Peer review of: Micro triboelectric ultrasonic device for acoustic energy transfer and signal communication (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
This report on a micro triboelectric ultrasonic device is clearly an interesting and thorough examination of 
the topic. As far as I know, it is the first time a triboelectric device is completely fabricated based on MEMS 
technology, much improving its miniaturization and integration. And optimization methods were also 
proposed to improve the output. The fact that the device can demonstrate signal communication via 
ultrasound is certainly noteworthy. Overall, this manuscript should be taken into consideration of the 
publication in Nature Communications after clarifying the following comments. 
1. The structure of the µTEH is quite similar to that of capacitive micromachined ultrasonic transducer 
(CMUT). Please give some details about the difference between them regarding structure, working 
mechanism and fabrication process. 
2. The operating mode the author is using for the µTEHs is highly asymmetric since on the negative phase of 
the cycle there is contact between the SiO2 and the Si and in the positive phase there is no contact. 
However, none of the measurements show any anharmonicity and appear quite sinusoidal (e.g. Figure 4c, 
5e, 5f, S2). Please give some explanation. 
3. The plot of Figure 4d seems fairly noisy. Please explain where the noise comes from. 
4. The traveling time in Figure 5e/f and Figure S2 seems different. Please give some explanation. 
5. Based on the acoustic experimental setup, the standing wave effect may also significantly affect the 
output of the device. I would like the author to give some explanation. 
 
 
 
Reviewer #2 (Remarks to the Author): 
 
The manuscript by Chen Chen et al. reports on a microtriboelectric energy harvester (μTEH) developed to be 
applied for acoustic energy transfer and signal communication via an ultrasound link. The novelty of the 
paper mainly relies on the fabrication method which is based on silicon-based MEMS technologies. This 
approach extends to silicon MEMS the possibility to use the triboelectric effect, making it possible a better 
integration and miniaturization of implanted device. On the other end, however, it is not exploiting the main 
advantage of triboelectric μTEH device which comes from the application of cheap and flexible polymeric 
materials and technologies. 
It is a comprehensive study which is more interesting for the approach than for the results and their 
applicability. It could be of interest for the MEMS community but it is far from any real clinical application. I 
am not totally convinced that it is suited for Nature communication. 
 
A few comments and questions: 
• The cavity is under vacuum. This prevents some sort of squeeze dumping. It would be interesting to know 
the advantage of this approach over having atmospheric pressure on both sides of the membrane (with 
holes on it), or vacuum on both sides. 
• There is a discrepancy between analytical calculation and COMSOL simulation which is attributed to the 
presence of the gold electrode in the comsol simulation only. The authors should state if comsol simulation 
without gold has been also performed and what is the result. 
• There should be a trade-off between contact area, sticking force and membrane elastic modulus. Can the 
authors comment on how this affects EH efficiency? 
• Figure 5b is totally unclear. 
• The authors states that a rectifying efficiency of 89% is possible, citing a paper by Ozeri et al. However, 
based on the harvested power and voltage levels of the authors’ technology (a few mVolts), the harvested 
energy can be pratically zero. Indeed, voltage threshold of AC-DC rectification is typically around hundreds 
of mV. This must be better specified. 
• What is the experimental efficiency achieved? Can you estimate the theoretical maximum transfer 
efficiency? 
 English syntax and grammar must be carefully checked throughout the manuscript (e.g. in line 146 “which 
3D structure” must be “whose 3D structure”). In particular abstract and intro must be rewritten to better 
contextualize and describe the work. 
On line 360 the degrees symbols are not well shown. 
 
 
 
Reviewer #3 (Remarks to the Author): 
 
1. This work is quite similar to the paper on Science by Prof Sang-Woo Kim (Science, 365. 6452, 491-494.), 
Comparing the output of the two articles, the device of Kim's team reached 6.74mW, which is 4 orders 
higher than this work (243 nW). With such an inferior performance, what is the advantage of the µTEH on 
energy harvesting? 
2. The µTEH is designed for the implanted device, but there is no experiment proof to demonstrate the such 
application or its biocompatibility. Please consider such demonstrations to improve the manuscript. 
3. The energy harvesting experiment was performed in oil of uniform density, but for an implanted device, 
the skin interface and muscle tissue can cause significant attenuation of ultrasound, more relevant 
experiments should be performed to verify the µTEH’s performance in sound-attenuating medium. 
4. In Figure 4, the piezo transducer was actuated by a continuous wave with the amplitude of 10 V, why didi 
the actual driving signal decrease to ~5 V? In addition, the manuscript does not explain how Figure 4b was 
tested, and the angle in Figure 4b is not explained. 
5. The top electrode with charge should be given in the schematic of Figure 1a to better explain the working 
principle. In addition, what is the bottom electrode of Figure 3C? Is that silicon? How can it work as an 
electrode? Please provide a complete device schematic, including all materials (friction layer, electrode, 
etc.), and draw a charge transfer diagram of the TENG workflow. 
6, In fig.1b-c, the KPFM data of the surface potential is poor and lacking proper level labeling. In addition, 
such a surface potential comparison cannot help support the conclusion without a reference like aluminum or 
PTFE. 
7, It is interesting to find that the input impedance of such a TENG is below 1k ohm, which is different from 
the usual range of previously reported TENGs. Why is the input impedance so low? It is suggested the 
authors should also provide the corresponding output current data of their device. 
8, It is interesting to see that the output power of this TENG device is still limited to nW level while the 
output voltage is up to several volts with low impedance. Why is the power output so low?