Heart Attack on a Chip: Scientists Model Conditions of Ischemia on a Microfluidic Device

The microfluidic device containing HL-1 cardiac cells is capable of modeling conditions observed during a heart attack, including a reduction in levels of oxygen. Credit: Tufts University

Researchers led by biomedical engineers at Tufts University invented a microfluidic chip containing cardiac cells that is capable of mimicking hypoxic conditions following a heart attack—specifically when an artery is blocked in the heart and then unblocked after treatment.

The chip contains multiplexed arrays of electronic sensors placed outside and inside the cells that can detect the rise and fall of voltage across individual cell membranes, as well as voltage waves moving across the cell layer, which cause the cells to beat in unison in the chip, just as they do in the heart. After reducing levels of oxygen in the fluid within the device, the sensors detect an initial period of tachycardia (accelerated beat rate), followed by a reduction in beat rate and eventually arrhythmia which mimics cardiac arrest.

The research, published in Nano Letters, is a significant advance toward understanding the electrophysiological responses at the cellular level to ischemic heart attacks, and could be applied to future drug development. The paper was selected by the American Chemical Society as Editors’ Choice, and is available with open access.

Cardiovascular disease (CVD) remains the leading cause of death worldwide, with most patients suffering from cardiac ischemia—which occurs when an artery supplying blood to the heart is partially or fully blocked. If ischemia occurs over an extended period, the heart tissue is starved of oxygen (a condition called “hypoxia”), and can lead to tissue death, or myocardial infarction. The changes in cardiac  and tissues induced by hypoxia include changes in voltage potentials across the cell membrane, release of neurotransmitters, shifts in gene expression, altered metabolic functions, and activation or deactivation of ion channels.

The  used in the microfluidic chip combines multi-electrode arrays that can provide extracellular readouts of voltage patterns, with nanopillar probes that enter the membrane to take readouts of voltage levels (action potentials) within each cell. Tiny channels in the chip allow the researchers to continuously and precisely adjust the fluid flowing over the cells, lowering the levels of oxygen to about 1-4 percent to mimic hypoxia or raising oxygen to 21 percent to model normal conditions. The changing conditions are meant to model what happens to cells in the heart when an artery is blocked, and then re-opened by treatment.

“Heart-on-a-chip models are a powerful tool to model diseases, but current tools to study electrophysiology in those systems are somewhat lacking, as they are either difficult to multiplex or eventually cause damage to the cells,” said Brian Timko, assistant professor of biomedical engineering at Tufts University School of Engineering, and corresponding author of the study. “Signaling pathways between molecules and ultimately electrophysiology occur rapidly during hypoxia, and our device can capture a lot of this information simultaneously in real time for a large ensemble of cells.”

When tested, the extracellular electrode arrays provided a two-dimensional map of voltage waves passing over the layer of , and revealed a predictable wave pattern under normal (21 percent) oxygen levels. In contrast, the researchers observed erratic and slower wave patterns when the oxygen was reduced to 1 percent.

The intracellular nanoprobe sensors provided a remarkably accurate picture of action potentials within each cell. These sensors were arranged as an array of tiny platinum tipped needles upon which the cells rest, like a bed of nails. When stimulated with an electric field, the needles puncture through the cell membrane, where they can begin taking measurements at single cell resolution. Both types of devices were created using photolithography—the technology used to create integrated circuits—which allowed researchers to achieve device arrays with highly reproducible properties.

The extracellular and intracellular sensors together provide information of the eletro-physiological effects of a modeled ischemic attack, including a “time lapse” of cells as they become dysfunctional and then respond to treatment. As such, the  could form the basis of a high throughput platform in drug discovery, identifying therapeutics which help cells and tissues recover normal function more rapidly.

“In the future, we can look beyond the effects of hypoxia and consider other factors contributing to acute heart disease, such as acidosis, nutrient deprivation and waste accumulation, simply by modifying the composition and flow of the medium,” said Timko. “We could also incorporate different types of sensors to detect specific molecules expressed in response to stresses.”

Explore further

Study reveals how low oxygen levels in the heart predispose people to cardiac arrhythmias

More information: Haitao Liu et al, Heart-on-a-Chip Model with Integrated Extra- and Intracellular Bioelectronics for Monitoring Cardiac Electrophysiology under Acute Hypoxia, Nano Letters (2020). DOI: 10.1021/acs.nanolett.0c00076

Journal information: Nano Letters

Water Purification at the Molecular Level: Research at Tufts University

1-water nano water-filter2Fracking for oil and gas is a dirty business. The process uses millions of gallons of water laced with chemicals and sand. Most of the contaminated water is trucked to treatment plants to be cleaned, which is costly and potentially environmentally hazardous.

A Tufts engineer is researching how to create membranes for filters that may one day be able to purify the water right at a fracking site. Ayse Asatekin, an assistant professor of chemical and biological engineering, is designing materials for sophisticated filters that would be more cost-effective and use less energy than current methods. They would work not only at fracking sites, but could also be used to clean industrial waste from manufacturing and pharmaceutical companies and to provide clean drinking water.

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Ayse Asatekin is experimenting with polymers that could one day be used in filters to distinguish between different chemicals. Photo: Kelvin Ma

Using filters to purify water isn’t new. Hippocrates, in the fourth century B.C., invented a bag filter to trap sediments that caused water to smell and taste bad, while Sanskrit writings from 2000 B.C. describe sand and gravel filtration. Water is still filtered using the same basic principle: force it through a porous membrane that traps large particles while allowing clean water to pass through.

But to catch certain chemicals, you need a membrane with pores that measure just one nanometer across. For perspective, a strand of human hair is some 60,000 nanometers wide.

For this nanotechnology, Asatekin has turned to polymers—molecules strung together to form long chains. “A polymer is like a necklace of beads,” Asatekin says. “You can make a long chain or a short chain; you can make branches going off it. By playing with all these things, you can control a polymer’s configuration and its properties.”

Ayse Asatekin holds a sample of a water filtration membrane in her lab. Photo: Kelvin Ma >>>

She’s using the polymer chains to create a grid of ultra-small pores capable of snaring the tiniest pollutants. These nano-membranes are working in the lab, and will soon be ready to be designed for specific uses, manufactured and tested in the field.

But someday, in addition to being small, Asatekin’s polymer filters will also be smart: she’s experimenting with polymers that could distinguish between different chemicals. “So even if two molecules were the same size, the polymer would ‘know’ that one has certain functional groups that the other lacks, and be able to block it,” she says.

Right now she is testing polymers that can recognize the difference between molecules using characteristics that define their structures. This could allow, for example, a smart membrane to separate a pharmaceutical from the chemical compounds that catalyze the reactions to create the drug.2-water nano nano1

Filters Go Mobile

Asatekin’s inspiration for the polymers came from observing bacteria. All bacterial cell walls, cell membranes and membranes that separate the nuclei from the cytoplasm have structures that allow one type of molecule to pass through their “doorways” while blocking others, she says.

“For example, there is one structure that allows a sugar to come in, one that allows calcium ions but no other molecules,” she says. “Each cell’s wall or membrane structure has its own target, and is very selective—this is what I am hoping my polymers will be able to do maybe 10 to 15 years from now.”

A polymer membrane looks like a piece of slightly shiny paper. To create a membrane, Asatekin takes her polymers and paints them onto a large-pore, paper-like material that is itself an acrylic polymer specially manufactured to suit each project. They might not look exciting, but it is these membranes that will allow filtration to go mobile.

For the membranes to be turned into actual transportable units to be used in the field at fracking, manufacturing or other sites, a company would need to scale up their production to make them as wide, long sheets. Then, several flat membranes would be rolled into large cylinders that could be one inch in diameter by one foot long or as large as eight inches in diameter by 40 inches tall, depending on the use. These pipe-like structures would be attached to a pump and secured on a rig, sometimes singly and sometimes stacked, and pressurized water would be forced through them, coming out clean on the other side.

Whether they are used for pharmaceutical purification, cleaning industrial wastewater or producing drinking water, the membranes Asatekin’s group is designing could be cleaned and last longer than current filters. More field testing is needed to define exactly how these systems would function. At fracking or industrial sites, the filtering process eliminates the need to transport contaminated water to a treatment facility. The purified water could be reused without ever leaving the site.

And the membranes’ mobility means they could purify water in remote areas of the world, a boon for the estimated 780 million people with no access to clean water, according to the World Health Organization and UNICEF’s Joint Monitoring Programme.

The nano-membranes would also save energy by eliminating the need boil water to turn it into vapor and then distill it.

“The Department of Energy estimates that these industrial purification and separation processes account for 40 to 70 percent of energy costs generated by a chemical manufacturing process,” Asatekin says. “What we are working on is expanding the applications for polymer membranes that would improve the energy efficiency of many manufacturing processes by not having to use distillation, but instead, passing it through our selective filters.”

Asatekin’s polymer membranes have another quality important for industrial use—something called fouling resistance. This means that oil and other heavy substances can’t clog the membrane pores and foul the purification process. Clean Membranes, the Tyngsboro, Massachusetts, company that Asatekin co-founded and now consults for, is working with oil and gas companies across the country to develop polymer membrane applications tailored to their needs.

Source: Tufts University