A Biological Battery
Cambridge, MA (November, 2012) Plugging into sources of energy within our body — such as heat, internal motion or metabolites — to power implanted medical devices has long been the goal of biomedical engineers. Now researchers based in Cambridge, Massachusetts have demonstrated that a sensing device embedded in the ear can be powered by the ear’s own electrochemical battery.
Our auditory system picks up external sounds and sends information to the brain in the form of neural signals. When the sound wave hits the ear, the eardrum vibrates in response. This mechanical energy must be converted into an appropriate electrochemical impulse.
Deep inside the ear, the cochlea perceives the frequency of the vibration. It maintains a gradient of potassium and sodium ions across a delicate membrane via a system of pumps and channels. This natural battery, which makes neurotransmission of sound possible, generates a net positive voltage.
Researchers have known about the existence of this endocochlear potential (EP) for decades but had not devised ways of using this voltage without interfering with the mammal’s hearing, says Konstantina Stankovic, otologic surgeon at Massachusetts Eye and Ear Infirmary, medical lead of the collaborative team. “New electrodes and new electronics had to be developed to make safe harvesting of energy possible,” she says.
Prof. Anantha Chandrakasan’s group at Massachusetts Institute of Technology designed the chip to extract current from the ear, keeping in mind the many physiological constraints. In the prototype, the harnessed power drives a wireless sensor that can monitor the value of the EP. A radio transmitter relays data to the clinician who uses the numbers to gauge the ear’s condition. Though our ear functions on EP ranging from 70-100 millivolts, this voltage is not enough for electronic implants. “Since the power from the source is so small, we accumulate energy on a capacitor. Once the capacitor fills up, it can drive a higher power electronic circuit,” says Chandrakasan. “We power a 2.4 Gigahertz radio in this case.”
But transistor-based electronics need hundreds of millivolts to start. A wireless receiver on the integrated circuit gets a short burst of radio waves to kick-start the system. The setup, implanted in the ear of a guinea pig, could transmit data for five hours without compromising normal hearing. Design optimization and more testing lie ahead.
“Thus far, we have demonstrated feasibility of sensing the EP, powered by the EP,” says Stankovic. “But we are eager to couple this energy-harvesting chip to a variety of molecular and chemical sensors to sense the inner ear and its environment and identify the most promising biomarkers relevant for the ultimate human application.”
The device cannot power multichannel cochlear implants or hearing aids as yet. But Charley C. Della Santina, professor of Otolaryngology and biomedical engineering at Johns Hopkins University, who is unconnected to the research team, points out that there is a real need for a system that can monitor the EP in animal models of Meniere’s disease — an inner ear disorder that affects balance and hearing. And this device may just fit the bill. Plus, the data collected in vivo could transform our understanding of how the mammalian ear works, says Stankovic. The paper that describes the findings appears in the latest issue of Nature Biotechnology.
Notes:
Recent Advances in Energy Harvesting from the Human Body for Biomedical Applications
Feb 8, 2008
An energy-efficient microchip
A cell phone dying at a crucial moment, a digital camera that warns of low power at a perfect photo opportunity or a laptop inopportunely running out of juice — many of us have had such vexing experiences firsthand.Researchers at the Massachusetts Institute of Technology and Texas Instruments (TI) have designed a chip, ten times more energy-efficient than existing ones, to keep portable devices going longer.
Because of the lower power consumption, the batteries of these mobile devices and vital medical implants do not have to be recharged or replaced as frequently as they are now.
Although users tend to be excited by new features on gadgets, every additional feature — like the ability to watch video clips or take snapshots on a cell phone — puts a burden on the battery’s limited power.
Clearly, a new approach to chip design was in order because saddling such portable devices with bigger and better batteries can only take us so far. In such devices, the key to turn down power consumption was the voltage, says Prof. Anantha Chandrakasan, Director of MIT’s Microsystems Technology Laboratories .
The longevity of the battery comes from the chip’s lower voltage requirement.
The operating voltage for circuits on a chip is typically 1 volt but the new chip can operate at just 0.3 volts. An important feature of this new chip’s design is a high-efficiency DC-to-DC converter — akin to an in-situ step-down transformer — that slashes the input voltage to one third of its strength.
Traditionally, the circuits on such chips have been optimised to operate at a supply voltage of around 1 volt. Scaling down the supply voltage has huge implications.
Memory and logic circuits had to be redesigned to operate at 0.3 volts, a voltage low enough to disrupt the chip’s very functionality, says Chandrakasan, the leader of the chip’s re-architecture team.
Besides, the team also had to overcome the variability inherent in chip manufacturing. At this lower supply voltage, variations and imperfections in the silicon chip could throw the electronic circuit out of whack; in the binary system what was registered as 1 could become 0 and vice versa. Designing the chip to minimize its vulnerability at the new voltage was a huge challenge.
“These design techniques show great potential for TI’s future low-power integrated circuit products and applications including wireless terminals, battery-operated instrumentation, sensor networks and medical electronics,” says Dr. Dennis Buss, chief scientist at Texas Instruments.
Networking devices
Portable and embedded medical devices, such as hearing aids and retinal implants, communications and networking devices based on such chips will consume only a tenth of the current power.
A person who now watches short clips on a cell phone, for instance, can view longer videos without the need to recharge. There could also be a variety of military applications in the production of tiny, self-contained sensor networks that could be dispersed in a battlefield.
Eventually, in medical devices the idea is to make the power requirement so low that they could run on ‘ambient energy’ — using the body’s own heat or movement to provide the requisite power.
The ultimate goal is to design a generator on a chip, eliminating the need for a battery, says Chandrakasan.