Carbon Nanofiber-Based Wearable Patches for Bio-Potential Monitoring
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We fabricated a wearable patch including a novel patch-type flexible dry electrode based on carbon nanofibers (CNFs) and biocompatible PDMS (MED-6215) for long-term bio-potential monitoring.
The patch comprises dry electrode parts for recording bio signals and sticky parts for skin mounting. To evaluate electrical characteristics and electrocardiograph (ECG) recording, the CNF concentrations and dry electrode thickness were varied. The electromechanical properties of the dry electrode were tested under cyclic load conditions. Cytotoxicity tests and long-term wearing tests were conducted to prove biocompatibility.
The fabricated wearable patch is flexible, stretchable, easily skin mountable, and directly connectable with the measuring system. CNF concentrations is the most important variable compared with diameter, thickness for obtaining high-quality ECG signals. The electromechanical tests revealed excellent recovery performances in resistivity and durability. Cytotoxicity tests showed no effects on cells and long-term wearing tests showed no skin reactions.
The fabricated patch can be utilized for long-term bio-potential monitoring such as electromyography and electroencephalograms. The proposed dry electrode has flexibility, conductivity, and biocompatibility and is widely applicable to wearable sensors.
KeywordsCarbon nanofibers Electrocardiograph Wearable Dry electrode
Recently, the wearable devices market has grown rapidly. Healthcare applications receive particular focus because many people want to be able to monitor their physical condition anywhere and at any time [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. For successful monitoring with no skin irritation, long term wearable biocompatible electrodes are critical and electrocardiography is a key bio signal . However, there are many remaining issues related to the battery, miniaturization, weight, and long-term biocompatibility.
The latest research solves these issues by recording bio-signals without the use of additive gels. Conventional electrodes are wet electrodes that include gels to improve adhesion onto the skin and resist external environmental factors; however, with time, dehydration of the gel causes decreasing signal quality, increasing signal noise, and skin irritation. Extensive research is attempting to solve these issues [2, 4, 6, 16, 17, 18, 19, 20, 21, 22, 23].
In this study, we propose a carbon nanofiber (CNF)-based wearable patch that is directly connectable to conventional electrocardiograph (ECG) measuring devices. It is fabricated by assembling dry electrode and sticky patch components. CNFs are dispersed in a Polydimethylsiloxane (PDMS) matrix in order to develop a flexible nano-composite material as an electrode. PDMS, a well-known superior elastic, is flexible, has optically transparent properties, and has recently become a popular choice for biomedical applications because of its non- toxicity, high gas-permeability, and amenable fabric ability [24, 25]. Sylgard-184 is the most used PDMS in these applications but its biocompatibility must be evaluated because it was developed specifically for electronic and lighting uses. MED-6215 was developed for medical and implantable PDMS uses and is considered acceptable for human implantation for periods greater than 29 days according to biocompatibility tests [26, 27].
Conversely, carbon-based nanomaterials such as carbon nanotubes (CNTs) and CNFs show excellent mechanical, electrical, and thermal properties and are inert in the context of biomedical applications.
For these reasons, these materials have been widely used in a variety of applications. However, CNTs have questionable biocompatibility with the human body [28, 29] and CNFs are more easily uniformly dispersed in elastomer than CNTs because of aggregation conditions [30, 31, 32, 33].
In this study, the dispersion protocol and concentrations were parametrized in order to achieve very good dispersion. To optimize electrical performance and the diffusion degree of uniformity, we developed a unique mixing and baking process. Secondly, sticky patch parts were made by patterning and detaching from a smooth surface substrate after spin-coating soft skin adhesive. In this process, the attachable and detachable strengths of the sticky patch are measured and optimized for use in a monitoring system. The electrical properties and biocompatibilities of the wearable patch were evaluated and ECG signals were measured by changing the CNF ratio and the thickness of the electrode.
2 Materials and Methods
CNFs (GNF-100, length 10–30 μm, diameter 50–200 nm, purity > 95%) were purchased from Carbon Nano-material Technology (Korea); Methyl group-terminated PDMS (MEP) which has properties [Serasense sf5, viscosity 5 mPa*s, relative density 0.95, specific gravity 0.915 (25 °C)] was purchased from KCC Corporation (Korea); and biocompatible PDMS [MED-6215, viscosity 3800 mPa*s, specific gravity 1.03 (25 °C)] and adhesive PDMS (MG 7-9850, viscosity 2900 mPa*s, adhesion 1.1 N/2.5 cm) were purchased from AvantorTM (USA) and Dow Corning (USA), respectively.
2.2 CNF Dispersion in PDMS
Specific quantitative volume
PDMS A (g)
PDMS B (g)
2.3 Fabrication of Wearable Patch
2.4 Characterization of Thermal Properties
To confirm thermal stability of the PDMS while heating process for fabricate dry electrode, we carried out the thermal gravimetric analysis (TGA). TGA of samples (weight: approximately 5–6 mg) was performed on a TGA Q50 (TA Instruments) under N2 gas atmosphere at a flow rate of 10 mL/min using platinum pans. Temperature profiles ranged from 25 to 400 °C with a heating rate of 10 °C/min.
2.5 Characterization of Electrical Properties
The sheet resistivity of the prepared samples was measured to evaluate the effect on electrical properties of CNF composition, thickness, and contact area. The CNF weight fraction and the contact area of the electrodes are thought to be critical factors determining the electrical properties. Fabricated CNF/PDMS electrodes are flexible, stretchable, and have no fixed shape, in contrast to commercial ECG electrodes (Ag/AgCl). Therefore, we measured the sheet resistivity according to different CNF ratios to investigate the effect of weight fraction.
2.6 Characterization of Electromechanical Properties
To evaluate the electromechanical properties of dry electrodes, the electrodes were tested under cyclic load conditions. Specimens with dimensions of 20 mm × 40 mm × 1 mm were prepared with stretching area dimensions of 20 mm × 20 mm × 1 mm. Prepared samples were fixed on a homemade motorized stretching machine and electrical resistivity was measured using a digital multimeter (Keysight 34410A) and a data acquisition program (Keysight BenchVue). The stretching length of the specimen was 1–6 mm (approximately 5–30%), and the electrical resistivity was measured in situ (sampling rate: 120 Hz). The hysteresis performance and strain profile of the dry electrodes were measured by incremental stretching (stretching speed: 1 mm/s). After stretching and releasing at each degree of strain (5, 10, 15, 20, 25, 30%), the apparatus was stopped for 15 s to relax the dry electrodes. The long-term durability of the dry electrodes were also tested by measuring the electrical resistivity sequentially over 10,000 cycles (stretching speed: 3 mm/s).
2.7 ECG Measurement
To acquire an ECG signal using the wearable patch, patches were placed on both wrists and the left ankle of human subjects. The ECG signal was amplified and acquired through a data acquisition system (BIOPAC systems MP36). Fabricated patches were evaluated through the measurement of ECG signals. These tests were performed for 60 s at a rate of 200 samples per second. To decrease the noise induced by the extension and movement cable of the ECG amplifier, cables from patches were fixed onto examinee’s wrists and ankles using compression bandages.
2.8 Biocompatibility Tests
L-929 (Mouse Fibroblasts) cells were purchased from ATCC (USA) and cultured in a minimum essential medium supplemented with 10% fetal bovine serum (FBS) and 1% penicillin–streptomycin (P/S) in a humid incubator at 37 °C and 5% CO2. In order to test the toxicity of the materials to cells, biocompatibility was verified by MTT test (ISO 10993-5:2009). To compare with conventional electrodes, we prepared both manufactured wearable patches and conventional electrodes. For these tests, confluent monolayer cells were adjusted to a density of 1 × 105 cells/mL, and 0.1 mL were added to each of 96 well plates. The wells were incubated for 24 h to obtain an 80% confluent monolayer of cells. The growth medium was discarded from the wells and replaced with 2 mL of the reagent control, negative control, positive control, and test sample. The MTT test measures the degree of cell growth at an absorbance of 570 nm. A decrease in number of living cells results in a decrease in the metabolic activity in the sample. This decrease directly correlates to the amount of blue-violet formazan formed, as monitored by the optical density at 570 nm. The lower the value of Viab.%, the higher the cytotoxic potential of the test sample. If viability is reduced to < 70% of the blank, it is judged to have cytotoxic potential.
The skin compatibility test was conducted on subjects wearing the wearable patch on their forearm for 7 days. During this time, observations were conducted of the side effects on the skin. To prevent inconvenience to the volunteer in their daily life, such as during bathing and exercise, a compression band was used to secure the wearable patch in case of necessity.
3 Results and Discussion
To more clearly observe the difference, we fabricated circular samples. As expected, the sheet resistivity decreased with an increase in CNF weight fraction and thickness and diameter of the dry electrode. However, the thickness and diameter effects were almost negligible compared to the CNF weight fraction. For a small diameter dry electrode, the change ratio of resistivity is larger than for a large diameter dry electrode. Also, no ECG signal was recorded at a CNF ratio of < 25 wt% ratio in the PDMS electrode.
In this study, we fabricated a simple carbon nanofiber-based wearable patch. The fabricated wearable patch was stretchable, skin-mountable, and biocompatible. The wearable patch was tested on three test subjects. The wearable patch showed potential for long-term measurement of ECG signals. The analysis of electrical characteristics showed that the ratio of CNFs is a key factor influencing electrical performance and ECG signal detection. In the ECG test, even though the amplitude was slightly lower than that of the Ag/AgCl electrode, PQRST waves were clearly visible and baselines were stable. In addition, we discovered that our electrode has no side effects such as itching or irritation even after 1 week of continuous wear. We expect that the proposed CNF/PDMS composite electrode has important applications for daily, universal, and mobile health care technology. The fabricated dry electrode may have other useful applications including strain, pressure, and temperature gauges in a variety of bio-signal monitoring equipment.
This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) Grant No. [NRF-2016R1A5A1921651].
Compliance with Ethical Standards
Conflicts of interest
The authors declare that they have no conflict of interest.
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