High-Sensitivity Low-Cost 2.61 GHz DGS Sensor for Non-Invasive Glucose Level Monitoring^†

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

  General comments.

1. The numbers of Figures and tables in the text are given in square brackets [].References are also given in square brackets.Therefore, square brackets around numbers figure and table in the text should be removed throughout the text.
2. In the titles of the vertical ordinate axes in Figures 4, Figure 7, Figure 11, Figure 12, and Figure 13, the unnecessary comma must be removed (replace “S2,1” on the “S21”).
3. In the titles of the horizontal abscissa axes in Figures 4, Figure 7, Figure 11, Figure 12, and Figure 13, the full name of the quantity must be given (replace “Freq(GHz)” on the “Frequency(GHz)”).

 

Comments in order of pages.

Page 1. Lines 21-22. – The sentence "A set of measurements were conducted to compare the sensitivity of the structure with and without the DGS." should be transpose and placed before the next to last sentence in the abstract (Line 29).

        Lines 22-23. – The phrase "These simulations" should be substituted for the "The conducted theoretical simulations."

 

Page 3. Line 86. – The phrase "by altering the current distribution within the device [10]." should be substituted for the "by altering the displacement current distribution within the device [10]." if we are talking about the displacement current.

        Lines 103-104. – The phrase "of DUT (Figure [2]) (Device under Test)" should be substituted for the "of DUT (Device under Test) (Figure 2)".

 

Page 4. Lines 133, 137. – In the equations, the point should be substituted for the multiplication sign or removed.

        Line 142. – The Table 1 title should be capitalized.

        Lines 142-143. Table 1. – Substitute the title of the fourth (last) column "transmitted (%)" for the "transmitted power (%)".

 

Page 5. Lines 157. Figure 3. – It would be better to use arrows to indicate where the metallization and where the substrate.

 

        Lines 164. Figure 4. – In the figure, the data points on the graphs should be slightly moved so that they do not overlap each other or the curves.

        Lines 165. – If the graph shows theoretical dependencies, the caption to the Figure 4 should be substituted for the "The theoretical frequency dependences of coefficient S21, for microwave resonator (DGS vs. no DGS)".

        Line 172. – It's not entirely clear what improvements in rejection performance are being referred to. If it's about noise suppression, it's not entirely obvious. If it's about increasing the resonance Q factor and peak depth, then that should be stated.

 

Page 6. Lines 177. – The paper "Mansour E; Allam A; Abdel-Rahman A.B. A novel approach to non-invasive blood glucose sensing based on a defected ground structure. 15th European Conference on Antennas and Propagation (EuCAP), Germany, 22-26 March 2021." presents theoretical formulas for calculating the relative dielectric permittivity. Do the authors know of any work with experimental confirmations of these calculations?

        Lines 180-184. – In this context, it is advisable to indicate the relative dielectric permittivity of the ABS plastic used and its loss tangent at the given frequency.

        Page 6. Lines 179-188 and Page 7. Lines 189-194. – This entire section of the article should be transpose to page 10, starting with Line 244.

 

Page 7. Lines 191. – At the end name and nomenclature of the equipment, it's best to add information about the manufacturer-developer and country of manufacture, for example: Bambu Lab X1 Carbon printer (Bambu Lab, China).

 

         Pages 7-8. Figure 7 – In Figure 7 (a), (c), (d), the points values should be moved so that they do not overlap each other and the minus sign is visible. In the upper right corner of Figure 7 (a), (b), (c), (d), on the explanatory picture with the microwave sensor and cuvette, all axes should be labeled so that they and their designations are clearly visible and legible. The numbers and letters at the bottom of the black-and-white scale bar should be enlarged.

 

Page 8. Line 200. – The full title of Figure 7 must be provided. Substitute the phrase "Figure 7. S21" for the "Figure 7. The theoretical frequency dependences of coefficient S21", if these are theoretical dependencies.

         Line 222. – The abbreviation "PCB" in the text must be expanded.

         Line 225. – Add information about the manufacturer and country of manufacture: Silhouette Cameo (manufacturer, country).

Page 9. Lines 232. – If the transverse dimensions of the substrates and radiating elements of all eight samples were the same, this must be noted in the text (in a short phrase).

        Lines 238. – Perhaps a better title for the Figure 9 would be: “Figure 9. (a) Radiating element of the fabricated structure (top side), (b) Defective ground plane (reverse side).

 

Page 10. Line 247 – After the device name, please add information about the manufacturer and country of manufacture (manufacturer, country).

        Figure 12. – In Figure 12, please provide the peak depth values, as is done in the other graphs.

        Lines 257-259 – With which graphs should Figure 12 be compared?

        Lines 260-261 – For which peak is the frequency shift value provided? It would be better to provide the corresponding difference often for both peaks.

 

Page 11. Line 270 – For the reader’s convenience, it would be better to occasionally remind the reader that “solution” is “water-glucose solution.”

         Figure 13 – In the graph, please provide the values ​​of all peaks so that they are clearly visible and do not overlap.friend.

         Figure 13. Line 273 – It is necessary to give the full name of the graph, namely “Figure 13. The measured frequency dependences of coefficient S12 for: (a) Structure with DGS and solution placed on the ground plane; (b) Structure without DGS and solution placed on the ground plane; (c) Structure with DGS and solution placed on top of the structure; (d) Structure without DGS and solution placed on top of the structure.”

        Lines 276-279 – Figures 13(a) and (c) show that in the first case, the cuvette is located in the center of the microwave sensor, while in the second (Figure 13(c)) it is closer to the edge. How does positioning accuracy affect the measurement results?

        Lines 284 – What solutions are we talking about? If we are talking about those shown in Figure 4, this little difference is there.

 

Page 12. Line 825 – It would be better to change the title of Table 4 for the "Table 4. Coefficient S21 measured results for the first resonance frequency."

        Lines 285-286 – It is necessary to compare the values ​​shown in Figure 13(d) and in the last column of Table 4.

 

         If the authors selected only the first peak, corresponding to a frequency of 2-2.6 GHz, for analysis, then the reason for this choice should be briefly stated in the text, since a similar table for the second peak is not provided.

        Lines 295-299 – The case where the cuvette was positioned above the emitting element (top of the structure) shows no worse results. This could be mentioned in one sentence and explain why the case shown in Figure 13(a) is preferable with the case shown in Figure 13(c), in the authors' opinion.

         Line 301 – Sensitivity is better denoted here and below by a different symbol (letter).

         Line 304 – The missing parenthesis should be added, or the parentheses should be deleted.

         Line 304 – The phrase "DGS structure" should be substituted for "Structure with DGS."

         Lines 319-320 – The title and first line of Table 5 should be moved to the next page, so that Table 5 appears entirely on one page.

 

 Page 13. Table 5 – The title of "this work" on the first page and its title in the last line of Table 5 are not corresponding to each other.

        Table 5 – After the work title, in the first column, the corresponding bibliography reference should be provided in square brackets . For example: "PCA-Assisted Blood Glucose Monitoring Using a Metamaterial-Inspired Sensor [14]".

        Lines 333-334 – If sample positioning relative to the microwave sensor side has no effect, then why is emphasized in the work the positioning of sample? (Page 12. Lines 281-284).

 

        Pages 13-14. Table 6 – Since the data presented in Table 6 duplicate the data presented in Table 5, Table 6 and its mention in the text can be removed.

 

Page 14. Lines 347-348 – The phrase "test on the humans" should be possibly better removed, as it sounds unethical. The fact that any methodology and devices (devices, sensors, and their materials) are ready for testing on the humans or animals most likely requires the conclusions of medical council, and regulations from specialized organizations.

        Lines 358-369 – The "Author Contribution" section must be completed in accordance with the guidelines of the journal "Micromachines." You can also see how it is completed in the published works of other authors in this journal.

        Line 370 – Remove unnecessary quotation marks.

         Lines 371-376 – It might be better to see how the "Data Availability Statement" section is completed in the published works of other authors in this journal. (https://www.pdpi.com/journal/mcromachines/instructions).

 

Page 15. Lines 384-420 – The bibliography must be formatted according to the journal’s requirements (https://www.pdpi.com/journal/mcromachines/instructions).

Author Response

Thank you very much for taking the time to review this manuscript. We have addressed the comments point by point, and the entire manuscript has been thoroughly revised to improve the English, grammar, and overall clarity. Please find our detailed responses below, and the corresponding revisions have been highlighted using track changes in the resubmitted files.

General Comments:

1. The numbers of Figures and tables in the text are given in square brackets []. References are also given in square Therefore, square brackets around numbers figure and table in the text should be removed throughout the text.

Author’s Response: We thank the reviewer for this valuable observation.
Author’s Action: We have removed the square brackets from all citations to figures and tables throughout the manuscript.

2. In the titles of the vertical ordinate axes in Figures 4, 7, 11, 12, and 13, the unnecessary comma must be removed (replace "S2,1" with "S21").

Author’s Response: We agree with the reviewer and thank them for pointing out this issue.
Author’s Action: The vertical axis labels in Figures 4, 7, 11, 12, and 13 have been corrected by removing the unnecessary comma, so that “S2,1” now appears correctly as “S21.”

3. In the titles of the horizontal abscissa axes in Figures 4, 7, 11, 12, and 13, the full name of the quantity must be given (replace "Freq(GHz)" with "Frequency(GHz)").

Author’s Response: We appreciate the reviewer’s suggestion, which improves the clarity and readability of the plots.

Author’s Action: The horizontal axis labels in Figures 4, 5 (Figure 7 in the original submission), 11, 12, and 13 have been revised by replacing “Freq(GHz)” with “Frequency (GHz)”.

 

Comments in order of pages:

4. Page Lines 21-22. - The sentence "A set of measurements were conducted..." should be transposed and placed before the next to last sentence in the abstract.

Author’s Response: We thank the reviewer for this helpful suggestion and agree that the proposed change improves the logical flow of the abstract.

Author’s Action: The abstract has been reorganized accordingly, and the sentence “A set of measurements was conducted...” has been moved to the position before the next-to-last sentence.

5. Lines 22-23. The phrase "These simulations" should be substituted for "The conducted theoretical simulations."

Author’s Response: We agree that this wording should be revised for greater precision. In this case, however, the intended term was “measurements” rather than “simulations.”
Author’s Action: The abstract has been corrected accordingly, and the phrase has been revised to reflect the intended meaning more accurately.

6. Page 3. Line 86. - The phrase "by altering the current distribution" should be substituted for "by altering the displacement current distribution."

Author’s Response: We thank the reviewer for this technical clarification and agree that the original wording should be revised for greater accuracy.

Author’s Action: The sentence has been corrected by replacing “altering the displacement and distribution of current”.

 

7. Lines 103-104. The phrase "of DUT (Figure [2]) (Device under Test)" should be substituted for "of DUT (Device under Test) (Figure 2)".

Author’s Response: We thank the reviewer for pointing out this issue and agree that the revised phrasing is clearer and more appropriate.

Author’s Action: The text has been corrected exactly as suggested.

 

8. Page 4. Lines 133, 137. In the equations, the point should be substituted for the multiplication sign or removed.

Author’s Response: We thank the reviewer for this observation and agree that the mathematical notation should be made more consistent.

Author’s Action: The equations have been revised accordingly, and the dots previously used as multiplication signs have been removed.

 

9. Lines 142-143. The Table 1 title should be capitalized. Substitute the title of the fourth column "transmitted (%)" for "transmitted power (%)".

Author’s Response: We thank the reviewer for pointing out these formatting issues.
Author’s Action: The title of Table 1 has been capitalized, and the fourth column heading has been corrected from “Transmitted (%)” to “Transmitted Power (%)”.

10. Page 5. Lines 157. Figure 3. It would be better to use arrows to indicate where the metallization and where the substrate.

Author’s Response: We thank the reviewer for this helpful suggestion and agree that the figure should more clearly distinguish the different regions.

Author’s Action: Figure 3 has been revised to improve its visual clarity. Although arrows were not added, the metallized regions and the substrate are distinguished using different colors, with the metallized areas shown in orange and the bare FR4 substrate shown in green.

11. Lines 164-165. Figure 4. The data points on the graphs should be slightly moved so they do not overlap. The caption should be substituted for "The theoretical frequency dependences of coefficient S21..."

Author’s Response: We thank the reviewer for this helpful suggestion and agree that improving the spacing of the data-point labels enhances the readability of the figure. We also agree that the caption should be revised for greater clarity and consistency.

Author’s Action: In Figure 4, the data-point labels were slightly repositioned to avoid overlap and improve legibility. In addition, the caption has been revised so that it now begins with “The theoretical frequency dependences of coefficient S21...”, as suggested.

 

12. Line 172. It’s not entirely clear what improvements in rejection performance are being referred to... If it’s about increasing the resonance Q factor and peak depth, then that should be stated.

Author’s Response: We thank the reviewer for this important observation and agree that the expression “improved rejection performance” was not sufficiently specific in its original form.

Author’s Action: The manuscript has been revised to clarify this point explicitly. The text now states that the improvement refers to an increase in the resonance quality factor and peak depth, as follows: “In this context, improved performance is associated with an increase in the resonance quality factor and peak depth, which ultimately enhances the sensor resolution.”

13. The paper "Mansour E; Allam A; Abdel-Rahman B. A novel approach to non- invasive blood glucose sensing based on a defected ground structure. 15th European Conference on Antennas and Propagation (EuCAP), Germany, 22-26 March 2021." presents theoretical formulas for calculating the relative dielectric permittivity. Do the authors know of any work with experimental confirmations of these calculations?

Author’s Response: The reviewer raises a very relevant point regarding the experimental validation of the theoretical formulas reported in the work by Mansour et al.

Author’s Action: To the best of our knowledge, we are not aware of published studies that provide direct experimental confirmation of those specific calculations for relative dielectric permittivity in the context discussed by Mansour et al. In the revised manuscript, we have therefore avoided making any claim of experimental validation for those formulas and have cited the work only as a theoretical reference.

 

14. Lines 180-184. It is advisable to indicate the relative dielectric permittivity of the ABS plastic used and its loss tangent.

Author’s Response: We thank the reviewer for this useful suggestion and agree that the dielectric properties of the ABS plastic should be stated explicitly for completeness and reproducibility.

Author’s Action: The manuscript has been revised accordingly. We added the dielectric properties of the ABS plastic used in the model, namely a relative permittivity () of 2.8 and a dielectric loss tangent () of 0.0053.

 

15. Page 6. Lines 179-188 and Page 7. Lines 189-194. This entire section should be transposed to page 10, starting with Line 244.

Author’s Response: We thank the reviewer for this helpful suggestion and agree that relocating this section improves the overall structure and logical flow of the manuscript.

Author’s Action: The indicated paragraphs describing the fabrication procedure were moved to the later section, as suggested, in order to improve the organization and continuity of the manuscript.

16. Page 7. Line 191 & Page 8. Line 225. Add information about the manufacturer and country for the Bambu Lab printer and Silhouette Cameo.

Author’s Response: We thank the reviewer for this observation and agree that including the manufacturer and country information improves the completeness of the experimental description.

Author’s Action: The manuscript has been revised accordingly, and the manufacturer details were added for both devices, namely “(Bambu Lab, China)” and “(Silhouette America, USA).

17. Pages 7-8. Figure Points values should be moved so they do not overlap. Label all axes on the explanatory picture, and enlarge numbers on the scale bar.

Author’s Response: We thank the reviewer for this helpful suggestion and agree that these changes improve the readability and interpretability of Figure 5 (Figure 7 in the original submission).

Author’s Action: Figure 5 (Figure 7 in the original submission) has been revised accordingly. The point-value labels were repositioned to avoid overlap, all axes in the explanatory diagram were explicitly labeled, and the numbers on the scale bar were enlarged to improve legibility.

 

18. Page 8. Line 200. Figure 7 full title must be provided: "Figure 7. The theoretical frequency dependences of coefficient S21".

Author’s Response: We thank the reviewer for this observation and agree that the figure caption should be stated in its complete form for clarity and consistency.

Author’s Action: The caption of Figure 5 (Figure 7 in the original submission) has been corrected accordingly and now appears as “Figure 5. The theoretical frequency dependences of coefficient S21”, as requested.

 

19. Line 222. The abbreviation "PCB" in the text must be expanded.

Author’s Response: We thank the reviewer for pointing out this omission.

Author’s Action: The abbreviation “PCB” has been expanded to “Printed Circuit Board” in the text at its first occurrence.

20. Page 9. Line 232. If transverse dimensions of the substrates/elements of all samples were the same, this must be noted.

Author’s Response: We thank the reviewer for this relevant observation and agree that this point should be stated explicitly in the manuscript.

Author’s Action: A clarifying sentence was added to indicate that all fabricated samples have identical transverse dimensions.

21. Line 238. Perhaps a better title for Figure 9 would be: "(a) Radiating element... (b) Defective ground plane...".

Author’s Response: We thank the reviewer for this helpful suggestion and agree that the proposed caption is clearer and more descriptive.

Author’s Action: The caption of Figure 7 (Figure 9 in the original submission) has been revised accordingly and now identifies the two subfigures as: “Figure 7. (a) Top side of the fabricated resonator, showing the metallic resonant element; (b) Bottom side of the fabricated resonator, showing the defected ground plane.” This wording was adopted to avoid the term “radiating element,” since the proposed structure is a resonator rather than an antenna, and the term “metallic resonant element” is therefore more accurate.

22. Page 10. Line 247. After the device name (VNA), add information about the manufacturer and country.

Author’s Response: We thank the reviewer for this observation and agree that the manufacturer information should be provided for completeness.

Author’s Action: The text has been revised accordingly, and the manufacturer and country information were added after the device name as “(Rohde & Schwarz, Germany)”.

 

23. Figure 12. Provide peak depth values.

Lines 257-261: Which graphs compare Figure 12?

Provide frequency shift values for both peaks.

Author’s Response: We thank the reviewer for this important observation and agree that Figure 12 should provide a clearer comparison between the simulated and measured results.

Author’s Action: Figure 12 has been revised to include the peak-depth values for the compared curves. In addition, the corresponding discussion in the manuscript was clarified to explicitly identify the graphs being compared, namely, the simulated and measured responses of the structures with and without DGS. The frequency shifts were also stated explicitly for both resonance peaks. In particular, for the structure with DGS, the measured response shows a frequency shift of approximately 0.45 GHz for the first resonance peak and 0.26 GHz for the second resonance peak relative to the simulated response.

24. Page 11. Line 270 & 284. Remind the reader that "solution" is "water-glucose solution".

Author’s Response: We thank the reviewer for this helpful observation and agree that the terminology should be made more explicit to avoid any possible ambiguity.

Author’s Action: The text has been revised accordingly, and the term “solution” was replaced with “water–glucose solution” in the relevant passages.

 

25. Figure 13. Provide values of all peaks so they do not overlap. The full name of the graph must be given.

Author’s Response: We thank the reviewer for this helpful suggestion and agree that the readability of Figure 13 can be improved by better positioning the peak-value labels and by using a more complete caption.

Author’s Action: Figure 13 has been revised accordingly. The values of all peaks were added and repositioned to avoid overlap, thereby improving the clarity of the graph.

26. Lines 276-279. How does positioning accuracy affect measurement results?

Author’s Response: We thank the reviewer for raising this important practical point. We agree that the positioning accuracy of the cuvette can influence the measured response and should therefore be discussed explicitly.

Author’s Action: A brief clarification was added to the manuscript stating that slight off-center variations in cuvette placement may induce minor shifts in the resonance frequency, which can affect the repeatability of the measurements. The text now reads: “...it is important to note that the placement of the cuvette can induce minor shifts in the resonance frequency, highlighting the need for consistent placement.”

27. Page 12. Line 285. Change the title of Table 4. Compare values Figure 13(d) and Table 4. State why only the first peak was selected.

Author’s Response: We thank the reviewer for this important observation and agree that the title of Table 4, its consistency with Figure 13(d), and the rationale for selecting only the first resonance peak should be clarified more explicitly.

Author’s Action: Table 4 and the corresponding discussion in the manuscript were revised accordingly. The title was changed to “Table 4. Measured Results of the S21 Coefficient at the First Resonance Frequency.” In addition, the values reported in Table 4 were checked against those shown in Figure 13(d), and the text was revised to clarify that only the first resonance frequency was selected because it was the only peak that exhibited a clear, stable, and concentration-dependent response for the investigated water–glucose solutions. The manuscript now explicitly states: “it is observed that only the first resonance frequency exhibits the capability to sense water–glucose solutions.”

28. Lines 295-299. Mention why the case shown in Figure 13(a) is preferable to the case in Figure 13(c).

Author’s Response: We thank the reviewer for this relevant observation and agree that the preference for the configuration shown in Figure 13(a), compared with Figure 13(c), should be stated more explicitly.

Author’s Action: The manuscript has been revised accordingly. We added an explanation clarifying that the configuration in Figure 13(a) is preferable because placing the cuvette on the DGS ground plane promotes stronger interaction with the concentrated electric field in the defected gap, which results in higher sensing capability and a more consistent response pattern. The text now states: “This configuration also exhibits a more consistent response pattern than the one with the water–glucose solution placed on top of the structure, because the DGS ground plane enables stronger interaction with the concentrated electric field in the defected gap, thereby providing higher sensing capability.”

29. Line 301. Sensitivity is better denoted by a different symbol.

Author’s Response: We agree that using “S” to denote sensitivity may create confusion with the scattering parameters.

Author’s Action: The notation was revised accordingly, and the symbol used for sensitivity was changed from “S” to “Se” throughout the manuscript.

30. Line 304. Missing parenthesis should be added, and substitute "Structure with DGS" for "DGS structure".

Author’s Response: We thank the reviewer for pointing out these issues and agree that both the syntax and the terminology should be corrected for clarity and consistency.

Author’s Action: The missing parenthesis was added, and the expression “DGS structure” was replaced with “Structure with DGS” in the revised manuscript.

31. Table 5 Formatting (Lines 319-320). Move title so it appears on one page. "This work" title must match. Add references in square brackets.

Author’s Response: We thank the reviewer for these formatting observations and agree that Table 5 should be presented more clearly and consistently.

Author’s Action: Table 5 was revised accordingly. The title was repositioned so that it appears entirely on a single page, the entry corresponding to this work was updated to match the manuscript title, and the references in the first column were formatted in square brackets for consistency with the rest of the table.

32. Lines 333-334. If sample positioning has no effect, why is it emphasized?

Author’s Response: We thank the reviewer for this important observation and agree that the original wording could suggest a contradiction regarding the role of sample positioning.

Author’s Action: The text was revised to clarify this point more precisely. We now distinguish between minor placement deviations, which have a negligible effect on the measured response, and the overall positioning configuration of the sample, which has a significant influence on sensor performance. In particular, positioning the sample on top of the structure or over the DGS region leads to different electromagnetic interactions and, consequently, different sensing behaviors.

33. Pages 13-14. Table 6. Remove duplicate Table

Author’s Response: We thank the reviewer for pointing out this redundancy.

Author’s Action: The duplicate Table 6 was removed from the revised manuscript.

34. Page 14. Lines 347-348. The phrase "test on humans" should be removed as it sounds unethical.

Author’s Response: We thank the reviewer for this important observation and fully agree that the original wording was inappropriate.

Author’s Action: The phrase “test on humans” was removed and replaced with more appropriate wording consistent with standard biomedical research ethics and scientific reporting.

35. Lines 358-376 & 384-420. Formatting for "Author Contribution", "Data Availability", and Bibliography.

Author’s Response: We thank the reviewer for pointing out these formatting issues and agree that the closing sections should strictly follow the journal’s required style.

Author’s Action: The Author Contributions, Data Availability Statement, and References sections were thoroughly revised to comply with the formatting requirements of Micromachines. Unnecessary quotation marks were removed, and these sections were reformatted in accordance with the journal’s template and style guidelines.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This paper examines the problem of determining the amount of glucose in a water solution. As noted in the introduction, this problem is extremely practically important, for example, for determining glucose level in human blood. The use of a noninvasive measurement method simplifies and accelerates the determination of glucose level in the blood of people with diabetes, thereby improving their quality of life. The authors demonstrated that their proposed method, based on a DGS sensor, has the highest sensitivity compared to other monitoring methods. The paper presents a theoretical and experimental analysis of the DGS-based hairpin sensor at a fairly high scientific and engineering level. However, several criticisms can be made of this work:
1) The formulas in the paper are not numbered sequentially. This makes it difficult to understand.
2) The paper is insufficiently structured. Subsections such as "Theory of S-parameters," "Geometry of the sensor," "The electrical properties of glucose and water solutions," "Structure fabrication," "Experimental setup," and so on should be added.
3) The formula SdB = −20*log10(|s|) (page 4, line 133) is incorrect; the minus sign is unnecessary. For a value of |s|<1, the value of SdB < 0 should be observed, i.e., in the presence of attenuation, the S-parameter value is negative. This is confirmed by Table 1 and Figure 4.
4) The title of Table 1 is incorrect. These are not typical values of the S11 parameter, but the relationship between the Scattering Parameter value in decibels, the Magnitude, and the value of the Transmitted and Reflected power, which follows from the previous formulas. This also applies to other S-parameters, such as S21. The actual value of S21 can reach -60 dB, as follows from Table 4.
5) Figure 3 is not entirely clear. Parts of the figure (a) and (b) should be labeled. It should be clearly stated that: The left figure (Figure 3a) shows the geometry of the hairpin sensor on one side of the FR4 epoxy substrate plate, adds the locations of electrical ports 1 and 2 should be added; The right figure (Figure 3b) shows the reverse side of the same plate, in the center of which the DGS may or may not be present as a circle without metallization. Orange color indicates metallization (Copper film), green color indicates no metallization. The origin of all specific sensor dimensions in the left figure should be separately explained.
6) In Figure 4, it should be stated that this is a theoretical result. The text should explain which electromagnetic parameters of FR4 epoxy substrate and metallization (thickness and conductivity) were used in the calculations.
7) The values of parameters of the glucose and water solutions used in the calculations were taken from [10]. What are the actual values of these parameters for the solutions used in the experiments? It should be stated how (with what equipment) and with what accuracy they were determined. The height of the liquid column in the container during the experiments should also be specified.
8) The dimensions of the ABS liquid container are questionable. Table 3 shows that the outer radius of the container (11 mm) exceeds the radius of the defect (10 mm), which is confirmed by Figure 7. However, photographs of the actual device (Figure 12) show that the container size is much smaller than the defect size. The authors should provide the correct container dimensions and double-check that the container dimensions in the calculations and measurements correspond to each other. This may explain the large discrepancy between the simulated and measured S21 parameters in Figure 12.
9) In the sentence (Page 7, lines 191-194), references to Figures 5 and 4 should be replaced with Figures 6 and 5, respectively.
10) A different notation should be used for the Sensitivity Parameter of the resonant sensor (page 12, line 301 and later), since the letter S is already used in this article to denote the scattering parameter (page 4, line 133).
11) The title of the presented paper in Table 5 should be corrected. The description of this table (page 12, lines 312-317) should indicate that the proposed method has the highest sensitivity compared to other methods, but it was tested over a narrower range of glucose concentrations (only 50-250 mg/dL).
12) The presented results indicate that the highest sensitivity of the device is achieved for the structure with DGS and a solution placed on the ground plane. The glucose solution is separated from the metallic sensor by a layer of FR4 epoxy and ABS plastic. In the Discussion or Conclusion section, the authors should mention the obvious possibility of placing the liquid container directly on the surface of a non-metallized substrate using a bottomless plastic cell. This will bring the liquid closer to the sensor and may further improve the device's performance.
Overall, the presented work can be published after the above deficiencies are corrected.

 

Author Response

Thank you very much for taking the time to review this manuscript. We have addressed the comments point by point, and the entire manuscript has been thoroughly revised to improve the English, grammar, and overall clarity. Please find our detailed responses below, and the corresponding revisions have been highlighted using track changes in the resubmitted files.

1. The formulas in the paper are not numbered sequentially. This makes it difficult to understand

Author’s Response: We thank the reviewer for pointing out this issue and apologize for the oversight in the original submission. We agree that non-sequential equation numbering may hinder the readability and interpretation of the manuscript.

Author’s Action: All equations in the revised manuscript have been renumbered to follow a consistent sequential order throughout the text.

2. The paper is insufficiently structured. Subsections such as "Theory of S-parameters," "Geometry of the sensor," "The electrical properties of glucose and water solutions," "Structure fabrication," "Experimental setup," and so on should be added

Author’s Response: We thank the reviewer for this important observation. We agree that the original version of the manuscript was not sufficiently structured and that the addition of explicit subsection headings substantially improves the organization, readability, and logical progression of the paper.

Author’s Action: The Materials and Methods section was thoroughly reorganized in the revised manuscript through the introduction of dedicated subsections. In particular, the following subsections were added: “2.1. Theory of S-parameters,” “2.2. Geometry of the sensor,” “2.3. The electrical properties of glucose and water solutions,” and “2.4. Structure fabrication.” This revised structure makes the methodological development clearer and separates the theoretical background, sensor design, material properties, and fabrication procedure into distinct and more accessible parts.

3. The formula SdB = -20*log10(|s|) (page 4, line 133) is incorrect; the minus sign is unnecessary. For a value of |s|<1, the value of SdB < 0 should be observed, i.e., in the presence of attenuation, the Sparameter value is negative. This is confirmed by Table 1 and Figure 4.

Author’s Response: We thank the reviewer for pointing out this important error. We agree that the minus sign in the original expression was incorrect and inconsistent with the standard definition of scattering parameters in decibels. As correctly noted, for |s| < 1, the corresponding value in decibels must be negative, which is fully consistent with the attenuation behavior shown in Table 1 and Figure 4.

Author’s Action: Equation 5 has been corrected accordingly and now follows the standard form: SdB = 20 log10|s|.

4. The title of Table 1 is incorrect. These are not typical values of the S11 parameter, but the relationship between the Scattering Parameter value in decibels, the Magnitude, and the value of the Transmitted and Reflected power, which follows from the previous formulas. This also applies to other S-parameters, such as S21. The actual value of S21 can reach -60 dB, as follows from Table 4.

Author’s Response: We thank the reviewer for this important observation and agree that the original title of Table 1 was inaccurate. The table does not present typical values of the S11 parameter itself, but rather the mathematical relationship between the scattering parameter expressed in decibels, its magnitude, and the corresponding transmitted and reflected power values. We also agree that this relationship is not limited to S11 and applies equally to other scattering parameters, such as S21.

Author’s Action: The title of Table 1 has been revised accordingly and now reads: “Table 1. Relationship Between the Scattering Parameter S11 in Decibels, Its Magnitude, and the Corresponding Transmitted and Reflected Power Values.”

5. Figure 3 is not entirely clear. Parts of the figure (a) and (b) should be labeled. It should be clearly stated that: The left figure (Figure 3a) shows the geometry of the hairpin sensor on one side of the FR4 epoxy substrate plate, adds the locations of electrical ports 1 and 2 should be added; The right figure (Figure 3b) shows the reverse side of the same plate, in the center of which the DGS may or may not be present as a circle without metallization. Orange color indicates metallization (Copper film), green color indicates no metallization. The origin of all specific sensor dimensions in the left figure should be separately explained.

Author’s Response: We thank the reviewer for this detailed and helpful observation. We agree that Figure 3 required a clearer description of both subfigures and a more explicit explanation of the meaning of the colors, ports, and dimensions.

Author’s Action: Figure 3 and its caption were revised accordingly. The caption now explicitly states that: “(a) Geometry of the hairpin sensor on the top side of the FR4 epoxy substrate, indicating the locations of electrical Ports 1 and 2. (b) Bottom side of the substrate showing the ground plane, where the Defected Ground Structure (DGS) is represented by a circular non-metallized region. The orange areas correspond to copper metallization, while the green areas represent the exposed FR4 substrate.

6. In Figure 4, it should be stated that this is a theoretical result. The text should explain which electromagnetic parameters of FR4 epoxy substrate and metallization (thickness and conductivity) were used in the calculations.

Author’s Response: We thank the reviewer for this valuable observation. We agree that Figure 4 should explicitly indicate that it presents theoretical results, and that the material parameters adopted in the simulations should be stated clearly in the manuscript.

Author’s Action: The caption of Figure 4 was revised accordingly and now begins with “The theoretical frequency dependences...”. In addition, the simulation description was expanded to specify the electromagnetic properties used in the model, namely an FR4 epoxy substrate with a thickness of 1.6 mm and relative permittivity εr = 4.4, while the metallization was defined as copper with a thickness of 0.035 mm.

7. The values of parameters of the glucose and water solutions used in the calculations were taken from [10]. What are the actual values of these parameters for the solutions used in the experiments? It should be stated how (with what equipment) and with what accuracy they were determined. The height of the liquid column in the container during the experiments should also be specified.

Author’s Response: We thank the reviewer for this important observation. We agree that the experimental section should more clearly distinguish between the dielectric properties adopted from the literature for the simulations and the preparation procedure used for the solutions in the measurements. In the present work, the dielectric parameters of the experimental water–glucose solutions were not independently measured with dedicated dielectric characterization equipment; instead, the solutions were prepared at controlled mass concentrations based on the values reported in the literature, and the experimental protocol was designed to ensure repeatability of the measurements.

Author’s Action: The manuscript was revised to clarify the preparation procedure and the experimental control conditions. We added that precise aliquots of glucose powder were dissolved in distilled water to obtain the desired concentrations, that the solutions were weighed using a precision analytical balance to ensure concentration accuracy, and that the liquid volume was dispensed with a micropipette. We also specified that the liquid column height inside the ABS container was kept approximately constant at 5 mm throughout all measurements in order to ensure consistent testing conditions.

8. The dimensions of the ABS liquid container are questionable. Table 3 shows that the outer radius of the container (11 mm) exceeds the radius of the defect (10 mm), which is confirmed by Figure 7. However, photographs of the actual device (Figure 12) show that the container size is much smaller than the defect size. The authors should provide the correct container dimensions and double-check that the container dimensions in the calculations and measurements correspond to each other. This may explain the large discrepancy between the simulated and measured S21 parameters in Figure 12.

Author’s Response: We thank the reviewer for this careful observation and apologize for this misunderstanding. The inconsistency identified between Table 3, Figure 7, and the photographs of the fabricated device was due to an error in the way the container dimensions were reported in the manuscript.

Author’s Action: The dimensions in Table 3 were corrected accordingly. The values previously indicated as radii actually corresponded to diameters, and this has now been fixed in the revised manuscript. In addition, Figure 5 was updated to show the correct image of the structure actually used in the simulations. We also carefully checked the consistency between the dimensions adopted in the simulations and those of the fabricated container used in the measurements.

9. In the sentence (Page 7, lines 191-194), references to Figures 5 and 4 should be replaced with Figures 6 and 5, respectively .

Author’s Response: We thank the reviewer for pointing out this inconsistency.

Author’s Action: The figure citations in the specified sentence were updated accordingly in the revised manuscript.

10. A different notation should be used for the Sensitivity Parameter of the resonant sensor (page 12, line 301 and later), since the letter S is already used in this article to denote the scattering parameter (page 4, line 133).

Author’s Response: We thank the reviewer for this important observation and agree that using the same symbol for both the scattering parameter and the sensitivity parameter may cause confusion.

Author’s Action: The notation was revised accordingly, and the sensitivity parameter is now denoted as “Se” throughout the manuscript in order to clearly distinguish it from the scattering parameter “S”.

11. The title of the presented paper in Table 5 should be corrected. The description of this table (page 12, lines 312-317) should indicate that the proposed method has the highest sensitivity compared to other methods, but it was tested over a narrower range of glucose concentrations (only 50-250 mg/dL).

Author’s Response: We thank the reviewer for this relevant observation and agree that Table 5 and the corresponding discussion should present a more accurate and balanced comparison with the literature.

Author’s Action: The title of the entry corresponding to the present work in Table 5 was corrected to match the revised manuscript, and the discussion was updated accordingly. The text now states: “As shown in Table 5, the proposed method achieves the highest sensitivity among the compared studies. However, it should be emphasized that our sensor was evaluated over a narrower glucose concentration range (50–250 mg/dL), which directly corresponds to the physiologically relevant range for diabetes monitoring, whereas some of the other reported works consider broader, but less clinically representative, concentration intervals.”

12. The presented results indicate that the highest sensitivity of the device is achieved for the structure with DGS and a solution placed on the ground plane. The glucose solution is separated from the metallic sensor by a layer of FR4 epoxy and ABS plastic. In the Discussion or Conclusion section, the authors should mention the obvious possibility of placing the liquid container directly on the surface of a non-metallized substrate using a bottomless plastic cell. This will bring the liquid closer to the sensor and may further improve the device's performance.

Author’s Response: We thank the reviewer for this excellent and constructive suggestion. We agree that reducing the distance between the water–glucose solution and the sensing region is a physically meaningful strategy to further enhance the interaction with the concentrated electric field and, consequently, to improve the sensor performance.

Author’s Action: This recommendation was incorporated into the Conclusion section as a direction for future improvement. The revised manuscript now states: “As a point of future improvement to maximize the device's performance, replacing the solid-base ABS container with a bottomless plastic cell is highly recommended. By placing this bottomless cell directly on the surface of the non-metallized FR4 substrate, the water–glucose solution will be brought into direct physical contact with the dielectric region of greatest electric field concentration (the DGS gap), eliminating the intermediate ABS layer and potentially increasing the overall sensitivity of the sensor.”

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

• Table 5: Remove the authors and title columns, and replace them with reference numbers (e.g., [2], [3], etc.).
• Add equation numbers to properly label all equations.
• Include references for the equations.
• Specify the impedance used to feed the sensors.
  What is the advantage of using DGS technic rather than meander line [R2] to miniture the proposed sensors 
• [R2] A. Lamkaddem, A. E. Yousfi, V. González-Posadas and D. Segovia-Vargas, "Miniaturized Dual Band Implantable Antenna for Implanted Biomedical Devices," in IEEE Access, vol. 12, pp. 15026-15036, 2024, doi: 10.1109/ACCESS.2024.3357488.
• Provide more details about the simulation scenario in Ansys.
• What is the advantage of using the proposed sensor rather than an implantable antenna [R1]?
  [R1] A. Lamkaddem, A. El Yousfi, Y. Huang, V. González Posadas and D. Segovia-Vargas, "Miniaturized Wireless Power Transfer System With a Dual-Band Implantable Antenna for Multifunctional Biomedical Devices," in IEEE Sensors Journal, vol. 25, no. 16, pp. 31473-31488, 15 Aug.15, 2025, doi: 10.1109/JSEN.2025.3587334.
• What is the permittivity of the plastic used?

Author Response

Thank you very much for taking the time to review this manuscript. We have addressed the comments point by point, and the entire manuscript has been thoroughly revised to improve the English, grammar, and overall clarity. Please find our detailed responses below, and the corresponding revisions have been highlighted using track changes in the resubmitted files.

1. Table 5: Remove the authors and title columns, and replace them with reference numbers (e.g., [2], [3], etc.).

Author’s Response: We thank the reviewer for this helpful suggestion and agree that this modification makes Table 5 more concise, clearer, and easier to read.

Author’s Action: Table 5 was revised accordingly. The “Authors” and “Title” columns were removed, and the studies are now identified in the first column by their corresponding reference numbers in square brackets.

2. Add equation numbers to properly label all equations.

Author’s Response: We thank the reviewer for pointing out this issue and apologize for the oversight in the original submission. We agree that non-sequential equation numbering may hinder the readability and interpretation of the manuscript.

Author’s Action: All equations in the revised manuscript have been renumbered to follow a consistent sequential order throughout the text.

3. Include references for the equations.

Author’s Response: We thank the reviewer for this observation and agree that the equations should be properly supported by references.

Author’s Action: The relevant equations were revised to include the appropriate citations. In particular, the expressions related to the S-parameters, their decibel representation, and the transmitted and reflected power calculations are now accompanied by the corresponding reference to Pozar, D.M., Microwave Engineering.

4. Specify the impedance used to feed the sensors.

Author’s Response: We thank the reviewer for this important observation and agree that the feeding impedance should be stated explicitly in the manuscript.

Author’s Action: A clarification was added in Section 2.2 specifying that the sensor is fed through asymmetric feed lines with a characteristic impedance of 50 Ω.

5. What is the advantage of using DGS technic rather than meander line [R2] to miniature the proposed sensors? ([R2] Lamkaddem et al., IEEE Access 2024).

Author’s Response: We thank the reviewer for this relevant observation. In order to clarify the advantage of using the Defected Ground Structure (DGS) technique instead of other miniaturization approaches, such as meander lines, we added an explicit discussion in the Introduction section of the revised manuscript.

Author’s Action: The following explanation was incorporated into Section 1: “Furthermore, when compared with other miniaturization techniques, such as meander lines [13], the Defected Ground Structure (DGS) offers the advantage of modifying the equivalent capacitance and inductance of the circuit without significantly increasing the geometric complexity of the top metallic layer. For sensing applications, this approach helps reduce undesired radiation losses while promoting a stronger concentration of the electric field in the defect region, which is essential for maximizing the interaction with the test samples and, consequently, enhancing sensitivity.”

6. Provide more details about the simulation scenario in

Author’s Response: We thank the reviewer for this important observation and agree that the description of the simulation scenario in ANSYS should be more detailed in order to improve reproducibility and technical clarity.

Author’s Action: The simulation description in the revised manuscript was expanded accordingly. We now specify that the proposed structure was modeled in ANSYS® Electronics Desktop using the High-Frequency Structure Simulator (HFSS) with a driven modal solution setup. The sensor was enclosed within an air-filled radiation boundary box extending at least λ/4 from the radiating elements to minimize artificial reflections. Excitation was applied through 50 Ω lumped ports assigned to the feed lines, and an interpolating frequency sweep from 1 GHz to 6 GHz was defined to accurately identify the resonance peaks. The material parameters used in the simulations were also stated explicitly: the FR4 epoxy substrate was modeled with relative permittivity εr = 4.4 and a thickness of 1.6 mm, while the metallization was defined as copper with a thickness of 0.035 mm.

7. What is the advantage of using the proposed sensor rather than an implantable antenna [R1]? ([R1] Lamkaddem et al., IEEE Sensors Journal 2025).

Author’s Response: We thank the reviewer for this relevant observation. To clarify the advantage of the proposed sensor over implantable antenna-based approaches, we added an explicit discussion in the Introduction section of the revised manuscript.

Author’s Action: The following text was incorporated into the Introduction: “Although recent advances in implantable antennas and miniaturized wireless systems [11] have demonstrated strong potential for continuous in vivo biomedical monitoring, they inherently require surgical intervention. This invasive procedure involves risks such as infection, biocompatibility issues, and physical discomfort. In contrast, the proposed DGS resonator provides a completely non-invasive alternative, enabling robust glucose detection without the need for surgical implantation. This approach is considerably safer and more comfortable, thereby improving patient adherence to continuous monitoring.”

8. What is the permittivity of the plastic used?

Author’s Response: We thank the reviewer for this observation and agree that the dielectric properties of the plastic should be stated explicitly.

Author’s Action: In addition, the revised manuscript now specifies the properties adopted for the ABS plastic container used in the simulations, namely a relative permittivity εr of 2.8 and a dielectric loss tangent (tan δ) of 0.0053.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have addressed all my comments.