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Page 2 of 34 Xi et al. Soft Sci 2023;3:26 https://dx.doi.org/10.20517/ss.2023.13

Furthermore, this review also identifies the key directions and challenges for future research.

Keywords: Self-powered sensor, internet of things, human-machine interfaces

INTRODUCTION
A wearable device is a portable device that is directly worn on the body or integrated into the clothing or
[1-3]
accessories of the user . It is designed to harvest energy from its environment to power the sensor,
typically from sources such as light, heat, or motion . In recent years, wearable devices have made rapid
[4,5]
[6-9]
progress, which shows great application prospects . Some common examples of wearable devices include
smartwatches, fitness trackers, virtual and augmented reality headsets, and smart clothing [10,11] . These devices
often collect data and communicate with other devices, such as smartphones, to provide users with
information, notifications, and other features [11,12] . The advantages of wearable devices are portability and
convenience, especially for disease diagnosis. The diagnosis of many diseases requires long-term and
continuous monitoring in order to make an accurate assessment. For instance, long-term electrocardiogram
(ECG) tests can detect arrhythmia diseases without requiring prolonged hospitalization. This approach can
significantly enhance patient comfort [13,14] . Another advantage of wearable devices is their integration.
Traditional devices can only complete one function, while wearable devices often integrate multiple
functions, for example, the bracelet integrated with blood pressure and pulse monitoring, the monitoring
equipment integrated with respiratory intensity frequency and gas component analysis, and the electronic
skin integrated with multiple sweat component analysis [15,16] . However, wearable devices are widely used and
usually include batteries. The lifespan of a self-powered wearable Internet of Things (IoT) sensor is
determined by the capacity and longevity of its battery. If the battery no longer contains any energy, the
device will stop working unless it is repaired (battery replacement). In addition, the size of the battery also
has a great impact on the size of the sensor device. Therefore, self-powered wearable electronic devices have
become one of the important research fields [1,17] .

Self-powered wearable devices are wearable devices that are designed to function without the need for an
external power source [18-20] . One of the advantages of self-powered wearable devices is convenience. Since no
external power supply is required, users can wear the device all day without worrying about charging, which
is especially important for the equipment for long-term disease monitoring [21,22] . Self-powered sensors play a
[23]
prominent role in many cases where it is difficult to charge or replace batteries . From the perspective of
environmental protection, since the self-powered devices have no battery, the recycling of this sensor is
more environmentally friendly [24,25] . In addition, the use of a battery-free device can also prevent the device
with a battery from remaining in place even if the service life of the battery has ended . In some cases, the
[26]
self-powered devices can extend the service life of the device . However, self-powered wearable devices
[27]
also have some potential disadvantages. One problem is that their capabilities are limited because the
energy-saving technology used to power the device may not support all features and functions of traditional
wearable devices [28-30] .

Self-powered wearable sensors generate their own power through various mechanisms, such as harvesting
energy from the environment, utilizing body heat, or using kinetic energy from motion [30,31] . Self-powered
wearable sensors have many potential applications, such as in healthcare, sports, and environmental
monitoring [24,32,33] . They offer the advantages of being wireless, autonomous, and unobtrusive, which can
lead to more comfortable and convenient monitoring of various parameters [34,35] . There are several types of
self-powered wearable sensors, including piezoelectric sensors, thermoelectric sensors, and triboelectric
sensors [36-38] . It also has the potential to enable new applications in health monitoring and personalized