Bio-printing has brought new perspectives to cell research. So far, however, other 3D printing methods have fallen short of expectations. A special bio-ink has now been developed at TU Wien (Vienna) that solves the current problems.
The new bio-ink enables:
- extremely fast and high-resolution 3D printing;
- integration of living cells directly into microstructures during the printing process.
Bio-printing of microstructures provides cell research with models whereby it can be observed how diseases spread via cells and how their behaviour can be controlled. Nevertheless, the challenges for 3D printing are considerable. Not only do the structures have to be tiny, they also need to reflect the natural environments of cells. As it is the mechanical and chemical properties as well as the geometries of the cell environments that influence cell proliferation.
In concrete terms, this means that cell environments must be permeable for nutrients so that the cells can survive and multiply. It is also important whether the structures are rigid or flexible. And whether they are stable or if they degrade over time.
Problems related to bio-printing
Manufacture of microscopic 3D objects is nowadays relatively straightforward. Living cells are embedded in the structure as part of the 3D procedure using bioprinting technology – a special additive 3D printing process. The drawbacks of this technique have on occasion been a lack of precision. As well as a time frame that is very brief for processing living cells. The cells are damaged if the time frame is exceeded.
Precision vs. speed
The biggest technical challenge in bioprinting was at times the low resolution that conventional technologies offered. Lithography-based approaches such as two-photon polymerization (2PP) are able to overcome this limitation.
Researchers at the TU Vienna have many years of practical experience in the application of this method. This is based on a chemical reaction that only becomes active when a molecule of the material simultaneously absorbs two of the laser beam’s photons. This is the case if the laser beam has a particularly high intensity and causes a selective and very specific hardening of the substance. These properties are conducive to high precision manufacture of the finest of structures.
The disadvantages of the two-photon polymerization is the slow printing speed. This ranges at times from micrometers to a few millimeters per second.
According to Professor Aleksandr Ovsianikov, head of the 3D Printing and Biofabrication research group at the Institute of Materials Science and Technology at TU Wien, the slower print speed in bioprinting is the result of certain chemical substances. His team achieved a speed of one meter per second with cell-friendly materials. The process must be completed in a few hours in order for the cells to survive and continue to develop.
This represents a major breakthrough when it comes to embedding living cells for two-photon polymerization, Ovsianikov explains.
“The high level of speed achievable in laser scanning makes it possible to quickly generate structures for statistical analysis during cell culture experiments as well as for large-scale production”. Aleksandr Ovsianikov
Another advantage of this method is that the cell environments can be individually adapted. Depending on the structure, they can be made more rigid or softer. Even delicate, continuous transitions are possible. The laser intensity can also be used to adjust the degradation of the structure relative to time.
The bio-ink is based on a a gelatin norbornene hydrogel, whereby dithiothreitol was used as a thiol cross-linking agent together with a special biocompatible photoinitiator based on diazosulfonate (doi: 10.1039/C8PY00278A).
Compatible with stem cells
The discovery of the cell-friendly bio-ink is not only a technical breakthrough, but also a major contribution to cell research. The microstructures that result from this process provide unprecedented accuracy. New insights can be gained into the spread of diseases throughout the body.
“Furthermore, the material is also compatible with stem cells and has already been tested with obese human stem cells in a laboratory. As with the L929 cells used in the publication*, these cells can be embedded directly into the 3D matrix and printed in accordance with a suitable architecture. This leads to excellent cell viability.” Aleksandr Ovsianikov
The research project constitutes a transnational and interdisciplinary collaboration. Besides the TU Wien, several Belgian research institutes were also involved: the Polymer Chemistry and Biomaterials Group in Gent, the Brussels Photonics Campus, the Department of Applied Physics and Photonics at the University of Brussels, Flanders Make in Lommel and Vrije Universiteit Brussel.
Three institutes were involved at the TU Vienna: The Institute of Materials Science and Technology, the Institute of Applied Synthetic Chemistry and the Institute of Lightweight Structures and Structural Biomechanics.
The high-resolution 3D printing technology and the requisite materials will be provided by UPNano, a young and successful spin-off from TU Vienna.