Smart bracelets now have charging freedom?

In the technological wave of the 21st century, wearable electronic devices are gradually becoming an indispensable part of our lives. These devices, with their portability, functionality, and user-friendliness, are changing the way we interact with the digital world.

There are many types of wearable devices, from simple pedometers to complex health monitoring systems, each with its own specific uses and functions. For example, smart watches can not only tell the time, but also monitor heart rate, sleep quality, steps, and receive notifications. Health monitors can track key physiological indicators such as blood sugar, blood pressure, and electrocardiogram in real time. Virtual reality glasses bring users an immersive experience and are widely used in games, education, and training simulations.

Figure 1 Wearable electronic devices developed based on flexible electrode materials Image source: Popular Science Times Although the convenience of wearable devices is self-evident, they still face challenges in energy supply . Traditional batteries not only need to be charged regularly, but also have life span limitations and environmental issues. Therefore, researchers are exploring how to make these devices energy self-sufficient, such as powering them through solar energy, kinetic energy , or even human body heat .

In this context, thermoelectric wearable devices and fingertip wearable microgrid technologies have emerged. Thermoelectric wearable devices use thermoelectric materials to convert human body heat into electrical energy, while fingertip microgrids collect and store energy through biofuel cells and stretchable batteries to achieve continuous power supply for wearable devices. These technologies not only improve energy efficiency, but also reduce the impact on the environment.

Thermoelectric wearable devices

Thermoelectric wearable devices are based on the thermoelectric effect , which is the ability to generate voltage and current when a temperature difference exists across a thermoelectric material. The key advantage of these devices is that they can use the body's natural heat , such as body heat, to generate electricity, thereby reducing reliance on traditional batteries.

Figure 2 Wearable thermoelectric materials and devices for self-powered electronic systems Image source: Advanced Materials

Thermoelectric materials: The core of these devices is thermoelectric materials, which can be inorganic semiconductors such as bismuth telluride and antimony telluride, or organic polymers. The thermoelectric figure of merit of these materials determines their conversion efficiency. The higher the thermoelectric figure of merit, the better the conversion efficiency.

Flexibility and comfort: In order to adapt to the irregular surface and dynamic movement of the human body, thermoelectric wearable devices need to have good flexibility and stretchability. This is usually achieved by combining thermoelectric materials with flexible substrate materials (such as polymers).

Design Innovations : To increase the power output of thermoelectric devices, researchers have used a variety of design strategies, including increasing the surface area of ​​thermoelectric materials, optimizing the nanostructure of thermoelectric materials, and designing more efficient thermal management systems to enhance temperature gradients.

Wearable microgrids on your fingertips

Figure 3 Principle and design of integrated fingertip wearable microgrid

(a) Schematic diagram of the fingertip wearable microgrid system, including biofuel cells, silver chloride-zinc batteries, flexible printed circuit boards, and wearable sensors with osmotic sweat extraction auxiliary paper fluidic system. The combination of biofuel cells and silver chloride-zinc batteries is constructed as an energy module, including two silver chloride-zinc batteries in series, each of which is charged by two biofuel cells in series. The inset (in red circle) zooms in on the components in contact with the fingertip, including four biofuel cells and a central osmotic pump system for sweat extraction: (i) main schematic, (ii) ventral, and (iii) dorsal.

(b) Schematic diagram of the working principle of the fingertip-mounted microgrid for energy harvesting, energy storage, and electrochemical sensing with wireless data transmission and smartphone display. Fingertip sweat provides biofuels and biomarkers for passive energy harvesting and continuous sensing, respectively. The microcontroller unit powers the four sensors, and the generated signals are converted into readable data through analog-to-digital converters and transmitted via Bluetooth low energy for further analysis. Image source: Nature Electronics

Fingertip wearable microgrids are a more specific wearable energy solution that focuses on harvesting bioenergy using the high sweat gland density of fingertips. Such systems typically include the following key components:

Biofuel cells: Biofuel cells use chemicals in fingertip sweat, such as lactic acid, as fuel to generate electricity through biochemical reactions. The design of these cells often includes enzyme catalysts to increase the efficiency of energy conversion.

Stretchable batteries: Such as silver chloride-zinc batteries, they can store electricity generated by biofuel cells. These batteries are designed to be stretchable and flexible to adapt to the shape and movement of the fingertips.

Microfluidic systems: To effectively direct sweat to the sensor, fingertip microgrids typically include a microfluidic system, such as laser-engraved microfluidic paper channels. These channels use capillary action to transport sweat to the sensor.

Electrochemical Sensors: These sensors are used to detect specific chemicals in sweat like glucose, vitamin C, lactic acid, etc. They convert the chemical signals into electrical signals for processing by the microcontroller.

Low-power electronics: The Fingertip Microgrid includes a low-power microcontroller and wireless transmission module to process sensor signals and transmit data wirelessly to smartphones or other devices.

The future development of thermoelectric wearable devices and fingertip wearable microgrids will focus on improving energy conversion efficiency, optimizing device design to enhance user experience, and expanding their applications in health monitoring, environmental monitoring, and human-computer interaction . As the technology matures, these devices are expected to play an important role in many fields such as healthcare, sports science, military, and entertainment, bringing more convenience and health protection to people's lives.

References:

[1] Ding, S., Saha, T., Yin, L. et al. A fingertip-wearable microgrid system for autonomous energy management and metabolic monitoring. Nat. Electron. (2024).

[2] Y. Jia, Q. Jiang, H. Sun, P. Liu, D. Hu, Y. Pei, W. Liu, X. Crispin, S. Fabiano, Y. Ma, Y. Cao, Wearable Thermoelectric Materials and Devices for Self-Powered Electronic Systems. Adv. Mater. 2021, 33, 2102990.

[3] Gong S, Lu Y, Yin J, et al. Materials-driven soft wearable bioelectronics for connected healthcare[J]. Chemical Reviews, 2024, 124(2): 455-553.

[4] Li Chuanfu. Soft electrode materials make wearable electronic devices no longer rigid[N]. Popular Science Times, 2024-07-19 (06).