However, for the category of patients with disabilities, these interfaces are not suitable

due to the problems on the part of the patients to clearly express their intentions either

by touching or by language. Zhang et al., in 2019, reported a breath-driven sensor to

transmit control commands to the HMI through breathing [20]. This sensor used the

TENG-based technology, and the device’s total size and weight were measured to be

2.47 g and 0.5 cm × 2 cm × 3.5 cm. The mechanism of the device was that when the airflow

is passed through the pipe, the PET film is set to vibrations and cyclical contact with the

electrode, thus generating electrical energy. Based on the breathing intensity, the device

can produce different electrical signals and identify normal breathing or enhanced

breathing. Enhanced breathing has higher electrical signals than normal breathing, with

the maximum value of output voltage and current reaching 342 V and 2.3 µA, respec­

tively. The HMI interface can later convert the electrical signals based on human

breathing to control electrical applications [20].

Several improvements have been made from the previous designs. In one of the latest

designs in 2021, Araz et al. proposed a cellulose nanofibril-based TENG that used diatom

biosilica to improve the output characteristics while maintaining the biocompatibility (see

Figure 20.4(d–f)) [21]. The biocompatible cellulose nanofibril and PTFE are used as the

friction material in the TENG. Compared to cellulose, the nanofibrils used in the TENG

provide flexibility, robustness, transparency, and most importantly, unmatched tribo­

electric performance giving it an edge [21]. Needless to say, the effect of SiO2 is considered

the best triboelectric material in the output performance of this device. The TENG based on

this configuration can produce an output voltage (Voc) of 88 V and current (I )

sc ^{of 18.6 µA}

under a constant loading of 8N. Based on the breathing (fast or intense), a maximum vol­

tage of 0.08–0.12 V can be generated using the smart TENG-based devices [21].

Similarly, Xue et al. proposed a wearable PyNG using PVDF thin film that was

integrated into the N95 mask to harvest the energy generated during breathing (see

Figure 20.4(g–i)) [22]. The PyNG is a three-layered device that includes a PVDF film and

two Al film electrodes. The PyNG installed on the mask was able to harness the tem­

perature fluctuation caused by human respiration to generate the voltage of 42 V (Voc) and

current (Isc) of 2.5 µA. Thus, the maximum power reached up to 8.31 µW, that however

depends upon the intensity of breathing and allows the real-time monitoring of the health

patterns of the patient [22].

20.4.3 Implantable Photovoltaic Cells

The PV cells have a long development history and have achieved many milestones since

their inception (Figure 20.5). A significant development occurred in 2012, as before this

year, PV cells employed CMOS technology that occupied larger areas that limited its ap­

plication to only a few devices [17]. With time, further developments occurred, and recently

the PV cells started to be manufactured from organic compounds with no encapsulation

requirements, even for biocompatibility. A significant development in implantable PV cells

was achieved in 2004 when Laube et al. tested it in a rabbit [23]. The implantable PV cell was

developed in an intraocular microsystem and was encapsulated in a resin. A NIR source

powered the device, and it was tested for seven months inside the rabbit, which is still a

record for the most extended period any device has been tested inside a subject [23].

Flexible implantable PV cells are a viable option considering the patient’s comfort level,

and such a system was first designed by Song et al. [24]. This device successfully gen­

erated power of 8 mW/cm², which was supplied to a commercial pacemaker implanted

in a rat [24]. In the field of flexible PV cells, organic photovoltaic (OPV) cells have started

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Bioelectronics