**4. Conclusions**

In order to seek an intelligent life, trillions of electronic device for the Internet of Things are requisite with higher personal, portable, complex, multifunctional, and smart. Aiming to maintain the normal working status of these small electronic devices sustainably, an effective technology to harvest small-scale energy from renewable natural resources is highly desirable. Given the collection characteristics of simple structure, flexibility, low cost, light weight, high efficiency, high power density, and environmental friendly, the invention of TENG is served as an promising small-scale energy harvester who can convert mechanical motions into electricity, even at low frequency. Futhermore, TENGs can also be utilized to transform physical parameters such as pressure, sliding, and other physiological variables into electronic signals, which directly reflected the information of mechanical stimuli and environmental conditions without an external power source. By extensively investigating, TENG can effectively harvest mechanical energy in almost any form based on the four fundamental modes, and thus can regard as the self-powered sensors for a wide application under diffident mechanical triggerings. In the future, the continuous endeavors on TENGs will lagerly enhance their output performance. Based on deeply investigating the fundamental menchanism of triboelectrification, it is possible to realize the ultrahigh charge density of TENG via material modification, structure design, or condition optimization. Besides the output perfoermance, the durability and output stability is the other bottleneck that limited the application of TENG, especially comparing with the traditional generator. It might overcome through fabricating new materials or coupling modes of operations. Based on the above discussion and analysis, it can be anticipated that TENG will soon become an ideal small-scall energy haverter with broad application as self-powered sensors through the world wide efforts.
