$SSOLHG6XUIDFH6FLHQFH Contents lists available at ScienceDirect
Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
High contact surface area enhanced Al/PDMS triboelectric nanogenerator
using novel overlapped microneedle arrays and its application to lighting
and self-powered devices
⁎
C.K. Chung , K.H. Ke
Department of Mechanical Engineering and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan
A R T I C L E I N FO
A B S T R A C T
Keywords:
Triboelectric nanogenerators
TENG
Mechanical energy
CO2 laser
Microneedle
Morphology
One key to enhance the output performance of triboelectric nanogenerators (TENG) is how to increase the
effective contact area that strongly depends on the feature morphology and pattern density. Conventional
morphologies of TENG including pyramids, cubes, lines, pillars, and domes with insufficient feature surface area
are generally fabricated using an expensive-and-time-consuming lithography and etching process. Here, we
propose a novel morphology of overlapped microneedles (OL-MN) arrays for high total contact surface area to
enhance the output performance of aluminium/polydimethylsiloxane (Al/PDMS) TENG under low operation
frequencies using hand tapping. Two kinds of separate low-density and high-density microneedles arrays,
namely LD-MN and HD-MN, are comparatively studied. The integrated process of low-cost CO2 laser ablation
and PDMS casting is used for rapid prototyping. The OL-MN has the highest total contact surface area compared
to the LD-MN and HD-MN at the constant laser power and scanning speed. The output performance of opencircuit voltage (Voc) and short-circuit current (Isc) of OL-MN-TENG are 123 V and 109.7 μA those are 3.66 and
3.71 times the Voc and Isc of LD-MN-TENG, respectively. The excellent OL-MN-TENG can light on 103 LEDs
connected in series and store energy in capacitors for application to various self-powered devices.
1. Introduction
With the development of global economy, green energy is an essential strategy for all countries in the future because the current fossil
fuels have the limitation and shortcomings. The scientists are actively
seeking environmentally friendly and sustainable green energy [1–4].
Current advances in various energy harvesters including the thermoelectric, solar, piezoelectric and triboelectric effects are used to store
the converted electricity in capacitors for the self-powered electronic
devices [5–8]. The triboelectric nanogenerators (TENG) are of sustainable energy, environmentally friendly, and for harvesting pluralistic
mechanical energy. It can produce the converted electric performance
from a wide range of daily mechanical energy, such as vibration,
rolling, and sliding [9–12]. Therefore, the TENG as a new green energy
has attracted great attentions because of the simple mechanical-electrical energy conversion for potential applications to commercial products [13–18]. The TENG may harvest the amount of mechanical energy that exists below 5 Hz from the human activities such as walking,
shaking and hand tapping. Increasing the output performance of TENG
is helpful for enlarging more applications. Different techniques
⁎
including the chemical treatment of surfaces [19,20] physical modification of surface morphology [21–26] are used for improving the
performance. The strategy of chemical treatment is to control the
chemical composition of material for enhancing the triboelectric polarity difference between two tribo-layers while the physically modified
surface morphology is to adjust the contact area and surface roughness
of triboelectric layer for generating more triboelectric charge during
friction under a constant force and frequency. Moreover, the fabric
based surface-embossed polydimethylsiloxane (PDMS) TENG has
higher output performance than the flat PDMS one [27]. Therefore one
key to enhance the output performance of TENG is to increase the effective contact area that strongly depends on the feature morphology
and pattern density. From partial contact to full contact, the featured
morphology can increase the effective contact area and triboelectric
charge for enhancing output performance [28–33]. Conventional
morphologies of TENG such as nano line, cube, pyramid [23], nano
pattern [29], nano pillar and dome [25], and micro pillar [22] are
operated under low frequencies below 5 Hz and with insufficient feature surface area. For comparison, the output performance of opencircuit voltage (Voc), short-circuit current (Isc), and current density (Jsc)
Corresponding author.
E-mail address: ckchung@mail.ncku.edu.tw (C.K. Chung).
https://doi.org/10.1016/j.apsusc.2020.145310
Received 14 November 2019; Received in revised form 23 December 2019; Accepted 6 January 2020
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Table 1
Comparison of surface morphology, materials, fabrication methods, operation condition, and electrical characteristics.
Morphology
Materials
Fabrication method
Operation condition
Electrical characteristics
Voc (V)
Pyramid
Line
Cube
Nano pattern
Textile
Nano-pillar
Nano-Dome
Micro pillars
Isc (μA)
Lighting
(LEDs)
J (μA/cm )
ITO/PET-PDMS/
PET/ITO
Photolithography, etching
Linear motor
18
0.7
0.13
Pyramid > Cube > line > flat film
Ag/PDMS-ZnO/Ag
[23]
Mechanical force
stimulator (10 kgf)
130 N
3 Hz
10 N
5 Hz
Hand tapping, 3 Hz
Hand tapping, 3 Hz
120
65
–
60
83
72
2.5
3.2
8.3
–
102.8
33.6
43.1
29.5
1.5
0.98
53
18
Microneedle
LD-MN
Al/PDMS/Al
Al/PDMS/Al
Chemical treatment, dipcoating
ICP etcher, Thermal
oxidation
Soft lithography plasma
treated
CO2 laser ablation
CO2 laser ablation
HD-MN
Al/PDMS/Al
CO2 laser ablation
Hand tapping, 3 Hz
110.4
62.7
2.09
91
OL-MN
Al/PDMS/Al
CO2 laser ablation
Hand tapping, 3 Hz
123
109.7
3.6
103
Au/PDMS/Au
Al/PDMS/Al
of TENG follows the ascending order of flat film < line < cube <
pyramid [23]. For instance, the voltage and the current of the pyramid
TENG forced by a linear motor are 18 V and 0.7 μA, and corresponding
to the Jsc of 0.13 μA cm−2, which is almost 5–6 times enhancement
compared with the flat film [23]. Besides, the Voc and Isc of the nano
dome TENG are 83 V and 3.2 μA, while the Voc and Isc of the nano pillar
are 60 V and 2.5 μA, using a force of 130 N at 3 Hz [25]. The Voc and Isc
of the micropillar aluminium (Al)/PDMS TENG increases from 42 V to
72 V and 4.2 μA to 8.3 μA, respectively, after the argon plasma treatment, using 10 N at 5 Hz [22]. Table 1 lists the comparison of TENGs
with various feature-morphology structures. In brief, most structures of
lines, cubes, pyramids, pillars, domes and various micro-nano patterns
generally have some fabrication limitations including complicated,
time-consuming, and high-cost lithography and etching process. Also,
the materials of high-cost Au and Ag, and brittle ITO are highly selected
as the electrode layer. Recently, Trinh and Chung proposed a unique
microneedles (MN) array introduced into TENG [30] as MN-TENG to
enhance performance through a bending–friction–deformation (BFD)
behavior in the contacting and friction. The Al-PDMS MN-TENG can
easily generate the bending, friction and deformation between the microneedles and Al under the applied force. It occurs not only between
top Al electrode and PDMS MN but also more strongly between the MN
and MN together with the MN and PDMS substrate during pressing state
for enhancing effective contact surface area and performance. The BFD
behavior is related to the density and total contact surface area of MN
morphology to influence the output performance of Al/PDMS TENG.
The separated microneedles array in [30] with low-to-medium pattern
density of 159–388 MN/cm2 has a maximum output performance of
102.8 V (Voc) and 43.1 μA (Isc). Comprehensive research on promoting
the output performance of the separated pattern array is lacking. It is of
thus interest to study if there is some MN pattern array for more effective contact area to benefit the output performance.
In this article, we propose a simple and innovative overlapped MN
array method to increase the total effective contact surface area of
TENG for enhancing output performance under low operation frequency using hand tapping. Moreover, the integrated process of lowcost CO2 laser ablation and PDMS casting is used for rapid prototyping.
Two separated array types of the microneedle density with low density
(LD-MN, 229 MN/cm2) and high density (HD-MN, 433 MN/cm2) are
used as references for comparison with the overlapped microneedle
density (OL-MN, 654 MN/cm2) fabricated at the constant laser power
and scanning speed. The feature density of OL-MN is 2.85 time LD-MN
and 1.51 time HD-MN, respectively. The total surface area calculated is
17.6 × 103, 19.94 × 103 and 22.91 × 103 mm2, respectively, for the
LD-MN, HD-MN, and OL-MN as listed in Table 2. The OL-MN-TENG
Ref
2
[29]
75
–
[25]
[22]
[30]
This
article
This
article
This
article
exhibits the best performance of Voc and Isc for 123 V and 109.7 μA,
respectively, those are 3.66 and 3.71 times the Voc and Isc of LD-MNTENG, respectively. The OL-MN-TENG can light on 103 LEDs connected
in series and charge a 0.47 μF capacitor above 2.0 V in 3.6 sec. The OLMN-TENG merits with good mobility and sustainability that is suitable
for self-powered devices. Various self-powered devices are demonstrated including the self-powered e-watch for portable electronic devices, the self-powered buzzer as a safety sound for seeking help in
emergency, the ME-NCKU LEDs flickering as an advertising board for
communication and the self-powered humidity sensor for potential
environmental Internet of Things (IoT) sensors network which is estimated that in 2020, there will be trillions of sensor units distributed on
the earth [31].
2. Experimental procedures
2.1. Design and fabrication of the separated and overlapped microneedle
array
Fig. 1 shows the schematic diagram of the separated and overlapped
master mold of polymethyl methacrylate (PMMA) fabricated by CO2
laser ablation. The low-cost commercial CO2 laser (VL-200, Universal
Laser System Inc., USA) of 10.6 μm in wavelength has a maximum
power of 30 W and a maximum scanning speed of 1140 mm/s. The
integrated system with the CorelDraw software was used for controlling
CO2 laser parameters. The linear separated and overlapped array of
PMMA master mold was related to the ablated spacing (Fig. 1(a)). The
spacing (S) is dependent on pulse per inch (PPI) under the relationship
of S = 25,400 (μm)/PPI. Because of the same power and speed, the spot
radius (R) is the same. When the spacing is greater than the sum of the
two radius (S > 2R), it will be separated; and if S < 2R it will
overlap, as shown in Fig. 1(a1–a2). The interspacing between rows and
rows is non-overlapped (Fig. 1(a3–a4)) while the cross section of the
separated and overlapped ones is shown in Fig. 1(b).
Fig. 2 shows the schematic fabrication flow of the overlapped microneedles arrayed PDMS and the assembly of Al/PDMS MN-TENG. The
completed female PMMA master mold (Fig. 2(a)) is used for the casting
of PDMS with the solution (Sylgard 184, Dow Corning) containing both
the elastomer and curing agent in a mass ratio of 10:1 and then degassed the air bubbles of PDMS via a vacuum about 1 h; and followed
by curing in the oven at 85 °C for 1 h as shown in Fig. 2(b). The size of
PDMS is 6 cm × 5 cm and the thickness is about 300 μm by a scraper.
Finally, the cured PDMS was placed at room temperature for cooling,
and peeled out of the female mold to form OL-MN arrays (Fig. 2(c)) for
the assembly of Al/PDMS TENG (Fig. 2(d)).
$SSOLHG6XUIDFH6FLHQFH C.K. Chung and K.H. Ke
Table 2
Separated array microneedle and overlapped array microneedle related parameters and overlap estimation at the constant laser power of 6.9 W and a scanning speed
of 22.8 mm/s.
Samples
Ablated
spacing (μm)
LD-MN
716
HD-MN
394
OL-MN
245
Average Height and
Width (μm)
H: 2171
W: 358
H: 1431
W: 301
H: 1311
W: 268
Surface area
(mm2)
Density (MN/
cm2)
Junction area overlapped parameter calculated
Single ablation
area (μm2)
Appraised the ablation area
between two folded OL-MN
(μm2)
Estimated percentage of one
overlapped area between two circles
(% μm2)
17604.8
229
100659.7
Non overlap
Non overlap
19945.6
433
71157.8
Non overlap
Non overlap
22916.8
654
56410.4
1680.2
1.51%
characterization of the surface morphology of PDMS was recorded by
optical microscopy (OM, Olympus BX 51M, Japan). The Voc and Isc
signal generated by the TENG were recorded by an oscilloscope (HIOKI
Memory HiCorder MR8870-20, Japan), the electric signals of Voc and Isc
are generated due to the externally applied force using a hand tapping.
3. Results and discussions
3.1. The separated and overlapped microneedles array
The PMMA mother molds of three types of microneedles array
namely separated LD-MN, HD-MN and overlapped OL-MN are ablated
by the CO2 laser processing at a constant power of 6.9 W, a constant
scanning speed of 22.8 mm/s, and the different ablated spacing of
716 μm, 394 μm and 245 μm, respectively, as shown in the top three
optical micrographs of Fig. 3(a–c). The corresponding cast PDMS microneedles are shown in the bottom three optical micrographs of
Fig. 3(a–c). The height and width of LD-MN are 2171 μm and 358 μm,
respectively, while the HD-MN are 1431 μm and 301 μm. However, the
height and width of OL-MN can have two types of expressions as shown
in Fig. 4(a), that is, the conical tip area above the overlapped bottom
area denoted as HT and WT, and the other is the height and width of
single microneedle as H and W. The measured HT and WT of the conical
tip area are 1080 μm and 235 μm, respectively, while the H and W of
the OL-MN are 1311 μm and 268 μm. The cross-sectional OM clearly
exhibits the difference between the separated and the overlapped MN
arrays. The overlapped area A0 (μm2) in the geometrical relationship of
Fig. 4(b) can be estimated by formula (1):
Ao =
1 ⎛ πθ 2
πθ 2
R1 − R12sinθ +
R2 − R22sinθ⎞
2 ⎝ 180
180
⎠
(1)
where R1 and R2 are the radius of the ablated holes in an unit of μm,
respectively, and θ (°) the corresponding overlapped angle in an unit of
degree. Here, R1 and R2 are the same, so formula (1) is simplified as
formula (2):
Fig. 1. Schematic illustration of the CO2 laser fabrication process of the separated and overlapped PMMA master mold: (a) The separated and overlapped
with the defined inter-hole spacing; (a1–a2) The spacing of single row is controlled by the pulse per inch of CO2 laser parameter; (a3–a4) The interspacing
between rows and rows is non-overlapped; (b) The cross-sectional view of the
ablated process for the separated or overlapped ones.
Ao =
πθ 2
R − R2sinθ
180
(2)
The Ao of OL-MN calculated is 1680.2 μm , and the overlap ratio is
1.5%. The density (MN/cm2) of the separated and overlapped microneedles is defined as formula (3) and can be calculated by the OM
images in Fig. 3:
2
2.2. The material selection, assembly and electrical measurements
The Al foil and flexible PDMS are selected as the electrode and the
friction materials for a sizeable triboelectric charge affinity difference.
In addition, Al is low-cost and easy to access while PDMS is of good
elastomer for reversible large deformation. The assembled Al/PDMS
MN-TENG contains an separated or overlapped MN-PDMS film, two Al
electrodes at the top and bottom attached to two PMMA plates, respectively, and four springs that are fixed at four corners for two PMMA
separation distance of about 10 mm, as shown in Fig. 2(d). The
Density =
N
A
(3)
where N is the total number of microneedles related to ablated spacing,
and A is the total area of the MN-PDMS specimen in an unit of cm2. The
density of LD-MN and HD-MN are 229 MN/cm2 and 433 MN/cm2, respectively. Moreover, the density of OL-MN is raised to 654 MN/cm2,
which is 2.8 times and 1.5 times that of LD-MN and HD-MN, respectively. The increased MN density benefits for enhancing the contact
$SSOLHG6XUIDFH6FLHQFH C.K. Chung and K.H. Ke
Fig. 2. Schematic diagrams of microneedle PDMS casting and assembly of Al/PMDS TENG: (a) the illustrated PMMA master mold substrate, (b) the casting PDMS
solution on PMMA master mold and curing in oven, (c) the cured PDMS was placed at room temperature for cooling and peeled out of female mold, and (d) the
schematic image of the assembled Al/PDMS TENG.
area during the contact deformation [30]. It is highly significant in the
OL-MN as the comparison in Fig. 4(c) and (d). In addition, we estimate
the surface area of three samples of LD-MN, HD-MN and OL-MN by 3D
parametric Solidworks software. The calculated surface area of OL-MN
is 22.91 × 103 mm2, the HD-MN is 19.94 × 103 mm2, and LD-MN is
17.6 × 103 mm2 as listed in Table 2.
Fig. 5(a) and (b) show the OM micrographs of the ablated PMMA
and the merged cast three kinds of MN-PDMS i.e. LD-MN, HD-MN and
OL-MN at the same laser power and scanning speed for comparing the
physical geometry and the measured average dimensions of microneedles, respectively. Decreasing the ablated spacing from LD-MN to
HD-MN to OL-MN increases the pattern density but reduces the average
MN height (H) and width (W) as shown in Fig. 5(c)–(d) and Table 2.
This is attributed to the contributed laser energy per spot that decreases
at a shorter ablation spacing. The output of the laser energy per spot
during the laser ablation is expressed by formula (4):
Energy per Spot =
Linear Energy Density
Pulses Per Inch
number of PPI and the energy per spot is dependent on a linear energy
density divided by the number of pulses. The magnitude of energy per
spot in order is LD-MN > HD-MN > OL-MN. So the evolution of the
ablated H and W of single MN in order is the same as listed in Table 2.
However, the total contact surface area is multiplied by total MN
number. It results in the result that OL-MN has the highest total contact
surface area, the HD-MN the second and the LD-MN the lowest. It is
noted that the highest pattern density in OL-MN combined with the
appropriate height and width can contribute to the highest total contact
surface area at the constant force.
3.2. Working mechanism of the OL-MN-TENG
Fig. 6 shows the basic contact-separation operation mechanism of
the OL-MN-TENG. In the initial position, the upper electrode (Al) and
the friction layer PDMS are electrically neutral without any charge
transfer as shown in Fig. 6(a). When a mechanical force is continuously
downward applied to the TENG, a higher degree of deformation occurs.
The contact surface area of the two materials increases with deformation to enhance the triboelectric effect. According to the polarity difference order of the triboelectric series, the maximum transfer of
electrons from the Al surface to PDMS with a microstructured surface at
(4)
where the linear energy density (P/S) is derived from the ratio of laser
power (P: Watt) to scanning speed (S: mm/s). The P/S is equal at the
same P and S, therefore the pattern density is proportional to the
Fig. 3. Optical micrographs (OM) of the master mold PMMA (top three) and cast PDMS (bottom three) microstructure: (a) LD-MN, (b) HD-MN, and (c) OL-MN to
compare the variation of three morphologies.
$SSOLHG6XUIDFH6FLHQFH C.K. Chung and K.H. Ke
Fig. 4. Schematic diagram of an overlapped array compared with a separated array: (a) The overlapped microneedle has the conical tip area above the overlapped
bottom area denoted as HT and WT, and the other is the total average height and width of single microneedle as H and W; (b) An illustrated diagram of estimating the
overlap area; (c1–c4) Separated arrays and (d1–d4) overlapped arrays in (1) front view, (2) side view, (3) top view, (4) an isometric view, respectively.
Fig. 5. The merged OM micrographs of (a) the ablated holes of PMMA and (b) the cast MN-PDMS of three ablation spacing at the same linear energy density for
comparing the physical geometry; the relationship of the ablated spacing with the measured: (c) average height and (d) average width of MNs.
$SSOLHG6XUIDFH6FLHQFH C.K. Chung and K.H. Ke
Fig. 6. The working principle of the OL-MN-TENG
with cross-sectional optical micrographs under cycled pressing–releasing operation: (a) the initial
position of OL-MN-TENG between Al and PDMS are
electrically neutral without any charge transfer, (b)
the complete contact of the Al and OL-MN leads to
a high degree of triboelectric charge transfer
through the Al friction with the greatest MN deformation, (c) the restoring of two contact surfaces
generates a current through the external circuit
when the force is releasing, (d) the complete separation results in a new electrical equilibrium, and
(e) the electrostatic induction again drives the
electrons to flow in the opposite direction when
pressing again.
a complete contact produces a greatest triboelectric charge as shown in
Fig. 6(b). The two contact surfaces starts to separate by spring when the
force is releasing. Since the triboelectric charge creates a potential
difference between the two electrodes, in order to balance the potential
difference, the electrostatic induction drives the electrons of the bottom
electrode, flowing to the top electrode through the external loop to
maintain an electrical equilibrium state as shown in Fig. 6(c). The
generated electricity continues until the complete separation and the
open-circuit voltage reaches its maximum value as shown in Fig. 6(d).
Subsequently, when pressing downward again, the electrostatic induction drives the electrons to flow in the opposite direction as shown in
Fig. 6(e) and reach a maximum deformation at a full contact (Fig. 6(b)).
The cycling contact-separation operation generates an alternating current.
during the contact-separation is shown in Fig. 6. Fig. 6(b–e) shows the
OL-MN subjected to a cycled force produced a wide range of contact
areas and deformation behavior, at partial contact. In addition,
Fig. 6(b) with full contact shows all vacancies between the OL-MNs and
Al electrode are filled with the largest area. The Voc of TENG is proportional to the triboelectric charge density (σ) at a constant initial
position was given by formula (5):
Voc =
σ·x (t )
ε0
(5)
where x(t), σ, and ε0 represent the distance between the upper and
lower friction layers at a time t, the triboelectric charge density of
PDMS surface, and vacuum permittivity, respectively [34]. Fig. 8 shows
the conceptual illustration of the induced charge density of the separated and overlapped array with the deformed contact area. Increasing
the total contact-surface-area from the deformation and pattern density
can induce more contact charge transfer to enhance triboelectric charge
density as well as triboelectric performance [35,36].
3.3. The performance of OL-MN-TENG
To evaluate the effect of the separated and overlapped MN-PDMS
films on the output performance of the MN-TENG, the three kinds of the
density MN-TENG consisting of LD-MN, HD-MN, and OL-MN PDMS
films were used for the electrical test. All sample size of MN-PDMS is
6.0 cm × 5.0 cm, and the test condition of initial position between the
upper and lower friction layers was set about 10 mm at a difference of
pattern density. Fig. 7 shows the electrical signals of the Voc and Isc of
the separated array (LD-MN, HD-MN) and overlapped array (OL-MN).
The Voc of HD-MN is 110.4 V and the Isc is 62.7 μA, corresponding to Jsc
of 2.09 μA/cm2, while the Voc of LD-MN is 33.6 V, and Isc is 29.5 μA,
corresponding to Jsc is 0.98 μA/cm2. Moreover, the OL-MN has the best
performance characteristics, the peak Voc of 123 V, Isc of 109.7 μA and
Jsc of 3.6 μA/cm2. In comparison, the Voc and Isc of OL-MN are 3.66
times and 3.71 times LD-MN, respectively, while the Voc and Isc of HDMN are 3.2 times and 2.12 times LD-MN. It is noted that the magnitude
of H and W of MN in order is LD-MN > HD-MN > OL-MN but the
pattern density of OL-MN (654 MN/cm2) is 2.8 times LD-MN (229 MN/
cm2) and 1.5 times HD-MN (433 MN/cm2), respectively. The pattern
density has a higher influence than H and W for the total contact surface area that is the featured surface area multiplied by the total MN
number. In addition, more MN number may enhance the output performance for more triboelectric charge density because of the more
deformed effective contact area that is the key to performance enhancement [25,32,33]. The real deformation process of the OL-MN
3.4. OL-MN-TENG: energy storage, lighting emission, and self-power
application
The ability to store energy is important for the real application.
Fig. 9(a) uses a bridge rectifier circuit design to convert AC power to DC
power for charging capacitors and lighting LEDs. Fig. 9(b) shows the
charging behavior of LD-MN, HD-MN and OL-MN on a 0.47 μF capacitor. The charging voltage (Vc) of OL-MN-TENG comes to 2 V in 3.6 s.
Furthermore, the OL-MN-TENG has a stable Vc of about 2.2 V while the
average Vc of HD-MN is 1.8 V, and the LD-MN-TENG is 1.2 V. The
higher charging voltage of the OL-MN-TENG indicates that the capacitor stores a higher amount of charge than the separated LD-MN and
HD-MN. The stored charge in a capacitor can be calculated by the capacitance formula (6):
C=
Q
V
(6)
The amount of stored charge is proportional to the magnitude of
charged voltage at the same capacitor. The charge stored on a 0.47 μF
via the OL-MN-TENG is 1.03 μC compared with the HD-MN-TENG of
0.84 μC, and LD-MN-TENG of 0.56 μC. The stored charge of OL-MNTENG and HD-MN-TENG is 1.83 and 1.5 times higher than LD-MN
$SSOLHG6XUIDFH6FLHQFH C.K. Chung and K.H. Ke
Fig. 7. Measurement of open-circuit voltage and short-circuit current of separated and overlapped arrays: (a) voltage and (b) current was measured during the realtime using hand tapping (LD-MN, OL-MN), (c) peak voltage and (d) peak current comparing the separated arrays (LD-MN, HD-MN) and overlapped arrays (OL-MN)
performance.
TENG, respectively. Fig. 9(c) shows the lighting emission application of
OL-MN-TENG, HD-MN-TENG, and LD-MN-TENG by the colored LEDs
connected in series. The OL-MN-TENG, HD-MN-TENG, and LD-MNTENG drove 103, 91, and 18 bright LEDs, respectively, by hand tapping
as shown in Fig. 9(c1–c3). The lighting behavior of OL-MN-TENG is
shown in the supporting information Video S1. Fig. 10(a) demonstrated
the all-blue LEDs arranged in the ME-NCKU advertising board for
flickering communication application as the supporting information
Video S2. Moreover, Fig. 10(b) shows the circuit design for self-powered e-watch using three capacitors in parallel for the total capacitance
of over 10 μF. The OL-MN-TENG realizes the ability of mechanicalelectrical energy conversion and stored energy in the capacitors for
applications to the self-powered e-watch, buzzer, and humidity sensor
at a low frequency using hand-tapping as shown in Fig. 10(c), (d) and
(e), respectively. We not only let the e-watch start working, and the ewatch will be gradually extinguished after stop pressing the OL-MNTENG; then we restart hand-pressing the OL-MN-TENG to make the ewatch restart normally working in a short time of about 10 s as the
supporting information Video S3. We also demonstrated a self-powered
buzzer via OL-MN-TENG to store energy in a 47 μF capacitor, and when
pressing the button to drive the buzzer to make sound sirens attracting
attention for seeking help in emergency as shown in Fig. 10(d) as the
supporting information Video S4. Furthermore, the IoT network sensors
like people’s five senses can detect information received from the surrounding environment information. We successfully demonstrate the
self-powered humidity sensor by two capacitors in parallel for the total
capacitance of 80 μF to supply the power for triggering the humidity
sensor with displaying an environment humidity (53%), as shown in
Fig. 10(e) as the supporting information Video S5. In brief, we successfully demonstrate various applications of the self-powered devices
using the OL-MN-TENG including the LED lighting, communication
with the advertising board, the portable e-watch, the buzzer of alarm
sound for seeking help, and the driven humidity sensor to detect the
humidity for potential environment IoT.
4. Conclusion
The performance of TENG is dependent on the featured morphology
that is related to total contact area. The conventional morphology such
as pyramids, cubes, lines, pillars, and domes generally uses an expensive, complicated, and time-consuming fabrication process as well
as insufficient feature surface area. Here, we demonstrate the novel OLMN-TENG for improving the total contact surface area via the featured
surface area of single high-aspect-ratio microneedle multiplied by the
total MN number during deformation of the MN-PDMS and Al friction
layers. Moreover, the OL-MN-TENG is fabricated by a low-cost CO2
laser and PDMS casting process for rapid production. The Voc (123 V)
and Isc (109.7 μA) of OL-MN-TENG are 3.6 times and 3.7 times that of
LD-MN-TENG (Voc of 33.6 V, Isc of 29.5 μA), respectively. The OL-MNTENG can light up the ME-NCKU advertising board and 103 LEDs
$SSOLHG6XUIDFH6FLHQFH C.K. Chung and K.H. Ke
Fig. 8. Conceptual illustration of contact area and deformation behavior expected effects of the separated (left) and the overlapped microneedle array (right) on the
friction layer.
Fig. 9. Energy storage and lighting application: (a) the circuit diagram design: (a1) capacitor charging and (a2) LED, (b) the charging voltage curve on a 0.47 μF
capacitor of LD-MN, HD-MN, and OL-MN (c) LED lighting situation: (c1) OL-MN-TENG drove 103 LED images; (c2) HD-MN-TENG drove 91 LEDs; (c3) LD-MN-TENG
drove 18 LEDs.
$SSOLHG6XUIDFH6FLHQFH C.K. Chung and K.H. Ke
Fig. 10. Applications of OL-MN-TENG to: (a) ME-NCKU logo on the advertising board (supporting information Video S2), (b) the circuit design using 2.2, 3.3, and 4.7
μF capacitors in parallel for charging e-watch, (c) the OL-MN-TENG driven self-powered e-watch. (c1–c2) the electronic watch from start work to normally work
process (supporting information Video S3). (d) The OL-MN-TENG driven self-powered buzzer (supporting information Video S4), and (e) self-powered humidity
sensor (supporting information Video S5).
Acknowledgement
connected in series as well as the self-powered e-watch, buzzer, and
humidity sensor for various successfully practical applications through
harvesting mechanical energy in the living environment.
This work is partially sponsored by the Ministry of Science and
Technology (MOST), Taiwan, under contract No MOST 108-2221-E006-187.
CRediT authorship contribution statement
Appendix A. Supplementary material
C.K. Chung: Conceptualization, Supervision, Validation, Writing review & editing, Investigation, Methodology. K.H. Ke:
Conceptualization, Investigation, Methodology, Data curation, Formal
analysis, Writing - original draft.
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.apsusc.2020.145310.
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