Transparent Epidermal Antenna for Unobtrusive Human-Centric Internet of Things Applications

The concept of optical transparency in antennas for epidermal electronics is demonstrated in this work as a means of improving the long-term comfort-of-wear level and possibly opening up a wider range of applications. In contrast to previous attempts, the epidermal antenna transparency is achieved by employing dielectric and conductive materials that are both transparent and flexible (i.e., polydimethylsiloxane transparent conductive textile composite) via a nonclean room procedure that is relatively simpler and less expensive. To demonstrate the concept, a modified rectangular loop epidermal antenna for an arm-worn wireless sensing system operating at 868-MHz ultra high-frequency (UHF) band is designed. Through a systematic numerical investigation, an interesting radiation response of the loop epidermal antenna as the result of two opposing mechanisms of radiation and loss is revealed, which dictates a specific design guideline for the loop when attached to the body compared to that in free space. Two antenna prototypes were fabricated with the developed transparent composite and its nontransparent counterpart. Then, comprehensive characterizations comparing both epidermal antenna prototypes were carried out, including antenna return loss and far-field tests on a human forearm phantom, and indoor wireless connectivity tests using a human test subject. By showing similar performance between the two prototypes, the study provides a convincing demonstration of the applicability of the developed transparent composite for the class of epidermal antenna and the capability of a transparent antenna to enable wireless connectivity in the context of epidermal electronics.

Abstract-The concept of optical transparency in antennas for epidermal electronics is demonstrated in this work as a means of improving the long-term comfort-of-wear level and possibly opening up a wider range of applications.In contrast to previous attempts, the epidermal antenna transparency is achieved by employing dielectric and conductive materials that are both transparent and flexible (i.e., polydimethylsiloxane transparent conductive textile composite) via a nonclean room procedure that is relatively simpler and less expensive.To demonstrate the concept, a modified rectangular loop epidermal antenna for an arm-worn wireless sensing system operating at 868-MHz ultra high-frequency (UHF) band is designed.Through a systematic numerical investigation, an interesting radiation response of the loop epidermal antenna as the result of two opposing mechanisms of radiation and loss is revealed, which dictates a specific design guideline for the loop when attached to the body compared to that in free space.Two antenna prototypes were fabricated with the developed transparent composite and its nontransparent counterpart.Then, comprehensive characterizations comparing both epidermal antenna prototypes were carried out, including antenna return loss and far-field tests on a human forearm phantom, and indoor wireless connectivity tests using a human test subject.By showing similar performance between the two prototypes, the study provides a convincing demonstration of the applicability of the developed transparent composite for the class of epidermal antenna and the capability of a transparent antenna to enable wireless connectivity in the context of epidermal electronics.Index Terms-Conductive textile, epidermal antenna, flexible antenna, Internet of Things (IoT), polydimethylsiloxane (PDMS), transparent antenna, unobtrusive, wearable antenna.

I. INTRODUCTION
T HE PHRASE epidermal electronics refers to an electronic system in which all functionality and associated active and passive components, ranging from electrodes, sensors, electronics, communication, and power supply modules, are embodied in a skin-compatible ultrathin form factor.The concept was introduced in 2011 [1], with the goal of revolutionizing human physiological sensing technologies, which have historically relied on wired interconnections between skin-attached sensors/electrodes and external entities containing signal conditioning units, data storages, communication modules, and power supplies.The notion of epidermal electronics is to create a device that features as many characteristics of human skin as possible (e.g., ultra thinness, flexibility, stretchability, self-healing, and air/water permeability, to mention a few) while still fulfilling certain functions.With such distinct qualities, epidermal electronics have opened up a plethora of new possibilities, notably in the fields of real time and truly noninvasive human physiological monitoring and human-machine interface, addressing problems that traditional wearable electronics cannot [1], [2], [3].
In the context of electronics mounted on human skin, producing an epidermal antenna with the highest comfort-of-wear level is just as critical as developing a high-performance antenna.Being typically the most space-occupying component, the less disruption created by the antenna's physical existence in the users' daily activities, the more likely people are to wear the epidermal device [24].One key engineering approach to achieving comfort-of-wear is to impart skin conformability to the epidermal antenna.Adding to that, if both of the antenna substrate and radiator can be made optically transparent, it will enhance further the wearers' comfort for long-term usage and aesthetic appearance, thus increasing the proliferation of the epidermal electronics and even renders vast areas of applications.In the case of smart wound dressing [25], for instance, the transparent attribute of the antenna and integrated electronics would enable clinicians to obtain real-time visualization and a more accurate evaluation of the wound healing progress without having to remove the dressing.For wireless sensing and monitoring devices equipped on the body of mental health patients and elderly [26], [27], visually imperceptible wearable devices will improve the reliability of long-term health monitoring and surveillance.Despite the benefits, the majority of the aforementioned studies on epidermal antenna have not demonstrated the concept of transparency in its entirety.For instance, in [13], [14], [15], [16], [17], [22], and [28], while the works employ transparent materials as the antenna substrate, the conductor is still realized with opaque materials.This scarcity may be attributable to the challenges of fabricating optically transparent flexible antennas [29].In particular, finding conductive materials with an optimal balance of conductivity, optical transparency, and mechanical robustness is notoriously difficult.In addition, the fabrication process is often costly and entails a series of complex procedures.
We recently proposed a new method for realizing transparent flexible antennas in [30] through layer-by-layer assembly of polydimethylsiloxane (PDMS) and patterned transparent conductive textile.The demonstrated concept is comparatively simpler and less expensive than other popular approaches based on transparent conductive oxides, transparent conductive polymers, silver nanowires, and 2-D materials [such as graphene, MXene, and molybdenum disulfide (MoS2)], which typically involve complex processes (e.g., physical/chemical vapor deposition, RF sputtering, lithography, electrospinning, and spray pyrolysis, to name a few) [30].The resultant antennas displayed noteworthy qualities of high transparency (i.e., >70% optical transmittance), flexibility, durability against humidity and deformation, while concurrently showing a good reproducible RF performance.
In this work, to further advance the field of epidermal electronics, we investigate for the first time the potential of implementing our proposed concept, PDMS-transparent conductive textile composite, for the realization of optically transparent epidermal antennas.To do this, we design, fabricate, and characterize a modified rectangular loop antenna operating at 868-MHz ultra high-frequency (UHF) band made of the aforementioned composite material.First, we present a systematic numerical examination of the antenna response as a function of different design parameters when attached on a phantom representing human arm tissue.By revealing an interesting response of a loop antenna in a close proximity to a human body, this study offers insight and general guidelines toward producing an epidermal antenna with the optimum performance.A comprehensive evaluation of the antenna performance was conducted, including reflection coefficient, far-field, wireless communication, and specific absorption rate (SAR) numerical tests.The wireless communication tests were carried out in an indoor long-range wide area network (LoRaWAN) testbed, which involved a number of LoRa gateways and a battery-powered wireless module with the developed epidermal antenna connected and worn on the forearm of a human subject.The results show a good performance in terms of antenna input impedance matching, radiation and hence wireless connectivity, as well as RF exposure, demonstrating the applicability of the developed composite for the class of epidermal antenna.
In this study, we also constructed an opaque prototype counterpart of the developed design out of PDMS and nontransparent conductive textile composite [31].For the first time, we present a thorough examination comparing the performance of opaque and transparent epidermal antennas, numerically and experimentally.Despite the variation in conductive textile quality, the results show no appreciable difference particularly in the radiation performance and hence the wireless link quality, corroborating the earlier research by [6].This discovery is significant, particularly in the context of epidermal antenna development, as it confirms that during the construction of an epidermal antenna, the designer may focus their efforts on other goals than perfecting the conductor quality to boost the radiation, if the σ is larger than 10 4 S/m [6].More importantly, the findings provide a compelling demonstration of the capability of the transparent antenna for enabling wireless functionality of epidermal electronics in the context of human-centric IoT.

II. MATERIALS
The fabrication of the proposed transparent epidermal antenna employs Less EMF VeilShield and Dow Corning Sylgard 184 for the antenna conductive and nonconductive parts, respectively.On the other hand, for the nontransparent counterpart, Less EMF Nickel(Ni)-Copper(Cu)-coated ripstop is used.VeilShield is a commercial woven conductive textile made of monofilament polyester threads that are coated with Cu and Zinc (Zn)-blackened Ni.According to the supplier's data sheet, the textile has a thickness of 57 μm with a sheet resistance of 0.1 / .The warp and weft threads interlace in one-to-one ratio at approximately 0.17-mm distance, resulting in a see-through textile with 132/inch mesh.Using an Agilent Carry 5000 UV-vis-NIR spectrophotometer, we assessed that this particular textile has an optical transmittance of approximately 72% in the visible light spectrum between 350 and 750 nm.Unlike VeilShield, the Ni-Cu-coated ripstop is a commercial woven textile constructed of multifilament threads covered with metal.The distance between the warp and weft groups intertwining in a one-to-one ratio is less than 0.04 mm, resulting in a good resemblance to a solid metallic plate.The supplier's data sheet describes this textile as having a thickness of 80 μm and a sheet resistance of 0.03 / .The coating material of both textiles contain Ni, a common allergen that can induce contact dermatitis [32].However, this problem is mitigated by the fact that in this work the textile is completely encapsulated in PDMS, which is biocompatible.Sylgard 184, a silicone elastomer supplied as a two-component liquid kit, was used to prepare the PDMS solution by mixing the base and the curing agent of the kit in a ten-to-one weight ratio.PDMS has an approximate relative permittivity (ε r ) of 2.8 and tanδ of 0.015 at 868 MHz as determined by experiments conducted with an Agilent 85070E Dielectric Kit.Noting that PDMS is a suitable material for epidermal electronics due to its unique properties, such as high flexibility/stretchability, transparency, water-and heat-resistance, and chemical stability [31].

A. Topology
The proposed epidermal antenna is based on a rectangular loop antenna, with one of its sides expanded, resulting in an unbalanced loop structure (see Fig. 1).This transformation is intended to mitigate common mode currents that typically flow when a balanced antenna or an antenna with a small ground plane is fed with an unbalanced feeding line (e.g., the coaxial cable used in the antenna characterizations described in the following sections) [33], [34].These currents flowing in the outer conductor of the coaxial cable cause the cable to become an integral part of the antenna, which can lead to measurement errors.The proposed antenna radiator made of the conductive textile is fully encapsulated by PDMS.A number of square-shaped holes are punched into the expanded side to facilitate air and water permeability, hence alleviating the nonbreathability issue of PDMS.Simulations revealed that the holes have no significant influence on the input impedance and radiation performance of the antenna (approximately 0.1-dB gain difference between antennas with and without holes).The antenna is envisaged for an arm-worn wireless sensing application operating at the European LoRaWAN frequency band, i.e., 863-870 MHz.LoRaWAN was chosen as the radio technology because of its attractive features of long-range communication with cost-effective infrastructure, high-network reliability (packet delivery ratio (PDR) > 0.995), and low-power consumption (< 105 mW) [35].
It should be noted that when a loop antenna is electrically large, the distinction between its magnetic and electric  fields is less obvious.In our case, denoting the effective permittivity sensed by the antenna when placed on a phantom as ε eff = (ε phantom +1)/2 [6], [36], the aperture/perimeter of the loop (4s) is electrically large, i.e., 2.05λ eff .Consequently, in terms of gain and efficiency, this loop antenna typically demonstrates no distinct advantages over its electric counterparts in operation near the human body.Nevertheless, in contrast to other antenna structures widely used for epidermal electronics (e.g., dipole and slot antennas), a loop antenna exhibits a smaller optimal size and a more stable input impedance [6].In addition to that, the selection of the loop topology took into account the convenience of implementation on the target part of the human body.

B. Simulation Setup
The design and optimization of the antenna were performed using full-wave electromagnetic simulators ANSYS HFSS and CST Microwave Studio 2020.Given the target implementation, the simulations were done with the antenna placed directly on a forearm phantom.During the design and optimization phases, the elliptical cylinder phantom model depicted in Fig. 2(a) was used in simulations.The model is a simplified replica of the SHO-GFPC-V1 forearm phantom from SPEAG [37] shown in Fig. 2(b), which was used for the antenna characterizations described in Section V. SHO-GFPC-V1 is a commercial anthropomorphic forearm phantom developed by SPEAG in conjunction with cellular telecommunications and Internet association (CTIA) for the over-the-air (OTA) performance test plan of forearm-mounted transmitters/receivers [37], [38].The phantom is mainly composed of silicone-carbon powder compound based on [39] that surrounds and fills a tube-supporting structure made of prepreg material.The silicone-carbon composition was optimized so that its RF dielectric properties matched those of the human forearm as identified by coaxial probe-based measurements done in [40] and [41].Following the construction of SHO-GFPC-V1, the inner layer of the phantom model in Fig. 2(a) represents the silicone-carbon filled-tube and was modeled with r of 30 and conductivity (σ ) of 2.5 S/m.The outer layer, which was modeled with r of 30 and σ of 0.7 S/, represents the silicone-carbon compound surrounding the tube.These electrical properties are based on the data provided by the SPEAG representatives for our target operating frequency of 868 MHz.In simulations, the nonconductive parts of the Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.antenna were modeled with the measured properties of PDMS described in Section II.On the other hand, the conductive parts were modeled as a section having the thickness of the associated textile and an updated sheet resistance to account for the percolation of PDMS into the pores of the chosen textile, i.e., 0.7 / and 0.23 / for the transparent and nontransparent textiles, respectively [30], [31].

C. Epidermal Antenna Parametric Study
With an aim to identify the optimal configuration of the proposed transparent antenna, the antenna responses as a function of dimensions, particularly the loop aperture (4s), trace width (w), expanded side width (g), and bottom PDMS layer thickness (h b ), were investigated.The study was performed numerically with the antenna mounted on the phantom model specified previously.The antenna responses were recorded in terms of the antenna input impedance, total gain, and radiation efficiency.Though only the performance of the transparent version is provided in this section, the same trend was observed in the nontransparent counterpart.
Fig. 3(a) and (b) show, respectively, the antenna input impedance and total gain/radiation efficiency with varying aperture (4s).For a comparison, Fig. 3(c) depicts the radiation performance as a function of 4s when the phantom is nonexistent.As expected, the antenna input impedance fluctuates [see Fig. 3(a)] as the antenna physical dimensions and, consequently, the antenna coupling to the phantom change.Comparing Fig. 3(b) and (c), a low-radiation level is immediately noticed when the phantom is present.This is understandable for the case of a nonshielded antenna placed in a very close proximity to a human-body-emulating phantom.With the impedance matching aspect neglected, such a low radiation is primarily attributed to the significant power losses in the phantom.It is, however, more interesting to note that in the case of with phantom, the antenna radiation performance exhibits a bell-shaped trend [see Fig. 3(b)].As 4s expands, initially the gain and radiation efficiency increase rather linearly, then reach a peak, before declining somewhat.Similar  occurrence was also reported in [6].Such a behavior is in contrast to the case when the loop antenna operates in free space.As can be seen in [Fig.3(c)], the free-space antenna gain and radiation efficiency typically have a positive correlation with the antenna aperture.
To further comprehend Fig. 3(b), simulations were carried out to analyze the antenna overall power distribution at 868 MHz, and the results are presented in Fig. 4(a).As can be seen, the antenna radiated power rises as 4s increases upto 120 mm.On the other hand, the VeilShield loss shows a continuous drop as 4s grows.From these two plots, it can be deduced that the antenna radiation resistance does increase as a function of loop aperture, which is the reason for the early increase in the antenna gain and radiation efficiency depicted in Fig. 3(b) till 4s = 120 mm.However, as the antenna aperture grows larger, the phantom loss, which appears to be very dominant, increases as the probability of reactive and radiative fields coupling to the phantom also increases [indicated by the power loss density distribution in Fig. 4(b)].Beyond 4s = 120 mm, the phantom loss appears to be more prominent, causing the decline in the radiated power plot, and thus the gain and radiation efficiency plots [see Fig. 3(b)].
Figs. 5 and 6 show the antenna performance with varying trace width (w) and expanded side width (g), respectively.The results indicate that unlike 4s, the variation of w and g has an insignificant effect on the antenna radiation performance.Wider w and g yield similar radiation performance as that of the narrow case [see Figs.Finally, the antenna performance as a function of bottom PDMS layer thickness (h b ), which essentially governs the loop-phantom distance, is shown in Fig. 7.As predicted, the distance between the antenna and phantom has a substantial impact on the antenna input impedance and radiation performance even with a small change.As the antenna is placed further away from the phantom (the thickness of the PDMS bottom encapsulation increases), the reactive fields coupling to the tissue decreases, which impacts the antenna overall impedance calculated at the feed point.Less coupling to the tissue also results in less power loss in the phantom and a larger optimal size of the antenna, which together lead to a significant rise in the antenna gain and efficiency.
It is important to note that, with the initial liquid state of PDMS, nearly any thicknesses can be achieved.The thickness of the PDMS encapsulation layer is then determined by a compromise between the expected electrical and mechanical properties of the resulting epidermal antenna.
The above-stated findings can then be summed up to constitute practical design guidelines for the proposed antenna.First, the aperture of the loop and the thickness of bottom PDMS layer can be employed as a key antenna radiation performance control.Second, the optimum loop aperture is the balance point between the opposing mechanisms of radiation and loss.Third, the loop width and the expanded side width can be used to tune the antenna input impedance to achieve matching.With the above in mind, the antenna was optimized, and its final dimensions are given in the caption of Fig. 1.

IV. EPIDERMAL ANTENNA FABRICATION
As illustrated in Fig. 8, the antenna prototypes were constructed through a layer-by-layer polymer-textile assembly process, starting from the bottom to the top encapsulation layer.Two customized ring molds each having a thickness of 0.5 mm were used to achieve the thickness of the PDMS encapsulation layers.It is important to note that with the initial liquid nature of PDMS, there is a high flexibility to further reduce the thickness of the encapsulation layer for a higher antenna mechanical compatibility with the skin.Upon degassing in a vacuum desiccator for 20 min to remove the trapped air bubbles, the PDMS solution was poured in the mold and cured in the oven at 80 • C for 30 min to make the encapsulation layer.The conductive textile was patterned manually using a razor blade following the design in Fig. 1.The attachment of the textile on the cured bottom encapsulation layer was done using the uncured PDMS followed by curing in the oven at 80 • C for 5 min.For the transparent textile, prior to the textile attachment, a sticky tape was adhered to the feeding point to resist the PDMS to PDMS bonding at that particular location.This was required to facilitate the attachment of a 50 U.FL connector detailed below, which is needed for antenna measurement purposes.
Upon curing of the top encapsulation layer, the fabricated prototype was peeled from the mold and the excess PDMS was trimmed from the edges.The PDMS was also cut to create the holes in the middle of the loop and on the antenna expanded side.The top PDMS layer was scratched Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.at the feeding point location slightly to expose the textile for the U.FL connector attachment.In the transparent prototype case, this would not have been possible without the sticky tape due to the strong integration achieved between PDMS and the transparent conductive textile.The U.FL connector was attached to the exposed textile (upon detaching the tape in the transparent case) with conductive silver epoxy (Chemtronics CircuitWorks), followed by curing in the oven at 80 • C for 10 min.PDMS encapsulation (10 min oven curing time at 80 • C) was applied at the connector-textile interface to strengthen the interconnection.Fig. 9 shows the photographs of the fabricated prototypes.In particular, the transparent antenna demonstrates the wearability aspects of transparency (approximately 70% measured optical transmittance) [Fig.9 V. ANTENNA PERFORMANCE Holistic investigations were carried out to evaluate the performance of the developed antenna prototypes, comparing the transparent and nontransparent counterparts.They included the testings of the antenna input reflection coefficient (|S 11 |) and far-field characteristics, as well as indoor wireless communication tests.In addition, a numerical analysis was conducted on the impact of the antenna on the underlying human body tissue in terms of RF exposure.

A. |S 11 | Tests
Fig. 10 shows the setup for the |S 11 | measurements conducted using an MS2038C vector network analyzer (VNA) from Anritsu.Two measurement scenarios were investigated: 1) before [Fig.10(a)] and 2) after [Fig.10(b)] connecting the antenna to a customized LoRa module, which was developed for the purpose of wireless communication tests.In both scenarios, the antenna was mounted on the SHO-GFPC-V1 forearm phantom.The measurement setup was mimicked in the simulation as can be seen on the right side of Fig. 10.In the second scenario, the LoRa module was placed on the opposite side of the antenna to minimize the impact of the module on the antenna impedance matching.This is considering that the epidermal antenna was designed and optimized as a stand-alone component as opposed to the LoRa module-integrated antenna.To note that rather than proposing a system, the emphasis of this work is on the component level, specifically the development of antennas.In addition, the wireless module that was developed solely for testing purposes is still physically incompatible for a seamless integration with the antenna.Tape was used to attach the antenna and the module to the phantom.A U.FL-to-SMA cable was used to connect the antenna to the VNA during the measurements.Nonconductive glue was applied at the feed location to strengthen the interconnection between the U.FL connector and the cable.For the second scenario, the outer part of the U.FL-to-SMA cable was connected to the ground of the wireless module board to mimic the actual implementation during the wireless tests, i.e., the antenna and the board's ground is connected through the feed point.
In both scenarios of |S 11 | tests, we performed a quick check by touching the coaxial cable connecting the antenna (and the wireless module in the second scenario) to the VNA.We observed stable |S 11 | results, which led us to believe that any common mode currents are negligible.We conducted additional simulations using the second scenario, i.e., antenna with the wireless module.We compared the antenna radiation performance of cases with and without the cable connecting the antenna and the board [see Fig. 10(b)], which replicates the ground connection explained before.The simulated result of the case with cable revealed a peak realized gain of no more than 0.2 dB higher than the case without cable, confirming the insignificance level of currents and the consequent effect of cable.Despite these findings, we still employed ferrite beads on the coaxial cables as a precaution against the common mode currents [33], [42].
Fig. 11 shows the |S 11 | performance of both the transparent and nontransparent prototypes for the two measurement scenarios mentioned above.Both antennas exhibit a wide 10-dB return loss bandwidth of more than 1 GHz covering the target frequency 868 MHz, which is in a good agreement with the simulated results.This performance is maintained even upon the integration with the LoRa module.The changes in antenna input impedance [see Fig. 11(b)] are anticipated with the antenna and ground of the module board connected, allowing  slight currents to flow to the board's ground, as observed in simulation.The discrepancies between the measured and simulated results might be attributed to slight differences in the geometry of the phantom model used in the simulation versus the actual phantom, as well as manufacturing defects.The latter is more probable in the transparent antenna case due to the nature of VeilShield, which is thinner, stretchier, and more prone to fraying, particularly during manual cutting.Errors in fabrication could possibly be mitigated by employing a cutting machine that can pattern the textile with greater precision and control.

B. Far-Field Tests
The antenna radiation characterizations were performed using the AMS-8050 antenna measurement system from ETS-LINDGREN for both scenarios indicated in Fig. 10.For the same reason described previously, multiple ferrite beads were added to the feed cable.The radiation patterns of both antennas are depicted in Fig. 12 and their radiation performance is summarized in Table I, showing a good agreement between the simulated and measured results.The differences in the results might be due to the same variables identified in the |S 11 | tests.Changes in the antenna pattern, as well as a slight increase in the antenna realized gain and radiation efficiency, result directly from the minor currents flowing to the board, as described in the previous section.Typically, the radiation efficiency of antennas constructed using PDMS-conductive textiles varies by more than 10% between these two textiles   (i.e., VeilShield and Ni-Cu-ripstop) [43].However, this does not appear to be the case when the antenna is in direct contact with human tissue.Due to the dominating power losses in the human body tissue beneath, the quality of the antenna conductor appears to have a minor impact on the radiation performance.

C. Indoor Wireless Communication Tests
To demonstrate the viability of the developed epidermal antenna for IoT applications, a simple LoRaWAN testbed Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.network (Fig. 13) was deployed on the second floor of Tyndall National Institute's Block B Building.The testbed network consists of one LoRaWAN sensor node and three LoRaWAN gateways (i.e., G 1 , G 2 , G 3 ).TTN-GW-868 [Fig.14(a)] [44], a commercial LoRaWAN gateway with an 868-MHz whip antenna was employed as the gateway.The sensor node, on the other hand, comprises the developed epidermal antenna connected to the wireless module using a U.FL-to-U.FL cable.Both the antenna and the module were mounted on the arm of a 34 year old male test subject weighing 70 kg and measuring 169 cm in height [see Fig. 14(b)].The antenna was attached on the top side of the forearm, whereas the module was attached on the other side of the forearm.This configuration is in accordance with the simulations and measurements setups using the SPEAG arm phantom shown in Fig. 10.Inside the wireless module, there is also a temperature sensor.Biocompatible skin adhesives (LOCTITE DURO-TAK 222 from Henkel) were used for the attachment of the antenna and the module to the skin.The adhesives are very thin (i.e., 45 μm) and thus have an insignificant impact on the antenna performance as verified through simulations.
The LoRa module in the node was programmed to broadcast 10 packets containing the measured temperature and received signal strength indicator (RSSI) data to the gateways.As the temperature sensor is located on the circuit board, which is not in direct contact with the skin, we refer to the measured data as the ambient temperature near the test subject's arm.These packets were transmitted with a 10-s interval between each packet and an adaptive spreading factor (SF) setting between 7 to 12.The gateway was connected to the things network's (TTN) LoRaWAN network server, which was responsible for network and data management, such as performing security checks and adaptive data rate changes, filtering redundant received packets, and forwarding data packets to the user data portal for visualization and storage.
The test subject was subsequently instructed to move to five different locations labeled as A to E in Fig. 13.At each location, RSSI values from the three gateways were recorded for two distinct scenarios: 1) test subject standing with both arms at the side of the body [Fig.14(c)] and 2) test subject standing with both arms raised [Fig.14(d)].Also calculated was the average of the RSSI values acquired from the same gateway.For comparison, the nontransparent antenna prototype was also subjected to this procedure.Out of the three average RSSI values obtained at each location, the maximum RSSI value, which was typically from the gateway nearest to the node and had PDR = 1, was noted.The results are presented in Fig. 15(a) and (b) for scenarios (1) and ( 2), respectively.The average ambient temperature measurements for each location are also shown.The results displayed in blue text (top line) pertain to the transparent prototype, whereas those displayed in green text (bottom line) correspond to the nontransparent prototype.
RSSI metric is a relative measurement of RF power level (in dBm) that in this case displays how effectively the gateway can receive the signal from the wirelessly connected sensor node.Notably, a LoRaWAN network can offer a maximum receiver sensitivity of −137 dBm at SF12 [45].From the results, it is validated that the developed transparent epidermal antenna is capable of enabling the wireless functionality of the developed module.Despite the lossy operating environment, the antenna successfully establishes a communication link with at least one gateway in each of the predefined locations for both test cases.Following the trend shown in the far-field tests results, generally there is no substantial difference in the wireless link quality provided by the transparent and nontransparent prototypes.

D. SAR Numerical Analysis
The RF exposure of the antenna on the target human body part was evaluated numerically through calculated SAR distribution in CST Microwave Studio 2020.For this purpose, the antenna was mounted on the human forearm phantom shown in Fig. 2. The SAR distribution at 868 MHz was computed for an input power of 30 mW (maximum RF power supplied from the wireless module) and averaged over 10 g of tissue.
As shown in Fig. 16, in terms of RF exposure, both the transparent and nontransparent epidermal antennas conform with the IEEE C95.1-2019 safety guideline for body extremities (i.e., SAR < 4 W/kg) [46] for the specified maximum input power, frequency, and position on the body.The quality of the antenna conductor seems to have little impact on the RF exposure of an antenna in direct contact with human body tissue.The transparent antenna exhibits only a marginally lower SAR peak value (i.e., 0.688 W/kg) than its nontransparent counterpart (i.e., 0.703 W/kg), which may indicate slightly less power deposition on the body due to the lower textile conductivity.Nevertheless, this tiny less power loss on the body is compensated by a higher metal loss, resulting in the insignificantly lower radiated power observed in the results of the far-field tests.

VI. CONCLUSION
The common engineering strategy for achieving high comfort-of-wear in antennas for epidermal electronics is to impart skin conformability, which includes flexibility, stretchability, air/water permeability, biocompatibility, and ultrathinness.To complete the above qualities toward a truly unobtrusive epidermal antenna, we have successfully demonstrated the concept of optical transparency achieved through a simple and cost-effective layer-by-layer PDMS-textile assembly technique.Proof-of-concept arm-worn modified rectangular loop antennas (transparent and nontransparent prototypes) demonstrating good agreement between measurements and simulations were used to validate the idea.The findings reveal that due to the major power loss to the human body, the transparent epidermal antenna performs similarly to the nontransparent counterpart, despite the substantial difference in the quality of the conductive material.This suggests that one can increase the comfort-of-wear level of epidermal electronics by employing a transparent epidermal antenna without worrying about a major decline in wireless performance.A unique approach needs to be taken though when designing an epidermal antenna due to its strong interaction with the lossy human body.In the case of a modified loop antenna as presented in this study, the radiation performance can be controlled by the thickness of the underlying membrane, with conformability as a tradeoff, and the loop aperture.For the latter, one should aim for the optimal aperture size, which is the point of equilibrium between the opposing mechanisms of radiation and loss.This is in contrary to the free space case, in which a larger aperture always results in better radiation.Once the aforementioned parameters have been established, the width of the loop strip and its expanded side can be tuned to tailor the input impedance while preserving the radiation performance.

Fig. 3 .
Fig. 3. Antenna performance at 868 MHz as a function of loop aperture (4s): (a) input impedance and (b) total gain/radiation efficiency when mounted on the phantom, and (c) total gain/radiation efficiency when the phantom was removed.When 4s was varied, other parameters were set as: g = 30 mm, w = 1 mm, f = 1 mm, e = h t = h b = 0.5 mm and the holes were removed for simplicity.The inset shows the topology of the antenna as 4s varies.

Fig. 4 .
Fig. 4. (a) Antenna power analysis at 868 MHz as a function of loop aperture (4s).(b) Power loss density of the phantom at 868 MHz as 4s increases.
5(b) and 6(b)].On the other hand, as can be seen in Figs.5(a) and 6(a), the antenna input resistance and reactance can be tuned by varying w and g.

Fig. 5 .Fig. 6 .Fig. 7 .
Fig. 5. Performance of the antenna on the phantom at 868 MHz as a function of trace width (w): (a) input impedance and (b) total gain/radiation efficiency.When w was varied, other parameters were set as: s = 45 mm, g = 30 mm, f = 1 mm, e = h t = h b = 0.5 mm and the holes were removed for simplicity.The inset shows the topology of the antenna as w varies.

Fig. 8 .
Fig. 8. Schematic illustration of the antenna fabrication process through layer-by-layer PDMS-textile assembly.

Fig. 9 .
Fig. 9. (a) Transparent epidermal antenna prototype compared to its nontransparent counterpart.(b) Transparent epidermal antenna when laid on an index finger, bent and rolled by hand, and mounted on a forearm.A printed Tyndall logo is added underneath the antenna to illustrate its see-through feature.

Fig. 10 .
Fig. 10.Antenna setups for |S 11 | and far-field measurements: (a) without and (b) with the wireless module connected.

Fig. 11 .
Fig. 11.Measured and simulated |S 11 | of the developed epidermal antennas, both the transparent and nontransparent counterpart, comparing the scenarios (a) without and (b) with the wireless module connected.

Fig. 12 .
Fig. 12. Measured and simulated radiation patterns of the developed epidermal antennas, both the transparent and nontransparent counterpart, comparing the scenarios (a) without and (b) with the wireless module connected.

Fig. 14 .
Fig. 14.(a) LoRa gateway and (b) node with transparent and nontransparent epidermal antennas.Two wireless testing scenarios: (c) test subject standing with both arms at the side of the body (scenario 1), (d) test subject standing with both arms raised (scenario 2).

Fig. 15 .
Fig. 15.Maximum average RSSI values recorded at each node location for: (a) scenario 1 and (b) scenario 2. The results with the transparent antenna are shown in blue text, whereas those with the nontransparent antenna are shown in green text.

TABLE I PEAK
REALIZED GAINS AND RADIATION EFFICIENCIES OF THE DEVELOPED EPIDERMAL ANTENNAS AT 868 MHZ WITHOUT AND WITH THE WIRELESS MODULE CONNECTED