Gut instinct, gut feeling or gutting it out - what exactly is the human gut? It's the workhorse of the digestive system and home to a vast, dynamic community of microorganisms that influence human health. In fact, the human body contains roughly equal numbers of non-human (microbial) and human cells.
Now, we are poised to learn much more about the gut and its pivotal role, thanks to a breakthrough by a team of researchers at Lawrence Livermore National Laboratory (LLNL). They replicated the gut in three dimensions, with all its spectacular architecture and activity, on a fluidic chip the size of a microscope slide.
The results are described in a paper, "Development of a gut-on-a-chip microfluidic device with three-dimensionally printed human intestinal tissue for studying human-microbe interactions," published recently in Biofabrication. The work was funded through a Laboratory-Directed Research and Development (LDRD) Strategic Initiative.
"The gut is an incredibly complex system, home to trillions of microorganisms, so it's very difficult to study directly," said Lindy Jang, the paper's lead author. "That's why we developed a simpler research device that mimics the human gut as closely as possible."
Organ-on-a-chip devices are novel in-vitro micro-scale biomimetic platforms that reproduce the physiological environment of human organisms. They're not new. They've been around for over 15 years, accelerating drug discovery and disease modeling and reducing the need for animal testing. But they are often highly simplified systems lacking the three-dimensional complexity of the human system.
In this case, gut refers to the small intestines: the workhorse of the human digestive system in a long narrow tube, measuring 8 or 9 meters with a 2-centimeter diameter. It packs a whopping 250 meters of surface area, about the size of a doubles tennis court, made possible by its unusual architecture.
"The gut has thousands of finger-like protrusions extending from its wall to maximize the absorptive surface area and cavities that regulate cell behavior," said research engineer Rick Hynes, who led the bioengineering thrust area of the project. "Replicating this architecture in three dimensions on a chip has been a challenge. Growing these structures from cells, which is how it's done for other organs, is time-consuming with a high failure rate."
This project leveraged LLNL's deep expertise in bioengineering and additive manufacturing. The research team used stereolithographic 3D printing of soft Gelatin-methacrylate (GelMA)-based bioresins and employed enzyme (transglutaminase)-mediated crosslinking of proteins onto the scaffold surface, significantly improving cell health and promoting gut-like behavior.
"We engineered a wildly complex biological system to behave like the gut in a reproducible laboratory environment," said Nicholas Be, the project principal investigator. "Recreating the environment of these cells was a substantial challenge. This is exactly the kind of problem where the Lab excels, bringing together different disciplines and expertise to accomplish something that would be difficult elsewhere."
They printed layers of what became the human gut architecture and then seeded cells onto that structure. Unlike other organ-on-a-chip designs, this chip has a removable gasket that houses the human tissue and simultaneously seals the two sides while still allowing chemical communication. This three-part design gives more flexibility in building the tissues and conducting experiments while potentially enabling the chip to be adapted for other organ systems or even combined organ systems.
"With this approach we can cultivate the cells separately off-chip until they are acting like gut cells," said Hynes. "And we can do this in a simple culture system in an incubator, rather than in extensive laboratory equipment."
The LLNL chip also allows frequent and extensive temporal sampling during an experiment, something that is not possible with in vivo models, such as mice, or human patients.
"Usually, you get one set of results at the end of an experiment," said Hynes. "It's kind of like attempting to study the Amazon by only looking at the river delta once a day. You'll never get a complete picture. This pulls that veil back on those intricate events that happen upstream."
The researchers credit the LDRD funding, which gave the team experimental freedom to explore a range of chip designs and additive manufacturing techniques. The project also included generating and curating datasets that enable predictions about microbiome health and developing machine learning models to perform those predictions.
"It's exciting to think about the questions we can now ask and test," Be said. "We can now poke at it in a lot of different ways and watch the results as they unfold."
In addition to Jang, Be and Hynes, other authors are: Claire Robertson, Michael Triplett, Aubree Hinckley, Monica Moya, Car Reen Kok, Mariam Mohagheghi, Tracy Weisenberger, Feliza Bourguet, Erica Bowers, Michael Morrison, Yaqing Wang, Aleksandr Noy and Ju An Park.
