Conformal, mobile e-tattoo for the ambulatory monitoring of whole-body hydration

Abstract

A stacked-design, conformable and mobile sensor that, when worn on an epidermis of a person can be used to continuously detect and monitor whole-body hydration in real time comprising a first flexible, stretchable insulating substrate comprising two or more current injection electrodes and one or more voltage electrodes comprised of biocompatible materials and formed on a first side of the flexible, stretchable insulating substrate and each electrode is configured to flex and stretch with the flexible, stretchable insulating substrate.

Description

CONFORMAL, MOBILE E-TATTOO FOR THE AMBULATORY MONITORING OF WHOLE-BODY HYDRATIONRELATED APPLICATION

[0001] This PCT application claims priority to, and the benefit of, U.S. Provisional Patent Application No. 63 / 634,140, filed April 15, 2024, entitled “CONFORMAL, MOBILE E- TATTOO FOR THE AMBULATORY MONITORING OF WHOLE-BODY HYDRATION,” which is incorporated by reference herein in its entirety.GOVERNMENT SUPPORT

[0002] This invention was made with government support under Grant no. N00014-20- 1-2112 awarded by the Office of Naval Research. The government has certain rights in the invention.BACKGROUND

[0003] Adequate fluid intake and maintenance of total body water homeostasis are crucial for optimal physiological function and human well-being. When these conditions are not maintained, however, various symptoms may manifest. This condition is commonly referred to as “dehydration.” Unlike the symptoms experienced among other health conditions (e.g., heart disease), that become noticeable over long periods of time, the onset of dehydration can happen in as little as a few hours. As a result, dehydration could be easily treated and managed if the body’s hydration status was monitored in real-time. Wearable, ambulatory whole-body hydration monitoring is crucial for the early detection of dehydration and can be leveraged for timely intervention to prevent symptom onset and potential future complications. Moreover, continuous whole-body hydration monitoring may also provide important information related to other systems of the body, such as the cardiovascular system and the body’s ability to regulate core temperature.

[0004] Dehydration is a common condition that affects individuals of all ages. It is known to cause complications when left untreated for an extended amount of time, and when other medical conditions are present, and can cause hospitalization and even death in certain circumstances. Dehydration has been reported to occur in 17-28% of adults in the United States. Based on this statistic alone, of the 260 million adults in the US (aged 18 or over), it is estimated that between 44 and 73 million individuals are dehydrated, to some extent, at any given time. However, many of the mild and severe symptoms can be prevented if a robust, accurate, and wearable whole -body hydration monitor were to be developed. It is worth noting that there are certain groups ofindividuals that would derive great benefit from such a device including but not limited to the elderly, athletes, people exposed to extreme environments, first responders, hospital patients, etc.

[0005] The elderly: The number of elderly individuals (age 65 or over) reached a staggering 55.8 million (16.8% of the population) in the United States in 2020. The elderly naturally have a lower volume of water in their bodies, have a higher risk of already suffering from various health conditions, and are more likely to be taking various medications (which may have a diuretic effect). These factors mean that such individuals are more vulnerable to becoming dehydrated.

[0006] Athletes: High-performance athletes, whether at the amateur, semi-professional, or professional levels of sport, all seek to maximize athletic performance. Consequently, athletes are always looking for ways to optimize their fluid status and prevent the onset of dehydration. Moreover, an optimal hydration state helps prevent injury and promotes muscle recovery postexercise.

[0007] Extreme environment individuals: Individuals who work in extreme environments (e.g., armed-forces personnel, construction workers, etc.) are also more susceptible to dehydration. For example, these individuals often find themselves in harsh environments, where extreme heat and humidity are present, and may suffer from physical and mental impairment (i.e., fatigue) due to dehydration. For example, in 2022, the United States had 1.4 million active military personnel. Moreover, as of 2023, there are approximately 10 million individuals in the United States that are employed in the construction industry. These individuals would derive great benefit from a wholebody hydration monitor to ensure they are staying well-hydrated in these extreme environments, as well as training scenarios.

[0008] First responders: Similar to the grouping above, first responders (particularly firefighters) are often in extreme environments in which they are more susceptible to dehydration. As of 2022, there are an estimated 4.6 million individuals that serve as career and / or volunteer firefighters, police, emergency medical technicians, and paramedics in the United States.

[0009] Hospital patients: Monitoring the fluid status of hospital patients is extremely important. Currently, there is no convenient and non-invasive means of assessing whole-body hydration in the hospital setting. In 2019, there were over 36.2 million hospital admissions in the United States.

[0010] Current techniques relied on to monitor whole-body hydration status are either invasive, only capture hydration at a single snapshot in time (i.e., discrete measurements), require the use of a bulky device that is impractical for long-term operation, or some combination of thesedrawbacks. Evidently, these disadvantages do not permit continuous whole-body hydration monitoring in a manner that is non-invasive and convenient.

[0011] Moreover, as greenhouse gas emissions blanket the globe and continue to increase, the world is warming faster than at any point in history. Global warming is starting to play a significant role in high heat and humid locations, making dehydration and monitoring fluid status an ever more important challenge.

[0012] Therefore, what is needed is a device, system and method that overcomes challenges in the art, some of which are described herein. In particular, what is desired is a device, system and method that provides a completely mobile and wearable solution that utilizes non-invasive sensing for determining and monitoring whole-body hydration.SUMMARY

[0013] Disclosed and described herein are embodiments of a device, system and method of a thin, light, flexible, and skin-conformal form factor e-tattoo that laminates onto the body (e.g., the upper arm region), is soft and stretchable, and comfortable to wear for long-term operation. Having ultra-low power consumption and an ability to wirelessly stream data wirelessly (e.g., Bluetooth) to host devices, the disclosed device monitors whole-body hydration in an ambulatory setting.

[0014] In some instances, the device comprises a plurality (e.g., four) of probes that provide four-terminal (i.e., kelvin) bioimpedance sensing and enabling non-invasive continuous monitoring of whole-body hydration. The wireless streaming capabilities enable data offloading for real-time data analysis and condition detection, as well as long term storage.

[0015] In some instances, the e-tattoo has an effective thickness (without battery) of 1.3mm. All sensing, computation and wireless systems are integrated into the wearable e-tattoo.

[0016] In some instances, the device leverages four-probe bioimpedance sensing in order to measure the segmental cross-arm bioimpedance in a non-invasive, wearable, and continuous manner. Biocompatible graphite based dry electrodes allow monitoring for days without degradation in signal quality or any skin irritation.

[0017] The foregoing illustrative summary, as well as other exemplary objectives and / or advantages of the disclosure, and the manner in which the same are accomplished, are further explained within the following detailed description and its accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Various other objects, features and attendant advantages of the present invention will become fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, and wherein:

[0019] Figure (FIG.) 1A illustrates an on-body application of an exemplary wearable bioimpedance-based e-tattoo for whole-body hydration assessment.

[0020] FIG. IB illustrates four-terminal cross-arm bioimpedance sensing of an exemplary wearable bioimpedance-based e-tattoo.

[0021] FIG. 1C is an illustration of the flow of current in human cells at low and high frequencies.

[0022] FIG. ID illustrates an exemplary bioimpedance vector analysis (BIVA) plot with various identified body-composition and hydration states.

[0023] FIG. IE is an illustration of an exemplary system for measuring and / or monitoring whole-body hydration using the disclosed e-tattoo.

[0024] FIG. 2A illustrates a finite element model of the brachium and its electrical simulations using a three-dimensional brachium model, with four different tissue layers (i.e., skin, fat, muscle, bone) and electrodes with complex electrical characteristics defined for each material type.

[0025] FIG. 2B illustrates electric field plots across two slices (XY and YZ axes) of the finite-element model of the brachium.

[0026] FIG. 2C illustrates simulated resistance, reactance, and impedance as a function of electrode spacing parameter, d.

[0027] FIG. 2D illustrates experimental data for resistance, reactance, and impedance as a function of electrode spacing parameter, d.

[0028] FIG. 2E illustrates impedance as a function of muscle resistivity for varying electrode spacings.

[0029] FIG. 2F illustrates the change in impedance as a function of muscle resistivity for varying electrode spacings.

[0030] FIG. 2G illustrates impedance as a function of muscle resistivity for cross-arm and single- side electrode configurations.

[0031] FIG. 2H illustrates the change in impedance as a function of muscle resistivity for cross-arm and single-side electrode configurations.

[0032] FIG. 3A illustrates an exemplary block diagram of the hardware components of an embodiment of the disclosed bioimpedance e-tattoo.

[0033] FIG. 3B illustrates an exploded 3D view of the flexible printed circuit (FPC) layer and electronic components of an exemplary bioimpedance e-tattoo.

[0034] FIG. 3C illustrates active power draw of an exemplary bioimpedance e-tattoo during operation. Average current in this example is 0.879 mA.

[0035] FIG. 3D illustrates contact impedance as a function of frequency for graphite polyurethane (GPU) film and wet-gel red dot (Red Dot Electrodes, 3M) electrodes for electrophysiological sensing.

[0036] FIG. 3E illustrates an exemplary resistance- strain curve for an embodiment of a bioimpedance e-tattoo.

[0037] FIG. 3F illustrates arm Bio-Z data collected for 45 minutes, during which a single participant engaged in muscle contraction exercises.

[0038] FIG. 3G are images of an exemplary Ecoflex™ encased bioimpedance e-tattoo and characterizing features, namely (I) size of the FPC layer relative to a single USA quarter, (II) flexibility, and (III) inherent stretchability of serpentine interconnects.

[0039] FIG. 4A illustrates diuretic-induced dehydration experimental protocol for wholebody hydration assessment. Blue [-60, 0] represents no data collection, orange [0, 30] represents data omitted due to electrode settling effects, and green [30, 180] represents useful data.

[0040] FIG. 4B illustrates per-participant normalized arm impedance versus time following the protocol presented in FIG. 4A.

[0041] FIG. 4C illustrates per-participant arm-diameter-normalized arm impedance versus percent body weight loss.

[0042] FIG. 4D illustrates a BIVA plot for arm impedance data.

[0043] FIG. 4E illustrates per-participant height-normalized body impedance versus percent body weight loss.

[0044] FIG. 4F illustrates a BIVA plot for whole-body impedance data.

[0045] FIGS. 4G - 4N each illustrates a BIVA plot for an individual participant corresponding to discrete measurements shown in FIG. 4A for arm impedance and whole-body impedance data.

[0046] FIG. 40 illustrates the relationship between BIVA vector length and percent body weight loss for all participants for arm impedance (left) and whole-body impedance (right) data.

[0047] FIGS. 5A-5D illustrates a 24-hour daily living experiment and analysis for data collected from N=1 human participant using the exemplary system.

[0048] FIG. 5A illustrates arm bioimpedance versus time.

[0049] FIG. 5B illustrates arm impedance versus percent body weight loss during the diuretic dehydration period denoted in FIG. 5 A.

[0050] FIG. 5C illustrates a BIVA plot for arm impedance data with data points corresponding to the dashed lines illustrated in FIG. 5A.

[0051] FIG. 5D illustrates an exemplary e-tattoo hydration sensor under various muscle contractions showcasing its ability to monitor in an ambulatory environment.

[0052] FIGS. 5E - 5H illustrate a 24-hour daily living experiment and analysis for data collected from N=1 human participants using a rigid system with commercial gel electrodes, where FIG. 5E illustrates an arm bioimpedance versus time graph; FIG. 5F illustrates an arm impedance versus percent body weight loss during the diuretic dehydration period denoted in FIG. 5E; FIG. 5G illustrates data points corresponding to the dashed lines in FIG. 5E; and FIG. 5H illustrates the rigid system with commercial gel electrodes.

[0053] FIGS. 51 - 5L illustrate the diuretic-protocol results conducted using a single- side electrode configuration set up on two participants, where FIG. 51 illustrates the single-side electrode configuration setup using a hydration e-tattoo; FIG. 5J illustrates post-processed arm resistance, reactance, and impedance data for the two participants undergoing diureticdehydration protocol; FIG. 5K illustrates a linear relationship between per-participant arm- diameter-normalized arm impedance and percent body weight loss; and FIG. 5L illustrates a BIVA plot for arm impedance data for the two participants.

[0054] FIGS. 5M - 5P illustrate diuretic -protocol results conducted using a rigid wrist- worn cross-arm configuration setup on a single human participant, where FIG. 5M illustrates the crossarm configuration setup; FIG. 5N illustrates post-processed wrist resistance, reactance, and impedance data undergoing diuretic-dehydration protocol, where large spikes in data can be dueto sudden wrist movements; FIG. 50 illustrates a linear relationship between wrist impedance and percent body weight loss; and FIG. 5P illustrates a serial BIVA plot for wrist impedance data, showcasing displacement in line with a minor ellipse axis due to minimal reactance variations.

[0055] FIG. 5Q illustrates a schematic representation of a sensing paradigm of the exemplary system.

[0056] FIGS. 6 A - 6C illustrate conventional methods for dehydration assessment.DETAILED DESCRIPTION

[0057] Before the present methods and systems are disclosed and described, it is to be understood that the methods and systems are not limited to specific synthetic methods, specific components, or to particular compositions. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.

[0058] As used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes-i from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0059] “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.

[0060] Throughout the description and claims of this specification, the word “comprise” and variations of the word, such as “comprising” and “comprises,” means “including but not limited to,” and is not intended to exclude, for example, other additives, components, integers or steps. “Exemplary” means “an example of’ and is not intended to convey an indication of a preferred or ideal embodiment. “Such as” is not used in a restrictive sense, but for explanatory purposes.

[0061] Disclosed are components that can be used to perform the disclosed methods and systems. These and other components are disclosed herein, and it is understood that when combinations, subsets, interactions, groups, etc. of these components are disclosed that while specific reference of each various individual and collective combinations and permutation of these may not be explicitly disclosed, each is specifically contemplated and described herein, for allmethods and systems. This applies to all aspects of this application including, but not limited to, steps in disclosed methods. Thus, if there are a variety of additional steps that can be performed it is understood that each of these additional steps can be performed with any specific embodiment or combination of embodiments of the disclosed methods.

[0062] The present methods and systems may be understood more readily by reference to the following detailed description of preferred embodiments and the Examples included therein and to the Figures and their previous and following description.

[0063] In some aspects, disclosed and described herein is a wearable, stretchable, thin, and lightweight, body (e.g., arm) laminated electronic tattoo (“e-tattoo”) that uses a plurality of probes for four-terminal (i.e., kelvin) bioimpedance sensing to monitor whole-body hydration. In some instances, the e-tattoo attaches to the upper arm (i.e., brachium) region of the body to capture the segmental cross-arm bioimpedance and evaluate whole-body hydration status. Bioimpedance sensing is a non-invasive sensing paradigm which relies on injecting a small amplitude, high frequency current into the body and sensing the voltage response due to this stimulation. This response is directly related to the fluid content of, and body fluid shifts that occur in the sensing region. As the name suggests, bioimpedance relates to the effective resistance of biological tissue to alternating current. Thus, as the fluids in the human body decrease, the bioimpedance is expected to increase, and vice-versa. Furthermore, existing techniques, like bioimpedance vector analysis (BIVA), are employed in order to assess hydration state changes. The disclosed e-tattoo utilizes biocompatible electrodes (e.g., graphite polyurethane-based), cut into serpentine- structured patterns for stretchability and higher conformability, that interface with human skin to capture electrophysiological data. The device is completely mobile and capable of wireless streaming (e.g., over Bluetooth) data in real time to one or more host devices. The combined hardware and software features of the e-tattoo provide ultra-low power consumption and prolonged battery life. By measuring the segmental arm bioimpedance, the e-tattoo captures dehydration and rehydration periods. Dehydration has been shown to increase the burden on the body’s cardiovascular system and increase core body temperatures. The capabilities of this e- tattoo enable continuous tracking of hydration status, which has significant implications for various individuals, especially those who are more prone to becoming dehydrated. For example, the elderly, who naturally have a lower volume of water in their bodies and have a higher risk of already suffering from various health conditions, are more susceptible to dehydration. Armed- forces personnel and construction workers often work in extreme environments (e.g., high heat and humidity conditions) where dehydration is more common. Furthermore, body hydration statusis a major concern for high-performance athletes, who seek to optimize their fluid intake in order to maximize performance, prevent injury, and aid recovery.

[0064] Conventional hydration detection methods / devices used to evaluate whole-body hydration (e.g., blood osmolality, urine osmolality, whole-body impedance devices, etc.) are either invasive, only capture hydration at a single snapshot in time (i.e., discrete measurements), require the use of a bulky device that is impractical for long-term operation, or some combination of these drawbacks. In contrast, the disclosed device, system and method utilizes non-invasive technology in a completely wearable and mobile format, in a manner that is soft, stretchable, light, conformal, and that can be easily laminated on the arm. Signals are captured in a continuous manner that enables monitoring of arm bioimpedance with high resolution.

[0065] Continuous monitoring and the wearable and completely mobile form factor of the disclosed device provides better time resolution, and therefore better sensitivity to fluid loss, as well as long-term operation for capturing the human body fluid status variation in a daily living environment (i.e., >24 hours).

[0066] Furthermore, conventional technologies that claim to be able to monitor hydration are actually capturing the skin hydration at the sensing region, typically using a two-probe sensing method. In fact, these conventional technologies are only able to capture the skin hydration state (usually by measuring the skin contact impedance) at the local point of application (e.g., skin hydration state on the wrist) as opposed to the hydration state of the entire human body. Local skin hydration measurements are heavily influenced by external factors, such as creams, sweat, and other fluids. In contrast, the disclosed device, system and method assesses whole-body hydration by capturing the true cross-limb bioimpedance (i.e., deep tissue) using the disclosed four-probe sensing method, and is therefore not limited by these external factors.

[0067] Some instances of the disclosed e-tattoo comprise a polyimide substrate with copper traces comprising the on-board electronics and the electrodes are made from biocompatible materials such as dry carbon-based graphite film. The e-tattoo is a multilayered device, with only the electrodes being in contact with the skin, and the electronics being completely isolated from the human body by being encased. An electrical connection is made between the layers (e.g., electrodes and electronics) via a conductive layer such as anisotropic conductive film (ACF). This ensures that the electrodes are detachable and can be disposed of, and the electronics can be reused with a new electrode layer. The disclosed e-tattoo has an extremely low power consumption (approximately 1.6 mW) compared to other contemporary wearable devices. Testing of exemplaryembodiments of the disclosed e-tattoo to measure arm impedance continuously and correlating this to body weight loss, which is considered a gold standard when capturing dehydration periods of less than 12 hours, yielded high correlation (R=0.95) among eight human subjects during a diuretic-induced dehydration study.

[0068] Conventional hydration monitoring devices are rigid and bulky and uncomfortable for long term use. Conversely, the disclosed device overcomes these issues by providing a flexible, stretchable, and small wearable form-factor solution. Coupled with the small, light, soft, stretchable, and unobtrusive form factor, it provides more user comfort which can help with user compliance. Due to the lightweight and thin conformal design, it is almost imperceptible to the user and can be worn under everyday clothing. Long duration testing of the device on a human subject demonstrated good signal quality over a long period (for example, 25 hours). Further, using industrial circuit manufacturing substrates and processes with just structural and design changes to achieve stretchability and conformability makes manufacturing the device cost effective and easy to manufacture.

[0069] Though generally described herein as being attached to the upper arm of a subject, it is to be appreciated that the disclosed e-tattoo can be attached to other parts of the body (for example, other limbs of the body (e.g., leg)) that possess enough muscle mass to reflect body hydration. Furthermore, the e-tattoo device can be attached to other parts of the body to sense other physiological parameters in addition to hydration such as electrocardiogram, respiration, human activity detection, etc. These other attachment locations and additional uses of the e-tattoo are considered within the scope of this disclosure.

[0070] As shown in FIG. 1A, a wearable bioimpedance-based electronic tattoo (e-tattoo) is disclosed that continuously and non-invasively monitors whole-body hydration (WBH). In some instances, the disclosed e-tattoo is affixed to the upper arm (brachium) using two pairs of temporary tattoo electrodes to capture segmental cross-arm bioimpedance in a kelvin sensing configuration as shown in Fig. IB.

[0071] A high frequency alternating current is utilized that passes through both extracellular (ECF) and intracellular (ICF) fluids (see Fig.lC). Bioimpedance vector analysis (BIVA), and / or various other analyses, is then employed to evaluate whole -body hydration change, as shown in Fig. ID. Bioimpedance for hydration sensing typically employs frequencies in the 5-500 kHz range. In one instance, the disclosed device, and the analysis provided, employs a 40 kHz alternating current, although this frequency can be programmed to other values. Additionally,BIVA, which is typically utilized in whole-body impedance studies, employs a frequency of 50 kHz (although any other frequency value can be realized). The e-tattoo is characterized by its flexible sensor design, which enables it to conform to the natural curvature of the skin, ensuring seamless operation and minimizing any inconvenience to the wearer. The raw bioimpedance signal (Bio-Z) is wirelessly transmitted in real-time via wireless protocols such as Bluetooth Low Energy (BLE), ZigBee, etc. to a host device 102. Through post-processing of the bioimpedance signal, participants’ hydration status change can be assessed following diuretic-induced dehydration and 24-hour daily living protocols (see Fig. IE). The disclosed hydration e-tattoo demonstrates monitoring of WBH in various conditions, highlighting the ability and feasibility of wearable hydration monitoring in everyday life.

[0072] The disclosed e-tattoo is used to monitor bioimpedance. Relative changes in bioimpedance (from some starting point) over time are correlated to percent body weight loss over time. It has been found that upper arm bioimpedance can serve as a reliable, continuous proxy of whole-body hydration that is cost-effective and highly accessible.

[0073] Additionally, BIVA is employed to highlight that the disclosed device and the resistance / reactance changes that occur during dehydration, are consistent with trends associated with the loss of body water and previous literature utilizing whole-body impedance (an established method).

[0074] Additionally / optionally, the disclosed device may be used to attain an approximation of whole-body hydration status using different methods. For example, this may be achieved by:• Day 1: Perform a calibration period of the device by wearing the device for one day and periodically logging body weight, while tracking arm bioimpedance with the e-tattoo.• Data analysis: Perform similar analysis to that presented in this work and correlate the relative change in arm bioimpedance to percent body weight loss over time and / or the BIVA vector length to percent body weight loss over time.• Day 2+: Continue wearing the device and provide estimated body weight loss metrics based on interpolating arm bioimpedance data / BIVA vector lengths according to the regression performed in the “Data analysis” stage.

[0075] While this is just an example of using the disclosed device for assessing WBH by performing some form of calibration, and then using the equation / relationship identified duringthat calibration period to track the absolute body- weight and / or fluid loss occurring, this and other methods of use are contemplated within the scope of this disclosure.

[0076] Bioimpedance sensing necessitates strategic placement of distinct pairs of electrodes for both injection and sensing purposes. These electrodes serve the function of stimulating the tissue through the application of a high-frequency AC signal and concurrently measuring the resultant potential difference, which fluctuates in response to impedance variations within the underlying tissue. When deployed around the brachium, these sensors exhibit sensitivity to various factors such as shifts in body fluids (e.g., blood flow, tissue hydration) and motion. To develop a hydration sensor sensitive to these fluid shifts, simulations were conducted to ascertain: 1) the optimal sensing configuration, and 2) the most favorable current-to-voltage electrode spacing. The results of these simulations guided the design of the device described herein.

[0077] Previous applications of bioimpedance sensing typically adopt an electrode arrangement where all electrodes are positioned linearly along various arteries of the body (e.g., wrist, ankle, neck), with the outer electrodes serving for current injection. This configuration is typically used so that temporal features (e.g., pulse transit time) can be captured. However, the disclosed embodiments have no such requirements.

[0078] Some of the embodiments disclosed herein utilize a “cross-arm configuration.” This setup involves placing a single current injection and single voltage sensing electrode on both sides of the arm, and is illustrated in Fig. 2A.

[0079] To carry out these simulations, a four-layer Finite Element Model (FEM) of the brachium was developed (Fig. 2A). To characterize the electrical properties of each tissue type, frequency-dependent parameters derived from Cole-Cole equations were employed. The entire geometry illustrated in Fig. 2A was subjected to simulation under various configurations by manipulating the electrode spacing through a parametric sweep of the parameter d. For these simulations, 100 pA of current, at an operating frequency of 40 kHz, was employed

[0040] . Voltage responses were captured via two voltage domain probes. A total of four electrodes (i.e., one pair for injection and one pair for sensing) are used to initiate four-terminal (Kelvin) Bio-Z sensing. Electrical simulations were conducted using the AC / DC physics module of COMSOL Multiphysics v6.1. The conductivity and relative permittivity of skin were chosen as 0.00025 S / m and 1128.3, respectively. The conductivity and relative permittivity of fat were chosen as 0.024 S / m and 215.2, respectively. The conductivity and relative permittivity of muscle were chosen as 0.350 S / m and 10988, respectively. The conductivity and relative permittivity of bone were chosenas 0.021 S / m and 281.1, respectively. Voltage domain probes were assigned to the sensing electrodes to capture the voltage potential as a function of electrode spacing and configuration.

[0080] Table 1 shows the tissue and geometric properties used in the COMSOL simulation for the four-layer FEM model of the brachium shown in Fig. 2A.Table 1

[0081] Fig. 2B depicts electric field density plots across two slices of the model. Notably, the cross-arm configuration showcases current passing through the entire geometry of the arm. With voltage sensing electrodes close by, the response due to stimulation of the entire arm would be captured. In contrast, however, a linear arrangement appears to have current pass through only half the geometry, which influences the captured response and, as a result, does not provide a complete picture of arm impedance changes when undergoing dehydration.

[0082] In addition to the sensing configuration, the distance (d) between the voltage sensing electrodes and the current injection electrodes plays a role in capturing the response due to stimulation. As expected, the current density is highest directly over the injection sites and reduces abruptly as you move away (see Fig. 2B). The captured response is therefore likely to significantly reduce as the voltage sensing electrodes are displaced away from the injection site. Fig. 2C highlights this behavior by illustrating the simulated resistance, reactance, and impedance as functions of the d parameter. Namely, simulated responses are at their maximum at smaller values of d and decline as the d parameter increases. This behavior is attributable to the inability of the voltage sensing electrodes to capture a substantial portion of the response generated by current stimulation when the voltage sensing electrodes are positioned at a considerable distance from the injection electrodes.

[0083] To confirm the simulated outcomes, this setup was replicated in an experimental setting, where four-terminal sensing was conducted using commercial wet-gel electrodes (Red Dot Electrodes, 3M) on a single human participant, for varying electrode spacings. As depicted in Fig. 2D, the experimental data closely resembled the simulated trends.

[0084] Additionally, simulations were performed to model the dehydration process with different electrode spacings in both configuration types. This was achieved by parameterizing the resistivity (p) of muscle tissue and conducting a parametric sweep of this parameter. Muscle resistivity was the sole tissue parameter under consideration, given that muscle tissue consists of about 75% water, whereas in fat tissue, this proportion is approximately 10%. The outcomes of these simulations, depicted in Figures 2E-2H, suggest that employing the smallest electrode spacing in the cross-arm configuration effectively captures a greater portion of the elicited response as the tissue undergoes dehydration.

[0085] Deriving insights from the collective findings of Figs. 2A-2G, it was inferred that the ideal device design should integrate the smallest achievable separation distance between the injection and sensing electrodes, in a cross-arm configuration. The selected setup, elaborated upon below, features a center-to-center electrode spacing of approximately 2 cm, maximizing the sensitivity to variations within the underlying tissue.

[0086] Generally, as shown in Figs. 3A and 3B, embodiments of the e-tattoo device comprise a flexible arm-laminated e-tattoo, capable of measuring bioimpedance in a completely mobile manner. Such an e-tattoo is generally comprised of an electrode layer, a flexible printed circuit (FPC) layer, and typically, a cover layer.

[0087] The electrode layer comprises two or more current injection electrodes and one or more voltage sensing electrodes that are made from biocompatible materials such as graphite polyurethane film and laminated onto a first side of a first flexible, stretchable insulating substrate. Typically, there are two current injection electrodes and two voltage sensing electrodes. Each electrode forms an electrode pattern on the first side of the first flexible, stretchable insulating substrate and each electrode is configured to flex and stretch with the first flexible, stretchable insulating substrate to conform to the epidermis of a wearer. In some instances, the electrode pattern on the first side of the first flexible, stretchable insulating substrate is serpentine shaped, and each electrode pattern generally includes one or more terminal pads for connection to an interconnect. In some instances, the first flexible, stretchable insulating substrate may be comprised of transparent or substantially transparent materials.

[0088] In some instances, the first flexible, stretchable insulating substrate comprises a polyurethane film medical dressing, such as Tegaderm™ (3M, Saint Paul, MN), having an adhesive layer on the first side of the first flexible, stretchable insulating substrate. Electrical contact is made with the electrodes from a second side of the first flexible, stretchable insulatingsubstrate through holes defined by the first flexible, stretchable insulating substrate and a conductive material such as an anisotropic conductive film (ACF) acts as an adhesive and conductor between electrodes and electronics.

[0089] Further comprising embodiments of the device is the flexible printed circuit (FPC) layer having a first side and a second side. In some instances, the second side of the FPC layer may be covered with a third flexible substrate that covers the first flexible, stretchable insulating substrate and the FPC layer. In some instances, this third layer of material may comprise Tegaderm™. In some instances, a portion of the third flexible substrate may be removed to define one or more holes that expose a power source (e.g., a battery) mounted on the second side of the FPC layer. In this way, the power source can be replaced as needed without having to replace the entire device. In some instances, the FPC layer may be comprised of transparent or substantially transparent materials.

[0090] Generally, the FPC layer comprises electronics disposed at least partially on the second side of the FPC layer. In some instances, the FPC layer comprises or at least partially comprises polyimide. Typically, the first side of the second flexible insulating substrate is in substantial contact with the second side of the first flexible, stretchable insulating substrate. Generally, the electronics comprise a processor such as a nRF52832 (Nordic Semiconductor, USA) central processing unit (CPU). Further comprising the electronics may be a communications interface such as a Bluetooth Low Energy (BLE) transceiver that may, or may not, be integrated into the CPU. In some instances, BLE may be chosen due to its ultra-low power consumption, though other communications devices may be used. Integrated BLE functionality additionally facilitates real-time transmission of data from the device to a designated receiver and / or host, for example the designated receiver may comprise a smartphone such as an Android or IOS-based smartphone running a custom-designed application. In some instances, the smartphone may comprise one or more processors that are used to analyze or at least partially analyze the data. In some instances, the designated receiver may further transmit the data to a host comprising one or more processors for analysis. In some instances, that data may be transmitted directly to the host comprising one or more processors.

[0091] Further comprising the electronics of the exemplary e-tattoo shown in Figs. 3A and 3B is an analog front end (AFE). For example, the AFE may comprise a MAX30002 AFE (Maxim Integrated), which serves as a single-channel Bio-Z sensor and is linked to the CPU via the serial peripheral interface (SPI). Power is supplied to the e-tattoo from a battery such as, for example, a small form factor 3.7 V 40 mAh lithium polymer (LiPo) battery, with all circuit elements beingpowered via a 1.8 V linear low-dropout (LDO) regulator (NCP161, onsemi). The device features a FPC layer that has been patterned into islands and connected via serpentine tracks, illustrated in exploded and interfaced views in Fig. 3B. The central island, housing almost all passive components (e.g., resistors, capacitors, inductors) and integrated circuits (IC) responsible for sensing, processing, and communication, is strategically positioned to sit centrally on the head of the bicep, while interconnecting pads are situated on separate islands and interface with the electrode layer to initiate the cross-arm configuration on the brachium. This island-based approach imparts stretchability, minimizes the number of interconnections, and provides a straightforward interface, all while maintaining optimal electronic density and promoting ease of operation.

[0092] Bio-Z sensing demands robust electrical contact with the skin, a requirement achieved through the utilization of graphite polyurethane film (GPU, Mineral Seal Corporation) laser cut into serpentine patterns and transferred onto commercial medical dressing (Tegaderm, 3M). Dry electrodes offer a number of advantages over conventional gel electrodes, such as immunity from signal degradation induced by electrode dehydration, an ultra-thin profile, and convenience for long-term wear. In order to establish electrical connectivity between the FPC layer pads and GPU electrodes, anisotropic conductive film (ACF) was employed. Small holes were punctured onto the medical dressing to expose part of the electrodes where a piece of ACF was attached. Upon mounting the electronics onto the ACF, uniform pressure was applied to facilitate activation and the formation of an electrical connection between the electrodes and the Bio-Z AFE. Double-sided insulating tape was also affixed to the underside (skin side) of the electrodes to prevent any injection or sensing at the interface location. This fabrication technique creates a temporary connection between the device and the electrodes, allowing for the convenient disposal of electrodes post-use, while enabling the recycling of the electronics. It is important to note that only the medical dressing and bio-compatible GPU electrodes come in direct contact with the skin, ensuring isolation from the electronics.

[0093] The embodiments of the disclosed e-tattoo prioritize user comfort, device re-usability, and long-term operation. In some instances, the disclosed e-tattoo employs an AC current of approximately 100 pA, or less, at 40 kHz for the purpose of injected stimulation. Moreover, a sampling frequency of approximately 0.2 Hz was chosen given that human hydration status is expected to undergo gradual rather than instantaneous variations. This choice of sampling frequency also serves to minimize power consumption. Fig. 3C shows the active power draw of the e-tattoo during operation, with an average current draw of approximately 0.879 mA.

[0094] The skin-electrode interface (i.e., “contact”) impedance is a factor for sensors interfacing with human skin. Fig. 3D illustrates the contact impedance for the GPU film electrodes and commercial wet gel electrodes. At the 40 kHz operating frequency, the contact impedance for the GPU film electrodes and wet gel electrodes measured approximately 4.1 kQ and 0.39 kQ, respectively. For an injection current of 100 pA, this impedance level remains well within the acceptable limits of the system.

[0095] When attaching to human participants, accurate impedance measurement is desired, especially when the FPC layer undergoes considerable strain due to muscle contractions and movement. Fig. 3E illustrates the FPC layer’s measured resistance under varying levels of strain. The FPC layers demonstrates accurate measurement with minimal deviation, even when subjected to strains surpassing the established maximum threshold of 30% tolerated by human skin.

[0096] The images in Fig. 3E showcase a serpentine interconnect before (left) and after (right) the strain experiment. The right- most image depicts the serpentine at approximately 45% strain, displaying plastic deformation but measuring resistance with only 1.65% deviation.

[0097] To comprehensively characterize the accuracy of the device across varying muscle deflections, a supplementary test was conducted by a single human participant. Fig. 3F shows the arm Bio-Z data collected for 45 minutes, during which a single participant engaged in muscle contraction exercises. These exercises included medium and full bicep flexion interspersed with rest intervals. Medium muscle flexion periods involved the participant flexing their bicep muscle to maintain their arm at a 45-degree angle relative to a flat surface. Similarly, full muscle flexion was executed without constraining the arm angle, resulting in complete bicep flexion. Each flexion period lasted five minutes and consisted of alternating 15- second intervals of flexion and rest, repeated until the completion of the five-minute duration. This process was repeated twice.

[0098] Analysis of the data in Fig. 3F reveals that during the medium muscle flexion period, the average percent signal deviation from baseline was approximately 3.3%, whereas during full muscle flexion, the average percent signal deviation was approximately 9.8%. It is noteworthy, however, that the signal consistently returned to its baseline value following each exercise period. This underscores the e-tattoo’s capability to revert to baseline readings when not subjected to strain or motion.

[0099] The design characteristics of the embodiments of the e-tattoo described herein collectively contribute to a compact, pliable, and stretchable device, facilitating the e-tattoo’s ability to seamlessly adapt to the natural contours of the human body (see Fig. 3G). Theseattributes ensure user comfort while also permitting the development of an exceptionally lightweight device, weighing approximately 2 grams.

[0100] As shown in Fig. 3G, the exemplary system is constructed using a double-layer FPC, where a polyimide (PI) substrate houses copper tracks on both the top and bottom layers. The FPC comprises an island-serpentine configuration, incorporating two islands that house electronics and copper pads for electrode interfacing. These islands are connected to the main board via stretchable serpentine interconnects. Each serpentine interconnect comprises the copper tracks required to facilitate the connection between current injection and voltage sensing pad connections and the electronics. The overall thickness of the FPC, inclusive of circuit elements, measures approximately 200 pm. To enhance mechanical stability, the central region of the FPC, which houses the majority of circuit components, is designed using rounded edges, like the shape of an eye. This eye-shaped design, in addition to the stretchable serpentine, provides better conformity to human skin and improved resistance against mechanical deformation. To ensure safety for use on human participants, the circuit elements were affixed to the FPC using lead (Pb) free soldering paste and the device was encapsulated in Ecoflex.

[0101] To evaluate the capabilities of the disclosed hydration e-tattoo, a diuretic-induced experimental protocol, as outlined in Fig. 4A, was performed. Sessions commenced with the preparation of the participant’s upper arm skin, which served as the sensing site. This preparation procedure involved applying an exfoliating gel, wiping away any residue, wiping the area with a saline wipe, and drying it with a delicate task wipe. Subsequently, the device was affixed to the participants’ left arm. Attachment on the left arm was chosen given that all participants were rightarm dominant and to maintain uniformity across the participant cohort. During the data collection period, participants were permitted to engage in minor tasks, such as operating a phone or laptop device or reading a book, using their dominant (i.e., right) hand.

[0102] Following the attachment, participants were provided an oral diuretic (furosemide) to induce fluid loss. Furosemide is classified as a loop diuretic, which functions by inhibiting the reabsorption of sodium and chloride in the kidneys, thereby reducing water reabsorption and increasing urine production. Consequently, significant solute and water losses occur, resulting in dehydration with minimal change in (ECF) osmolality. This type of dehydration, known as isotonic dehydration, is considered a suitable representation of the dehydration that occurs due to conditions such as diarrhea and vomiting.

[0103] Data collection commenced approximately 60 minutes following device attachment and intake of furosemide, primarily to allow enough time for the oral diuretic to take effect, but also to account for the electrode settling effect and the initial high contact impedance encountered when attaching electrodes to biological tissue. Data was then collected over a three-hour period, in which participants were asked to urinate in 30-minute intervals. To account for fluid loss, body weight measurements were conducted before and after each urination event using commercially available bath scales. Changes in body weight are commonly utilized as a reliable indicator of changes in body water and are considered a gold standard in measuring body water fluctuations over a duration of up to 12 hours. It is important to note that during dehydration, the only other bodily constituent that undergoes significant changes is the bodily carbohydrate (glycogen), and fat stores due to oxidation for energy. However, these reductions in substrates amount to a mere 1.5 grams per minute and are offset by an equivalent amount of water production from substrate oxidation. Consequently, the reduction in body weight reliably reflects the loss of the original quantity of body water, making it an accurate representation of the extent of fluid loss.

[0104] Along-side the body weight measurements, whole-body impedance measurements were acquired from participants approximately 5 minutes preceding each urination event using a high precision LCR meter (E4980AL, Keysight Technologies), as denoted by the white circles in Fig. 4A. These measurements served a dual purpose: firstly, to enable comparison with existing studies; and secondly, to establish a baseline against which to compare impedance measurements from the arm.

[0105] Arm- impedance, whole-body impedance, and body-weight data were collected from N = 8 presumably healthy participants (7 male, 1 female, age: 26.3 ± 3.3 years, weight: 75.7 ± 12.4 kg, height: 179.8 ± 6.3 cm, BMI: 23.3 ± 3.1), adhering to the experimental protocol described above. Across all participants, the average body weight loss was 1.65 ± 0.30 kg (3.64 ± 0.66 lbs), corresponding to an average percent body weight loss of 2.30 ± 0.40 %. Arm resistance and reactance data were continuously collected, and arm impedance was computed as follows:

[0106] Fig. 4B illustrates the normalized arm impedance as a function of time for N = 8 participants; data and analysis for an additional, participant 4 (male, age 25), is not presented due to a minimal total mass loss of 0.5 lbs at the conclusion of the protocol and noisy impedance data arising from interface issues between the ACF and the GPU film electrodes. The presented data trends in Fig. 4B are post-processed, involving the application of a median filter and mean filterto the raw data. The median filter is employed to eliminate outliers or significant spikes caused by abrupt movements or motion. Following this, a mean filter is applied to smooth the data and emphasize long-term trends for subsequent hydration assessment. The normalization incorporated in Fig. 4B utilizes the initial arm impedance per participant. Namely,2 —Normalized ZnLIrImIII. = — f~7 ( Xalso denoted as Z„ (r / mIII.-)' [ I21 J o where Zo is the impedance at time = 30 (i.e., the initial impedance during the useful data period). The use of this normalization ensures that data from different participants can be directly compared on the same scale, and so that common trends and variations can be identified across participants. In particular, as illustrated, the arm impedance for all participants increased throughout the duration of the protocol.

[0107] Furthermore, to enable direct comparisons between continuously acquired Bio-Z data and discrete body weight measurements, a windowing scheme was implemented. A 5-minute average window was applied on the Bio-Z data collected between two urination events to extract a single Bio-Z value for comparison, as depicted by the black squares in Fig. 4A. Each averaged value was compared to the body weight measurement recorded at the preceding urination event. For instance, the Bio-Z value extracted at the 45-minute mark, obtained by averaging all Bio-Z data points between [42.5, 47.5] minutes, was directly compared to the body weight measurement result at 30 minutes. Following this windowing approach, Fig. 4C depicts the arm impedance normalized by arm diameter versus percent body weight loss for each participant. Strong positive linear correlations were observed across all participants, supported by Pearson’s correlation coefficients. Across the entire participant cohort, correlation coefficients ranged from 0.899 to 0.992, with the mean correlation coefficient calculated as 0.956 ± 0.033. The observed increase in arm impedance aligns with expectations, considering the concurrent dehydration of participants and the consequent reduction in their total body water (TBW) content.

[0108] Fig. 4E illustrates the per-participant height-normalized whole-body impedance versus percent body weight loss. Similar to Fig. 4C, positive linear correlations were evident across all participants. In this case, however, Pearson’s correlation coefficients exhibited a broader range, spanning from 0.611 to 0.989. The mean correlation coefficient for the cohort was computed as 0.870 ± 0.117, and the notable increase in variance, relative to its arm impedance counterpart, may be attributable to several reasons. First, whole-body impedance data is more susceptible to posture- related variations and captures substantial fluid shifts occurring in the gastrointestinal and other regions of the human body. Second, the conformability of the e-tattoo to the body and its localized sensing capabilities make it more robust in the presence of motion. Third, the arm sensingconfiguration is less susceptible to posture-related variations and does not capture any fluid shifts unrelated to those occurring in the underlying tissue of the brachium.

[0109] Whole-body hydration assessment as described herein involves the utilization of BIVA. BIVA uses whole-body resistance and reactance values, normalized for standing height and plotted on the RXCgraph. The resulting vector has both length and direction. The length of the vector is inversely related to TBW, while the combination of vector length and direction serve as an indicator of tissue hydration status.

[0110] BIVA enables classification and ranking of hydration, along with soft-tissue mass, by analyzing an individual vector’ s position relative to a healthy reference population. The variability among individuals in terms of impedance vector is depicted through a bivariate normal distribution with elliptical probability regions (50, 75, and 95%). Vector position on the RXCgraph is interpreted relative to the two directions on the RXCplane. Vector displacements along the major axis of the tolerance ellipse signify progressive alterations in tissue hydration; dehydration is indicated by lengthy vectors outside the upper region of the 50% tolerance ellipse, while fluid overload with apparent edema is characterized by short vectors outside the lower pole of the 50% ellipse.

[0111] In order to compare arm impedance data collected using the disclosed hydration e- tattoo and the elliptical probability regions generated using the healthy reference population, a previously devised RXc-score graph was utilized. This graph transforms vectors to z scores, enabling comparison of data across different impedance analyzers and different populations. Figures 4D and 4F depict BIVA plots for arm impedance and whole-body impedance data, respectively. The elliptical probability regions, in both figures, were generated using the standard reference interval and data for healthy young individuals, as provided by Piccolli et al. (A. Piccoli, L. Pillon, F. Dumler, Nutrition 18, 153-167 (2002), which is fully incorporated by reference). This reference interval utilizes whole-body resistance and reactance values derived from a 50-kHz signal, normalized for height.

[0112] Following the application of the z-transform to each participant’ s arm impedance data, vector displacements, relative to the healthy reference population, could be observed. Figure 4D illustrates paired vectors for each participant, where one vector indicates the participant’s status at the initiation of the protocol, and the other vector indicates the participant’ s status at the conclusion of the protocol. As illustrated, discernible alterations in the RXCvectors were observed for all participants. The directional shifts in the vectors for all participants, except participant 8, were consistent with trends associated with the loss of body water and previous literature utilizing whole-body impedance. Specifically, these changes were characterized by an upward andrightward displacement of resistance and reactance values. In the case of participant 8, the arm impedance BIVA plot exhibited inconsistent directional changes, attributed to minimal changes in reactance over the course of the protocol. Participant 8’s reactance demonstrated relative stability throughout the protocol, while their resistance exhibited trends comparable to those observed in the other participants.

[0113] Figure 4F displays paired vectors representing each participant, using their gathered whole-body impedance data. Similar patterns are evident compared to the arm impedance data, highlighting the inherent capability of arm impedance to monitor hydration status. Moreover, as arm impedance data was continuously collected and BIVA does not necessitate body weight assessment, vectors can be plotted for any given time point, offering a temporally relevant portrayal of hydration status. The benefit of depicting arm impedance and whole-body impedance vectors at various protocol stages using the BIVA plot is that it demonstrates vector displacement toward an increasingly dehydrated state over time (see Figs. 4G - 4N). Figs. 4G - 4N each illustrates a BIVA plot for an individual participant corresponding to discrete measurements shown in Fig. 4A for arm impedance and whole -body impedance data. More transparent data points represent a more dehydrated state.

[0114] Another use of the BIVA plot is that the location of each individual marker relative to the reference ellipses indicates the hydration status of the person at that moment. For example, markers in the third quadrant indicate fluid overload and markers in the first quadrant indicate a dehydrated state. Moreover, the length of the BIVA vector from the starting marker to the end marker can be inversely related to the loss of TBW

[0047] , with shorter vectors indicating minor fluid loss / gain while longer vectors suggesting a larger shift in hydration status. Several studies have found that sweat, skin temperature, electrode placement consistency across applications, and posture variations, can easily influence absolute Bio-Z measurements. Consequently, considering that the length of the vector is inversely related to TBW, previous studies have attempted to leverage vector lengths estimating hydration status change. In this study, vector lengths were computed utilizing the Euclidean distance formula: d = V(%2 - ^i)2+ (y2- yi)2[3] where (xi, yi) represents the vector calculated at the beginning of the protocol for each participant. As noted herein, the relationship between vector length and percent body weight loss across all participants can be derived using equation [3] above. Fig. 40 illustrates the relationship between vector length and percent body weight loss for all participants for arm impedance (left) and wholebody impedance (right) data.

[0115] The relation between TBW change and vector length was assessed using Pearson’s correlation, revealing a strong positive linear correlation for arm data (0.93 ± 0.07, see Fig. 40 left). Conversely, vector lengths determined using whole-body data exhibited weak correlations and significant variability (0.76 ± 0.27, see Fig. 40 right), likely stemming from the same aforementioned factors. A notable advantage of utilizing vector lengths over absolute impedance measurements is their resilience to absolute impedance changes induced by inconsistencies in the placement of the hydration e-tattoo on the brachium, from application to application.

[0116] In addition to the diuretic-induced dehydration investigation, a 24-hour daily living study was conducted on a single human participant. The primary objective of this experiment was to demonstrate the long-term monitoring capabilities of the hydration sensor, assessing its efficacy in monitoring hydration status continuously during routine daily activities Furthermore, the study sought to evaluate the device’s comfort and feasibility of wear during sleep and other daily tasks, as well as to ascertain the sensor’s ability to track the rehydration process subsequent to dehydration.

[0117] Figure 5A presents the bioimpedance data plotted against time for the 24-hour daily living experiment. Body weight measurements were collected using commercial bath scales following any major activity or whenever the participant urinated, ate food, or drank a beverage. The graph delineates various activities, such as sleep, bus travel, and major meal consumption, aiming to elucidate patterns and trends within the data. To account for the common usage of medications in individuals’ daily routines, the diuretic-induced dehydration protocol, as previously discussed, was integrated into the 24-hour daily living protocol. Importantly, the participant’s dietary and fluid intake remained unrestricted, excluding the diuretic-induced dehydration period. Data collection encompassed the entire span of rehydration and food ingestion to capture pertinent information. The red 502 and blue 504 dashed lines in Fig. 5A correspond to the respective red 506 and blue 508 data points illustrated in Figures 5B and 5C.

[0118] As depicted in Fig. 5B, a strong positive linear relationship was discerned during the diuretic dehydration interval, consistent with the trends observed in Fig. 4. Furthermore, Fig. 5C presents the BIVA plot for both the diuretic-induced dehydration and subsequent rehydration phases over the course of the 24-hour experimental protocol. Notably, vector displacements consistent with dehydration and rehydration were observed; transitioning from a hydrated to a dehydrated state during diuretic-induced dehydration, and conversely, from a dehydrated to a more hydrated state as the participant underwent rehydration and ingested food. A noteworthy observation pertains to the plateau observed in arm impedance several hours following the conclusion of the diuretic dehydration protocol. This same behavior is also observed by theapparent grouping of BIVA data points in the upper-right comer of the BIVA plot. These two observations can be attributed to the sustained presence of the oral diuretic in the participant’s body, following the diuretic protocol’s conclusion, coupled with the inherent delay in tissue rehydration subsequent to fluid ingestion.

[0119] Several hours subsequent to the diuretic protocol, and following the participant’s rehydration, a gradual convergence of arm impedance towards the baseline value recorded prior to the initiation of diuretic-induced dehydration was observed. These findings underscore the hydration e-tattoo sensor’s capability to effectively monitor both dehydration and rehydration events within the context of daily living, over extended temporal durations.

[0120] To validate the findings of Figs. 5A - 5D, the same participant underwent a repetition of the protocol three weeks later, utilizing a rigid PCB and commercially available gel electrodes. Figs. 5E - 5H illustrate the outcomes of this repeated test, demonstrating a close alignment with the results depicted in Figs. 5A - 5D. However, the rigid system exhibits more fluctuations in arm impedance data, as indicated by the large error bars. This variability is likely attributed to the system's rigidity, which impedes its ability to stretch or flex in response to the arm’s movements. Additionally, the participant reported electrode delamination around the 20-hour mark despite the absence of a notable spike in the data. This observation suggests that continued use could potentially lead to signal degradation. These outcomes underscore the advantages of employing a flexible sensor equipped with dry electrodes, as it mitigates issues related to rigidity and electrode delamination.

[0121] Figs. 51 - 5L illustrate the diuretic-protocol results conducted using a single-side electrode configuration set up on two participants. Fig. 51 illustrates the single-side electrode configuration setup using a hydration e-tattoo. Fig. 5J illustrates post-processed arm resistance, reactance, and impedance data for the two participants undergoing the diuretic-dehydration protocol. FIG. 5K illustrates a linear relationship between per-participant arm-diameter- normalized arm impedance and percent body weight loss. Fig. 5L illustrates a BIVA plot for arm impedance data for the two participants.

[0122] Figs. 5M - 5P illustrate diuretic -protocol results conducted using a rigid wrist- worn cross-arm configuration setup on a single human participant. Fig. 5M illustrates the cross-arm configuration setup. Fig. 5B illustrates post-processed wrist resistance, reactance, and impedance data undergoing the diuretic-dehydration protocol. Large spikes in data can be due to sudden wrist movements. Fig. 5C illustrates a linear relationship between wrist impedance and percent body weight loss. Fig. 5D illustrates a serial BIVA plot for wrist impedance data, showcasing displacement in line with a minor ellipse axis due to minimal reactance variations.

[0123] Fig. 5Q illustrates a schematic representation of a sensing paradigm of the exemplary system. As shown, contact impedance decreases as a function of frequency (left). Single-channel bioimpedance sensors (e.g., MAX30002) measure tissue impedance by capturing the voltage response due to fixed amplitude alternating current stimulation. Skin-electrode interface impedance along voltage pathways can be avoided due to zero current down these pathways.

[0124] Discussion

[0125] Discussion #1. Water is vital for the human body as it constitutes 45-70% of human body mass, facilitates oxygen and nutrient transport, maintains proper organ function, regulates body temperature, eliminates waste, and supports virtually all other vital physiological processes[I], [2]. Dehydration, defined as a deficit in total body water (TBW) can arise from inadequate fluid intake, excessive sweating, vomiting, and / or diarrhea [3]. Initial symptoms, such as headache and dry mouth, can escalate to more serious conditions if left untreated. The severity of dehydration can be quantified by the percentage change in total body weight caused by water loss. For example, life-threatening symptoms, such as altered respiratory activity or various cardiovascular conditions (e.g., low blood pressure, elevated heart rate), can arise with a 1-2% body weight loss and become pronounced with a 3-5% loss [4]. Considerable cognitive impairment has been observed to begin with a 2% reduction in body weight due to water loss [5], [6]. Dehydration also affects the thermoregulatory capacity of the body by decreasing sweating and cutaneous blood flow. For instance, core body temperature increases by 0.15-0.20 °C for every 1% decrease in body weight due to fluid loss [7]. Reduced thermoregulation increases the risk of developing heat-related injuries, such as muscle cramps, fatigue, or heatstroke [8]. Chronic mild dehydration is associated with an increased risk of kidney stone formation [9],

[0010] .

[0126] While the negative effects of dehydration and the importance of rehydration are known, a portion of the population is still at risk. For example, persons working in extreme temperature environments (e.g., firefighters, armed forces personnel) are more likely to experience dehydration. The estimated fluid loss for these individuals may exceed 1 L / h solely by perspiration[I I]. Athletes are also susceptible to dehydration due to increased sweat production, which can exceed 1 L / h [3]. When combined with heat stress, dehydration impairs cognitive functions, athletic performance, and technical skills related to sports

[0012] . Even minor levels of dehydration can be critical, particularly at the elite level. Consequently, athletes, firefighters, and armed forces personnel would benefit from continuous hydration assessment to optimize performance, prevent injury, and aid recovery.

[0127] FIGS. 6 A - 6C each illustrates the conventional method for dehydration assessment. Common practices for assessing dehydration typically involve qualitative methods, such ascomparing the physical appearance of urine with a color chart (see Fig. 6A) [2]. However, these methods are susceptible to contamination (e.g., food, medicines) and human errors and are unsuitable for monitoring rapid changes in hydration status. Quantitative assessment of a person’s whole body hydration (WBH) level relies on blood or urine tests, such as blood, plasma, or urine osmolality tests in labs (see Fig. 6B). Although these results are accurate, they are time-consuming and costly and only offer intermittent data points. The lack of a real-time, mobile WBH assessment approach hinders decision-making and initiation of necessary rehydration measures.

[0128] Bioelectrical impedance (i.e., bioimpedance signal, Bio-Z) provides a quantitative and non-invasive approach to evaluate WBH [2],

[0013] . Bio-Z capitalizes on the deep tissue penetration capability of alternating current (AC) to extract valuable physiological information and can distinguish between intracellular and extracellular fluids [14-16]. The setup of commercial equipment for the detection of WBH involves placing electrodes on the hands and feet to measure the whole-body Bio-Z as the AC current flows through the body from hands to feet (Fig. 6C). High-frequency AC can pass through both extracellular fluids (ECF) and intracellular fluids (ICF) (see Fig. 1C)

[0017] . However, the equipment is stationary and expensive, making it impractical for daily use. Furthermore, the precision of Bio-Z analysis (BIA), which uses multiple regression equations to predict fluid volumes, including TBW and extracellular water (ECW), experiences biased estimates due to being linked to normal body weight and composition and relying on a combination of parameters (e.g., gender, age, weight) that may induce errors

[0018] . Consequently, detecting changes in body water using BIA is limited in both healthy individuals and clinical populations

[0019] ,

[0020] . An improved approach, named bioelectrical impedance vector analysis (BIVA), separates the resistance (R) and reactance (Xc) components of Bio-Z and utilizes the vectorial changes in the RXCplot to chart changes in hydration and body composition (see Fig. ID) [21-27]. BIVA, first introduced by Piccoli et al., uses the bivariate distribution of the whole-body electrical resistance and reactance recorded from a healthy reference population of 7722 males and 8181 females, known as the National Health and Nutrition Examination Survey III (NHANES III), to provide a qualitative indication of hydration, without any assumptions about body components or prediction models

[0022] . However, the BIVA approach is based on resistance and reactance measured from hands to feet or wrists to ankles, which limits its practicality for continuous ambulatory monitoring. The potential of using localized deep tissue BIVA for the continuous assessment of WBH has never been explored due to the lack of suitable sensors.

[0129] Advances in skin-soft and skin-conformable epidermal electronics, i.e., e-tattoos, have spurred the development of wearable Bio-Z sensors for the purpose of measuring skin hydration [28-34]. Skin-conformable gold nanomembranes

[0032] and breathable nano-mesh electrodes

[0035] have been used to detect skin hydration levels without altering skin functions. Despite their effectiveness, their bipolar skin-impedance measurement configuration and small electrode spacing restricted them from tracking the hydration of superficial skin layers. Other types of wearable biosensors (e.g., those based on micro-fluidics) have been developed for sweat and hydration sensing

[0036] ,

[0037] .

[0130] The study introduced the exemplary system based on tetrapolar Bio-Z sensing with strategically placed electrodes across the arm (see Fig. 1A). Two pairs of temporary tattoo-like, skin-conformable electrodes for high-frequency AC injection and voltage sensing allow cross-arm Bio-Z measurement in the transmission mode (see Fig. IB). The measured resistance and reactance are wirelessly transmitted to a smartphone via Bluetooth Low Energy (BLE) in real time. To avoid complications in Bio-Z detection caused by temperature or motion, the study designed a diuretic- induced dehydration protocol that requires minimal movements and temperature fluctuation in an indoor setting (see Fig. IE). The cross-arm Bio-Z and BIVA obtained by the exemplary system match the whole-body Bio-Z and BIVA measured by a state-of-the-art impedance analyzer. To demonstrate the wearability and continuous sensing capability of the exemplary system, the study also conducted a 24-hour measurement under free-living conditions (see Fig. IE). For the first time, the study demonstrated the feasibility and effectiveness of a crossarm Bio-Z sensor for continuous WBH assessment, improving awareness of hydration and facilitate proactive health management in everyday life or for high-risk and high-activity populations.

[0131] Discussion #2. Compared to BIA, BIVA does not rely on multiple regression equations used to predict fluid volumes, including TBW and extracellular water (ECW), and soft tissue composition (FM and FFM). The use of prediction equations for TBW and FFM relies on parameters such as Height2 / Resistance, body weight, age, gender, reactance, and height

[0050] , and additionally relies on the assumption of constant hydration of the FFM

[0019] . Moreover, the models and equations for BIA are derived in healthy people and tend to yield inaccurate values of soft tissue composition for individuals with fluid variation. Due to these inherent limitations of BIA, Piccoli et al. introduced the phase-sensitive RXCgraph method that is based on the analysis of the bivariate distribution of the impedance vector in healthy populations, obese individuals, and individuals suffering from various renal diseases. Unlike BIA, BIVA does not rely on making assumptions about body composition

[0022] .

[0132] BIVA is a noninvasive property-based diagnostic method used to assess body composition and hydration status by measuring the resistance ( / ?) and reactance (Xc) of biological tissue in a similar manner to BIA. Also similar to BIA, BIVA involves the application of electrodes to the arms and legs that pass a current through the body, with the resulting Bio-Z being acomposite measure of both resistance, which reflects the opposition to current flow due to ICF and ECF, and reactance, which represents the capacitive properties of cell membranes. However, in contrast to BIA, BIVA considers the R and X(components separately. The measured values are plotted on the RXCgraph, providing a graphical representation that provides a qualitative indication of fluid distribution and body cell mass. The RXCgraph is constructed using a reference dataset and generates elliptical probability regions to qualitatively indicate hydration status and soft tissue composition. In this study, the elliptical probability regions are generated using the standard reference interval of the NHANES III dataset

[0023] for healthy young individuals, as provided by Piccolli et al. (51 ). NHANES III includes whole-body resistance and reactance recordings from approximately 15,900 adults derived from a 50-kHz signal, normalized for height to control for the different stature of individuals. Repeated RXCmeasurements, collected over long temporal durations, capture the intrasubject variability by using the intersubject variability of data from the reference population.

[0133] Previous studies employed BIVA to assess extracellular and intracellular dehydration

[0021] , monitor body composition during pregnancy

[0024] , discern variations in hydration status and body composition among university athletes

[0025] ,

[0026] , as well as monitor the treatment of severe acute malnutrition in children

[0027] .

[0134] Discussion #3. A cohort of nine healthy participants, aged 19 to 29, participated in the diuretic-induced dehydration experiment in the study, while a single male, aged 26, was selected for the long-term daily living study. Participant selection considered diverse body mass index and musculature to explore their potential impact on the performance of the e-tattoo. Individuals who were selected to participate in the diuretic-induced experiment protocol were asked to refrain from drinking alcohol and caffeine 24 hours prior to data collection to prevent any external influence on hydration status. Following the completion of the study, participants were provided with an electrolyte solution (Pedialyte) to replenish lost fluids and restore electrolyte balance.

[0135] Conclusion

[0136] In summary, described herein are embodiments of a wearable, bioimpedance-based, electronic tattoo capable of assessing whole-body hydration change in a non-invasive manner. COMSOL FEM electrode configuration and spacing simulations, as well as experimental validation, were conducted for FPC layer design and optimizing bioimpedance sensitivity in underlying human tissue. A diuretic-induced dehydration pilot study involving eight participants demonstrated the device’s applicability and heightened sensitivity for on-demand, wearable, and non-invasive whole-body hydration assessment. Results from a 24- hour daily living experimentfurther emphasized the device’s capability to capture both dehydration and rehydration trends on a single participant.

[0137] In the specification and / or figures, typical embodiments have been disclosed. The present disclosure is not limited to such exemplary embodiments. Those skilled in the art will also appreciate that various adaptations and modifications of the preferred and alternative embodiments described above can be configured without departing from the scope and spirit of the disclosure.

[0138] The use of the term “and / or” includes any and all combinations of one or more of the associated listed items. The figures are schematic representations and so are not necessarily drawn to scale. Unless otherwise noted, specific terms have been used in a generic and descriptive sense and not for purposes of limitation.

[0139] While the methods and systems have been described in connection with preferred embodiments and specific examples, it is not intended that the scope be limited to the particular embodiments set forth, as the embodiments herein are intended in all respects to be illustrative rather than restrictive.

[0140] Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that an order be inferred, in any respect. This holds for any possible nonexpress basis for interpretation, including: matters of logic with respect to arrangement of steps or operational flow; plain meaning derived from grammatical organization or punctuation; the number or type of embodiments described in the specification.

[0141] Throughout this application, various publications may be referenced. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which the methods and systems pertain.

[0142] It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope or spirit. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit being indicated by the following claims.

[0143] The following patents, applications and publications as listed below and throughout this document are hereby incorporated by reference in their entirety herein.[1] R. A. Oppliger, C. Bartok, Sports Medicine 32, 959-971 (2002).[2] D. C. Garrett et al., IEEE reviews in biomedical engineering 11, 233-248 (2017).[3] E. J'equier, F. Constant, European journal of clinical nutrition 64, 115-123 (2010).[4] S. M. Kleiner, Journal of the American Dietetic Association 99, 200-206 (1999).[5] P. Gopinathan, G. Pichan, V. Sharma, Archives of Environmental Health: An International Journal 43, 15-17 (1988).[6] N. A. Masento, M. Golightly, D. T. Field, L. T. Butler, C. M. van Reekum, British Journal of Nutrition 111, 1841-1852 (2014).[7] S. J. Montain, E. F. Coyle, Journal of applied physiology 73, 1340-1350 (1992).[8] T. Nishiyasu, X. Shi, G. W. Mack, E. R. Nadel, Journal of Applied Physiology 71, 1382— 1386 (1991).[9] L. E. Armstrong, Nutrition reviews 70, S121-S127 (2012).

[0010] A. M. El-Sharkawy, O. Sahota, D. N. Lobo, Nutrition reviews 73, 97-109 (2015).

[0011] G. P. Bates, V. S. Miller, D. M. Joubert, Annals of occupational hygiene 54, 137-143 (2010).

[0012] R. P. Nuccio, K. A. Barnes, J. M. Carter, L. B. Baker, Sports Medicine 47, 1951-1982 (2017).

[0013] O. R. Barley, D. W. Chapman, C. R. Abbiss, Journal of the International Society of Sports Nutrition 17, 1-13 (2020).

[0014] S. N. Cheuvront, R. W. Kenefick, Comprehensive Physiology 4, 257-285 (2011).

[0015] K. Mange et al., Annals of internal medicine 127, 848-853 (1997).

[0016] S. F. Khalil, M. S. Mohktar, F. Ibrahim, Sensors 14, 10895-10928 (2014).

[0017] U. G. Kyle et al., Clinical nutrition 23, 1226-1243 (2004).

[0018] A. Piccoli et al., International journal of obesity 22, 97-104 (1998).

[0019] A. C. Buchholz, C. Bartok, D. A. Schoeller, Nutrition in Clinical Practice 19, 433-446 (2004).

[0020] C. O’brien, A. Young, M. Sawka, International journal of sports medicine 23, 361-366 (2002).

[0021] K. R. Heavens, N. Charkoudian, C. O’Brien, R. W. Kenefick, S. N. Cheuvront, The American Journal of Clinical Nutrition 103, 724-729 (2016).

[0022] A. Piccoli, B. Rossi, L. Pillon, G. Bucciante, Kidney international 46, 534-539 (1994).

[0023] U. D. of Health, H. Services, et al., Centers for Disease Control and Prevention (1996).

[0024] A. Moroni et al., European Journal of Clinical Nutrition 76, 604-609 (2022).

[0025] P. C. Martins, L. A. Gobbo, D. A. S. Silva, Journal of the International Society of Sports Nutrition 18, 1-8 (2021).

[0026] M. Carrasco-Marginet et al., PloS one 12, e0178819 (2017).

[0027] T. Girma et al., Clinical Nutrition 40, 624-631 (2021).

[0028] H. Li et al., Chemical Reviews 124, 3220-3283 (2024).

[0029] S. Yang et al., Adv. Mater 27, 6423-6430 (2015).

[0030] Y. Wang et al., npj Flexible Electronics 2, 6 (2018).

[0031] X. Huang et al., Advanced Functional Materials 24, 3845-3845 (2014).

[0032] X. Huang et al., IEEE transactions on biomedical engineering 60, 2848-2857 (2013).

[0033] H. Cheng, Y. Zhang, X. Huang, J. A. Rogers, Y. Huang, Sensors and Actuators A: Physical 203, 149-153 (2013).

[0034] X. Huang, W.-H. Yeo, Y. Liu, J. A. Rogers, Biointerphases 7 (2012).

[0035] R. Matsukawa, A. Miyamoto, T. Yokota, T. Someya, Advanced Healthcare Materials 9, 2001322 (2020).

[0036] J. C. Spinelli et al., npj Digital Medicine 8, 76 (2025).

[0037] S. Cho et al., Science translational medicine 16, eado5366 (2024).

[0038] D. Kireev et al., Nature Nanotechnology 17, 864-870 (2022).

[0039] K. Sei et al., npj Digital Medicine 6, 59 (2023).

[0040] C. Gabriel, S. Gabriel, Compilation of the dielectric properties of body tissues at RF and microwave frequencies, 1996.

[0041] I. Lorenzo, M. Serra-Prat, J. C. Y'ebenes, Nutrients 11, 1857 (2019).

[0042] S. Bhattacharya et al., A Chest-conformable, wireless electro-mechanical E-tattoo for measuring multiple cardiac time intervals. Advanced Electronic Materials. 2023: 2201284.

[0043] I. E. Commission et al., IEC 60601-1: 2005 (2005).

[0044] J. R. Caton, P. A. Mole, W. C. Adams, D. S. Heustis, Medicine and science in sports and exercise 20, 489-491 (1988).

[0045] M. J. Buono et al., Physiological Measurement 25, 119 (2004).

[0046] A. Piccoli, Cardiovascular Disorders in Hemodialysis 149, 150-161 (2005).

[0047] H. C. Lukaski, C. B. Hall, W. A. Siders, Nutrition 23, 543-550 (2007).

[0048] B. Sun et al., Advanced Materials 30, 1804327, eprint: https : / / advanced. onlinelibrary .wiley . com / doi / pdf / 10.1002 / adma.201804327 , (https: / / advanced.onlinelibrary.wiley.com / doi / abs / 10.1002 / adma.201804327) (2018).

[0049] B. Zhang et al., en, Nature 628, 84-92 (Mar. 2024).

[0050] L. B. Houtkooper, T. G. Lohman, S. B. Going, W. H. Howell, The American journal of clinical nutrition 64, 436S-448S (1996).

[0051] A. Piccoli, L. Pillon, F. Dumler, Nutrition 18, 153-167 (2002).

Claims

AMENDED CLAIMS received by the International Bureau on 12 August 2025 (12.08.2025)CLAIMSWhat is claimed is:

1. A stacked-design, conformable, and mobile sensor, comprising: a first flexible, stretchable insulating substrate comprising a first side and a second side opposite the first side, wherein the first side adheres to an epidermis and the first flexible, stretchable insulating substrate conforms to the epidermis; two or more current injection electrodes and one or more voltage sensing electrodes, wherein each of the two or more current injection electrodes and one or more voltage sensing electrodes are comprised of biocompatible materials and wherein each electrode of the two or more current injection electrodes and one or more voltage sensing electrodes forms an electrode pattern on the first side of the first flexible, stretchable insulating substrate and each electrode is configured to flex and stretch with the first flexible, stretchable insulating substrate to substantially be in contact with and conform to the epidermis; an electrically-insulated flexible printed circuit (FPC) layer having a first side and a second side, said first side substantially in contact with the second side of the first flexible, stretchable insulating substrate; electronics disposed at least partially on the second side of the FPC layer, said electronics comprising at least a wireless communications interface, a central processing unit (CPU), an analog front-end (AFE) linked to the CPU, and a power source, wherein components of the electronics are separated from one another on the second side of the FPC layer and connected to one another using conductive traces configured to flex and stretch with the first flexible, stretchable insulating substrate and / or the FPC layer, and to conform to the epidermis, wherein the electronics are connected to the two or more current injection electrodes and one or more voltage sensing electrodes using conductive material in the first flexible, stretchable insulating substrate, or conductive material in both the first flexible, stretchable insulating substrate and the FPC layer.

2. The stacked stacked-design, conformable, and mobile sensor of claim 1, wherein one or more of the two or more current injection electrodes or one or more of the voltage sensing electrodes are comprised of graphite polyurethane film andlaminated onto the first side of the first flexible, stretchable insulating substrate, and wherein the two or more current injection electrodes and the voltage sensing electrodes are spaced apart by a distance, d, and d is approximately 2 cm.

3. The stacked-design, conformable and mobile sensor of any one of the above claims, wherein the second side of the FPC layer is covered with a third flexible substrate that covers the first flexible, stretchable insulating substrate and the FPC layer, and wherein the third flexible substrate defines one or more holes that expose the power source mounted on the second side of the FPC layer so that the power source can be replaced.

4. The stacked-design, conformable and mobile sensor of any one of the above claims, wherein the wireless communications interface is used to wirelessly transmit data from the sensor to a designated receiver, and the designated receiver comprises a smartphone.

5. The stacked-design, conformable and mobile sensor of any one of the above claims, wherein the electronics mounted on the second side of the FPC layer have been patterned into a plurality of islands and connected via conductive serpentine tracks, and wherein the sensor is configured to attach to the epidermis on a brachium of a person and one of the plurality of islands comprises a central island housing passive components of the electronics, said central island configured to sit centrally on a bicep of the person.

6. The stacked-design, conformable and mobile sensor of claim 5, wherein the two or more current injection electrodes are used to inject a low-amplitude, high-frequency AC current into the epidermis, and wherein the one or more voltage sensing electrodes are used to sense voltage induced in a body of a person by the injected AC current.

7. The stacked-design, conformable and mobile sensor of claim 6, wherein the injected current and the induced voltage are used to determine bioimpedance, wherein saidbioimpedance fluctuates in response to shifts in body fluids of the person and the bioimpedance is used to determine whole-body hydration of the person.

8. The stacked-design, conformable and mobile sensor of claim 7, wherein the bioimpedance is used to determine whole-body hydration of the person by correlating a relative change in bioimpedance (from some starting point) over time to a percent body weight loss over time.

9. The stacked-design, conformable and mobile sensor of any one of the above claims, wherein the two or more current injection electrodes and the one or more voltage sensing electrodes and the first flexible, stretchable insulating substrate can be separated from the electronics and FPC layer and discarded such that the electronics and the FPC layer can be reused to form a second stacked-design, conformable and mobile sensor.

10. A method for using a stacked-design, conformable and mobile sensor to determine whole-body hydration, the method comprising: attaching a stacked-design, conformable and mobile sensor to an epidermis, said stacked-design, conformable and mobile sensor comprising: a first flexible, stretchable insulating substrate comprising a first side and a second side opposite the first side, wherein the first side adheres to an epidermis and the first flexible, stretchable insulating substrate conforms to the epidermis; two or more current injection electrodes and one or more voltage sensing electrodes, wherein each of the two or more current injection electrodes and one or more voltage sensing electrodes are comprised of biocompatible materials and wherein each electrode of the two or more current injection electrodes and one or more voltage sensing electrodes forms an electrode pattern on the first side of the first flexible, stretchable insulating substrate and each electrode is configured to flex and stretch with the first flexible, stretchable insulating substrate to substantially be in contact with and conform to the epidermis;an electrically-insulated flexible printed circuit (FPC) layer having a first side and a second side, said first side substantially in contact with the second side of the first flexible, stretchable insulating substrate; electronics disposed at least partially on the second side of the FPC layer, said electronics comprising at least a wireless communications interface, a central processing unit (CPU), an analog front-end (AFE) linked to the CPU, and a power source, wherein components of the electronics are separated from one another on the second side of the FPC layer and connected to one another using conductive traces configured to flex and stretch with the first flexible, stretchable insulating substrate and / or the FPC layer, and to conform to the epidermis, wherein the electronics are connected to the two or more current injection electrodes and one or more voltage sensing electrodes using conductive material in the first flexible, stretchable insulating substrate, or conductive material in both the first flexible, stretchable insulating substrate and the FPC layer; and injecting, using the one or more current injection electrodes, a low-amplitude, high- frequency AC current into the epidermis; sensing, using the one or more voltage sensing electrodes, voltage induced in a body of a person by the injected AC current; receiving, by one or more processors, data transmitted from the stacked-design, conformable and mobile sensor, said data comprising at least the injected current and the induced voltage sampled periodically and concurrently; and determining, by the one or more processors, bioimpedance of the person using the injected current and the induced voltage, wherein the bioimpedance is used to determine whole-body hydration of the person.

11. The method of claim 10, wherein two or more current injection electrodes or one or more of the voltage sensing electrodes are comprised of graphite polyurethane film and laminated onto the first side of the first flexible, stretchable insulating substrate, wherein the current injection electrodes and the voltage sensing electrodes are spaced apart by a distance, d, and d is approximately 2 cm.

12. The method of claim 10, wherein the second side of the FPC layer is covered with a third flexible substrate that covers the first flexible, stretchable insulating substrate and / or the FPC layer, wherein the third flexible substrate defines one or more holes that expose the power source mounted on the second side of the FPC layer so that the power source can be replaced.

13. The method of claim 10, wherein the wireless communications interface is used to wirelessly transmit data from the sensor to a designated receiver, wherein the designated receiver comprises a smartphone.

14. The method of claim 10, wherein the electronics mounted on the second side of the FPC layer have been patterned into a plurality of islands and connected via conductive serpentine tracks, and wherein the sensor is configured to attach to the epidermis on a brachium of a person and one of the plurality of islands comprises a central island housing passive components of the electronics, said central island configured to sit centrally on a bicep of the person.

15. The method of claim 14, wherein the two or more current injection electrodes and the one or more voltage sensing electrodes are configured to be attached to the brachium in a cross-arm configuration.

16. The method of claim 10, wherein the bioimpedance fluctuates in response to shifts in body fluids of the person, wherein bioimpedance is used to determine whole-body hydration of the person by correlating a relative change in bioimpedance (from some starting point) over time to a percent body weight loss over time.

17. The method of claim 10, wherein the two or more current injection electrodes and the one or more voltage sensing electrodes and the first flexible, stretchable insulating substrate can be separated from the electronics and FPC layer and discarded such that the electronics and the FPC layer can be reused to form a second stacked-design, conformable and mobile sensor.

18. A system for using a stacked-design, conformable and mobile sensor to determine whole-body hydration, the system comprising: a stacked-design, conformable and mobile sensor, wherein the stacked-design, conformable and mobile sensor is attached to an epidermis of a person, said stacked-design, conformable and mobile sensor comprising: a first flexible, stretchable insulating substrate comprising a first side and a second side opposite the first side, wherein the first side adheres to the epidermis and the first flexible, stretchable insulating substrate conforms to the epidermis; two or more current injection electrodes and one or more voltage sensing electrodes, wherein each of the two or more current injection electrodes and one or more voltage sensing electrodes are comprised of biocompatible materials and wherein each electrode of the two or more current injection electrodes and one or more voltage sensing electrodes forms an electrode pattern on the first side of the first flexible, stretchable insulating substrate and each electrode is configured to flex and stretch with the first flexible, stretchable insulating substrate to substantially be in contact with and conform to the epidermis; an electrically-insulated flexible printed circuit (FPC) layer having a first side and a second side, said first side substantially in contact with the second side of the first flexible, stretchable insulating substrate; electronics disposed at least partially on the second side of the FPC layer, said electronics comprising at least a wireless communications interface, a central processing unit (CPU), an analog front-end (AFE) linked to the CPU, and a power source, wherein components of the electronics are separated from one another on the second side of the FPC layer and connected to one another using conductive traces configured to flex and stretch with the first flexible, stretchable insulating substrate and / or the FPC layer, and to conform to the epidermis, wherein the electronics are connected to the two or more current injection electrodes and one or more voltage sensing electrodes using conductive material in the first flexible, stretchable insulating substrate, orconductive material in both the first flexible, stretchable insulating substrate and the FPC layer, wherein the two or more current injection electrodes are used to inject a low-amplitude, high-frequency AC current into the epidermis, and wherein the one or more voltage sensing electrodes are used to sense voltage induced in a body of the person by the injected AC current; and one or more processors, wherein the one or more processors receive data transmitted from the stacked-design, conformable and mobile sensor, said data comprising at least the injected current and the induced voltage sampled periodically and concurrently, and determine bioimpedance of the person using the injected current and the induced voltage, wherein the bioimpedance is used by the one or more processors to determine whole-body hydration of the person.

19. The system of claim 18, wherein the sensor is configured to attach to the epidermis on a brachium of the person, and wherein the two or more current injection electrodes and the one or more voltage sensing electrodes are configured to be attached to the brachium in a cross-arm configuration.

20. The system of claim 18, wherein the bioimpedance fluctuates in response to shifts in body fluids of the person, and wherein the bioimpedance is used to determine whole-body hydration of the person by correlating a relative change in bioimpedance (from some starting point) over time to a percent body weight loss over time.

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