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Fabrication, characterization and applications of graphene electronic tattoos

Nature Protocols volume 16, pages 2395–2417 (2021)Cite this article

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Abstract

Numerous fields of science and technology, including healthcare, robotics and bioelectronics, have begun to switch their research direction from developing ‘high-end, high-cost’ tools towards ‘high-end, low-cost’ solutions. Graphene electronic tattoos (GETs), whose fabrication protocol is discussed in this work, are ideal building blocks of future wearable technology due to their outstanding electromechanical properties. The GETs are composed of high-quality, large-scale graphene that is transferred onto tattoo paper, resulting in an electronic device that is applied onto skin like a temporary tattoo. Here, we provide a comprehensive GET fabrication protocol, starting from graphene growth and ending with integration onto human skin. The methodology presented is unique since it utilizes high-quality electronic-grade graphene, while the processing is done by using low-cost and off-the-shelf methods, such as a mechanical cutter plotter. The GETs can be either used in combination with advanced scientific equipment to perform precision experiments, or with low-cost electrophysiology boards, to conduct similar operations from home. In this protocol, we showcase how GETs can be applied onto the human body and how they can be used to obtain a variety of biopotentials, including electroencephalogram (brain waves), electrocardiogram (heart activity), electromyogram (muscle activity), as well as monitoring of body temperature and hydration. With graphene available from commercial sources, the whole protocol consumes ~3 h of labor and does not require highly trained personnel. The protocol described in this work can be readily replicated in simple laboratories, including high school facilities.

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Fig. 1: Use of GETs for electrophysiological sensing.
Fig. 2: Monitoring skin temperature and hydration via GETs.
Fig. 3: Schematics of GET-based electrooculography and its use for HMI.
Fig. 4: Schematics of the CVD growth process and an approximate timeline recipe for graphene growth.
Fig. 5: Preparation of the copper foil (Steps 1–4).
Fig. 6: Preparation of a bare tattoo substrate (Steps 6–9).
Fig. 7: Transfer of PMMA/graphene flake from the copper etchant (light blue) into clean DI water (Steps 13–15).
Fig. 8: The graphene flip process (Step 18).
Fig. 9: Shaping GETs via the cameo plotter (Steps 20–22).
Fig. 10: Placement of conductive wires and transfer of GETs.
Fig. 11: Electrical performance of the GETs on skin.
Fig. 12: A typical biGET’s temperature response.
Fig. 13: ECG measurement setup via Open BCI ganglion board.
Fig. 14: Impedance vs. sheet resistance comparison for mono- (red), bi- (green), and tri- (blue) layer configurations of graphene from three different suppliers (triangle, circle, and square).

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References

  1. Flores, M., Glusman, G., Brogaard, K., Price, N. D. & Hood, L. P4 medicine: how systems medicine will transform the healthcare sector and society. Per. Med. 10, 565–576 (2013).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  2. Becker, S. et al. mHealth 2.0: experiences, possibilities, and perspectives. JMIR mHealth uHealth 2, e24 (2014).

    Article  PubMed  PubMed Central  Google Scholar 

  3. Chow, C. K., Ariyarathna, N., Islam, S. M. S., Thiagalingam, A. & Redfern, J. mHealth in cardiovascular health care. Heart Lung Circ. 25, 802–807 (2016).

    Article  PubMed  Google Scholar 

  4. Price, M. et al. mHealth: A mechanism to deliver more accessible, more effective mental health care. Clin. Psychol. Psychother. 21, 427–436 (2014).

    Article  PubMed  Google Scholar 

  5. Ray, T. R. et al. Bio-integrated wearable systems: a comprehensive review. Chem. Rev. 119, 5461–5533 (2019).

    Article  CAS  PubMed  Google Scholar 

  6. Mukhopadhyay, S. C. Wearable sensors for human activity monitoring: a review. IEEE Sens. J. 15, 1321–1330 (2015).

    Article  Google Scholar 

  7. Heikenfeld, J. et al. Wearable sensors: modalities, challenges, and prospects. Lab Chip 18, (2017).

  8. Neethirajan, S. Recent advances in wearable sensors for animal health management. Sens. Bio-Sensing Res 12, 15–29 (2017).

    Article  Google Scholar 

  9. Guk, K. et al. Evolution of wearable devices with real-time disease monitoring for personalized healthcare. Nanomaterials 9, 1–23 (2019).

    Article  CAS  Google Scholar 

  10. Wearable Technology Market—Growth, Trends, and Forecasts (2020–2025). https://www.researchandmarkets.com/reports/4591296/wearable-technology-market-growth-trends-and

  11. Wang, C. et al. Monitoring of the central blood pressure waveform via a conformal ultrasonic device. Nat. Biomed. Eng. 2, 687–695 (2018).

    Article  PubMed  PubMed Central  Google Scholar 

  12. Kim, D.-H. et al. Epidermal electronics. Science 333, 838–843 (2011).

    Article  CAS  PubMed  Google Scholar 

  13. Gutruf, P. et al. Fully implantable optoelectronic systems for battery-free, multimodal operation in neuroscience research. Nat. Electron. 1, 652–660 (2018).

    Article  Google Scholar 

  14. Miro, P. et al. An atlas of two-dimensional materials. Chem. Soc. Rev. 43, 6537–6554 (2014).

    Article  CAS  PubMed  Google Scholar 

  15. Ferrari, A. C. et al. Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems. Nanoscale 7, 4598–4810 (2015).

    Article  CAS  Google Scholar 

  16. Chhowalla, M., Jena, D. & Zhang, H. Two-dimensional semiconductors for transistors. Nat. Rev. Mater 1, 16052 (2016).

    Article  CAS  Google Scholar 

  17. Bhimanapati, G. R. et al. Recent advances in two-dimensional materials beyond graphene. ACS Nano 9, 11509–11539 (2015).

    Article  CAS  PubMed  Google Scholar 

  18. Kireev, D., Offenhaeusser, A. & Offenhäusser, A. Graphene & two-dimensional devices for bioelectronics and neuroprosthetics. 2D Mater 5, 042004 (2018).

    Article  CAS  Google Scholar 

  19. Hess, L. H., Seifert, M. & Garrido, J. A. Graphene transistors for bioelectronics. Proc. IEEE 101, 1780–1792 (2013).

    Article  CAS  Google Scholar 

  20. Kireev, D. et al. Graphene transistors for interfacing with cells: towards a deeper understanding of liquid gating and sensitivity. Sci. Rep. 7, 6658 (2017).

    Article  PubMed  PubMed Central  Google Scholar 

  21. Kireev, D. et al. Graphene multielectrode arrays as a versatile tool for extracellular measurements. Adv. Healthc. Mater. 6, 1601433 (2017).

    Article  Google Scholar 

  22. Kabiri Ameri, S. et al. Graphene electronic tattoo sensors. ACS Nano 11, 7634–7641 (2017).

    Article  CAS  PubMed  Google Scholar 

  23. Ameri, S. K. et al. Imperceptible electrooculography graphene sensor system for human–robot interface. npj 2D Mater. Appl. 2, 19 (2018).

    Article  Google Scholar 

  24. Sel, K. et al. Electrical characterization of graphene-based e-tattoos for bio-impedance-based physiological sensing. BioCAS 2019 - Biomed. Circuits Syst. Conf. Proc. 1–4 (2019). https://doi.org/10.1109/BIOCAS.2019.8919003

  25. Zhang, X. et al. Ultrasensitive field‐effect biosensors enabled by the unique electronic properties of graphene. Small 16, 1902820 (2020).

    Article  CAS  Google Scholar 

  26. Huang, H. et al. Graphene-based sensors for human health monitoring. Front. Chem. 7, 1–26 (2019).

    Article  Google Scholar 

  27. Wassei, J. K. & Kaner, R. B. Graphene, a promising transparent conductor. Mater. Today 13, 52–59 (2010).

    Article  CAS  Google Scholar 

  28. Das, T., Sharma, B. K., Katiyar, A. K. & Ahn, J.-H. H. Graphene-based flexible and wearable electronics. J. Semicond. 39, 011007 (2018).

    Article  Google Scholar 

  29. Park, D. et al. Fabrication and utility of a transparent graphene neural electrode array for electrophysiology, in vivo imaging, and optogenetics. Nat. Protoc. 11, 2201–2222 (2016).

    Article  CAS  PubMed  Google Scholar 

  30. Song, J.-K. et al. Wearable force touch sensor array using a flexible and transparent electrode. Adv. Funct. Mater. 27, 1605286 (2017).

    Article  Google Scholar 

  31. Yang, S. et al. “Cut-and-paste” manufacture of multiparametric epidermal sensor systems. Adv. Mater. 27, 6423–6430 (2015).

    Article  CAS  PubMed  Google Scholar 

  32. Wang, Y. et al. Low-cost, μm-thick, tape-free electronic tattoo sensors with minimized motion and sweat artifacts. npj Flex. Electron. 2, 6 (2018).

    Article  Google Scholar 

  33. Stier, A. et al. Stretchable tattoo-like heater with on-site temperature feedback control. Micromachines 9, 170 (2018).

    Article  PubMed Central  Google Scholar 

  34. Bae, S. et al. Roll-to-roll production of 30-inch graphene films for transparent electrodes. Nat. Nanotechnol. 5, 574–578 (2010).

    Article  CAS  Google Scholar 

  35. DiMarco, J. P. & Philbrick, J. T. Use of ambulatory electrocardiographic (Holter) monitoring. Ann. Intern. Med. 113, 53–68 (1990).

    Article  CAS  PubMed  Google Scholar 

  36. Montain, S. J. & Ely, M. Water Requirements and Soldier Hydration (Borden Institute). https://ke.army.mil/bordeninstitute/other_pub/hydrationpdf.pdf

  37. Sawka, M. N., Cheuvront, S. N. & Kenefick, R. W. High skin temperature and hypohydration impair aerobic performance. Exp. Physiol. 97, 327–332 (2012).

    Article  PubMed  Google Scholar 

  38. Kumari, P., Mathew, L. & Syal, P. Increasing trend of wearables and multimodal interface for human activity monitoring: a review. Biosens. Bioelectron. 90, 298–307 (2017).

    Article  CAS  PubMed  Google Scholar 

  39. Barea Navarro, R., Boquete Vázquez, L. & López Guillén, E. EOG-based wheelchair control. in Smart Wheelchairs and Brain-Computer Interfaces 381–403 (Elsevier, 2018). https://doi.org/10.1016/B978-0-12-812892-3.00016-9

  40. Sel, K., Ibrahim, B. & Jafari, R. ImpediBands: body coupled bio-impedance patches for physiological sensing proof of concept. IEEE Trans. Biomed. Circuits Syst. 14, 757–774 (2020).

    Article  PubMed  Google Scholar 

  41. Ibrahim, B. & Jafari, R. A novel method for continuous blood pressure monitoring using wrist-worn bio-impedance sensors. 2018 IEEE Biomed. Circuits Syst. Conf. BioCAS 2018 - Proc. 3–6 (2018). https://doi.org/10.1109/BIOCAS.2018.8584783

  42. Masvidal-Codina, E. et al. High-resolution mapping of infraslow cortical brain activity enabled by graphene microtransistors. Nat. Mater. 18, 280–288 (2019).

    Article  CAS  PubMed  Google Scholar 

  43. Polat, E. O. et al. Flexible graphene photodetectors for wearable fitness monitoring. Sci. Adv. 5, eaaw7846 (2019).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  44. Miao, P. et al. Graphene nanostructure-based tactile sensors for electronic skin applications. Nano-Micro Lett 11, 71 (2019).

    Article  CAS  Google Scholar 

  45. Saccomandi, P. et al. Microfabricated tactile sensors for biomedical applications: a review. Biosensors 4, 422–448 (2014).

    Article  PubMed  PubMed Central  Google Scholar 

  46. Chortos, A., Liu, J. & Bao, Z. Pursuing prosthetic electronic skin. Nat. Mater. 15, 937–950 (2016).

    Article  CAS  PubMed  Google Scholar 

  47. Chang, T. H., Li, K., Yang, H. & Chen, P. Y. Multifunctionality and mechanical actuation of 2D materials for skin-mimicking capabilities. Adv. Mater. 30, 1–13 (2018).

    Article  Google Scholar 

  48. Akinwande, D., Petrone, N. & Hone, J. Two-dimensional flexible nanoelectronics. Nat. Commun. 5, 5678 (2014).

    Article  CAS  PubMed  Google Scholar 

  49. Qiao, Y. et al. Graphene-based wearable sensors. Nanoscale 11, 18923–18945 (2019).

    Article  CAS  PubMed  Google Scholar 

  50. Amjadi, M., Kyung, K.-U., Park, I. & Sitti, M. Stretchable, skin-mountable, and wearable strain sensors and their potential applications: a review. Adv. Funct. Mater. 26, 1678–1698 (2016).

    Article  CAS  Google Scholar 

  51. Jang, H., Dai, Z., Ha, K.-H., Ameri, S. K. & Lu, N. Stretchability of PMMA-supported CVD graphene and of its electrical contacts. 2D Mater 7, 014003 (2019).

    Article  Google Scholar 

  52. Yetisen, A. K., Martinez-Hurtado, J. L., Ünal, B., Khademhosseini, A. & Butt, H. Wearables in medicine. Adv. Mater. 30, e1706910 (2018).

    Article  Google Scholar 

  53. Tricoli, A., Nasiri, N. & De, S. Wearable and miniaturized sensor technologies for personalized and preventive medicine. Adv. Funct. Mater. 27, 1–19 (2017).

    Article  Google Scholar 

  54. Griss, P., Tolvanen-Laakso, H. K., Meriläinen, P. & Stemme, G. Characterization of micromachined spiked biopotential electrodes. IEEE Trans. Biomed. Eng. 49, 597–604 (2002).

    Article  PubMed  Google Scholar 

  55. Kirkup, L. & Searle, A. A direct comparison of wet, dry and insulating bioelectric recording electrodes. Physiol. Meas. 21, 271 (2000).

    Article  PubMed  Google Scholar 

  56. Giffney, T., Bejanin, E., Kurian, A. S., Travas-Sejdic, J. & Aw, K. Highly stretchable printed strain sensors using multi-walled carbon nanotube/silicone rubber composites. Sensors Actuators A Phys 259, 44–49 (2017).

    Article  CAS  Google Scholar 

  57. Cheng, Y., Wang, R., Zhai, H. & Sun, J. Stretchable electronic skin based on silver nanowire composite fiber electrodes for sensing pressure, proximity, and multidirectional strain. Nanoscale 9, 3834–3842 (2017).

    Article  CAS  PubMed  Google Scholar 

  58. Herbert, R., Kim, J. H., Kim, Y. S., Lee, H. M. & Yeo, W. H. Soft material-enabled, flexible hybrid electronics for medicine, healthcare, and human-machine interfaces. Materials (Basel) 11, 187 (2018).

    Article  Google Scholar 

  59. Kang, J., Tok, J. B. H. & Bao, Z. Self-healing soft electronics. Nat. Electron. 2, 144–150 (2019).

    Article  Google Scholar 

  60. Bauer, S. et al. 25th anniversary article: A soft future: From robots and sensor skin to energy harvesters. Adv. Mater. 26, 149–162 (2014).

    Article  CAS  PubMed  Google Scholar 

  61. Pang, C., Lee, C. & Suh, K. Y. Recent advances in flexible sensors for wearable and implantable devices. J. Appl. Polym. Sci. 130, 1429–1441 (2013).

    Article  CAS  Google Scholar 

  62. Nezakati, T., Seifalian, A., Tan, A. & Seifalian, A. M. Conductive polymers: opportunities and challenges in biomedical applications. Chem. Rev. 118, 6766–6843 (2018).

    Article  CAS  PubMed  Google Scholar 

  63. Bihar, E. et al. Fully printed all-polymer tattoo/textile electronics for electromyography. Flex. Print. Electron. 3, 034004 (2018).

    Article  Google Scholar 

  64. Ferrari, L. M. et al. Ultraconformable temporary tattoo electrodes for electrophysiology. Adv. Sci. 5, 1700771 (2018).

    Article  Google Scholar 

  65. Yeo, W. H. et al. Multifunctional epidermal electronics printed directly onto the skin. Adv. Mater. 25, 2773–2778 (2013).

    Article  CAS  PubMed  Google Scholar 

  66. Huang, X., Yeo, W. H., Liu, Y. & Rogers, J. A. Epidermal differential impedance sensor for conformal skin hydration monitoring. Biointerphases 7, 1–9 (2012).

    Article  Google Scholar 

  67. Gutruf, P. & Rogers, J. A. Implantable, wireless device platforms for neuroscience research. Curr. Opin. Neurobiol. 50, 42–49 (2018).

    Article  CAS  PubMed  Google Scholar 

  68. Ferrari, L. M., Ismailov, U., Badier, J.-M., Greco, F. & Ismailova, E. Conducting polymer tattoo electrodes in clinical electro- and magneto-encephalography. npj Flex. Electron. 4, 1–9 (2020).

    Article  Google Scholar 

  69. Webb, R. C. et al. Ultrathin conformal devices for precise and continuous thermal characterization of human skin. Nat. Mater. 12, 938–944 (2013).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  70. Bariya, M., Nyein, H. Y. Y. & Javey, A. Wearable sweat sensors. Nat. Electron. 1, 160–171 (2018).

    Article  Google Scholar 

  71. Chung, H. J. et al. Stretchable, multiplexed pH sensors with demonstrations on rabbit and human hearts undergoing ischemia. Adv. Healthc. Mater. 3, 59–68 (2014).

    Article  CAS  PubMed  Google Scholar 

  72. Miyamoto, A. et al. Inflammation-free, gas-permeable, lightweight, stretchable on-skin electronics with nanomeshes. Nat. Nanotechnol. 12, 907–913 (2017).

    Article  CAS  PubMed  Google Scholar 

  73. Gong, S. et al. Local crack-programmed gold nanowire electronic skin tattoos for in-plane multisensor integration. Adv. Mater. 31, 1–8 (2019).

    Article  Google Scholar 

  74. Ershad, F. et al. Ultra-conformal drawn-on-skin electronics for multifunctional motion artifact-free sensing and point-of-care treatment. Nat. Commun. 11, 3823 (2020).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  75. Pan, C. et al. Visually imperceptible liquid-metal circuits for transparent, stretchable electronics with direct laser writing. Adv. Mater. 30, 1706937 (2018).

    Article  Google Scholar 

  76. Childres, I., Jauregui, L. A., Tian, J. & Chen, Y. P. Effect of oxygen plasma etching on graphene studied using Raman spectroscopy and electronic transport measurements. New J. Phys. 13, 025008 (2011).

    Article  Google Scholar 

  77. Liu, N. et al. Ultratransparent and stretchable graphene electrodes. Sci. Adv. 3, e1700159 (2017).

    Article  PubMed  PubMed Central  Google Scholar 

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Acknowledgements

This work was supported in part by the Office of Naval Research grant N00014-18-1-2706. We also acknowledge the support, in part, of National Science Foundation (NSF) grant 2031674. We thank F. Qing of UESTC, and NASCENT-Grolltex collaboration for providing us with large-scale CVD-grown graphene.

Author information

Authors and Affiliations

  1. Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, TX, USA

    Dmitry Kireev, Alena Nederveld, Jameson Kampfe, Nanshu Lu & Deji Akinwande

  2. Microelectronics Research Center, The University of Texas at Austin, Austin, TX, USA

    Dmitry Kireev & Deji Akinwande

  3. Department of Electrical and Computer Engineering, Queen’s University, Kingston, Ontario, Canada

    Shideh Kabiri Ameri

  4. Texas Materials Institute, The University of Texas at Austin, Austin, TX, USA

    Hongwoo Jang, Nanshu Lu & Deji Akinwande

  5. Department of Biomedical Engineering, The University of Texas at Austin, Austin, TX, USA

    Nanshu Lu

  6. Department of Aerospace Engineering and Engineering Mechanics, The University of Texas at Austin, Austin, TX, USA

    Nanshu Lu

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Contributions

S.K.A., N.L. and D.A. conceived the idea and performed initial experiments. D.K. and S.K.A. optimized the procedure. D.K. and S.K.A. developed the protocol. D.K., S.K.A., A.N., H.J. and J.K. performed the experiments and analyzed the data. D.K. complied the data, wrote the manuscript, and designed the video supplements. All authors discussed the results and contributed to the editing of the manuscript.

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Correspondence to Dmitry Kireev.

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The authors declare no competing interests.

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Peer review information Nature Protocols thanks Mario Caironi, Wenlong Cheng and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Related links

Key references using this protocol:

Kabiri Ameri, S. et al. ACS Nano 11, 7634–7641 (2017): https://doi.org/10.1021/acsnano.7b02182

Ameri, S. K. et al. npj 2D Mater. Appl. 2, 1–7 (2018): https://doi.org/10.1038/s41699-018-0064-4

Sel, K. et al. BioCAS 2019 - Biomed. Circuits Syst. Conf. Proc. 1–4 (2019): https://doi.org/10.1109/BIOCAS.2019.8919003

Supplementary information

Supplementary Information

Supplementary Tables 1 and 2.

Reporting Summary

Supplementary Video 1

Visual aid to help understanding procedure for Step 18 of the protocol – reversing graphene

Supplementary Video 2

Visual aid to help understanding procedure for Step 21 of the protocol – shaping GETs via Cameo plotter

Supplementary Video 3

Visual aid to help understanding procedure for Step 22 of the protocol – removing excess of graphene.

Supplementary Video 4

Visual aid to help understanding procedure for Step 24-Option A of the protocol – contacting GETs via copper tape

Supplementary Video 5

Visual aid to help understanding procedure for Step 24-Option B of the protocol – contacting GETs via silver epoxy

Supplementary Video 6

Visual aid to help understanding procedure for Step 25 – troubleshooting the GET transfer on skin

Source data

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Source Data Fig. 2

Data files supporting the graphs, figures, plots.

Source Data Fig. 11

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Source Data Fig. 12

Data files supporting the graphs, figures, plots.

Source Data Fig. 13

Data files supporting the graphs, figures, plots.

Source Data Fig. 14

Data files supporting the graphs, figures, plots.

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Kireev, D., Ameri, S.K., Nederveld, A. et al. Fabrication, characterization and applications of graphene electronic tattoos. Nat Protoc 16, 2395–2417 (2021). https://doi.org/10.1038/s41596-020-00489-8

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