Resistive Sensing in Soft Robotic Grippers: A Comprehensive Review of Strain, Tactile, and Ionic Sensors

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper reviews resistive sensing technologies in soft robotic grippers, focusing on strain, tactile, and ionic sensors. It systematically compares their working principles, materials, fabrication methods, and performance metrics. The authors highlight the trade-offs among sensitivity, linearity, durability, and scalability, emphasizing the emerging potential of ionic sensors for multimodal sensing. Key contributions include identifying current challenges such as hysteresis, long-term stability, and signal processing, and proposing future directions like hybrid material systems, additive manufacturing, and AI-enhanced signal interpretation to enable reliable, real-world applications in healthcare, food handling, and human–robot collaboration. This paper is well-structured and well-written. Before publication, there are some questions to be solved.

1. The authors mentioned “Soft robots, as opposed to conventional rigid robots, are able to continually deform, adapt to uneven surfaces, and work securely around people or delicate goods.”, more state-of-the-art can be cited: DOI: 10.34133/cbsystems.0103; DOI: 10.34133/cbsystems.0105.
2. Regarding the comparison between resistive, capacitive, optical, and piezoelectric, what are quantitative benchmarking protocols.
3. The gauge-factor, hysteresis, and drift value are important for strain-sensors, if references reported the exact value, it would better present these in table 2.
4. For sensor fabrication, what can be controlled, e.g., elastomer curing times, temperatures, and humidity levels.
5. How about the ionic-sensor dehydration and leakage quantified under prolonged ambient.
6. Details for the experimental protocol used to cycle-test sensors is required to provide.
7. How about the biocompatibility tests for hydrogel and ionogel materials when intended for food or healthcare contact.
8. For the fingertip ionic sensor, what’s the minimum detectable slip velocity and force resolution.

Author Response

Comment 1: 

The authors mentioned “Soft robots, as opposed to conventional rigid robots, are able to continually deform, adapt to uneven surfaces, and work securely around people or delicate goods.”, more state-of-the-art can be cited: DOI: 10.34133/cbsystems.0103; DOI: 10.34133/cbsystems.0105.

Response 1:

Thank you for providing the papers. We added the requested state-of-the-art citations to Section 1.2 and updated the reference list (DOI:10.34133/cbsystems.0103; DOI:10.34133/cbsystems.0105).

Comment 2: 

Regarding the comparison between resistive, capacitive, optical, and piezoelectric, what are quantitative benchmarking protocols.

Response 2:

We agree that benchmarking protocols are critical. However, as this paper is a review, we have summarized and discussed comparative performance indicators already reported in the literature (e.g., gauge factor, response time, hysteresis, and drift) rather than conducting new tests. Quantitative benchmarking across different transduction mechanisms currently lacks universal standards, a challenge highlighted in Section 1.8. We will clarify this by explicitly noting that systematic quantitative benchmarking (using standardized deformation profiles, calibration loads, and frequency ranges) remains an open research need in the field.

 

Comment 3: 

The gauge-factor, hysteresis, and drift value are important for strain-sensors, if references reported the exact value, it would better present these in table 2.

Response 3: 

Thank you for the observation. Several of these values are already qualitatively compared in Section 3.4. We will revise Table 2 to include explicit numerical values for gauge factor, hysteresis, and drift where they are available from the cited references, ensuring that all added data are directly extracted from existing publications rather than new measurements.

 

Comment 4: 

For sensor fabrication, what can be controlled, e.g., elastomer curing times, temperatures, and humidity levels.

Response 4: 

We appreciate this comment. Section 1.7 already discusses curing times and fabrication conditions for representative elastomers and hydrogels. To improve clarity, we will briefly emphasize that environmental factors such as curing temperature, humidity, and polymerization time are typically controlled within reported ranges (e.g., Ecoflex™ curing at room temperature for 3–4 h). These parameters will be summarized without expanding the section beyond its current scope.

 

Comment 5: 

How about the ionic-sensor dehydration and leakage quantified under prolonged ambient.

Response 5:

This is an important point that we already mentioned in Section 1.8 as a key durability challenge. Quantitative studies in the literature report mass-loss rates or conductivity decay over time; however, there is no standardized protocol for these tests. We will include a short clarifying statement noting that while dehydration and leakage are often qualitatively reported (e.g., conductivity drop after several days in ambient air), systematic quantification remains limited and is identified as a future research need.

 

Comment 6:

 Details for the experimental protocol used to cycle-test sensors is required to provide

Response 6:

Since this paper is a review, no original experimental protocols were conducted. We will clarify this explicitly in the text and summarize typical cycling test conditions reported in prior works (e.g., repeated bending up to 1,000–10,000 cycles at 10–30% strain). This will ensure the reader understands that all cycling data are literature-based, not from new experiments.

 

Comment 7:

 Details for the experimental protocol used to cycle-test sensors is required to provide

Response 7:

An excellent point; we will include a short addition in the discussion of ionic and hydrogel materials (Section 5) to mention that most reported hydrogels (e.g., Alg–PAAm, PVA–glycerol systems) have demonstrated cytocompatibility or are derived from food-grade precursors, but comprehensive biocompatibility testing for long-term or repeated use is still limited. This clarification will reference established studies without adding new experimental data.

 

Comment 8:

For the fingertip ionic sensor, what’s the minimum detectable slip velocity and force resolution.

Response 8:

The fingertip ionic sensor performance values are derived from prior works (e.g., Truby et al., Nat. Commun. 2018). Reported minimum slip velocity is on the order of a few mm/s and force resolution in the 10–100 mN range, depending on geometry and ionic composition. We will include these reference-based figures in Section 5.3 and cite the relevant original sources. No new measurements will be introduced.

Reviewer 2 Report

Comments and Suggestions for Authors

This review provides a structured overview of resistive sensing technologiesfor soft robotic grippers, covering working mechanisms, materials, fabrication, and applications. The scope aligns with the interdisciplinary focus of Sustainability, and the discussion of trade-offsadds value to the field. However, several issues need to be addressed.

1.The abstract and Section 1.4 note that resistive sensors are preferred for their "ease of use, scalability, and compatibility with deformable materials" compared to capacitive, optical, or magnetic alternatives. However, the comparison lacks quantitative benchmarks 

2.Section 1.5 classifies resistive sensors into three categories but does not explicitly define how "ionic sensors" differ from traditional resistive sensors in terms of transduction mechanism.

3.Table 1 compares materials for resistive sensors but omits critical practical metrics: (a) cost per gram of conductive material, (b) shelf life, and (c) compatibility with large-scale manufacturing . Would adding these metrics help readers select materials for specific applications?

4.The review notes that liquid metals face leakage issues, but it does not explore recent solutions. Are there emerging fabrication techniques that mitigate liquid metal leakage, and if so, why were they not included?

5.Table 2 compares strain sensor designs but lacks consistency in performance metrics: some entries report "linearity up to X% strain," while others do not specify linearity ranges. Could the authors standardize metrics  for all entries to enable direct comparison?

6. The integration strategies in Figure 5 do not address how sensor placement affects measurement accuracy. For example, does embedding a strain sensor in a pneumatic gripper’s wall distort actuation or improve signal stability?

7.Table 3 is incompleteand lacks a "Cross-Sensitivity" column 

8.Table 7 compares ionogel and hydrogel sensors but does not include "Response Time". For example, how quickly do hydrogel strain sensors respond to sudden finger bending compared to ionogel sensors?

9.The future directions mention "hybrid multimodal sensors" but do not address how to resolve signal interference between modalities

10. The review uses inconsistent terminology . Could the authors define key terms in a glossary or use consistent language throughout?

Author Response

We sincerely thank the reviewer for the careful reading and valuable comments. The feedback has helped us refine the clarity, consistency, and depth of the review while keeping its original focus as a comprehensive synthesis. Our detailed responses are provided below.

 

Comment 1:

The abstract and Section 1.4 note that resistive sensors are preferred for their "ease of use, scalability, and compatibility with deformable materials" compared to capacitive, optical, or magnetic alternatives. However, the comparison lacks quantitative benchmarks.

Response 1:
We agree that quantitative benchmarking strengthens the comparison. Since this paper is a review, we do not perform new measurements, but we already note (Section 1.8) that standardized benchmarking protocols are lacking across sensing modalities. We will expand Section 1.4 with a short statement emphasizing that while existing literature provides representative quantitative ranges (e.g., capacitive sensors: 0.05–1 pF/kPa sensitivity; resistive sensors: gauge factors typically 2–200; piezoelectric sensors: 0.1–10 V/N response), systematic cross-modal benchmarking is still not standardized. This clarification will better contextualize the comparison without expanding the paper’s scope.

 

Comment 2:

Section 1.5 classifies resistive sensors into three categories but does not explicitly define how "ionic sensors" differ from traditional resistive sensors in terms of transduction mechanism.

Response 2:
Thank you for pointing this out. We will add one clarifying sentence in Section 1.5 stating that ionic sensors differ from traditional resistive sensors in that charge transport occurs primarily through mobile ions rather than electrons, leading to combined electrochemical and mechanical coupling. This distinction will make the classification clearer to the reader.

 

Comment 3:

Table 1 compares materials for resistive sensors but omits critical practical metrics: (a) cost per gram of conductive material, (b) shelf life, and (c) compatibility with large-scale manufacturing. Would adding these metrics help readers select materials for specific applications?

Response 3:
We appreciate the suggestion. As this is a literature-based review, detailed cost and shelf-life data are rarely reported consistently across sources. However, we agree that these are relevant design considerations. Therefore, we will add a short paragraph following Table 1, noting qualitative trends, e.g., carbon-based materials and textiles are low-cost and scalable, while liquid metals and conductive polymers are higher-cost and more specialized. We will also add a “Practical Considerations” note summarizing relative cost and manufacturability qualitatively rather than inserting an additional column of uncertain numerical data.

 

Comment 4:

The review notes that liquid metals face leakage issues, but it does not explore recent solutions. Are there emerging fabrication techniques that mitigate liquid metal leakage, and if so, why were they not included?

Response 4:
This is an excellent point. We will add a short note in Section 1.8 mentioning recent approaches such as microchannel encapsulation with stretchable elastomers, surface oxide stabilization, and microstructured confinement within polymer matrices that mitigate leakage. These methods are mentioned in the literature but were omitted initially to maintain brevity. We will now include a concise reference to these advances without creating a separate subsection.

 

Comment 5:

Table 2 compares strain sensor designs but lacks consistency in performance metrics: some entries report "linearity up to X% strain," while others do not specify linearity ranges. Could the authors standardize metrics for all entries to enable direct comparison?

Response 5:
We thank the reviewer for this observation. We will revise Table 2 to include consistent parameters across all entries, specifically gauge factor, linearity range, and hysteresis, using the values available from the cited sources. This will standardize the comparison and improve the table’s readability without adding new experimental data.

 

Comment 6:

The integration strategies in Figure 5 do not address how sensor placement affects measurement accuracy. For example, does embedding a strain sensor in a pneumatic gripper’s wall distort actuation or improve signal stability?

Response 6:
This is a valuable observation. We will add one explanatory paragraph beneath Figure 5 clarifying that surface-mounted sensors offer higher sensitivity but may experience slippage, whereas embedded sensors within actuator walls improve mechanical coupling and signal stability with minimal effect on actuation deformation (as confirmed in literature). This addition will provide conceptual guidance without introducing new experiments.

 

Comment 7:

Table 3 is incomplete and lacks a "Cross-Sensitivity" column.

Response 7:
We agree that cross-sensitivity (to temperature, humidity, etc.) is important. We will update Table 3 to include a new column summarizing qualitative cross-sensitivity information (e.g., “moderate temperature sensitivity,” “low humidity dependence,” “high mechanical isolation”) extracted from the cited works. This addition will improve comprehensiveness while remaining concise.

 

Comment 8:

Table 7 compares ionogel and hydrogel sensors but does not include "Response Time". For example, how quickly do hydrogel strain sensors respond to sudden finger bending compared to ionogel sensors?

Response 8:
Thank you for pointing this out. Typically, hydrogel sensors respond within 100–300 ms, while ionogel sensors exhibit faster responses (~10–100 ms) due to higher ionic mobility. 

 

Comment 9:

The future directions mention "hybrid multimodal sensors" but do not address how to resolve signal interference between modalities.

Response 9:
We agree that this is a critical issue. We will expand Section 1.8 (Challenges and Future Directions) to include one or two sentences explaining that signal interference in multimodal sensors can be mitigated by decoupling signal pathways through frequency-domain filtering, differential electrode design, or machine learning-based signal separation, as discussed in recent hybrid sensing works. This addition will address the comment without requiring new data.

 

Comment 10:

The review uses inconsistent terminology. Could the authors define key terms in a glossary or use consistent language throughout?

Response 10:
We acknowledge this feedback and will carefully review the manuscript to ensure consistent terminology for key concepts such as “strain,” “tactile,” and “ionic” sensors. Instead of adding a full glossary (to preserve conciseness), we will define these terms clearly upon first use and maintain uniform phrasing throughout the manuscript.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript presents a timely, well-structured, and comprehensive review of resistive sensing technologies for soft robotic grippers, focusing on strain, tactile, and ionic sensors. The scope is clear, the literature coverage is broad and up to date, and the analysis highlights both established foundations and emerging directions. The inclusion of comparative tables, figures, and a timeline adds significant value for readers. Overall, this work will be useful to both new researchers entering the field and experienced scholars seeking an overview of recent advances.

That said, several areas could be improved to further strengthen the manuscript:

• The English is overall clear and professional, but there are minor grammatical issues and typographical errors (e.g., “Grippers play a rolethat…” should be “Grippers play a role that…”). A careful proofreading or light language polish is recommended.
• The applications section is valuable but remains somewhat general. Including more detailed case studies or industrial demonstrations beyond laboratory prototypes (e.g., food handling, healthcare, rehabilitation robotics) would emphasize the practical impact of resistive sensing.
• The review notes the lack of standardized metrics for comparing resistive sensors (e.g., gauge factor, hysteresis %, response times, cycle stability). It would strengthen the paper if the authors proposed or highlighted a potential framework for benchmarking performance across sensor categories.

Author Response

We sincerely thank the reviewer for the thoughtful and encouraging feedback. We are pleased that the reviewer found the manuscript comprehensive, timely, and valuable for the community. We address each point below.

Comment 1:

The English is overall clear and professional, but there are minor grammatical issues and typographical errors (e.g., “Grippers play a rolethat…” should be “Grippers play a role that…”). A careful proofreading or light language polish is recommended.

Response 1:
We appreciate this observation. The entire manuscript will undergo a thorough language and typographical review to correct minor grammatical inconsistencies and spacing errors (such as “rolethat,” “tech-niques,” etc.). These edits will ensure fluency and readability without altering the technical content.

 

Comment 2:

The applications section is valuable but remains somewhat general. Including more detailed case studies or industrial demonstrations beyond laboratory prototypes (e.g., food handling, healthcare, rehabilitation robotics) would emphasize the practical impact of resistive sensing.

Response 2:
We agree that practical demonstrations reinforce the review’s relevance. To address this, we will expand the Applications subsection (Section 1.6) by adding a concise paragraph highlighting representative industrial and translational case studies reported in recent literature, including resistive-sensor-based grippers for food handling (e.g., fruit packaging), healthcare and rehabilitation (assistive exosoft devices), and human–robot collaboration. These additions will be drawn from existing studies already cited or newly referenced, requiring no structural expansion or new experimental data.

 

Comment 3:

The review notes the lack of standardized metrics for comparing resistive sensors (e.g., gauge factor, hysteresis %, response times, cycle stability). It would strengthen the paper if the authors proposed or highlighted a potential framework for benchmarking performance across sensor categories.

Response 3:
We appreciate this excellent suggestion. We will include a short conceptual paragraph in Section 1.8 (Challenges and Future Directions) proposing a benchmarking framework for resistive sensing. This will outline key standardized parameters, such as gauge factor (GF), hysteresis (%), response/recovery time, and cycle stability (N cycles), and suggest that future studies adopt uniform deformation amplitudes and loading frequencies for comparison. This brief proposal will strengthen the paper’s methodological insight without adding new experiments.

 

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

accept

Reviewer 2 Report

Comments and Suggestions for Authors

The author has addressed my issues.