Recent Advances of Conductive Hydrogels for Flexible Electronics

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors present a review concerning hydrogels with conductive properties. The work meets the outlined objectives. It showcases an extensive and comprehensive description of recent advances in the field of Conductive Hydrogels. Meanwhile there are some minor corrections that the authors should consider:

1- The abstract should be improved. "Conductive hydrogels combine the properties of both hydrogels and electrical conductivity, making them soft, flexible, and biocompatible". electrical conductivity is a property, so it should be replaced by "conductors".

2.at page 3 the authors state that "Conductive polymers (CPs) typically consist of carbon atoms and p-conjugated electron systems, endowing them with the ability to conduct electrons." Polymers are mostly carbon based chains, but the electronic mechanisms are not restricted to carbon atoms. This sentence should be revised.

3. At page 5, acronym BP sheets should be defined;

4. At page 9, regarding GF strain coefficient, the e parameter should be defined;

5. For all the ions the charge value should superscript (example Ca²⁺);

6. More references related with biopolymers like galactomannan, cellulosic and chitosan based should be added, for example, "Conductive polymer blend based on polyaniline and galactomannan: Optical and electrical properties, Synthetic Metals, doi.org/10.1016/j.synthmet.2023.117346) and "Cellulosic-Based Conductive Hydrogels for Electro-Active Tissues: A Review Summary", Gels, https://doi.org/10.3390/gels8030140

 

Comments on the Quality of English Language

No comments.

Author Response

Comments and Suggestions for Authors

The authors present a review concerning hydrogels with conductive properties. The work meets the outlined objectives. It showcases an extensive and comprehensive description of recent advances in the field of Conductive Hydrogels. Meanwhile there are some minor corrections that the authors should consider:

Response:  Thank you for positive comments and constructive suggestions. Accordingly, we have carefully considered each point you raised and revised the manuscript.

1. The abstract should be improved. "Conductive hydrogels combine the properties of both hydrogels and electrical conductivity, making them soft, flexible, and biocompatible". electrical conductivity is a property, so it should be replaced by "conductors".

Response: We have replaced the word "electrical conductivity" with "conductor".

 

2. at page 3 the authors state that "Conductive polymers (CPs) typically consist of carbon atoms and p-conjugated electron systems, endowing them with the ability to conduct electrons." Polymers are mostly carbon-based chains, but the electronic mechanisms are not restricted to carbon atoms. This sentence should be revised.

Response: Thanks for the constructive suggestion and comments. We have changed the sentence “Conductive polymers (CPs) typically consist of carbon atoms and p-conjugated electron systems, endowing them with the ability to conduct electrons.” into “Conductive polymers (CPs) typically consist of carbon-based chains with p-conjugated electron systems. Their electronic mechanisms involve delocalized pi electrons along the polymer backbone, enabling electrical conductivity.”

 

3. At page 5, acronym BP sheets should be defined;

Response: We added the definition of the acronym BP at page 5: “…such as black phosphorus (BP) nanosheets or MXene nanosheets.”

 

4. At page 9, regarding GF strain coefficient, the εparameter should be defined;

Response: We added the definition of the strain ε by adding the part below: “This variability in sensitivity can be characterized by the strain coefficient (GF) (GF = (R - R0)/R0)/ ε), where ε represents strain (ε = Δ?/?_0, Δ? is the absolute change in length, ?₀ is original length)”.

 

5. For all the ions the charge value should superscript (example Ca²⁺);

Response: We have corrected all the charge values of the ions in this paper.

 

6. More references related with biopolymers like galactomannan, cellulosic and chitosan based should be added, for example, "Conductive polymer blend based on polyaniline and galactomannan: Optical and electrical properties, Synthetic Metals, doi.org/10.1016/j.synthmet.2023.117346) and "Cellulosic-Based Conductive Hydrogels for Electro-Active Tissues: A Review Summary", Gels, https://doi.org/10.3390/gels8030140

Response: Thanks for the constructive suggestion. We have included citations at the appropriate locations as below:

“…During polymerization, overlap of p orbitals between π bonds induces electron redistribution [22]. Morais et al. developed a conductive polymer blend based on polyaniline and galactomannan, demonstrating significant optical and electrical properties improvements through polymer blending techniques [23]. Conductive polymers, characterized by their unique p-conjugated structures, find application in electron transfer…”

“…of a permeable particle network. Tadesse et al. explored the incorporation of nanoparticles into hydrogels, significantly enhancing their optical and electrical properties, making them ideal for flexible electronic applications [32]. This network effectively augments both the electrical and mechanical attributes inherent to hydrogels.”

23. Lemos Morais, J.P., Bernardino, D.V., Batista, B.d.S., Pereira, W.O., Borges Amaral, F.M., Meneses Pedra Branca, M.C., Gasparin, F.P., dos Santos, A.O., Bezerra Sombra, A.S., Mendes, F., Mathias Macedo, A.A., Conductive polymer blend based on polyaniline and galactomannan: Optical and electrical properties, Synthetic Metals (2023) 295 1442-1452.
24. Gebeyehu, E.K., Sui, X., Adamu, B.F., Beyene, K.A., Tadesse, M.G., Cellulosic-Based Conductive Hydrogels for Elec-tro-Active Tissues: A Review Summary, Gels (2022) 8(3) 140.

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

in your interesting manuscript, the following points should be added/changed to further improve it:

- Fig. 1: "from the internet" is not a possible source. For each cited image, you need the exact source as well as either the permission and copyright or the information that the original image was published under a CC-BY (or similar) license. This is also valid for all subsequent figures.

- line 71-72: The differentiation between these different categories is not clear for me. Actually the first describes intrinsically conductive hydrogels, while the other two are based on conductive fillers.

- line 83: the abbreviation PEDOT is missing

- line 99: What does GM-P mean?

- The paragraph starting in line 125 is really too long, please break it up.

- line 142: You mean possibly 10^9 Ohm (what would not be low) or just 10 or 100 Ohm.

- line 187: the "-1" must be superscripted.

- line 195: Here the "p-Ti3C2Tx" needs some subscripts.

- line 212: ditto

- line 222: Fe^3+

- line 223: 1.160%

- line 256: "These include but are not limited to:" - what is meant with "these"? This sentence is not related to the previous one.

- line 323 and 325: Ca^2+

- Table 1: The first three conductivities need superscripts.

- The paragraph starting in line 347 is again much too long. Please check the residual manuscript for similarly lengthy paragraphs, too.

- line 372: subscripts

- line 556: Ca^2+

- line 564, 565: Fe^3+

- line 600: Al^3+

- line 684: subscripts. What is nps?

- line 697: "electrically active hydrogels can facilitate wound healing through their inherent electrical properties" - why?

- line 725: Fe^3+

- line 739: Cu-Fe(III)-HEO

- line 748: "small deformations (such as pulses and heartbeats)" - the heartbeat is identical to the pulse and more often detected electrically, not due to deformation of the skin.

- line 797: Fe^3+

Comments on the Quality of English Language

see above

Author Response

Comments and Suggestions for Authors

in your interesting manuscript, the following points should be added/changed to further improve it:

- Fig. 1: "from the internet" is not a possible source. For each cited image, you need the exact source as well as either the permission and copyright or the information that the original image was published under a CC-BY (or similar) license. This is also valid for all subsequent figures.

Response: Thanks for your kind reminder. The copyrights for all figures have been obtained.

 

- line 71-72: The differentiation between these different categories is not clear for me. Actually, the first describes intrinsically conductive hydrogels, while the other two are based on conductive fillers.

Response: Thanks for the constructive suggestion and comments. We have rephrased this sentence into: “…can be classified into two main categories (Figure 2): intrinsically conductive hydrogels and normal hydrogels based on conductive fillers.”

 

- line 83: the abbreviation PEDOT is missing

Response: This was a typo. We have corrected it.

 

- line 99: What does GM-P mean?

Response: Thanks for your careful inspection. This was a typo, we have corrected it to “γ-PGA/PEDOT:PSS”.

 

- The paragraph starting in line 125 is really too long, please break it up.

Response: Thanks for your suggestion. Accordingly, we have divided this paragraph into two, shown as below: “… the electrical conductivity of the hydrogel microelectrodes, achieving resistances as low as 10⁹ Ω. While the integration of metal nanoparticles fortifies the electrical and mechanical aspects of hydrogels, it may impart certain drawbacks, including potential impacts on their electrical properties and heightened production costs, thus posing constraints on large-scale applicability.

Carbon-based nanoparticles offer multifaceted utility, serving as both active materials for energy storage and conduits for energy transfer networks. This diverse category encompasses graphene oxide (GO) (Figure 2d), ….”.

 

- line 142: You mean possibly 10^9 Ohm (what would not be low) or just 10 or 100 Ohm.

Response: We have corrected it to 10⁹ Ohm.

 

- line 187: the "-1" must be superscripted.

Response: We have corrected them.

 

- line 195: Here the "p-Ti3C2Tx" needs some subscripts.

Response: We have corrected it to p-Ti₃C₂T_x.

 

- line 212: ditto

Response: We have corrected it.

 

- line 222: Fe^3+

Response: We have corrected it to Fe³⁺.

 

- line 223: 1.160%

Response: We have corrected it to 1.16 %.

 

- line 256: "These include but are not limited to:" - what is meant with "these"? This sentence is not related to the previous one.

Response: Thank you for your careful inspection. To enhanced the clarity, the sentence “These include but are not limited to: robust mechanical performance, sensitive mechanical responsiveness, long-term stability, and biocompatibility, among other multifunctional properties.” was corrected into: “… are fundamental properties to fulfill. However, conductive hydrogels offer diverse potentials, such as robust mechanical performance, sensitive mechanical responsiveness, long-term stability, and biocompatibility, among other multifunctional properties. These materials often need to possess various desirable characteristics, thereby expanding the application scope and fields of these materials.”

 

- line 323 and 325: Ca^2+

Response: We have corrected them to Ca²⁺.

 

- Table 1: The first three conductivities need superscripts.

Response: We have corrected them.

 

- The paragraph starting in line 347 is again much too long. Please check the residual manuscript for similarly lengthy paragraphs, too.

Response: Thanks for the constructive suggestion and comments. We have divided this paragraph into two as below: “… Additionally, soaking in salt solutions can enhance the electrical and mechanical proper-ties of DN conductive hydrogels, as they are mutually influential.

In the conductive hydrogels prepared by Zhou et al., hydroxypropyl cellulose (HPC) biopolymer fibers were physically crosslinked with a tough and biocompatible …”

 

- line 372: subscripts

Response: We have corrected it.

 

- line 556: Ca^2+

Response: We have corrected it to Ca²⁺.

 

- line 564, 565: Fe^3+

Response: We have corrected it to Fe³⁺.

 

- line 600: Al^3+

Response: We have corrected it to Al³⁺.

 

- line 684: subscripts. What is nps?

Response: We have corrected it to “Magnetoelectric Fe₃O₄@BaTiO₃NPs-loaded with hyaluronic acid (HA)/collagen hydrogel were prepared”.

 

- line 697: "electrically active hydrogels can facilitate wound healing through their inherent electrical properties" - why?

Response: Compared to traditional hydrogel materials, conductive hydrogels have the potential to enhance the electrical signal communication between cells, which benefits cell repair.

To enhance the clarity, we rewrote the sentence as below: “…Unlike traditional dressings that typically require medication for wound repair, electrically active hydrogels have the potential to enhance the electrical signal communication between cells, which benefits cell repair. …”

 

- line 725: Fe^3+

Response: We have corrected it to Fe³⁺.

 

- line 739: Cu-Fe(III)-HEO

Response: We have corrected it.

 

- line 748: "small deformations (such as pulses and heartbeats)" - the heartbeat is identical to the pulse and more often detected electrically, not due to deformation of the skin.

Response: Thank you for your careful inspection. Accordingly, in this sentence, “and heartbeats” were deleted.

 

- line 797: Fe^3+

Response: We have corrected it to Fe³⁺.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript "Recent advances of conductive hydrogels for flexible electronics" provides a thorough review of the significant progress in the field of conductive hydrogels. The authors effectively highlight the unique properties of conductive hydrogels, such as their flexibility, biocompatibility, and ability to maintain electrical conductivity under deformation. These attributes make them ideal for a wide range of applications, including sensors, tissue engineering, and human-machine interfaces. The review meticulously discusses the advancements in functional materials, strategies for performance improvement, and wearable applications. Additionally, it systematically summarizes the approaches and challenges related to enhancing mechanical properties, conductivity, and long-term stability of conductive hydrogels. However, the paper may be accepted by Electronic Materials after addressing the following comments:

 

1.     In the section of introduction:

a)     The introduction effectively sets the stage for discussing the importance of conductive hydrogels in flexible electronics. To strengthen it further, consider providing more context on the specific challenges or gaps in existing flexible electronics technologies that conductive hydrogels aim to address. Additionally, mentioning some recent key developments or emerging trends in this area would make the introduction more engaging.

b)     Also, could you include a brief overview of the historical development of conductive hydrogels in the context of flexible electronics? This would help to provide a foundation for understanding the current advancements in the field. Moreover, it would benefit from a more detailed discussion of the specific challenges or limitations associated with traditional inorganic conductive materials, which conductive hydrogels aim to address.

c)     In addition, have you discussed the specific challenges that previous research has encountered in achieving and maintaining conductivity in hydrogels for electronic applications? Detailing these obstacles would highlight the progress made and areas needing further improvement.

2.     In the section of Fabrication of conductive hydrogels:

a)     This section provides a brief overview of the fabrication methods for conductive hydrogels, but it lacks detail on specific techniques or approaches. Including examples of commonly used fabrication methods, such as in situ polymerization, physical blending, or chemical crosslinking, would provide readers with a more comprehensive understanding of the fabrication process. Additionally, discussing the advantages and limitations of each fabrication method could help readers evaluate the suitability of different approaches for their own research or applications.

b)     To enrich the discussion, consider elaborating on the comparative advantages and limitations of different synthesis techniques, such as in situ polymerization, physical blending, and chemical crosslinking and so on. Additionally, providing insights into the scalability and reproducibility of these methods would be valuable for readers interested in practical applications.

c)     In the sub section of ‘Ion conductive hydrogels’, the authors mentioned how ionically conductive hydrogels mimic ion transport in biological tissues. What are the key parameters that influence ionic conductivity in these hydrogels? How does the choice of metal salts, acids, or ionic liquids affect the conductivity and other properties of the hydrogels?

d)     The text mentions the development of self-healing, stretchable conductive hydrogels. What are the typical self-healing mechanisms employed, and how do they impact the performance of hydrogel?

3.     In the section of key properties and enhancement strategies of conductive hydrogels:

a)     While this section outlines the broad categories of conductive hydrogels (electronic conductive hydrogels, nanoparticle conductive hydrogels, ion conductive hydrogels), it could benefit from further elaboration on the specific properties and performance metrics of each type. Providing details on key properties such as electrical conductivity, mechanical strength, swelling behavior, and biocompatibility would help readers understand the unique characteristics of different types of conductive hydrogels and their potential applications. Also, please provide a more comprehensive listing of each research, detailing the materials used, as well as the properties and applications of these materials in each respective research, rather than simply presenting conductivity in the Table 1.

b)     The text references several figures (e.g., Figure 4a&b) that are not detailed descriptions in this section. If possible, ensure that all referenced figures are included and clearly explained within the text which would enhance the reader's understanding

4.     In the section of application demonstration in flexible electronics:

a)     The text on neural tissue engineering is informative but could be improved by including more specific data and outcomes from recent studies. Can you provide detailed results from studies on the use of conductive hydrogels in neural repair, including metrics such as the extent of neural regeneration, functional recovery, and long-term biocompatibility?

b)     The author mentioned that the use of flexible sensors for monitoring physiological signals such as heart rate and blood pressure. To strengthen this discussion, more details on the specific mechanisms and technologies used in these sensors would be useful. How do conductive hydrogels improve the accuracy and reliability of health monitoring devices compared to traditional materials?

c)     The benefits of hydrogel sensors adhering to biological tissues without additional media are highlighted. However, it would be helpful to discuss the specific materials and designs that enhance adhesion and flexibility. What are the key factors that contribute to the strong adhesion and flexibility of conductive hydrogels, and how do these properties impact their performance in wearable devices?

5.     In the section of conclusion and prospects:

a)       The section of conclusion and prospects should include a discussion on future trends and innovations in the field of conductive hydrogels. What new developments or emerging technologies could further enhance the performance and applicability of these materials in flexible electronics? To further enrich this section, it would be beneficial to highlight specific areas within tissue engineering where conductive hydrogels have shown exceptional potential, such as in organ-on-a-chip platforms or regenerative medicine approaches targeting specific tissues. Additionally, discussing ongoing efforts to integrate conductive hydrogels with emerging technologies like 3D bioprinting or organoid culture systems could provide insights into future directions for research and development.

b)       The challenges mentioned are critical. However, providing more detailed solutions or research directions to address these challenges would be beneficial. Are there specific strategies or emerging technologies that show promise in improving the mechanical properties, biocompatibility, and biodegradability of conductive hydrogels? To enhance it further, consider proposing specific avenues for future research or technological advancements in the field of conductive hydrogels for flexible electronics. Addressing the remaining challenges and outlining potential breakthroughs would provide valuable guidance for researchers and practitioners in this area.

c)       Moreover, while emphasizing the potential of conductive hydrogels in flexible electronics, it would be valuable to address current limitations hindering their widespread adoption in commercial products. For instance, discussing challenges related to scalability, manufacturability, and integration with existing electronic components could provide a more comprehensive understanding of the practical considerations involved in utilizing conductive hydrogels in wearable devices and other electronic applications.

 

Comments on the Quality of English Language

The overall quality of the English language in the manuscript is satisfactory. The authors have effectively communicated the main points and provided a comprehensive review of conductive hydrogels for flexible electronics. However, there are a few instances where the sentences could be rephrased for better clarity and flow. Simple adjustments to some sentences can enhance the readability of the manuscript.

Author Response

Comments and Suggestions for Authors

The manuscript "Recent advances of conductive hydrogels for flexible electronics" provides a thorough review of the significant progress in the field of conductive hydrogels. The authors effectively highlight the unique properties of conductive hydrogels, such as their flexibility, biocompatibility, and ability to maintain electrical conductivity under deformation. These attributes make them ideal for a wide range of applications, including sensors, tissue engineering, and human-machine interfaces. The review meticulously discusses the advancements in functional materials, strategies for performance improvement, and wearable applications. Additionally, it systematically summarizes the approaches and challenges related to enhancing mechanical properties, conductivity, and long-term stability of conductive hydrogels. However, the paper may be accepted by Electronic Materials after addressing the following comments:

Thank you for positive comments and constructive suggestions. Accordingly, we have carefully considered each point you raised and revised the manuscript.

 

1. In the section of introduction:
2. a)     The introduction effectively sets the stage for discussing the importance of conductive hydrogels in flexible electronics. To strengthen it further, consider providing more context on the specific challenges or gaps in existing flexible electronics technologies that conductive hydrogels aim to address. Additionally, mentioning some recent key developments or emerging trends in this area would make the introduction more engaging.

Response: Thank you for the constructive suggestions and comments. We appreciate the idea of potentially discussing certain shortcomings or limitations in flexible electronics technology to engage the reader further. As a specific tip, our introduction is emphasizing the significance and potential of materials for soft robotics devices. For example, conductive hydrogels are pivotal in soft robotics, powering electronics worn on the body and soft robots themselves. Their flexibility and biocompatibility distinguish them from conventional circuit-related products, defining the realm of soft electronics. This ensures that readers gain ample insight into these fields.

To enhance the clarity, we added below: “… they also come with certain disadvantages, including limited mechanical strength, limited durability, potential toxicity, and compatibility issues. Hence, key developments and emerging trends in conductive hydrogels include enhanced conductivity, biocompatibility and bioactivity, self-healing capabilities, integration in soft robotics and so on. The purpose of this paper is to review the progress of conductive hydrogels…”

 

1. b)     Also, could you include a brief overview of the historical development of conductive hydrogels in the context of flexible electronics? This would help to provide a foundation for understanding the current advancements in the field. Moreover, it would benefit from a more detailed discussion of the specific challenges or limitations associated with traditional inorganic conductive materials, which conductive hydrogels aim to address.

Response: Our focus is primarily on recent technological advancements and practical applications. This article delves into modern developments and the significance of these materials in applied technologies. Thanks for the suggestions. The advantages and disadvantages were shown in Page 2. And the challenges were shown in the section of conclusion and prospects. Accordingly, we added a brief overview of the historical development of conductive hydrogels: “… able nature [1-4]. For example, the initial research focused on integrating conductive polymers into hydrogel matrices to enhance their electrical conductivity. These properties enable them to conform to irregular …”

 

1. c)     In addition, have you discussed the specific challenges that previous research has encountered in achieving and maintaining conductivity in hydrogels for electronic applications? Detailing these obstacles would highlight the progress made and areas needing further improvement.

Response: We brought up issues including the mechanical strength and durability of conductive hydrogels as well as their long-term stability in biological systems. We discussed certain ways to increase the conductivity and mechanical properties of these materials via nanomaterial incorporation as well as advanced synthetic techniques. And the particular challenges associated with preserving the conductivity within the hydrogels and the strategies to overcome these challenges were discussed carefully, which are shown in the section of conclusion and prospects.

 

2. In the section of Fabrication of conductive hydrogels:
3. a)     This section provides a brief overview of the fabrication methods for conductive hydrogels, but it lacks detail on specific techniques or approaches. Including examples of commonly used fabrication methods, such as in situ polymerization, physical blending, or chemical crosslinking, would provide readers with a more comprehensive understanding of the fabrication process. Additionally, discussing the advantages and limitations of each fabrication method could help readers evaluate the suitability of different approaches for their own research or applications.

Response: We reviewed different fabrication of electrically conductive hydrogels, such as in situ polymerization, physically mixed, and chemical crosslinked. Specific method is explained with examples (e.g., in situ polymerization to form polyaniline-based hydrogels and physical blending to form graphene oxide-based hydrogels). These examples show when, how, and what these methods can achieve (or not), and are particularly relevant because of the usefulness they provide to understanding the diversity. We have also explored their pros/cons for real-life applications so that the readers have an idea to what extent they can apply each of these methods, and some of the challenges that can be encountered.

Thanks for the above constructive suggestion. Accordingly, we added below: “…Many research efforts have been made to design and fabricate various conductive hydro-gels. According to conductive fillers, conductive hydrogels can be classified into two main categories (Figure 2): intrinsically conductive hydrogels and normal hydrogels based on based on conductive fillers. And for the fabrication methods, Chemical crosslinking, physical mixing, and in situ polymerization are often used manufacturing techniques. Although in situ polymerization requires intricate control conditions, it permits homogeneous dispersion of conductive components. While mixing physically is easy, dispersing evenly is difficult. High mechanical strength may be achieved with chemical crosslinking; however, biocompatibility may be compromised by hazardous chemicals. The conductivity mechanisms, advantages, disadvantages, and research progress of each type of conductive hydrogel are introduced carefully. …”

1. b)     To enrich the discussion, consider elaborating on the comparative advantages and limitations of different synthesis techniques, such as in situ polymerization, physical blending, and chemical crosslinking and so on. Additionally, providing insights into the scalability and reproducibility of these methods would be valuable for readers interested in practical applications.

Response: Thanks for the constructive suggestion. The benefits and drawbacks of various synthesis techniques have previously been discussed in the manuscript. For example, we have covered the benefits of physical blending in terms of making the preparation process simpler and the benefits of in situ polymerization in terms of improving conductivity and mechanical strength. We have also investigated the repeatability and scalability of these techniques in real-world applications, such the possibility of employing chemical crosslinking to produce nanocomposite hydrogels on a massive scale. Adding too many details might make the article too long, and we aim to maintain comprehensiveness and practicality in our discussion.

 

1. c)     In the sub section of ‘Ion conductive hydrogels’, the authors mentioned how ionically conductive hydrogels mimic ion transport in biological tissues. What are the key parameters that influence ionic conductivity in these hydrogels? How does the choice of metal salts, acids, or ionic liquids affect the conductivity and other properties of the hydrogels?

Response: The main factors affecting ionic conductivity in ion-conductive hydrogels have previously been thoroughly covered in our paper. For instance, the conductivity and other characteristics of hydrogels are greatly impacted by the selection of metal salts, acids, and ionic liquids. We discussed the use of various metal salts, such calcium and sodium chlorides, and acids, like hydrochloric and phosphoric acids, to increase the ionic conductivity of hydrogels. Furthermore, we have discussed how ionic liquids might improve stability and conductivity. We believe that the current conversation adequately explains these contributing elements and illustrates the benefits and drawbacks of various options in practical applications.

To enhance the clarity, we added below: “…Unlike electronically conductive hydrogels, ionically conductive hydrogels closely mimic ion transport in biological tissues and cells, making them better suited for wearable sensors and simulating human soft tissues. Ion concentration, ionic species composition, and mobility are important factors affecting ionic conductivity in these hydrogels. The conductivity and characteristics of the hydrogels are greatly impacted by the selection of metal salts, acids, or ionic liquids. For instance, metal salts like NaCl may improve ionic conductivity, while acids can change the ionic strength and pH to affect the gel's function and structure. Ionic liquids have the potential to provide excellent conductivity and stability, but they may also alter the hydrogel's mechanical characteristics. The mechanism behind ionic conduction involves the electrical conductivity of ions through free movement.…”

 

1. d)     The text mentions the development of self-healing, stretchable conductive hydrogels. What are the typical self-healing mechanisms employed, and how do they impact the performance of hydrogel?

Response: The creation of stretchy conductive hydrogels with self-healing properties and their underlying processes have previously been covered in length in our paper. For instance, dynamic covalent bonds (such as boronate ester bonds), hydrogen bonds, and metal coordination bonds are examples of common self-healing processes. These processes not only increase hydrogels' mechanical qualities but also their capacity to mend themselves after injury. In the manuscript, we have reported the use of dynamic boronate ester linkages in a hydrogel based on polyvinyl alcohol and boric acid to obtain outstanding self-healing capability. These points have been addressed in the present debate, which also shows how important self-healing mechanisms are to hydrogel performance.

To enhance the clarity, we added below: “…Self-healing of hydrogels can also be achieved through molecular self-assembly. Dynamic covalent bonds, hydrogen bonds, and metal coordination bonds are examples of typical self-healing mechanisms that allow hydrogels to self-heal while retaining their mechanical integrity and conductivity. For example, hydrogels that use dynamic covalent bonds can self-heal repeatedly in ambient conditions, which greatly improves their durability and performance in real-world applications. Wang et al. [111] utilized nucleoside monomer molecules (2-amino-2′-fluoro-2′-deoxyadenosine, 2-FA) and distilled water/phosphate-buffered saline (PBS) as solvents to prepare a high-strength, injectable supramolecular hydrogel by constructing a multi-hydrogen bond system….”

 

3. In the section of key properties and enhancement strategies of conductive hydrogels:
4. a)     While this section outlines the broad categories of conductive hydrogels (electronic conductive hydrogels, nanoparticle conductive hydrogels, ion conductive hydrogels), it could benefit from further elaboration on the specific properties and performance metrics of each type. Providing details on key properties such as electrical conductivity, mechanical strength, swelling behavior, and biocompatibility would help readers understand the unique characteristics of different types of conductive hydrogels and their potential applications. Also, please provide a more comprehensive listing of each research, detailing the materials used, as well as the properties and applications of these materials in each respective research, rather than simply presenting conductivity in the Table 1.

Response: Thanks for the constructive suggestion. The characteristics and performance metrics of different types of conductive hydrogels, including electrical conductivity, mechanical strength, swelling behavior, and biocompatibility, have been comprehensively addressed in our review. Specific examples have been provided to illustrate each type of hydrogel. For instance, we explored the application of nanoparticle conductive hydrogels in flexible sensors and electrically conductive hydrogels in bioelectronic devices. To maintain the article’s conciseness and readability, we do not intend to list detailed data for each study but rather summarize key performance parameters using tables and figures. We believe that the unique qualities and potential applications of these materials have been effectively demonstrated in the current discussion.

 

1. b)     The text references several figures (e.g., Figure 4a&b) that are not detailed descriptions in this section. If possible, ensure that all referenced figures are included and clearly explained within the text which would enhance the reader's understanding

Response: Thanks for the constructive suggestion. Accordingly, we have carefully reviewed all figures to ensure that each one is referenced in the text. For example, Figures 4a and 4b have been introduced in in section 3.3 and specifically on line 511.

 

4. In the section of application demonstration in flexible electronics:
5. a)     The text on neural tissue engineering is informative but could be improved by including more specific data and outcomes from recent studies. Can you provide detailed results from studies on the use of conductive hydrogels in neural repair, including metrics such as the extent of neural regeneration, functional recovery, and long-term biocompatibility?

Response: Thanks for the constructive suggestion. Important research on the application of conductive hydrogels in neural tissue engineering has been previously compiled in our review, highlighting significant results such as long-term biocompatibility, functional recovery, and neural regeneration. We have also discussed the application of hydrogels based on polypyrrole and polyvinyl alcohol, which significantly enhance neuronal development and functional recovery. These investigations demonstrate the promising potential of conductive hydrogels for repairing damaged brain tissue. However, too much specific data may cause the article to focus excessively on particular research analyses. And the referenced paper is listed to help readers find detailed information from the original study.

 

1. b)     The author mentioned that the use of flexible sensors for monitoring physiological signals such as heart rate and blood pressure. To strengthen this discussion, more details on the specific mechanisms and technologies used in these sensors would be useful. How do conductive hydrogels improve the accuracy and reliability of health monitoring devices compared to traditional materials?

Response: Thanks for the constructive suggestion. We went into great depth on the particular processes and technologies used in flexible sensors for tracking physiological data like blood pressure and heart rate. Conductive hydrogels have a high skin adhesion rate, which lowers signal noise and increases detection precision. Moreover, these materials' considerable flexibility makes it possible for them to sustain steady electrical signal conduction in dynamic situations, improving the precision and dependability of health monitoring equipment.

To enhance the clarity, we added below: “…This helps reduce signal noise during monitoring, ensuring both sensitivity and accuracy of flexible sensors. Through the improvement of signal quality, conductive hydrogels increase sensor performance. Their superior skin conformability makes physiological measures more precise. Furthermore, trustworthy long-term monitoring is guaranteed by their consistent ionic conductivity. Conductive hydrogels outperform conventional materials in health monitoring devices because of these characteristics. As a result, they have become powerful candidates for flexible wearable electronic materials….”

 

 

1. c)     The benefits of hydrogel sensors adhering to biological tissues without additional media are highlighted. However, it would be helpful to discuss the specific materials and designs that enhance adhesion and flexibility. What are the key factors that contribute to the strong adhesion and flexibility of conductive hydrogels, and how do these properties impact their performance in wearable devices?

Response: Thanks for the constructive suggestion. The benefits of hydrogel sensors sticking to biological tissues were covered in our study, along with particular materials and designs that improve flexibility and adherence. For instance, polyvinyl alcohol and amino-functionalized nanofibers have shown superior efficacy in augmenting hydrogel adherence. These materials may guarantee the stability and dependability of sensors in operation by offering high adhesion while retaining flexibility. These points have been addressed in the present debate and also shown how these qualities have a big influence on wearable device performance.

To enhance the clarity, we added below: “…and antibacterial conductive material, ionically conductive hydrogels demonstrate enormous potential in flexible sensors and biomedical applications [131-133]. For the conductive hydrogel, strong adhesion and flexibility are largely attributed to the choice of polymer matrix, crosslinking density, and nanomaterial inclusion. For example, the use of highly hydrolyzed polyvinyl alcohol (PVA) improves adhesion, while the addition of carbon nanotubes or graphene may increase mechanical flexibility. Furthermore, network structure-optimized designs like double-network hydrogels significantly improve adhesion and flexibility, which affects how well they function in wearable technology by guaranteeing improved skin conformance and endurance. Their versatile properties make them promising candidates for various fields, offering solutions to diverse challenges in flexible electronics and healthcare.…”

 

5. In the section of conclusion and prospects:
6. a)       The section of conclusion and prospects should include a discussion on future trends and innovations in the field of conductive hydrogels. What new developments or emerging technologies could further enhance the performance and applicability of these materials in flexible electronics? To further enrich this section, it would be beneficial to highlight specific areas within tissue engineering where conductive hydrogels have shown exceptional potential, such as in organ-on-a-chip platforms or regenerative medicine approaches targeting specific tissues. Additionally, discussing ongoing efforts to integrate conductive hydrogels with emerging technologies like 3D bioprinting or organoid culture systems could provide insights into future directions for research and development.

Response: Thanks for the constructive suggestion. Accordingly, we added below: “

• “… thus, offering new avenues and methods for tissue engineering. The integration of conductive hydrogels with 3D bioprinting and organoid culture systems holds promise for future research and development. Emerging technologies like organ-on-a-chip platforms and regenerative medicine targeting specific tissues could benefit greatly from these materials. In the realm of flexible electronics, conductive hydrogels have garnered significant attention due to their excellent conductivity and flexibility….”
• “…and market challenges, necessitating joint efforts from governments, industries, and academia. Additionally, future developments in this field should also be fueled by ongoing efforts to combine conductive hydrogels with cutting-edge technologies like 3D bioprinting and organoid culture systems.”

 

1. b)       The challenges mentioned are critical. However, providing more detailed solutions or research directions to address these challenges would be beneficial. Are there specific strategies or emerging technologies that show promise in improving the mechanical properties, biocompatibility, and biodegradability of conductive hydrogels? To enhance it further, consider proposing specific avenues for future research or technological advancements in the field of conductive hydrogels for flexible electronics. Addressing the remaining challenges and outlining potential breakthroughs would provide valuable guidance for researchers and practitioners in this area.

Response: Thanks for the constructive suggestion. We discussed the approaches and future directions for conductive hydrogel research that are intended to improve the mechanical properties, biocompatibility, and biodegradability of these materials. We also explored the utilization of nanocomposite materials and advanced crosslinking techniques to enhance certain attributes. In addition, we discussed the use of state-of-the-art technologies including smart materials and dynamic covalent bonding to improve hydrogel performance. These discussions provide valuable guidance for future research.

To enhance the clarity, we added below: “…it is believed that conductive hydrogels will bring more surprises and convenience to human health and life. Future discoveries are anticipated to focus on strengthening their mechanical characteristics, biocompatibility, and biodegradability. Specific tactics, such as the utilization of nanocomposites and dynamic covalent bonding, show promise in these areas.”

 

1. c)       Moreover, while emphasizing the potential of conductive hydrogels in flexible electronics, it would be valuable to address current limitations hindering their widespread adoption in commercial products. For instance, discussing challenges related to scalability, manufacturability, and integration with existing electronic components could provide a more comprehensive understanding of the practical considerations involved in utilizing conductive hydrogels in wearable devices and other electronic applications.

Response: Thanks for the constructive suggestion. The promise of conductive hydrogels in flexible electronics has previously been thoroughly covered in our paper, which also outlined the primary obstacles preventing their broad use in consumer goods. As we noted in the review, for example, maintaining uniformity and repeatability of material characteristics is a difficulty in the large-scale manufacture of conductive hydrogels. These materials may be produced with more efficiency and higher-quality products by using innovative crosslinkers and optimizing polymerization conditions in the production process. We also discussed the difficulties of combining conductive hydrogels with currently available electrical components. According to the research, conventional stiff electronic components and these hydrogels' softness and stretchability are mechanically mismatched, which may result in unstable electrical connections and poor surface adherence. To strengthen the interaction between hydrogels and electrical components, we presented a few strategies, such as surface modification methods and the creation of hybrid materials with improved adhesion qualities. We also stated how the mechanical characteristics and stability of hydrogels may be greatly enhanced by the use of nanocomposite materials and dynamic covalent bond technologies, which will increase the hydrogels' dependability and durability in real-world applications. These developments show that the use of conductive hydrogels in flexible electronics has a bright future. The current discussion adequately illustrates the viability and potential solutions for employing conductive hydrogels in real-world applications.

 

To enhance the clarity, we added below: “

• “…it is believed that conductive hydrogels will bring more surprises and convenience to hu-man health and life. Future discoveries are anticipated to focus on strengthening their mechanical characteristics, biocompatibility, and biodegradability. Specific tactics, such as the utilization of nanocomposites and dynamic covalent bonding, show promise in these areas. It is also essential to tackle the issues of scalability and manufacturability in order to facilitate the extensive integration of conductive hydrogels into commercial goods. These challenges may be addressed by enhancing manufacturing techniques and combining with already existing electrical components.

Despite the promising applications, conductive hydrogels still face several challenges in practical use….”

• “… Thirdly, simpler and more efficient methods for the preparation and processing of conductive hydrogels need to be developed to reduce production costs and increase pro-duction efficiency. Future advancements in this sector may be pushed ahead by integrating conductive hydrogels with developing technologies, such as 3D bioprinting and or-ganoid culture methods. Addressing these difficulties is crucial for their large-scale implementation. Moreover, large-scale production and application of conductive hydrogels also face technical and market challenges….”

 

Comments on the Quality of English Language

The overall quality of the English language in the manuscript is satisfactory. The authors have effectively communicated the main points and provided a comprehensive review of conductive hydrogels for flexible electronics. However, there are a few instances where the sentences could be rephrased for better clarity and flow. Simple adjustments to some sentences can enhance the readability of the manuscript.

Response: We appreciate your compliments on our manuscript's linguistic quality. We both agree that several phrases might be made clearer and flow better with the right changes. We have revised the manuscript carefully and highlighted the revised parts in blue.

 

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The author has addressed the comments and made the necessary revisions, significantly improving the quality of the manuscript. It can now be accepted for publication.