Fig.1 Schematic illustration of fabrication process of the porous elastomer dielectric layer by using salt cube template method[48].(Copyright 2019, Materials Research Express)

Fig.2 (a) Schematic illustration of the fabrication procedure of the porous PDMS sponge using sugar particles. (b) Conceptual model of the triboelectric sponge(TES) with an embedded generator. (c) Various types of sugar particles were used as templates for the PDMS sponge films. All of the scale bars(black) shown in the figures are 200 μm. (d) Morphologies of three types of TESs using scanning electron microscope. All of the scale bars(white) shown in the figures are 500 μm [53].(Copyright 2016, Wiley Online Library)

Fig.3 Fabrication of the micro-structured PDMS film and the capacitive pressure sensors.(a) Schematic illustration of one-step processing of the micro-structured PDMS film based on a mixture of PDMS prepolymer and its curing agent with ammonium bicarbonate(NH4HCO3) and its seamless integration into the process flow for fabricating a flexible capacitive sensor.(b) The photo image of the fabricated large area micro-structured PDMS film(the inset shows the cross-sectional photo image of the micro-structured PDMS film).(c) The cross-sectional photo images of the micro-structured PDMS film clipped by a tweezer without pressure(upper) and with pressure(bottom)[56].(Copyright 2016, American Chemical Society)

Fig.4 Schematic illustration of the STNG.(a) Structure and fabrication process of the STNG.(b) FE-SEM images of the sponge-structured film[59].(Copyright 2016, Wiley Online Library)

Fig.5 Fabrication process of the double dielectric layer composed of the porous and dense PDMS films via vapor encapsulation casting. (i) Spin-coating of uncured PDMS solution.(ii) Sealing the sample with D.I. water in the autoclave.(iii) Heating the autoclave to produce water vapor. Magnified image shows that water vapor penetrates into the uncured PDMS film.(iv) Curing of the double dielectric layer[63].(Copyright 2019, Elsevier)

Fig.6 Schematic illustration of the tactile sensor device fabrication. (a) Si masters with recessed pyramid microstructures are fabricated by conventional lithography methods. (b) PDMS precursor is cast on the Si masters. (c) Freestanding PDMS films with microstructures are cured and peeled off from Si masters. (d) SEM image of PDMS with uniform pyramid pattern arrays[66]. (Copyright 2014, Wiley Online Library)

Fig.7 (a) Schematic process for the fabrication of the paper-based capacitive pressure sensor. (b) Photograph of the fabricated pressure sensor. (c) Schematic depiction of the pressure sensor under applied pressure[72]. (Copyright 2010, Wiley Online Library)

Fig.8 Detection of various mechanical stimuli. (a) Capacitance and film resistances as a function of time under repeated normal pressure of 2 kPa. (b) Capacitance and film resistances as a function of bending angle from 0 to 65°. (c) Capacitance and film resistances as a function of time at incrementally increasing and decreasing bending angle from 0 to 65°. (d) Capacitance and film resistances as a function of percent strained laterally and (e) as a function of time at repeated strain of 15%. (f) Capacitance as a function of time under sound vibration due to hitting a guitar string. (a~e) Change in capacitance, change in top electrode resistance, and change in bottom electrode resistance are represented as blue circles, black diamonds, and red triangles, respectively[13].(Copyright 2014, Wiley Online Library)

Fig.9 Electrical output characteristics of the fabricated LI-TENGs:(a) open-circuit voltage,(b) short-circuit current of the fabricated LI-TENGs with laser power levels ranging from 0 to 132 mW.(c) RL dependency of the output voltage and current of the LI-TENG(29 mW).(d) Open-circuit voltage of the bare TENG and the LI-TENG(29 mW) according to the external force[83].(Copyright 2017, Elsevier)