The healthcare system requires bioelectric electrodes for long-term monitoring of patients' vitals. However, one of the main difficulties associated with bioelectric signal monitoring systems is the high contact impedance of human skin (stratum corneum), at the electrode-skin contact interface, which inhibits the proper transmission of electrical signals.
Study: Ordered Nanopillar Arrays of Low Dynamic Noise Dry Bioelectrodes for Electrocardiogram Surface Monitoring. Image Credit: pedrosala/Shutterstock.com
Researchers have sought to control contact impedance, basing structures on contact area and skin properties. Recently, scientists have developed dry electrodes with ordered micronanopillar arrays for dynamic electrocardiogram (ECG) surface monitoring. This study is available in ACS Applied Materials and Interfaces.
Bioelectric Signal Monitoring System
Human skin is composed of multiple layers of micro- and nano-scaled rough structures. At a macroscopic level, micron-scale-sized fluctuations are visible on the skin surface, while at a microscopic level, many nano-scale fluctuations are present, whose micro-size depths range between 200 nm and 1 μm. These microsize fluctuations are studied via several superimposed sinusoidal waves.
The contact impedance in commercially available wet electrodes is lowered by using a conductive gel. Additionally, skin pre-treatment processes, associated with the reduction of air gap at the electrode-skin contact interface, also lower contact impedance. However, these processes have some limitations including gradual drying of conductive gel over a period of time and incidence of skin inflammation and allergy after skin pre-treatments.
Recently, wet electrodes have been replaced by dry electrodes, especially for long-term monitoring of patients with special medical needs. One of the key advantages of penetrating dry electrodes in the stratum corneum, without damaging skin tissues, is the decrease in contact impedance. However, this method might cause skin inflammation and infection.
Scientists stated that instead of penetrating dry electrodes, utilizing flexible micro- and nanostructured electrodes, which can fit the skin more naturally, is a beneficial approach. Additionally, as this process does not require the electrodes to be penetrated into the skin, it is more comfortable and stable, with a lowered impedance performance.
Previous studies have shown that flexible dry electrodes exhibit extraordinary biocompatibility, which significantly decreases the possibility of inflammation. These reports strongly support the use of dry electrodes for long-term monitoring of health.
Scientists compared similar-sized dry electrodes that are flat with those containing micro- and nanosurface. They found that micro- and nano-surface dry electrodes contained greater surface area, which can be in contact with the skin. These offer lowered contact impedance and a superior quality signal can be acquired.
Development of Bioelectric Dry Electrodes with Micronanostructures
In the current study, scientists developed a novel method for developing flexible bioelectric dry electrodes with micro and nano-structures. They designed dimensionally tunable anodized aluminum oxide (AAO) templates, i.e., single-layer AAO (SAAO) and double-layer AAO (D-AAO) templates, which contain micro- and nanostructured pores, to produce ordered nanoarray structured films (e.g., nanopillars, nanorods, and nanotubes).
Using these templates, scientists prepared single-layer micronanopillar structured arrays of polyaniline/thermoplastic polyurethane dry electrodes (PANI/TPU-SE) and double-layer micronanopillar structured arrays of structured polyaniline/thermoplastic polyurethane dry electrodes (PANI/TPU-DE).
Researchers used phosphoric acid/oxalic acid as the electrolyte and controlled the oxidation time and number, to develop different structures of AAO. AAO template served as the reaction vessel, where in situ polymerization technique was used to polymerize the aniline monomer into PANI nanopillars on the pore walls of the AAO template. The authors fabricated AAO/PANI/TPU by the spin-coating technique. Subsequently, the AAO template was removed and, thereby, the PANI/TPU conductive film was developed. PANI/TPU exhibited significant flexibility and was easily attached to the surface of human skin.
Scientists used a silver paste that possesses good conductivity to assemble the PANI/TPU conductive film, with a snap, polyimide tape, medical tape, and copper foil tape, to produce the ECG electrode patch.
The PANI polymerization process was analyzed using various tools including scanning electron microscopy (SEM), X-~Ray photoelectron spectroscopy (XPS), and Fourier Transform Infrared Spectroscopy (FTIR), which showed the formation of polyaniline on AAO. Researchers studied the electrochemical performance of PANI/TPU-PE (planar structure), PANI/TPU-SE, and PANI/TPU-DE.
It was observed that the contact impedance values, at the electrode−skin interface, decreased with the increasing frequency. Scientists reported that in the entire frequency range, PANI/TPU-DE300 exhibited the smallest impedance, closer to that of wet electrodes. However, PANI/TPU-SE showed lower contact impedance values compared to PANI/TPU-PE.
The authors further reported that the micro nanostructured morphology of dry electrodes enabled easy conformation to human skin. Compared to flat dry electrodes, the double-layer structure of dry micro nanostructured electrodes exhibited a 210.7% increase in peel strength.
Analysis of Newly Developed Dry Bioelectrodes for ECG Monitoring
Researchers analyzed the ECG signals generated by the electrodes in the resting state and in different motions. In the resting state, all electrodes were able to accurately acquire ECG signals. However, when the human body was in motion, different levels of noise appeared in ECG signals generated via different electrodes. In motion, PANI/TPU-DE300 electrodes exhibited marginal drift in signal waveforms, whereas, PANI/TPU-SE10 revealed major baseline drift.
The findings of the current study strongly indicate that PANI/TPU-DE300 is more similar to conventional wet electrodes and, therefore, suitable for measuring ECG signals. These can be effectively applied in long-term healthcare monitoring systems.
Reference
Niu, X. et al. (2022) Ordered Nanopillar Arrays of Low Dynamic Noise Dry Bioelectrodes for Electrocardiogram Surface Monitoring, ACS Applied Materials and Interfaces. https://pubs.acs.org/doi/10.1021/acsami.2c08318
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