Peer review of: Quantum effect-based flexible and transparent pressure sensors with ultrahigh sensitivity and se (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
In this manuscript, entitled “Quantum effect-based flexible and transparent pressure sensors with 
ultrahigh sensitivity and sensing density”, the authors reported a flexible pressure sensor based on 
the Fowler-Nordheim tunnelling effect, which is claimed showing advantages of high sensitivity and 
temperature noninterference. It may be interesting for the readers. However, I suggested to 
consider following comcerns before publication. 
Here are my concerns: 
1. In table S1,the ‘pressure range above 1kPa-1’ is misleading. And it should provide the unit. 
Moreover, it does not have much meaning to have a high sensitivity for large pressure. Besides, 
the response time is not so impressive. I feel it is over claimed in the main text. 
2. As the authors reported, the pressure sensor is based on the Fowler-Nordheim (F-N) tunnelling 
effect, which is related to electric field strength. Therefore, what is the relationship between the 
sensor’s sensitivity and the voltage? More discussion or results will be helpful. 
3. As the authors reported, the pressure sensor needs a preloading process. How does the preload 
process affect the sensor’s performance and reliablity ? Could this be avoided by further design? 
Besides, since the signal of the loading and unloading are not the same, it is not good for practical 
applications. This is an big drawback. I suggest the authors comment on not only the advantages 
but also the drawbacks. 
4. As shown in Fig. 2b, the sensor’s performance varies greatly in different testing cycles, how to 
solve this problem? It is a obvious drawback. 
As the authors reported, they claimed the possibility of using this thin-film sensor as an 
implantable device only by immersing in water. It is kindly of too weak. There seems to be a lack 
of reliable biocompatibility tests. 
5. As shown in Fig. 4a, the tunneling current is clearly marked in the figure. Why is there no 
horizontal tunneling current? 
6. As shown in Supplementary Movie 3, the result of the position test, move test, and shape test 
were not ideal. How to solve this problem? 
7. Although the authors show a 400 per cm-2 matrix, the applciations does not the show the 
corresponding resolution. As seen in the Figure 5h. 
8. Besides, the manuscript should be checked to correct some grammar errors and spelling 
mistakes (Page 2, Line 31, “from the the current transduction mechanisms”). 
 
 
 
Reviewer #2: 
Remarks to the Author: 
The authors presented an interesting study to design and fabricate a highly sensitive pressure 
sensor by using urchin-like hollow carbon spheres mixed in PDMS. Although there have been 
numerous results in literature about developing sensitive pressure sensors, this paper gives a 
simple system with extremely high pressure sensitivity. They have also showed this pressure 
sensor has very good repeatability, stability and dynamic response. Temperature was also shown 
to not interfere the pressure sensing, superior to most similar systems. They also demonstrated 
that high-density arrays of such pressure sensors can be fabricated, showing excellent sensing 
performance. While the experimental results show very promising phenomena, the explanation of 
the mechanism is flawed, therefore it's not recommended to publish in its current form. Here are 
some comments that need clarification: 
1. In Fig. 2b, there is a significant plateau where the resistance doesn't decrease with increasing 
pressure, why? what's the reason of the plateau? Also why the curve goes down rapidly again after 
the plateau? why these sudden changes in the slope of resistance curve not reflected in the 
sensitivity curve? 
2. the authors claim the insensitivity to temperature is due to the hollow carbon spheres, but temp 
increase also induces significant thermal expansion in PDMS, causing increase in spacing between 
carbon spheres, why this effect is not affecting the sensing? 
3. The explanation of the mechanism is largely based on the assumption that the concentration of 
UHCSs increase with external pressure, however, PDMS's Poisson's ratio is very close to 0.5, it is 
nearly incompressible, so the concentration of UHCSs should not change during uniaxial 
compression, this assumption could be problematic. 
4. what level of deformation could be induced into the film? given the formula and ratio of PDMS 
used, the modulus of PDMS is ~1 MPa, even when 1kPa pressure is applied, it only causes 0.1% 
strain, how such a small strain induces such a large change in resistance and sensitivity as shown 
in Fig. 2? 
5. the authors claim insulation along horizontal directions and conduction along vertical direction 
due to pressure loading, what's the ratio of the resistivity along different directions? what's the 
mechanism? 
 
 
 
Reviewer #3: 
Remarks to the Author: 
The authors report a novel strategy using F-N tunneling effect to improve the sensitivity of a PDMS 
composite pressure sensor. The design of urchin-like hollow carbon spheres in PDMS is the key to 
the high performance. A ultrahigh sensitivity of ~260 kPa-1 at 1 Pa is remarkable, especially with 
high transparency and temperature noninterference of the thin-film sensor. However there are 
some concerns that need to be explained. This paper may be considered for publication after 
major revisions. 
1. The urchin-like hollow structure of the carbon spheres seems fragile. A large pressure generated 
by deformation of the substrate might destroy the thin hollow sphere. The authors are suggested 
to explain the relevance of applied pressure (1-10,000 Pa) to the strain of the PDMS film, and how 
does the strain affect the sphere through squeezing. 
2. If the hollow sphere or the tip is kind of soft (compressive or bendable) to endure the substrate 
deformation, should the theoretical model be modified? 
3. How to explain the variance between forward-R/R0 and backward-R/R0? 
4. Figure 3 shows only good flexibility of the PDMS film, but no evidence of “implantable”. The 
current results imply PDMS-based thin-film pressure sensors are all “implantable”, then what is the 
novelty of this work? 
5. Comparing to the theoretical sensing density of over 2718000 per cm2, the reported value is 
400 per cm2, which fails to reflect the advance of the sensor. Besides, the sensing density of 400 
per cm2 is already achievable by industrial method, then what is the contribution of the PDMS 
sensor?