Ultralight Iontronic Triboelectric Mechanoreceptor with High Specific Outputs for Epidermal Electronics

Highlights An ultralight, ultrathin iontronic triboelectric mechanoreceptor (ITM) is proposed with high specific outputs for multi-functional epidermal applications. Precise vital signals and physical activities monitoring of human bodies are noninvasively implemented by the ITM. Acoustic-to-electrical energy conversion is realized by the ITM, and biometric applications such as voices differentiation and the noise dosimeter are demonstrated. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-022-00834-4.


Supplementary Figures
The fabrication procedures of the ITM through electrostatic spinning process; The ITM is fabricated by two-step electrospinning technology; (i) The electrospinning process of the ionic electrode layer; (ii) The electrospinning process of the TPU layer. The TPU nanofibers were electrospun directly on the above electrode layer    It was observed that stress-strain curves decrease when increasing the concentration of EMITFSI in ionic nanofiber electrodes, and we ascribe the decreases to the finer nanofibers diameter. As shown in Fig. 2e-f, it was found observed that the addition of ionic liquid gives rise to the finer nanofibers. Under the same conditions for the electrospinning process, the resultant diameter for the pure TPU nanofiber is around ~610 nm, the diameter for the ionic electrode nanofiber (with 60% ionic liquid) is around 150~210 nm. In this regard, we infer that reduced mechanical strength of the ionic electrode film results from finer nanofibers.

Fig. S6
Cyclic performance of the ionic electrode; a The relative resistance variations of the ionic electrode against cyclic strain at 40%; b The relative resistance variations of the ionic electrode against cyclic strain at 60%; c Conductivity variations of the ionic electrode after storage for three weeks at room temperature   Durability test of the ITM after contacting for 30000 cycles; a Open-circuit voltage of the ITM after contact-separation for 30000 cycles; b SEM images of the ITM surface after contact-separation cycles (Scar bar is 10 μm) Figure S10 shows the open-circuit voltage of the ITM, and the SEM characterization of the ITM surface morphology after 30000 contacting cycles. Results showed that the output can maintain over 68% of the initial level after contact-separation for over 30,000 cycles. And the SEM images imply that, applied pressure caused by frequent contacts would make the nanofibers film become more tight and compacted, but no fiber fracture is observed after 30,000 contacts

Fig. S11
Durability test of the ITM after sliding for 40000 cycles; a Open-circuit voltage of the ITM after sliding for 30000 cycles; b SEM images of the ITM surface after sliding cyclic test (Scar bar is 10 μm) Figure S11 shows the open-circuit voltage of the ITM, and the SEM characterization of the ITM surface morphology after 40000 sliding cycles. Results showed that the output can maintain over 70% of the initial level after sliding for 40,000 cycles. Although showing a little attenuation, it's reasonably to believe that this output attenuation is acceptable and reasonable, since no encapsulation strategy was utilized in order to maintain the great breathability of our device. And due to the simple and cost-effective fabrication strategy, the ITM can also be used as disposable product for short-time usage. In this sense, the durability/stability for the ITM (under tens of thousands of cycles) is enough to support short-time (for one or several days) service. Indeed, we know that there's room for improvement of the ITM in stability and durability performance. And it's believed to be further optimized by materials modification and structure design in future explorations.

Fig. S12
Durability test of the ITM after bending for 30000 cycles; a Open-circuit voltage of the ITM after bending for 30000 cycles; b SEM images of the ITM surface after bending cyclic test (Scar bar is 10 μm) Figure S12 shows the open-circuit voltage of the ITM, and the SEM characterization of the ITM surface morphology after 30000 bending cycles. Results showed that the output can maintain over 90% of the initial level after bending for 30000 cycles. And the SEM images imply no obvious morphology changes of the nanofibers film, and no fracture is observed in the fibers after 30000 bending tests. For the stretching cyclic tests, we measured the output performance of the ITM under different stretched states (from 0% to 60%), the open-circuit voltage can maintain 52% of the initial value when the ITM was stretched to 60% state (Fig. S13a). Then, we tested the performance of the ITM after stretching to 50% for 20,000 cycles, the output is 86% of the initial level, as demonstrated in Fig. S13b. SEM images in Fig. S13c demonstrate that, when the nanofiber film is stretched under external strain, the nanofibers are driven to directional alignment while no fibers fracture is observed. The contrastive SEM images of the ITM surface before (Fig. S13d) and after stretching (Fig. S13e) are consistent, indicating the great stability of the ITM.  Figure S14 presents the output of the ITM under different humidity conditions, from 30% to 90%. Since triboelectrification is mainly surface phenomenon, therefore open-circuit voltage of the ITM is distinctly affected by the environmental humidity and perspiration. But, the ITM also maintain over 28% of the initial value even under 90% humidity, and the output can recover to the initial value after the humidity recovered. The output performance of the ITM under perspiration condition is shown in Fig. S15 Under sweating conditions, our ITM also demonstrates decent output after working for 3000 cycles. All these results prove that our ITM is suitable for on-skin and epidermal devices, it not only has the imperceptible and breathable structure, but also demonstrates excellent stability during daily activities.    To better convey the advantages of our ITM, the differences between the developed ITM and other reported triboelectric mechanoreceptors are listed as below: 1. The ITM can achieve versatile epidermal applications in single device. We demonstrate the ITM applying for mechanical energy harvesting, radial artery pulse monitoring, human activities monitoring, acoustic energy harvesting and biometric applications. More importantly, all these functions are performed in one single ultralight and breathable device. However, previous triboelectric mechanoreceptors are reported to achieve only one or two of these functions. For instance, some mechanoreceptors are unable to detect subtle pulse signals (with three distinct peaks), and others mechanoreceptors show inability for biometric applications.

The ITM shows high weight specific power density in harvesting mechanical energy.
For previous mechanical energy harvesters, they need either sophisticated and bulk structure design or extra power management circuits to achieve desirable power density. [31-36] For our ITM, we achieve high-performance with an ultrathin and ultralight device. As summarized in Table S1, we compared the different energy harvesters. As a result, our ITM shows high weight specific power density compared with other mechanical energy harvesters; and also holds comparable performance advantages when referring to other cutting-edge environmental energy technologies.