Graphene Electronic Tattoos

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Photograph of a Graphene Electronic Tattoo placed on human skin (wrist).

Graphene Electronic Tattoos (GETs) - refer to the unique category of skin-wearable electronic devices specifically designed for personalized healthcare applications. Although there are multiple types of graphene-based wearables, some of which are also called e-tattoos, GETs represent a distinct subset within the broader realm of graphene-based wearables due to their unique composition and exceptional characteristics. The GETs are primarily composed of a single graphene plane, a remarkable two-dimensional material characterized by an atomic monolayer arranged in a nanostructure resembling a honeycomb lattice. Notably, these devices differ from other graphene-based wearables that incorporate graphene particles mixed in silicones, polymers, or rely on laser-induced graphene. The manufacturing process for GETs employs chemical vapor deposition, ensuring the utilization of high-quality graphene for optimal performance. The graphene layers in GETs are extraordinarily thin, measuring less than 1 nanometer, and are supported by an ultra-thin layer of polymer, such as PMMA, which has a thickness below 500 nanometers. The GETs are optically transparent, lightweight, flexible, self-adhesive (via Van der Waals forces), making them perfectly conformal to the micro-curvature of skin. Moreover, the self-adhesive nature of GETs, accomplished through the utilization of Van der Waals forces, ensures seamless attachment to the skin surface. The versatility of GETs is a key advantage, as they have been successfully employed in detecting a diverse range of physiological activities. These include monitoring brain activity through electroencephalography (EEG), measuring muscle activity using electromyography (EMG), analyzing heart activity via electrocardiography (ECG), observing ocular activity with electrooculography (EOG), as well as tracking vital parameters such as temperature and hydration levels. GETs exhibit remarkable electrical, optical, and mechanical properties, positioning them as promising devices for personalized healthcare applications.

GETs were initially pioneered by researchers Shideh Kabiri, Nanshu Lu, and Deji Akinwande at the University of Texas-Austin.

Fabrication

The fabrication process of Graphene Electronic Tattoos (GETs) starts with the growth of high-quality graphene through chemical vapor deposition (CVD) on a copper substrate. Once grown, the graphene is coated with a thin layer of PMMA and transferred onto temporary tattoo paper after copper etching as detailed in the Protocol. The tattoo paper is then cut into the desired shape and size using a mechanical cutter plotter. To apply the GETs, they are transferred onto the skin using water, similar to decal-style temporary tattoos. Once the graphene is on the skin, it is self-adhesive and remains on the desired locations. The process is easily replicated in simple laboratories, as it requires less than three hours of labor and no highly trained personnel.

Properties

GETs possess desirable properties such as optical transparency, lightweight, and flexibility. They conform well to the skin's micro-curvature and are less affected by motion artifacts. Earlier versions had limitations like impermeability to sweat and scattered electrical properties. To address these issues, GETs 2.0 have been developed by Dmitry Kireev, featuring superior electrical properties, sweat permeability, and robustness. Microsized holes are embossed in GETs 2.0 to enable sweat evaporation without significantly affecting electrical properties.

Most importantly, the GETs 2.0 are made with few-layer graphene structures, which exhibit decreased sheet resistance, lower skin impedance, and reduced standard deviations compared to monolayer graphene. They can function as skin-wearable electronic heaters and demonstrate efficient heating properties. Currently, 2L-GETs are considered the most suitable components for future graphene-based electronic tattoos due to their performance, device-to-device consistency, and ease of fabrication.

Significant applications

General Electrophysiology

The GETs find significant applications in various wearable electrophysiology cases. For each case, specific locations are chosen for placing the graphene tattoos, along with reference electrodes:

1. Measuring EEG from the forehead. Requires a set of graphene tattoos placed on the subject’s forehead, most commonly Fp1 and Fp2 locations. The reference electrode is placed close to the bone, typically behind earlobe.
2. Measuring EMG from any muscle. Measurements taken in differential amplification mode. The muscular contractions will result in the generation of a net electrical potential that is recorded by the tattoos. Due to GET dimensions and intimate contact to skin, they can be used for precise muscle activity monitoring and complex HMIs. The reference electrode for EMG monitoring is placed near a bone.
3. Measuring ECG from the chest or two hands. Requires ≥2 GETs to be placed onto the subject. Typically, the most common location is the chest (close to the heart) or on the two forearms. The ECG measurements are taken in differential amplification mode. The reference electrode is placed by the abdomen.

Other significant applications are mentioned below.

Electrooculography

Electrooculography or EOG - is a sub-field that involves recording electrical signals resulting from the depolarization between the retina and cornea. These signals allow for the distinction of specific electrical patterns when a person shifts their gaze in different directions. To measure the EOG signals accurately, the GETs are strategically placed above, below, and on the sides of the eyes. Hence, GETs provide highly precise eye tracking capabilities. The captured EOG information is indicative of the direction of the gaze (up, down, left, or right), and can be further processed in real-time and transmitted to a robot or flying quadcopter, enabling responsive movement based on eye movements. Additionally, the transparency of GETs enhances visual aesthetics, which is not achievable with standard electrodes like gold or Ag/AgCl gel electrodes. While EOG applications can be realized with various electrode types, graphene tattoos offer exceptional advantages, particularly for on-face use, where transparency and imperceptibility are crucial factors.

Electrodermal Activity

GETs are also used to monitor electrodermal activity (EDA) on the palm in a continuous, non-invasive and unrestrained manner. EDA serves as a widely used indicator of mental stress. Monitoring EDA on the palms is particularly recommended due to the high density of eccrine sweat glands, which become activated during psychological stimuli, making palm measurements highly sensitive. While commercial wearable EDA sensors have limitations in terms of ambulatory use, recent advancements involve the placement of serpentine-shaped GETs, overlapping with a gold serpentine tape, directly on the palm without additional adhesive. This unobtrusive setup enables a 15-hour ambulatory monitoring of EDA, encompassing activities such as studying, exercising, driving, eating, and sleeping. Results from the study indicate that the GET-based EDA sensor exhibits similar physiological event detection capabilities as gel electrodes, particularly when motion is involved. GET-based sensors demonstrate superior stability in capturing EDA signals during ambulatory scenarios, surpassing the obstructive nature and susceptibility to detachment associated with gel electrodes.

Blood Pressure

Graphene electronic tattoos offer a unique solution for continuous and non-invasive method to monitor arterial blood pressure. The combined efforts of Kaan Sel and Roozbeh Jafari from Texas A&M University, and Dmitry Kireev and Deji Akinwande from the University of Texas at Austin and colleagues have showcased the effectiveness of GETs, bioimpedance measurements, and machine learning in achieving continuous blood pressure tracking. To implement this approach, an array of GETs is placed on the wrist, positioned above the radial and ulnar arteries branching out from the brachial artery. An AC current is injected through the outer GETs, while the inner pairs record the corresponding changes in biopotentials. Frequency-dependent bioimpedance spectroscopy is employed to measure the variable portion of the bioimpedance, representing blood volume changes in the artery caused by pulse pressure waves. By using the characteristic points extracted from the dataset, a machine learning regression algorithm is constructed, incorporating a multi-variable non-linear mathematical framework to establish the mapping between the recorded waveforms and blood pressure. The accuracy of graphene tattoos satisfies the highest level, IEEE Grade A classification, for wearable blood pressure monitoring. The GETs successfully measure arterial blood pressure for extended periods exceeding multiple hours, withstanding diverse activities and environmental conditions, including exposure to light and heat, contact with water or sweat, and strenuous exercise. This renders GETs an ideal solution for the continuous monitoring of blood pressure in ambulatory settings.

Future

Looking ahead, the future of wearable technology envisions the utilization of not just graphene but also various other 2D materials, which offer distinctive properties and advantages, in the creation of imperceptible wearables and tattoos. For instance, skin-attached membranes of MoS2 and PtSe2 exhibit semiconductive transistor properties, while PtTe2 tattoos demonstrate superior metallic interfaces with the skin. As the field of 2D materials continues to expand, it is highly likely that the next generation of wearables will extensively incorporate these materials. A potential outcome of this progression is the achievement of entirely imperceptible communication between these wearables and the human body. The atomically thin multilayer heterostructures of 2D materials can be employed to construct measurement, amplification, and communication components, allowing futuristic e-tattoos to seamlessly integrate with the human body. This promises a future where wearables become an integral part of our daily lives, imperceptibly enhancing our interactions with technology.

References

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