives: No technology exists solely on its own. Alternatives can spring up nearly overnight and decimate established players similar to how smartphones have largely replaced personal digital cameras. While 3D bioprinting is being researched, so are other technologies that could intentionally or unintentionally threaten it. These disruptions can come in the form of either mechanically or bioengineered alternatives. Potential mechanical alternatives could be ventricular assist devices, dialysis machines, and insulin pumps that continue to improve over their current versions as to potentially lower the need for engineered hearts, kidneys, and pancreases, respectively. In terms of bioengineered solutions, the genetic modification of pigs to remove the threat of donor rejection in humans could potentially solve many organ shortage problems by effectively relying on the pig as the bioprinter and incubator. While none of these solutions are guaranteed, the potential still exists. Even if one or several solutions end up a reality, it will come at the cost of bioprinting, but it is undoubtedly an immense gain for society.
The combination of both addressable and fundamental problems runs in opposition to the frequent claim that artificial organs are “5 to 10 years away” just as they allegedly were 5 to 10 years ago. Even if funding to the field continues to increase, I am reminded of the phrase “9 women cannot produce a child in 1 month”, as money is not the only barrier to technological innovation. The persistent stiff-arming of expectations runs the risk of a funding ‘winter’ for bioprinting as has happened in other fields. Notable funding winters occurred in artificial intelligence (AI) which went through two in the 1970s and 1990s as promises about AI potential did not match with the computational power required at the time (Fig. 2).
There is a concerning parallel between the claims touted in media headlines and the current ability to 3D bioprint organs just as AI researchers in the 1970s claimed a Jetsons-style future was only years away. Although 3D bioprinting is a new and endlessly exciting field, I ask that researchers demonstrate the courage to be realistic, lest we risk trapping our expectations in the Land of Oz, where bioprinted “organs” are forever and always 5 years away.
About the Author:
Chief Operations Officer, Fluidform
Andrew is a co-founder of FluidForm and leads the development, manufacturing, and scale-up efforts of LifeSupport™. His research focuses on developing the next generation of techniques for vascularizing 3D-bioprinted tissues to improve the clinical translational potential of tissue-engineered therapies.