European Observatory for Nanomaterials

Graphene Electronic Tattoos (GETs) are a type of skin-wearable electronic device that can be used for personalized healthcare by transmitting the bio-electrical activity of the human body in measurable electrical signal [1], [2]. The GETs are made from high-quality single-atom-thick graphene (atomic monolayer of carbon atoms tightly bound in a honeycomb lattice nanostructure) and were first developed in 2017 [3]. The GETs are optically transparent, lightweight, and flexible, making them adhere and conform to the micro-curvature of the skin, which helps them to remain at the exact positions during use and transmit bioelectrical signals during movements which would not be possible with other, thick and rigid materials such as gold or silver/silver chloride (Ag/AgCl) gel electrodes. GETs have been used to monitor electrophysiological signals such as the brain (EEG), heart (ECG), muscle (EMG), and ocular (EOG) activities, as well as skin temperature and hydration level [3], [4]. 

The overall fabrication of the GETs is relatively straightforward, as explained in the step-by-step protocol [5]. Shortly, the procedure for making GETs starts with the growth of high-quality, large-scale graphene via chemical vapor deposition (CVD) on a copper substrate, which is well-established and yields a one-atom-thick graphene structure [6]. After the graphene is grown on copper, it is covered with a thin layer of PMMA (common e-beam resist) and placed onto copper etchant solution. When the copper is etched away, the graphene/PMMA stack is transferred onto a temporary tattoo paper and then diced into an arbitrary shape using a mechanical cutter plotter (see video). The graphene is then transferred onto the skin (see video) using a wetting agent, such as water. Once the graphene is on the skin, it is self-adhesive and non-irritative. 

It is essential to mention that graphene tattoos are entirely biocompatible and do not cause any inflammatory or toxic effects. In the early works, the exfoliated, nanometer-sized graphene was re-suspended in liquid and brought in direct contact with the living cells causing cellular toxicity[7]. In contrast, our graphene tattoos do not cause any cellular toxicity. First, this is because the graphene used here is large-scale and supported by a substrate; hence it cannot interact closely with the human cells or inner organs [8]–[10]. This is also affirmed by the fact that the GETs can be easily removed from the human skin using a soft adhesive, and none of the human subjects have complained of any side effects. 

Recently, the performance of the GETs was significantly improved and GETs 2.0 have been developed with superior electrical properties, permeability to sweat, and robustness [11]. When evaluating a wearable electrode's performance, two significant parameters are reported: the overall sheet resistance of the material and the electrode-skin impedance (the response of a specific skin region under the tattoo to an externally applied electrical current). The latter is essential, as the lower interface impedance means that the measured signal will be of higher quality. One always aims to create wearable systems with the lowest interface impedance.  In this regard, the GETs 2.0, made with multilayer graphene, exhibit 3.5-fold decreased sheet resistance and 2.5-fold lower skin impedance than monolayer GETs. When implementing any wearable device, it is not only the performance that matters but the deviation in the performance within the batch of identical devices. Unfortunately, the performance of the monolayer graphene-based tattoos varies significantly, which can be attributed to the presence of grain boundaries that limit charge transport and the existence of micro and macro cracks in the cm-scale samples. Multilayer GETs 2.0, however, do not suffer from the same problem, and we show an almost 5-fold reduction in their performance deviation, assuring that each device performs almost identically. 

Besides, microsized holes have also been embossed in the topological structure of the GETs 2.0 to enable healthy sweat evaporation without substantially decreasing electrical properties. GETs 2.0 can also be used as efficient skin-wearable electronic heaters, exhibiting heating efficiency of ~6 mW/oC. 

Ultimately, the GETs were also used to monitor arterial blood pressure continuously and non-invasively [2]. In the recent work, the GETs are placed on the wrist over the radial and ulnar arteries, and the bioimpedance-based measurements (using an external circuitry) were performed, which then measure the impedance of the underlying artery. The measured bioimpedance signal is then directly correlated to the arterial volume and the dynamics of the volume change (which is correlated to Blood Pressure). Using a machine learning regression algorithm, it was possible to accurately measure blood pressure in an entirely non-invasive, continuous, cuff-less manner with the highest-grade accuracy. The GETs play an essential role in this work due to the fact that they self-adhere and provide the highest quality electrical interface for 4-probe bioimpedance [2], [12]. 

The main weaknesses of graphene electronic tattoos (GETs) are their vulnerability to scribing, scratching, or excessive shear pressure. Additionally, establishing interconnection to GETs is another essential problem to solve. GETs are only 200 nm thick, but they should be connected to a back-end circuit via much thicker and stiffer electrical connectors. Due to the stiffness mismatch, the weakest point under deformation will be the interconnection. 

Conclusions and Future Outlooks

Furthermore, other 2D materials besides graphene are being used to create wearables and tattoos that are almost invisible to the human eye. These materials have unique properties that make them ideal for use in wearables and tattoos, such as semiconductive transistors in Molybdenum disulfide (MoS2) membranes[13] and Platinum diselenide (PtSe2) tattoos [14], and superior metallic interfaces in Platinum ditelluride (PtTe2) tattoos [14]. As more 2D materials become available, it is likely that these wearables and tattoos will become even more advanced, allowing for seamless integration with the human body. In the future, these e-tattoos could enable entirely imperceptible communication with the human body.

References

[1]    D. Kireev and D. Akinwande, “Electronic Tattoos,” in Encyclopedia of Sensors and Biosensors, vol. 1, R. Narayan, Ed. Elsevier, 2023, pp. 103–114. doi: 10.1016/B978-0-12-822548-6.00132-1.

[2]    D. Kireev, K. Sel, B. Ibrahim, N. Kumar, A. Akbari, R. Jafari, and D. Akinwande, “Continuous cuffless monitoring of arterial blood pressure via graphene bioimpedance tattoos,” Nat. Nanotechnol., vol. 17, no. 8, pp. 864–870, Aug. 2022, doi: 10.1038/s41565-022-01145-w.

[3]    S. Kabiri Ameri, R. Ho, H. Jang, L. Tao, Y. Wang, L. Wang, D. M. Schnyer, D. Akinwande, and N. Lu, “Graphene Electronic Tattoo Sensors,” ACS Nano, vol. 11, no. 8, pp. 7634–7641, Aug. 2017, doi: 10.1021/acsnano.7b02182.

[4]    S. K. Ameri, M. Kim, I. A. Kuang, W. K. Perera, M. Alshiekh, H. Jeong, U. Topcu, D. Akinwande, and N. Lu, “Imperceptible electrooculography graphene sensor system for human–robot interface,” npj 2D Mater. Appl., vol. 2, no. 1, p. 19, Dec. 2018, doi: 10.1038/s41699-018-0064-4.

[5]    D. Kireev, S. K. Ameri, A. Nederveld, J. Kampfe, H. Jang, N. Lu, and D. Akinwande, “Fabrication, characterization and applications of graphene electronic tattoos,” Nat. Protoc., vol. 16, no. 5, pp. 2395–2417, May 2021, doi: 10.1038/s41596-020-00489-8.

[6]    X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S. K. Banerjee, L. Colombo, and R. S. Ruoff, “Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils,” Science (80-. )., vol. 324, no. 5932, pp. 1312–1314, May 2009, doi: 10.1126/science.1171245.

[7]    R. Arvidsson, S. Molander, and B. A. Sandén, “Review of Potential Environmental and Health Risks of the Nanomaterial Graphene,” Hum. Ecol. Risk Assess. An Int. J., no. 2011, Mar. 2013, doi: 10.1080/10807039.2012.702039.

[8]    A. Fabbro, D. Scaini, V. León, E. Vázquez, G. Cellot, G. Privitera, L. Lombardi, F. Torrisi, F. Tomarchio, F. Bonaccorso, S. Bosi, A. C. Ferrari, L. Ballerini, and M. Prato, “Graphene-Based Interfaces Do Not Alter Target Nerve Cells,” ACS Nano, vol. 10, no. 1, pp. 615–623, Jan. 2016, doi: 10.1021/acsnano.5b05647.

[9]    A. Bendali, L. H. Hess, M. Seifert, V. Forster, A. Stephan, J. a Garrido, and S. Picaud, “Purified Neurons can Survive on Peptide-Free Graphene Layers,” Adv. Healthc. Mater., vol. 2, no. 7, pp. 929–933, Jul. 2013, doi: 10.1002/adhm.201200347.

[10]    N. P. Pampaloni, M. Lottner, M. Giugliano, A. Matruglio, F. D’Amico, M. Prato, J. A. Garrido, L. Ballerini, and D. Scaini, “Single-layer graphene modulates neuronal communication and augments membrane ion currents,” Nat. Nanotechnol., vol. 13, no. 8, pp. 755–764, Aug. 2018, doi: 10.1038/s41565-018-0163-6.

[11]    D. Kireev, J. Kampfe, A. Hall, and D. Akinwande, “Graphene electronic tattoos 2.0 with enhanced performance, breathability and robustness,” npj 2D Mater. Appl., vol. 6, no. 1, p. 46, Dec. 2022, doi: 10.1038/s41699-022-00324-6.

[12]    K. Sel, D. Kireev, A. Brown, B. Ibrahim, D. Akinwande, and R. Jafari, “Electrical Characterization of Graphene-based e-Tattoos for Bio-Impedance-based Physiological Sensing,” in 2019 IEEE Biomedical Circuits and Systems Conference (BioCAS), Oct. 2019, pp. 1–4. doi: 10.1109/BIOCAS.2019.8919003.

[13]    Z. Yan, D. Xu, Z. Lin, P. Wang, B. Cao, H. Ren, F. Song, C. Wan, L. Wang, J. Zhou, X. Zhao, J. Chen, Y. Huang, and X. Duan, “Highly stretchable van der Waals thin films for adaptable and breathable electronic membranes,” Science (80-. )., vol. 375, no. 6583, pp. 852–859, Feb. 2022, doi: 10.1126/science.abl8941.

[14]    D. Kireev, E. Okogbue, R. R. T. Jayanth, T. Ko, Y. Jung, and D. Akinwande, “Multipurpose and Reusable Ultrathin Electronic Tattoos Based on PtSe 2 and PtTe 2,” ACS Nano, vol. 15, no. 2, pp. 2800–2811, Feb. 2021, doi: 10.1021/acsnano.0c08689.

Biography of the author

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