HUMAN-on-a-CHIP
Will putting the equivalent of a human being on a plastic chip be the future of pharmaceutical testing? Read on for a summary and explanation of an actual research study that furthered human-body-on-a-chip technology…
A lot of pharmaceutical testing and drug research is currently done using animals like mice. But there's a growing need for alternatives: not only because of ethical issues, but also because animal responses don't always predict what will happen in humans. We need more ways to test potential disease-treating drugs for efficacy and toxicity in human organs. Of course, we can't just test out lots of different medications and potential drug candidates directly in people. Cell culture of human cells is one way we could study how drugs might perform in the body. However, growing cells in a single layer in a petri dish obviously lacks the complexity and variables present in the human body.
Organoids are made up of 3D cultures of cells that resemble many aspects of human organs on a much smaller scale. Organoids can be created to mimic many organ systems like brain, heart, and lungs. Another alternative is organs-on-a-chip. In this model, either organoids or cells are cultured in the lab within small plastic devices that are separated into different compartments to more accurately mimic their environment in the body, like a liquid-air interface or blood-brain-barrier membrane. But in the body, all these systems are connected, not in separate culture dishes…
What if we connected the different cells or organoids somehow? Then we'd have a sort of “human-on-a-chip.” Believe it or not, this futuristic-sounding idea is an actual research tool used today. This paper discussed in this article developed a robotic system that furthered this technology.
Prior to the publication of this study in 2020, research efforts were already underway towards linking types of lab-grown cell culture organ systems, known as human-on-a-chip or human-body-on-a-chip. But the types of systems under experimentation had some flaws. In order to culture cells of different types in the lab (cardiac cells, neuronal cells, and lung cells, for example) different culture environments are needed. These cell types each have different requirements such as different formulations of liquid they are growing in called growth media, and the percentage of gasses like oxygen and carbon dioxide they’re exposed to in the air surrounding them. In the body, these different organs are all compartmentalized and connected by a complex vasculature. So in the lab, it doesn’t necessarily work to just toss everything in the same petri dish. But it also isn’t practical to try to connect tons of huge cell culture plates and petri dishes. That’s where the field of microfluidics comes in.
Microfluidics essentially means the process of constructing tiny chambers to hold fluid. Think micro = small, and fluidics = fluid. As mentioned earlier, cells can be cultured on microfluidic chips, or organs-on-a-chip. But why?
Just growing cells or organoids in a dish isn’t always enough. The environment around the cells also influences them and how they’ll behave. For example, imagine being suddenly transported to a totally different environment, like a hot desert, or the arctic, or having to live in a tiny tiny room, or being surrounded by people you don’t like. Cells react to all of these things too, both in terms of chemicals and other molecules around them, and also physical things like being compressed or stretched. All of these things contribute to the cellular microenvironment, or the tiny environment surrounding the cells which contains all of the factors that could influence them.
To be able to more closely mimic the microenvironment that cells experience in the body, we need to be able to control it in the lab. For example, just squirting a drug we want to test directly onto cells isn’t generally what happens in the body. More often, the drug first travels through the bloodstream before reaching the cells of an organ. For example a microfluidic device could be constructed that contains two compartments: a chamber that holds liver cells, and another chamber that holds a blood substitute that carries nutrients. The two chambers are connected by a membrane that the liquid can pass through, and we can even line the membrane with endothelial cells, the cells that make up blood vessels in the body. Then, we have a system where we can inject our drug into the channel with the blood substitute, let it perfuse through the membrane, and then see how it interacts with the liver cells. We can have organs-on-chips for many other systems too of course, like heart, lungs, and neuronal cells. Each of these chips can be designed to more accurately mimic the conditions found in the body, like a liquid-air interface for lung-associated cells, or a blood-brain barrier, or the ability to stretch and compress certain organs as might occur in the body.
Still, a drug we want to test might not necessarily be injectable, but taken orally. In this way, the drug would first pass through the gut, then be absorbed into the bloodstream, and then reach the target organ for example. So, we need some way to connect greater numbers of these organs-on-chips in more complex arrangements. In addition, we need to enable sample collection and changing out of blood substitute and growth media. Of course, we also need the ability to take microscope images and study the cells and tissues in the different compartments. For such a setup, we also need the flexibility to link organ chips differently to simulate different treatments. For example, we may wish to study how a drug differs when taken orally versus being injected. This system would also benefit from the ability to study different conditions, like if an organ became damaged or stopped functioning.
The researchers that wrote the study we’re discussing today felt that prior attempts were lacking. Systems using different types of automated pumps to perfuse and link the microfluidic chambers made it difficult to observe and study the cells. Manual attempts at changing fluids between chambers were also insufficient. For example, transferring fluids directly between the different compartments doesn’t quite simulate conditions in the bloodstream. Further, repeatedly opening and closing the incubator where the cells are growing causes frequent fluctuations in temperature and air quality. All of these factors make complex experiments not really doable. So the researchers here presented a new approach using liquid-handling robotics. They designed an instrument they named the “Interrogator” that is small enough to fit within a standard cell culture incubator.
This interrogator instrument automatically circulates and replenishes the fluids between all the chips, avoiding opening and closing the incubator and disturbing the cells. It's also programmable in JavaScript by a computer interface located outside of the tissue culture hood, so that experimental conditions can be more easily modified. A miniature mobile microscope enables taking images of the cells, and samples of fluid can be collected automatically. The instrument can support up to ten organ chips, and the robotic fluid handling program can be customized to change where a chemical is introduced into the system. This could be useful if you wanted to test different drug delivery methods, for example: orally, interacting first with gut cells, subcutaneous, interacting first with skin, or intravenous, injected into the blood substitute. To further mimic the cellular microenvironment, some of the organ chips were designed to allow mechanical stretching or squeezing of the attached cells.
This robotic system sounds great, but does it actually work as planned?
To test it out, the researchers set up a trial with eight linked organ chips: intestine, liver, kidney, lung, heart, skin, blood-brain-barrier, and brain. Each of the chips contained the organ cells in one compartment and vascular endothelial cells in the other compartment. They then programmed the instrument to mimic oral intake of a substance. This was done by robotically transferring small volumes of the blood substitute from the outlet of an organ chip to the inlet of another. This effectively made the liquid circulate through the chips in the order of first interacting with the gut chip, then the liver chip, then the kidney and heart chips, then the lung, skin, and brain chips. They were able to perfuse the blood substitute through all of these chips, and maintain cellular growth for three weeks. They also infused a dye to more closely trace the circulation of the blood substitute through the system, mimicking for example a small drug or compound being absorbed into the intestines and entering the bloodstream. They then measured the concentrations of tracer dye in the different organ chips and found that they could consistently repeat their results. They could even create detailed simulations in silico (computer models) that matched the results of their experimental trials. This confirmed that the interrogator was circulating the blood substitute consistently and reliably. This is essential for experimental success.
In addition to monitoring drug delivery, this system could be useful for many other types of experiments. For example, organ chips can be made to mimic diseases like COPD, asthma, and infections. Effects of toxic exposures or radiation could be studied in these disease models, and different drugs tested for efficacy and off-target effects, or unwanted effects on other organs or cells. The researchers here mentioned that they published a companion paper, where they were able to successfully obtain data on two drugs that matched data obtained from clinical trials.
What do you think? Will human on chip systems ever be able to completely replace research done in animals, or even clinical trials? Although this research could potentially alleviate many concerns associated with animal testing, could human-on-a-chip technology raise any ethical issues of its own?
Reference for the paper discussed in this article:
Novak, Richard, et al. "Robotic fluidic coupling and interrogation of multiple vascularized organ chips." Nature biomedical engineering 4.4 (2020): 407-420.
