A Bioinspired Artificial Injury Response System Based on a Robust Polymer Memristor to Mimic a Sense of Pain, Sign of Injury, and Healing

Abstract

Flexible electronic skin with features that include sensing, processing, and responding to stimuli have transformed human-robot interactions. However, more advanced capabilities, such as human-like self-protection modalities with a sense of pain, sign of injury, and healing, are more challenging. Herein, a novel, flexible, and robust diffusive memristor based on a copolymer of chlorotrifluoroethylene and vinylidene fluoride (FK-800) as an artificial nociceptor (pain sensor) is reported. Devices composed of Ag/FK-800/Pt have outstanding switching endurance >10⁶ cycles, orders of magnitude higher than any other two-terminal polymer/organic memristors in literature (typically 10² -10³ cycles). In situ conductive atomic force microscopy is employed to dynamically switch individual filaments, which demonstrates that conductive filaments correlate with polymer grain boundaries and FK-800 has superior morphological stability under repeated switching cycles. It is hypothesized that the high thermal stability and high elasticity of FK-800 contribute to the stability under local Joule heating associated with electrical switching. To mimic biological nociceptors, four signature nociceptive characteristics are demonstrated: threshold triggering, no adaptation, relaxation, and sensitization. Lastly, by integrating a triboelectric generator (artificial mechanoreceptor), memristor (artificial nociceptor), and light emitting diode (artificial bruise), the first bioinspired injury response system capable of sensing pain, showing signs of injury, and healing, is demonstrated.

Keywords: FK-800; artificial nociceptor; electronic skin; flexible memristor; memristor.

Figure 1

Characteristics of FK‐800 based memristors. a) Schematic illustration of the FK‐800 memristor cross‐bar arrays with the structure of Pt/FK‐800/Ag. b) Plan view SEM image of the FK‐800 memristors with a junction size of ≈100 µm × 100 µm. c) Cross‐sectional SEM image of the FK‐800 memristor; the thickness of FK‐800 film is ≈200 nm. d) I–V cycles (0 →5 V → −5 V → 0) of an FK‐800 memristor at a sweep rate of 0.3 V s⁻¹. e) The corresponding I–V sweeps on a log scale. f) Endurance test of the FK‐800 memristor with 10⁶ cycles that switch between the “on” and “off” states; the testing on and off voltage pulses applied were 4.5 and 0.2 V with a pulse width of 0.5 ms pulse and pulse interval of 2 ms. g) Comparison of the endurance of the high‐performance organic/polymer memristors reported in literature.^{[ 41} , ⁴² , ⁴³ , ⁴⁴ , ⁴⁵ , ⁴⁶ , ⁴⁷ , ^{48 ]}

Figure 2

Comparing the morphological integrity of FK‐800 and PVDF‐based memristors before and after switching via conductive AFM. Topography image of the FK‐800 a) before applying a voltage and b) after cycling. c) Corresponding current map in the on‐state using a bias of 5 V and a compliance current of 10 nA. d) Corresponding current map in the off‐state using a bias of 0.2 V. e) I–V sweep at the active spot indicated in the green circle in (a) and inactive spot indicated by the grey circle in (a). There are no obvious changes in morphology due to switching. To compare, a PVDF device was scanned under the same conditions. Topography image of the PVDF film f) before applying a voltage and g) after cycling. h) Corresponding current map in the on‐state using a bias of 5 V and a compliance current of 10 nA. i) Corresponding current map in the off‐state using a bias of 0.2 V. j) I–V sweep at the active spot indicated in the blue circle in (f) and inactive spot indicated in the gray circle in (f). White circles in (f) and (g) highlight active spots where there is evidence of irreversible morphological changes due to switching. All images are 2 µm × 2 µm. The vertical scale is provided as an inset.

Figure 3

An artificial nociceptor based on an FK‐800 memristor with four signature nociceptive characteristics: threshold, no adaptation, relaxation, and sensitization. a) Pulse level measurement: a train of 200 ms wide voltage pulses (black) with different amplitudes (1, 2, 3, 4, and 5 V) and the corresponding output currents (green). The interval time between pulses was with 3 s. b) Pulse width measurement: a train pulse of 4 V (black) with different pulse width (0.5, 1, 10, 200, and 500 ms) and the corresponding output currents (green). The interval time between pulses was with 3 s. c) Relaxation: the current response at 2 V (200 ms) after a train pulse of 4 V (200 ms) with different interval time: 0.1, 1, 30, 100, and 3000 ms. d) Current response of the device to 100 voltage pulses (pulse width of 20 ms and pulse interval of 2 ms) with different amplitude (3, 4, and 5 V). The higher the pulse level, the shorter the incubation time. e–j) Demonstration of the allodynia and hyperalgesia features. (e) The output currents at 200 ms voltage pulses with different amplitudes (1, 2, 3, 4, and 5 V) 3 min after different injury pulses (8.5 V – blue, 7 V – yellow, 0 V – green/no injury). The corresponding maximum output currents at different pulse amplitudes in (f) log scale and (g) linear scale. (h) The output currents at 200 ms voltage pulses with different amplitudes (1, 2, 3, 4, and 5 V) after the injury pulse of 7 V (200 ms) with different time intervals, 500 ms (blue), 30 min (yellow), and 5.5 h (green). The corresponding maximum output currents at different pulse amplitudes in (i) log scale and (j) linear scale. The threshold voltage shifted to a lower end and the current intensity increased under the same injury pulses, emulating the allodynia and hyperalgesia effect of a nociceptor.

Figure 4

Demonstration of the first bioinspired artificial injury response system with advanced functions, including a sense of pain, sign of injury, and healing, under different scenarios. a) Schematic illustration of the physiological protection modality of human bodies under noxious stimuli, including a sense of pain, sign of injury, and healing. b) Schematic illustration of the bioinspired artificial injury response to emulate a sense of pain, sign of injury and healing based on the integration of a triboelectric generator (artificial mechanoreceptor), a memristor (artificial nociceptor), and a light emitting diode (LED) (artificial bruise) and a power source of 2.4 V. c) Circuit diagram of the corresponding artificial injury response system. d–g) Demonstration of the injury response under a gentle touch. (d) The voltage output generated by a gentle touch on the triboelectric generator, (e) the corresponding memristor circuit current (≈10⁻⁸A), and (f–g) the photographs of the corresponding state of the LED (off), which suggests a gentle touch would cause no pain nor harm. h–j) Demonstration of the injury response under a mild hit. (h) The voltage output generated by a mild hit on the triboelectric generator. (i) The corresponding memristor circuit current and (j) the photographs of each state of the LED over time, which suggests a mild hit would cause a mild pain and light bruise, but it would heal fast (2 s). k–m) Demonstration of the injury response under a hard hit. (k) The voltage output generated by a hard hit on the triboelectric generator. (l) The corresponding memristor circuit current and (m) the photographs of each state of the LED over time, which suggests a hard hit would cause a more intensive pain (high current) and more serious bruise (higher brightness), and it would take a longer time to recover.