CHAPTER 4. RESULTS AND DISCUSSION
4.1 Silk fibroin based biodegradable piezoelectric composite sensor
4.3.1 Ag NWs-embedded within PDMS film for stable performance
Figure 25.(a) The photographs of wrinkle test of Al film (top side) and Ag-PDMS stretchable film (bottom side), (b) the snapshot of stretchable test of Al film (left side) and Ag-PDMS stretchable film (right side) (c) The schematic images of fully- and partially-exposed Ag NWs on PDMS. The photographs and SEM images show the stability of the two films. Fully-exposed film was peeled off after friction several times. (d) Sheet resistance of Ag NWs-PDMS as a function of Ag NW concentration. The photographs of the films are also shown in the inset. (e) The output voltage and current of the TENG fabricated with the stretchable film as a function of the concentrations of Ag NWs.
Figures 25a and 25b show the durability test of the stretchable film, compared with the Al foil typically used as a positive triboelectric material. As expected, the Al is irreversibly deformed by wrinkling on both sides of the Al foil. The stretchable film is reversible to the extreme deformation, as shown in Figure 25a. Figure 25b shows the photographs of stretching test with a length increase of over 20 %. It is clearly seen that the stretchable film can almost perfectly recover its original shape after release of the tensile force, while the Al foil is broken, unable to withstand the tensile force. This will cause the
non-uniform contact, thereby, non-uniform charges will be generated. With these durability tests, in TENG, a periodic external force is applied to the electrode for the energy harvesting, which requires the stability of the film during the physical contact for a long time. As seen clearly, the Ag NWs are partially embedded within the PDMS, leaving the rest of the NWs exposed. The accurate control of the rest is found to be critical to improve the stability of the TENG. Actually, when the exposed NWs are too long, i.e. if the NWs are coated on the surface of the PDMS, the film is found to peel-off after several pushing-release cycles. However, our film showed very stable performance over 1 day at large pushing force of 90 N, as shown in the Figure 25c.
Figure 26.The output voltage of TENG fabricated with PDMS film fully embedded with Ag NWs.
As the concentration of Ag NWs increases, the sheet resistance decreases up to ~ 10 Ω/sq, as shown in Figure 25d. Although it is still larger than that (~ 0.1 Ω/sq) of Al film, it is quite conductive, comparable to those reported previously.[89-91] We measured the output voltage and current of the TENG fabricated with the stretchable films as a function of the concentration of the NWs under a compressive force of 50 N and a frequency of 10 Hz, as shown in Figure 25e. As a negative triboelectric material, PDMS was used. Open-circuit voltage (Voc) and short-circuit current (Isc) were measured to characterize the TENGs’ electric performance. The size of the active area is approximately 1.5 cm × 1.5 cm. Compared with that (~ 0.045 mA) of the TENG fabricated by the Al film, the smaller output currents
As OH-modified Au NPs were decorated on the Ag-PDMS film and the TENG was fabricated.
However, there is no significant enhancement in the output voltage and current although the Voc
increased by 10 %, as shown in Figures 27a and 27b. However, when the Au NPs were capped with DMAP, it was clearly seen that the output current was enhanced to 0.082 mA and 80 V in the Iscand Voc, respectively. To investigate the output power of the TENG, resistors were used as external loads from 10 Ω to 1 GΩ, the instantaneous power of the external resistance for TENG reaches a peak value of 2.6 mW at a resistance of 10 MΩ, as shown in Figure 27c. By using the TENG, 60 blue LEDs can be lit up during the pushing force in the TENG, compared with that (35 LEDs) by the TENG with Al film (Figure 27d). The long-term stability of the TENG was also evaluated by using a pushing tester for 24 h. The consistent output current of the TENG were maintained, as shown in Figure 28.
Figure 27.(a) The output voltages and (b) currents of the TENG with Al film, PDMS film with Ag NWs, and OH- and DMAP-Au NPs supported Ag-PDMS film. (c) The output voltage and current, and output power with resistance of external loads from 10 Ω to 1 GΩ. (d) Snapshots of 60 commercial blue LEDs connected in series by the TENG fabricated with Al, and DMAP-Au NPs supported Ag-PDMS film.
Compared with the OH-coated Au NPs, the outer surface of Au NPs carrying a nucleophile such as DMAP has a positive charge and excess electron is transferred to the interior of the NPs, becoming negative. This will induce a permanent dipole at the DMAP–Au interface, pointed away from the surface of Au NPs. The dipole introduces a dipole-induced potential step at the interface when the Au NPs contact with the PDMS, up-shifting the vacuum level in DMAP-coated Au NPs. The change in the work function, ΔΦS, is generally related to the dipole through the Helmholtz equation;[92]
∆Φ = 4 cos
∙
where P0 is the dipole moment of the free molecular in vacuum state, N/A is the number of dipoles/molecules per surface area, ε= (P0/P) is the effective dielectric constant of a molecular monolayer and ε0 is the permittivity in vacuum. θ is the tilted angle of interfacial dipoles/molecule relative to the surface plane. Thus, the potential step will decrease the effective metal work function in DMAP-coated Au NPs.
Figure 28.Long-term stability test under repeatedly contact separation cycle of the TENG with Ag- PDMS with DMAP-Au over 24 h.
the sodium citrate was capped, it shifted toward lower binding energy by 0.15 eV. Based on these results, it is clearly seen that the smallest work function of approximately 4.43 eV was measured in DMAP- coated Au NPs (Figure 29c). Figure 29d shows the energy band diagram at the interface when the Au NPs contact with the PDMS. The permanent dipole of citrate-modified Au NPs, in which the dipole are oriented toward the Au NPs, down-shifts the vacuum level, increasing the effective metal work function, whereas the outer surface of DMAP-Au NPs has positive charges, which decreases the effective metal work function. This will account for the increased potential difference with the Fermi level of the PDMS (Figure 29d).
Figure 29.(a) The Zeta potential of DMAP-, OH-, Citrate-Au NPs. (b) UPS spectra and (c) the change of work function of DMAP-, OH- and Citrate-Au NPs. (d) Energy band diagrams at the Au/PDMS interface during the contact.
Finally, the Au NPs with the various capping agents are decorated on the Al films and TENGs are fabricated, the electrical output voltages and currents were measured, plotted in Figures30a and 30b.
Compared with the TENG with Al film, the TENG with a citrate as a capping agent showed larger enhancement in the power generation, which is due to the increase in the effective contact area by the
Au NPs coating. As the Au NPs were capped with DMAP, it is obvious that the output voltage and current density significantly increase and reach a record value of 63 V and 0.068 mA, respectively. For the transferred charge density, as expected, TENG with Au NPs capped with the DMAP showed the largest value, reaching up to 19 μC/m2under the same mechanical force, as shown in Figure 30c.
Figure 30. (a) The output voltage and (b) current of the TENGs with pristine Al film and Al film decorated with Au NPs capped with Citrate, OH, and DMAP. (c) The measured transferred charge density. (d) The calculated electrostatic potentials of the TENGs with various capping agents, simulated by the COMSOL multi-physics software. The inset show potential distributions in the TENGs.
= ε0V/dgap= 22.14 μC/m2, in which V is the output voltage (150 V) of TENG with Au NPs capped with the DMAP. The maximum surface charge density (σ′max) accumulated on the top electrode can be expressed as below [93]
=
where εPDMSand εAuare the relative permittivity of PDMS (3) and Au (6.9), respectively, dPDMSand dAuare the thickness of PDMS film (100 μm) and diameter of Au NP (15 nm), respectively. The σ′max
can be determined by the gap distance dgapbetween top PDMS film and bottom electrode, because dPDMS, dAu, εPDMSand εAuare constants. Therefore, the maximum surface charge densities on the top electrode of TENG with Au NPs capped with the DMAP, OH, and citrate can be obtained as σ′max(DMAP) = 21.86 μC/m2, σ′max(OH) = 16.05 μC/m2 and σ′max(citrate) = 14.60 μC/m2can be obtained, respectively.
By using the COMSOL multi-physics software, we calculated electrostatic potentials of the Au NPs- decorated TENGs with the above results, as shown in Figure 30d. It is clearly seen that TENG with Au NPs capped with the DMAP shows a large difference in electrostatic potentials, compared to those of TENGs with other capping agents. The enhancement was also observed when polyimide film, as a negative triboelectric material, was used, as plotted in Figure 31.
Figure 31.(a) The output voltage and (b) current of TENG fabricated with polyimide and pristine Al film, Al film with Citrate-Au, OH-Au and DMAP-Au.
In order to verify the increase of the transferred charge density, we measured the surface potential of polyimide films before and after friction with the OH- and DMAP-Au NPs, compared with the Al film.
Figure 32a shows the schematic illustration of KPFM system that measures the surface potential difference of the polyimide films compared to the tip before and after friction. Before the friction, the surface potential was measured to be approximately -134 mV, as shown in Figure 32b. It was changed to -41 mV after the friction with the Al film (Figure 32c), indicating that the electrons are transferred to
the polyimide film. By DMAP-Au NPs, it was measured that the polyimide film had the surface potential of approximately 111 mV, larger 30 mV than that by OH-Au NPs. (Figures 32d and 32e) This clearly shows that more electrons are transferred to the polyimide film by the friction with DMAP-Au NPs.
The enhancement in output power of the TENG with the stretchable film coated by DMAP-Au NPs can be explained by the improvement of the contact uniformity and the change of the surface potential.
We systematically measured the output performance of the TENG with the stretchable film as a function of external force from 1 to 90 N, compared with that with Al foil, plotted in Figures 33a and 33b. For the TENG with the stretchable film, as the force increases, the output voltage and current increase to 80 V and 0.082 mA, respectively, enhanced by approximately 60 % compared with that with Al foil. Here, it is noteworthy that under lower force than 10 N, the output performance steeply increases with the force in the TENG fabricated with the stretchable film, while that with Al foil does not. This indicates that the enhancement is much larger at low forces in the TENG with the stretchable film. Actually, in Figure 33c and 33d, the Al foil shows non-uniform contact with PDMS even at a force of 10 N, while the Ag-PDMS film shows quite uniform contact irrespective of the applied force. The non-uniform contact makes the non-uniform charging, decreasing the output power.
Figure 33. The output voltages (a) and currents (b) of the TENGs with Al film, Ag-PDMS film coated by DMAP-Au NPs as a function of compressive force. The photographs of contact surface between PDMS and (c) Al, and (d) Ag-PDMS film.
4.3.4 Practical application in self-powered devices
To show the practical application, we evaluated the charging characteristic of the TENG, which was
integrated with AC to DC converting circuit and buck-boost circuit, as shown in Figure 34a. The converting circuit consists of one rectifier, three low capacitors (3´0.001 µF) and one capacitor (1, 10, 100, 1000 µF) to convert AC to DC output signal. When the vertical compressive force of 50 N under frequency of 10 Hz was applied, the capacitors were found to be charged, as shown in Figure 34b. The measured voltage of the capacitor is also boosted up a constant voltage of 2.6 V using buck-boost circuit, as shown in Figure 34c. Additionally, an integrated wireless sensing system was developed by integrating the TENG with a signal-processing circuit, as shown in Figure 35. The wireless sensing system can be operated by the output voltage from the Au NPs-decorated TENG to trigger an IC timer (NE 555) that controls a wireless transmitter for remotely switching a siren between an emergency and a normal state. When a human hand contacted the TENG, the generated output voltage operates the remote controller, resulted in turning on a siren with flashing light of sensor, as shown in Figure 34d (also see the movie of Figure 36).
Figure 35.A circuit of integrated wireless sensing system which is composed of the TENG with Ag- PDMS with DMAP-Au, a signal-processing circuit and rectifier circuit.
Figure 36.The video shows a wireless sensing system which is composed of the TENG with Ag-PDMS with DMAP-Au, a signal-processing circuit and rectifier circuit.
4.4 Wearable self-powered sensor based on triangular prism shaped supporter 4.4.1 Angle optimization of triangular prism shaped supporter
Figure 37 shows the real photos and schematic pictures when walking on the rigid surface. It is clearly seen that the step starts with an angle of approximately 25 o, as shown in Figure 37b.
It may mean that the force exerted by the human body is initially applied with an oblique angle of 25 o, not vertically. The pressure then moves from the heel to the toe, that is, the wave- like pressure. This will induce the non-uniform contact inside the TENG, resulted in the decrease of the output power. According to the way people walk, the triangular prism shaped supporter was introduced for calibrating the angle with the ground under natural human walking. Figure 38a shows the schematic fabrication diagram of the TENG embedded with the supporter. The supporter has the sponge-like mesoporous structures of the PDMS film for making comfortable walking shoes, as shown in Figure 38b. The supporter was fabricated by mold-casting method by using a FDM based 3D printer (see the movie of Figure 39). And the supporter was attached on the bottom side of the curved PET/PI substrate. The Al film is then introduced on the supporter and the top side of the substrate, followed by the attachment of PDMS film. The PU/PDMS sponge is also positioned at inside of the insole for the enhancement in the restoring force and maintain an air gap of approximately 2 cm between the top and bottom parts of the insole.
The supporter has an average area of 2 cm × 2 cm and fabricated with various angles of 5 oto 35 oto find the optimized condition for high-output power generation. To see the effect of the angle on the output power, we systematically measured the output voltage and current as a function of the angle from 0 oto 35 o, plotted in Figure 40. For practical application, the external pressure was generated by the human body when people walk and applied onto the TENGs. The flat TENG without the supporter generated the output voltage and current of 8 V and 6 μA, respectively. When the supporter with an angle of 5 owas used, the output voltage and current increased to 38 V and 30 μA, respectively. However, the output signals were decreased when the angle was increased to 35 o. Further increase in the angle to 25 o increased the voltage and current to 64 V and 55 μA, respectively, an enhancement of approximately 600 %, compared with the flat TENG.
Figure 38. Schematic diagrams of the fabrication process for the TENG with mesoporous PDMS supporter. (b) The SEM image of sponge-like mesoporous PDMS at low (top) and high (bottom) magnitude.
Figure 40. (a) The output current signal of flat TENG and with various angle from 0 o to 35 o of triangular prism shaped supporter. (b) The voltage and current of TENG with the supporter as a function
of angle.
Figure 39. The video shows a fabrication of mold using FDM 3D printer for casting a sponge-like mesoporous PDMS which is inclined supporter.
The change in the output power with the angle is explained via the uniformity of the contact between the Al and PDMS. Figure 41 shows how the Al and PDMS is contacted when walking. Without the supporter, the wave-like contact is clearly observed in Figure 41a. However, the supporter can make the uniform contact possible between them, as shown in Figure 41b. This will bring about higher output performance in TENG with the supporter. To support the results, the cycled compressive force around 30 N at an applied frequency of 3 Hz was applied vertically to the TENGs without and with the supporter, the output voltages and currents were measured. For the TENG with the supporter, the electrical output current of approximately 7 μA was generated, while 16 μA was generated in the flat TENG under the same mechanical force. The decrease of the output current in the TENG with the supporter was due to
Figure 41. The optical images showing contact mechanism of (a) the flat TENG and (b) the TENG with inclined supporter (25 o) under human walking for comparing uniformity of contact. (c) And the current signal of TENG with and without inclined supporter under vertical force using pushing tester.
4.4.2 Weight-response output performance change
Based on the above results, the angle was chosen to be approximately 25 o for the highest output power.
We measured the output current of the TENGs with the weight from 74 to 92 kg, plotted in Figures 42a.
The output voltages and currents increase with the weight which may ascribe to the increase in the effective contact area [94-96]. At 92 kg, the output voltage and current reached 69 V and 65 µA, respectively. The output current as a function of walking speed from 1.2 m/s to 2.4 m/s was also measured and plotted in Figures 42b. It is also clearly seen that the output voltages and currents increase with the speed. The enhancement in the output signals at higher frequency is reported to be due to the effective compensation effects of the electrons lost by scattering with the molecules in air [97, 98]. To investigate the output power of the TENG with the supporter, resistors were used as external loads from
1 Ω to 1 GΩ, the instantaneous power of the external resistance for the TENG reaches a peak value of 1.5 mW at a resistance of 1 MΩ, as shown in Figure 42c. To show the practical application, we evaluated the charging characteristic of the TENG, which was integrated with AC to DC converting circuit, as shown in Figure 43. The converting circuit consists of one rectifier, three low capacitors (3 × 0.001 µF) and one capacitor (100 µF) to convert AC to DC output signal. When the human weight of 74 kg under frequency of 1 Hz was applied, the capacitor was found to be charged up to 0.6 V and 1 white LED bulb connected to the output terminals of the circuit were continuously powered with comparable brightness for 5 seconds.
Figure 42. The output current of TENG with supporter under (a) various human weight from 74 kg to
Figure 43.A circuit of charging system which is composed of the rectifier circuit, 3 × 10 nF, 100 μF capacitors and 1 white LED.
4.4.3 Self-powered pressure distribution sensor for monitoring local pressure action
We also demonstrated the self-powered pressure distribution sensor for monitoring the local pressure actions of human foot. The TENG arrays (5 × 5) with an area of 1 × 1 cm2 were fabricated and the output power was measured when walking, as shown in Figures 44. In general, the patterns of the human gait are characterized by differences in walking velocity, kinetic and potential energy, changes in the contact with the surface. Physical and psychological state of human will also influence on the gait patterns, thus, understanding of the fundamentals of gait can assist individuals with disabilities. Under a constant human weight of 74 kg, the insole with TENG arrays reveals relative pressure distribution to extract and integrate each data sources when walking normally or abnormally such as in-toeing and out- toeing, as shown in Figures 45a, 45b, and 45c. Figures 45d, 45e, and 45f show a two-dimensional contour plot of normal gait, in-toeing gait and out-toeing gait, respectively. In case of normal gait, the pressure distribution across the foot is quite balanced during walking. However, the abnormal gaits induces the changes of pressure distribution due to the difference of concentration point in foot during walking. In detail, the highest pressure values were obtained at the medial forefoot and hallux region in left-foot when in-toeing gait as shown in Figure 45e. Also, in case of out-toeing gait, as shown in Figure 45f, the pressure distribution was concentrated at the lateral forefoot region. This successfully shows its potential applications such as object recognition, ultra-sensitive e-skin, and self-powered health and activity monitoring system.