Medical Device Daily National Editor

Electronic devices, and the batteries that power them, intrinsically require a variety of channels, connectors, insulating materials, assorted hardware and a box to hold all of this stuff. This puts a clear spatial limit on how small we can make these devices, notes David LaVan, PhD, an engineer with the National Institute of Standards and Technology (NIST; Gaithersburg, Maryland).

Well, no ... not if you're an electric eel, a creature that can produce significant electrical charge at the cellular level.

This creature, LaVan told Medical Device Daily, was the starting point for his research at NIST, in collaboration with Jian Xu of the department of chemical engineering at Yale University (New Haven, Connecticut).

Their target goal: demonstration that an artificial eel-like cellular battery can be made and used as an energy source for a tiny retinal prosthetic device within the eye, a project under development by Mark Humayan, MD, PhD, of the University of Southern California (Los Angeles) and associate director of research at USC's Doheny Retina Institute.

The result of the work by LaVan and Xu is a paper demonstrating the use of modern engineering design processes that provide direction for creating artificial cells that replicate the electrical behavior of electric eel cells and actually improve on them ("Designing artificial cells to harness the biological ion concentration gradient," Nature Nanotechnology, Sept. 21 issue).

The paper, according to LaVan, is an example of how the relatively new approach called systems biology can be used to design such power cells for energizing medical implants and other tiny devices.

Electric eels channel the output of thousands of specialized cells called electrocytes to generate electric potentials of up to 600 volts, biologists have found, with that mechanism similar to that in nerve cells.

LaVan and Xu developed a numerical model to represent the conversion of ion concentrations to electrical impulses within electrocyte cells and tested it against previously published data on electrocytes and nerve cells to verify accuracy. Then they considered how to optimize the system to maximize power output by changing the overall mix of channel types.

Calculations presented in the paper show that substantial improvements are possible.

One design for an artificial cell generated 40% more energy in a single pulse than a natural electrocyte. Another would produce peak power outputs of more than 28% higher.

In principle, the authors say in the paper, stacked layers of artificial cells in a cube are capable of producing continuous power output of about 300 microwatts to drive small implant devices. The size of this "box" of cells would be only slightly more than 4 mm on a side.

The individual components of such artificial cells — including a pair of artificial membranes separated by an insulated partition and ion channels that could be created by engineering proteins — already have been demonstrated by other researchers. Like the natural counterpart, the cell's energy source would be adenosine triphosphate (ATP), synthesized from the body's sugars and fats using tailored bacteria or mitochondria.

The arrival of a chemical signal triggers the opening of selective channels in a cell membrane, causing sodium ions to flow in and potassium ions to flow out. The ion swap increases the voltage across the membrane, which causes even more channels to open. Past a certain point, the process becomes self-perpetuating, resulting in an electric pulse traveling through the cell. The channels then close and alternate paths open to "pump" the ions back to their initial concentrations during a "resting" state.

In all, according to LaVan, there are at least seven different types of channels, each with several possible variables to tweak, such as their density in the membrane. Nerve cells, which move information rather than energy, can fire rapidly but with relatively little power.

LaVan told MDD that he and Xu chose the retinal prosthetic device as an interesting application for this proposed battery source, essentially as a hypothetical target to guide their research (and noting that this reverses the all-too-common process of developing a neat new technology and then searching around for an application).

However, he said that the proposed cellular battery is unlikely to be used in this device because the actual application will move to completion faster than the basic science.

But he said he is continuing with his research in cellular battery development to create a "similar system," but declined describing it, saying he didn't want to let "the cat out of the bag."

LaVan said that while the concept of a cellular battery may not show up in a retinal device, it clearly has the potential for powering a range of mechanisms for controlling bodily processes.

"I don't want to give a false impression this [work] is very early," he said.

But he said the research opens a window into the understanding that such devices can be created, and "how to predict and understand them."