Today, wearables are important medical devices for detecting disease and routine health monitoring. Some being developed can administer drugs, heal wounds and facilitate gene expression in patients. And, with so many different options available, there’s a variety of approaches to powering them up. There are typically two types of health wearable: fitness and medical-based wearables. Fitness wearables tend to be bulkier systems and include consumer products such as Fitbits and smart running shoes. Medical wearables tend to be thinner, more flexible and conformal devices that adhere closer to the user’s skin.
Wearables are used in a range of health monitoring and delivery applications. Some of these include monitoring gait, foot motion and position, heart rate, pulse, sweat (metabolic molecules, nutrients and medical substances), body temperature and bacteria.
According to Jun Yao, associate professor at the Institute for Applied Life Sciences, University of Massachusetts, Amherst: “Triboelectric generators (TENGs), piezoelectric generators (PENGs), batteries and solar cells are the essential devices for powering health wearables.”
TENGs, PENGs and solar cells all harvest energy from their surroundings, while batteries are an energy storage device. Each of these options comes with its own advantages and disadvantages for powering wearables.
Battery storage
Batteries are the current standard power device for many health wearables – especially bulkier devices like fitness and health watches. Glucose monitors are also powered by batteries, adds Yao. However, the bulky size of commercial batteries means that they’re not as suitable for thinner, more conformal wearable devices, such as biosensing platforms that wrap around the user’s skin to monitor their health conditions at the molecular level (as sweat contains a lot of molecules and ions that can tell a lot about a person’s state of health). Li-ion batteries are the current gold standard and can be produced on a large scale. They have known capabilities and limitations – such as the ability to store a lot of charge for the average wearable, being rechargeable and safe enough to use in health applications. Though batteries require charging, they “can maintain the continuous operation in various conditions”, says Yao. “This is still the mainstream powering option for wearables.”
While there’s a market for bulkier wearables, in future many will be smaller, conformable and more flexible. Here, traditional batteries would be too rigid, but there are currently a number of flexible battery architectures being developed – such as those which can fold and be twisted at will.
These options include flexible Li-ion, Li-sulphur, Li-air, Zn-ion and Zn-air, and graphene batteries. Nanofabrication techniques are often used to create thin film and flexible batteries, to construct flexible and lightweight electrodes that have a better capacity, cycling stability and safety compared to traditional batteries. While there are still technical challenges to solve here – especially around low energy densities compared to their bulk counterparts – it’s a developing area that will help in the creation of thinner wearables that can be used continuously for long periods of time.
Flexible solar cells
Flexible solar cells are another option for emerging wearable devices. While traditional and more bulky solar cells are not the most suitable option – because they can’t conform their shape to the wearer – a number of flexible solar cells based on different active materials are emerging.
“Traditional photovoltaic devices are typically brittle and not suitable for flexible wearable integration. Recent progress has demonstrated flexible photovoltaic devices that are suitable,” says Yao. These options show good stability under bending and can be easily integrated into flexible materials, but there are wide variations in power conversion efficiencies (PCEs) depending on the materials used. Flexible silicon solar cells are one potential solution. These are a lot less rigid than bulky devices and are fabricated by depositing thin layers of silicon onto flexible polymer substrates or by embedding silicon directly into the substrate.
There are also organic solar cells, which have been around for many years in the wearable space because they are made from naturally flexible materials. However, they have lower PCEs than inorganic solar cells, so while they are still an option – thanks to advances in nanomaterials and nanofabrication techniques – they’re up against flexible inorganic solar cells with higher PCEs. Two key examples of this are graphene and thin film perovskite solar cells.
Perovskites have gathered a lot of interest because the bulk single junction solar cells have PCEs of over 25% and are one of the most exciting solar technologies emerging today. They can be made into lightweight and low-cost thin films, and because perovskites are a class of materials, many material compositions can be chosen to tailor the properties of the solar cell to the wearable. Their PCEs are not as high as their bulk counterparts, but they’re still higher than a lot of other flexible solar cells.
Graphene is known for its high electrical conductivity, charge carrier mobility, flexibility and mechanical strength – a lot of properties that contribute to producing a highly efficient flexible solar cell. While it is a zero-bandgap material (that is, a conductor not a semiconductor), graphene can easily be doped to become a semiconducting material. Plus, the mechanical stability of graphene means that the devices have the potential to last a lot longer than other flexible solar cells. There is also flexible dye-sensitised solar cells (DSSCs) that are compatible with commercial roll-to-roll manufacturing technology. However, while they are made from abundant materials, these are in a state of infancy compared to other solar cells and currently have very low PCEs – but they may hold future potential. One of the challenges of using solar cells is that they only work in sunlight, so their service can be intermittent, and they don’t work the best in indoor environments. “Solar devices are subject to lighting conditions and office illumination drastically reduces the energy output, but they are the most technologically mature among the energy harvesting device categories,” says Yao. The solution to this problem is combining solar cells with flexible battery technologies, so the solar cells can harvest energy and store it for when there isn’t an abundance of sunlight.
Triboelectric nanogenerators (TENGs)
Out of the two nanogenerators mentioned earlier, TENGs are the most mature option and are already being used to power a number of wearables. They have been used in drug delivery systems, pacemakers, nerve stimulation devices and more. “TENGs have also been incorporated into running shoes,” adds Yao.
TENGs harvest mechanical motion from their surroundings and convert it into an electrical output via a contact-induced electrification mechanism. They’re lightweight devices that can provide constant power (so long as there’s motion) and generate a high peak-to-peak output power. Importantly, there’s no reliance on external charging. Here’s how they work. TENGs use a triboelectric material, which becomes electrically charged when interacting with an external stimulus. This generates frictional forces that produce electrical charges on the surface of the TENG, which then get redistributed across the TENG, producing an electrical current that can then power the components of the wearable. With TENGs, the small scale of the device and need for a good output has meant that many 2D nanomaterials – such as graphene, MXenes and transition metal dichalcogenides (TMDCs) – have become popular choices as the active triboelectric materials.
“TENGs and PENGs are similar in the sense that they both convert mechanical energy from bodily motion into electricity,” Yao explains. “TENGs typically have a higher energy output than PENGs. However, because they rely on motion, they cannot sustain energy when the body is still – such as sitting in an office or laying in a bed.” Because of this, wearables that use TENGs can also benefit by having a battery system attached, though not all TENGbased wearables require this extra energy storage.
Piezoelectric nanogenerators (PENGs)
PENGs are similar to TENGs but they aren’t as well-established. PENGs use the piezoelectric effect to create electricity – which happens when a piezoelectric material is subjected to a mechanical force (such as movement of the wearer). This force causes the ions in the material to rearrange at the atomic level, causing a charge imbalance within the material lattice: with positive charges on one face of the material and negative charges on the other. This charge imbalance creates a current that can be used by the components of the wearable. Like TENGs, PENGs are using nanomaterials (or nanostructured materials) as the active piezoelectric materials due to their small size and efficient electrical properties, with materials such as hexagonal boron nitride (h-BN), TMDCs, doped graphene and group III and IV monochalcogenide materials being the prime choices. However, there are reasons why TENGs are preferred over PENGs, Yao explains. “PENGs can only be deployed on body parts of frequent movement [or] motion, such as the bottom of the feet, and mechanical abrasion of the layers can affect the lifetime of these products. PENGs cannot also sustain energy when the body is still.”
Plus, TENGs currently provide a higher output, primarily because they can work with smaller motions than PENGs can. However, PENGs do still have a high energy conversion, and can harvest energy from a wide range of human motion, which makes them suitable for a lot of different applications.
As new small-scale piezoelectric materials get discovered, more efficient PENGs might be developed in the future. However, when it comes to health wearables, both TENGs and PENGs are the two front-running technologies in the nanogenerator space – there are other nanogenerators as well, but they are nowhere near as mature as these two.
“It’s not necessarily about a single type of device, but rather the integration of multiple devices to leverage their complementary benefits,” says Yao. “For instance, combining a battery with TENG and solar devices can provide alternating power solutions tailored to different environments, enhancing energy sustainability.”
Harvesting energy using multiple technologies is going to be the way forward for developing robust wearable devices that can work in all fitness and medical environments. As different ways of producing energy mature, we will start to see a wider range of sustainable medical devices with advanced wireless capabilities becoming available that can be constantly attached to a user for as long as needed.
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