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Page 2 of 12 Jan et al. Soft Sci 2024;4:10 https://dx.doi.org/10.20517/ss.2023.54

INTRODUCTION
Wearable electronics have received considerable attention due to their important applications in healthcare
[1-4]
monitoring, rehabilitation, human-machine interfaces, and artificial intelligence . Flexible and skin-
wearable pressure sensors, in particular, have become integral components of wearable devices used in
[5]
healthcare and physiological motion monitoring . These sensors work by transducing mechanical pressure
into electrical signals, such as triboelectricity, piezoelectricity, capacitance, and resistance . Among these,
[6-9]
the sensors based on triboelectric nanogenerators (TENGs) have gained attention as self-powered devices
for dynamic pressure sensing due to their ease of fabrication, a wide choice of materials, cost-effectiveness,
and ability to operate efficiently at low frequencies [10,11] . TENGs work on the cooccurrence of
triboelectrification and electrostatic induction and are capable of harvesting energy by converting
[12]
mechanical energy from physiological motions into electrical energy [6,13,14] . Harvesting energy from daily life
physical activities, such as walking and jogging, is essentially promising for powering the small-sized
electronics [15,16] .

Based on basic working modes, TENGs can harvest energy in contact-separation (CS) [17,18] , single-
electrode , linear sliding , and freestanding arrangements. The CS mode has been widely studied due
[19]
[20]
[21]
to its simple fabrication and compatible design with multi-layer integration [17,18,22] . Fundamentally, in this
mode, two dielectric layers of different polarities, namely, tribo-negative and tribo-positive layers, generate
surface charges by contact electrification. The charges are then redistributed on the electrodes attached to
opposite sides of dielectric layers, which creates a potential difference and flow of charge across the two
terminals upon separation of the dielectric layers . The performance of TENG-based sensors has been
[22]
widely investigated using various fabrication strategies, such as surface texturing, core-shell fiber mats, and
surface crumpling, mostly focusing on the tribo-negative materials [23-26] . For example, in the previous
[25]
work , the pressure sensitivities of 1.67 V/kPa (0-3 kPa) and 0.20 V/kPa (3-32 kPa) were reported by
enhancing the surface charge potential and charge trapping by polyvinylidene fluoride/silver nanowire
nanofibrous membranes. On the other hand, the tribo-positive layer typically has limited choices for flexible
TENGs, which would otherwise enable improvements in the performance of these flexible devices [27,28] .
Achieving biocompatibility while maintaining the capability for motion monitoring and energy harvesting
is another critical issue for wearable devices [29,30] . Furthermore, upholding a high sensitivity across a broad
range of pressures is a critical criterion for the sensors in next-generation electronics .
[31]
In this study, we fabricated a laminated flexible-TENG (LF-TENG) having a dielectric-to-dielectric
configuration with a simple fabrication strategy of layer-by-layer deposition. LF-TENGs are composed of
polytetrafluoroethylene (PTFE) and polymethyl methacrylate (PMMA) films encapsulated in flexible and
biocompatible polydimethylsiloxane (PDMS) for demonstration of motion monitoring and energy
harvesting. A textured PDMS substrate was used to deposit the layered indium tin oxide-copper (ITO/Cu)
electrode by a sputtering and tribo-positive PMMA thin layer over it. The introduction of the ITO layer
onto the layered ITO/Cu electrode significantly improved the overall quality and performance of the
electrode. The primary function of the electrode is to efficiently transfer the charges generated within the
triboelectric layer to the external circuit while minimizing any losses [32,33] . The LF-TENG displayed decent
pressure-sensing capabilities, featuring a pressure sensitivity of 7.287 V/kPa in the low-pressure range
(0.0245 to 1.23 kPa) and 0.663 V/kPa in the higher-pressure range (2.45 to 23.3 kPa). Along with the fairly
2
decent self-powered sensing, the LF-TENG also exhibited a high power density of 306.2 mW/m . The
LF-TENG was then encapsulated between thin layers of PDMS with two acrylic spacers. The encapsulated
TENG (E-TENG) sensor effectively traced the physiological motions, such as wrist and finger bending.