Self-adhesive epidermal carbon nanotube electronics for tether-free long-term continuous recording of biosignals

Abstract

The long-term, continuous, inconspicuous, and noiseless monitoring of bioelectrical signals is critical to the early diagnosis of disease and monitoring health and wellbeing. However, it is a major challenge to record the bioelectrical signals of patients going about their daily lives because of the difficulties of integrating skin-like conducting materials, the measuring system, and medical technologies in a single platform. In this study, we developed a thin epidermis-like electronics that is capable of repeated self-adhesion onto skin, integration with commercial electronic components through soldering, and conformal contact without serious motion artifacts. Using well-mixed carbon nanotubes and adhesive polydimethylsiloxane, we fabricated an epidermal carbon nanotube electronics which maintains excellent conformal contact even within wrinkles in skin, and can be used to record electrocardiogram signals robustly. The electrode is biocompatible and can even be operated in water, which means patients can live normal lives despite wearing a complicated recording system.

Figure 1. ECG electrode based on CNT/aPDMS.

(a) The aggregated CNTs are mixed with aPDMS by using flow stress. The CNTs are in electrical contact, which means that CNT/aPDMS is conductive. (b) A SEM image of CNT/aPDMS after removal of some aPDMS with THF solution. (c) Conformal contact of CNT/aPDMS with wrinkles and rough skin and (d) a SEM image of a CNT/aPDMS electrode attached to a PDMS skin replica. (e) Structure of an ECG electrode composed of a PDMS base, a metal-patterned layer (Au/Ti/PI), a frame layer (aPDMS), and a CNT/aPDMS interfacial layer. (f) Upper surface of the ECG electrode and serpentine lines. (g) Bottom surface of the ECG electrode, which is the surface attached to the chest.

Figure 2. Adhesiveness and penetrative properties of CNT/aPDMS.

(a) The adhesiveness of CNT/aPDMS. (b) Adhesion force measurements during repeated attaching and detaching processes. The CNT/aPDMS surface was cleaned every five cycles. (c) The Young's modulus of CNT/aPDMS. The modulus was calculated to be 27.5 kPa. (d) SEM image of the PDMS skin replica. (e) Black tape, CNT/PDMS, and CNT/aPDMS were attached to the skin replica to determine their contact areas. Images of the contact areas on (f) black tape, (g) CNT/PDMS, and (h) CNT/aPDMS; the percentage contact areas were calculated to be 30.9%, 61.9%, and 99.7% respectively. (scale bar: 3 mm, pressure was applied to CNT/PDMS).

Figure 3. Electrical properties of CNT/aPDMS.

(a) Schematic diagram of the impedance from the heart to the preamplifier, which is composed of the impedance from the heart to the epidermis (Z_HE), the contact impedance between CNT/aPDMS and the epidermis (Z_CE), the intrinsic impedance of CNT/aPDMS (Z_C), and the contact impedance between CNT/aPDMS and the metal-patterned layer (Z_CM). (b) The conductivity of Z_C increases as the concentration of CNTs increases. (c) Z_CM is also resistive and its impedance decreases with the concentration of CNTs. (d) Z_CE is much higher than the other impedances between the heart and the amplifier. The impedance of the dry electrode is the highest and that of the Ag/AgCl electrode is the lowest.

Figure 4. Motion artifact tests and ECG measurements.

(a) Experimental set-up for motion artifact testing. Dry, CNT/aPDMS, and Ag/AgCl electrodes were attached to the left arm in separate tests. In each test, the arm was placed on the shaker, which rotates in a circle (d = 3 cm) with a frequency of 60, 120, or 180 rpm; these frequencies correspond to the 1, 2, and 3 Hz motion artifacts respectively. (b) Recorded motion artifacts for dry, CNT/aPDMS and Ag/AgCl electrodes for motions with frequencies of 1, 2, and 3 Hz and (c) their RMS values. (d) Attachment of the ECG electrode to the left chest. Triangle shaped RA, LA and LL electrodes represent leads I, II and III. (e) The thin and flexible ECG electrode was robustly attached. (f) ECG waveforms recorded with leads I, II, and III for the Ag/AgCl, dry, and CNT/aPDMS electrodes. The waveforms of CNT/aPDMS are comparable to those of the Ag/AgCl electrode whereas noise and artifacts (arrow) are evident in the signals of the dry electrode.

Figure 5. ECG measurements in watery environments.

(a) The ECG electrode remains robustly attached to the chest even when the electrode is submerged in bath water. The ECG acquisition module was affixed to the left arm with an elastic band and a Bluetooth communication system transmitted the ECG signal to a (b) distant notebook wirelessly. (c) The ECG waveform recorded during the bath. An EMG signal (arrows) arose intermittently when the subject changed his body posture.