An epidermal electronic system for physiological information acquisition, processing, and storage with an integrated flash memory array

Abstract

Epidermal electronic systems that simultaneously provide physiological information acquisition, processing, and storage are in high demand for health care/clinical applications. However, these system-level demonstrations using flexible devices are still challenging because of obstacles in device performance, functional module construction, or integration scale. Here, on the basis of carbon nanotubes, we present an epidermal system that incorporates flexible sensors, sensor interface circuits, and an integrated flash memory array to collect physiological information from the human body surface; amplify weak biosignals by high-performance differential amplifiers (voltage gain of 27 decibels, common-mode rejection ratio of >43 decibels, and gain bandwidth product of >22 kilohertz); and store the processed information in the memory array with performance on par with industrial standards (retention time of 10⁸ seconds, program/erase voltages of ±2 volts, and endurance of 10⁶ cycles). The results shed light on the great application potential of epidermal electronic systems in personalized diagnostic and physiological monitoring.

Fig. 1.. Design and architecture of the epidermal electronic system.

(A) Schematic illustration and (B) photograph of an epidermal electronic system conformally laminated on the skin. Scale bar, 1 cm. (C) Spatial layout of the whole system and the architecture of (D) sensors, (E) sensor interface circuits, and (F) an integrated flash memory array (24 bit by 16 bit). (G) Information flow of the epidermal electronic system.

Fig. 2.. Device structure and characteristics of CNT-based flash memory.

(A) Schematic illustrated device structure and (B) pseudo-colored optical image of a flash memory. Scale bar, 100 μm. S, D, and G refer to the source, drain, and gate electrodes of the device, respectively. (C) Scanning electron microscopy image of the CNT thin films in the channel. Scale bar, 1 μm. (D) TEM, (E) STEM image, and EDS analysis results taken from a cross-sectional area of the stacked heterogeneous structure in the memory. Scale bars, 20 nm. (F) Schematic band diagrams of the flash memory during programming and erasing. (G) Transfer characteristics of the flash memory after being programmed and erased. (H) I_prg and I_ers of the flash memory for different program (V_prg) and erase (V_ers) voltages. (I) Retention characteristics of the flash memory. (J) Endurance of the flash memory with repetitive program/erase operations. (K) Program/erase speed characteristics of the flash memory.

Fig. 3.. Uniformity, mechanical robustness, and array integration of flexible flash memories.

(A) Transfer characteristics of 100 flash memories in programmed and erased states. Corresponding statistical distribution of the (B) threshold voltage V_th under programmed and erased states, (C) memory window, (D) readout current under the programmed (I_prg) and erased (I_ers) states, and (E) programmed/erased current ratio (I_ers/I_prg). (F) Pseudo-colored optical image and (G) circuit diagram of an integrated 24 bit by 16 bit flash memory array. Scale bar, 100 μm. (H) Transfer characteristics of C_NB, C_NW, C_NS, and C_S during programming of C_S. (I) Photograph of memories during bending test. Scale bar, 1 cm. I_ers and I_prg of the flash memory (J) when bent with different bending radii of curvature and (K) during cyclic bending test.

Fig. 4.. High-performance CNT-based differential amplifiers.

(A) Pseudo-colored optical image and (B) schematic illustrated device structure of CNT TFTs. Scale bar, 100 μm. (C) Transfer characteristics of 50 CNT TFTs. The inset shows the transfer characteristics of a typical CNT TFT with a small hysteresis voltage. Corresponding statistical distribution of (D) hysteresis voltage and (E) current on/off ratio (I_on/I_off). (F) Voltage transfer curve and voltage gain of an inverter. The inset shows the pseudo-colored optical image and circuit diagram of an inverter. (G) Pseudo-colored optical image of a CNT differential amplifier. Scale bar, 100 μm. (H) Circuit diagram and operation modes of the differential amplifier. Input-output characteristics of the differential amplifier working in the (I) differential and (J) common modes. Bode plots of the small-signal (K) gain and (L) phase shift of the differential amplifier.

Fig. 5.. Physiological information recording using the epidermal system.

(A) Block diagram of the epidermal system for physiological signal recording. (B) Photograph of the ECG sensor with serpentine geometrical electrodes. (C) Processed ECG signal obtained from a participant before exercise. (D) Processed ECG signals from the participant measured every 20 s before and after exercise. Optical image and response characteristics of (E) the temperature and (F) humidity sensor. (G) Physiological information extracted from real-time monitoring results of the three sensors. (H) Encoding map of the information obtained in (G) written into the flash memory array. Data retrieved from the memory array (I) immediately after writing and (J) 1 day after writing.