The cells of the heart can be stretched by as much as 100 percent with every beat. But traditional platforms for studying cells are static, limiting researchers’ ability to study these cells in a realistic way in the lab. Now researchers at Purdue University and Stanford University have developed stretchable electrode arrays for studying these cells. These arrays should help develop tissue-engineered grafts to repair the damage caused by heart attacks, and could serve as bio-friendly electrical interfaces in implantable devices. They’re also being used to study how the mechanical stress inflicted during traumatic brain injury changes neurons’ electrical activity over the long term.
The new system, developed by a team led by Babak Ziaie, a professor of electrical and computer engineering at Purdue, consists of a stretchy polymer containing a small array of gold-coated pins. These pins act as microelectrodes that can send and record electrical signals. In the past, the difficulty in designing these electrode arrays has been developing electrical connections for the electrodes that can be stretched without degrading their performance. In the Purdue system, electrical current is carried to and from the electrodes by a liquid metal alloy that flows through channels within the polymer.
The stretchy electrode arrays maintain their electrical properties better than any flexible electrode previously developed. Using a liquid alloy means that resistance to electrical current does not drop when the array is stretched.
This makes them a useful tool for studying and stimulating cardiac cells, says Rebecca Taylor, one of the Stanford researchers working on the project. The heart’s muscle cells receive regular electrical stimulation that causes them to beat. They also experience regular mechanical stresses caused by the beating of the tissue around them. These stimuli tell heart cells to keep acting like heart cells, so mimicking them in the lab is an important first step toward engineering tissue to repair the damage caused by heart attacks or congenital heart defects.
Taylor says that the new electrodes make it possible to stretch and stimulate heart-muscle cells in vitro: “You trick them into thinking they’re in the heart.” These long, beating muscle cells normally lose their shape after about a day in culture, growing still and pulling into small, round shapes. The Stanford researchers’ first goal is to use the stretchy, electrically stimulating cell platform to maintain the cells in their normal state. After that, they plan to use the approach to grow patches of healthy heart tissue for reparative grafts.
Another team of researchers, at Columbia University and Princeton University, are using stretchable electrode arrays to study traumatic brain injury (TBI). This kind of injury results from an acute event like a car accident or a battlefield explosion, but the ill effects grow much worse over the long term as cells in the brain react to the injury by changing their gene-expression patterns, and subsequently their electrical activity. So the hope is that better understanding how mechanical stresses lead to molecular changes in the brain could, in turn, lead to life-saving therapies. “The deformation of brain tissue sets into motion cellular signaling cascades that take some time to develop; very often, that’s what kills you,” says Barclay Morrison, a biomedical engineer at Columbia. Morrison is studying TBI in the lab using stretchable electrodes developed by Sigurd Wagner, a professor of electrical engineering at Princeton.
In Wagner’s electrode arrays, thin gold electrical contacts printed using standard lithography techniques play the same role as Ziaie’s liquid alloy contacts. These electrodes do experience a spike in resistance when stretched, but this lasts less than a second, says Morrison. The stretchiness of the electrodes allows the Columbia team to inflict TBI-like mechanical stress on neurons grown in culture, and monitor their electrical activity over the long term.
“One of the big issues right now in TBI is that we aren’t sure of the thresholds of injury,” says Kevin Kit Parker, a U.S. Army Reserve captain and a biomedical-engineering professor at Harvard University. Electrodes like those being developed by Ziaie and Morrison “would allow us to precisely determine what kind of blast forces are required to acutely disrupt the electrical activity in the brain,” Parker says.
Morrison says that his studies have shown that TBI-like damage can be initiated in cells grow in the lab by a 10 percent strain inflicted over 50 milliseconds. In a forthcoming paper to be published in the Journal of Neurotrauma, he and Wagner describe the effects of simulated trauma on cells from different regions of the brain. “We’ve shown that, depending on the brain region, cell death is responsive to the rate and magnitude of stretch,” says Morrison.
Over the long term, the researchers hope that stretchable electrode arrays will prove suitable for more than just studying cells, and can be used to make implantable devices for studying and treating disease. Morrison says that stretchable electrodes may prove friendlier interfaces for neural prosthetics that connect to the brain, such as implants that allow quadriplegics to control their wheelchair or use a cursor on a computer screen just by thinking about it. Because these implants are flexible, they ought to cause less scarring than a rigid silicon chip. Scarring interferes with the performance of an implant.
Stretchable electrode arrays also show promise as an electrical interface to other kinds of muscle. A compliant electrode array wrapped around the smooth muscles of the bladder might be used to send electrical signals that allow the muscle to move again, helping to treat patients suffering from incontinence.
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