A microscope image of a 3D printed sugar template used for creating vasculature in living tissues. (Credit: Jordan S. Miller)
Researchers are hopeful that new advances in tissue engineering and regenerative medicine could one day make a replacement liver from a patient's own cells, or animal muscle tissue that could be cut into steaks without ever being inside a cow. Bioengineers can already make 2D structures out of many kinds of tissue, but one of the major roadblocks to making the jump to 3D is keeping the cells within large structures from suffocating.
University of Pennsylvania researchers have developed an innovative solution to this perfusion problem: they've shown that 3D printed templates of filament networks can be used to rapidly create vasculature and improve the function of engineered living tissues.
The research was conducted by a team led by postdoctoral fellow Jordan S. Miller and Christopher S. Chen, the Skirkanich Professor of Innovation in the Department of Bioengineering at Penn, along with Sangeeta N. Bhatia, Wilson Professor at the Massachusetts Institute of Technology, and postdoctoral fellow Kelly R. Stevens in Bhatia's laboratory.
Without a vascular system, living cells on the inside of a 3D tissue structure quickly die. Bioengineers have therefore explored 3D printing as a way to prototype tissues containing large volumes of living cells.
The most commonly explored techniques are layer-by-layer fabrication, or bioprinting, where single layers or droplets of cells and gel are created and then assembled together one drop at a time, somewhat like building a stack of LEGOS.
Rather than trying to print a large volume of tissue and leave hollow channels for vasculature in a layer-by-layer approach, Chen and colleagues focused on the vasculature first and designed free-standing 3D filament networks in the shape of a vascular system that sat inside as a mold. The team's approach allowed for the mold to be removed once the cells were added and formed a solid tissue.
''Sometimes the simplest solutions come from going back to the basics,'' Miller said. ''I got the first hint at this solution when I visited a Body Worlds exhibit, where you can see plastic casts of free-standing, whole organ vasculature.''
After much testing, the team found the perfect mix of material properties in a humble material: sugar. Sugars are mechanically strong and make up the majority of organic biomass on the planet in the form of cellulose, but their building blocks are also typically added and dissolved into nutrient media that help cells grow.
''We tested many different sugar formulationss until we were able to optimize all of these characteristics together.'' Miller said.
The formula they settled on -- a combination of sucrose and glucose along with dextran for structural reinforcement -- is printed with a RepRap, an open-source 3D printer with a custom-designed extruder and controlling software. An important step in stabilizing the sugar after printing, templates are coated in a thin layer of a degradable polymer derived from corn. This coating allows the sugar template to be dissolved and to flow out of the gel through the channels they create without inhibiting the solidification of the gel or damaging the growing cells nearby. Once the sugar is removed, the researchers start flowing fluid through the vascular architecture and cells begin to receive nutrients and oxygen similar to the exchange that naturally happens in the body.
The team believe that their 3D printing method will be a scalable solution for a wide variety of cell - and tissue - based applications because all organ vasculature follows similar architectural patterns.
This whole process is quick and inexpensive, allowing the researchers to switch with ease between computer simulations and physical models of multiple vascular configurations. Chen Said: ''This new platform technology, makes tissue formation a gentle and quick jouney.''
In addition to Miller, Chen, Bhatia and Stevens, the research was conducted by Michael T. Yang, Brendon M. Baker, Duc-Huy T. Nguyen, Daniel M. Cohen, Esteban Toro, Peter A. Galie, Xiang Yu and Ritika Chaturvedi of Penn Bioengineering, along with Alice A. Chen of MIT. Bhatia is also a Howard Hughes Medical Institute investigator.
This research was supported by the National Institutes of Health, the Penn Center for Engineering Cells and Regeneration and the American Heart Association-Jon Holden DeHaan Foundation.