A press release issued in April 2013, by Organovo Holdings, Inc., USA, a manufacturer of 3D human tissues, announces the creation of a three dimensional human liver tissue using a technology called “bioprinting”.
These tissues were engineered using a bioprinting platform branded “NovoGen” by the Organovo. As Dr. Sharon Presnell, Chief Technology Officer and Executive Vice President of Research and Development, at Organovo has stated in the press release; “They actually look and feel like living tissues."
The printed liver tissues which are comprised of about 20 cell layers, successfully imitate the characteristic cellular patterns of the natural liver tissues. Furthermore, these tissues could carry out essential liver functions such as production of albumin, fibrinogen and transferrin, inducible enzyme activities etc. Organovo claims that their bioprinted liver tissues were even capable of carrying out Cholesterol biosynthesis, which has not previously shown feasible in vitro in a multi-cellular 3D human liver system.
A stained Cross-section of the human liver tissue bioprinted by Organovo
Bioprinting- An Introduction
Bioprinting is a process essentially similar in principle to the digital 3D printing technology. Instead of ink droplets or materials like plastic or metal, the printer heads of these “bioprinters” deposit several layers of cellular aggregates-termed as “bioink”- along with a cell-free dissolvable supporting medium-termed as “bio-paper’-in both horizontal and vertical directions, creating a three-dimensional cellular organ.
Bioink particles are the building blocks of the bioprinting process. These bioink units are masses of cells of either spherical or cylindrical shapes. These can be composed of a single cell type (homogenous) or of different types of cells (heterogeneous). Bioink can be prepared using several methods. One such method involves culturing cells in an appropriate medium followed by centrifuging into pellets. These pellets are then transferred into micropipettes and incubated in order to re-establish the cellular interactions. Finally, these cell pellets are extruded into cylindrical bioink particles. Alternatively, these extruded cylinders can be cut into fragments of uniform size and placed overnight in a gyratory shaker allowing them to be shaped into spherical bioink droplets.
The type of cells in the bioink depends upon the type of tissue to be printed. A bioprinter can also carry different kinds of bioinks in different cartridges in the same way a conventional printer contains inks of different colours.
Bio-paper-a support matrix usually composed of biocompatible hydrogel- acts as the framework and protects the cells during the printing process. However, this is different from the traditional solid scaffold-based tissue engineering, where cells are seeded into a natural or synthetic scaffold followed by culturing in a bioreactor. In the bioprinting process, the bio-paper is ultimately removed, thus making the printed organ ‘scaffold-free’, implying that it does not depend on the scaffold for its three-dimensionality. The hydrogel particle may also be used to as gap-fillers in the printing process to create the hollows and grooves within organs where necessary.
Bioprinting- The Process
In the process of printing the tissue, ‘an image’ i.e. a computer-based model of the target tissue has to be first created. Then, the bioink particles required for printing have to be developed using a suitable protocol. The types of bioink are determined based on the type tissue that is to be printed. For example, for the printing of the liver tissue, three main types of cells found natural liver tissues-namely hepatocytes, endothelial cells and hepatic stellate cells- were used as bioinks. Then these bioinks are loaded into the bioprinter along with the bio-paper. The cell aggregates and the supporting hydrogel matrix are deposited layer-by-layer as directed by the digital design. After the printing process is completed the cells-by themselves-rearrange and fuse into a functioning tissue. This self-assembling, which is entirely a natural phenomenon, increases the efficiency of the bioprinting by making it sufficient to deposit the bioink particles roughly at the required position without having to print all the intricate details of the natural tissue. After the post–printing assembly has taken place, the tissue is further matured in an incubator. The supporting gel matrix is either dissolved away or removed by some other means.
Layer-by-layer approach of bioprinting Image source: http://www.explainingthefuture.com/bioprinting.html
Since the printed tissues are scaffold-free, they eliminate the risk inducing of immunogenic reactions which are common with other engineered tissues. There is the possibility of manufacturing bioink particles using the cells of the recipient, thus making them better candidates in organ transplanting. Bioprinting enables the creation of complex organs composed of multiple cell types with vascular architecture. Furthermore, bioprinting is an automated process making the fabrication of engineered tissues reproducible, scalable and economical.
Bioprinting- The Future Promises
With the ultimate goal of printing a complete, functioning human organ that can be used in organ transplant, scientists are envisioning an array of possibilities bioprinting will bring about. One of the proposed uses is to fabricate tissues that are to be used as models in drug testing and development. Rather than the currently used animal models or the two dimensional cell cultures, these 3D organs will provide better models for such experiments since they closely resemble the natural tissues in their structure and functionality. There is also hope of using this technology in the fields of regenerative medicine. Cosmetics is also another prospective area which will benefit from the bioprinting technology. Moreover, there is the prospect of developin in-situ bioprinting that will enable tissue regeneration by directly depositing the bioink on the body of the patient. Studies are also ongoing about manufacturing food products such as artificial raw meat using the bioprinting technique.
In-situ bioprinting Image source: http://www.explainingthefuture.com/bioprinting.html
Bioprinting in Cosmetics Applications Image source: http://www.explainingthefuture.com/bioprinting.html
1.Jakab, K., Norotte, C., Marga, F., Murphy, K., Vunjak-Novakovic, G., & Forgacs, G. (2010). Tissue engineering by self-assembly and bio-printing of living cells. Biofabrication, 2(2), 022001.
2.Marga, F., Jakab, K., Khatiwala, C., Shepherd, B., Dorfman, S., Hubbard, B., ... & Gabor, F. (2012). Toward engineering functional organ modules by additive manufacturing. Biofabrication, 4(2), 022001.
The following videos will provide a better understanding of the bioprinting process and its applications: