Tumors are complex little environments. Blood vessels, cancer cells, support cells that make a squishy matrix of proteins, and more, all communicate through a subtle ebb and flow of biochemical signals. These complex mechanical and chemical interactions play a key role in whether a tumor will respond to a drug or not, and whether and when a cancer cell will escape, travel elsewhere in the body, and form another tumor—a process called metastasis. For scientists, these environments are devilishly difficult to mimic in lab models of cancer. Now, researchers have used 3-D printing to improve so-called tumors-on-a-chip that mimic the complex biochemistry and cellular interactions of the tumor microenvironment. They hope to use these models to study the underlying biology of metastasis, and to test new drugs (Adv. Mater. 2019, DOI: 10.1002/adma.201806899).
Bioengineers have developed many tools and techniques for building more lifelike tumors, says Angela Panoskaltsis-Mortari, a research professor at the University of Minnesota Medical School. In collaboration with mechanical engineer Michael C. McAlpine, Panoskaltsis-Mortari and her team combined the techniques they found most promising, and added their own twist. For example, they lined microfluidic channels with blood vessel cells so they could mimic the blood vessels in tumors, while giving researchers a means to apply drugs to the tumors and capture escaping cancer cells. The team also used cell-friendly 3-D printers to create structures made up of support cells, called fibroblasts, in a gel matrix on either side of the main blood vessel. This mimics the 3-D environment a cell would encounter in a tumor inside the body, and encourages more natural behavior than the hard, flat surface of a culture dish.
The main novelty in their model is its ability to release biochemicals called growth factors on demand and at realistic concentrations. These molecules encourage the tumor itself to grow, and coax support cells to build new blood vessels to feed the tumor. During the printing process, the team seeds the tumor model with microcapsules filled with these biomolecules. The capsules are decorated with gold nanoparticles whose size and shape are tuned to heat up and burst open the microcapsule when illuminated with particular frequencies of light. The researchers simply scan the right color of laser over a microcapsule and it will start releasing the growth factors. McAlpine developed these microcapsules, which he calls “a 3-D printed chemical toolbox,” in 2015 (Nano Lett. 2015, DOI: 10.1021/acs.nanolett.5b01688); this is the first time they’ve been incorporated into a tissue model.
Panoskaltsis-Mortari says the capsules allow the researchers to create more realistic biochemical gradients in their tumor models. “We can time it so we’re creating a chemical gradient, and doing it slowly,” she says. Scientists usually flood a tumor model with growth factors, which, leads to rapid, and not necessarily realistic, growth.
In one experiment, they printed a spot of cancer cells about a micrometer in diameter on their chip. The tumor cells expressed a green fluorescent protein and the blood vessel cells expressed a red one. The researchers could watch the first steps in metastasis, as bright green cells moved towards the main blood vessel wall. Some cells broke through the wall, and the researchers could capture them using the chip’s microfluidics system. The team plans to sequence these escaping cells to try to understand why they went metastatic. And in future tests, the researchers say they could watch whether a drug causes cancer cells to stay put.
Panoskaltsis-Mortari says that compared with typical tissue models, their chips better reflect the rate at which tumors grow and respond to drugs, as well as how blood vessels develop in a tumor. She attributes these results to their chip’s more modest growth factor gradients.
The metastatic models are “an interesting approach to study one of the most important aspects of cancer,” says Ali Khademhosseini, a bioengineer at the University of California, Los Angeles. Lab-on-chip devices, he says, will be useful for discovering new drugs as well as for studying the biology of disease.
The Minnesota team has made lung cancer and melanoma models. Panoskaltsis-Mortari says the next step is to mimic other aspects of the microenvironment, for example adding immune cells to the mix.