Surface Textured Double Layer Triboelectric Nanogenerator for Autonomous and Ultra‐Sensitive Biomedical Sensing

Triboelectric nanogenerators (TENGs) have demonstrated great promise especially for the realization of self‐powered biomedical sensors. Nevertheless, developing TENG sensors able to detect the broad range of biomechanical movements experienced on the human body is still a challenge. Herein, a unique ridge‐structured device sensitive to wide range of forces is reported (i.e., low‐forced pulse monitoring to high‐forced gait monitoring). The device is composed of thermoplastic polyurethane layer sandwiched between two textured silicon elastomeric layers. Compared to non‐textured surface configurations, the proposed ridged‐structure provides an increased frictional contact area between the triboelectric materials, while also acting as a spacer between the triboelectric materials. The influence of ridge dimensions on the output performance is investigated by mechanical simulations and electromechanical experimental tests. The optimized device shows a maximum peak output power and current densities of 490 mW m−2 and 1750 µA m−2, respectively at 30 N and 7 Hz of compressive forces. The proposed device exhibits stable electrical output for 10000 cycles. As a proof of concept, the proposed device is used as wearable sensors for monitoring pulse rate, breath patterns, and gait movements. The study suggests the possibility of utilization of novel‐structured sandwich‐type elastomer ridge‐based TENG in different aspects of biomedical sensing and smart wearable application.


Introduction
Recent advancements in flexible electronic technologies have contributed to the tremendous growth of the market of portable electronic devices for various fields of applications, especially those related to the health, sports, and lifestyle industries. The plethora of devices demonstrated include for example physical activity trackers [1] (e.g., smartwatches), physiological signal monitoring systems [2,3] (e.g., heart rate sensors, breath rate sensors), as well as implantable healthcare devices. [4,5] Although the power requirements of sensing elements have kept reducing, they still require power sources to fulfill their energy needs. Commercial batteries, being the common source to power these devices can only provide a limited quantity of energy and depend on charging sources or replacements for their continuous use. Besides this, batteries also have a detrimental impact on the environment as major sources of electronic waste (e-waste) and toxic material pollution. [6] These issues can be resolved by replacing the batteries with sustainable, miniaturized, and portable energy harvesting sources. In this context, researchers have been actively working on exploiting energy sources able to transform into electrical energy in an efficient and sustainable manner, including thermoelectric, [7] photo-electric, [8,9] and electro-mechanical, [10,11] etc. In this context, biomechanical energy is one of the abundantly available energy resources in our living environments. Piezo-electric and triboelectric nanogenerators are noted as promising conversion technologies to harvest these mechanical energies and fulfill the power requirement of low-power electronics. [12] Recently, triboelectric nanogenerators (TENGs) are attracting attention owing to their advantageous features such as costeffectiveness, easy fabrication, broad selection of materials, and ability to generate high instantaneous output powers. [13] These TENGs generate electricity based on the principle of contact electrification [14,15] and electrostatic induction [16,17] that occurs between the surfaces of two different triboelectric materials when they encounter each other, under an applied mechanical force.
Since, Wang's group exploited this concept in 2012, [18] it has attained many breakthroughs in district research fields including the conversion of mechanical energy into electricity, self-driving electronic systems, human-machine interface, autonomous motion, and biomedical sensors, etc. [2,19,20] However, to employ the TENGs for motion and biomedical sensing applications, the accuracy or sensitivity of the device is most crucial, which can be achieved only by the high performance and well-designed device architectures. The performance of TENG depends on several parameters, such as the choice of optimistic triboelectric material selection, [21] the TENG design, [22,23] its impedance, [24] and the effective surface roughness or contact area. [25] As mentioned above, the surface roughness/morphology is in fact very important for TENG performance, due to the impact of interfacial phenomena on the generated charge. Indeed, various researchers [26,27] have shown that increased surface area induced by nano/micro-structures on the surface of triboelectric materials result in increased output performance. These surface modifications are usually achieved by surface texturing of micro/nano patterns using laser engravings, [28] lithography-based methods, [29] etc. Some literature also suggests the implementation of multilayer structures [30,31] to increase the device performance by increasing the interacting surfaces. Even if all these surface modifications proved the possibility to ensure the TENG's performance, the techniques used for their realization are complicated and require expensive equipment, limiting the scalability of the devices. Therefore, there is an urgent need of cost-effective and simple method able to effectively increase the surface area of TENGs.
Another key aspect to attaining the high performance of TENGs, specifically in vertical contact operation mode [32,33] is the effective/continuous contact and separations between the triboelectric active materials. The contact separation is an analogy used for the distance by which the contact between the active triboelectric materials is avoided. These contact separations can be achieved by bending the triboelectric films in archedshape [34,35] or by introducing spacers of non-active materials. [33,36] Researchers have found that in absence of spacers (i.e., contact distance), the output energy is poor [32] because the decrease in the separation distance increases the capacitance and ultimately results in a decrease in potential across the triboelectric layers. On the other side, there is also evidence that the presence of too many spacers, made of a non-active material, can interfere with the interaction of two triboelectric surfaces, leading to a low device output. [37] This situation demands for the design of a spacer structure that can keep the distance between the triboelectric materials and at the same time does not interfere with the interaction of triboelectric materials. Another parameter on which the triboelectric performance depends is the application of the force on the triboelectric materials to establish the contact between them. In this case, the output performance increases with the increase in the applied force as suggested by the literature. [38,39] This aspect of increased triboelectric performance in relation to the increased applied force opens the way to TENGs to be used as a force/pressure sensor. Perhaps, the researchers have exploited this possibility of TENGs by utilizing them for applications like e-skin, [40,41] motion detection, [19,42] breath sensing, [2,3] etc. Some examples of TENG based pressure sensors involve gait analysis to observe human movement, [43] as well as pulse detection. [44] The limitation with these TENG structures is that they are either viable for high forces or low forces, but not for both. Real world applications clearly call for new TENG structures, that can be able to sense the bio-mechanical energies (generated by the human and muscular motions) over a broad range from mW to several Watts, including pulse motion, breath rate, gait movement, etc.
To overcome all the above-mentioned limitations and to further contribute to the optimal utilization and exploitation of TENGs also as sensors, this work presents a sandwich-type elastomer ridge-based TENG (SER-TENG), characterized by a unique design (Figure 1c), based on a double layer of triboelectric materials characterized by ridged structures. The proposed ridged structure allows to overcome two of the limitations that have been discussed previously. First, the ridged surface increases the effective surface area as compared to flat surfaces. Furthermore, these ridges, made from the same active triboelectric material, keep the separation between negative and positive triboelectric layers without the need for an external separator made of a different material. All these advantages have been achieved by a simple and cost-effective fabrication protocol based on the molding process. Moreover, the proposed SER-TENG, due to its unique structural design and interlocking ridge arrangements on the triboelectric surfaces is capable of sensing forces ranging from very low to high amplitude. Such advantageous features of the SER-TENG allow it to be used as a unique design for significantly sensing the bio-mechanical forces/pressures ranging from very low to very high intensities. Besides, most materials used to develop the SER-TENG are soft and biocompatible such as artificial skin-like silicone elastomer and stretchable silver, which can undeniably be embedded into the wearables of wearers and sense their activities without creating any discomfort. Ultimately, the biomechanical sensing ability of the ultra-sensitive SER-TENG device is demonstrated by employing it as a breath monitoring, real real-time pulse detection, and gait monitoring system.

Results and Discussion
The proposed SER-TENG consists of three alternating layers, in which, a positive triboelectric layer (thin films of thermoplastic polyurethane (TPU)) is sandwiched between two negative triboelectric layers (i.e., silicone elastomer) with the ridges on its inner surfaces. The reason behind the selection of these active triboelectric materials is that both the TPU and silicone elastomer possess flexible structures with good mechanical properties. [45,46] Besides that, TPU acts as a good electro-positive layer while its counterpart triboelectric layer is strongly electronegative as per the triboelectric series. [47] Furthermore, there are several work mentioning the triboelectric nanogenerators with PDMS and TPU pairing as triboelectric materials while there is no work that studies the TPU with Ecoflex pairing as triboelectric materials. Besides that, the better stretchability and biocompatibility of Ecoflex as compared to PDMS [48] and cost effectiveness of Ecoflex paved the way to investigate the triboelectric output with this material pair. Figure 1 schematically represents the facile and cost-effective fabrication process of the SER-TENG and its structural arrangement. As shown in Figure 1a, the silicone elastomeric layer was achieved by a simple replication process of a 3D printed mold, which consisted of the desired ridged pattern. Further, the contact electrode of the TENG was developed on the planar side of the ridged silicone elastomer by coating a thin silver (Ag) layer via a doctor blading method. To protect the electrode from mechanical wear, another layer of silicone elastomer was coated on top of it (as shown in Figure 1a). Herein, the ridges on the negative triboelectric material (silicone elastomer) act as a spacer, which helps to retain the silicone elastomer to its original positions in the absence of the compression force, while keeping the positive and negative triboelectric layers completely separated from each other. Whereas, the intermediate positive tri-boelectric material was realized by first coating with an Ag ink on one-side of two very thin TPU sheets, and both these Agcoated TPU layers were laminated by placing one on top of another followed by the hot press treatment (as shown in Figure 1b). Herein, the Ag electrode in between the two TPU layers was employed as another contact electrode of TENG. The detailed fabrication process of the proposed SER-TENG components is mentioned in Experimental Section. The schematic representation and photographic image of the as-fabricated SER-TENG is shown in Figure 1c,d. The photographic image clearly illustrates the sandwich-type arrangement of the SER-TENG device with the TPU layer placed between the two ridged silicone elastomer layers. The ridges on the internal side of the two-silicone elastomer layers are arranged in such a manner that the crest of one layer corresponds to the trough of the opposite silicone elastomer layer. Under the application of compressive forces, these crests and troughs attempts to come into contact and lead to compress the TPU layer between them (as shown in Figureb 2a). As mentioned above, first, these ridges serve as a separator, which allows to overcoming the need for external separators between the two triboelectric active layers. Second, they are also used to increase the contact area, as compared to the planar surface (as shown in Table 1), which result in a consequent improvement of the output performance. The proposed SER-TENG has been designed to work in the presence of dynamic vertical compressive forces, as shown in Figure 2a. Figure 2a-i mimics the initial condition of the device in compressed mode. In this condition, the triboelectric layers are at the maximum contact (or friction) with each other and lead to the generation of equal and opposite triboelectric charges across their surfaces owing to triboelectrification. Hence the surface charges are completely neutralized, and result in no current flow across the external circuit. When the compressive force is removed (as shown in Figure 2a-ii), the two layers start moving away from each other. In this instance, the charges on the surface of the triboelectric materials are disturbed. These disturbed surface charges create the polarity (positive and negative) on the electrodes of the respective positive and negative triboelectric layers, resulting in the flow of current from the electrode underneath the negative (silicon elastomer) to the electrode underneath the positive (TPU) triboelectric layer. In completely relaxed conditions, the surface charge distribution comes in equilibrium, as depicted in Figure 2a-iii, and hence no flow of current is observed. When the compressive force is applied again, the equilibrium of surface charge distribution is again disturbed. Thus, to restore it, the silicone elastomer (negative triboelectric layer) collects the electrons from the TPU through the passage of current in the external circuit. Likewise, under the continuous cyclic compressive forces applied and released on top of the SER-TENG can result in the generation of alternating current (AC) as its output.
To investigate the influence of surface morphology on the output performance of SER-TENG and to identify the optimum ridge size of the proposed device, an initial characterization was performed on SER-TENG devices with different ridge size, as shown in Figure 2b-e. This analysis was performed by measuring open-circuit voltages (V OC ), short-circuit charge (Q SC ), and shortcircuit current (I SC ) of several devices with different ridge sizes (flat, 0.25, 0.5, 1, 2, and 3 mm) and constant ridge width (4 mm) to 30 N compressive force at 1 Hz, as shown in Figure 2c-e. These results are clearly illustrating an exponential increase in all electrical output parameters of SER-TENG by increasing the ridge size until 1 mm ridge size devices, after which the output values gradually decrease. This phenomenon can be explained by the decrease of the contact area with the increase in ridge size after the particular level, due to the viscoelasticity of the elastomeric material. The silicone elastomer ridges exert the pressure on TPU and push it up to a certain level. after which the stiffness of the TPU layer causes the increase in normal stress on the ridge surface and ultimately resulting in the compression of viscoelastic Eco-flex ridges rather than further pushing the TPU layer. To validate the above explained results, mechanical structural simulation was performed on the design structure of the proposed SER-TENG device. The material properties of both silicone elastomer and TPU were acquired from literature, [49,50] as shown in Table 2. To depict the real-world conditions, the 3D structures of the devices with different ridge sizes were subject to the incremental compressive forces from 2 to 70 N. The details about the 3D model, the boundary conditions and the force application are explained later in Experimental Section.
The simulated results of the maximum principal stress exerted on both the TPU layer and the silicone elastomer ridges for all the ridge size devices are shown in Figures 3a and 3b, respectively. The maximum principal stress (as shown in Figure 3a,b) exerted on both the silicone elastomer ridges and the TPU layer increased with the increase of the ridge size. This is due to the fact that the ridges with increased sizes exert more pressure on the TPU layer in order to achieve complete contact between the silicone elastomer (ridged) surface and TPU surface. While the maximum principal stress on the TPU surface is localized on the contact points between silicone elastomer layer and the TPU for all the ridge size devices, the distribution of stress on the sili- Table 2. Mechanical properties of triboelectric materials for FEM mechanical simulation.

Model
Properties cone elastomer layer for all the devices is worth an observation ( Figure 3b). What can be seen is that the devices with small ridge sizes (0.25-1 mm) have the stress distributed almost equally over the entire surface, implying the complete contact between the TPU and silicone elastomer layer surfaces. In the devices with ridge sizes greater than 1 mm, the stress is more localized on the ridges rather than the valley. This effect can be explained by the fact that the layers of the SER-TENG are arranged in such a way that, when the compressive force is being applied to it, the ridges of both silicone elastomer layers pushed the TPU layer between them. This pushing action went on until the surface of TPU is in complete contact with the ridged surface of silicone elastomer on both sides. This pushing action increases the stress on the crest of the ridges. After reaching a certain stress level, the ridges start getting compressed rather than pushing the TPU layer, which is being observed in devices with greater ridge sizes than 1 mm. This ultimately resulted in a decrease in the contact area between the silicone elastomer and the TPU layers. The detailed visual representation of the stress localization at different forces can be seen in Video S1, Supporting Information. As explained in the literature, the increase in force applied on the triboelectric layers resulted in the increase of electrical output. [38,51] Similarly, the increase in force applied on the contact area (i.e., contact force) results in increased electrical performance. To validate this theory, we derived the values of the contact area between the two triboelectric layers and normal stress exerted on the TPU layer at 30 N for all the ridge sizes. The relationship between the structure size (ridge height), contact area and the contact force can be explained by observing Figure 3c,d. In Figure 3c, it can be clearly seen that until the ridge height of 1 mm (0.25, 0.5, and 1 mm) the contact area between the triboelectric layers is 100% and it starts decreasing beyond that ridge size. The theory behind this is that when the force is applied, the ridges of Ecoflex start putting pressure on the TPU layer resulting in the bending of TPU layer between the ridges of Ecoflex as illustrated in the Figure S2, Supporting Information. This bending goes on until the stiffness of TPU exceeds the force being applied to it. After this point, the force exerted is utilized in the compressive deformation of the ridges of Ecoflex. For the devices with lesser ridge sizes (0.25-1 mm), the stiffness of the TPU does not reach that extent and the bending deformation of the TPU continues until the whole surface of the TPU comes in contact with Ecoflex ridged surface. The stiffness of the TPU exceeds the applied force in the devices with ridge sizes greater than 1 mm resulting in less contact area of the TPU surface with Ecoflex ridges, and ultimately less electrical output as compared to 1 mm ridge size device. The theory is illustrated in the Figure S2, Supporting Information. With the combination of nor-mal stress on the effective contact area, contact force has been calculated (as shown in Figure 3d. The maximum values of contact force were achieved on the device having 1 mm ridge size. From these values, we imply that the device with more contact force will have better power output. This implication also supports the measured output (as shown in Figure 2c-e) in the validation of the 1 mm ridge size to be optimum. The contact forces calculated at different compressive forces (as shown in Figure S3, Supporting Information) show that 1 mm devices are the most performing ones, for all the values of the force tested. The effect of the force on the power output of the device is discussed below.
The detailed characterization of the SER-TENG was performed, as shown in Figure 4. The ability of the proposed device to respond to various forces is important to investigate. Thus, to study the relationship between the external mechanical loadings and the electrical output performance of the proposed devices, the output (V OC , Q SC , and I SC ) of the devices with different ridge sizes was measured by applying compressive forces from 2 to 70 N. As shown in Figure 4a-c, the output magnitude for all the ridged devices increases with the increase in compressive forces. It is worth highlighting that the 1 mm device, which is deemed to be the optimized device at 30 N, performed best at all the other force magnitudes. This result is consistent with the recent literature. [38,[51][52][53] Specifically, Vasandani et al. [51] mentioned the role of contact force in the charge generation process and implied that greater contact forces increase the triboelectric surface charge densities and hence the output performance. Furthermore, it was also observed that after 30 N of compressive force, the V OC and Q SC started saturating. This phenomenon has also been discussed in literature, [54,55] which explains this compressive force as a force that is enough to extract maximum number of charges on the contacting surfaces. Indeed, literature [56] also suggests the increase in the output current with the increase of the force frequency. This is due to the fact that the current depends on the rate of flow of charge. As the force frequency is increased, the charge also follows resulting in the increase of the output current. Also in this case, 1 mm ridge size showed the highest current value (as shown in Figure 4d) for all the different frequencies used. The reproducibility of the device is an important aspect to be studied. Despite simple fabrication process, the proposed SER-TENG exhibited high level of reproducibility. Figure S4, Supporting Information represents the range of V OC , Q SC , and I SC of four devices of 1 mm ridge size. The output values were measured at 30 N and 1 Hz of compressive forces. It is clear from Figure S4, Supporting Information, that the variation in output values at all forces and frequencies lies within the acceptable range (≈10%) and hence support our claim for the SER-TENG being reproducible. This reproducibility factor can also be visualized in Video S2, Supporting Information. Four different devices with were applied with compressive forces of 30 N (for device 1 and device 3) and 25 N (for device 2 and device 4) respectively. It can be observed that these devices shows similar output on the application of same force magnitudes, i.e., ∼70 and ∼60 V for 30 and 25 N respectively, and hence they are evidently reproducible.
The power density of the 1 mm ridge size SER-TENG was investigated by connecting it in series with external variable resistors (from 10 to 10 Ω) and testing it at 30 N and constant frequency (1 Hz). As shown in Figure 5a the voltage (V load ) increases with the increment of the resistance from 1 Ω, reaching the highest value (88 V) at 10 Ω, while current (I load ) showed the opposite trend with the highest value at 10 Ω. These two quantities cross each other at ≈100 Ω, indicating this value as the approximate internal resistance of the device. Since the device has a high internal resistance, it works as a current source, as noted in the literature [57,58] . For this reason, the peak output power density was calculated by using the formula P = I 2 R. Where I is the output current at fixed load resistance. The maximum measured peak output power density was found to be 360 mW m −2 (as shown in Figure 5b) at 100 Ω, which is therefore considered the approximate value of internal resistance of the proposed SER-TENG. The average power output is an important aspect for powering commercial IOT devices, [59] therefore, the average output power density of 1 mm ridge size device has been calculated. The graph in Figure 5b shows the value of average power output density of the proposed SER-TENG at 1 Hz of compressive force cycles. The peak output power density of the device was analyzed also at different frequencies. As expected, the peak output power density of the device (as shown in Figure S5, Supporting Information) increased by increasing the frequency of the applied force cycles (≈406 and 490 mW m −2 for 5 and 7 Hz respectively). Further, the long-term stability of the device was evaluated by applying the compressive force of 30 N at 1 Hz for 10000 cycles. As shown in Figure 5c stable peaks of output voltages (V OC ) are observed, clearly demonstrating the practical value of the proposed TENG for long-term cyclic application. Furthermore, the output performance of the proposed SER-TENG has been compared with previously proposed flexible TENG structures as shown in Table S1, Supporting Information. Table S1, Supporting Information summarizes the comparable or superior output performance of the SER-TENG as compared to other TENG devices. [60][61][62][63] Besides that, the hassle-free, cost-effective, and reproducible fabrication procedure gives an advantage to SER-TENG over mentioned TENG devices.
The proposed SER-TENG was later investigated to supply energy to a real-world commercial load. At first, it was utilized to power 118 commercial LEDs (5 mm) connected in series as shown in Figure 6a. Figure 6a-i represents the schematic of the connections of LEDs with the proposed SER-TENG device.
The SER-TENG device was subjected to the compressive forces of 30 N at 5 Hz resulting in the glow of LEDs as shown in Figure 6a-ii,iii. The real-time powering of the LEDs can be seen in Video S3, Supporting Information. To further verify the powering performance of the proposed SER-TENG, it was used to turn on a commercially available digital calculator, as shown in Figure 6b. To achieve that, the output from the SER-TENG (at 30 N and 5 Hz compressive force) was first rectified and then used to charge a capacitor (23.5 μF) up to 2.2 V. Later, the charged capacitor was used to power the calculator. The schematic of the explained connections is shown in Figure 6b-i, while the powered calculator can be seen in Figure 6b-ii. Figure 6b-iii represents the voltage of the capacitor versus time, where the charging slope determines the time it took to charge the capacitor from the rectified output of the SER-TENG. The powering curve represents the instance of powering the commercial calculator. The charging and powering event of the calculator is shown in detail in Video S4, Supporting Information. The abovementioned applications demonstrate how the proposed SER-TENG is a promising candidate for powering applications. To employ the SER-TENG for mechanical sensing applications, the sensitivity and the response time of the SER-TENG were calculated using the voltage output of 1 mm ridge size device as shown in Figure 4a. The voltage output shows the linear response for the logarithmic force values (as shown in Figure S6, Supporting Information) with the sensitivity of 18.211 V N −1 . Similarly, the response and recovery time was calculated from the voltage output at 1 Hz and 30 N of compressive force (as shown in Figure S7, Supporting Information). The response time for the 1-s force pulse was ≈500 ms while the recovery time was found to be 330 ms. It can be observed that the device output increased and decreased gradually in real-time. This gradual behavior rather than the pulse behavior is due to the fact that the applied compressive force followed the sinusoidal pattern rather than the pulsatile pattern as shown in Figure S7a, Supporting Information. Similarly, the voltage response/recovery time was also calculated at 4 Hz compressive force cycles to check the real-time response/recovery of the device at higher frequencies. As expected, the response and recovery time decreased with the increase of frequency and was found to be 100 ms and 89 ms for response and recovery respectively, as shown in Figure S8, Supporting In-formation. Similarly the current response and recovery time at 4 Hz of compressive force ( Figure S9, Supporting Information) was found to be 100 and 76 ms respectively. The linear sensitivity and the real-time response and recovery makes the proposed SER-TENG suitable for being used as mechanical sensors. To verify that, use of the proposed SER-TENG was later evaluated for biomedical sensing applications. A real-time breath monitoring belt, a real-time pulse count monitoring band, and a sole for gait analysis have been realized. The 3D-printed housing along with a textile-based belt was used to hold the SER-TENG against the lower thoracic region (as shown in Figure 6a-i). The signals from the SER-TENG based breath monitoring device were then acquired in real-time by LabView-based program using a digital multimeter. The waveform graph as shown in Figure 6c-ii clearly represents the distinguished waveforms for tachypnea (hyperbreathing), apnea (no breathing), and eupnea (normal breathing) conditions. The detailed working of the breathing monitor can be seen in Video S5, Supporting Information. To utilize the SER-TENG device as a real-time pulse count monitor, it was resized to the measurements of 12 × 12 mm 2 as shown in Figure 7a. Moreover, a 3D-printed housing along with a band was utilized www.advancedsciencenews.com www.advmattechnol.de to clamp the device to the wrist (as shown in Figure 7a-i). The signals from the device were then acquired using a digital multimeter controlled by a LabView-based program. The program involves real-time peak detection resulting in the display of realtime pulse count and the pulse waveform as shown in Figure 7aii. The real-time working of our pulse detector can be seen in Video S6, Supporting Information.
Gait analysis is an important analysis method in delaying and treating various diseases, such as cerebral palsy, total knee arthroplasties, etc. [64][65][66][67] Keeping that in mind, researchers have made the use of triboelectric nanogenerators as pressure detectors for gait analysis application. [20,43] For this reason, the proposed unique SER-TENG structure was employed to realize an array of sensors in a sole for the real-time gait monitoring application, as shown in Figure 7b. The advantage of this setup is the ability to record the pressure map of the human feet in real-time. Figure 7b-i shows the SER-TENG sensor arrangement in the sole, which was defined following the pressure points of the human feet mentioned in ref. [68]. The multi-channel acquisition was realized by using a DAQ card and LabView-based program, which was employed for plotting. Figure 7b-ii,iii represents the application and removal of pressure (red being applying pressure and blue shows the pressure removal) at each device at the particular instance. The detailed gait application is being demonstrated in Video S7, Supporting Information. In the video, it can be clearly seen how the gait monitoring device was subjected to both walking and running paradigms. Figure 7b-iv,v shows the slow and quick frequency in the waveform instances during the walking and running paradigm respectively. Furthermore, the force localization of in different regions of the sole (as shown in Video S7, Supporting Information) can be used to observe and improve the walking and running postures. This shows the viability of the proposed SER-TENG devices to be successfully implemented for gait and posture monitoring application.

Conclusion
In conclusion, we introduced a novel double-layered TENG with a unique ridge-based design. More specifically, the ridged surface significantly influenced the SER-TENG power output and overall device performance, due to increased frictional contact area. To achieve maximum power output, different ridge dimensions were investigated through mechanical simulation and electromechanical experimental results, which are discussed in detail. The ridge size of 1 mm was found to be optimized for maximum power output. Under the compressive force of 30 N and 7 Hz of cyclic frequency, the device exhibited the maximum peak output power and current density values of 490 mW m −2 and 1750 μA m −2 . The device also demonstrated long-term output stability for up to 10000 cycles. The unique SER-TENG exhibited a linear increment in its electric output for the range of applied compressing forces sweeping from 2 to 70 N, which makes the device suitable for wide range of biomechanical sensing applications. To demonstrate the proof of concept, the proposed SER-TENG devices were successfully employed in a smart chest band, a smart wrist band, and smart sole for monitoring the breath pattern, pulse rate, and gait movements respectively, and were tested on a human subject. In the future, further studies on the optimization of the SER-TENG's other geometrical parameters, like ridge-width and ridge-shape, will be conducted to further improve the device's electrical output performance. In addition, the SER-TENG devices are planned to be integrated with supporting electronics and intelligent algorithms (such as machine learning and AI) to exploit them further as a wearable functional sensor (primarily for gesture recognition and motion detection) for enhanced user experience in AR/VR based next-generation technologies.

Experimental Section
Fabrication of SER-TENG Device:: The silicone elastomer with the ridges on its surface and the TPU film were used as the negative and positive triboelectric materials of the SER-TENG, respectively. To produce the ridge-like architectures on the surface of the silicon elastomer, first, molds with the negative of the ridged patterned were developed by utilizing a 3D printing technique (Ultimaker BV, S5). Molds with various ridge dimensions (as mentioned in Table 1) were produced with poly-lactic acid (PLA) over an area of 4 × 4 cm 2 . Afterward, the mixture of type-A and type-B elastomer components of ecoflex 00-30 (Smooth-on, Inc.), at equal volume were prepared and poured onto the 3D printed molds, followed by a curing treatment at 60°C for 4 h in an oven (as shown in Figure 1a). After the curing treatment, silver (Ag) ink (DuPont PE874; Insulectro Printed Electronics) was deposited on its surface by doctor blading method via 3.6 × 3.6 cm 2 mask. Subsequently, another elastomer layer was coated on top, and it was utilized as a protective cover of the Ag electrode followed by the curing treatment at 60°C for 4 h. The whole layer of negative triboelectric layer (consisting of the ridge-type elastomer along with the Ag electrode and the protecting elastomer layer) was then peeled-off from the 3D printed mold. Such a whole elastomer ridged stack was further heated at 120°C in oven for 20 min to completely cure the Ag ink. Besides, the as-purchased TPU layer (Platilon U073 Covestro Ag.; thickness of 100 μm) with an area of 4 × 4 cm 2 was also printed with the Ag ink as an electrode, by doctor blading method followed by the curing treatment at 120°C for 20 min. Afterward, another TPU layer was placed and laminated on top of TPU/Ag, by the hot press technique to attain a unified double sided TPU layers along with the Ag electrode in the middle. Ultimately, the asfabricated TPU/Ag/TPU stack was further sandwiched between two elastomer ridged stacks to realize the SER-TENG device, as shown in Figure 1.
Theoretical Simulations:: To validate the experimental results and to identify the optimized ridge sizes, a finite element method (FEM) theoretical simulation was executed using the Ansys workbench (ANSYS, Inc.). The 3D device structures of SER-TENG with various ridge sizes of the silicone elastomer were designed on SolidWorks (SolidWorks Corporation), and then imported into the Ansys workbench. The mesh arrangement along with the boundary conditions are shown in Figure S1, Supporting Information. Static structural analysis was performed by applying incremental forces on the top side of the SER-TENG structure (as shown in Figure S1b, Supporting Information) with the fixed support on the bottom side (as shown in Figure S1d, Supporting Information). The side boundaries were implemented with frictionless support (as shown in Figure S1c, Supporting Information) to let the layers move in vertical direction. The stress and the deformation in the layers (silicone elastomer and TPU) were observed to calculate the contact force on the application of applied force.
Electrical Output Characterization of SER-TENG:: The performance of TENG was analyzed by applying distinct ranges of compressive forces and frequencies, using a bespoke linear motor based customized dynamic mechanical setup. The electrical output performance including the V OC , I SC , and Q SC of SER-TENG under distinct compression conditions were measured using an electrometer 6514 (Keithley instruments) and a low-noise preamplifier integrated with a digital phosphor oscilloscope DPO-4104 (Tektronix Inc.).
SER-TENG for Powering Applications:: The powering capacity of the proposed SER-TENG was analyzed by powering commercial LEDs and a calculator, by applying the compressive force. To power the LEDs, the LEDs www.advancedsciencenews.com www.advmattechnol.de were simply connected in series with each other and with the proposed SER-TENG. For powering the calculator, the output of the SER-TENG was connected to a bridge rectifier integrated circuit (IC). Two capacitors of 47 μF were connected in series to store the rectified output. The calculator was then powered via a parallel connection to the capacitor.
The participant(s) were made fully aware of the data collection protocols and written consent has been obtained to publish the clinical data obtained from this study prior to the collection of data.
SER-TENG for Breath Monitor:: Data acquisition for breath monitor applications was performed by using digital multimeter DAQ-6510 (Keithley instruments) and integrating it with the customized application developed on LabView platform (National Instruments Corp.). For the breath monitor, the 4 × 4 cm 2 SER-TENG was encased in a 3D printed case that was tied to the lower thoracic region. The breath wave signals from SER-TENG were then acquired by the LabView based program.
SER-TENG for Pulse Detection and Gait Analysis:: Data acquisition for pulse detector and gait analysis applications were performed by using USB-DAQ 6343 (National Instruments Corp.) and integrating it with the customized application developed on LabView platform (National Instruments Corp.). For the pulse sensor realization, a 3D printed case was used to place the SER-TENG in it and tie it against the wrist. The pulse signals from the wrist tied SER-TENG were then acquired by LabView based program. This program processed the acquired signal with peak detection module in real time. For gait analysis application, seven TENG devices with the dimensions of 1.6 × 1.6 cm 2 were placed in the customized shoesole made by the laser cutting the flexible cork sheet. LabView based intensity plotting module was used to record, plot the real-time activation, and deactivation of multiple devices.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.