Electronics
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
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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,
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Batteries are the standard power device for many health wearables – especially bulkier devices such as fitness watches.
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