A wireless forehead e-tattoo for decoding mental workload

Abstract

Disclosed and described herein are systems, methods and devices comprising a non-invasive, thin, and wireless forehead EEG and EOG wearable (forehead e-tattoo) designed to be as flexible and conformable to the skin as a temporary tattoo. The disclosed device integrates adhesive PEDOT:PSS composite coated graphite-deposited polyurethane electrodes (APC-GPU), flexible electronics, and wireless data monitoring and recording capabilities.

Description

Attorney Docket No.10046-619WO1 Client Ref. No.8422 LU A WIRELESS FOREHEAD E-TATTOO FOR DECODING MENTAL WORKLOAD CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to and benefit of U.S. provisional patent application number 63 / 667,903 filed July 5, 2024, which is fully incorporated by reference and made a part hereof. GOVERNMENT SUPPORT

[0002] This invention was made with government support under Grant no. W911NF-19-2- 0333 awarded by the Army Research Office. The government has certain rights in the invention. BACKGROUND

[0003] Real-time monitoring of an operator’s cognitive state through psychophysiological indices is critical to enhancing the safety and performance of human-in-the-loop systems. However, traditional laboratory-grade electroencephalography (EEG) and electrooculography (EOG) devices, with their extensive wired electrodes and time-intensive applications, present limitations for monitoring during daily activities.

[0004] Human mental workload is a key variable of interest in the fields of human-machine interaction and ergonomics due to their direct implications on human performance. Although its definition varies, mental workload can be generally described as the level at which a person’s working memory capacity and cognitive processes are engaged by the task at hand. At low mental workload levels, the person is not engaged and could be careless; at high mental workload levels, the person is overwhelmed and loses control. As such, it is of interest to designers of human-in- the-loop systems to manage a user’s mental workload level to optimize task performance. This becomes particularly important in high-complexity, safety-critical tasks where reductions in task performance can and have resulted in devastating losses of lives and assets. In such contexts, mental workload has been directly correlated with accident probability. This has motivated various studies in mental workload assessment, especially in drivers, aircraft pilots, air traffic controllers, and the like, and includes development of the NASA Task Load Index (NASA-TLX) questionnaire, which is a self-reported questionnaire still serving as the gold standard of mental workload estimation.

[0005] Unfortunately, while the importance of mental workload in human performance is well established, its measurement is not. Mental workload is a multidimensional and nonstationaryAttorney Docket No.10046-619WO1 Client Ref. No.8422 LU variable that is determined by a combination of factors including: the inherent nature and difficulty of the task, the circumstances under which a person performs the task, as well as the current skill and fatigue level of the operator. Currently, survey-based self-reported measures and observation of task performance are two typical ways to evaluate mental workload quantitatively. Self- assessment suffers from extremely low temporal resolution, intrusiveness to task performance, subjective bias, and conflict of interest as a person may be incentivized to report workload levels deemed suitable for their performance. Meanwhile, mere observation of task performance can inherently only be assessed after a task attempt, making it unacceptable in any safety-critical context.

[0006] Combining multiple physiological indices has been proposed as a promising alternative to quantify mental workload due to their high sensitivity, temporal resolution, and lack of subjective bias. Mental workload has been linked to peripheral nervous system signals such as heart rate and electrodermal activity, which is convenient but lacks direct relevance to mental workload. Electroencephalography (EEG) is the primary signal modality for physiological mental workload monitoring, but conventional EEG measurement systems are bulky, wired, and uncomfortable. Ocular activity such as blinks, saccades, and pupillary response has also been strongly associated with mental workload assessment. Optical eye-tracking is commonly used but has restrictive requirements such as the fixation of the operator’s general head direction towards external sensor(s) and the lack of optical confounders such as certain types of eyewear, which may be unrealistic in real-world task performance scenarios. Meanwhile, electrooculography (EOG) has no such restrictions while providing rich blink and saccade information2425 in high temporal resolution, which have been correlated with drowsiness / fatigue, vigilance, and mental workload.

[0007] Despite the desirability and demonstrated success of electrophysiology-based mental workload estimation, recording hardware has limited its widespread use. Wired recording systems restrict the user’s movement, both physically and by the occurrence of significant motion artifacts, and are also uncomfortable. Discomfort and restrictions on user movement also inherently modify the task environment, reducing task performance and undermining the purpose of mental workload estimation. Systems based on conventional gel electrodes require long setup times and suffer from signal quality degradation after a few hours, limiting potential applications.

[0008] Recent progress in the development of wireless wearable devices based on dry electrodes has increased the practical applicability of EEG. These devices are commonly based on a headband-like form factor, which is convenient and comfortable but allows relative dislocation of electrodes between and during recordings, which increases the risk of bad channels and furtherAttorney Docket No.10046-619WO1 Client Ref. No.8422 LU reduces the already poor spatial resolution of the EEG collected. Recent studies have proposed devices with alternative form factors for EEG and EOG measurement such as headphones for mental workload estimation. These devices are also not low-profile, making them incompatible with external hardware such as helmets in the case of vehicle operators, much like the case of traditional EEG caps.

[0009] To combat the abovementioned issues, many epidermal electronics-based devices capable of measuring forehead electrophysiology have been developed. Such systems feature dry electrodes integrated on a soft substrate to form stable and conformal contact with the skin. However, they are not always integrated with the appropriate electronics needed to complete data acquisition, not in line with standard EEG measurement practices: absence of human body grounding, have a very low number of recording channels, or rely only on a single signal modality. These factors are disadvantageous for successful mental workload estimation in real-world task environments.

[0010] 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 an ultrathin flexible wireless e-tattoo system for EEG and EOG-based mental workload estimation which can be easily applied on the forehead. SUMMARY

[0011] Disclosed and described herein are embodiments of a device, system and method of an ultrathin flexible wireless e-tattoo system for EEG and EOG-based mental workload estimation that can be easily applied on the forehead. In some aspects, adhesive PEDOT:PSS composite coated graphite-deposited polyurethane (APC-GPU) electrodes are integrated on a highly conformal medical dressing and provide ideal electrode-skin interfaces for long-term, low-noise forehead EEG and EOG measurements. In some instances, a stretchable flexible printed circuit board is integrated within the e-tattoo and features the lowest power consumption among similar flexible devices. Aspects of the device provide an all-in-one systemic solution for wearable long- term mental workload monitoring, and this capability is demonstrated herein by the successful estimation of known mental workload levels in six human subjects induced by a dual N-back task, a gold standard for standardized mental workload testing.

[0012] 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.Attorney Docket No.10046-619WO1 Client Ref. No.8422 LU BRIEF DESCRIPTION OF THE DRAWINGS

[0013] 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:

[0014] Figures 1A and 1B illustrate photographs of an exemplary wireless forehead EEG and EOG e-tattoo for mental workload estimation where Figure 1A is the front and Figure 1B is a side view.

[0015] Figure 1C illustrates a photograph of an exemplary wireless forehead EEG and EOG e-tattoo for mental workload estimation showing EEG electrodes and flexible data acquisition (DAQ) circuits on the forehead.

[0016] Figure 1D illustrates a photograph of an exemplary wireless forehead EEG and EOG e-tattoo for mental workload estimation showing EOG electrodes.

[0017] Figure 1E illustrates a photograph of an exemplary wireless forehead EEG and EOG e-tattoo for mental workload estimation showing reference and ground electrodes that were conformally applied to the mastoids.

[0018] Figure 1F illustrates an exploded view of an exemplary wireless forehead EEG and EOG e-tattoo for mental workload estimation showing layers of the flexible DAQ circuits, where island-bridge design and silicone encapsulation increase the stretchability and flexibility by strain isolation.

[0019] Figure 1G is an exemplary illustration of an exploded assembly view of the sensor patch and the flexible DAQ circuits.

[0020] Figure 1H illustrates a comparison of an embodiment of the disclosed wearable system with other forehead EEG or EOG wearable systems.

[0021] Figures 2A-2I illustrate characteristics of an exemplary APC-GPU electrode, where Figure 2A is a schematic of the molecular structures and interactions of the APC, including PEDOT:PSS, citric acid (H3Cit), poly (vinyl alcohol) (PVA), and β-cyclodextrin (β-CD), and the involved molecular interactions; Figure 2B is a SEM image of the cross-sectional view of the APC-GPU electrode showing conformable coating of APC on GPU; Figure 2C is a SEM image of APC-GPU electrode gaplessly conformed on pig skin due to the APC coating; Figure 2DAttorney Docket No.10046-619WO1 Client Ref. No.8422 LU illustrates stress-strain curves of GPU and APC-GPU films with various PEDOT:PSS mass ratios (1.8%, 3.6%, 7.1%, 13.2% and 22.3%); Figure 2E illustrates a comparison of the skin adhesion forces of gel and APC-GPU films measured by the 90°peel test; Figure 2F illustrates sheet resistance of GPU and APC-GPU films; Figure 2G illustrates area-specific skin contact impedance of conventional gel electrode and APC-GPU electrodes (with PEDOT:PSS mass ratio of 3.6%) right after application; Figure 2H illustrates skin-electrode contact impedance sweep of APC-GPU electrode at 0 hr, 1 hr, and 5 hr after application; and Figure 2I illustrates change of contact impedance of APC-GPU electrode over 5 hours with confidence interval set at 95% (shadowed area).

[0022] Figures 3A-3D illustrate characteristics of flexible DAQ circuits, where Figure 3A is an image of an exemplary flexible DAQ circuit, which comprises a power management module, analog front-end (AFE) and analog to digital converter (ADC) module, and Bluetooth (BLE) module with a low profile FPC connector to connect the power management module to a rechargeable Li-Po battery; Figure 2B is an exemplary block diagram of physiological DAQ, BLE data transmission, and data analysis for mental workload estimation; Figure 3C illustrates a comparison of EEG signal power spectra showing that an implementation of the closed-loop right- leg drive (RLD) configuration achieved an 85.7 % reduction in the body-coupled 60 Hz noise compared to the open-loop RLD alternative under heavy powerline contamination conditions expected in out-of-the-laboratory environments; Figure 3D illustrates that a 150 mAh battery could last beyond 28 hours given a system average current draw of 5.25 mA (average 5.23 mA in advertising phase, 5.26 mA in active data transmission).

[0023] Figures 4A-4F illustrate signal quality validation of an exemplary embodiment of the disclosed forehead e-tattoo and comparison against a lab-grade EEG device (i.e., Brain Vision EEG device), where Figures 4A-4D illustrate Alpha band filtered EEG and spectrograms from (A, B) Brain Vision and (C, D) forehead e-tattoo simultaneously measured during eye open-closed test; Figure 4E illustrates horizontal and vertical EOG measured from forehead e-tattoo under various eye movements and blinks; and Figure 4F illustrates Brain Vision (red – higher amplitude lines) and e-tattoo (light blue – lower amplitude lines) measured EEG under various head movements, facial expressions, and ambulatory movements.

[0024] Figures 5A-5E illustrate experimental results of a dual N-back experiment paradigm and the feature analysis of an exemplary embodiment of the disclosed forehead e-tattoo, where Figure 5A is a diagram of experiment paradigm and dual N-back trial design; Figure 5B illustrates average self-assessed workload level based on the sum of all six NASA-TLX questionnaire ratingsAttorney Docket No.10046-619WO1 Client Ref. No.8422 LU across all trials and subjects showing all differences between the N-back difficulties are significant, except for the difference between 0 and 1-back (p = 0.053); Figure 5C is a comparison of TLX- based self-assessed performance and N-back task accuracy; Figure 5D illustrates changes in average behavioral performances across trials and subjects for each N-back task difficulty; and Figure 5E illustrates average EEG band powers and EOG features for each N-back task difficulty showing the only significant differences (p < 0.05) between the conditions are indicated. ABP (Average blink peak), ASD (Average saccade duration), where all error bars show 95% confidence interval constructed by taking a percentile interval of the bootstrap distribution. All significance between the N-back difficulties is tested by two-sided Mann-Whitney U tests. ns(p ≥ 0.05), *(p < 0.05), **(p < 0.01), ***(p < 0.001).

[0025] Figures 6A-6D illustrate mental workload estimation, where Figure 6A illustrates an overall N-back dataset structure and approach for mental workload estimation where the dataset is split into either trials for trial-level regression or further into individual stimuli for stimulus-level classification, extracted EEG and EOG features are listed, all evaluations are based on a 3-fold cross-validation; Figure 6B illustrates a confusion matrix for all subjects from the 4-class stimulus- level classification of N; Figure 6C illustrates a micro-averaged one-versus-rest (OvR) receiver operating characteristic (ROC) curves of each subject compared to that of a random unskilled classifier, defined as the chance level; and Figure 6D illustrates a predicted mental workload over time (trials) by the random forest regression model for the best-performing subject (Subject 6).Attorney Docket No.10046-619WO1 Client Ref. No.8422 LU DETAILED DESCRIPTION

[0026] 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.

[0027] 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 includesfrom 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.

[0028] “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.

[0029] 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.

[0030] 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 all methods 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.

[0031] 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.Attorney Docket No.10046-619WO1 Client Ref. No.8422 LU

[0032] Figures 1A and 1B present front and side views of an example of a forehead e-tattoo. The flexible and wireless forehead wearable integrates film dressing like a sensor patch for EEG and EOG on the forehead and flexible circuits for wireless data monitoring and recording capabilities. The e-tattoo measures multiple (e.g., four channels) of EEG (AF7, Fp1, Fp2, AF8) on the forehead and the additionally multiple (e.g., two) channels of EOG (vertical EOG, horizontal EOG) to estimate the level of mental workload from prefrontal brain and eye activities. The stretchable and (optionally) transparent sensor patch includes serpentine patterned graphite- deposited polyurethane (GPU) interconnects and adhesive PEDOT:PSS composite (APC) coated on GPU (APC-GPU) electrodes on sensing locations. The strongly adhesive and flexible APC- GPU allows intimate contact of the electrodes on various skin curvatures (see Figures 1C, 1D, and 1E), securing reliable signal measurement during mobile and real-world applications. The flexible circuit system attached to the electrode patch (Figure 1F) wirelessly transmits data through a wireless protocol such as Bluetooth to estimate the level of mental workload in a mobile device. Island-bridge circuit design increases the stretchability and flexibility of the on-board circuits required to endure stretching and wrinkling of skins during facial expressions. In some embodiments, a gradual increase of thickness from GPU interconnects (40 μm) to the pads of the flexible circuits (150 μm) reduces stress concentration, securing a reliable connection between the soft sensor patch and the circuits).

[0033] Figure 1G is an exemplary illustration of an exploded assembly view of the sensor patch and the flexible DAQ circuits. The sensor patch may be comprised of three layers: a top carrying Tegaderm layer, an electrode layer, and a bottom Tegaderm layer. Each electrode of the sensor patch may be connected to the exposed pads of the circuits by double sided ACF (anisotropic conductive film) tape.

[0034] One non-limiting example of a wireless forehead e-tattoo such as the one shown in Figure 1G comprises a soft sensor patch and a flexible DAQ module comprising a flexible printed circuit (FPC). The tattoo-like soft sensor patches may be configured to fit each study subject’s facial proportions. The soft sensor patch comprises a carrying top 3M Tegaderm layer (3M, USA), an electrode layer, and an insulating bottom Tegaderm layer. The stretchable filamentary- serpentine-shaped electrode layer made of GPU (Mineral Seal Corporation,382 China) was fabricated using a cut-and-paste method. APC was made by adding supramolecular solvents (citric acid and β-cyclodextrin) and elastic polymer networks (chemically crosslinked PVA networks with glutaraldehyde) to the aqueous solution of PEDOT:PSS (Heraeus, USA). A homogeneous mixture of APC was obtained by DAC 330-100SE (FlackTek Speed Mixer, USA). APC solutionAttorney Docket No.10046-619WO1 Client Ref. No.8422 LU was blade-coated on GPU at the active sensing locations and evaporated water in an oven at 70 degrees. Then, the electrode layer is transferred to the carrying Tegaderm layer and encapsulated by the bottom Tegaderm layer while only exposing the APC-coated sensing locations and connecting pads to the DAQ circuits.

[0035] The FPC is divided into several islands connected by serpentine traces to improve stretchability and also for isolation between power, analog, and digital signals. The exemplary FPC is powered by a 150mAh LiPo battery modified with an FPC connector (5034800400, Molex, USA), into which the FPC is directly inserted. For power management, low-noise low-dropout regulators TPS7A2025 and LM27761 (both from Texas Instruments, USA) are used to create 2.5V and 5V rails. The analog front end comprises an ADS1299 24-bit analog-to-digital converter (Texas Instruments, USA), which is connected via the serial peripheral interface (SPI) to a nRF52832 Bluetooth Low Energy system-on-a-chip (Nordic Semiconductor, Norway). The nRF52832 is programmed with a low-power design firmware with the serial wire debug interface. All the components are mounted on a 0.1 mm-thick double-layer FPC with electroless nickel immersion gold treatment for exposed copper. The thickness of the FPC may be brought to 0.3 mm at select areas by a polyimide stiffener (for the battery input terminal) or a FR-4 stiffener (for under the large ICs). Chip components may be hand-soldered on the FPC (manufactured by PCBWay, China). The circuit components include the ADS129924-bit analog-to-digital converter (Texas Instruments, USA) for signal amplification and conversion and nRF52832 Bluetooth Low- Energy SoC (Nordic Semiconductor, Norway) for wireless transmission of data. The circuit features a low-noise design with the use of ground isolation, shielding and low-noise linear regulators. All components may be encapsulated in silicone compound (Silbione, Elkem, Norway) for better strain isolation and electrical shielding. An anisotropic conductive film (9703, 3M, USA) can be used to bind and electrically connect the electrode layer to the channel pads on the FPC. In some instances, a skin-safe medical adhesive may be applied under the silicon casing to fix the circuits on the patch.

[0036] The modular architecture of the forehead e-tattoo eases the customization of the sensor patch for individuals and allows for quick reassembly of the e-tattoo when replacing the disposable sensor patch after use. Figure 1H and Table 1, below, compare embodiments of the disclosed wearable system with other forehead EEG or EOG wearable systems. The disclosed forehead e- tattoo’s low contact impedance and low power consumption stand out, confirming its feasibility in mobile and daily wearable applications.Attorney Docket No.10046-619WO1 Client Ref. No.8422 LUTable 1 Comparison of Forehead EEG and EOG Wearables

[0037] Figures 2A-2I show mechanical and electrical properties of an exemplary APC-GPU electrode. As noted above, the disclosed adhesive PEDOT:PSS composite is comprised of PEDOT:PSS, β-Cyclodextrin, citric acid, poly(vinyl alcohol), and glutaraldehyde-crosslinked polyvinyl alcohol (PVA) to achieve low modulus, high stretchability, and stable interface adhesion, as shown in Figure 2A. A cross-sectional view of scanning electronmicroscopy (SEM) image shows that APC is uniformly blade-coated and dried on the GPU, resulting in a 30±4-μm- thick layer (Figure 2B). APC coating decreases the average surface roughness (Sa) of the electrode from approximately 0.527 μm to approximately 0.421 μm. Although increased thickness and reduced surface roughness decrease Van der Waals’ interactions, the supramolecules formed by β-cyclodextrin and citric acid in APC can enhance the hydrogen bonding between the APC and the surface of the skin which is covered by stratum corneum comprising keratin and lipids, enabling conformal and stable contact of the electrode on the skin (see Figure 2C.

[0038] Additionally, the PEDOT:PSS content of APC-GPU electrodes can be adjusted to balance softness and conductivity. Due to the high rigidity (Young’s modulus, E > 500 MPa) of PEDOT:PSS, increasing the mass ratio of PEDOT:PSS enhances conductivity but compromisesAttorney Docket No.10046-619WO1 Client Ref. No.8422 LU the softness of the electrodes. Therefore, the mechanical properties of GPU and APC-GPU were compared with PEDOT:PSS mass ratios of 1.8%, 3.6%, 7.1%, 13.2%, and 23.3% (Figure 2D).

[0039] As shown in Figure 2D, the fracture strain decreased from 138% for 1.8%-APC-GPU to 82% for 23.3%-APC-GPU, while the effective Young’s modulus increased from 108 MPa to 147 MPa. Figure 2E shows that APC-GPU with a PEDOT:PSS mass ratio of 3.6% has the strongest adhesion of 80 N / m, while the APC-GPU with PEDOT:PSS concentration of 23.3% has the weakest adhesion of 10 N / m. In particular, the APC-GPU exhibited an adhesion force 12 times larger than that of the commercial liquid gel EEG electrode (6 N / m), suggesting its higher stability under motion. Furthermore, the APC layer with (3.6% PEDOT:PSS) functions as both an adhesive and a mechanically compatible interface, achieving perfect conformability to the skin even in the absence of an adhesive substrate. In contrast, bare GPU cannot fully conform to the skin even if supported by an adhesive Tegaderm substrate. Thus, results show that APC-GPU is clearly superior to GPU and gel electrode in adhesion.

[0040] In terms of electrical properties, the sheet resistances of bare GPU and APC-GPU are nearly identical, around 50 Ω / sq (Figure 2F). This suggests that the thin APC has little effect on the overall resistance of the GPU because the GPU is more electrically conductive than the thin APC layer, whose sheet resistance was measured to be 68.34±38.12 Ω / sq. Therefore, uncoated GPU serpentine ribbons served as stretchable, low-resistance interconnects, which are also low cost and easy to manufacture. Although APC coating does not affect the sheet resistance of GPU, it can significantly reduce contact impedance with the skin due to the simultaneous electrical and ionic conductivity of PEDOT:PSS, which can facilitate the conversion of ionic current to electronic current. The contact impedance of the APC-GPU electrode at different PEDOT:PSS mass ratios shows that APC-GPU outperforms gel-based electrodes, with a PEDOT:PSS mass ratio of 3.6% showing the lowest contact impedance. Given its high stretchability, strong adhesion, and low contact impedance, APC-GPU with a 3.6% PEDOT:PSS mass ratio was used in all the following experiments.

[0041] To further evaluate the performance of APC-GPU under dynamic conditions, impedance hysteresis and cyclic loading tests were conducted up to 30% strain.64 The impedance responses during loading and unloading show that impedance variations remained within 300 Ω, with a hysteresis of 59 Ω. Additionally, impedance changes over 1800 cycles, where the impedance stabilized after 500 cycles, remain between 0.86 kΩ and 1 kΩ. Figure 2G compares the contact impedance of the APC-GPU electrodes with that of the bare GPU electrodes and the gel electrodes (Kendall disposable surface electrode, USA). APC-GPUAttorney Docket No.10046-619WO1 Client Ref. No.8422 LU electrode achieved a remarkably low contact impedance of 8.03 kΩ·cm2 at 10 Hz, which was even lower than that of the commercial gel electrode (12.06 kΩ·cm2). The contact impedance of the APC-GPU electrode decreased over time in a typical EEG bandwidth of 1-100 Hz as shown in Figure 2H. At 10 Hz, the contact impedance of the APC-GPU electrode gradually decreased from 8.03 kΩ·cm2 to 2.53 kΩ·cm2 after 5 hours due to slight sweat secretion. The reduction in contact impedance after exercise further confirmed the effect of sweat on reducing the contact impedance. Despite excessive sweating during exercise, the adhesion of the APC-GPU is still sufficient to maintain good contact with the skin, which is important for monitoring EEG under dynamic motion. Figure 2I shows that the average contact impedance of APC-GPU electrodes always remained under 25 kΩ during the test period, ensuring high EEG and EOG signal quality. In contrast, the contact impedance of the bare GPU electrodes increased with time as sweat plays little role with non-ionically conductive GPU but sebum secretion worsened the mechanical and electrical contacts between the electrode and the skin. In conclusion, the APC coating significantly enhances the performance of GPU-based filamentary serpentine electrodes by improving adhesion and reducing electrode-skin contact impedance. These innovations ensure reliable, high-quality EEG and EOG signal acquisition even under dynamic movements, making APC-GPU electrodes a promising solution for large-area, accessible, and disposable forehead e- tattoos.

[0042]

[0043] GPU exhibits the softest characteristics with Young’s modulus of 109 MPa and stretchability of 144% as the uncoated GPU has the thinnest thickness of 40 μm. As the mass ratio of PEDOT:PSS increases from 3.6% to 22.3%, Young’s modulus increases from 129 MPa to 147 MPa, and stretchability decreases from 135% to 84%. Since low sheet resistance is desired for interconnects, it was also examined. Figure 2E indicates that APC coating slightly increases the sheet resistance, and PEDOT:PSS content does not significantly contribute to the sheet resistance. Therefore, the interconnects remain uncoated to minimize the resistance. Lastly, the adhesion force of APC-GPU on the skin through a 90-degree peel test was measured. The skin adhesion force of APC-GPU is 80 N / m and 6 N / m with PEDOT:PSS mass ratios of 3.6% and 22.3%, respectively (Figure 2F). Based on these characterizations, it was determined that an APC-GPU with 3.6% PEDOT:PSS mass ratio provides high stretchability and strong adhesive interface, ensuring secure contact on curved skin, though APC-GPU with higher or lower PEDOT:PSS mass ratios are contemplated within the scope of this disclosure. The skin conformability and conductivity of APC-GPU electrodes was evaluated through skin-electrode contact impedanceAttorney Docket No.10046-619WO1 Client Ref. No.8422 LU measurements. Lowering the contact impedance improves signal quality. Herein, the contact impedance of APC-GPU electrodes with GPU electrodes and a solid gel electrode (Kendall disposable surface electrode, USA) (Figure 2G) were compared. APC-GPU electrodes have average contact impedance of 8.03 kΩ·cm2, which is lower than the conventional gel electrode impedance of 12.06 kΩ·cm2at 10 Hz. Figures 2H and 2I show the long-term reliability of APC- GPU. The contact impedance gradually decreases from 8.03 kΩ·cm2to 2.53 kΩ·cm2over 5 hours since the accumulation of sweat improves skin conductivity. On the other hand, the contact impedance of GPU electrodes increases over time as the accumulation of sebum and sweat between the electrode and skin worsens the contact. Lastly, the average contact impedance of APC-GPU electrodes is maintained under 25 kΩ, ensuring high EEG and EOG signal quality.

[0044] Although the maximum signal quality of an EEG-EOG system is determined by the electromechanical properties of the electrode-skin interface, care must be taken in the design of the associated data acquisition (DAQ) system to achieve high-quality signal processing. Figure 3A shows a top-down view of an embodiment of a flexible printed circuit (FPC) serving as the data acquisition platform for an exemplary e-tattoo. The shown double-layer FPC was manufactured according to the schematic shown in Figure 1F. The circuit comprises several isolated island modules connected by stretchable serpentine traces. This island-bridge design provides both mechanical isolation to maximize conformability to the curved forehead, as well as mechanical robustness and electrical isolation between power, analog and digital signals to improve circuit stability. The power management module has a serpentine extension that slides directly into an ultra-low profile FPC connector mounted on a small rechargeable battery (e.g., a 3.7V lithium polymer battery, see Figure 3A, inset). In some instances, the isolation of a rigid battery away from the board allows a small total FPC area of approximately 858.37 mm2as well as increased device flexibility. In some instances, the battery may comprise a 150 mAh battery, which could last beyond 28 hours given a system current draw of 5.25 mA (average 5.23 mA in advertising phase, 5.26 mA in active data transmission) (Figure 3D). The central board is connected to three signal acquisition islands containing ten copper pads (4 EEG electrodes, 4 EOG electrodes forming 2 bipolar channels, 1 common EEG reference electrode, and 1 grounding electrode) which each interface with the corresponding trace in the GPU layer via a z-axis anisotropic conductive film (ACF). The signals are low-pass filtered for antialiasing and then amplified and converted to digital signals by a 24-bit analog-to-digital converter (Texas Instruments ADS1299). For EEG data acquisition, a closed-loop right-leg drive (RLD) configuration was implemented, although this disclosure contemplates the use of otherAttorney Docket No.10046-619WO1 Client Ref. No.8422 LU configurations. In one embodiment of the disclosed e-tattoo, this configuration achieved an 85.7 % reduction in the body-coupled 60 Hz noise (Figure 3C) compared to the open-loop alternative under heavy powerline contamination conditions expected in out-of-the-laboratory environments. The ADS1299 converter communicates, via a Serial Peripheral Interface (SPI), with the Nordic nRF52832 Bluetooth Low Energy (BLE) system-on-a-chip on the wireless transmission module which includes a 2.4-GHz antenna and probe points for in-circuit device programming via a Serial Wire Debug (SWD) interface. The overall device connects wirelessly to a custom application (Android, IOS, etc.), which marks every acquired sample with a unique timestamp based on the Lab Streaming Layer architecture. This allows EEG and EOG data to be temporally synchronized with sub-millisecond resolution, as required in cognitive neurophysiology experiments, for various activity and event measurements in the disclosed dual N-back task study. The stability of the wireless transmission was characterized by the low mean packet loss of 0.07% at distances below 1 m and 0.5% at distances below 10 m. Overall, the disclosed embodiment of the data acquisition system had a low self-noise of 0.079 μVrms, 0.634 μVpp, and a low noise corner frequency of 10- 1 Hz, well below the frequencies of interest in EEG and EOG signals. Herein, the accuracy of the physiological signals collected with the described embodiment of the device is further demonstrated through comparisons with a gold-standard device. Validation of Signal Quality and Comparison against Lab-Grade EEG Device

[0045] Figures 4A-4F compare motion artifacts and signal quality between an exemplary embodiment of the disclosed wireless forehead e-tattoo and a wireless lab-grade EEG device, the Brain Vision actiCap electrodes, with the LiveAmp system (Brain Products, Germany). In this comparison, gel-based active electrodes (actiCAP, Brain Products, Germany) were placed adjacent to the APC-GPU electrodes of the forehead e-tattoo. The gel electrodes were connected to the LiveAmp amplifier using standard DIN 1.5mm EEG cables. Both the Brain Vision system and the forehead e-tattoo were configured to record signals simultaneously at a sampling rate of 250 Hz.

[0046] Figures 4A-4F compare the signal quality and motion artifacts between the two. All EEG electrodes were applied to AF7, Fp1, Fp2, and AF8. In this comparison, solid gel-based active electrodes (actiCAP, Brain Products, Germany) were placed right above the APC-GPU electrodes with an offset of 1 cm. The reference and ground electrodes were also placed adjacent to each other on the mastoids. The gel electrodes were connected to the LiveAmp amplifier through standard DIN 1.5 mm EEG cables. The Brain Vision system and the forehead e-tattoo were configured to record signals simultaneously at a sampling rate of 250 Hz.Attorney Docket No.10046-619WO1 Client Ref. No.8422 LU

[0047] To assess the ability of the forehead e-tattoo to record basic neural activities, the synchronization and desynchronization of the alpha EEG band (8-12 Hz) during open and closed eyes was measured. Alpha synchronization (i.e., elevated EEG alpha band power) is expected when eyes are closed. Alpha bandpass-filtered EEG signals from both the Brain Vision system (Figure 4A) and the forehead e-tattoo (Figure 4C) indicated similar levels of alpha activity during closed and open eyes. Similarly, Figures 4B and 4D show the average spectrograms across all four EEG channels obtained by the Brain Vision system and the e-tattoo, respectively. The EEG signals and the average spectrograms are virtually indistinguishable, validating the EEG sensing capabilities of the wireless e-tattoo. The disclosed e-tattoo also captured the apparent alpha synchronization during eye closing in all subjects. By accurately measuring the visual stimuli-modulated changes in alpha activity, the feasibility of using the e-tattoo for applications requiring accurate neural spectra information was validated.

[0048] The disclosed e-tattoo also clearly distinguishes horizontal EOG (hEOG) and vertical EOG (vEOG) during eye movements and blinks (Figure 4E). During vertical eye movements, there was a lack of hEOG signals and vice versa, which can be attributed to the precise placement of the hEOG and vEOG electrodes owing to the high customizability of the e-tattoo to fit individual user’s face. Distinctive orthogonal EOG measurements enable a wider range of EOG analysis than combined EOG measurements in other wearable platforms. Weak and strong blinks were also captured accurately by the e-tattoo as evident in the vEOG channel. They are important for regression-based EOG artifact removal from the forehead EEG signals. Many motion artifact removal methods for ambulatory EEG rely on a considerable number of EEG channels, which can be impractical for wearable form factors. Additionally, these approaches often incorporate external sensing modalities, such as accelerometers, which can add complexity to the system. On the other hand, a significant advantage of cable-free wireless EEG sensing is suppression of motion artifacts without extensive signal processing. Figure 4F compares the EEG measured by the Brain Vision system (red) and the forehead e-tattoo (blue) during head rotations (looking up / down / left / right), facial muscle movements (raising eyebrows, smiling, swallowing), and outdoor activities (walking and running). EEG signals recorded by the Brain Vision system were significantly corrupted by dynamic movements such as head rotations, walking, and running, while the e-tattoo EEG remained unaffected. The average root mean square (RMS) values of the EEG signals under different motion conditions were 1847.26 μV and 68.04 μV for the Brain Vision and forehead e-tattoo, respectively. The RMS value of the forehead e-tattoo EEG falls within the typical EEG amplitude range of 10-100 μV. Only duringAttorney Docket No.10046-619WO1 Client Ref. No.8422 LU eyebrow-raising did the e-tattoo EEG fluctuate more than the Brain Vision EEG due to the relative proximity of the e-tattoo electrodes to the eyebrows. Other subtle facial expressions, such as smiling and swallowing, did not induce significant motion artifacts in either system. The relative immunity to motion artifacts indicates that the wireless forehead e-tattoo holds promise for real-life mental workload monitoring.

[0049] EXPERIMENTAL RESULTS - Dual N-back tasks

[0050] The N-back task has been widely adopted to parametrically evoke different levels of working memory load by adjusting the ”N” value, directly taxing the prefrontal cortex, which is a key brain region associated with cognitive control and mental effort. Described herein are the results of a study to estimate the mental workload of six human subjects wearing and interacting with the wireless forehead e-tattoos. To that end, a dual N-back trial was designed for the subjects to exercise mental workload by memorizing two different sequences of stimuli: the positions of the cells and the alphabet characters within them. The trial’s difficulty escalates as the load factor N increases from 0-back to 3-back. Figure 5A illustrates the experimental paradigm and the sequence of the disclosed dual N-back trial. Each experimental session comprises three runs with 5-minute breaks between them. Within each run, 16 N-back trials are evenly distributed across four difficulty levels (0-, 1-, 2-, and 3-back) in a randomized order.

[0051] During each N-back trial, subjects are presented with 20 stimuli, each consisting of a visual stimulus of 0.6 s duration, followed by a black screen for 1.4 s, totaling 2.0 s per interval. When a ”matching stimulus” appears at the specified n-turn, subjects are instructed to respond by clicking the right mouse button for a location match and the left mouse button for an alphabet match. In the 0-back scenario, subjects are given a target alphabet and a cell position to match before the stimulus cycle begins. After each trial, subjects evaluate their task load and performance using the NASA-TLX test.

[0052] The combination of self-assessed task load, behavioral performance, and physiological indices were examined to provide a wholesome analysis of the mental workload induced by the dual N-back test. Figure 5B shows that the averaged self-assessed total TLX index increases with test difficulty, indicating distinct difficulty levels in the dual N-back design. The difference in total TLX between 0-back and 1-back levels has no significance (p = 0.053), meaning the subjects didn’t find an apparent difference in terms of difficulty between 0-back and 1-back. The average NASA-TLX ratings for six subjective subscales indicate that increasing the N factor does notAttorney Docket No.10046-619WO1 Client Ref. No.8422 LU increase the physical demand to accomplish the tasks. Accuracy and response time were also examined to assess subjects’ behavioral performance differences.

[0053] Figure 5C compares the average N-back performance assessed by the normalized performance subscale of TLX (100-Performacne Rating / 100) and task accuracy. Both metrics indicate that the performance decreases as difficulty N increases. However, the self-assessed performance reflects sensitivity rather than accuracy, as subjects prioritized identifying the matching stimuli (hits) over non-matching stimuli (correct rejections). The effects of N-back difficulty for average detection rates, false alarm rates, and match reaction times are shown in Figure 5D. As predicted, the detection rate increases, while the false alarm rate also increases with N. Although there were no incentives or penalties based on reaction times, subjects reacted faster to the matching stimuli at lower difficulties. Conclusively, the NASA-TLX ratings and behavioral performance analysis reveal negligible differences in mental workload between 0-back and 1-back tasks. Additionally, while the total TLX rating shows a significant difference between 2-back and 3-back, task accuracy, reaction time, and false alarm rates indicate that the performance difference was insignificant between these two levels. EEG neural power spectra are processed to be normalized and extracted for delta (1-4 Hz), theta (4-8 Hz), alpha (8-12 Hz), beta (12-30 Hz),and gamma (30-50 Hz) bands. EOG features include the number of eye blinks and saccades, the average blink peak (ABP), the average blink duration (ABD), and the average saccades duration (ASD). We also computed several ratios of EEG power bands. Several neuroscience studies have reported a correlation between workload and EEG band power, especially in theta and alpha bands. An increase in theta power is also correlated with the increase in memory-required brain activities, and is most clearly observed in the frontal brain region which the forehead EEG system can detect easily. A decrease in alpha power indicates arousal, attention, or workload. An increase in frontal theta power and a decrease in alpha power indicates an increase in mental workload, as reported in previous studies. A decrease in beta power, coinciding with low performance and slow reaction times, is associated with increased memory load during the maintenance of visual information. EXPERIMENTAL RESULTS - Estimating Mental workload from forehead EEG and EOG signals

[0054] A random forest model was built to predict the level of mental workload experienced by subjects during their performance during dual N-back tests. For this purpose, the known N label was used as the ground truth for mental workload, as validated in the previous section and commonly done in the literature. Figure 6A shows the structure of the entire dataset recording during this study, where each channel-run-subject block containing approximately 20-25 minutesAttorney Docket No.10046-619WO1 Client Ref. No.8422 LU of e-tattoo data (with variable duration due to NASA-TLX administration) was divided into either 16 epochs of 40 s for trial-level regression or further divided into 16 × 20 epochs of 1.8 s for stimulus-level classification. The latter method provides a more systematic evaluation of the predictive power of the extracted EEG features through a 4-class classification of subject data collected during thousands of individual N-back stimulus presentations. The former method provides a more reliable and continuous estimate of mental workload by using longer data segments, which allowed for the meaningful inclusion of EOG features, as opposed to the stimulus-level approach where the majority of the 1.8 s data segments were lacking blinks or saccades, resulting in a sparse feature matrix. Both approaches were evaluated with subject specific 3-fold cross-validation, in which the model was always predicting the task difficulty N only based on unseen training data. [To further reduce the risk of overfitting, the number of trees in the random forest classifier was increased to 300, which decreases the generalization error. Figure 6B shows the confusion matrix of the stimulus-level classification, summed across all subjects. To verify that the mental workload classifier achieved above-chance accuracy (1) for every subject and (2) for every N level, we plotted the micro-averaged one-versus-rest (OvR) receiver operating characteristic (ROC) curves of each subject and compared them to those of a random unskilled classifier (Figure 6C). In other words, each ROC curve in Figure 6C is a weighted average of 4 ROC curves for that subject (N = 0 vs. N = 1 or 2 or 3, N = 1 vs. N = 0 or 2 or 3, and so on). To clarify, the OvR paradigm is used only for visualizing the two-dimensional ROC curves; the actual classification results presented in Figure 6B are the result of a true 4-class classification. Finally, to validate mental workload estimation of a subject over time, we performed trial-level regression of N using a random forest regressor. Figure 6D shows the 3-fold cross- validated predictions of Subject 6’s mental workload throughout a 2.5-hour experimental session, showing a high Pearson’s correlation coefficient of 0.89 with the actual N levels that the subject experienced. Together, these results show that the EEG and EOG data collected by the e-tattoo system contained sufficient features for a reliable estimation of the mental workload evoked by the dual N-back task in each of the 6 subjects. CONCLUSION

[0055] Disclosed herein are embodiments of a wireless EEG and EOG forehead e-tattoo for mental workload decoding. The e-tattoo platform offers convenient and comfortable wear for extended periods, facilitated by noble APC-GPU electrodes and a flexible assembly of electrodes and circuits. The remarkably low contact impedance and strong skin adhesion of the APC-GPU electrodes ensure reliable and accurate multimodal physiological measurements, even duringAttorney Docket No.10046-619WO1 Client Ref. No.8422 LU dynamic body movements. Compared with standard lab-grade systems, the disclosed e-tattoo system has demonstrated high signal quality and the feasibility of obtaining accurate neural spectra from the forehead. The automatic estimation of mental workload was performed through a dual N-back experiment in a controlled lab setting. A random forest-based workload estimation model, trained on forehead EEG and EOG features, indicates the e-tattoo’s potential as a low-profile cognitive state monitoring platform for various human-machine interaction applications.

[0056] Non-limiting examples of uses of the disclosed forehead e-tattoo platform include online estimation of mental workload during real-world human-machine interaction tasks that require optimized task performance and engagement, and monitoring various mental states and disorders, such as vigilance, stress, and sleep disorders, in the future.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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 non- express 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.

[0061] 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.Attorney Docket No.10046-619WO1 Client Ref. No.8422 LU

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[0159] 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.

Claims

Attorney Docket No.10046-619WO1 Client Ref. No.8422 LU CLAIMS 1. A wireless forehead e-tattoo for mental workload estimation comprising: a sensor section; and a data acquisition (DAQ) section; and wherein the sensor section further comprises: a first flexible, stretchable insulating substrate comprising a first side and the 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; a plurality of sensors configured on the second side of the first flexible, stretchable insulating substrate, said plurality of sensors comprising one or more electrodes, each electrode comprising a conductive base layer, and a conductive adhesive covering at least a portion of one surface of the base layer, wherein the base layer and the conductive adhesive layer are formed in one or more patterns on the second side or the first flexible, stretchable insulating layer and each electrode is configured to flex and stretch with the first flexible, stretchable insulating substrate to conform to the epidermis; and a second flexible, stretchable insulating substrate comprising a first side and a second side opposite the first side, wherein the first side substantially covers and adheres to the plurality of sensors and / or a portion of the second side of the first flexible, stretchable insulating substrate and the second flexible, stretchable insulating substrate conforms to the epidermis; and wherein the DAQ section further comprises: electronics, said electronics comprising a power management module, an analog front-end (AFE), an analog to digital converter (ADC) module, a communication module, and a power source, and an electrically-insulated flexible printed circuit (FPC) layer having a first side and a second side, said first side substantially in contact with a second side of the first flexible, stretchable insulating substrate, wherein said electronics are disposed at least partially on the second side of the FPC layer, wherein components of the electronics are separatedAttorney Docket No.10046-619WO1 Client Ref. No.8422 LU 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 in communication with the electrodes of the sensor section through one or more conductive windows in the FPC layer.

2. The wireless forehead e-tattoo for mental workload estimation of claim 1, wherein the second flexible, stretchable insulating substrate forms the FPC layer, and wherein the plurality of sensors comprise at least an electroencephalography (EEG) sensor and a electrooculography (EOG) sensor.

3. The wireless forehead e-tattoo for mental workload estimation of claim 2, wherein the EEG sensor measures four channels of EEG (AF7, Fp1, Fp2, AF8) on the forehead and the EOG sensor measures two channels of EOG (vertical EOG, horizontal EOG) to estimate the level of mental workload from prefrontal brain and eye activities.

4. The wireless forehead e-tattoo for mental workload estimation of any one of claims 1- 3, wherein the sensor section is configured to fit a study subject’s facial proportions.

5. The wireless forehead e-tattoo of any one of claims 6-10, wherein the base layer of one or more of the sensors of the plurality of sensors is comprised of graphite-deposited polyurethane (GPU), and wherein the conductive adhesive covering of one or more of the sensors of the plurality of sensors is comprised of an adhesive PEDOT:PSS composite (APC).

6. The wireless forehead e-tattoo of claim 5, wherein the APC is made by adding supramolecular solvents (citric acid and β-cyclodextrin) and elastic polymer networks (chemically crosslinked PVA networks with glutaraldehyde) to an aqueous solution of PEDOT:PSS.

7. The wireless forehead e-tattoo of claim 5 or claim 6, wherein the PEDOT:PSS has a mass ratio of from 3% to 24%.Attorney Docket No.10046-619WO1 Client Ref. No.8422 LU 8. The wireless forehead e-tattoo of any one of claims 1-7, wherein the one or more patterns on the second side or the first flexible, stretchable insulating layer comprises filamentary-serpentine-shaped patterns.

9. The wireless e-tattoo of claim 8, wherein the FPC layer and the electronics are divided into several islands connected by serpentine conductive traces to improve stretchability and also for isolation between power, analog, and digital signals.

10. The wireless forehead e-tattoo of any one of claims 6-17, wherein the power source comprises a battery modified with a FPC connector, into which the FPC connector is directly inserted into the power management module, wherein the power management module comprises two low-noise low-dropout regulators that are used to create 2.5V and 5V rails.

11. The wireless forehead e-tattoo of any one of claims 6-19, wherein the AFE and ADC comprises a 24-bit analog-to-digital converter, which is connected via the serial peripheral interface (SPI) to the communication module, and wherein the communication module comprises a Bluetooth Low Energy system-on-a-chip for wireless transmission of data.

12. The wireless forehead e-tattoo of any one of claims 1-11, wherein components of the electronics are encapsulated in a silicone compound for better strain isolation and electrical shielding.

13. The wireless forehead e-tattoo of any one of claims 1-12, wherein the one or more conductive windows in the FPC layer comprise an anisotropic conductive film (ACF), wherein the ACF acts as an adhesive and conductor between the electrodes and the electronics.

14. The wireless forehead e-tattoo of any one of claims 1-13, wherein the FPC layer comprises or at least partially comprises polyimide.

15. The wireless forehead e-tattoo of any one of claims 1-14, wherein the communication module is used to wirelessly transmit data from the e-tattoo to a designated receiver, wherein the designated receiver comprises a smartphone.Attorney Docket No.10046-619WO1 Client Ref. No.8422 LU 16. The wireless forehead e-tattoo of claim 15, wherein the designated receiver further transmits at least a portion of the data to a host comprising one or more processors for analysis.

17. The wireless forehead e-tattoo of any one of claims 15-16, wherein the designated receiver comprises one or more processors and is used to at least partially analyze the data.

18. The wireless forehead e-tattoo of any one of claims 1-17, wherein the sensor section can be separated from the DAQ section and discarded such that the DAQ section can be reused to form a second wireless forehead e-tattoo.

19. A method for using a wireless forehead e-tattoo for mental workload estimation, the method comprising: attaching a wireless forehead e-tattoo to an epidermis of a person, said wireless forehead e-tattoo comprising: a sensor section; and a data acquisition (DAQ) section; and wherein the sensor section further comprises: a first flexible, stretchable insulating substrate comprising a first side and the 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; a plurality of sensors configured on the second side of the first flexible, stretchable insulating substrate, said plurality of sensors comprising one or more electrodes, each electrode comprising a conductive base layer, and a conductive adhesive covering at least a portion of one surface of the base layer, wherein the base layer and the conductive adhesive layer are formed in one or more patterns on the second side or the first flexible, stretchable insulating layer and each electrode is configured to flex and stretch with the first flexible, stretchable insulating substrate to conform to the epidermis; andAttorney Docket No.10046-619WO1 Client Ref. No.8422 LU a second flexible, stretchable insulating substrate comprising a first side and a second side opposite the first side, wherein the first side substantially covers and adheres to the plurality of sensors and / or a portion of the second side of the first flexible, stretchable insulating substrate and the second flexible, stretchable insulating substrate conforms to the epidermis; and wherein the DAQ section further comprises: electronics, said electronics comprising a power management module, an analog front-end (AFE), an analog to digital converter (ADC) module, a communication module, and a power source, and an electrically-insulated flexible printed circuit (FPC) layer having a first side and a second side, said first side substantially in contact with a second side of the first flexible, stretchable insulating substrate, wherein said electronics are disposed at least partially on the second side of the FPC layer, 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 in communication with the electrodes of the sensor section through one or more conductive windows in the FPC layer; and receiving, by one or more processors, data transmitted from the wireless forehead e-tattoo; and determining, by the one or more processors, mental workload estimation of the person using data.

20. A system for using wireless forehead e-tattoo for mental workload estimation, the system comprising: a wireless forehead e-tattoo to an epidermis of a person, said wireless forehead e- tattoo comprising: a sensor section; andAttorney Docket No.10046-619WO1 Client Ref. No.8422 LU a data acquisition (DAQ) section; and wherein the sensor section further comprises: a first flexible, stretchable insulating substrate comprising a first side and the 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; a plurality of sensors configured on the second side of the first flexible, stretchable insulating substrate, said plurality of sensors comprising one or more electrodes, each electrode comprising a conductive base layer, and a conductive adhesive covering at least a portion of one surface of the base layer, wherein the base layer and the conductive adhesive layer are formed in one or more patterns on the second side or the first flexible, stretchable insulating layer and each electrode is configured to flex and stretch with the first flexible, stretchable insulating substrate to conform to the epidermis; and a second flexible, stretchable insulating substrate comprising a first side and a second side opposite the first side, wherein the first side substantially covers and adheres to the plurality of sensors and / or a portion of the second side of the first flexible, stretchable insulating substrate and the second flexible, stretchable insulating substrate conforms to the epidermis; and wherein the DAQ section further comprises: electronics, said electronics comprising a power management module, an analog front-end (AFE), an analog to digital converter (ADC) module, a communication module, and a power source, and an electrically-insulated flexible printed circuit (FPC) layer having a first side and a second side, said first side substantially in contact with a second side of the first flexible, stretchable insulating substrate,Attorney Docket No.10046-619WO1 Client Ref. No.8422 LU wherein said electronics are disposed at least partially on the second side of the FPC layer, 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 in communication with the electrodes of the sensor section through one or more conductive windows in the FPC layer; and one or more processors, wherein the one or more processors receive data transmitted from the wireless forehead e-tattoo, wherein the data is used by the one or more processors to determine the mental workload estimation of the person.

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