Mechanical Properties of Differently Nanostructured and High-Pressure Compressed Hydroxyapatite-Based Materials for Bone Tissue Regeneration

Round 1

Reviewer 1 Report

The article "Mechanical properties of differently nanostructured and high-pressure compressed hydroxyapatite-based materials for bone 3
tissue regeneration" is devoted to synthesis of hydroxyapatite (HAp) by solid state synthesis (SHAp), sonochemical synthesis (EHAp) and ultrasonication (CHAp) and analysis of its mechanical properties. To my mind the level of study as well as its topic is suitable for Minerals journal. But there are some issues that can be improved.

1) the chapter 2.2 "Synthesis of Hydroxyapatite (HAp) nanoparticles" confuses me. It seems that two paragraphs (lines 106-119 and lines 120-130) tells about the same but contradict each other. So this chapter should be corrected

2) Is the Vickers hardness tests the same as Vickers indentation method? If no, the link to Vickers hardness tests explanation should be added

3) At the lines 219-220 it can be seen that sizes of nanoparticles are EHAP<CHAP. But in the table 1 crystallite sizes are CHAP<EHAP. Is it correct?

4) In the conclusion the application of considered structures should be discussed.

Author Response

Dear Sir/Madam,

Please find the attached word file of Reviewer 1 response attached herewith. Kindly do the needful.

Thank you

Kind regards

Author Response File: Author Response.doc

Reviewer 2 Report

The manuscript presents the fabrication and characterization of nanocrystalline hydroxyapatite-based bulk samples, varying parameters such as the precursors, the synthesis method and the compacting pressure. The authors conduct a detailed microstructural and mechanical characterization to draw some conclusions on the best precursor and best process to obtain optimal performance, especially in terms of fracture toughness, which is known to be the weak point oh HAp materials.

However, the scope of the manuscript is not clear, nor is the innovative aspect of their research, as compared to the extensive literature available on the topic. The specific application is also not clear, other than a general bone regeneration field: HAp is normally proposed as a coating, and the use as a bulk material is not clear.

For any of such application, moreover, a proper toughness characterization would be needed, as the one measured by indentation is, as the authors point out, semi-quantitative at the best.

Finally, the conclusions are clear but limited to a description of the results without indication of the novelty and of the impact of the research. In my opinion this work needs to be restructured in its scope and an actual target application should be defined

Author Response

Dear Sir/Madam,

Please find the attached word file of Reviewer 2 response attached herewith. Kindly do the needful.

Thank you

Kind regards

Author Response File: Author Response.doc

Reviewer 3 Report

Manuscript ID No. minerals-1453596

Title: Mechanical properties of differently nanostructured and high-pressure compressed hydroxyapatite-based materials for bone tissue regeneration

This work is about

“The brittle nature of hydroxyapatite (HAp) biomaterial, constrained by its low fracture 11 toughness (f up to 1.2 vs. 2–12 MPam1/2 of human bone), remains one of the significant factors impairments to their use in bone regeneration. In the present study, HAp nanoparticles synthesized by the solid-state (SHAp) and sonochemical (EHAp) approaches using eggshell-derived calcium hydroxide and ammonium dihydrogen orthophosphate as precursors are compared with those synthesized using commercially available calcium hydroxide and ammonium dihydrogen orthophosphate as precursors (CHAp) employing sonochemical method. The HAp samples were then compressed into compact materials using a uni-axial high-pressure compression technique at a pre-optimized load and mechanically characterized using the Vickers indentation method and compressive strength testing. The analysis showed that more minor (30-40 nm) and more crystalline EHAp and CHAp resulted in mechanically more robust materials (σm= 54.53 MPa and47.72 MPa) with higher elastic modulus (E= 4011.1 MPa and2750.25 MPa) and density/hardness-dependent fracture toughness (σf=4.34 MPa m1/2and 6.57 MPa m1/2) than those prepared from relatively more significant, but heterogeneously nanostructured SHAp (σm=28.40 MPa, E=2116.75 MPa, σf=5.39 MPa m1/2). The CHAp material was found to be the most suitable for applications in bone regeneration.”

I recommend the following suggestion and comments before the acceptance of the manuscript.

Title & Abstract:

• In the interesting paper, however, no single assay was carried out to prove the Bone Tissue Regeneration potential of the material and this part should be discussed in the last section as a shortcoming of this paper.

Introduction:

• A manuscript is started with the title “Mechanical properties of differently nanostructured and high-pressure compressed hydroxyapatite-based materials for bone tissue regeneration” no information on bone regeneration, and natural bone physiology in this part. Adding the said information may impact paper publication.
• More information on hydroxyapatite is required as you have reported the synthesis of hydroxyapatite including biochemical structure and bone regeneration approach.

Materials and methods:

This section is well written.

Results:

• Figure quality (magnification, font size, and style, etc.) must be maintained, especially Figures 1, 2, 3 & 4 as they have been differently described.
• The author claimed the “uniformly relatively agglomerated spherical nanoparticles”, but I can’t see the uniformity of agglomeration of nanoparticles.
• The author also claimed the nanostructure system and to prove nanoparticle synthesis TEM analysis may be required to enhance publication impact.

Results:

• The discussion section is well written explaining mechanical findings through different angles; however, the shortcoming of the paper is that there is no test done to perform the biocompatibility and bone regeneration potential of the prepared nanostructured hydroxyapatite material.

Conclusion:

• The study conclusion is very lengthy. It needs to be completely rewritten again to summarize everything in two paragraphs.

References:

• Only very few references are available in the last five years, it is recommended to include the latest reference in last 3/5 years to put an impression on readers a novel study.
• It is recommended to remove old references to improve manuscript quality and research novelty.
• Try to add more references in the discussion with recent citations.

English editing:

• The manuscript requires English editing (typo errors, spelling and grammar, etc.) by a professional English native as the manuscript contains several English issues.

Comments for author File: Comments.pdf

Author Response

Dear Sir/Madam,

Please find the attached word file of Reviewer 3 response attached herewith. Kindly do the needful.

Thank you

Kind regards

Author Response File: Author Response.doc

Round 2

Reviewer 2 Report

The authors responded satisfactorily to the points risen

Reviewer 3 Report

The author have revised the manuscript as per suggestions and now the manuscript in a position for publications.