Date Published: January 03, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Laura M. Ferrari, Sudha Sudha, Sergio Tarantino, Roberto Esposti, Francesco Bolzoni, Paolo Cavallari, Christian Cipriani, Virgilio Mattoli, Francesco Greco.
Electrically interfacing the skin for monitoring personal health condition is the basis of skin‐contact electrophysiology. In the clinical practice the use of stiff and bulky pregelled or dry electrodes, in contrast to the soft body tissues, imposes severe restrictions to user comfort and mobility while limiting clinical applications. Here, in this work dry, unperceivable temporary tattoo electrodes are presented. Customized single or multielectrode arrays are readily fabricated by inkjet printing of conducting polymer onto commercial decal transfer paper, which allows for easy transfer on the user’s skin. Conformal adhesion to the skin is provided thanks to their ultralow thickness (<1 µm). Tattoo electrode–skin contact impedance is characterized on short‐ (1 h) and long‐term (48 h) and compared with standard pregelled and dry electrodes. The viability in electrophysiology is validated by surface electromyography and electrocardiography recordings on various locations on limbs and face. A novel concept of tattoo as perforable skin‐contact electrode, through which hairs can grow, is demonstrated, thus permitting to envision very long‐term recordings on areas with high hair density. The proposed materials and patterning strategy make this technology amenable for large‐scale production of low‐cost sensing devices.
The recent surge of “epidermal electronics”1 field has been enabled by advancements in technology of electronic materials: devices go thinner and more flexible (or even stretchable) than ever.2, 3, 4, 5 These innovations have seen the introduction of organic electronic materials in the portfolio of materials scientists and electronic engineers, besides the well‐known inorganic semiconductors, metals, and oxides.6, 7 Progresses in printed electronics provided suitable techniques for fabrication of devices above novel plastic or paper substrates with large‐area/high‐throughput capability.8, 9 On the other hand, interfacing devices with skin can open new pathways for applications, like the development of unprecedented personal health monitoring systems, sensors, drug delivery systems, among the others.10, 11, 12 Indeed, electrically interfacing with the outer layer of our body—the skin—can provide important information about the health condition, through the monitoring of chemical and physical variables. For instance muscles, brain, or heart activity, as well as other vital parameters,5, 13, 14, 15 could be tracked and appropriately processed. Skin‐contact electrophysiology, with its well‐established and multifacet techniques—such as electromyography (EMG), electrooculography, electrocardiography (ECG), electroencephalography (EEG)—consists in the recording of the electrical activity related to functioning of cells, tissues, organs, with the principal purposes of diagnosis and monitoring. By means of electrodes placed on the skin, voltage or current recording permits to analyze the functioning of different tissues and organs, at different resolution levels, depending on the technique and number/density of electrodes used. Most skin‐contact electrodes in use for clinical practice or research (Ag/AgCl) operate with an electrolytic gel for establishing a stable electrode–skin wet interface.16, 17 Although their intensive use, reported drawbacks of wet (gelled) electrodes are: limited signal stability over time (as the gel dries out),18 short circuit in high density recording (due to the gel leakage across neighboring electrodes), discomfort, heft/rigidity against skin compliance, and the requirement for skin cleaning/preparation. In the quest for unobtrusive, lightweight yet reliable electrodes interfacing with skin we recently proposed conducting polymer‐based temporary tattoo nanosheets.19 Temporary tattoo paper was used as an unconventional substrate for electrodes fabrication. This substrate included a thin (around 500 nm thickness) ethylcellulose (EC) layer on top of which a layer for poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) was spin coated. The tattoo electrode was then transferred onto the target surface (skin) by dissolution of a water‐soluble sacrificial layer. The same substrate has been recently proposed for fabricating skin‐worn sensors for metabolites.5, 20, 21 However, in order to meet the requirements toward practical applications, like vital parameters monitoring in healthcare and sport, such tattoo electrode technology needs to be advanced. On the one hand, patterning of electrodes is strongly required, as multielectrode array tattoo is needed in any of the mentioned applications. Moreover, integration of stable interconnections and external connectors is mandatory, while maintaining an easy and reliable transfer on the skin. The main challenge is to optimize functionality while retaining ultralow thickness and flexibility, thus enabling a seamless interface.22, 23, 24 Lately, Hanein and co‐workers25 elaborated on a similar strategy as Zucca et al.19 and developed patterned electrodes on temporary tattoo paper by using screen printing of carbon ink followed by plasma polymerization of PEDOT:PSS. Recording of EMG signals and operation up to 3 h were demonstrated. However such tattoo electrodes were relatively thick (substrate 1.2 µm, overall thickness of electrodes around 100 µm), thus preventing a conformal contact with the complex texture of skin, made of 10–100 µm thick valleys and ridges. Recently, a “wet transfer, dry patterning strategy” has been proposed for ultrathin graphene electronic tattoo sensors,26 in which graphene is supported on a thin (around 440 nm) polymethylmethacrylate or thicker (around 13 µm) polyimide layer. These are first transferred on tattoo paper (few µm thick) or medical adhesive (few mm thick), cutted out in the form of ribbons, then transferred on skin by wetting the tattoo paper, and used for electrophysiological recordings. Actually seamless interfacing is sought in skin‐contact electrophysiology applications, in order to improve the quality of the recording. An ultrathin, therefore ultraconformable, sensor interface can maximize the contact area (hence signal amplitude) and may avoid movement artifacts arising from the relative displacement between skin and electrodes. Moreover, comfort for the user and long‐term stability are important requirements especially in long‐term recordings or whenever complex or delicate anatomical districts have to be investigated (e.g. the face). Intrinsic obtrusiveness and weight of the electrodes severely curtail such recordings, and can hamper or modify the natural skin stretching/displacement caused by the activation of underlying muscles. As a matter of fact, an unperceivable electrode technology can open new possibilities for skin‐contact sensing, both for clinical diagnosis/monitoring and for research. In order to address the above‐mentioned requirements and starting from our previous results, we elaborated a strategy for the fabrication and multilayer assembling of temporary single or multielectrode tattoo. Inkjet printing was adopted for fabrication, as this noncontact manufacturing method can allow for low‐cost, large‐area, reliable, high resolution device production, and possibly future integration with other printed electronic components.9 In this paper we present the materials and processes used to manufacture various kinds of ultrathin and ultraconformable single electrode tattoos and temporary tattoo multielectrodes arrays (TTMEAs), suitable for different electrophysiological measurements. Their thickness, electrical properties, stability, and interface with skin are assessed and discussed. In particular, the electrode–skin contact impedance of the tattoo electrodes is compared with state‐of‐art electrodes. Electrode–skin contact impedance was recorded up to 2 days, and its substantial stability was demonstrated. This suggests the use in long‐term continuous applications, e.g. monitoring of vital parameters. Several proofs of concept for application in face or limb EMG, as well as ECG are presented and discussed. Finally, we demonstrate hair growing through tattoo electrodes over 24 h, with no visible effect on the quality of the recording. This result allows to envision unprecedented applications, e.g. long‐term monitoring of EEG activity on shaved scalp, which is currently not possible with wet (gelled) electrodes due to the drying of the electrolyte and eventual displacement of the electrodes caused by hair growing.
The results presented so far provide a comprehensive characterization of tattoo electrodes as regards their structure, their conformal adhesion and their dry electrical interfacing with skin. The use of dry contact eliminates drawbacks arising from a gel layer and its stability over time. With an overall thickness lower than 1 µm, we presented the thinnest skin‐contact electrodes to the best of our knowledge. This guaranteed excellent unperceivability. Impedance records highlighted the adoption of tattoo electrodes over long term and with performance comparable with standard electrodes. EMG and ECG recordings permitted to appreciate good recording capabilities in combination with dedicated laboratory measurement setup and also with commercially available portable devices. The adopted manufacturing method, based on inkjet printing and lamination, enables to fabricate multielectrodes arrays with freedom in design. The latter is desirable in view of specific applications in diagnostics, human machine interfacing and personal health monitoring in sport, among others. Based on EMG records, TTMEAs can be thought as ideal electrodes for facial recognition; indeed, soft and lightweight tattoo electrodes don’t modify natural movements of users as in the case of bulky electrodes. Future directions of this research should also consider recording of long‐term EEG on scalp, which is impossible to perform with standard electrodes due to the displacement of electrodes during hair growth. On the contrary, tattoo electrodes can be perforated by hairs growing through it without a visible loss of recording capability, and remain stably adhered to skin. While two different strategies for wired connection with external devices are presented, further efforts should be dedicated to establish wireless communication, e.g. through printed RF antenna and analogic front‐end on the same tattoo substrate.
Materials: PEDOT:PSS aqueous dispersion (Clevios PJet 700 by Heraeus) had been used to fabricate the sensing part of the electrodes. Commercially available temporary transfer tattoo paper kit (Tattoo 2.1, by The Magic Touch Ltd., UK), composed of two sheets, decal transfer paper and glue sheet, had been employed as unconventional substrate and passivation layer, respectively (Figure S1, Supporting Information). The decal paper, used as substrate for electrodes fabrication, was made up of three layers: a paper carrier, a starch–dextrin water soluble layer, and an EC (thickness ≈ 450 nm) layer. The glue part was a three‐layered sheet composed of a silicone paper carrier sheet, acrylic glue (thickness ≈ 700 nm), and a plastic liner. The glue part was used for providing tattoo adhesion while preventing direct contact of interconnection lines with skin. Polyimide film (Kapton by Goodfellow, thickness 13 µm) had been employed as support layer for the external electrical connection, in case of planar connectors. Standard acrylic transparent sheet (thickness 0.3 mm) was used to build up laser‐cut masks.
The authors declare no conflict of interest.