Published on 02/23/22
Abstract
To avail the future wearables for electrophysiological signal monitoring, on-skin flexible electrodes must be researched and developed. The key requirements for such electrodes are high electrical conductivity, flexibility, and adhesion to skin. Through several months of research and development, we successfully designed a novel method of making on-skin flexible electrodes by combining silver nanowire (AgNW) with a custom-made flexible and tacky silicone film. Unlike previous models of on-skin flexible electrodes, this novel process provides an affordable and simple alternative, which would increase manufacturability and usability. Due to the high electrical conductivity of silver, AgNW provided an electrode with high stability and reliability. The silicone film offered an adhesive surface that was malleable to ensure low skin contact impedance and comfort. The final design met all of the aforementioned electrical and mechanical requirements, demonstrating its feasibility as a potential electrode for future wearable EP monitoring devices.
Introduction
The entire idea of creating an on-skin electrode transpired from a physical education project, where students were required to measure their heart rates in the morning. As high schoolers, waking up in the morning was difficult enough, and staying focused for 1 minute to calculate the resting heart rate was a hassle. So, my partner and I set out to design an on-skin flexible electrode that will measure people’s heart rates. To understand heart rates, electrodes, and current technologies, we conducted extensive research.
Human body movements are controlled by low-voltage electrical potentials called electrophysiological signals which are measured by electrodes. Conventional Ag or AgCl electrodes had many drawbacks to their design. The requirement of a conducting gel caused skin irritations for those with sensitive skin and did not provide users with reliable data due to the rigidity of the electrode.1
Based on research, it was obvious that there were plenty of attempts to create on-skin electrodes over the past few years.2-5 Electrodes can monitor heart signals, track abnormalities in heart rates, and detect possible health issues in the human anatomy. However, all of the electrodes from prior research from professionals utilized expensive equipment and materials and relied on extremely complicated processes. Such electrodes included metal film electrodes, nanocarbon (carbon nanotube) based electrodes, and metallic nanomaterial-based electrodes.6 To counter the complicated fabrication processes such as metal sputtering and using organic solvents, this project aimed to develop a novel and simple method.
With the intent to replace the current Ag/AgCl electrodes, we aimed to create an on-skin electrode using silver nanowires through a simple and effective transfer process. The electrodes were designed to have high conductivity to ensure the reliability and accuracy of data. The on-skin electrodes also had to be comfortable to the skin and stretch more than 15% of their original shape to match the tensile strength of human skin.6
Essentially, the objective of the project was to create an on-skin electrode using silver nanowire all the while ensuring a soft, self-adhering, stretchable, and conductive material for electrophysiological monitoring and human-machine interfaces.
AgNW Electrode Fabrication Process
Three methods were explored in the process of finding the most effective way to make AgNW on-skin electrodes.
Method 1: Spray the AgNW-IPA (Isopropyl alcohol) from a spray bottle onto a flat piece of water-soluble PVA (polyvinyl alcohol film) under a fume hood. Repeat spraying 5 times to ensure a uniform and continuous layer of AgNW. Rest for 1 minute to let the IPA evaporate before respraying. Dispense liquid silicone onto the PVA with AgNW facing up. Cure the silicone adhesive in a heating oven for 10 minutes at 100°C. Silicone-AgNW-PVA stack was submerged in hot water at 90°C for 6 hours to fully dissolve the PVA layer, leaving only the AgNW on the silicone film.
Method 2: Spray AgNW-IPA directly onto glass sheets precleaned with IPA. Repeat spraying 5 times to ensure a uniform and continuous layer of AgNW. Allow rest for 1 minute to let the IPA evaporate before respraying. Dispense liquid silicone to the glass slide with AgNW facing up and cure the product following the same steps as method 1. Carefully detach the silicone film by hand.
Method 3: Spray AgNW-IPA directly onto a filtration paper with a tape mask (used to outline the final AgNW pattern). Spray 5 times with 1-minute rests in between to evaporate the IPA. After removing the masks, gently press a pre-cured silicone film (30mm x 30 mm x 2mm) onto the AgNW for a few seconds to transfer the AgNW. Carefully peel off silicone film along with the AgNW from the filtration paper for the final product. Shown in figure 1.
Figure 1 Schematic process flow of AgNW transfer.
Results and Discussion
AgNW Electrode Prototype Fabrication
Methods 1 and 2 proved not to be viable options as uniform and continuous layers of AgNW were unable to form on the silicone strips, resulting in low surface electrical conductivity (measured resistance in mega ohms). Detaching the silicone-AgNW complex from the glass strip proved to be a difficult process for method 2.
The sources of error for methods 1 and 2 were due to the high reactivity of the nanowire surfaces as they tended to embed themselves below the polymeric resins. The liquid silicone may have fully covered and embedded the nanowires, leaving minimal nanowires left to monitor EP signals.
Method 3 proved to be an effective option as the porous texture of the filtration paper eased the detaching process. As a result, most of the nanowires stayed intact (Figure 2a). The AgNW electrode dimensions were around 30mm x 30 mm x 2 mm (Figure 2b).
(a) (b)
Figure 2 AgNW transfer process (a) and fabricated AgNW electrode prototypes (b).
The SEM (Scanning Electron Microscope), SU 3800 from Hitachi, was used to observe the AgNW on silicone strips. Figure 3(a) shows that the AgNW formed a uniform layer on the surface while Figure 3(b) shows that some of the AgNW penetrated through the surface due to its high reactivity. This certainly will be an important point of improvement for the future. More research should be conducted to ensure the tackiness of silicone film affects AgNW penetration.
(a) (b)
Figure 3 SEM photos of the transferred AgNWs on the surface of the electrodes at 500 magnification (a) and 5000 magnification (b).
Electrical and Mechanical Performance of AgNW Electrode Prototype
The electrical impedance of the AgNW electrode was measured using two pieces of copper foil tapes (30 mm apart), which acted as connecting leads for the multimeter. The two copper foil tapes were stuck on the forearm after cleaning with alcohol, then aligned and attached with two AgNW electrodes (Figure 4).
The electrical impedance was measured from frequencies 100 Hz to 10,000 Hz and was collected for both the AgNW electrode prototype and the store-bought Ag/AgCl electrode (Figure 5). The figure shows that the electrical impedance of the AgNW prototype was similar to that of the store-bought electrode, showing that the electrical performance was satisfactory.
(a) (b)
Figure 4 Electrical Impedance Testing of AgNW Electrodes.
Figure 5 Electrical impedance (Z) with frequency for a conventional Ag/AgCl electrode and AgNW electrodes.
Mechanical Performance of the AgNW Electrode Prototypes
The stretchability was measured following ASTM D638.7 The silicone film was cut into dog bone dimensions to set a constant shape and was measured using the Instron tester (Figure 6a). Figure 6b is the resulting graph of force applied versus strain % of four specimens. Table 1 shows that the average elongation is around 184.95%, far exceeding the 15% elongation requirement to meet human skin stretching.
(a) (b)
Figure 6 Dog done shaped silicone film on an Instron tester (a) and force and strain data of the silicone material (b).
Table 1 Tensile strength and elongation of the silicone material.
Feasibility Demonstration of the AgNW Electrode Prototypes
An electrocardiogram (ECG) was used to determine the feasibility of the AgNW electrode. Two AgNW electrodes were attached to two forearms, and one to the waist for reference.
The collected ECG graph below (Figure 8) shows a clear rhythm and a relatively high signal-to-noise ratio (SNR) – which indicates an efficacious electrode. The ECG proved that the AgNW electrode prototypes are potential options for collecting EP signals and future wearables.
(a) (b)
Figure 8 ECG testing set up (a) and graph data collected using the AgNW electrode prototype (b).
Conclusion
Through this project, we developed a simple and novel method to cure a silicone film with AgNW carefully transferred onto the surface. The prototype yielded satisfactory electrical and mechanical performance and was comparable to the standard Ag/AgCl electrodes. The ECG further corroborated the feasibility of this AgNW electrode fabrication process.
Acknowledgements
The authors would like to thank Mrs. Anne Love to be the sponsor for this project, Dr. Daniel Lu from Henkel to be the adult supervisor and Prof. Zhuo Li from Fudan University for her help with the ECG testing.
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