Peer review of: Employing a MEMS plasma switch for conditioning high-voltage kinetic energy harvesters (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
The manuscript reported an efficient energy management system for TENGs using a MEMS plasma 
switch. The energy harvesting efficiency is improved two orders of magnitude compared to a full-
wave rectifying circuit. TENGs are new energy harvesting devices that can produce high voltage but 
low current, and the energy management has been a challenge. This work provide a feasible solution 
and is of interests to researchers in the field. I recommend the publication of this manuscript. The 
manuscript has some minor errors and author might want to fix them. For example, the caption of 
fig 3. "The figure between (g) and (h)" should be "The figure betwen (d) and (e)". 
 
 
Reviewer #2 (Remarks to the Author): 
 
This an interesting paper, showcasing a promising approach to implement useful energy harvesters 
harnessing the triboelectric effect. The increase in harvesting efficiency of two orders of magnitude 
is significant. Please take care of the following issues to make your paper stronger and acceptable for 
publication: 
 
1-There seems to be a reference missing, the reference should fall between Ref. [35] and [36]. You 
quote it on page 2 and 5. Please look into and either add the reference or remove it (so the word 
processor does not show an error). 
 
2-The authors should comment on the suitability of their approach (microplasma switch) to 
environmental changes, e.g. humidity. The switch seems to need some kind of specialized packaging, 
e.g. hermetic, inert atmosphere. What would be the specifications of this controlled environment? 
please comment. 
 
3-Page 7, Fig. 2 (c) and (d). Please comment on why your modeling underestimates the voltage and 
overestimates the current (the opposite effects are good, they try to compensate each other when 
calculating power). Also, it is good that modeling and experiment agree on the lower bound (what 
you call "bottom envelope"), but there are features in the experimental data that are not captured 
by your model, e.g. the current ripples that seem to have a slower timescale than the natural 
oscillations Fig. 2(d). Where does this come from? Is this an artifact in the experiment of is inherent 
to your approach? please explain. 
 
4-page 8. You are missing a paragraph on microplasmas to provide context. In nutshell, by 
miniaturizing the inter-electrode separation, plasma sources can operate stably at less vacuum, even 
at atmospheric pressure, which opens very exciting opportunities, e.g. creating excited species that 
are otherwise only possible to create at extreme conditions. You should also some key references on 
micro plasmas. I suggest to include these: 
 
-K H Becker et al 2006 "Microplasmas and Applications", J. Phys. D: Appl. Phys. 39 R55 
and key recent work (focused on agile manufacturing, but still relevant to the microplasma "big 
picture") 
 
-Y S Kornbluth et al 2018 "Microsputterer with integrated ion-drag focusing for additive 
manufacturing of thin, narrow conductive lines" J. Phys. D: Appl. Phys. 51 165603 
 
-S. Ghosh et al. 2014 "Fabrication of Electrically Conductive Metal Patterns at the Surface of Polymer 
Films by Microplasma-Based Direct Writing" ACS Appl. Mater. Interfaces, 6, 5, 3099-3104 
 
5-Page 8. Spring softening (the spring constant is reduced) due to gap reduction is a well known 
phenomenon in MEMS mechanical switches. How spring softening affects (or improves?) your 
approach? please explain. 
 
6-Somewhere in the text, the authors should clearly state that solid-state switches are not good for 
this application becasue they are inherently leaky. You need a mechanical switch that physically 
disconnects the circuit. 
 
7-Please comment on reliability issues. Your devices are made of Si. Are you concerned about 
lifetime? would it help to make the devices in other materials, e.g. tungsten? please help us 
understand the trade-offs, e.g. energy function, fatigue, stability of physical properties (e.g. single 
crystal vs, multi-crystal). This is a significant issue for your technology to be adopted as mainstream. 
 
8-Based on your data, the choice of go = 9 um seems to be a fortunate coincidence; what would it 
take to optimize the design? In other words, maybe there is a better value for go, with even better 
performance. What would it take to find it? 
 
9-Page 11, you mention that 80 pairs of tips are working in the fixed-fixed switch. I respectfully 
disagree. The microplasma is a non-linear phenomenon, it is very sensitive to the tip radii (tip 
electric fields depend on their tip radius), my guess is that only a few of them are working during the 
discharge due to the tip radii spread (it is unavoidable when you make arrays of anything, and when 
one makes arrays of very small features, the spread tends to have long tails). 
 
10-Talking about the multi-tip switch, why do you have so many tips? are you concerned about 
lifetime (so as soon as the sharper tips get damaged, the duller tips start working)? please comment 
 
11- Supplementary video 2. Maybe is my browser, but your video is upside down! can you please 
check you uploaded the video in the right orientation? 
 
12-Thinking some more about your multi-tip device, the authors should point out that you might 
need current regulators in each emitter to uniformize the current in the device, to protect the tips 
from burning/damaging, to efficiently use the array of tips. In a nutshell, you can take care of the tip 
radii spread if you integrate negative feedback, i..e, having some ballast in series with each tip, so 
there is a maximum current per emitter, so the sharper tips can work at the same time the duller 
tips work. An ideal ballast has high impedance and high saturation current, e.g. an ungated field 
effect transistor (FET) operating in saturation, which is not hard to achieve given the voltage 
involved (you can get the FETs to saturate with volts). Given that you are making your structures in 
silicon, you can easily make long and narrow fingers ending in tips, which would make the fingers act 
as ungated FETs monolithically integrated to the sharp tips. Researchers have reported a similar idea 
for field emission of electrons: 
 
-L. F. Velasquez-Garcia et al. 2011 "Uniform High-Current Cathodes Using Massive Arrays of Si Field 
Emitters Individually Controlled by Vertical Si Ungated FETs—Part 1: Device Design and Simulation" 
IEEE Transactions in Electron Devices, vol. 58, No. 6, pp. 1775 - 1782. 
 
please add such discussion. Again, this is a significant issue for your approach to be adopted 
mainstream.