Inkjet and Extrusion Printed Silver Biomedical Tattoo Electrodes

Abstract

Ag/AgCl medical electrodes have served as the gold standard component for the acquisition of bioelectric signals in the human body (i.e. heart, muscle, and brain activity). Although ubiquitous, these electrodes exhibit an inability to sufficiently conform to the body and degrade in performance over time. As an alternative to conventional electrode fabrication methods, additive manufacturing methods such as inkjet and extrusion printing, can improve the efficiency of the fabrication process of electrical devices. These methods can also improve the performance of electrophysiological electrodes. To this end, tattoo-based electrodes were fabricated using inkjet and extrusion printing. These methods were optimized to print silver-based inks on tattoo paper, a porous substrate. The resulting electrodes were characterized in terms of sheet resistance and impedance as well as mechanical performance in terms of bending strain. The results for inkjet and extrusion printed tattoo electrodes are compared.

Full text

Inkjet and Extrusion Printed Silver Biomedical
Tattoo Electrodes
Yoland El-hajj1, Milad Ghalamboran1, Gerd Grau1
1 Department of Electrical Engineering and Computer Science, York University, Toronto, ON, Canada
yolandje@yorku.ca
Abstract—Ag/AgCl medical electrodes have served as the gold
standard component for the acquisition of bioelectric signals in the
human body (i.e. heart, muscle, and brain activity). Although
ubiquitous, these electrodes exhibit an inability to sufficiently
conform to the body and degrade in performance over time. As an
alternative to conventional electrode fabrication methods, additive
manufacturing methods such as inkjet and extrusion printing, can
improve the efficiency of the fabrication process of electrical
devices. These methods can also improve the performance of
electrophysiological electrodes. To this end, tattoo-based
electrodes were fabricated using inkjet and extrusion printing.
These methods were optimized to print silver-based inks on tattoo
paper, a porous substrate. The resulting electrodes were
characterized in terms of sheet resistance and impedance as well
as mechanical performance in terms of bending strain. The results
for inkjet and extrusion printed tattoo electrodes are compared.
Keywords— medical electrode, tattoo electrode, ECG, inkjet
printing, extrusion printing, silver ink
I. INTRODUCTION
Medical electrodes are a vital tool used to gain insight
toward internal processes occurring in the human body. These
electrodes function as a transducer converting a biological
stimulus into an electric signal. These components can possess
different structures and are primarily used to examine
functionality within the cardiac (ECG), muscular (EMG), and
neural (EEG) systems. With these measurements, disorders
within biological systems can be diagnosed (e.g. Parkinson’s
disease) and biomedical systems can be implemented with a
biological stimulus as an input (i.e. control of prosthetics).
The most commonly used electrodes are Ag/AgCl male
connector snap closures with an electrolytic gel, as they possess
suitable qualities needed for electrodes such as a low skin-
electrode impedance, and low noise [1]. Despite these
advantages, these electrodes are susceptible to motion artifacts,
can be uncomfortable for the user and are unsuitable for long
term use. Tattoos are potentially a superior material to monitor
such biological activities, as they are lightweight, conform to the
skin, and can be made with a variety of conductive materials.
Previous work has been done to develop tattoo-based
alternatives to conventional Ag/AgCl electrodes. Polymer
(PEDOT:PSS) based inkjet printed tattoo electrodes
demonstrated by Ferrari et al. [2] were implemented for the
acquisition of EEG signals. A cut-and-paste method was
implemented by Wang et al. [3] to form Cu and Ag based
temporary tattoos, used to examine skin temperature, skin
hydration, and ECG signals. Another promising material,
graphene, was used by Ameri et al. [4] to form an electronic
tattoo via a “wet-transfer, dry-patterning” method. The resulting
tattoos were used to examine multiple vitals, including ECG,
EMG, and EEG signals, as well as skin temperature, and skin
hydration.
Although these reports are promising, most of them have
relied on traditional fabrication processes (i.e. photolithography,
sputtering, etc.), which increase the cost and complexity during
fabrication. Compared to these traditional methods,
microfabrication processes such as inkjet printing and screen
printing can improve the efficiency of the fabrication process. In
the context of printed electronics, inkjet printing involves the
deposition of a functional (i.e. conductive, insulating) material
on a substrate in a droplet form that approximately corresponds
to the size of the nozzle diameter used [5]. Alternatively,
extrusion printing uses much higher viscosity comparable to
screen printing, but does not require a printing master. Inkjet
printing results in patterns that are thinner (~100 nm) and at a
higher resolution (50 µm) compared to extrusion printing (>10
µm and >100 µm, respectively) [5]. While the superior
resolution of inkjet printing can be beneficial, the lower
viscosity of the ink potentially leads to absorption and
unevenness in the resulting pattern on porous substrates such as
tattoo paper. This limitation can be overcome using extrusion
printing, with its higher viscosity precenting ink absorption.
Here, these two methods are compared for the fabrication of
tattoo electrodes as they are the two most common digital
printing methods for printed electronics.
There is a large potential for the fabrication of medical
electrodes using printing methods, and with the use of more
robust materials. Printing-based techniques can provide
numerous benefits for medical sensors, such as flexibility in the
materials used and patterns printed, and personalization of the
sensor structure. In addition, more electrically and mechanically
robust materials can be implemented using flexible materials
[8]. There is a gap in microfabrication of medical electrodes
using printing technology with metal-based materials, as a
majority of the reported electrode tattoos use organic materials.
To this end, we have developed inkjet and extrusion printed
tattoo medical electrodes to examine bioelectric phenomena
within the body. Silver-based inks were selected as the printing
medium due to their high conductivity, mechanical robustness,
biocompatibility, and moderate cost compared to other inks (e.g.
gold, graphene).
978-1-6654-4273-2/22/$31.00 ©2022 IEEE
2022 IEEE International Conference on Flexible and Printable Sensors and Systems (FLEPS) | 978-1-6654-4273-2/22/$31.00 ©2022 IEEE | DOI: 10.1109/FLEPS53764.2022.9781535
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II. METHODOLOGY
A. Electrode Fabrication
4 mm by 4 mm squares were printed on commercial tattoo
paper (Silhouette Temporary Tattoo Paper, USA) using inkjet
and extrusion printing. Prior to printing, the tattoo paper was
attached on a glass substrate (Fisherbrand Precleaned
Microscope Slide) with tape to create an even printing surface,
and purged with air to remove any dust.
For inkjet printing, silver nanoparticle ink (ANP DGP 40LT-
15 C, Advanced Nano Products, Co., Sejong, Korea) was used.
To deposit the ink on the tattoo substrate, a custom-built inkjet
printer with a 60 µm diameter nozzle (MJ-ATP-01-60-8MX,
Microfab Technologies, Inc. Plano, TX) was used. The jetting
parameters were taken from previous experimentation [6], and a
drop spacing of 80 µm was used to obtain the desired pattern.
Extrusion printing was performed with an extrusion printer
using a 225 µm diameter nozzle (Voltera NOVA, Kitchener,
Canada). The printing materials, including the tattoo paper and
silver flake ink (Creative Materials 120-07, Ayer, USA), were
utilized during the calibration of the printer, which calculates the
dispensing height for the nozzle relative to the substrate, and the
pressure required to move the ink out of the nozzle.
For both inkjet and extrusion printing, the samples were
dried at 100ºC for 30 minutes to sufficiently remove the solvents
in the ink mixtures and allow for sufficient sintering of the
flakes/particles. Figure 1 displays the fabrication and transfer
process of the tattoo electrodes.
Figure 1: Tattoo (a) fabrication and (b) transfer process. (c)
Cross sectional view of tattoo on phantom
B. Electrical and Mechanical Measurements
Sheet resistance measurements were conducted based on
the Van der Pauw method [7] using a Semiconductor Parameter
Analyzer (Keithley 4200A-SCS, Tektronix, Beaverton, OR).
The data was analyzed in MATLAB 2021a, to calculate the
sheet resistance of the squares. Analysis of the surface
morphology of the tattoo paper was completed with optical
profilometry (Bruker Contour GT-K 3D Optical Profiler).
To acquire impedance measurements of the electrodes, a
phantom model was used. The phantom model was made by
combining 15 g of gelatin (ClassiKool 240 Bloom Gelatine,
UK), 1 g of NaCl (Fisher Scientific), and 50 mL of deionised
water, stirred and heated at 60ºC for 15 minutes to dissolve the
granules in the mixture. The mixture was subsequently poured
into a 3D printed mold (6 mm × 6 mm × 2 mm) with a metal
cup electrode (Grass Disposable Deep Cup Electrode Ag/AgCl)
secured to the base of the mold. It was cured in the refrigerator
for 24 hours. The model was left at room temperature for 20
minutes prior to conducting impedance measurements.
Impedance measurements were conducted using a potentiostat
(Metrohm Autolab PGSTAT204 Compact and modular
potentiostat/galvanostat). A cross section of the printed
electrode (representative of the inkjet or extrusion printed
electrodes) on the phantom model can be seen in Figure 1(c).
Conventional electrodes (3M 2560 Red Dot Multi-Purpose
Monitoring Electrode) were used for comparison purposes.
The electrical performance of the electrodes with the
application of bending strain was also examined. Two extrusion
printed contacts (6 mm by 6 mm) were printed at the two ends
of 4 mm by 12 mm inkjet and extrusion printed patterns. The
resulting electrodes were placed on a latex platform which was
secured on plastic pipe segments with 9 mm, 12 mm, 22 mm,
and 25 mm diameters. The contacts were probed with a
multimeter (Keithley, 2100 series), and the resistance was
measured using a 2-point probe method.
III. RESULTS
The measured sheet resistance of the inkjet and extrusion
printed electrodes are 25.1 kΩ/sq and 9.4 mΩ/sq respectively.
Compared to the inkjet printed electrodes, the extrusion printed
electrodes possess a lower sheet resistance. The main
contributing factor to this is the higher quantity of ink deposited
during the extrusion printing process. In addition, a smaller sheet
resistance can be attributed to the silver flake ink sitting on the
surface of the tattoo paper, rather than partially absorbing into
the paper as seen with the silver nanoparticle ink. This is shown
in the profilometry results for the inkjet and extrusion printed
electrodes shown in Figure 2. The thickness of the extrusion
printed electrode was approximately 140 µm, while the
thickness of the inkjet printed electrode was not possible to
determine due to the apparent absorption into the paper, as seen
in Figure 2. Although partly absorbed, the inkjet printed tattoo
was sufficiently released from the tattoo paper post-printing and
the structure was supported with the glue layer placed on top.
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Figure 2: Profilometry scans of top left corner of (a)
extrusion printed and (b) inkjet electrodes on tattoo paper. Note
the different height scales as extrusion printed electrodes have
a larger thickness.
A non-invasive impedance measurement was obtained by
placing the electrodes on the surface of the phantom model. A
cross section of the tattoo applied to the phantom model can be
seen in Figure 1 (c). The impedance measurements are extracted
by fitting an equivalent circuit model containing an impedance
associated with the electrode-electrolyte interface (represented
as resistor Rd and capacitor Cd in parallel) and series resistance
(Rs) associated with interfacial effects and the resistance of the
electrode materials.
The impedance over contact area was measured from 0.1 Hz
to 1,000 Hz, with the results shown in Figure 3. The sizes of the
electrodes (16 mm
2
for the printed electrodes and 78 mm
2
for
the conventional electrode) were accounted for by multiplying
impedance with electrode area. As seen in the plots, the
extrusion printed electrodes possess a lower impedance and a
smaller error range (illustrated by the shaded region) compared
to the inkjet printed electrodes, while the conventional
electrodes maintain a fairly low and consistent impedance over
the frequency range. Since the extrusion printed electrodes are
not partially absorbed into the tattoo paper like the inkjet printed
electrodes, there is likely a larger contact area of conductive
material to the gelatin model at the tattoo-phantom interface,
resulting in a lower value for Rd and higher value for Cd.
Figure 3: Impedance of inkjet printed, extrusion printed,
and conventional tattoo electrodes
The results of the bending experiments (as seen in Figure 4)
indicate that the extrusion printed electrodes are more
susceptible to deformation with increased bending. The
resistance returns to the initial value after each bending test.
Due to the rigidity of the Ag component in the conventional
electrode, the bending results for these electrodes were not
included. Conversely, the inkjet printed electrodes maintained
a generally consistent performance as more strain was applied.
The latter can be attributed to the thinner nature of the inkjet
printed electrodes, allowing them to more easily conform to the
surface they are applied on.
Figure 4: Normalized resistance as a function of curvature for
inkjet and extrusion printed electrodes
IV. CONCLUSION
In this work, we report the performance of inkjet and
extrusion printed silver tattoo electrodes. Printing parameters
were optimized for use of functional silver-based inks on an
unconventional substrate (tattoo paper). The electrodes can be
used for the acquisition of biomedical signals, particularly in
applications that require long term monitoring, e.g. ECG
acquisition to monitor heart activity after an individual
experiences a stroke. Based on the current performance of the
electrodes, the inkjet printed electrodes would be more ideal for
acquiring signals in areas of the body engaged in higher
movement (e.g. EMG). Extrusion printed electrodes exhibit a
lower sheet resistance and impedance as these tattoos do not
absorb into the paper, a phenomenon which occurs for inkjet
printed tattoos. Although the extrusion printed electrodes have
a better electrical performance, the inkjet printed electrodes
demonstrate a more consistent electrical performance given
their ability to better withstand bending strain. Further work can
be done to optimize the electrical performance of the inkjet
printed electrodes (e.g. printing more layers) and enhance the
durability of extrusion printed patterns on tattoo paper. Future
work will study the reliability of the tattoo electrodes on skin.
A
CKNOWLEDGMENT
We acknowledge the support of the Natural Sciences and
Engineering Research Council of Canada (NSERC), funding
reference number STPGP 521480-18.
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