using MOF-PVDF composite [38]. They used CdI2-INH=CMe2 MOF as a nanofiller to

create pores in the β-phase PVDF matrix to obtain a synergistic effect of nanodiploes

(originated from β-phase PVDF) and microdipoles (originated from the porous structure)

by solution casting method. PVDF@MOF formed a porous and flexible film while PVDF

alone is rarely porous. The PVDF@MOF showed a fourfold increase in its dielectric

constant and a fivefold increase in the piezoelectric coefficient than the bare PVDF. The

PVDF@MOF film sandwiched between the silver electrodes showed an open circuit

voltage of 12 V and a short circuit current of 60 nA under vertical stress of 10 kPa. It

shows a quick response time of 8 ms and a sensitivity of 8.52 V/kPa. By utilizing this

property, MOF-PVDF ferroelectret film has been used as an ultra-sensitive pressure

sensor with mechano-sensitivity of 8.52 V/kPa within 1 kPa pressure range as well as a

high-power density of 32 µW/cm². The application of PVDF@MOF PENG as a wearable

sensor is explained in the next section.

In another work by the same group, they have developed CdI2-NAP MOF@PVDF

nanofibers by electrospinning [39]. The developed TENG delivered an open circuit vol­

tage of 22 V and a short circuit current of 0.1 µA under periodic stress of 22 kPa with a

quick response time of 5 ms. Along with mechanoelectrical conversion, acoustoelectric

conversion studies were also done by sandwiching the nanofiber sheet between the ITO-

coated PET sheet. A 1 cm diameter hole was made to the device and placed ahead of the

speaker. The device produced 6 V at 120 Hz frequency and 110 dB sound pressure level.

14.4 Wearable MOF-Based Sensors

The important requirements of a wearable electronic device are flexibility and lightweight

soft material, being able to make direct contact with the skin, the stretchability of the device,

and the stability of the material [40]. Considering the properties of MOFs such as ultrahigh

porosity, structural flexibility, and large surface areas, MOFs and MOF-derived materials

have been utilized for wearable sensor applications with enhanced performance.

A flexible and wearable sensor based on MOFs coupled with multiwalled carbon na­

notube (MWCNT) fibers has been developed for the sensitive detection of NO2 gas [41].

Here, the MOFs were introduced as the precursors of the metal oxides (MO), and well-

aligned MWCNT fibers were used as a support for gas-sensing nanocrystals. Both the

MOF/MWCNT and as-derived MO/MWCNT hybrid fibers showed a remarkable de­

tection sensitivity for NO2 down to 0.1 ppm without external heating. In addition, this

flexible fiber device can be bent into different angles without loss of sensor performance,

and hence it can be further intertwined into smart textiles for NO2 sensing. Also, the MO/

MWCNT hybrid fibers exhibited a high specific capacitance of 110 F/cm³ which can be

utilized in energy storage applications. The dual functions of MO/MWCNT hybrid fibers

validate the promising application for integrated wearable devices for safety and

healthcare purposes.

In a similar way, Rauf et al. developed a smart textile sensor using a MOF as an active

thin-film layer for humidity detection with high selectivity [42]. The deposition of a thin

layer of MIL-96 (Al) MOF particles onto interdigitated electrode-based fabrics was done

using the Langmuir-Blodgett method for the first time. The developed textile sensor

exhibited a sensitivity of 0.6 femtofarad (fF) per % relative humidity with a limit of de­

tection around 0.71% relative humidity. In addition, the sensor exhibited promising

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