5. ConclusionsIn this paper, we pro

5. Conclusions
In this paper, we provide a comprehensive review of the
piezoelectric nanogenerators in the last 8 years. According to
the mechanical configuration of the piezoelectric nanowires,
there are three major models for the nanogenerators, including
the lateral bending model, vertical compression model,
and the lateral stretching models. For some devices, the
working mechanism of the nanogenerators may be due to
the coupling of lateral bending and stretching models. Based
on these basic models, three major types of nanogenerators
were fabricated by researchers. Among them, the VINGs
were mostly investigated due to the higher output and the
proven fabrication technique of ZnO nanowire arrays; the
LINGs with the highest energy conversion efficiency have
also been investigated in recent years with the development
of electrospinning techniques; the nanowire-composite type
nanogenerators with simplest fabrication process and high
electrical output have exhibited great potentials. By harvesting
the mechanical energies such as the muscle movement,
heartbeats, air/water flow, and solid pressures and rain drops,
the nanogenerators can be used as the power source for
driving micro/nanodevices and self-powered sensors, as well
as the active sensors for the detection the variation of
strain-related physical phenomenon and chemical conditions.
Moreover, the piezoelectric nanogenerators can be
integrated with other energy harvesting devices for building
hybrid cells. Although numerous efforts have already been
done for promoting the practical application of piezoelectric
nanogenerators, there are some critical issues which still
need to be further resolved, for example, the energy integration
of nanogenerators for high output power sources,
the structural design for increasing the energy harvesting
efficiency in different conditions, and the development of
practicable integrated self-powered systems with improved
stability and reliability. Furthermore, the long-term stability
and standard testing method of the nanogenerators still need
to be concerned in the future studies.
Conflict of Interests
The authors declare that there is no conflict of interests
regarding the publication of this paper.