Researchers are hopeful that new advances in tissue engineering and regenerative medicine could one day make areplacement liver from a patient’s own cells, or animal muscle tissue that could be cut into steaks without ever being inside a cow. Scientists 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; organs have complicated 3D blood vessel networks that are still impossible to recreate.

Now, 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 led by Jordan Miller and Christopher Chen, along with Sangeeta Bhatia and Kelly Stevens of MIT.

It appears in the journal Nature Materials. Without a vascular system — a highway for delivering nutrients and removing waste products — living cells on the inside of a 3D tissue structure die. Thin tissues grown from a few layers of cells don’t have this problem, as all of the cells have direct access to nutrients and oxygen.

Bioengineers have therefore explored 3D printing as a way to prototype tissues containing large volumes of 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.

But many useful cell types, like liver cells, cannot readily survive the rigors of direct 3D bioprinting. To get around this problem, researchers turned the printing process inside out. Rather than trying to print a large volume of tissue and leave hollow channels in a layer-by-layer approach, Chen focused on the vasculature first and designed free-standing 3D filament networks in the shape of a vascular system that sat inside a mould. The approach allowed for the template to be removed once the cells formed a tissue.

This rapid casting technique hinged on developing a material that is rigid enough to exist as a 3D network of cylindrical filaments but which can also easily dissolve in water without toxic effects. They also needed to make the material compatible with a 3D printer so they could make reproducible vascular networks faster, and at larger scale and higher complexity, than possible in layer-by-layer bioprinting.

After much testing, the team found the perfect mix of properties in a humble material: sugar. Sugars are mechanically strong and make up the majority of organic biomass in the form of cellulose, but their building blocks are also typically added and dissolved into nutrient media that help cells grow.

They used a combination of sucrose and glucose along with dextran for structural reinforcement and printed it with a RepRap, an open-source 3D printer. 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 whole process is quick and inexpensive, allowing the researchers to switch with ease between computer simulations and physical models of multiple vascular configurations. The researchers showed that human blood vessel cells that were injected throughout the sugar-based vascular networks spontaneously generated new capillary sprouts to increase the vascular network’s reach, in the same way blood vessels in the body naturally grow.