Introduction:
Every ten seconds, a person is added to the organ transplant list. Currently, there are around 120,000 people on the organ transplant list, and every day, twenty-two people on that list die due to the shortage of organs. Countries have tried implementing different policies to increase the amount of organ donors, however, the incentives offered are not enough to motivate individuals to donate organs. The demand for organs significantly exceeds the supply, in fact, by the time you read this sentence, two people will have been added to the organ transplant list. With our current solutions, we are unable to meet the demand for organs, however, thanks to modern technology, in specific bioprinting, we can.
What is Bioprinting?
According to the Oxford Dictionary, bioprinting is defined as “The use of 3D printing technology with materials that incorporate viable living cells”. As you can conclude from the definition above, bioprinting is the process of using cells to create different tissues which can ultimately be used together to create vital organs.
How does Bioprinting work:
The process of bioprinting can be divided into three sections:Imaging and Design, Material and Cell selection, and Bioprinting.
Below is a detailed explanation of the three different processes:
Imaging and Design
When trying to replicate the intricate, heterogeneous architecture of tissues and organs it is imperative that one has a comprehensive understanding of the desired organ’s components. Thanks to medical imaging technology, such as MRIs or CT scans, bioprinting engineers are able to gain information about the desired organ on both a cellular and tissue level. Once the medical image has been captured from one of the said medical imaging modalities, the data must be processed using tomographic reconstruction to produce 2D cross-sectional imageswhich are later translated to 3D anatomical representations. In terms of creating these 3D anatomical representations, there are multiple Computer Aided Design(CAD) programs that can be used to create them, the most effective technique has been using mathematic modeling methods in addition to CAD programs to create these representations (Jakab). Using the followingmethod ultimately enables engineers to view the anatomy of the organ while retaining the voxel[1] information which can later be used for interpreting the volume of the organ. One important concept to understand is that when an organ is being bioprinted, the 3D representations being created should not be an exact match of the organ that is being duplicated. This is because the person seeking an organ transplantmight have some form of an injury or a disease that is altering the shape of their organ.Insuch scenarios, computer modeling techniques are employed to alter the medical image of the defected organ to create representations of how the organ would appear if it was healthy. Once the final model of the tissue or organ has been completed, the tissue or organ begins to prep for manufacturing. This is achieved by dividing the 3D rendered model by 2D horizontal slices that are ultimately imported to the bioprinter(Murphy 775). In order to better understand this process, I created an infographic[2] below that explains the technique:
In the above image, I am trying to bioprint my least favorite candy a “Mega Bruiser Jawbreaker” which can be seen in Image 1. Using tomographic reconstruction on the Jawbreaker, 2D horizontal cross sectionals are created which can be seen in Image 2, in actual practice, the 2D horizontal cross sectionals are far smaller, however, in the diagram above I only drew four cross sectionals to demonstrate the main idea. Then those cross sectionals, which can be seen in Image 3, are imported to the bioprinter. Finally, once the Material/Cell process (explained further down) is completed, the bioprinter prints each cross sectionals successively on top of each other which ultimately produces Image 4, a replicated version of the original jawbreaker.
Material and Cell Selection
When a tissue or organ is being bioprinted, there are various techniques that can be used to bioprint the tissue or organ[3]. Some systems (bioprinter) deposit a continuous bead of bioink[4] to form a 3D structure while other times, a system might deposit materials in defined spaces. The technique used is dictated by the material and cells that are chosen to create the bioink. Initially, 3D printers were not designed for biological applications and non-biological materials were used as the ink to create the different objects. Thus one of the main challenges in the field bioprinting has been finding a material that can provide the desired mechanical and functional properties of a tissue but also be compatible with biological materials.
Currently, the materials being used for regenerative medicine are predominately based on either naturally derived polymers such as alginate, gelatin, collagen and hyaluronic acid or synthetic molecules such as polyethylene glycol or PEG112.Both materials, natural polymers and synthetic molecules, have their advantages and disadvantages when being used in the field of bioprinting. Some benefits of natural polymers are their inherent bioactivity and their similarity to human ECM[5]. In comparison, the main advantage of using synthetic polymers is that they can be “tailored with specific physical properties to suit particular applications” which ultimately make them the better choice bioprinting as it easy for engineers to control their physical properties during synthesis (Murphy 775). However, there are drawbacks to using synthetic polymers such as having poor biocompatibility and that during the degradation of the polymer, it loses some of its mechanical properties. Overall as the variety of biological materials for bioprinting is increasing, the list of “desired traits” has also become more specific and complex. Some of the qualities that these materials must now possess are “suitable crosslinking mechanisms to facilitate bioprinter depositions, higher levels of biocompatibility, and short-term stability” (Murphy 776).
Once the material for the bioink has been selected, the next step is to choose the cells. The choice of cells for a tissue or organ is crucial for correct function of the fabricated construct.Tissues and organs consist of an array of cell types that must be recapitulated in the transplanted tissue in order to achieve correct functionality (Atala 776).
Bioprinting:
Once the Imaging/Design phase and the Material/Cell phase have been completed, the next step is to begin printing. There are three different methods to bioprint organs: Inkjet, Microextrusion, and Laser Assisted bioprinting. All three strategies are explained in detail below:
Inkjet:
Currently, inkjet printers are the most commonly used printers for both biological and non-biological applications. The main idea of inkjet printers is that they deposit controlled volumes of ink on predefined locations. When trying to understand how inkjet bioprinting works, the easiest way to understand it is by thinking about the printers you use at libraries which are typically inkjet printers.The difference for bioprinting is you replace the ink with biological materials (discussed in Materials and Cell selection section) and you replace the paper with a stage on which a 3D object can be printed on (you now have an X, Y, and Z axis). There are two forces that are primarily used when dealing with inkjet bioprinting, thermal and acoustic forces. Thermal inkjet printers function by electrically heating the nozzle of the ink dispenser which ultimately produces “pulses of pressure that force droplets from the nozzle”. Acoustic inkjet printers, on the other hand, have a piezoelectric crystal that “creates an acoustic wave inside the had to break the liquid intro droplets at regular intervals” (Murphy 778). The image[6]on the right
shows the two kinds of inkjet printing, thermal and acoustic (piezoelectric):
Microextrusion:
Microextrusion is the most common and affordable non-biological 3D printer being used today. These kinds of printers are commonly composed of a “temperature-controlled material handling and dispensing system and a stage”(Murphy 777). Microextrusion printer function by creating extrusions[7] of a material which is then stored in a nozzle which ultimately deposits the material onto a substrate. Unlike inkjet printing which prints out liquid droplets, Microextrusion printing prints continuous beadsof bioink along the Z-axis of the stage. There are ultimately two different methods that can be employed to extrude biological materials for bioprinting, pneumatic and mechanical. Pneumatically driven printers are advantageous in the sense that they have simple mechanism components[8] as the force is only limited by air-pressure capabilities of the system. On the other hand, mechanically driven mechanisms have more intricate components (piston, screw, and valve) that work in tandem, ultimately making it more complex. It is this complexity of the system however, that ultimatelyallows for this system to provide greater spatial control of the ink being deposited (Atala 778). The image[9] below shows the two kinds of microextrusion printing, Pneumatically and Mechanical:
Laser-Assisted:
Laser-assisted bioprintingrevolves around the concept of “Laser-Induced Forward Transfer”[10]to make copies of the original organ or tissue.The transfer in LIFT is induced by focusing one or more laser “pulses onto the support film interface (energy absorbing layer in the image below), where heating and phase change of the film provide the propulsion to propel material to a receiving substrate place nearby” (Eason). Laser-assisted bioprinting is not heavily employed by bioprinting engineers, as there are multiple factors that reducethe quality of the replicated tissue. Some of these factors are surface tension, the wettability of the substrate and the viscosity of the biological material/layer (Eason). Ultimately problems in the resolution of the printed organ lead to said organ being less functional hence making it inefficient to use. Despite these problems, there are also some advantages to using LAB (laser-assisted bioprinting) when printing tissues or organs. One benefit is that LAB is nozzle free, therefore there are no chances of nozzle being clogged. Overall, while LAB could produce promising tissues and organs, we currently lack the technology to fix the problems that occur with the resolution of the printed organ. The image[11]on the right shows Laser-assisted bioprinting:
Summary:
The chart below[12]summarizes the information above by showing all the steps (Imaging/Design, Material/Cell selection, and Bioprinting).
Future of Bioprinting: n terms of bioprinting applications today, we have been able to replicate some tissues, however we are still far away from being able to print out complex organs such as a kidney or a heart. The graphic below shows a timeline of what human body parts we will be able to create in the upcoming years[13]. This graphic was created in 2011 so we are currently in the “very soon” stage:
Treatment vs. Enhancements:
This section of the paper begins to investigate the implications in the future once bioprinting becomes possible. One of the main reasons why people disagree with the idea of bioprinting organs is because of the idea of treatment vs enhancement. People believe that instead of printing out organs for life-sustaining purposes, people will print out organs to enhance themselves whether it be internally or cosmetically (bioprinting body parts). For example, a cross-country runner might bioprint a new lung for himself so that he can become a better long distance runner. Many people find this to be unethical and believe enhancements ultimately corrupt the innate purpose of bioprinting.Some people might abuse themselves as they realize if any of their organs fail they can print out a new one (example would be an alcoholic continuously drinking).
Overall, while there are advantages present with bioprinting, there are also some possible consequences that can result from its development.
Other Solutions:
While there is large support for bioprinting, there are still many people who are against it. They believe that there are other solutions available that will be able to increase the supply of organs available for an organ transplant. Many countries have tested programs that offer organ donors stipends, tax breaks, and other financial incentives, but almost all have proven to be ineffective. One plan that has to be proven to be effective is the current plan being implemented by Israel. Israel’s organ donation plan relies on the concept of self-preservation. The program prioritizes organ allocation based upon willingness to be a donor, “If two people on the organ transplant waiting list are medically equally well matched as potential recipients, the organ will go to the person who previously agreed to be an organ donor” (Aptekar). This rule ultimately encourages people to become organ donors as by becoming an organ donor, in the event of them needing an organ, they would have a higher chance of receiving a transplant. This plan has been active in Israel for the past three years however, the results are showing a strong increase in the number of registered organ donors. While this may seem like an “answer”, this still does not truly erase the gap between the demand for organs and the supply of organs for transplants as it still depends on humans to provide the organs.
Conclusion:
Overall, while there are programs that encourage organ donations, I believe it is important to continue investing our resources into the field of bioprinting. Bioprinting would ultimately allow us to create organs for those in need in a time and cost efficient manner. In addition, it would ultimately enable us to expand further in the field of medicine which could benefit avariety of people, ranging from people with congenital organ defects to individuals with lifetime injuries. While we might still be far from being able to bioprint an organ, it is important to realize how great of an impact bioprinting can have as it has the “[bioprinting has the] potential to change the world”[14].
Works Cited
Aptekar. "How Can Organ Donation Rates Be Improved?" The Huffington Post. TheHuffingtonPost.com, n.d. Web. 27 Sept. 2016.
Eason, Rob. "IN THIS SECTION." Laser-Induced Forward Transfer. N.p., n.d. Web. 25 Sept. 2016.
F, Jeffery. "3D Bioprinting Becoming Economically Feasible." National University of Singapore, n.d. Web. 25 Sept. 2016.
Jakab, Karoly, Francoise Marga, Cyrille Norotte, Keith Murphy, Gordana Vunjak-Novakovic, and Gabor Forgacs. "Tissue Engineering by Self-assembly and Bio-printing of Living Cells." Biofabrication. U.S. National Library of Medicine, June 2010. Web. 25 Sept. 2016.
Murphy, Sean, Atala, Anthony. "3D Bioprinting of Tissues and Organs." Biotechnolgy. Nature, 25 June 2014. Web. 25 Sept. 2016.
Papavulur, Alexander. LASER INDUCED FORWARD TRANSFER FOR MATERIALS PATTERNING (n.d.): n. pag. Web. 25 Sept. 2016.
[1] A voxel is a value on a 3-D grid
[2]Image Created by me on Inkskape
[3] Different Techniques will be discussed later, in the last section- “Bioprinting”
[4]Ink that is used to create the organs-combination of the material and cells selected
[5]Extracellular Matrix- The non-cellular portion of a tissue produced by cells and used to provide support
[6]Image was adapted from Katie Vicari/Nature Publishing Group
[7]Extrusion- process to create objects at a fixed cross-sectional
[8]Simple in the sense that the only factor involved is air pressure
[9] Image was adapted from Katie Vicari/Nature Publishing Group
[10]LIFT- involves the pixelated transfer of material from a thin film onto the rear side of a transparent support substrate(Eason)
[11] Image was adapted from Katie Vicari/Nature Publishing Group
[12] Image Citation: Massachusetts Medical Society
[13]Image citation: University of Pittsburgh; developmental biologist Vladimir Mironov
[14]Direct Quote: Jeff Kowalski, CTO of Autodesk