A Stretchable Electronic Tattoo for Self-Powered Human–Machine Interfaces and Therapeutic Applications

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors propose a metal film-based conductor, and study mechanical properties, electrical performance. The application of self-powered sensors and thermal stimulation were also discussed. There were still many problems that needed to be addressed. I suggest the following points for consideration:

1. In fig. 2, based on this fabrication method, will the different Cu-graphene films have the same microcracked morphology structure under the same input force? Will this affect the resistance variation? This issue should be discussed, and error bars should be added to fig. 2ghjk.
2. In fig. 3a, the flow of mobile electrons should be indicated to satisfy charge conservation.
3. Why is there a two-order-of-magnitude discrepancy between the simulated potential difference in fig. 3b and the measured result in fig. 3c?
4. Referring to the waveforms in fig. 3f, each contact generates a downward peak and an upward peak. Then, how is the data in fig. 3e obtained? Why are some pixels positive while others are negative?
5. Biocompatibility and drug delivery were mentioned in the manuscript. Corresponding experiments and data should be supplemented to support and discuss this conclusion.

Author Response

The first referee report and response

The authors propose a metal film-based conductor, and study mechanical properties, electrical performance. The application of self-powered sensors and thermal stimulation were also discussed. There were still many problems that needed to be addressed. I suggest the following points for consideration.

Response: We are deeply grateful to the reviewer for their meticulous evaluation and constructive suggestions, which have significantly strengthened both the technical quality and scholarly impact of our work. Through careful consideration of each comment, we have implemented comprehensive revisions throughout the manuscript.

Comment 1: In fig. 2, based on this fabrication method, will the different Cu-graphene films have the same microcracked morphology structure under the same input force? Will this affect the resistance variation? This issue should be discussed, and error bars should be added to fig. 2ghjk.

Response 1: We sincerely appreciate the reviewer’s insightful suggestion. The bilayer electrode we constructed can maintain good conductivity under stretching. Its main advantages are its lightweight and thin nature, excellent adhesion, and independence from substrate material limitations. We analyzed the surface morphology and conductivity of different electrodes. Our method has limited controllability over surface cracks; although the surface cracks vary, their impact on the resistance change under stretching is minimal. Our approach primarily aims to generate random micro-cracks, suppress the formation of penetrating cracks, and utilize the graphene layer to connect cracks within the same layer. As a result, it ensures tensile conductivity stability for electrodes prepared on different substrates. In accordance with the reviewers' comments, we have revised the corresponding figures accordingly.

"We investigated the surface morphology and electrical conductivity of various electrodes. A primary limitation of our approach is the insufficient controllability over surface cracking, which leads to variations in crack patterns. Nevertheless, this method effectively generates random micro-cracks, suppresses the formation of through-thickness cracks, and utilizes the graphene layer to bridge cracks within the same plane. As a result, it ensures stable conductivity under mechanical stretching for electrodes fabricated on diverse substrates."

Figure 2. Mechanism of microcracked morphology structure and electrical characterization.

Thanks.

Comment 2: In fig. 3a, the flow of mobile electrons should be indicated to satisfy charge conservation.

Response 2: We sincerely appreciate the reviewer’s insightful suggestion. As requested by the reviewers, we have revised the corresponding figures.

Figure 3. Mechanism and electrical signals of the Cu-graphene film based on self-powered E-skin.

Thanks.

Comment 3: Why is there a two-order-of-magnitude discrepancy between the simulated potential difference in fig. 3b and the measured result in fig. 3c?

Response 3: We sincerely appreciate the reviewer’s insightful suggestion. During the simulation process, inaccuracies arising from dimensional constraints and interface charge configurations caused discrepancies in the initial results. These have been rectified in the revised figure.

Thanks.

Comment 4: Referring to the waveforms in fig. 3f, each contact generates a downward peak and an upward peak. Then, how is the data in fig. 3e obtained? Why are some pixels positive while others are negative?

Response 4: We sincerely appreciate the reviewer’s insightful suggestion. The triboelectric signal exhibits two peaks, and we plotted the data using absolute values (Figure 3e). The depth of the red color represents the magnitude of the peaks, while purple indicates regions with negligible signal. Due to the acquisition circuit and associated interference, achieving an absolute 0 V output is not feasible. Nevertheless, in Figure 3e, the red regions can still be correlated with the corresponding letters.

"We plotted the data using absolute values; the depth of red represents the peak magnitude, and the red regions are aligned with the corresponding letters."

Thanks.

Comment 5: Biocompatibility and drug delivery were mentioned in the manuscript. Corresponding experiments and data should be supplemented to support and discuss this conclusion.

Response 5: This paper primarily aims to present a low-cost method for fabricating flexible electrodes. Its main advantages are excellent adhesion and independence from the shape and material of the substrate. We have demonstrated various application scenarios for these flexible electrodes. Regarding drug delivery, due to limitations in our laboratory conditions, we have only conducted preliminary experiments to illustrate a potential future application. We fully agree with the reviewer regarding the necessity of biocompatibility experiments. However, considering the constraints of time and testing costs, we intend to conduct in-depth research on this aspect in our future work.

Thanks.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript requires major revision due to the following fundamental issues:

1. Lack of mechanistic-level analysis and data-driven validation.
   The manuscript mainly provides qualitative descriptions and schematic explanations, while detailed mechanistic analysis supported by quantitative data is largely absent. Key claims regarding crack control, dual conductive pathways, and strain-insensitive conductivity are not sufficiently substantiated by rigorous experimental or numerical evidence.

2. Absence of clearly identifiable innovation.
   The authors do not clearly articulate what the genuine innovation of this work is. From the perspectives of fabrication process, material selection, structural design, and application demonstrations, the presented approaches and results appear to rely on well-established strategies in the literature, with no evident conceptual, methodological, or functional novelty.

3. Unclear necessity and advantage of the proposed approach.
   It is not convincingly explained why the newly obtained results are necessary or advantageous compared to existing flexible and stretchable electronic systems. The manuscript does not demonstrate that the reported performance or functionalities are uniquely enabled by the proposed platform or offer clear advantages over alternative, widely used materials and designs.

Unless these issues are comprehensively addressed, the current work remains incremental in nature and does not yet meet the standard required for publication

Author Response

The manuscript requires major revision due to the following fundamental issues:

Response: We sincerely appreciate the reviewer's commitment to dedicating their valuable time and expertise to provide insightful feedback. We have carefully considered the reviewer’s comments and actively incorporated their suggestions, ensuring that every issue raised has been thoroughly addressed and all concerns effectively resolved. In response to the feedback, we have prepared a detailed point-by-point clarification and incorporated these revisions into the updated manuscript.

Comment 1:  Lack of mechanistic-level analysis and data-driven validation. The manuscript mainly provides qualitative descriptions and schematic explanations, while detailed mechanistic analysis supported by quantitative data is largely absent. Key claims regarding crack control, dual conductive pathways, and strain-insensitive conductivity are not sufficiently substantiated by rigorous experimental or numerical evidence.

Response 1: We sincerely appreciate the reviewer's insightful suggestion. Currently, there is no clear consensus on a detailed theoretical framework for flexible electrodes. Relevant studies are primarily interpreted based on conventional interface theories. To achieve low resistance and high mechanical stability, we selected the graphene film as the interlayer between the low-resistance Cu film and the tattoo paper to restrain cracks on the Cu film and establish a double conductive path (Figure S7). Another critical parameter for achieving resistance bending strain insensitivity is the small folding structures of tattoo papers, which reduce partial adhesion between the tattoo paper and flexible substrate (Figure S9). During the deformation process, the shear pattern undergoes three primary deformation stages. In the first stage, the structure composed of parallel and symmetrical incisions experiences external loading, causing each beam to rotate around its nodal points and bend in-plane, thereby elongating the entire shear sheet along the stretching direction. In the second stage, the beams continue to deform along the tensile direction, with stress rapidly increasing until fracture initiation occurs. The third stage is characterized by beam fracture, where cracks progressively accumulate, ultimately leading to complete struc-tural failure. Through experimental investigation and theoretical analysis of three distinct shear patterns, we conclude that an increase in the y of the shear configuration results in a more compact tensile structure, thereby reducing its stretchability. Conversely, an increase in the Lc leads to a looser tensile structure, enhancing its stretchability (Figure S13). In this paper, we have conducted a preliminary exploration of this aspect and hope the reviewers will recognize our findings.

Thanks.

Comment 2: Absence of clearly identifiable innovation. The authors do not clearly articulate what the genuine innovation of this work is. From the perspectives of fabrication process, material selection, structural design, and application demonstrations, the presented approaches and results appear to rely on well-established strategies in the literature, with no evident conceptual, methodological, or functional novelty.

Response 2: We sincerely appreciate the reviewer's insightful suggestion. Benefiting from their ultrathin geometry and the adhesion enabled by tattoo paper, the electrodes achieve intimate contact with diverse substrates—such as PDMS, glass, metal, fiber, and human skin—enabling the demonstration of an on-skin intelligent communication system for calling a cellphone and a thermal patch for skin heating via electrical and optical signals.

"Compared to these studies, our fabrication method is more straightforward and rapid, enabling the stable construction of electrodes on various substrate surfaces. The resulting electrodes are lightweight, thin, and exhibit excellent conformability. These electrodes can serve as conductive layers in flexible electronic devices—such as flexible photothermal devices and tactile sensors—meeting the demands for comfortable wearability."

Thanks.

Comment 3: Unclear necessity and advantage of the proposed approach. It is not convincingly explained why the newly obtained results are necessary or advantageous compared to existing flexible and stretchable electronic systems. The manuscript does not demonstrate that the reported performance or functionalities are uniquely enabled by the proposed platform or offer clear advantages over alternative, widely used materials and designs.

Response 3: We sincerely appreciate the reviewer’s insightful suggestion. Numerous studies have been conducted on flexible electrodes, yet achieving conformable conductive layers on diverse substrates remains a challenge. To address this, we focus on tattoo paper, leveraging its ability to form stable interfaces on both human skin and various other substrates. Combined with kirigami patterning, it serves as a conductive layer for diverse flexible devices. By integrating a dual-conduction pathway comprising a graphene and a metal layer, we not only enhance electrical stability but also exploit the photothermal properties of graphene—an effect that cannot be fully utilized with a single metal layer alone.

"A key challenge in the field of flexible electronics is fabricating conformable conductive layers on various substrates. Our approach addresses this by utilizing tattoo paper, which readily forms stable interfaces with human skin and other materials, in conjunction with kirigami cutting techniques. The resulting bilayer electrode, featuring both graphene and metal layers, provides dual conduction pathways. This design not only improves electrical stability but also capitalizes on the photothermal effect of graphene, a feature absent in single-metal electrodes."

Thanks.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have addressed some of the questions. Given that the biocompatibility and drug delivery experiments cannot be conducted at present, the discussions on these two aspects in the manuscript need to be revised accordingly.

Author Response

Comment: The authors have addressed some of the questions. Given that the biocompatibility and drug delivery experiments cannot be conducted at present, the discussions on these two aspects in the manuscript need to be revised accordingly.

Response: We are deeply grateful to the reviewer for their meticulous evaluation and constructive suggestions, which have significantly strengthened both the technical quality and scholarly impact of our work.

"These initial findings serve as a proof-of-concept for the device's potential in drug delivery. However, given the requirements for biomedical translation, a comprehensive biosafety evaluation is imperative. Our subsequent research will be dedicated to systematically evaluating the device for biomedical applications."

Reviewer 2 Report

Comments and Suggestions for Authors

With Modification and detailed explanation, now I believe that this manuscript is of some extent of importance and innovation, with the modification now, it is OK to be published now.

Only one more comment: in the paragraph about Key challenge in the field of flexible electronics (line 411-416), the discussion is proper, but there needs some reference or solid proof to confirm the claim.

Author Response

Comment: With Modification and detailed explanation, now I believe that this manuscript is of some extent of importance and innovation, with the modification now, it is OK to be published now. Only one more comment: in the paragraph about Key challenge in the field of flexible electronics (line 411-416), the discussion is proper, but there needs some reference or solid proof to confirm the claim.

Response: We sincerely appreciate the reviewer's commitment to dedicating their valuable time and expertise to provide insightful feedback. We have carefully considered the reviewer’s comments and have added the relevant references, ensuring that every issue raised has been thoroughly addressed and all concerns effectively resolved.