A chip that breathes

Living, breathing human lung-on-a-chip could be a potential drug-testing alternative

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CAMBRIDGE, Mass.—Harvard University researchers have created a tiny device the size of a coin that mimics the workings of a human lung on a computer chip and may provide a way to test drugs and assess the impact of environmental pollutants.

Researchers at Harvard's Wyss Institute for Biologically Inspired Engineering in Boston have combined microfabrication techniques from the computer industry with modern tissue engineering techniques, human cells and a plain old vacuum pump to create a living, breathing human lung-on-a-chip. The device mimics the most active part of the lung, the boundary between the air sac and the bloodstream.

The see-through lung-on-a-chip contains two chambers separated by a flexible, porous membrane lined with human lung cells on one side and cells from capillary blood vessels on the other. It acts like the air sacs that make up human lungs, rhythmically stretching and expanding to duplicate the effects of breathing.

According to Donald Ingber, the founding director of Harvard's Wyss Institute and lead designer of the device, the device may save drug companies time and money by enabling them to gauge the effects of inhaled medications. Ingber's work was published recently in the journal Science.

"The ability of the lung-on-a-chip device to predict absorption of airborne nanoparticles and mimic the inflammatory response triggered by microbial pathogens provides proof-of-principle for the concept that organs-on-chips could replace many animal studies in the future," says Ingber.

According to Ingber, the most critical advance made in this research is the demonstration of proof-of-principle for the concept of developing organ-on-chip drug testing devices with human cells that actually recapitulate whole-organ-level functions that are often extremely sensitive to mechanical forces in the in vivo environment.

"This is in contrast to most existing in vitro models," he says. "Our demonstration that new insights made with our device relating, for example, to the effects of breathing movements on nanoparticle absorption were actually confirmed in whole animal studies put this device a rung above any other organ-on-chip device, at least to our knowledge."

 "We were inspired by how breathing works in the human lung through the creation of a vacuum that is created when our chest expands, which sucks air into the lung and causes the air sac walls to stretch," says first author Dan Huh, a Wyss technology development fellow at the Institute."Our use of a vacuum to mimic this in our microengineered system was based on design principles from nature."

Ingber, also the Judah Folkman professor of vascular biology at Harvard Medical School and Children's Hospital Boston, points out that until now, tissue-engineered microsystems have been limited, either mechanically or biologically.

"We really can't understand how biology works unless we put it in the physical context of real living cells, tissues and organs," he says.

The lung-on-a-chip micro device takes a new approach to tissue engineering by placing two layers of living tissues—the lining of the lung's air sacs and the blood vessels that surround them—across a porous, flexible boundary. Air is delivered to the lung lining cells, a rich culture medium flows in the capillary channel to mimic blood and cyclic mechanical stretching mimics breathing. The device was created using a novel microfabrication strategy that uses clear rubbery materials. The strategy was pioneered by another Wyss core faculty member, George Whitesides, the Woodford L. and Ann A. Flowers University Professor at Harvard University.

With every human breath, air enters the lungs, fills microscopic air sacs called alveoli and transfers oxygen through a thin, flexible, permeable membrane of lung cells into the bloodstream. It is this membrane—a three-layered interface of lung cells, a permeable extracellular matrix and capillary blood vessel cells—that does the lung's heavy lifting. What's more, this lung-blood interface recognizes invaders such as inhaled bacteria or toxins and activates an immune response.

To determine how well the device replicates the natural responses of living lungs to stimuli, the researchers tested its response to inhaled living E. coli bacteria. They introduced bacteria into the air channel on the lung side of the device and at the same time flowed white blood cells through the channel on the blood vessel side. The lung cells detected the bacteria and, through the porous membrane, activated the blood vessel cells, which in turn triggered an immune response that ultimately caused the white blood cells to move to the air chamber and destroy the bacteria.

"The ability to recreate realistically both the mechanical and biological sides of the in vivo coin is an exciting innovation," says Rustem Ismagilov, professor of chemistry at the University of Chicago, who specializes in biochemical microfluidic systems.

The team followed this experiment with a "real-world application of the device," says Huh. They introduced a variety of nano-scaled particles into the air sac channel. Some of these particles exist in commercial products; others are found in air and water pollution. Several types of these nanoparticles entered the lung cells and caused the cells to overproduce free radicals and to induce inflammation.

"Most importantly, we learned from this model that the act of breathing increases nanoparticle absorption and that it also plays an important role in inducing the toxicity of these nanoparticles," Huh says.

For drug research, Ingber points out the major advantage would be to shorten the time and costs required for animal studies by carrying out studies in vitro with human cells and organ-level complexity.

"It might be possible to use HLA-restricted human cells in the future (or patient-specific cells using induced pluripotent stem cells strategies) that could lead to more effective predictions of results in humans than is possible in small animal studies at present," he notes.

Moreover, the increased nanoparticle absorption can be key in drug research efforts.

"There are increasing numbers of drug formulations that are effectively nanoparticles (e.g., liposomes, imaging agents, etc.)," Ingber says. "This will increase further over time as multifunctional nanomaterials enter the medical arena."

"This lung-on-a-chip merges a number of technologies in an innovative way," says Robert Langer, MIT Institute professor." I think it should be useful in testing the safety of different substances on the lung and I can also imagine other related applications, such as in research into how the lung functions."

According to Ismagilov, it's too early to predict how successful this field of research will be. Still, "the potential to use human cells while recapitulating the complex mechanical features and chemical microenvironments of an organ could provide a truly revolutionary paradigm shift in drug discovery," he says.

The relevance of Ingber's findings also are not limited to research on inhaled drug candidates.

"For example, we could explore the effects of intravenous drugs on inflammatory responses or vascular leakage in this model," he says.

Moving forward, the next challenge for Ingber's team is to identify corporate partners to identify their real-world needs for this type of screening device.  

"For example, if we can clearly demonstrate that we can carry out a study in our device that predicts results that currently only can be obtained in animals (or in humans)," he says. "Then I believe we would be ready to commercialize this technology for widespread use."

Ingber's team also is extending its analysis to quantitate gas exchange across the interface (e.g., oxygen, carbon dioxide, anesthetics).

"We are exploring better ways to mimic introduction of particles and drugs by nebulizer for analysis of aerosol-based therapeutics," he says.

At the Wyss Institute, Ingber heads the Biomimetic Microsystems Platform.

"This platform already has major activities involved with development of multiple other organs, including a peristalsing gut-on-a-chip, beating heart-on-a-chip, liver, bone marrow, cancer, kidney, brain etc., all with human cells," he explains. "In particular, we are currently working with another Wyss faculty member, Kit Parker, to integrate our breathing lung with his beating heart to create a "heart-lung micromachine" for testing of bioavailability and cardiotoxicity of aerosol-based drugs absorbed through the lung on a chip.  The long-term goal would be to integrated all of these by engineered microfluidic networks to produce an effective 'human-on-a-chip.'"

This research was funded by the National Institutes of Health, the American Heart Association and the Wyss Institute for Biologically Inspired Engineering at Harvard University.

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