SkinAid: A Wirelessly Powered Smart Dressing Solution for Continuous Wound-Tracking Using Textile-Based Frequency Modulation

In this article, SkinAid, a battery-free, low-cost, robust, and user-friendly smart bandage for electrochemical monitoring and sensing of chronic wounds is proposed. The working principle of the bandage is based on direct frequency modulation of a tri-electrode electrochemical sensing of wound data. The electronics and biotelemetry links were realized using low-cost manufacturing process of textile embroidery onto fabric substrate. The transmitter was represented by a bedsheet with novel corrugated crossed-dipole made of Elektrisola-7 embroidered onto gauze fabric. An input RF signal of 1 W was transmitted at 462 MHz from the bedsheet to the all-textile bandage featuring a rectifying circuit, a voltage-controlled oscillator (VCO), an electrochemical sensor, and a 915-MHz dipole for re-transmission of the modulated wound data. We demonstrate that for wound fluid emulated by various uric acid concentrations from 0.2 mM to 1.2 mM, corresponding modulated frequency varies from 1090 MHz to 1145 MHz for signals captured at 25 cm away from the bandage. For pH modulation ranging from 2 to 10, the corresponding modulated frequency was between 800 MHz and 830 MHz for signals received at more than 6 feet away from the bandage. For quick and reliable assessment, two empirical models were developed for the direct frequency modulation as a function of uric acid and pH. To the best of our knowledge, this is the first time an all-textile (fabric-integrated), battery-free and wirelessly powered smart bandage have been proposed for wound monitoring. This result can be used as a first step in developing RFID-type, battery-free, and low-cost 5G/6G smart bandages using millimeterwave and terahertz frequencies where the bedsheet can be host to a MIMO-aided beamforming.

Abstract-In this article, SkinAid, a battery-free, low-cost, robust, and user-friendly smart bandage for electrochemical monitoring and sensing of chronic wounds is proposed.The working principle of the bandage is based on direct frequency modulation of a tri-electrode electrochemical sensing of wound data.The electronics and biotelemetry links were realized using low-cost manufacturing process of textile embroidery onto fabric substrate.The transmitter was represented by a bedsheet with novel corrugated crossed-dipole made of Elektrisola-7 embroidered onto gauze fabric.An input RF signal of 1 W was transmitted at 462 MHz from the bedsheet to the all-textile bandage featuring a rectifying circuit, a voltage-controlled oscillator (VCO), an electrochemical sensor, and a 915-MHz dipole for re-transmission of the modulated wound data.We demonstrate that for wound fluid emulated by various uric acid concentrations from 0.2 mM to 1.2 mM, corresponding modulated frequency varies from 1090 MHz to 1145 MHz for signals captured at 25 cm away from the bandage.For pH modulation ranging from 2 to 10, the corresponding modulated frequency was between 800 MHz and 830 MHz for signals received at more than 6 feet away from the bandage.For quick and reliable assessment, two empirical models were developed for the direct frequency modulation as a function of uric acid and pH.To the best of our knowledge, this is the first time an all-textile (fabric-integrated), battery-free and wirelessly powered smart bandage have been proposed for wound monitoring.This result can be used as a first step in developing RFID-type, battery-free, and low-cost 5G/6G smart bandages using I. INTRODUCTION S MART, connected, and personalized health has been of a great interest since the birth of Internet of Things (IoT) and internet of health things (IoHT).One of the goals has been to track health conditions wirelessly and remotely.Wireless tracking of wound condition is associated to chronic wound management which has costed more than $20 Billion in treatment per year [1] in medical costs.A wireless monitoring solution is of particular interest for diabetic patients, who are more effected by long term wounds and chronic wounds.
State-of-the-art solutions for wireless wound monitoring have shown significant advancements in wound biochemistry analysis and associated electronics for power and wireless data management.Effective collection and analysis of wound fluid is done by optimizing the surface chemistry of the sensors, designed to look at the biomarkers.The intelligent bandages use biomarkers (or bio-modulators) including pH, uric acid concentration, temperature, oxygenation, moisture, cortisol, etc [2], [3], [4], [5], [6], [7].The most predominant biomarkers are the pH level (influencing the biochemical reactions involved in the healing process) and the uric acid concentration (an endogenous indicator that can be used to indirectly monitor the pH) of the wound fluid.However, the pH monitoring is preferred over the other bio-markers, because it is simple, fast, and can be easily done at an outpatient clinic.It is also used for skin grafting eligibility [8].The pH has been found to be a potent influential factor in the healing progress of the wound due to a modulation observed between its values of 4 and 8.9 [8].For that, the healing of the wound occurs when it shifts from neutral (pH ≈ 7) to acidic (pH ≤ 7) [9] and the severity of the damaged tissue escalates at pHs beyond 7.The uric acid has a threshold of 0.4 mM below which, the wound is in recovery phase and above it, the wound is getting severe [10].According to the findings from [11], [12], it can be used to indirectly evaluate wound pH.Finally, pH can be monitored as a function of glucose [13].
A critical part in wound monitoring system encompasses the electronics involved in the powering and extraction of data.Prior state-of-the-art solutions have shown sensor and electronic interface integration using integrated circuits by taking advantage of micro-controllers.These may or may not include battery-based operation.For example solutions that use battery are listed as [2], [3], [14], [15], [16], [17], [18], [19].More recently near field communication (NFC) has been demonstrated as an effective means for powering the electronics [7], [20], [21], [22], and avoid batteries [23].
The bulky battery-and micro-controller-based circuits shown in the past make them uncomfortable to wear and the fear of battery depletion can also be a drawback.To eliminate the obtrusive components of the systems, the use of bio-compatible flexible substrate and minimalist circuits is optimum.The best candidate among the bio-compatible substrates to be used in such systems for contact with the skin is fabric, which comes with challenges that are already addressed in [24].
In this article, SkinAid is wirelessly powered and it uses frequency modulation and back reflection to send the modulated wound data to a remote receiver (see Fig. 1).We propose sensor-electronics integration to provide battery-free and wireless operation, and also offer a level of simplicity that is sufficient to allow integration with fabric surfaces.In order to achieve this, we propose voltage-controlled-oscillator-based frequency modulation of the sensor data [25], [26], and near-zone RF power transfer enabled by single-diode RF to DC rectification [27].This solution mimics an RFID (also referred to as incident-reflect or back-scatter as shown in Fig. 1) operation while also employs active circuit for frequency modulation of the sensor data.Textile RFIDs have been proposed before in [19], [28], [29].However, they used "passive-type" RFID, which would not be a great choice for wound monitoring as the latter requires a very selective module, which is not the case in passive RFIDs.The proposed frequency modulation (as opposed to amplitude modulation RFID) solution is independent of the sensor biochemistry and different types of tri-electrode amperometric sensors (for pH, Uric Acid, among others) are compatible with the proposed electronics.This allows independent sensing of biomarkers with selectivity and specificity, and therefore is adaptable to multi-modal monitoring.This proves to be a unique advantage over traditional RFIDs which rely on impedance modulation of antennas to measure changes in the wound fluid, i.e. traditional RFID is not selective in sensing the specific chemical in the fluid.
The bandage unit (shown in Fig. 3) is equipped with receiving textile antenna, a textile RF-DC rectifier, an electrochemical sensor to be dipped into the wound fluid to detect the bio-indicators, a textile voltage-controlled oscillator (VCO) to convert the sensor data into a unique frequency signal, and a data-telemetry antenna for wireless transmission to an external interrogator.Our system is demonstrated to measure changing levels of uric acid and pH in a solution.Here we report the completion of the work started in [25] and [26].In [25], we conducted the analysis of the antenna in providing modulated wound data.In [26], a bandage was developed using a resistive load which emulated the wound fluid.The current work provides a comprehensive work on the testbed evolution that allows us to use uric acid with wired transfer and pH level with the wireless transfer.These experiments demonstrate that both possible modalities of data telemetry for any biomarker can be realized.The manuscript is organized as follows: Section II presents the smart system with all the modalities and capabilities, Section III lays out all the components, Section IV shows the integration of the bandage and the corresponding measurements, and Section V concludes the work.

II. THE PROPOSED SMART SYSTEM: "SKINAID"
Here we summarize the operation, features and design-motivations for the constituent components within the proposed smart bandage system.There are three main components of the smart bandage system, namely, (1) interrogator unit, (2) data collection unit or smart bandage, and (3) data retrieval and post processing.The block diagram of the system is shown in Fig. 2. A Photograph of the developed smart-bandage unit (i.e. component two) along with an associated schematic/operational diagram is shown in Fig. 3.

A. The Interrogating Unit
The interrogating unit consists of an external RF-power supply antenna (not shown in Fig. 3) optimized for operation between 347 and 590 MHz where the center frequency was approximately 500 MHz (see Fig. 4).The choice of frequency for the RF power supply operation is based on the available sizes for Tx and Rx antennas in the smart-bandage unit.The Associated RF signal generator, amplifier chain and antenna are part of the this unit, as shown in Fig. 2. In order to effectively transfer  RF-power in the near-zone, we use an X-shaped (realized by a 45 0 -rotation of the traditional cross-shaped) resonant dipole, which represents a modified form of the anchor shaped antenna that was proposed in [30].The selected antenna shape exhibits polarization diversity in the near field due to four orthogonal arms.In addition, this shape is also responsible for misalignment resilience in lateral or rotational directions due to a larger footprint and symmetry in E and H planes.This unit can be integrated into kiosks, patient beds, or handheld interrogators as shown in the Fig. 3.

B. Smart-Bandage Unit
This unit consists of a textile-based X-shaped receiving antenna to receive the RF power from interrogator unit, allow for RF-to-DC conversion using an textile integrated rectifier unit that converts the received RF signal into DC and thus provide power to the electrochemical sensor and the frequency modulator (VCO).The proposed system uses a minimalist single diode rectifier design for efficient and small-component operation of the rectifier with an RF-to-DC conversion performance better or similar to those published in [31], [32], [33], [34], [35], [36].The schematic operational diagram and the photograph of the unit is shown in Fig. 3.A tri-electrode amperometric sensor is activated using the available DC power, which is specifically activated for measurement of pH or uric acid using on enzymatic activation associated to the sensor surface chemistry.The electrochemical sensor provides varying DC current level as an output which varies as per the detected concentration level of uric acid or pH.The uric acid is detected by an enzymatic sensor that consumes about 24 μW and the pH level of the wound fluid is detected by a zero-power non-enzymatic sensor.The VCO, consuming 6 mW of power, converts the DC signal to a unique frequency by using the voltage dependent frequency operation of the VCO, thus providing a frequency signal associated to the biomarker.An empirical relation between the amperometric current output from the sensor unit and the uric acid concentration (C UA ) is found to be Corresponding voltage at the output (which is also the input of the VCO block) is given by (2) The VCO uses the DC current level (converted into a voltage level through a resister) as input and provides a frequency modulated (FM) signal in the range of 800 MHz-1700 MHz.Being an RF signal (not a DC signal) this FM output can be easily transmitted wirelessly, using antennas of reasonable size to an external processing unit (remote receiver).A dipole antenna, as shown in Fig. 3 right, is used for transmitting the output from the smart-bandage.

C. The Telemetry Link and Wound Assessment
The output of the dipole antenna wirelessly connects the wound data to the external kiosk, completing the wireless telemetry link.For long distance communication, the modulated wound data can be transferred to a local hub that sends the info to a hospital, a doctor's office or a local medical center.The concentration level of uric acid and pH level are interpreted based on the peak frequency of the signal spectrum in the 800 to 1700 MHz range.(1) For the case of uric acid concentration estimation, the acid concentration is compared to the threshold mark of 0.4 mM to asses the wound severity [10].For uric acid levels with concentration above the 0.4-mM threshold, the wound is said to be severe.For lower concentrations, the would is said to be healing.These concentration levels correspond to a unique frequency peak in the spectrum of the signal transmitted to the kiosk.( 2) Using the pH level, based on the acidity or basicity of the wound fluid, the severity of wound healing process can be monitored.Indeed, for pH level ranging from 5 to 5.5, the wound is said to be epithelial or healed and for pH level beyond 7, it is said to be severe or non-healing.Like the UA-assessment, each pH level corresponds to a unique frequency peaks and then interpretation of the pH level is made.For quick and reliable assessment, an empirical equation is developed for both electrochemical sensing schemes, as discussed in Section IV.

A. Near Field Wireless Power Transfer Using Fabric-Integrated Corrugated Crossed-Dipole Antenna
Wireless power to the smart bandage is provided through a near field RF link using a new misalignment resilient antenna configuration.Here, we propose the use of X-shaped dipole antenna, which is a variant of recently proposed misalignment resilient anchor-shaped antenna [37] that demonstrated an improved power transfer efficiency for practical scenarios as further demonstrated in [38].In this type of antenna, the improvement in misalignment resiliency occurs by using both, the inductive and the capacitive electromagnetic couplings between the receiver and transmitter antennas, which allows power transfer in different extents of misalignment [37].However, since the original anchor-shaped design lacks symmetry to establish similarity between the vertical and horizontal (X and Y axes, Z being the normal to plane of article) misalignments, as well as the rotational (elevation and azimuthal) misalignments, a new X-shaped dipole is used in this work.This misalignment resilience is important in real-life settings because the antenna is aimed at being used in a smart bandage to be worn by a patient susceptible to all the types of misalignments mentioned above.Before system integration, the X-shaped antenna is designed and its return loss and power transfer efficiency performance are measured under different misalignment scenarios.These results are discussed in the up-coming sections.
1) Design Optimization of X-Shaped Antenna: The antenna was designed and simulated using HFSS (version R2) and the design parameters are listed in Table II.PEC boundary conditions were assigned to the conductive traces of Elektrisola-7 (conductive) threads embroidered onto gauze fabric.We expected some reasonable losses that were reflected in the measurement results (see Fig. 6) as was the case in our previously published work [24].An airbox (open region) was used to emulate free space radiation, and a lumped port of 50 Ohms was assigned to each antenna's excitation port.The model consisted of 4 arms with a series of pixelated steps referred to as "corrugations" starting from 5 to 100 each.The arms are flared out making an angle ≈ 90 • .This is to establish symmetry across the two axes in the aperture of the antenna.The frequency of the antenna is dictated by the size of the each arm, i.e. by the number of corrugations.Fig. 5  and w is the width of the corrugation.w depends on the dielectric of the substrate.Frequency scaling of the proposed design can be established through the equation below In this work, the value of N and w were chosen to be 15 and 5.51 mm, respectively.This yields an operating frequency of around 500 MHz.Next, we evaluated the RF performance of the antenna by placing a transmitter and a receiver operating at the same frequency and misaligned them laterally, diagonally, and angularly.
2) Effects of Misalignment on the Power Transmission: In Fig. 6 the PTE performance under misalignment is shown.Several cases of misalignment are considered with different degrees of freedoms, such as rotational and lateral.Fractional power transfer efficiency (similar to shown in [30]) is reported by measurement of |S 21 | as shown in the figure.For lateral misalignment performance along the X-axis, a |S 21 | of 0.6 to 0.8 is observed when the misalignment is in the range of 1 cm to 10 cm.This is due to the fact that the EM fields spanned and coupled away from the aperture.For lateral misalignment performance along the Y-axis, a |S 21 | of 0.6 to 0.7 is observed from 1 cm to 10 cm.These values are similar to those of the misalignment along the X-axis.This is due to the fact that the topology creates a symmetry in the design.For the diagonal misalignment, the Rx and Tx are misaligned from distances equal to the euclidean distance between X and Y with same values.An average |S 21 | of 0.6 is obtained for distances ranging from 1.4 cm to 14.14 cm.The stabilization in the result is another confirmation of the symmetry created in the design model.For angular misalignments elevational and azimuthal, an average |S 21 | of 0.5 is obtained and the similarity in the results for both misalignment schemes further justifies the effects of the symmetry.
Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

TABLE I SUMMARY OF MISALIGNMENT TYPES, ALONG WITH THEIR TRANSFER DISTANCES, ANGLES, AND OPERATING FREQUENCIES
Fig. 6.Misalignment tests for the proposed antenna at broadside and when subject to misalignments: (a) power transfer performance at broadside and (b)-(f) performance when the transmitter and receiver are misalignment with respect to the X-axis, the Y-axis, diagonally (measurement is not reported due to absence of testing setup), with respect to the elevational and azimuthal angles, respectively.

B. Effects of Bending on the Power Transmission
To test the flexibility of the antenna, we studied the power transfer performance when it is subject to bending (mechanical deformation) and misaligned along the X and Y axes.The radius of the curvature was chosen to be 36 mm, which is the average radius of human's wrist.The simulation results in Fig. 7 showed an average |S 21 | of 0.7 and 0.65 where the misalignment is achieved along the Y and the X axes, respectively.On average, this performance is similar to the ones mentioned above.This is again a justification of the effects of the symmetry within the antenna.The performance of the new presented antenna is summarized in Table I.In summary, the X-shaped symmetrical antenna allows a stable power transmission performance when the Tx and Rx are misaligned.In all measured data, we find agreement with full-wave simulations, confirming our design approach and measurement data.For all the misalignment cases, the reflection coefficient values were −6 dB or better.

C. Fabrication
The antenna prototypes were fabricated using fabric substrates where the metalized regions were realized using embroidery of conductive metal thread similar to prior works [24], [38].

D. Textile-Integrated Rectifying Circuit
Fig. 8 shows chosen circuit configuration, fabricated prototype and associated test set-up for the RF to DC conversion circuit utilized for the proposed smart bandage.Due to challenges of flexible integration, the circuit should be of minimum number of lumped components, and yet provide the required high power conversion efficiency.An ideal circuit for such applications was earlier demonstrated by the authors [38], where single diode based rectifier using microstrip-line based inductive/capacitive elements has shown more than 50 % conversion efficiency.Using this topology, a new circuit is optimized, fabricated and tested for an operational frequency of 500 MHz to match with RF power transfer frequency.The circuit is shown in Fig. 8, and is described as following.The first λ/8-microstrip line is used as a bandpass filter.The second λ/8-microstrip, which is shorted, is used after the diode stage to introduce an inductive effect equal in magnitude to the capacitive effect provided by capacitive packaging of the diode.The two λ/8-microstrip lines on the either side of the diode provide a quarter wave resonance due to shorted end, such that the maxima lies at the position of the diode, thus allowing full wave rectification at the resonant frequency.This allows greater than 50% efficiency for the rectifier circuit.A DC block capacitor C 1 = 10 pF is added, a capacitor C 2 =3.9 pF was placed in parallel with the HSMS2820-diode for proving a matching with 50-Ω RF-input.The 33-μH inductor along with the third λ/8-stub were used to remove the ripples from the output waveform.
Rectifier model was based on full-wave simulation and non-linear model for the lumped components by using ADS simulation tool, and the prototype was fabricated using embroidery of Elektrisola-7 electrical metal fiber onto gauze fabric (as shown in Fig. 8(c)).Processes developed in prior works [24], [38] were used for the realization of the fabric integrated rectifier circuit.The RF-to-DC conversion efficiency was simulated and tested at 500 MHz.It was found that at 26 dBm input power, the rectifier shows peak efficiency of 76% for RF to DC conversion, and the corresponding output DC-voltage is 10 V.This efficiency value is better than or similar to recently published rectifying circuit designs [38].The optimization of the circuit was done to match the required power and voltage requirements for the VCO and electrochemical sensor.The performance of the rectifier remained the same when was subject to bending at a radius of 85 mm.This result was consistent with that of the work proposed in [39].Next, we discuss the sensing by looking at the electrochemical sensors as presented next.

E. Electrochemical Sensor Design and Fabrication 1) Uric Acid Sensor:
r Design and fabrication: The uric acid flexible sensors were fabricated in-house on wound dressings using a multilayer screen-printing approach.Conductive Ag/AgCl and carbon inks were screen-printed sequentially onto a thermoplastic polyurethane (TPU) film and cured at 80 • C for 15 min following each print.The carbon ink was used for the fabrication of working and counter electrodes, while the Ag/AgCl ink served as the reference electrode.The TPU film with the printed sensors was then heat laminated over a wound dressing followed by a protective TPU encapsulation layer to define the working area and the electrode pads of the sensor.
r Immobilization: The working electrode of the uric acid sensor was functionalized with nanomaterials and enzymes specific for uric acid detection.Multiwalled carbon nanotubes (2 μl, 5 mg/ml) were first drop-casted and dried at 70 • C.This was followed by drop casting and drying (@ 70 • C) of gold nanoparticle solution (1 μl, OD = 50).The horseradish peroxidase (2 μl, 5 mg/ml), and uricase enzyme (2 μl, 5 mg/ml), were then immobilized over the electrode by drop-casting and air drying.
r Characterization: The electrochemical current response of the fabricated sensors toward uric acid was determined in a range of 0 to 720 μM through amperometric measurements at a working potential of -0.6 V.An increase in the current was observed with an increasing concentration of uric acid (Fig. 1(a)).The sensitivity of the sensor was found to be −3.0 nA/μM (R2 = 0.99), showing the potential of the sensor to accurately measure uric acid in the clinically relevant range like previously developed sensors published in [40].Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

TABLE II SUMMARY OF THE CHARACTERISTICS AND DIMENSIONS OF THE ANTENNA AND RF MODULES/CIRCUITS
measurements using buffer solutions (pH = 4.0-12.0).The PANI deposited electrodes were dipped into the buffer solutions and OCP readings were carried out for 60 s.
Owing to the deprotonation of polyaniline in alkaline buffers, the OCP of the sensor decreased as the pH of the buffer increased (Fig. 2(a)).The sensitivity of the pH sensor was found to be 62.6 ± 3.26 mV/pH (Fig. 2(b)).Furthermore, the sensors showed good repeatability for successive measurements of pH in buffer solutions.

F. Textile-Integrated Voltage-Controlled Oscillator
To remotely access the wound-data information, a textilebased modulation circuit to modulate and transmit the corresponding data was employed.The design parameters of the modulation circuit, along with preceding components such as rectifier and antenna is shown in Table II.As the output of the electrochemical sensor is DC, there is a need to convert it into RF for feasible wireless transmission.Therefore the idea of using a voltage-controlled oscillator (VCO) is required for this project.The VCO's output frequency is tuned with the output voltage from the sensor and will convert it into a frequency signal that is transmittable through a wireless link to a remote receiver.Fig. 9 shows the characterization results of the VCO.The VCO, along with required lumped elements was integrated on the textile surface where embroidery of Elektrisola-7 unto gauze fabric was used and a COTS Crystek CVCO055BE integrated circuit was soldered with two capacitors C 1 = 1 nF and C 2 = 10 nF to smoothly modulate the output signal.The VCO was characterized with input voltages (V CT RL ) ranging from 0 V to 6 V and this selected range is based on (2), which provides the voltage output from the VCO block.The relation between the frequency output of the VCO block as a function of the control voltage is empirically given by (4) The tuning voltage emulates the sensor output, which contains the wound-health information in the form of uric acid concentration or pH level as supplied from the electrochemical sensor.
(1) For wound fluid emulated using different uric acid (UA) samples, the output of the sensor is a current signal whose values were between 5.357 μA and 6.286 μA and the empirical model is represented by the (1).A 350-kΩ resistor was placed across the leads of the sensor to convert that current into a changing voltage for the modulation.The VCO was powered by a power management circuit that features a voltage divider to share the output DC voltage with the VCO and the electrochemical UA-sensor.(2) For the wound fluid emulated by different pH solutions, a zero-power sensor was used (without the use of a power management circuit) and the sensor output was a variable voltage ranging from 0 to 1 V.The voltage was then used by the VCO for frequency conversion and modulation.The system integration and measurement are presented in the next section.

IV. SMART BANDAGE INTEGRATION AND DEMONSTRATIONS
The proposed smart bandage was developed by integration of various components that are shown in the prior Sections.This demonstration is conducted in two steps.In the first step, a preliminary bench-top set-up was established to illustrate the frequency modulation of sensor data along with the wireless telemetry function.In the second step, fully functional wireless power and data-telemetry links were demonstrated along with sensor and data-modulation functions on a textile substrate.

A. VCO Based Frequency Modulation and Wireless Link Demonstration
The first step involved is shown in Fig. 10.Fig. 10 top shows the blocks involved in this experiment.The goal of the Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.experiment is to illustrate (1) sensor function to detect uric acid concentration level and provide an amperometric DC data into the radio frequency signal, and (2) wireless telemetry link to send spectrum to external signal analysis unit.To illustrate these points, in this first step, we simplify the wireless power transfer by using wired RF power supply, rectification, power management to provide power to the system.
A Keysight E36312 A DC power supply was used to drive a TI BQ25504 power management circuit that provided DC power from RF source after rectification, and the power was split using a voltage divider (through resistors R 1 , R 2 ).The split power outputs were respectively fed to electrochemical tri-electrode sensor and a voltage controlled oscillator, respectively.The wound fluid is emulated by having UA modal solution.The tri-electrode sensor was dipped and output was provided to textile integrated VCO to modulate the sensor output, and a pair of transmitter/receiver PCB-patch antennas operating at 915 MHz were used for telemetry link.The remote wireless receiver is a Keysight PXA spectrum analyzer which displays the frequency spectrum of the received signal.We tested uric acid samples with concentrations varying from 0.2 mM to 1.2 mM and the output of the sensor was fed to a 350-kΩ load (R 3 in Fig. 10 (5) This equation yields a sensing resolution of 48.45 MHz/mM.This sensing resolution, as a function of the variable input voltage of the VCO can be controlled by changing the resistance value of R 3 in Fig. 10.The integration of the bandage, wireless power and data-telemetry modalities is presented next.

B. Fully Integrated Textile Based Smart Bandage With Wireless Power and Telemetry
With preliminary data which supported the VCO based modulation and wireless link approach, we next developed the textile integrated system with functionality of wireless power, sensing, data-modulation and telemetry link.The developed prototype is shown in Fig. 3.The experimental set-up to demonstrate the performance of the smart bandage is shown in Fig. 12.The proposed smart bandage was powered through near-field antennas connected to RF sources and positioned in the vicinity of the reciever antenna.For demonstration, we used integration of RF source antenna into bedsheet through embroidery of conductive fiber, and the design was again based on the corrugated-X shaped dipole embroidered onto gauze fabric.A practical wound monitoring scenario was emulated as shown in Fig. 12.The Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.novel antenna topology (see Fig. 5) exhibited high power transfer efficiency and resilience to all types of misalignments such as lateral, angular, and diagonal, as shown in Fig. 6(a)-(f).At 6 inches from the bed was the bandage (shown in Fig. 3), represented by a receiving corrugated-X-shaped dipole antenna as seen in Fig. 5, a rectifying circuit exhibiting an RF-to-DC conversion efficiency of 76% at 400 mW (26 dBm) of input RF power.
In addition to that, a non-enzymatic pH sensor was used and dipped into the wound fluid emulated by standard pH Sigma-Aldrich solutions bought from MilliporeSigma.The solutions ranged from pH = 2 to 10, a VCO tuned from 0 to 1 V to output an RF modulated signal with spectrum with peak frequency ranging from 836 MHz to 950 MHz, and a 915-MHz textile dipole to send the modulated signal wirelessly to a spectrum analyzer placed more than 6 feet away from the mannequin.Longer ranges can be achieved by using a long range antenna (LoRA).The textile based transmitting and receiving antennas were designed and fabricated to operate around 500 MHz (see Table I).The conductive threads were embroidered onto a gauze-substrate backed with sticky stabilizer with a dielectric constant, = 1.67 and a loss tangent tanδ = 0.07.The antenna had a footprint of 17 cm × 17 cm (0.366λ g × 0.366λ g ) where λ g is the guided wavelength.Our experimental results suggested that, using pH solutions ranging from 2 to 10, the frequency modulation would be between 800 and 830 MHz (see Fig. 13).As per the guidelines provided in [8] for wound monitoring, when the pH is less than 7, the wound is healing.That means, when the frequency is between 810 MHz and 830 MHz, the wound is healing and for frequencies lower than 810 MHz, the wound is severe.For quick and reliable assessment, an empirical model was developed and shown in This theoretical model corresponds to a sensing resolution of 3.375 MHz/pH, which can be further controlled by using a high-input-voltage VCO circuit.As for the power requirement of the system, we followed the guidelines provided by the the International Commission on Non-Ionizing Radiation Protection (ICNIRP) [41] for a limit of 1.475 mW/cm 2 considering our highest frequency.The transmitting RF power provided from the bedsheet was 1 W, which was chosen within the FCC and FDA requirements.This power level is needed to drive the COTS VCO.However, the concept can be scaled down in power if a customized RFIC/system-on-chip (SoC)/system-in-package (SiP) is considered.Furthermore, the size of the antenna can be scaled down as well as the frequency of operation using the strongly coupled magnetic resonance method [42] already implemented on textiles by Bao et al. [43] for highly efficient wireless power transfer systems.The area of the receiver was calculated by taking the area on the mannequin's leg where the bandage was located.The radius of the mannequin's leg was found to be R leg = 8.5 cm and the received power density was evaluated using the equation below Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

TABLE III COMPARISON WITH STATE-OF-THE-ART SMART BANDAGES USED FOR WOUND MONITORING/SENSING AND HEALING
where P inc = 1 W is the incident power and the A rec = π × R leg × h bandage = π × 8.5 cm × 17 cm = 454 cm 2 .Replacing these values back to the (7) combined with the highest value of |S 21 | from Fig. 6, the received power density will be at most 1.41 mW/cm 2 for misalignments of up to 10 cm.The setup is therefore compliant with the ICNIRP as the space between the bandage and the bed was 15.24 cm (6 inches), which would indicate a much lower |S 21 |.The SAR calculations from prior experiment [38] also apply here and is in compliance with ICNIRP.
In summary, the RFID-modality based electrochemical sensing system is proposed and features: 1) an input radio frequency signal from a transmitting antenna embedded into a bed sheet, 2) an electrochemical data collection with a tri-electrode uric acid and pH sensors, 3) an in-situ, analog modulation of the sensor-data into a radio frequency signal for transmission to a remote receiver, 4) the emulation and demonstration of real-life scenario representing a bedridden patient with the bandage of his leg, and 5) a novel approach allowing the elimination of ICs for RF transceivers, microprocessors and thus offering a simplistic footprint.The proposed approach therefore provides an all-textile, battery-less, and wireless smart bandage approach that can be used to continuously monitor the healing process of chronic wounds in real-time.Its comparison with state-of-the-art smart bandages using uric acid, lactate, NaCl, glucose, and pH modulation can be found in Table III , which shows that proposed method is the only RFID (back-scattertype) modality which also maintains selectivity and specificity of the sensor data.The system can be used for any type of chemical sensing by choosing appropriate chemical agent at the sensor surface.In addition, the proposed smart bandage can be powered by any transmitting source placed on the floor, ceiling, bench, hallway, bed, wheelchair, and chair, making the system's architecture more versatile compared to the state-of-the-art.
Given the possibility of miniaturizing the antenna, rectifying circuit, sensor platform, and frequency modulation using a system-on-chip or system-in-package, this system can be used in future 5G/6G communication links where data telemetry can be done using a modulation system achieving high data rate Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.
using FSK mm-wave wireless system [45].For reduced external required power, the strongly coupled magnetic resonance method can be used to model micro-or nanoscale antennas for high-efficiency mm-wave wireless power transceivers.

C. Secondary Applications
The proposed system demonstrates full-fabric integration and modulation of wound-health data for quick and reliable assessment.It can also be used to evaluate the status of the transient phases during the healing process.That is: 1) pH ≤ 7 is an indication that the wound is being reepitheliased [8], [9], [46], [47], an adjacent skin (barrier function is formed, the wound is a stage-I wound, there is high oxygen supply (Bohr-effect), there is bacterial growth suppression, there is a reduction of proteolytic activity, and an improvement in fibroblasts growth.2) pH ≈ 7 is an indication of stage-II chronicity of the wound.
It is also the transition phase from severity to recovery [46].3) pH > 7 is an indication of stage-III chronicity of the wound [8].It also suggests that the wound contains a high bacterial load, the wound is eligible for skin grafting (take-rate of ≈100% is expected), and the indication of effective biosurgery (use of larvae of lucilia sericata flies to debride the necrotic tissue of the wound).

V. CONCLUSION
In this article, we demonstrate a battery-free smart bandage system which can measure electrochemical data through traditional tri-electrode system using frequency modulation and maintains specificty and selectivity of the bio-marker.The backscatter approach is shown for bio-modulators such as uric acid and pH, however can be extended to many different chemicals.The smart system through the bench-top setup performs smart bandage function through a back-scattering of a unique signal representing the wound-health data when different uric acid concentrations are used.The back-scattering process was achieved by placing a remote receiver at 25 cm away from the bench.The assessment is made by taking the frequency and back-convert it into its corresponding uric acid concentration based on the empirical threshold (0.4 mM) found in [10].The pH-aware smart bandage is the first ever reported all-textile, standalone, and battery-less novel sensing approach suitable for all types of tri-electrode sensors for continuous monitoring of chronic wounds.The bandage works by achieving direct frequency modulation of wound-health data using a VCO.The bandage was powered wirelessly using a corrugated-crossed dipole rectenna and the data back-scattering was done through a textile dipole integrated in the bandage.Various pH solutions were used and modulated by the VCO.The signals were captured at more than 6 feet by a spectrum analyzer displaying each signal corresponding to its specific pH.The assessment was done using the threshold reported in [8], [46].For quick and reliable assessment of chronic wounds using uric acid and pH, empirical models were developed and reported.This smart sensing solution is attractive by its robustness, simplicity, its low-cost, ease to manufacture, and ease to use that are made possible by the textile-integration of reduced electronic achieving direct frequency modulation.

Fig. 1 .
Fig. 1.Applications of the smart textile-integrated wound monitoring circuit, entirely based on fabric integration (SkinAid).Graphics show applications using (a) a remote stationary remote kiosk and (b) a hand-held interrogator for mobile patients.

Fig. 2 .
Fig. 2. System diagram showing the powering and data-metry link for the proposed smart-bandage.
(a) shows the close-up of the antenna, Fig. 5(b) represents the resonant frequency as a function of the number of corrugations and Fig. 5(c) shows the reflection coefficient at the resonant frequency as a function of the number of steps.A fitting curve was developed to facilitate quick simulation model using the number of and the size of the step that depends on the substrate used.Equation (3) was empirically obtained, plotted in Fig. 5(c), and compared with the simulation result.In this equation, c is the speed of light, N is the number of corrugations,

Fig. 5 .
Fig. 5. Design and modeling of the proposed corrugated crossed-dipole: (a) HFSS design model, (b) simulated and theoretical resonant frequency as a function of the number of corrugations, and (c) simulated reflection coefficient at resonant frequency with changing number of corrugations.

Fig. 7 .
Fig. 7. Misalignment tests for the proposed antenna when subject to bending (R curvature = 36 mm) and misalignments: (a) Illustration of bending scenario and misalignment along Y, (b) illustration of bending scenario and misalignment along X, (c) and (d) power transfer performance corresponding to misalignment is achieved along the Y and X axes, respectively.

Fig. 8 .
Fig. 8. Textile-based rectifying circuit: (a) experimental setup, (b) the corresponding circuit model, (c) the finished product, (d) and (e) characterization results of the rectifying circuit: RF-to-DC conversion efficiency and Collected DC voltage, respectively.

2 )
pH Sensor: r Immobilization: The working electrode of the pH sensor was immobilized by electrodeposition of an electrically conductive polymer, polyaniline.Polyaniline (PANI) was deposited on the working electrode by cyclic voltammetry by sweeping the potential from −0.2 to 1 V at a scan rate of 100 mV/s for 20 cycles.

r
Characterization: The electrochemical response of the pH sensors was recorded using open circuit potential (OCP)

Fig. 9 .
Fig. 9. Textile-based voltage control oscillator: (a) experimental setup, (b) the corresponding circuit model, (c) the finished product, (d) and (e) characterization results of the rectifying circuit: RF-to-DC conversion efficiency and Collected DC voltage, respectively.

Fig. 10 .
Fig. 10.Bench-top experiment for data modulation and return wireless link.Top: System block diagram of the experimental setup for data-modulation and wireless data telemetry functions.Bottom: Desk-top bench set-up to illustrate the system with D = 25 cm distance for the data telemetry link.
(a)) to provide voltage output in a range of 1.875 V to 2.2 V.This is shown in Fig. 11.We observed associated frequency shift in the VCO output as shown in the figure as well.The normalized spectra are shown in Fig. 11(a) where the signal spectrum changed for different uric acid concentration values.The corresponding frequency change for the signals ranged from 1089 MHz to 1120 MHz.As mentioned in [10], for UA concentrations below 0.4 mM, the wound is healing.That means, the wound is healing when the remote receiver displays signals whose frequencies are between 1089 MHz and 1106 MHz.By the same token, the wound is severe when the UA concentration is more than 0.4 mM, which means the received signals are of frequencies greater than 1106 MHz.For assessment, an empirical equation is also developed which allows us to relate measured frequency with UA concentration by the a priori knowledge of the equation.f UA (MHz) = 1121.256+ 12.365 × log 2 (0.965 × (C UA )) .

Fig. 11 .
Fig. 11.Wound assessment using uric acid as bio-modulator: (a) modulated spectra captured from different wound fluids emulated by chemical solutions with uric acid concentrations and (b) wirelessly received response (measured values) for different concentrations of uric acid in the wound fluid during the bench-top experiment.

Fig. 12 .
Fig.12."In-Vitro" experimental setting for the complete wirelessly powered smart system emulating a bedridden patient with the bandage on the left leg for wound assessment.

Fig. 13 .
Fig. 13.Wound assessment using pH as bio-modulator: (a) modulated spectra captured from different wound fluids emulated by chemical solutions with various pHs and (b) plot showing the acidic and basic limits corresponding to the wound remission and chronicity.