attachable to the elbow joint to detect the biomechanics of human arms. In this sensor, the

authors observed a greater piezoelectric output with the increase of the bending angle of

the arms, leading to sensor deformation.

Another functional application in GaN is in UV sensors. Heo et al. [52] presented UV

light (sun exposure) and blue light (phototherapy in neonatal intensive care units) sensors

based on AlGaN and wireless modules on a flexible substrate. Although the microdevice

is composed of a chip for near-field communication, a radiofrequency antenna, photo­

diodes, supercapacitors, and a transducer, it is still compact, being easily used in sun­

glasses and earrings, for example.

As WBG materials have optoelectronic properties within the UV wavelength range, other

compounds can be used, such as ZnO, belonging to the II-VI compounds. Li et al. [53]

developed flexible UV photodetectors based on a metal-semiconductor-metal sandwich

structure. Vertically aligned ZnO nanowires were grown in silver nanowire networks, with

the deposition of another layer of silver nanowires on top of the ZnO one. With this

structure, the authors achieved a transparent and flexible material, with a transparency of

75%, and good mechanical stability. The photodetector could operate at a low voltage of

0.5 V and exhibited a high photocurrent to a dark current ratio of 9756, and fast photo­

response time (1.83 s rise time and 1.75 s decay time). Song et al. [54] also developed flexible

photodetectors using ZnO nanowires grown on polyester fabric. This controlled cultivation

can be carried out using a low-temperature hydrothermal method, and the photodetectors

exhibited a stable response with high photocurrent to dark current ratios.

Some of the aforementioned devices may require a power source to operate. However,

these battery-powered devices typically have a short operating time, which is a problem

for implantable devices as they may require additional surgery to replace this source. An

interesting source of energy would be from the movements of the human body, such as

muscle movement, which is a form of energy that can be easily converted by piezoelectric

materials, such as WBG compounds. ZnO is the most studied compound for this appli­

cation, and Voiculescu et al. [55] described an elastic device that can be attached to the

skin and produces energy through body movements. The system is based on a thin film of

ZnO deposited on a polymeric substrate coated with an elastic gold electrode. At 8%

voltage, the device output voltage was 2 V, with a power of 160 μW and power density of

1.27 mW/cm², demonstrating the potential of ZnO for manufacturing flexible energy-

harvesting devices. In contrast, Qin et al. [56] reported the use of ZnO nanowire to obtain

piezoelectric energy harvester. The device had an open-circuit voltage and short-circuit

current of 1–3 mV and 5 pA, respectively. Despite the great potential of the use of ZnO for

this application, its chemical instability in aqueous media can make it difficult to use these

devices in vivo [2]. Therefore, there is a need for further studies to overcome this problem

and allow the use of storage devices based on ZnO for practical applications.

References

1. R. Woods-Robinson et al., “Wide band gap chalcogenide Semiconductors”, Chem. Rev., vol. 120,

no 9, pp. 4007–4055, 2020, doi: 10.1021/acs.chemrev.9b00600

2. N.K. Nguyen et al., “Wide-band-gap semiconductors for Biointegrated electronics: Recent

advances and future directions”, ACS Appl. Electron. Mater., vol. 3, pp. 1959–1981, 2021, doi:

10.1021/acsaelm.0c01122

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