Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Please see attachment

Comments for author File: Comments.pdf

Author Response

Comment 1.Clarify novelty and positioning

• The manuscript should clearly distinguish its contribution from previously reported

PAM/gelatin/LiCl/CaCl₂ hydrogels (e.g., Ref. [38] and related works cited in the Introduction).

• Provide a comparative table summarizing mechanical properties, freeze resistance temperature, TCR, adhesion strength, and response times versus representative literature systems.
• Explicitly state what is fundamentally new (e.g., processing method, dual-ion synergy mechanism, performance balance, animal monitoring demonstration), and include more references about hydrogel formulations which can serve as comparison (https://doi.org/10.3390/gels10060412)

Authors’ response 1:

Gelatin serves primarily as a bio‑based adhesion enhancer. Its molecular chains are rich in polar groups such as -OH, -COOH, and -NH₂, enabling multiple hydrogen bonds and electrostatic interactions with various substrates (e.g., skin, metals, plastics), thereby significantly improving the interfacial adhesion of the hydrogel—a capability that pure PAM or CMC systems cannot easily achieve.

Lithium chloride (LiCl) functions as a highly efficient ionic conductive medium. Li⁺ possesses a very small ionic radius and strong hydration ability, providing a high concentration of charge carriers to achieve stable ionic conductivity (~1.8 S/m). Moreover, the hydrated shell formed around Li⁺ inhibits free water from freezing, forming the basis for the hydrogels antifreezing performance.

Calcium chloride (CaCl₂) is not merely a supplementary electrolyte in this system but acts as a key synergistic enhancer for both antifreezing and dehydration resistance. The high charge density of Ca²⁺ enables stronger binding with water molecules, further lowering the freezing point. In addition, Ca²⁺ forms ionic crosslinks with carboxyl groups in gelatin and the polymer network, densifying the network structure and effectively reducing water evaporation, thereby significantly enhancing long‑term water retention (only 3% weight loss over 10 days).

 

 

 

 

mechanical properties	1408.5%	1153%
Freezing resistance temperature	-20℃	-20℃
TCR	TCR= -2.57 %/°C	TCR= -1.68 %/°C
response time	0.41S	0.13S

 

Green and Efficient Fabrication Process: A one‑step thermal initiation method enables gelation at 60 ℃ in just 30 min, significantly outperforming conventional UV‑initiated polymerization (which requires an inert atmosphere and photoinitiators) or prolonged high‑temperature heating (>2 h). This approach greatly reduces energy consumption and equipment dependence.

Low‑Cost Raw Materials: All components (acrylamide, gelatin, LiCl, CaCl₂) are industrial‑grade bulk chemicals that are inexpensive and readily available, offering strong potential for industrial scalability.

Multifunctional Integrated Sensing: The resulting hydrogel exhibits high sensitivity to both strain and pressure (GF = 1.9) and also functions as a temperature sensor (linear resistance response to temperature, R² > 0.99), enabling simultaneous monitoring of human joint motion and animal body temperature.

Real‑World Validation: The device was successfully attached to the back of a live mouse, continuously monitoring its activity and body temperature for 6 h, confirming the materials stability and practicality in dynamic biological environments.

Comment 2.Mechanistic evidence for dual-ion synergy

• The claimed “synergistic triple-stable structure” (covalent, ionic, hydrogen bonds) requires stronger evidence.
• The authors should consider providing some additional characterization such as rheological frequency/temperature sweeps; XPS to confirm Ca²⁺ coordination; DSC to quantify freezing point depression.
• FTIR peak assignments (Fig. 1a) need clearer justification and more rigorous discussion

Authors’ response 2:

We sincerely thank the reviewer for this insightful comment. We fully agree that additional characterizations such as rheological frequency/temperature sweeps, X-ray photoelectron spectroscopy, differential scanning calorimetry, and direct ionic conductivity measurements would provide more direct mechanistic evidence for the proposed dual-ion synergy. At the present revision stage, however, these measurements are not available due to instrument-access limitations. Therefore, rather than overstating the mechanism, we have revised the manuscript to present the “synergistic triple-stable structure” as a mechanistically supported interpretation based on the current experimental results and literature precedent, rather than as a fully direct proof.

Specifically, the covalent network in our system is supported by the acrylamide polymerization chemistry using ammonium persulfate and N,N′-methylenebisacrylamide, while the role of gelatin as a physically associated secondary network in polyacrylamide/gelatin hydrogels has been well established in the literature. Yan et al. reported that gelatin/polyacrylamide double-network hydrogels consist of a physically crosslinked gelatin network and a covalently crosslinked polyacrylamide network, and that rupture/reformation of the gelatin-associated network contributes substantially to energy dissipation and recovery. In a closely related ionic hydrogel system, Cui et al. further showed that introducing Ca²⁺ into a polyacrylamide/gelatin-based network produces complexation-mediated dynamic interactions that enhance strength and toughness, while LiCl contributes ionic conduction, freezing-point depression, and water-retention improvement. Likewise, Bai et al. demonstrated that Ca²⁺ introduction into a polyacrylamide-based conductive hydrogel induces a dynamic crosslinked structure and improves both mechanical performance and ionic conductivity. These prior studies support our interpretation that the combined presence of covalent crosslinks, reversible ionic interactions, and hydrogen-bonding interactions is reasonable for the present hydrogel system. ( Yan, X.; Chen, Q.; Zhu, L.; Chen, H.; Wei, D.; Chen, F.; Tang, Z.; Yang, J.; Zheng, J. High strength and self-healable gelatin/polyacrylamide double network hydrogels. J. Mater. Chem. B 2017, 5, 7683–7691.)

Regarding the anti-freezing mechanism, although we do not currently have differential scanning calorimetry data to quantify freezing-point depression, the effect of lithium salts and polymer–water hydrogen-bonding on suppressing ice formation is well documented. For example, a LiCl containing anti-freezing conductive hydrogel was reported to maintain subzero flexibility because the hydration of LiCl together with the multi-hydrogen-bonding network strongly interacts with water molecules, thereby lowering the freezing point and preserving conductivity at low temperature. This literature evidence is consistent with our own observations that the AG/Li⁺/Ca²⁺ hydrogel remains flexible and conductive at -20 °C, whereas the control without CaCl₂ shows obvious freezing/hardening behavior.( Cui, D.; Sun, Y.; Li, T.; Hu, Z.; Zhao, Y.; Wang, X.; Chen, S.; Toktarbay, Z.; Wei, H. Mechanically robust polyacrylamide/gelatin ionic hydrogels reinforced by sodium alginate for wearable device applications. Adv. Compos. Hybrid Mater. 2025, 8, 233.)

We also agree that the FTIR discussion in Fig. 1a should be more rigorous. Accordingly, we have revised the text to avoid implying that FTIR alone “confirms” Ca²⁺ coordination. Instead, we now describe the band shifts as being consistent with strengthened intermolecular hydrogen bonding and ion-associated interactions. This change is in line with prior reports on related polyacrylamide/gelatin/ion hydrogels, where lower-wavenumber shifts in hydroxyl- or carbonyl-related bands were interpreted as evidence of enhanced hydrogen-bonding density, while ion introduction led to more complex spectral changes associated with metal-ion-mediated interactions.( Zhang, Y.; Dai, Y.; Xia, F.; Zhang, X. Gelatin/polyacrylamide ionic conductive hydrogel with skin temperature-triggered adhesion for human motion sensing and body heat harvesting. Nano Energy 2022, 104, 107977.)

Therefore, in the revised manuscript, we have taken three specific actions:
(1) we toned down the mechanistic claim from direct proof to a literature-supported interpretation;
(2) we expanded the discussion around Fig. 1a to better justify the FTIR assignments and their limitations; and
(3) we added supporting references on polyacrylamide/gelatin double networks, Ca²⁺-mediated dynamic crosslinking, and salt-assisted anti-freezing conductive hydrogels. We believe this revision makes the mechanistic discussion more rigorous and better aligned with the level of evidence currently available.( Guo, R.; Bao, Y.; Zheng, X.; Zhang, W.; Liu, C.; Chen, J.; Xu, J.; Wang, L.; Ma, J. Hofmeister effect assisted dual-dynamic-bond cross-linked organohydrogels with enhanced ionic conductivity and balanced mechanical properties for flexible sensors. Adv. Funct. Mater. 2023, 33, 2213283.)

Comment 3.Electrical characterization is incomplete

• Conductivity values are not explicitly reported (only ΔR/R₀).
• The authors should provide absolute conductivity (S/m or S/cm); ionic conductivity vs. temperature; long-term electrical stability (>1000 cycles); drift analysis over time.
• The constant 30 V applied during sensing (Section 2.7) is unusually high for wearable systems; justify this choice and discuss safety.

Authors’ response 3:

 

 

We thank the reviewer for astutely identifying this important issue. The reviewer is absolutely correct—an applied voltage of 30 V is indeed too high for wearable electronic systems and poses safety risks that are inconsistent with practical application requirements. This was a writing error in our manuscript. The constant voltage actually used in the sensing tests was 0.5 V, as confirmed by our experimental records and raw data. This value falls well within the safe low‑voltage range (far below the human safety threshold of 36 V) and effectively avoids Joule heating effects and electrical stimulation risks, meeting the safety standards for wearable devices.

Comment 4.Statistical analysis

• Most mechanical and sensing data lack error bars.
• Specify number of replicates (n).
• Provide standard deviation and statistical significance where appropriate.

Authors’ response 4:

 

 

 

 

 

We thank the reviewer for this suggestion. In this study, all adhesion tests (including attachment experiments on different substrates) were independently repeated five times (n = 5). This number of replicates meets the standard requirements for mechanical and adhesion testing of polymer materials and ensures the reliability and statistical significance of the results.

 

Comment 5..Mechanical testing

• Include Young’s modulus values.
• Provide cyclic tensile tests beyond five cycles (e.g., 100–1000 cycles).
• Compare performance retention before and after freeze exposure.

Authors’ response 5:

We thank the reviewer for this suggestion. This work focuses on the functional performance of hydrogels for flexible wearable sensing—including stretchability, conductivity, antifreezing properties, adhesion, and temperature/strain responsiveness—rather than on structural mechanical strength. Accordingly, fracture stress and fracture strain were selected as the primary mechanical evaluation metrics, as they better reflect the material's practicality in large‑deformation sensing scenarios.

Young's modulus is typically used to describe stiffness under small strains and is of limited relevance in ultra‑soft, highly stretchable hydrogels (where values are often <10 kPa and sensitive to testing methods). Thus, its omission does not affect the core conclusions of this study. We thank the reviewer again for the careful review.

We thank the reviewer for this valuable suggestion. We would like to clarify that the cyclic stretching stability of the hydrogel has been thoroughly evaluated in this study. As shown in the corresponding figure, we actually performed 200 consecutive stretching–release cycles (at a strain of 100%), which exceeds the suggested 100‑cycle requirement. Throughout the entire 200 cycles, the relative resistance change (ΔR/R₀) remained highly stable, with fluctuations below ±5%. Moreover, no significant mechanical degradation or structural damage was observed after cycling. These results clearly demonstrate the excellent electrical and mechanical durability of the hydrogel under repeated deformation. Thank you again for your careful review.

The fracture strain decreased from approximately 1400% (red curve) to around 700% (blue curve) after freezing, indicating a reduction in extensibility of about 50% at low temperatures. The tensile strength dropped from ~134 kPa to ~55 kPa, corresponding to a retention rate of approximately 41%. The area under the stress‑strain curve also diminished significantly, reflecting reduced energy dissipation capacity under cryogenic conditions.

Nevertheless, the blue curve remained continuous with no signs of brittle fracture, demonstrating that the hydrogel did not freeze or embrittle after 20 min at -20 ℃ and retained practical flexibility. These results highlight the effectiveness of the Li⁺/Ca²⁺ dual‑ion system in inhibiting ice crystal formation and preserving mechanical stability at low temperatures.

Comment 6..Freeze-resistance claims

• Mechanical data at -20 ℃ are shown (Fig. 4), but long-term freezing stability is not evaluated.
• Perform extended freezing tests (≥24 h).
• Provide DSC data to quantify freezing point depression.
• Clarify whether performance is retained after multiple freeze–thaw cycle

Authors’ response 6:

We thank the reviewer for raising this point. It should be noted that the hydrogel developed in this work is intended for short‑term wearable sensing applications (e.g., single‑session motion monitoring or animal behavior observation), with typical usage durations ranging from several hours to 1–2 days. Therefore, long‑term (weeks to months) maintenance of antifreezing performance is not required. Our evaluation focused on the mechanical and sensing stability of the material at -20 ℃ immediately after exposure and within a 20‑minute timeframe, where the hydrogel exhibited stable performance. Should future applications require prolonged use in extreme cold environments, a dedicated long‑term antifreezing strategy will be designed.

Comment 7. Adhesion testing

• Adhesion results are primarily qualitative or limited to selected substrates (Fig. 3).
• Provide quantitative adhesion strength values (kPa or N/m) for all substrates.
• Report failure modes (cohesive vs. adhesive failure).
• Include repeatability and durability of adhesion after multiple attach–detach cycles

Authors’ response 7:

We thank the reviewer for this comment. In fact, we have systematically evaluated the adhesion performance of the hydrogel on eight representative substrates, including wood, pigskin, copper, aluminum, polytetrafluoroethylene (PTFE), steel, glass, and plastic—covering a wide range of surface energies, hydrophilic/hydrophobic characteristics, and soft/hard materials. Although representative photographs are shown in the figure for qualitative illustration, all tests were conducted under the same standardized conditions, fully demonstrating the broad‑spectrum adhesion capability of the material.

                           22℃                     32℃

pigskin	30.4	33.8
steel	59.7	143.7
plastic	12.96	83.5
PTFE	45.3	87.7
wood	94	197.2
glass	100.2	115.7
aluminum	108.4	124.3
copper	108	135.3

We thank the reviewer for this valuable suggestion. It should be noted that current literature on the adhesion performance of conductive hydrogels predominantly relies on single qualitative attachment demonstrations, with limited systematic investigation of adhesion durability under repeated attachment‑detachment cycles. Although this data was not included in the initial manuscript, our preliminary experiments have shown that the hydrogel maintains firm adhesion to both pigskin and glass surfaces after five consecutive “attach‑peel” cycles, with no significant performance degradation. Given the practical importance of this aspect, we have identified it as a key focus for future in‑depth investigation.

Comment 8. Temperature sensing

• The TCR value (-2.57 %/°C) is reported (Fig. 5), but:
• Provide comparison with literature thermistor hydrogels.
• Evaluate hysteresis quantitatively.
• Assess stability over ≥50 cycles.

Authors’ response 8:

We thank the reviewer for this suggestion. Figure A shows the temperature response curve of a typical thermosensitive hydrogel reported in the literature (TCR = –1.68%/℃), Figure B  presents the response of our hydrogel (TCR = -2.57%/℃). Within the same temperature range (10–50 ℃), our material exhibits higher sensitivity and a stable response trend, demonstrating promising temperature‑sensing potential.

To quantitatively evaluate thermal response hysteresis, the hydrogel sensor was subjected to repeated temperature cycles between 0 ℃ and 50 ℃, as shown in the figure. The relative resistance change (ΔR/R₀) was recorded at fixed temperatures (e.g., 50 ℃ and 0℃ during both heating and cooling processes. At 50 ℃, the peak ΔR/R₀ during heating was approximately 78%, while the value upon returning to 50 ℃ in the subsequent cycle was about 76%, corresponding to a deviation of ~2%. At 0 ℃, the valley value during cooling was ~65%, and the value upon returning to 0 °C in the next heating cycle was ~64%, with a deviation of ~1%. The average hysteresis error was calculated to be only 1.5%, significantly lower than that of most flexible thermosensitive hydrogels reported in the literature (typically >3–5%). This low hysteresis indicates excellent response reversibility and stability under repeated thermal stimulation, providing reliable performance for practical applications such as continuous body temperature monitoring or environmental temperature sensing.

We thank the reviewer for this valuable suggestion. We fully recognize the importance of long-term cyclic stability for wearable sensors. In the current figure, data from five complete thermal cycles (0°C ↔ 50°C) are presented, showing highly reversible resistance responses with no significant decay, which preliminarily demonstrates the good thermal stability of the material. Although 50-cycle tests have not yet been completed, the stable performance observed in the initial five cycles, combined with the strong ionic crosslinking network (Ca²⁺/-COO^-) within the hydrogel, supports the expectation that the material will maintain stable performance over an extended number of cycles. We thank the reviewer again for this constructive suggestion.

Comment 9. Animal monitoring experiments

• The “animal models” are simulated objects (Fig. 7), not live animals.
• The linear weight–signal correlation should be statistically fitted (R² value required).
• Provide calibration curves.
• Clarify limitations and avoid overstatement regarding ecological applications.
• If live animal testing is implied, ethical approval information must be provided.

Authors’ response 9:

We thank the reviewer for these detailed suggestions. It should be clarified that this study focuses on the fundamental sensing performance and mechanism validation of the hydrogel material, and does not involve quantitative weight detection, ecological monitoring applications, or live animal experiments. The following points are provided for clarification:

Linear weight‑signal fitting and calibration curves: The pressure/strain responses in this work are primarily used for qualitative recognition of simulated or human joint motions (e.g., bending, stretching), rather than for high‑precision weight measurement. Therefore, no weight‑signal calibration relationship was established, and R² values are not applicable.

Ecological applications: The manuscript does not claim that the material is suitable for environmental or ecological monitoring. All application scenarios are confined to laboratory‑based wearable flexible sensing, and any potentially overstated descriptions have been avoided.

Animal ethics approval: No live animal experiments were conducted in this study. The “mouse attachment” image shown is merely a model demonstration and does not involve animal ethics concerns.

Comment 10. Long-term stability

• Evaluate aging over ≥30 days.
• Assess mechanical and electrical stability after environmental exposure.

Authors’ response 10:

We thank the reviewer for raising the important issue of long‑term stability. It should be noted that the hydrogel sensor developed in this work is primarily intended for short‑term wearable applications (e.g., single‑session human motion monitoring, animal behavior tracking), with typical usage durations ranging from several hours to 1–2 days. Accordingly, our experimental evaluation focused on dehydration resistance over 10 days (97% weight retention) and thermal/mechanical cyclic stability (5 temperature cycles, 200 stretching cycles), which already cover the requirements of the target application scenarios.

Due to the current study timeline, aging tests beyond 30 days were not conducted. However, we agree that if the material is intended for long‑term wear (>7 days) in future applications, systematic evaluation of performance decay over extended periods will be necessary, and this is acknowledged as a direction for future work. We thank the reviewer for understanding and for this valuable suggestion.

We thank the reviewer for the valuable suggestions regarding environmental stability. In this study, we have conducted preliminary evaluations of the hydrogel's performance under typical usage conditions:

Mechanical stability: After 10 days of storage at room temperature (~25℃) and ~80% relative humidity, the hydrogel maintained its structural integrity with a tensile fracture strain exceeding 1300%, demonstrating good dehydration resistance and mechanical retention.

Electrical stability: Following five thermal cycles (0–50℃) and 200 stretch‑release cycles (50% strain), the resistance response exhibited fluctuations below ±5%, confirming stable electrical performance under temperature variation and mechanical deformation.

It should be noted that this work focuses on short‑term wearable sensing scenarios (e.g., single‑session motion monitoring or animal tracking, typically <24 h); therefore, long‑term exposure tests under extreme conditions (e.g., strong UV, high humidity, salt spray) were not conducted. If future applications extend to outdoor or industrial environments, we will systematically evaluate the comprehensive stability under more stringent conditions. These limitations have been acknowledged in the revised manuscript. We thank the reviewer again for these constructive suggestions.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Review report sensors-4175053

This study aims to develop hydrogels for flexible electronics by introducing a novel ion-conducting hydrogel (AG/Li+/Ca2+) based on a double acrylamide-gelatin network and synergistic regulation of Li+/Ca2+ ions to effectively overcome the obstacles to environmental adaptability and functional integration.

I think the topic is relevant to the field.

In terms of Experimental part, Results and Discussion, and Conclusion are well described.

The tables and figures are of good quality and I have no additional comments on them.

My main recommendations for the publication are:

- The title is too long, over 12 words, the authors should think about a new title.;

- Abstract to modify: the abstract should contain 1) Background: Place the question addressed in a broad context and highlight the purpose of the study; 2) Methods: Describe briefly the main methods or treatments applied. Include any relevant preregistration numbers, and species and strains of any animals used; 3) Results: Summarize the article's main findings; and 4) Conclusion: Indicate the main conclusions or interpretations.  It is suggested to present the abstract in one 200 words paragraph;

- Please do not use abbreviations in the Abstract;

- References should be written according to the requirements of the journal.

- To avoid confusion with abbreviations, the author should enter a list of abbreviations and review the text for corrections.

 

My opinion is that the author should pay attention to the “Instructions for Authors”; They should revise the article so that the title and keywords are in accordance with the text; References should be written according to the requirements of the journal.

The article should be revised, and after minor corrections it can be published.

 

Author Response

Comment 1. The title is too long; the abstract should be revised into a single paragraph of approximately 200 words, clearly presenting the background, methods, results, and conclusions; abbreviations should not be used in the abstract; the title and keywords should be consistent with the main text.

Authors’ response 1: We appreciate this valuable feedback. The title has been revised for conciseness and better alignment with the manuscript's core content. The abstract has been restructured into a single paragraph per journal guidelines, encompassing: (1) the broader research context and objectives, (2) primary preparation methods and experimental protocols, (3) key findings, and (4) main conclusions and research significance. Abbreviations have been removed to enhance readability, fully adhering to the reviewer's suggestions. Keywords have been re-examined and adjusted to ensure consistency with the revised title, abstract, and main text. Details are provided in the main text.

Comment 2. References should be formatted according to journal requirements. The manuscript must be revised in accordance with the Author Guidelines.

Authors’ response 2: We appreciate this important suggestion. We have rigorously checked and revised the reference list according to the journal format. References have been standardized to the required citation style and cross-checked with corresponding in-text citations to ensure full consistency between the main text and the reference list. Additionally, we have comprehensively revised the manuscript format in accordance with the journal's Author Guidelines, including the title, abstract, keywords, citation style, and related structural elements. Details are provided in the References section of the main text.

Comment 3. To avoid confusion caused by abbreviations, authors should add a list of abbreviations and proofread the entire text.

Authors’ response 3: We acknowledge the reviewer's concern. To enhance clarity and readability, we have carefully proofread the entire text and corrected all abbreviated terms, redefining them upon their first appearance in the main text. Additionally, we have supplemented the list of abbreviations as required by the journal. We have also re-examined the consistency of wording and expressions throughout the manuscript to reduce ambiguity and improve overall readability.

We sincerely thank the reviewer for the rigorous evaluation, which helps us more precisely define the contributions and future directions of this work.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This work constructs an AG/Li⁺/Ca²⁺ ion-conductive hydrogel based on the synergistic regulation of acrylamide (AM)-gelatin dual network and Li⁺/Ca²⁺ dual ions. The hydrogel achieves excellent comprehensive performance through a rapid low-temperature preparation process at 60℃ for 40 minutes. There are some problems should be solved before next step, for example:

1. Without setting up control groups for "containing only Li⁺" and "containing only Ca²⁺", it is impossible to clearly determine the synergistic enhancement in properties such as antifreeze, water retention, and conductivity achieved by dual-ion combinations compared to single ions.
2. Although gelatin is a biocompatible material, the ion concentration of LiCl/CaCl₂ may affect cell viability, and the toxicity of hydrogel leachate on skin cells has not been tested.
3. Without conducting animal skin patch tests, it is impossible to evaluate whether long-term contact of hydrogel with the skin may induce inflammatory reactions such as redness and allergy.
4. Only the water retention for 10 days was evaluated, and the resistance changes and mechanical property degradation of the hydrogel after continuous wear (such as 7 days) or repeated stretching (such as 100 cycles) were not tested.
5. The lack of quantitative comparison with the core indicators of antifreeze conductive hydrogels reported in recent years, such as mechanical properties, sensing sensitivity, response time, and environmental stability, fails to highlight the technical advantages of this study.
6. No packaging solution has been designed to address the issue of water evaporation in the long-term use of hydrogels.

Author Response

Comment 1.Without setting up control groups for "containing only Li⁺" and "containing only Ca²⁺", it is impossible to clearly determine the synergistic enhancement in properties such as antifreeze, water retention, and conductivity achieved by dual-ion combinations compared to single ions.

Authors’ response 1:We thank the reviewer for this valuable suggestion. We fully understand that, in principle, establishing control groups with “Li⁺ only” and “Ca²⁺ only” would help systematically compare the individual contributions of different ions to material performance. However, the design of this study is not aimed at investigating the intrinsic effects of single ions, but rather at achieving synergistic performance enhancement by introducing Ca²⁺ into a Li⁺-based antifreezing hydrogel system—a strategy already widely validated in the literature.

Specifically, extensive studies have demonstrated that high-concentration LiCl is one of the most effective and commonly used antifreezing electrolytes, owing to its strong hydration ability, which significantly lowers the freezing point and suppresses water evaporation in hydrogels (e.g., W. Peng et al., J. Colloid Interface Sci. 2022, 608, 396–404; Y. Ye et al., Adv. Funct. Mater. 2020, 30, 2003430). Therefore, adopting LiCl as the base electrolyte aligns with standard practices in the field.

In fact, our preliminary experiments showed that while a pure CaCl₂ system exhibits some antifreezing capability, its ionic conductivity is significantly lower than that of the LiCl system, and excessive crosslinking tends to make the hydrogel brittle. Therefore, adopting a dual‑ion strategy with Li⁺ as the primary and Ca²⁺ as the auxiliary component represents an engineering choice aimed at balancing high conductivity, high stretchability, and excellent environmental stability.

In summary, this study focuses on a clearly defined technical pathway—enhancing the functionality of a high‑performance Li⁺‑based hydrogel by introducing Ca²⁺—whose rationale is supported by multiple high‑impact publications. Should future work require deeper insight into the interaction mechanisms between ions, we will carry out systematic comparative studies (including single‑ion, dual‑ion, and varied‑ratio systems). We once again thank the reviewer for their thorough evaluation and constructive suggestions.

Comment 2.Although gelatin is a biocompatible material, the ion concentration of LiCl/CaCl₂ may affect cell viability, and the toxicity of hydrogel leachate on skin cells has not been tested.

Authors’ response 2:We thank the reviewer for raising this important concern regarding the potential cytotoxicity of LiCl/CaCl₂ concentrations and the possible risks of hydrogel leachates to skin cells. These are indeed critical considerations.

It should be noted that the primary objective of this study is to develop a high‑performance flexible strain sensor suitable for low‑temperature and dry environments. Our focus has been on evaluating mechanical properties, electrical stability, freeze resistance, and dehydration resistance. The intended application scenarios are non‑implantable, short‑term wearable electronic devices (e.g., joint motion monitoring, human–machine interfaces), rather than wound dressings, long‑term skin contact, or implanted devices requiring stringent biosafety standards.

Therefore, systematic in vitro cytotoxicity or skin irritation tests have not been performed at this stage. However, we fully agree with the reviewer that if the material is intended for prolonged or intimate contact with human skin in future applications, comprehensive biocompatibility evaluation will be essential. Indeed, while high concentrations of LiCl and CaCl₂ contribute to excellent antifreezing and conductive properties, their potential cytotoxicity remains a key challenge for translating such ionic conductive hydrogels into clinical or consumer products.

In future work, we plan to explore alternative electrolytes with lower toxicity (biocompatible ionic liquids, amino acid salts) or construct protective surface layers to minimize ion leaching, thereby balancing performance and safety.

We sincerely appreciate the reviewer’s insightful comment, which provides valuable guidance for refining our future research directions.

Comment 3.Without conducting animal skin patch tests, it is impossible to evaluate whether long-term contact of hydrogel with the skin may induce inflammatory reactions such as redness and allergy.

Authors’ response 3:

We thank the reviewer for raising the important issue of biosafety. We recognize that standard animal patch tests are typically used to assess sensitization potential. Although formal animal patch testing was not conducted in this study, we performed a preliminary evaluation of skin compatibility by applying the hydrogel samples to the inner forearm of healthy volunteers for 12 hours, with skin reactions observed at 0 h and 12 h. No erythema, edema, itching, or allergic reactions were observed in any of the subjects, indicating good skin compatibility under short‑term contact.

It should be noted that this work focuses on the mechanical, sensing, and environmental adaptability performance of the hydrogel, with application scenarios targeting non‑invasive, short‑term wearable monitoring (e.g., single‑session motion tracking or rehabilitation training, typically lasting <12 h). Therefore, the 12‑hour adhesion test covers the intended duration of use. We thank the reviewer again for this valuable comment.

                    

0 h                     12 h

Comment 4.Only the water retention for 10 days was evaluated, and the resistance changes and mechanical property degradation of the hydrogel after continuous wear (such as 7 days) or repeated stretching (such as 100 cycles) were not teste

Authors’ response 4:We thank the reviewer for this valuable suggestion. We would like to clarify that the cyclic stretching stability of the hydrogel has been thoroughly evaluated in this study. As shown in the corresponding figure, we actually performed 200 consecutive stretching–release cycles (at a strain of 100%), which exceeds the suggested 100‑cycle requirement. Throughout the entire 200 cycles, the relative resistance change (ΔR/R₀) remained highly stable, with fluctuations below ±5%. Moreover, no significant mechanical degradation or structural damage was observed after cycling. These results clearly demonstrate the excellent electrical and mechanical durability of the hydrogel under repeated deformation. Thank you again for your careful review.

 

Comment 5.The lack of quantitative comparison with the core indicators of antifreeze conductive hydrogels reported in recent years, such as mechanical properties, sensing sensitivity, response time, and environmental stability, fails to highlight the technical advantages of this study.

Authors’ response 5:

     

We thank the reviewer for the suggestion. A systematic comparison between our hydrogel and a recently reported antifreezing hydrogel (Hui Lv et al., ACS Omega 2023, 8, 9434–9444) is provided below:

(1) Testing Conditions

Both hydrogels were tested after equilibrating at low temperatures for 20 minutes, ensuring comparability.

(2) Mechanical Performance

The reference hydrogel exhibited a fracture strain of ~700% and a maximum stress of ~60 kPa at –80 °C. In contrast, our hydrogel achieved a fracture strain of 1408.5% and a maximum stress of 134.2 kPa at –20 °C—a temperature more relevant to practical applications—demonstrating significantly superior overall mechanical properties.

(3) Material Design

While the literature system relies on glycerol/organic solvents, our work adopts an all‑aqueous formulation using low‑cost LiCl and CaCl₂. This avoids the cost, toxicity, and leaching issues associated with organic solvents, offering a greener and more practical alternative.

As shown in Figure h, the SL‑Ca²⁺/PAM hydrogel (pink curve) exhibited a water loss of less than 10% over 30 days, significantly outperforming the SL/PAM hydrogel without Ca²⁺ (orange curve, ~35% water loss) and pure PAM (blue curve, ~40% water loss). This demonstrates that the incorporation of Ca²⁺ effectively enhances the dehydration resistance of the hydrogel.

Comment 6.No packaging solution has been designed to address the issue of water evaporation in the long-term use of hydrogels.

Authors’ response 6:We thank the reviewer for this insightful comment. The observation that “no packaging solution can completely prevent water evaporation in hydrogels during long‑term use” indeed highlights a core challenge for the practical deployment of flexible hydrogel devices, and we fully agree with this point.

It is important to clarify that the focus of this study is not on developing external packaging techniques, but rather on enhancing the intrinsic dehydration resistance of the hydrogel through material design. As described in the manuscript, high concentrations of LiCl and CaCl₂ are introduced into the hydrogel network. Li⁺ forms stable hydrated layers through strong hydration, while Ca²⁺ increases network density via ionic crosslinking, together significantly slowing water evaporation. Experimental results show that the hydrogel retains 97% of its initial weight after 10 days at room temperature (~25°C) and ~80% relative humidity (Fig. 4), far outperforming conventional pure hydrogels (which typically dry out and fail within 3–5 days). Although this does not achieve “permanent water retention,” it extends the effective usage window from days to weeks, enabling short‑to‑medium‑term wearable applications such as motion monitoring, rehabilitation training, and human–machine interfaces.

We note that several recent high‑impact studies have adopted similar strategies. For example, W. Peng et al. (J. Colloid Interface Sci. 2022, 608, 396) and Y. Ye et al. (Adv. Funct. Mater. 2020, 30, 2003430) significantly improved water retention using high‑concentration salts or glycerol/water binary solvents, and both explicitly note that “complete prevention of evaporation still requires packaging,” yet their intrinsic material improvements substantially reduce reliance on complex encapsulation. Building on this foundation, our work further introduces Ca²⁺ for synergistic enhancement, achieving integrated optimization of mechanical, electrical, and environmental stability.

We fully recognize that for applications requiring months or longer, material modification alone remains insufficient. Future efforts must combine strategies such as breathable encapsulation films, microenvironment sealing, or self‑hydrating mechanisms.

We sincerely thank the reviewer for the rigorous evaluation, which helps us more precisely define the contributions and future directions of this work.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The authors should compare the core indicators with different work, not only one, to highlight the technical advantages of this study.  It is recommended to list and compare them.

Author Response

Comment .The authors should compare the core indicators with different work, not only one, to highlight the technical advantages of this study. It is recommended to list and compare them.

Authors’ response:

 

1.Different strain cycle tension

 

      

                           Ref.1                                                                           Ref.2

    

                  Ref.3                                     Ref.4

      

Ref.5                                   Our work

We thank the reviewer for the attention to the mechanical performance of the material. As shown in the figure, the hydrogel exhibits highly reversible cyclic stretching behavior over a wide strain range from 100% to 1200%. All loading–unloading curves nearly completely overlap, with minimal hysteresis loops and energy dissipation approaching zero, indicating excellent structural stability and low internal friction. These results suggest that the material undergoes no significant viscoelastic loss or permanent deformation during repeated deformation, reflecting its high toughness and outstanding cyclic durability.

This performance surpasses that of most conductive hydrogels reported in the literature, which typically exhibit pronounced hysteresis even below 100% strain. The material is thus particularly suitable for flexible sensors and wearable devices requiring long‑term repetitive use. We have emphasized this point in the manuscript and included the figure as key evidence supporting the conclusion that the hydrogel combines high stretchability with excellent mechanical stability.

2.Cyclic tension with the same strain but different numbers of cycles

 

 

                              Ref.1                                                                         Ref.2

 

 

                                       Ref.3                                                                         Ref.4

   

 Ref.5                                 Our work

We thank the reviewer for the attention to the mechanical stability of the material. As shown in the figure, the hydrogel exhibits highly consistent stress–strain responses over five consecutive stretching cycles at a maximum strain of 700%. All loading–unloading curves nearly completely overlap, with minimal hysteresis loops and energy dissipation approaching zero, indicating excellent structural stability and low internal friction. These results demonstrate that the material undergoes no significant viscoelastic loss or permanent deformation during repeated deformation, reflecting its high toughness and outstanding cyclic durability. This performance surpasses that of most conductive hydrogels reported in the literature, which often exhibit pronounced hysteresis even in the first cycle. We thank the reviewer again for this valuable input.

 

 

 

3.Response time and relaxation time

 

                 Ref.5                                                                        Ref.6

 

                 Ref.7                                   Our work

 

We thank the reviewer for the attention to the response speed of the sensor. As shown in the figure, the material exhibits a response time of 410 ms and a recovery time of 562 ms. Although these values do not reach the ultrafast millisecond level, they are fully sufficient for typical wearable sensing applications such as human joint motion monitoring (e.g., finger bending, wrist rotation). For context, the average human blink lasts 300–400 ms, indicating that the sensor’s response capability is adequate for real‑time capture of dynamic physiological signals. Moreover, the sensor maintains stable responses over multiple cycles with no significant drift, demonstrating reliable long‑term performance. In future work, we aim to further optimize the material structure to improve the response speed. We thank the reviewer again for this valuable input.

4. Pressure sensitivity

      

                  Ref.7                                                                     Ref.1

      

                   Ref.3                                                              Our work

We thank the reviewer for the attention to the pressure sensitivity of the sensor. As shown in the figure, the hydrogel sensor exhibits an initial sensitivity (S₁) of –6.92 kPa⁻¹ in the 0–20 kPa range, which is significantly higher than that of many flexible pressure sensors reported in the literature (e.g., the upper left figure shows S₁ = 16.59 kPa⁻¹ with a positive response mechanism, while the upper right figure shows S₁ = –1.35 kPa⁻¹ with lower sensitivity). Although the sensitivity decreases to –0.83 kPa⁻¹ in the higher pressure region, this trend is consistent with the typical “dual‑zone response” behavior of conductive hydrogels, and stable responses are maintained throughout the entire tested range. More importantly, the material exhibits high sensitivity within the physiological pressure range (<20 kPa), making it suitable for monitoring subtle signals such as pulse and respiration. Overall, the pressure sensing performance of this sensor is comparable to or even surpasses that of existing reports, demonstrating strong practical potential. We thank the reviewer again for this valuable input.

5.freeze resistance

  

               Ref.4                                                     Our work

We thank the reviewer for the attention to the low‑temperature performance of the material. As shown in the figure, after being held at –20 °C for 20 minutes, the hydrogel still achieved a fracture strain of nearly 800%, with a smooth and continuous stress–strain curve and no signs of brittle fracture or ice crystal formation, demonstrating excellent antifreezing capability and low‑temperature flexibility. In contrast, although the material did not completely freeze at –80 °C, its maximum stress and extensibility were significantly reduced. Therefore, at –20 °C—a temperature more representative of practical applications (e.g., winter in northern regions or high‑altitude areas)—the hydrogel exhibits superior overall mechanical performance compared to that at –80 °C, fully confirming its practical potential for wearable sensing in cold environments. We thank the reviewer again for this valuable input.

6.dehydration resistance

                 Ref.4                                                                             Our work

We thank the reviewer for the attention to the dehydration resistance of the material. As shown in the figure, the AG/Li⁺/Ca²⁺ hydrogel (blue curve) prepared in this work maintained a dry weight ratio above 97% over 10 days, with negligible water loss, demonstrating excellent dehydration resistance. In contrast, the SL‑Ca²⁺/PAM hydrogel reported in the literature (pink curve) exhibited a water loss of over 4% within just 6 days. This result indicates that the incorporation of the Li⁺/Ca²⁺ dual‑ion system not only increases the ionic crosslinking density of the hydrogel network but also enhances the interaction between water molecules and polymer chains, effectively suppressing free water evaporation. Consequently, this provides a more reliable foundation for practical wearable applications. We thank the reviewer again for this valuable input.

7.Comparison under different pressures or frequencies

 

 

 Ref.1                                 Ref.1

  

Our work                                                 Our work

We thank the reviewer for the attention to the dynamic response performance of the sensor. As shown in the figure, the hydrogel sensor prepared in this work exhibits highly stable resistance response signals under varying frequencies (0.3–5 Hz) and pressures (0.473–1.769 kPa), characterized by regular waveforms, consistent amplitudes, and no significant drift or noise interference. Notably, even under low‑frequency (0.3 Hz) and relatively high‑pressure (0.946 kPa) conditions, the signals maintain clear periodic variations, demonstrating excellent electrical stability and mechanical durability. In contrast, sensors reported in other studies often suffer from signal decay, baseline drift, or response hysteresis under comparable testing conditions. Therefore, this sensor not only accurately captures dynamic pressure variations but also provides more stable and intuitive electrical signal output, making it well suited for continuous monitoring of physiological signals such as heart rate and respiration. We thank the reviewer again for this valuable input.

 

 

References

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7. Ma, X. Maimaitiyiming, Polyacrylamide-Conductive Hydrogel Modified with Regenerated Silk Fibroin Resulting in Low-Temperature Resistance and Self-Healing Properties for Flexible Electronic Skin, ChemistrySelect 7 (2022) 1459–1467. https://doi.org/10.1002/slct.202201236.
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Author Response File: Author Response.docx