Human Body as Future Hearing Aid Power Supply?
Last week’s blog introduced the use of alternate energy to power hearing aids, and ended suggesting that perhaps the human body itself could serve as such a source, thereby possibly even eliminating the power cell (battery) altogether. Of course, such a direction was not suggested as something imminent, but at the same time, some body function monitoring devices are starting to employ energy from the body, or body heat, for continuous and sustainable performance. So, could hearing aids eventually operate from techniques that have been found to harness the energy of the body to serve as a power source?
A variety of techniques are available for energy scavenging, including solar and wind powers, ocean waves, piezoelectricity, thermoelectricity, and physical motions. This blog is more interested in those approaches most likely to be employed in using the human body to produce energy to power a hearing aid.
Three kinds of energy sources can be used for harvesting in wearable devices. These are the mechanical energy of people’s own moving or accelerations on transport, an electromagnetic energy that is mainly light energy, and the heat flow caused by the difference in temperature between the human body and the ambient. It is the latter that is of interest here.
Human Body Heat
Warm-blooded animals, or homeotherms, including humans, constantly generate heat as a useful side effect of metabolism. However, only a part of this heat is dissipated into the ambient as a heat flow and infrared radiation; the rest of it is rejected in a form of water vapor. Furthermore, only a small fraction of the heat flow can be used in a compact, wearer-friendly and unobtrusive energy scavenger. For example, nobody would like to wear a device on his or her face. Therefore, the heat flow from the face is not likely to be used.
The heat flow can be converted into electricity by using a thermoelectric generator (TEG), the heart of which is a thermopile. It is known from the thermodynamics that the heat flow observed on human skin cannot be effectively converted into electricity, although a human being generates more than 100Wof heat on average. Assuming that about 1%–2% of this heat can be used, an electrical power of the order of milliwatts can be obtained using a person as a heat generator. If we recall that watches consume about 1,000 times less power, it is fairly good power.
Body as a Heat Source for a Wearable Thermoelectric Power Supply
(Mechanical and Thermal (Heat Variations) Generated Energy)
It is no secret that engineers have been working for years on ways of reclaiming wasted energy, first through “dumb” methods that capture waste heat, and more recently using microelectronic and nanoscale devices that capture energy from your every movement – voluntary or involuntary–or from heat generated by the body. Until fairly recently, most of the work done in the energy harvesting field has been related with harvesting energy from the environment instead of getting it from the human body.
However, with today’s smart phones doing more, and implementing such features as multitasking, the issue of battery life has become a big topic of conversation. The iPhone™ and other communication devices have been vastly improved in managing power, but companies are always looking for innovative ways to get the most out of the limited power supply they have to work with. And, with smart phones emerging as the “control centers” of our activities and communications, including hearing systems, the race is on to produce power supplies that allow for continuous, and integrated, performance. Additionally, the need for ubiquitous electronics that help humans in everyday life is rapidly growing with increasing features and possibilities of modern mobile terminal devices. This would seem to be the case for hearing aids. A main drawback is the demand for power supplies that allow unlimited operating and stand-by times.
The most effective means today to mange this problem relies on the development of devices and circuits that transform energy from the user’s environment or directly from human power into electricity to supply electronic circuits and systems.
The Human Body as a Power Source
At this point, it is possible to draw a distinction between active and passive human energy harvesting methods.
Active vs. Passive Harvesting
Active powering of electronic devices takes place when the user of the electronic product has to do specific work in order to power the product that otherwise the user would not have done (pedaling, walking, specific exercise activity, etc.)
Passive powering of electronic devices takes place when the user does not have to do any task different from the normal tasks associated with the product – the user is not forced to change his/her habits to generate energy. Passive power assumes that an unobtrusive technique has to be adopted. Thus, power is harvested from the user’s everyday actions (walking, breathing, blood pressure, finger motion, etc.), or from body heat.
Body Energy Generating Systems That Might Apply to Hearing Aids
These systems harvest body energy either through temperature or movement.
Thermoelectric Generators (TEGs)
The human body continuously radiates heat. Devices with direct contact to the human body can harvest this wasted energy by means of thermogenerators (TEGs). This valuable technology for self-sustaining power supplies consists of a thermocouple module that employs the temperature gradient between the hot (body) and cold (ambient) side of the thermopair to generate electrical energy. The thermogenerator converts the heat flow existing between the body and the ambient, in electrical energy.
Thermoelectric energy sensors use human body heat. Attached to the body, they are capable of generating electric potential by sensing the differences between body and room temperature (Seebeck Effect). This technology has been used with ultra low-power biomedical applications (EEG, ECG, blood pressure, temperature, etc.). The sensors communicate with a personal server (PDA/cellphone) through short distance wireless protocols such as Bluetooth, Ultra-wideband, or Zigbee {{1}}[[1]] Khan, Q.A, and Band, S.J. Energy harvesting for self powered wearable health monitoring system, http://www.gsaglobal.org/documents/2009/OSU_EnergyHarvesting.pdf[[1]].
The difference in temperature of the surrounding environment and the human body is very important in generating electricity through heat energy of the body. Since this variation in internal and external temperature is of few degrees, it would normally produce only around 200 millivolts and would not be enough to power electronic devices that normally require about 1-2 volts, such as a hearing aid {{2}}[[2]] Generating electricity from body heat, Oct. 2, 2011, http://hassam.hubpages.com/hub/Generating-Electricity-From-Body-Heat[[2]].
A further downside, and one that would impact hearing aids, is low energy conversion. However, scientists have worked to combine a number of components in a completely new way for formulating circuits, which can function on 200 millivolts and will help in extracting heat energy from the body heat alone. Scientists also predict that in the near future, temperature differences of just about 0.5 degrees will be sufficient enough to generate electricity.
Nano-Generators
Advancements in nanotechnology and material sciences are causing energy requirements to fall, but at the same time increasing its production and transfer. In order to use the movements of human body to generate electricity, scientists have developed a new class of devices that can function to manufacture energy from body movements, like muscles stretching.
These nanoscale components are called ‘nano-generators’ (modification of thermoelectric generators), and are made from semi-conductor elements that are less heavy than the conventional energy sources like batteries. A prototype Zinc oxide nanowire extracts electrical energy simply from the change in temperature between a hot and a cold environment.
Since zinc oxide is non-toxic, nano-generators can safely be implanted onto a human body. Inside the body, our cells produce mechanical energy by burning chemical energy that is generated through burning complex molecules of glucose into simpler ones. Nano-generators will utilize the mechanical energy and convert it into electrical energy for empowering devices inside the body {{3}}[[3]] Generating electricity from body heat, Oct. 2, 2011 http://hassam.hubpages.com/hub/Generating-Electricity-From-Body-Heat [[3]].
Nanotechnologists are considering both external and internal power sources. Some designs rely on an implanted or injested nanorobot to use the patient’s own body as a way of generating power. Other designs include a small power source on board the robot itself. Finally, some designs use forces outside the patient’s body to power the robot.
Examples of Products That Have Used Thermoelectric Harvesting
Nokia has unveiled a concept phone that will recharge the battery by using body heat and is designed to eliminate docks and cables. The phone sports a copper back that transfers the energy from body heat into a thermogenerator for storage. The phone will utilize body heat when held in the hand or while in your pocket and can be quick-charged by placing the phone on a warm source. Nokia is hoping that the new charging technology will not only make cell phone use more convenient by eliminating the continuous need to “top off” your phones battery, but also eliminate “vampire energy” from chargers being plugged in all day.
A promising new technology called Power Felt, a thermoelectric device that converts body heat into an electrical current, soon could create enough juice to make another call simply by touching it {{4}}[[4]] Zimmer, L. Power felt charges your cellphone with body heat (and it costs $1!), Ecouterre, 2/24/2012 http://www.ecouterre.com/power-felt-charges-your-phone-with-body-heat-and-it-costs-1/[[4]]. Developed by researchers in the Center for Nanotechnology and Molecular Materials at Wake Forest University, Power Felt is composed of tiny carbon nanotubes locked up in flexible plastic fibers and made to feel like fabric. These are attached to the body, where desired. The technology uses temperature differences – room temperature versus body temperature, for instance – to create a charge.
The Seiko Thermic watch was a significant milestone in thermoelectronics {{5}}[[5]] Seiko Instruments Inc. http://www.sii.co.jp/info/eg/thermic main.html[[5]]. The watch used a thermoelectric generator to convert heat from the wrist into electrical energy. It was the first watch to be powered by the temperature gradient between the body and environment temperature. In this case the use of body heat is fairly straightforward if all one wants to do is check the time (as with a watch); but as soon as one tries to run something as power hungry as a cell phone, or even a hearing aid, the solution is more complex.
A practical energy harvesting system was successfully built using RF transmissions and measurements performed every second with only the heat radiated by the human body. An energy storage element can be attached as well to guarantee system functionality that does not have permanently available thermoelectrical power.
A body heat powered music player (Skinny Player MP3) has been designed to attach to the user’s skin. It does not require batteries, but obtains power from body heat created during movement. The flexible player has an on/off button and a flexible speaker so it does not require headphones, and has enough memory to hold an album of music {{6}}[[6]] Wang, Chih-Wei and Fu, Shou-His, A body heat powered music player, Energy Harvesting Journal, Dec. 10, 2010, http://www.envirogadget.com/mp3-gadgets/skinny-player-stick-on-music-player-powered-by-body-heat-concept [[6]]. Whether it works or not is overshadowed by the possibility that it offers.
A significant constraint on thermoelectric solutions relates to the relative low temperature gradient in the range of 5 degree to 10C, or even lower, between the human body and the environment. Therefore, low voltage differences are provided at the output of the TEG. Specific step-up DCDC converters have to be used by a power management unit to supply an electrical load. Temperature differences, hence electrical energy, are not constant over the operating time, such that two scenarios can be distinguished. On one hand, the electrical load (e.g., a wireless communication module) is supplied only as long as TEG-power is available. On the other hand, accumulators in the form of capacitors or batteries can be employed to ensure continuous power.
Because temperature difference between skin and air is too small to generate much electricity, researchers are working to develop ultra low power devices to go with their thermal energy harvesters. This means that, in the future, we will be able to operate our cellphones and other small portable devices with nothing but the heat of our own body! In short, human batteries are coming — it’s just a matter of time and research. The only reason such energy harvesting techniques aren’t used in hearing aids is due to the relative immaturity of the technology.
Piezoelectric Energy Harvesting
Many people have proposed capturing the energy in body motion through complex mechanical contraptions or piezoelectric material. It’s not a new idea. Some of the most capable mechanisms for inactively converting human body functions into electricity involves the piezoelectric effect.
The piezoelectric effect converts mechanical strain into electrical current or voltage. This strain can come from many different sources – human motion and acoustic noises are everyday examples. The piezoelectric substances are like ceramics that generate electricity from mechanical pull or press, and do not need any voltage to be applied to function. Except in rare instances, the piezoelectric effect operates in AC, requiring time-varying inputs at mechanical resonance to be efficient. Piezoelectric energy generators produce minuscule amounts of electricity that struggle to power a computer, let alone a sensor. The produced power is on the order of milliwatts, too small for many system applications, but OK for watches. Regardless, it is impossible to capture 100 per cent body energy under any circumstances. This is why body heat can only be converted in three per cent efficiency only with current thermoelectric materials. But, that means the body is throwing away energy that could be put to good use. As long as there’s a difference between the temperature of your skin and the surrounding environment, then things known as thermopiles can convert the temperature difference to electricity. And while it’s easy to capture body heat on a grand scale, there’s still no easy way to harvest large amounts of waste heat on a local, wearable scale. The only reason such energy harvesting techniques aren’t used in hearing aids at this time is due to the relative immaturity of the technology.
A recent device called a “thermoacoustic prime mover” has been created that generates power by converting heat into sound. The sound waves can then be converted into electricity using piezoelectric devices that create pressure and convert the sound into electrical current {{7}}[[7]] Dyball, C. 15 unbelieveable ways energy harvesting will change your life, Greener Ideal, April 10, 2012 [[7]].
Summary
Portable electronic systems include any autonomous device that is powered by a battery. The battery can be augmented by energy harvesting with secondary batteries being recharged from energy harvested outside of, or from the human body. As these devices are mainly at the research stage, many improvements will be needed concerning all their parameters. It is expected that this will be driven primarily by military or space applications where cost is not a primary concern, and not by assisted living applications.
