Novel Flexible Triboelectric Nanogenerator based on Metallized Porous PDMS and Parylene C

Triboelectric nanogenerators (TENGs) have recently become a powerful technology for energy harvesting and self-powered sensor networks. One of their main advantages is the possibility to employ a wide range of materials, especially for fabricating inexpensive and easy-to-use devices. This paper reports the fabrication and preliminary characterization of a novel flexible triboelectric nanogenerator which could be employed for driving future low power consumption wearable devices. The proposed TENG is a single-electrode device operating in contact-separation mode for applications in low-frequency energy harvesting from intermittent tapping loads involving the human body, such as finger or hand tapping. The novelty of the device lies in the choice of materials: it is based on a combination of a polysiloxane elastomer and a poly (para-xylylene). In particular, the TENG is composed, sequentially, of a poly (dimethylsiloxane) (PDMS) substrate which was made porous and rough with a steam-curing step; then, a metallization layer with titanium and gold, deposited on the PDMS surface with an optimal substrate–electrode adhesion. Finally, the metallized structure was coated with a thin film of parylene C serving as friction layer. This material provides excellent conformability and high charge-retaining capability, playing a crucial role in the triboelectric process; it also makes the device suitable for employment in harsh, wet environments owing to its inertness and barrier properties. Preliminary performance tests were conducted by measuring the open-circuit voltage and power density under finger tapping (~2 N) at ~5 Hz. The device exhibited a peak-to-peak voltage of 1.6 V and power density peak of 2.24 mW/m2 at ~0.4 MΩ. The proposed TENG demonstrated ease of process, simplicity, cost-effectiveness, and flexibility.


Introduction
Flexible, wearable electronics have attracted plenty of research and technology attention owing to their advantageous properties over rigid electronic systems and devices of softness, lightweight, comfort, and adaptability [1][2][3][4]. Combining these features with functional materials and specific architectures has led to the employment of flexible devices for several applications, e.g., physiological monitoring and chemical sensing [5][6][7][8][9], soft robotics [10][11][12], energy harvesting [13][14][15][16][17], and motion detection [18][19][20]. The recently increasing importance of self-powered electronics has led to the development of integrated systems with energy harvesting technologies. the device lies in the choice of materials: the porosity of metallized PDMS and the charge-retaining capability of pC play a crucial role in the triboelectric working mechanism.
Parylene C was chosen as friction layer by virtue of its desirable outstanding properties of optimal conformality, chemical inertness, and compactness: it is an excellent coating for different types of substrates and the synthesis process is very efficient in allowing complete control of the deposition parameters [59]. Parylene is used as a protective coating for biomedical devices and microelectronics [60], but it also reveals great potential as a charge accumulation layer during the triboelectrification process because surface charge densities higher than 2.0 mC/m 2 may be achieved [54,61].

Materials
Silicon wafers or glass slides were used as substrates for the deposition of PDMS by spin-coating. PDMS (Sylgard 184 Silicone Elastomer) was supplied by Dow Corning Corporation in two forms: a viscous uncured pre-polymer and a curing agent. Parylene C was provided by Specialty Coating Systems in form of dimer powders. Kapton HN 25 µm-thick foils were supplied by DuPont.

Fabrication of Flexible TENG
In Figure 1A,B a schematic representation and captured images of the fabricated devices are reported, showing an active area of the device of 2 × 2 cm 2 , while Figure 1C illustrates the steps of the fabrication process.
A flask filled with distilled water was placed on a hot plate (at 120 • C) and connected through a temperature-resistant rubber tube to a glass chamber containing the samples (see Figure 2A). The tube inlet was controlled by a valve which was opened when the water inside the flasks reached a constant temperature and pressure. The tube outlet was enlarged with a glass funnel in order to achieve a wide vapor jet. The steam-curing step, as previously described in [62], was adopted to produce asperities and pores which allow the adhesion of metal layers onto PDMS and favor the charge generation during contact of friction layers in the triboelectric device. The residual water droplets that condensed on the PDMS surface were removed under a nitrogen flow and by heating the sample (120 • C, 10 min).
The metal electrodes of the devices were made by metallizing the PDMS substrate by physical vapor deposition processes: titanium (50 nm) was first deposited as adhesion promoter ( Figure 1C(iii)) by reactive sputtering and then gold (50 nm) was thermally evaporated as exposed electrode ( Figure 1C(iv]). After peeling off the flexible patch ( Figure 1C(v)), it was attached onto a 25 µm-thick kapton foil and used as support for the connections ( Figure 1C(vi)): an electrical wire was connected using a rivet and a~2.5 µm-thick parylene C film was then deposited by room temperature chemical vapor deposition (RT-CVD) step ( Figure 1C(vii)).

Characterization of Flexible TENG
The morphology of the metallized flat patches were analyzed by means of a profilometer (Bruker Dektak Xt) and atomic force microscopy (AFM), and the surface porosity was observed with an optical microscope (Nikon Eclipse L200NSD Microscope) and by scanning electron microscopy (SEM, Helios NanoLab 600i, FEI). The electrical resistance of the flexible substrates were measured with a multimeter: the probes were positioned on the metal side of the metallized porous PDMS patches before pC deposition. Preliminary output generation tests were performed under finger tapping by connecting the device to an oscilloscope (Tektronix MDO 4104-3) and to a source meter (Keithley Series 2400), for measuring the open-circuit voltage and the short-circuit current. The power curve of the device was obtained by measuring the voltage drop on a variable resistive load connected to the device. Mechanical tensile tests were conducted by dynamic mechanical analysis (DMA Q800 TA Instruments).

Results and Discussion
This study shows the low-cost fabrication of a novel flexible single-electrode TENG operating in contact-separation mode and based on metallized porous PDMS and parylene C (pC).
The parylene film acts as friction layer, while the PDMS patch confers flexibility and adaptability, making the device suitable for smart clothes and wearable technology.

Characterization of Flexible TENG
The morphology of the metallized flat patches were analyzed by means of a profilometer (Bruker Dektak Xt) and atomic force microscopy (AFM), and the surface porosity was observed with an optical microscope (Nikon Eclipse L200NSD Microscope) and by scanning electron microscopy (SEM, Helios NanoLab 600i, FEI). The electrical resistance of the flexible substrates were measured with a multimeter: the probes were positioned on the metal side of the metallized porous PDMS patches before pC deposition. Preliminary output generation tests were performed under finger tapping by connecting the device to an oscilloscope (Tektronix MDO 4104-3) and to a source meter (Keithley Series 2400), for measuring the open-circuit voltage and the short-circuit current. The power curve of the device was obtained by measuring the voltage drop on a variable resistive load connected to the device. Mechanical tensile tests were conducted by dynamic mechanical analysis (DMA Q800 TA Instruments).
The topography images (see Figure 2C(i)), taken with the profilometer, provide information about the height of asperities produced by the water vapor on scanned areas of 1 × 1 mm 2 : a maximum roughness of 30-40 µm was measured, as shown by the colored bar. A lower-range morphology, at the inner surface of the pores, was observed with AFM, as reported by the micrographs in Figure  2C(ii): the roughness (Rms(sq)) of a scanned area of 5 × 5 µm 2 was 1.398 nm. The electrical resistance of the flat metallized PDMS patch (~2 × 2 cm 2 ) was ~3 Ω/sq and it was measured for different bending angles (as defined in the inset) and stretching ratios to test its flexibility (see Figure 3A,B respectively). It increases with decreasing bending angles and with increasing stretching ratios, since the micropores get wider, reducing the number of conductive pathways in the metal layers. However, the electrical resistance of unbent/unstretched substrate is recovered because of the strong adhesion between the Ti/Au metallization and the porous PDMS. Thus, the surface porosity induced by the steam-curing step contributes not only for the triboelectrification process, but primarily for allowing the adhesion of the metal layers onto the soft PDMS substrate.
Stress-strain curves obtained by dynamic mechanical analysis are reported in Figure 3C(ii) for the rough PDMS patch (PDMSr) and the patches metallized with Ti, Au/Ti, and covered with a parylene layer. The Young's modulus, tensile strength, and strain at break of the samples were deduced from the curves for each sample and summarized in Figure 3C(i): the range for the measured quantities is comparable for the differently metallized patches and a slight increase in the Young's

Results and Discussion
This study shows the low-cost fabrication of a novel flexible single-electrode TENG operating in contact-separation mode and based on metallized porous PDMS and parylene C (pC).
The parylene film acts as friction layer, while the PDMS patch confers flexibility and adaptability, making the device suitable for smart clothes and wearable technology.
The optical and SEM micrographs in Figure 2B(i,ii) show the surface morphology of the flat metallized PDMS patches: it can be observed that the steam-curing step produced a regular porosity and the pore diameters are in the range of 5-30 µm.
The topography images (see Figure 2C(i)), taken with the profilometer, provide information about the height of asperities produced by the water vapor on scanned areas of 1 × 1 mm 2 : a maximum roughness of 30-40 µm was measured, as shown by the colored bar. A lower-range morphology, at the inner surface of the pores, was observed with AFM, as reported by the micrographs in Figure 2C(ii): the roughness (Rms(sq)) of a scanned area of 5 × 5 µm 2 was 1.398 nm.
The electrical resistance of the flat metallized PDMS patch (~2 × 2 cm 2 ) was~3 Ω/sq and it was measured for different bending angles (as defined in the inset) and stretching ratios to test its flexibility (see Figure 3A,B respectively). It increases with decreasing bending angles and with increasing stretching ratios, since the micropores get wider, reducing the number of conductive pathways in the metal layers. However, the electrical resistance of unbent/unstretched substrate is recovered because of the strong adhesion between the Ti/Au metallization and the porous PDMS. Thus, the surface porosity induced by the steam-curing step contributes not only for the triboelectrification process, but primarily for allowing the adhesion of the metal layers onto the soft PDMS substrate.  The presented TENG was fabricated as single-electrode device for energy harvesting from a lowfrequency source, in particular, from human body motion. Performance preliminary tests were thus conducted by measuring the open-circuit voltage under finger tapping. The average applied force and frequency of the load were measured with a force resistive sensor (FSR 402, Interlink Electronics Sensor Technologies), resulting as ~2 N and ~5 Hz. The output voltage was measured with the oscilloscope for different parylene (pC) thicknesses: as illustrated by the plot in Figure 4C, the voltage decreases with increasing pC thickness, mainly due to the decrease in the capacity of the triboelectric device as well as the higher rigidity of the layered structure. In particular, the device exhibited a peakto-peak voltage of ~1.6 V for 1 µm-thick pC, and up to ~0.6 V for 6 µm-thick pC. Consequently, the generation tests were performed by choosing a pC thickness of 1 µm: the maximum peak-to-peak open-circuit voltage and short-circuit current, measured with oscilloscope and source meter respectively, were ~1.6 V and ~0.15 µA (Figure 4A,B). The power density curve is reported in Figure Stress-strain curves obtained by dynamic mechanical analysis are reported in Figure 3C(ii) for the rough PDMS patch (PDMSr) and the patches metallized with Ti, Au/Ti, and covered with a parylene layer. The Young's modulus, tensile strength, and strain at break of the samples were deduced from the curves for each sample and summarized in Figure 3C(i): the range for the measured quantities is comparable for the differently metallized patches and a slight increase in the Young's modulus can be observed, due to the deposition of metal and parylene films onto the PDMS patch. The PDMS-based triboelectric device without the kapton support exhibits a Young's modulus of 1.549 MPa, a tensile strength of 5.154 MPa, and a strain at break of 166.2%.
The presented TENG was fabricated as single-electrode device for energy harvesting from a low-frequency source, in particular, from human body motion. Performance preliminary tests were thus conducted by measuring the open-circuit voltage under finger tapping. The average applied force and frequency of the load were measured with a force resistive sensor (FSR 402, Interlink Electronics Sensor Technologies), resulting as~2 N and~5 Hz. The output voltage was measured with the oscilloscope for different parylene (pC) thicknesses: as illustrated by the plot in Figure 4C, the voltage decreases with increasing pC thickness, mainly due to the decrease in the capacity of the triboelectric device as well as the higher rigidity of the layered structure. In particular, the device Energies 2020, 13, 1625 7 of 12 exhibited a peak-to-peak voltage of~1.6 V for 1 µm-thick pC, and up to~0.6 V for 6 µm-thick pC. Consequently, the generation tests were performed by choosing a pC thickness of 1 µm: the maximum peak-to-peak open-circuit voltage and short-circuit current, measured with oscilloscope and source meter respectively, were~1.6 V and~0.15 µA (Figure 4A,B). The power density curve is reported in Figure 4D, as calculated after measuring the average peak-to-peak voltage drop on the resistive loads connected to the TENG, according to the following expression: where p is the output power density (mW/m 2 ) P is the output power (mW); S is the TENG active area (m 2 ); R Ω , V are the resistance (Ω) and the peak-to-peak voltage drop (V) on the resistive load, respectively. The maximum power is reached when the device is closed on an optimal load matching the device electrical impedance [63]: the peak of power density was 2.24 mW/m 2 at~0.4 MΩ ( Figure 4D).
Energies 2020, 13, x FOR PEER REVIEW 8 of 12 The TENG proposed in the present work is envisioned to be easily employed in distributed arrays to meet the power requirements of various wearable, flexible energy harvesting devices. Moreover, the adoption of parylene C as an external encapsulating friction layer makes the device suitable for employment in harsh environments; for instance, for scavenging energy from falling raindrops, since the protective anti-corrosion and anti-biofouling behavior of parylene have been demonstrated in previous works [60,[66][67][68][69]. In Figure 4E, the proposed working mechanism of the TENG is illustrated. The exact operation mode is defined as conductor-to-dielectric single-electrode vertical contact-separation TENG, and the conductor approaching the device is represented by the naked finger. At the initial state of the press-release cycle ( Figure 4E(i)), the finger and the upper layer of parylene C are brought into contact, such that surface charge transfer takes place due to the friction and contact electrification effect. According to the triboelectric series, human dry skin acts as a positive tribo-material [42,64,65] so positive charges are generated on the skin surface and the same amount of negative charges form on the surface of parylene coating according to the charge conservation principle. Therefore, positive charges are electrostatically induced on the primary electrode underneath parylene (Ti/Au metallization). The triboelectric charges on parylene are confined on the surface because it is an insulator, so during contact ( Figure 4E(i)), charges with opposite signs reside almost on the same plane, and no electrical potential difference is established between the two sides. When the finger starts to separate from the parylene surface ( Figure 4E(ii)), a potential difference arises between the primary electrode and the ground under open-circuit conditions, increasing with the vertical finger-parylene separation distance (up to 1-2 cm for complete release in Figure 4E(iii)). In short-circuit conditions, the voltage drop drives an electron transfer, thus generating an instantaneous current from the ground to the electrode, forcing them to have the same potential. A successive approach of the finger to parylene ( Figure 4E(iv)) triggers the same mechanisms, but in the opposite way, because the potential difference decreases and the electrons are driven from the primary electrode to the ground.
The TENG proposed in the present work is envisioned to be easily employed in distributed arrays to meet the power requirements of various wearable, flexible energy harvesting devices. Moreover, the adoption of parylene C as an external encapsulating friction layer makes the device suitable for employment in harsh environments; for instance, for scavenging energy from falling raindrops, since the protective anti-corrosion and anti-biofouling behavior of parylene have been demonstrated in previous works [60,[66][67][68][69].

Conclusions
Our research shows an easy fabrication method for a flexible contact-separation mode triboelectric nanogenerator (TENG) based on novel combinations of materials, in essence, a porous PDMS substrate metallized with titanium and gold and coated with a conformal thin film of parylene C.
The TENG is a single-electrode device, so it is characterized by simpler architecture and ease of process if compared to more complex operation modes. It is particularly suitable for energy generation by intermittent tapping onto the device and, thus, for applications involving the human body such as finger and hand tapping or walking on the floor.
Performance preliminary tests were conducted by measuring the open-circuit voltage and power density under finger tapping (~2 N) at~5 Hz. The device exhibited a peak-to-peak voltage of~1.6 V and the peak of power density was 2.24 mW/m 2 at~0.4 MΩ. The working mechanism of the TENG was proposed for contact with human skin based on the experimental results. The use of parylene C lets us envision the possibility to employ the device in harsh environments, where humidity or other aggressive agents are present.