Admittedly, we enjoy the smart watches and their great features. But there is still a problem that can make users feel uncomfortable, this problem happens to be charging them regularly.

The easiest solution is to increase the battery capacity, however, the larger size will increase and the volume will increase as well. All current wearable devices including sports bracelets, wireless headsets or virtual reality contact lenses have power problems. And with current battery technology, they are too hard as well as too bulky to bend or attach directly to the user's skin.

The power required for these devices ranges from 1 mW for a simple step counter to 10 mW for a more functional smartwatch. When using batteries with a size of only a few centimeters with a maximum capacity of about 300 mAh, the device lasts only a few days.

Researchers are facing challenges in developing new flexible batteries as well as new supercapacitors. However, it is difficult to produce such batteries with low cost screen printing technology.

Other developers are seeking to eliminate battery use altogether, instead using Near Field Communication chips (NFC - short-range wireless technology) to provide wireless power. But NFC technology requires an external power source, such as a phone, and it must be near the wearable within a few centimeters; Once the power source is out of this range, the wearable will stop working.

At the University of California's Wearable Sensor Research Center (San Diego), researchers have come up with a better solution to the power problem of future wearables. It is the collection of waste energy, especially bioenergy, from the wearer of the device. Wearables that use this power source may be so small that the user is completely unaware of its presence and the team calls them "invisible" wearables.
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Sweat: The First Source Of Bioenergy:

Photo: Sean McCabe
Collecting energy from our bodies or surrounding elements is not a new initiative. Before that, a number of energy sources were used such as motion, light and heat.

Since the 1770s, a self-propelled lake cave has used the body's natural movement to generate energy. In this first self-propelled device, a pendulum inside the watch will power itself.

The latest version uses a magnetic pendulum that runs through the coil core to generate electricity and charge the battery. Modern self-propelled watches use piezoelectric materials, the crystalline substances can release an amount of charge when bent or twisted.

Solar panels have been made the size of a fingernail and they can generate energy for decades. Most are solid, but researchers at the center have developed flexible and even twisted solar cells.

The third type of energy is heat from the body. In most climates, the body temperature is always higher than the ambient temperature. Small thermoelectric plants can operate based on this difference. The basic PowerWatch is the first device to use this power source and is on the surface in contact with the user's skin.

Unfortunately, only a few of the above methods of generating power supply can be applied to a small, flexible and useful wearable device. The motion power supply may be fine with the watch, but it cannot be used with other wearable devices such as the chest or the ear.

The heat source is effective only with the design of a large heat absorber, usually made of aluminum, so it can absorb the heat from the body and transfer it to the device. In addition, neither thermal nor photovoltaic can work effectively when the device is worn under clothing.

That is why the power source from sweat is the brightest candidate. Certain chemicals in human sweat can be used as fuel for fuel cells. These biofuel panels could generate enough power to power more practical devices. The team at the University of California has developed a prototype of wearable devices that use energy from sweat.
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How Does Sweat Become Energy?

Photo: Wearable Sensor Research Center, University of California (San Diego)
A fuel cell consists of two electrodes, an anode and a cathode, and an electrolyte between them. The fuel enters the anode, where a catalyst splits its molecules into electrons and protons. The protons pass through the membrane and reach the cathode while the electrons will enter the circuit. William Robert Grove, a Welsh scientist, created this system in 1839; he used hydrogen gas as fuel and oxygen as a catalyst to create water and electricity.

Hydrogen gas is not suitable as a fuel for wearables because they are highly flammable. On the other hand, sweat is a rich and accessible fuel, especially for sports or exercise players.

At the same time, active people are the most frequent and most frequently used customer file types. These are forecast to be the first customers interested in wearable devices using biofuel sources on the market.

In addition to water, sweat contains many minerals and some other substances like glucose and lactate. These substances, also known as metabolites, are a byproduct of metabolism in living organisms and become a potential source of bioenergy.

We are especially interested in lactate because their content in sweat increases when the body is working hard. In the biofuel battery, we create layers of enzymes that can react with the lactate present in sweat to separate electrons and protons, thereby generating electricity.

However, this study is not the first time body fluids have been used as fuel. Several pacemakers and implantable hearing aids since the 1970s have thought of using glucose as the energy source for biofuels. The use of biofuels in the body as a source of energy for transplant devices is also plausible because of their abundance.

The biggest drawback is that the enzymes used to break down fuel quickly decline and become ineffective after only a few days. The only way to recover is to have surgery to replace the implant and of course, no one will do so after only a few days of transplant.

To overcome this problem of enzyme degradation, the team focused on developing disposable body worn devices. The first biofuel panel was introduced by the group in 2014. The lactate biofuel panel was fabricated with mesh printing technology on a cloth frontal bandage and a wristband.

Participants in the experiment will wear a forehead and wristband while cycling. Each biofuel battery will be connected to a small DC-DC converter. The converter has shown enough volts to light a 1 microwatt LED light or for an electronic clock.
Photo: Chris Philpot

In the experiment, every square centimeter of a biofuel battery could generate up to 100 microwatts, enough to power both LEDs and clocks at the same time. The amount of electricity generated by a biofuel battery is larger than a small heat source or photovoltaic source with indoor lighting conditions.

So far, this is the first test version of a printable fuel cell in a wearable device on a common object. Moreover, because biofuel panels can be printed on flexible materials, the designs will help users feel comfortable to use as well as being able to work well even when bent many times.

However, this power source cannot provide enough power for a modern activity tracking sensor or a multi-function smart watch. And it also can not make us comfortable when exposed to the skin as expected.

Of course, a wearable actually has more components than an electronic watch. The simplest operating trackers include accelerometers, memory and Bluetooth. With the above components, the power source needs to reach a capacity of about 1 to 2 mW, 10 times more than the capacity of the first version of 2014.

Therefore, increasing the capacity for bio-energy batteries is one of the biggest challenges of the team. If these panels could not generate the power that technology products needed, they could not be applied in practice.

In 2017, the team at the University of California teamed up with Sheng Xu and his team to create a 3D carbon nanotube in small pellets that can attach to anodes and cathodes. These pellets have increased the surface area of ​​the electrode without increasing the device size. Thus, they allow the electrodes to have more catalysts, contact with more fuel and generate more energy.

Increasing the surface area of ​​the battery and adding chemical components to the catalyst helped the biofuel panels increase capacity by 10 times, reaching 1 mW per square centimeter. This is the equivalent of a small solar panel placed directly under sunlight.

However, the 3D electrodes used are not as flexible. The design that the group aims to ensure can be flexible as well as stretchable. Because the human body is not uniform and has a curvature, just being flexible is not enough to ensure that the biofuel panel can lie comfortably on the skin surface. The research team is still continuing to research the solution to this problem.

In addition, there is another obstacle in the process of transferring energy from sweat to the wearable device. Most of the time, the body does not sweat a lot, or at least the sweat is not enough to produce the required amount of energy. If the body does not sweat, the fuel cell will become dry and stop working. For sports lovers or athletes, this problem is not too big, but in other cases, it is the opposite.

There are three ways to solve this problem. One is to apply biofuel batteries only to devices that can ensure the required amount of perspiration. The other is to add an extra charge. And three is to add an additional power source besides the biofuel battery.

Economically, it is not appropriate to limit the equipment used. Therefore, the team focused its research on the following two solutions. For wearables that require constant power, such as a smartwatch, it's obvious that the solution will be to use batteries or supercapacitors as an auxiliary power source.

Of course, the auxiliary power supply must have the same physical properties as the rest of the wearable device, because otherwise the whole study would be meaningless.

To reduce dependence on auxiliary power supplies, the team also sought to develop a versatile power supply. Users of the device do not need to sweat continuously, instead combining three types of power sources from biofuels, light and heat will increase the amount of electricity generated at a specific time rather than when using only a single source. And of course, it still has to ensure the comfort and flexibility needed.

To date, not only has biofuel batteries replaced traditional wearable batteries have been successfully researched and applied, the team at the University of California has also made some progress in the past.

The battery manufacturing process by printing technology is smaller in size, cheaper and more flexible than traditional batteries. The findings, along with new generation supercapacitors, will help shorten the time that bioenergy batteries appear on future wearables.
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Self-sufficient Sensors:

In particular, this is the most special point of biofuel battery. They can act as a sensor and thus, it is something no other battery can do. They can do this because the capacity of the panels is usually proportional to the concentration of the chemical used as the fuel.

However, we can rely on this to create a self-powered sensor that can both track the index and generate energy. This idea was first launched in 2001 by a research team at Jerusalem Jewish University.

Creating energy from sweat is a new technology, so there are still many obstacles scientists face. For example, the amount of sweat each person produces is different, so the device must be suitable for many different conditions. Simultaneously, integrating this technology with other technologies to create a complete device is also a big challenge.

And finally, the life of the bioenergy panels also needs to be improved. Currently, the prototypes are designed for one-time use only, can be easily worn and removed as a personal medical dressing.

To be considered as a true wearable, they must be at least usable for a day, such as an athlete using to optimize exercise, or a patient who needs to monitor the temperature and hydration levels, or for individuals who need to monitor body activity.

Once the research is successful, scientists will be able to create a generation of wearable devices that are compatible, flexible, robust, rugged, cleanable and usable around the clock without charging.

Sweat is not the only biological material we can take advantage of from the body. Tears can also be a biofuel for contact lenses, saliva can power an intelligent mouth guard. Infant urine can be used to power smart diapers, or skin fluids obtained by micro needles can be used to power the dispensing device.

These devices, along with self-powered biosensors, can measure users' biological readings and send them to a wireless health monitoring app, and the data they promise will be much better compared to current wearable devices.
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