Conformable Wearable Electrodes: From Fabrication to Electrophysiological Assessment

Summary

Two recent technologies-tattoo and textiles-have demonstrated promising results in cutaneous sensing. Here, we present the fabrication and evaluation methods of tattoo and textile electrodes for cutaneous electrophysiological sensing. These electronic interfaces made of conductive polymers outperform the existing standards in terms of comfort and sensitivity.

Abstract

Wearable electronic devices are becoming key players in monitoring the body signals predominantly altered during physical activity tracking. Considering the growing interest in telemedicine and personalized care driven by the rise of the Internet of Things era, wearable sensors have expanded their field of application into healthcare. To ensure the collection of clinically relevant data, these devices need to establish conformable interfaces with the human body to provide high-signal-quality recordings and long-term operation. To this end, this paper presents a method to easily fabricate conformable thin tattoo- and soft textile-based sensors for their application as wearable organic electronic devices in a broad spectrum of surface electrophysiological recordings.

The sensors are developed through a cost-effective and scalable process of cutaneous electrode patterning using poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), the most popular conductive polymer in bioelectronics, on off-the-shelf, wearable substrates. This paper presents key steps in electrode characterization through impedance spectroscopy to investigate their performance in signal transduction when coupled with the skin. Comparative studies are required to position the performance of novel sensors with respect to the clinical gold standard. To validate the fabricated sensors' performance, this protocol shows how to perform various biosignal recordings from different configurations through a user-friendly and portable electronic setup in a laboratory environment. This methods paper will allow multiple experimental initiatives to advance the current state of the art in wearable sensors for human body health monitoring.

Introduction

Noninvasive biopotential recording is performed through skin-contact electrodes, providing a vast amount of data on the physiological status of the human body in fitness and healthcare¹. Novel types of wearable biomonitoring devices have been developed from the latest technological advances in electronics through the downscaling of integrated controlling and communicating components to portable dimensions. Smart monitoring devices pervade the market daily, offering multiple monitoring capabilities with the ultimate goal of providing sufficient physiological content to enable medical diagnostics². Therefore, safe, reliable, and robust interfaces with the human body present critical challenges in the development of legitimate wearable technologies for healthcare. Tattoo and textile electrodes have recently appeared as reliable and stable interfaces perceived as innovative, comfortable devices for wearable biosensing^3,4,5.

Tattoo sensors are dry and thin interfaces that, owing to their low thickness (~1 µm), ensure adhesive-free, conformable skin contact. They are based on a commercially available tattoo paper kit composed of a layered structure, which allows the release of an ultrathin polymeric layer on the skin⁶. The layered structure also allows for easy handling of the thin polymeric layer during the sensor's fabrication process and its transfer to the skin. The final electrode is fully conformable and almost imperceptible to the wearer. Textile sensors are electronic devices obtained from fabric functionalization with electroactive materials⁷. They are mainly integrated or simply sewed into clothes to ensure the user's comfort due to their softness, breathability, and evident affinity with garments. For almost a decade, textile and tattoo electrodes have been assessed in surface electrophysiological recordings^3,8,9, showing good results both in wearability and signal quality recordings and reporting high signal-to-noise ratio (SNR) in short- and long-term evaluations. They are also conceived as a potential platform for wearable biochemical sweat analysis^1,10.

The growing interest in tattoo, textile, and, in general, flexible thin film technologies (e.g., those made of plastic foils such as parylene or different elastomers) is mainly promoted by the compatibility with low-cost and scalable fabrication methods. Screen printing, inkjet printing, direct patterning, dip coating, and stamp transfer have been successfully adopted to produce such kinds of electronic interfaces¹¹. Among these, inkjet printing is the most advanced digital and fast prototyping technique. It is mainly applied to the patterning of conductive inks in a non-contact, additive fashion under ambient conditions and on a large variety of substrates¹². Although multiple wearable sensors have been fabricated through noble metal ink patterning¹³, metal films are brittle and undergo cracking when mechanically stressed. Different research groups have adopted different strategies to endow metals with the property of mechanical compatibility with skin. These strategies include reducing the film thickness and using serpentine designs or wrinkled and prestretched substrates^14,15,16. Soft and intrinsically flexible conductive materials, such as conductive polymers, found their application in flexible bioelectronic devices. Their polymeric flexibility is combined with electric and ionic conductivity. PEDOT:PSS is the most used conductive polymer in bioelectronics. It is characterized by softness, biocompatibility, sustainability, and printing processability¹⁷, which make it compatible with the widespread production of biomedical devices.

Devices, such as planar electrodes connected to an acquisition system, allow the recording of biopotentials in health monitoring. Human body biopotentials are electrical signals generated by electrogenic cells that propagate through the body up to the skin surface. According to where the electrodes are placed, it is possible to acquire data related to the electrical activity of the brain (EEG), muscles (EMG), heart (ECG), and skin conductivity (e.g., bioimpedance or electrodermal activity, EDA). The quality of the data is then assessed to evaluate the usability of the electrodes in clinical applications. A high SNR defines their performance¹⁸, which is typically compared with state-of-the-art Ag/AgCl electrode recordings. Although the Ag/AgCl electrodes also have high SNR, they lack long-term operationality and conformable wearability. High-quality biosignal recordings provide insights into human health status related to a particular organ's function. Thus, these benefits of comfortable tattoo or textile interfaces indicate their promise for long-term applications that can enable real-life mobile health monitoring and pave the way for the development of telemedicine¹⁹.

This paper reports how to fabricate and assess tattoo and textile electrodes in health biomonitoring. After its fabrication, a novel electrode must be characterized. Typically, electrochemical impedance spectroscopy (EIS) is adopted to study the electrical performance of the electrode with respect to a target interface (e.g., skin) in terms of the transfer function. EIS is used to compare the impedance characteristics of multiple electrodes and perform tests under different conditions (e.g., varying the electrode design or studying long-term responses). This paper shows the recording of surface biosignals through an easy setup and reports a user-friendly method to record different types of biosignals applicable to any novel fabricated electrode that needs to be validated for cutaneous biopotential recordings.

Protocol

NOTE: Experiments involving human subjects did not involve the collection of identifiable private information related to the individual's health status and are only used here for technological demonstration. Data were averaged over three different subjects. The electrophysiological recordings were extracted from previously published data^6,21.

1. Inkjet-printed PEDOT:PSS electrode fabrication

NOTE: The following protocol has been used to fabricate electrodes for electrophysiology on commercial, flexible substrates-tattoo paper⁶ and textile²¹. The same approach has been largely adopted to make electrodes on flexible substrates such as thin plastic foils²². In all cases, an inkjet printer was used for the patterning of PEDOT:PSS (see the Table of Materials).

1. Electrode substrate preprocessing
   1. Cut a piece of the substrate of interest.
      1. When using a tattoo substrate, wash it with water before printing to remove the topmost, water-soluble layer from the paper²³.
         NOTE: The tattoo paper kit is also provided with a glue sheet used in this work, both to enhance the tattoo adhesion and as a passivation layer. Tattoo paper has a layered structure (Supplemental Figure S1), including a supporting paper sheet, a water-soluble polyvinylalcohol (PVA) layer, a releasable polyurethane film, and a topmost PVA layer. The glue sheet has a layered structure composed of silicone paper as the support, water-based acrylic glue, and a top release liner.
   2. To fabricate wearable sensors, start cutting the substrate of interest. Place the substrate on the printer plate, taping its border to keep it flat.
2. Printing of PEDOT:PSS ink
   1. Prepare the design to print, such as a circle (12 mm diameter) with a rectangular pad at the bottom (3 mm x 7 mm), the latter to be used for the interconnection.
   2. Fill the printer cartridges (10 pl) with the PEDOT:PSS commercial ink after filtering it. This is an aqueous dispersion of the conductive polymer.
   3. Print the design on the substrate.
      1. When using tattoo paper and textile, which have moderate-high surface energy and absorbing properties, respectively, print with a drop spacing of ~20 µm.
      2. Print multiple PEDOT:PSS layers, either consecutively or by applying a drying process (110 °C for 15 min) between the layers to create a homogeneous and continuous conductive pattern.
         NOTE: This is especially required in the case of textile electrodes, where the 3D-like structure of textiles requires more ink content to create a continuous conductive path within the fabric.
   4. Dry the electrode at 110 °C for 15 min in the oven to complete solvent evaporation.
      NOTE: Electrodes obtained on textile, PET, and tattoo paper (Figure 1A-C) by printing multiple devices in one run (Figure 1D) can now be stored in a closed, clean, and dry environment before continuing with the next steps.
3. External connector fabrication
   1. Tattoo electrodes
      1. Cut a rectangular piece of polyethylene naphthalate (PEN) substrate (8 mm x 12 mm, 1.3 mm thickness).
      2. Print a rectangular design (3 mm x 12 mm) with three PEDOT:PSS layers on top of the substrate.
      3. Dry the printed sample in the oven at 110 °C for 15 min.
      4. Laminate the PEN interconnection onto the tattoo electrode, with the PEDOT:PSS rectangular parts facing each other.
      5. Cut a hole (diameter 11.3 mm) in the tattoo paper glue sheet. Align this hole of the glue sheet with the circular sensing part of the tattoo PEDOT:PSS electrode. Add a piece of polyimide tape (see the Table of Materials) onto the free end of the PEN interconnection.
   2. Textile and plastic foil electrodes
      1. Attach a piece of conductive tape (e.g., copper tape) around the rectangular printed connection to obtain a robust and stable interconnection.
      2. Plug a pogo pin connector into the copper tape and connect the pogo pin to the recording system.
4. Tattoo electrode transfer
   1. Remove the glue liner. Place the tattoo onto the desired portion of the skin.
   2. Wet the back support paper, keeping the tattoo in position. Once the back support paper is soaked, slide it to remove it, leaving only the electrode made of the transferable ultrathin film on the skin.
   3. Plug the flat PEN contact into the external acquisition unit. See section 1.3.
5. Textile electrode positioning
   1. Place the electrode on the skin. With the help of a fabric sports bracelet or medical tape, keep the electrode in stable contact with the skin to ensure high-quality signal recordings during movement.
6. Perform the desired surface electrophysiological recording. Wash the tattoo electrodes away after the recordings by rubbing them with a wet sponge.

2. Electrode characterization using electrochemical impedance spectroscopy

1. On-body measurement
   1. Ensure that the volunteer is comfortably seated with an arm placed on a table at rest.
      NOTE: No skin cleaning or scrubbing is needed.
2. Electrode placement
   1. Place one electrode on the skin and connect it to the working electrode-sensing electrode (WE-S) of the EIS.
   2. Place another electrode 3 cm apart from the first one and connect it to the counter electrode (CE) of the EIS.
   3. Place the third electrode on the elbow and connect it to the reference electrode (RE) of the EIS. See Figure 2A for the setup of the three electrodes .
      NOTE: The electrodes connected to the CE and RE of the EIS can be both Ag/AgCl electrodes or made of PEDOT:PSS, as is the case for the WE in this study.
3. Start the recording on the EIS potentiostat. Apply a current between the counter and the working electrodes. Measure the potential variation across the reference and sensing couple.
   ​NOTE: The tattoo and textile electrode connection with the acquisition system can be made with a clip to form a stable electrical connection with the potentiostat cables. The output impedance computed at each frequency consists of two contributions: skin impedance and skin-electrode contact impedance.

3. Surface electrophysiological recordings

NOTE: The following section describes the electrode placement for each biosignal of interest. Once the electrodes are correctly placed and well attached to the skin, they can be connected to the portable acquisition system to start the recordings. The video content of this article shows an example of electrophysiological monitoring using commercially available Ag/AgCl electrodes and a portable electronic unit.

1. For ECG, adopt a wearable configuration with two or three (one used as ground) electrodes. Place the electrodes in multiple body areas (e.g., chest, wrists, ribs) with a minimum interelectrode distance of 6 cm to get an appreciable signal.
   NOTE: A classical location entails the placement of two electrodes on the left and right clavicles; in this case, the ground electrode can be placed on the left iliac crest.
2. For muscle electrical activity recording (EMG), place the electrodes along the muscle of interest (e.g., on the biceps or the calf). Place the ground electrode on a static location such as an adjacent bone.
3. For brain electrical activity recording (EEG), place the electrodes in multiple locations on the head.
   NOTE: Comfortable locations are the forehead and around the outer ears. A reference electrode may be required, typically behind the ear on the mastoid bone.
4. For electrodermal activity measurements (EDA), place two electrodes on the palm of the left hand. Perform the recording when the subject is at rest or doing physical exercise.
   NOTE: Skin impedance can be measured over the whole body surface (e.g., the ribs, on the back, on the foot sole); a sufficient interelectrode distance of 6 cm ensures good monitoring.

Results

This paper shows the fabrication of comfortable skin-contact electrodes by inkjet printing and a method to characterize them and perform electrophysiology recordings. We reported the fabrication steps of PEDOT:PSS inkjet printing directly on different substrates, such as fabric (Figure 1A), PEN (Figure 1B), and tattoo paper (Figure 1C,D) for reference. The proposed designs in protocol step 1.2.1. and step 1.3.1.5. d...

Discussion

This paper describes an easy and scalable process to fabricate wearable electrodes and demonstrates a method for recording electrophysiological biosignals. It uses three examples of wearable substrates, such as tattoo, textile, and thin films. It introduces how to build a sensor on these substrates and characterize its performance prior to its application. To make the electrodes here, we employed PEDOT:PSS, a conductive polymer that stands out from metal-based conductors due to its cost-effectiveness, versatile processab...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Biosignalplux - Plux wireless device for electrophysiological recordings	PLUX Wireless Biosignals S.A		EEG, ECG, EMG, EDA sensors
Covidien Kendal Disposable electrodes, medical grade disposable electrodes (Pregelled, 24 mm)	Covidien / Kendal (formally Tyco) ARBO electrodes	H124SG	Commercial Ag/AgCl electrodes for electrophysiology
Dimatix inkjet printer	Fujifilm	DMP 2800	Inkjet printer
Laser Cutter	Universal Laser Systems	VLS 3.50, 50 W	Laser cutter to cut the glue sheet for tattoo electrodes fabrication
NOVA	Metrohm Autolab	NOVA 2.1	Electrochemistry software to control Autolab instruments
OpenSignals	2020 PLUX wireless biosignals, S.A.		Software suite for real-time biosignals visualisation, capable of direct interaction with PLUX devices
PEDOT:PSS inkjet printable ink	Heraeus Deutschland GmbH & Co. KG	CLEVIOS Pjet 700
Polyethylene naphthalene (PEN) foil	Goodfellow	thickness 1.3 μm	Used for tattoo electrodes interconnection fabrication
Polyimide tape	3M	Kapton tape by 3 M, thickness 50 μm	Used for tattoo electrodes interconnection fabrication
Potentiostat	Metrohm Autolab	Autolab potentiostat B.V.	Used for EIS measurements
Silhouette temporary tattoo paper kit	Silhouette Americ, Inc, US		Substrate for tattoo-based electrodes
Wowen textile 100% cotton and commercially available pantyhose			Substrate for textile-based electrodes