Injectability of Thermosensitive, Low-Concentrated Chitosan Colloids as Flow Phenomenon through the Capillary under High Shear Rate Conditions

Round 1

Reviewer 1 Report

The manuscript provides an experimental view of the injectability process of thermosensitive chitosan gels. The overall contents were generally appropriate for the subject but some important information is missing. Furthermore, the manuscript is lengthy and some parts can be significantly reorganized/shortened. However, there are numerous issues in the present manuscript that need to be addressed before publication and English native review is strongly advisable.

Below are specific comments:

• The abstract is poorly written. The abstract should include the methods and comparison clearly.

• The graphical abstract partially elucidates comprehension.

  • Example: Uman, Selen, Abhishek Dhand, and Jason A. Burdick. "Recent advances in shear‐thinning and self‐healing hydrogels for biomedical applications." Journal of Applied Polymer Science 137.25 (2020): 48668.
• Line 463: you mentioned "comprehensive theoretical and experimental", the presented work in terms of theoretical work is not comprehensive at all and needs huge improvement in the both introduction and body of the paper.
• The authors should elaborate/discuss the difference in mathematical modeling in the introduction. Also, there is a rich body of literature on the bioinspired designs area but the relevant discussion is lacking in the current manuscript. For example:
  • Example: Blacklow, S. O., et al. "Bioinspired mechanically active adhesive dressings to accelerate wound closure." Science advances 5.7 (2019): eaaw3963.
• Line 61: Please rephrase 'requirements...desired characteristics...'
• Lines 37 to 46 are too basic and lengthy.
• Line 304 - 321: These sections were too lengthy and should be condensed significantly.

General comments:

As a suggestion, please always add some schematics graphs to your article. That will help the readers to understand the concept better.

Author Response

Dear Reviewer,

Thank you very much for your review of our manuscript and valuable comments.We hope that our changes will improve the quality of our manuscript.Please find below our answers to your questions.

Point 1: The abstract is poorly written. The abstract should include the methods and comparison clearly.

Response 1: In line with your suggestion, we have improved the abstract focusing more on the research methodology.

Point 2:The graphical abstract partially elucidates comprehension.

Response 2:We have added a graphical abstract

Point 3: Line 463: you mentioned "comprehensive theoretical and experimental", the presented work in terms of theoretical work is not comprehensive at all and needs huge improvement in the both introduction and body of the paper.

Response 3: We humbly accept and appreciate the reviewer’s feedback.Nevertheless, we would like to summarize the theoretical analyzes that have been carried out and which have not yet been considered in the case of thermosensitive chitosan hydrogels.

•Theoretical description of the experimental data in the widest range of injection needles so far, with particular emphasis on high shear rates conditions (outside the measurement range of rotational rheometer).In our considerations, we took into account the second area of shear thinning, thus indicating a possible range of forces occurring during the injection.

•Theoretical discussion of the greater sensitivity of small needles to the applied crosshead speed.

•Discussion of the non-obvious phenomenon of easier application of scaffolds characterized by LCST, which are stored at room temperature.

•Theoretical discussion of the negligible in the literature but possible influence of the viscoelastic properties of the experimental material and the conscious proving that in the case of the tested systems it is not necessary to take into account the elastic properties (Cox-Merz rule).

•Analysis of the influence of high shear rates on the change of the rheological properties of the medium (thixotropic tests), which must be consider because they may lead to changes in the architecture of the experimental medium leading to their difficult recovery and, consequently, different gelation conditions.
At the same time, we have made efforts to improve the quality of the manuscript.

Point 4:The authors should elaborate/discuss the difference in mathematical modeling in the introduction. Also, there is a rich body of literature on the bioinspired designs area but the relevant discussion is lacking in the current manuscript.

Response 4: Thank you very much for pointing out interesting studies.However, we would like to emphasize that the aim of the project was to determine the possibility of injecting low-concentrated colloidal chitosan systems through hypodermic needles.Although these systems are very often studied both to explain the mechanism of the phase transition as well as their application potential, and they are commonly called injectable, there are almost no instrumental studies that would quantify their clinical usefulness.The only studies that comprehensively compare experimental injectability tests with theoretically determined force values are discussed.We have described the procedure that ensured the authors good agreement of both quantities.Most of the remaining published studies only use the theoretical basis of capillary rheometry of non-Newtonian fluids to determine flow conditions in the needle, mainly shear rate values, to further determine viscosity values.This information was added into the manuscript(lines 104-108).

Point 5:Line 61: Please rephrase 'requirements...desired characteristics...'

Response 5:Thank you for the correction, which we have made.

Point 6: Lines 37 to 46 are too basic and lengthy

Response 6:We shortened and reorganized the indicated part.

Point 7:Line 304 -321: These sections were too lengthy and should be condensed significantly.

Response 7:We shortened and reorganized the indicated part.We also eliminated part of the description by introducing equation 7.

Point 8:As a suggestion, please always add some schematics graphs to your article. That will help the readers to understand the concept better.

Response 8:Thank you very much for your suggestion.In the introduction section, we have added a schematic drawing of the concept of thermosensitive injectable hydrogels(Figure 1).

Reviewer 2 Report

This manuscript studies the flow phenomenon of chitosan colloids with thermo-sensibility and low concentration under high shear rate conditions. With analysis of the needle diameter, length and injection speed, the predicted injection force values are in good agreement with the experimental data. I suggest this manuscript to be accepted on Polymers after the following questions being solved.

1. As for invasive injectable scaffolds, what about the biocompatibility, cytotoxicity as well as biodegradablity of this system?
2. What about the effect of chitosan colloids’ diameter on the flow performance?

Author Response

Dear Reviewer,

Thank you very much for your review of our manuscript and its positive evaluation. We hope that our changes will improve the quality of our manuscript and will not change your opinion.Please find below our answers to your questions.

Point 1: As for invasive injectable scaffolds, what about the biocompatibility, cytotoxicity as well as biodegradablity of this system?

Response 1: Biodegradability, cytotoxicity, and other biological studies for chitosan systems are often conducted in the form of both cell culture and laboratory tests on animals.We have supplemented the missing information in the introduction and cited selected publications.Please look at lines 61-63.

Point 2:What about the effect of chitosan colloids’ diameter on the flow performance?

Response 2:The aim of the project was to determine the possibility of injecting low-concentrated colloidal chitosan systems through hypodermic needles. The considered systems were characterized by the lower critical solution temperature above room temperature at which the injectability tests were carried out. Under these conditions, precipitation of the lyophobic form of the polymer from the solution occurred, but the temperature is too low for the formation of stable nuclei whose dimension is larger than the critical size. Therefore, it is not possible to produce large aggregates or a lattice, the phenomenon of fast gelation does not take place. The material consists of suspended embryos and is equivalent to a fluid gel [1]. Since the internal diameters of the needles used were larger than 0.26 mm, the authors assumed that this dimension was much larger than that of the embryos suspended in a continuous phase. The study shows that assumption turned out to be correct. There was no sudden change (increase) in DGF value that would indicate blocked flow in the needle caused by an increase in chitosan aggregates to values comparable to the inner diameter of the needle.

[1] Fernández Farrés, I.; Moakes, R.J.A.; Norton, I.T. Designing biopolymer fluid gels: A microstructural approach. Food Hydrocolloids 2014, 42, 362–372