Influence of the Polarizing Magnetic Field and Volume Fraction of Nanoparticles in a Ferrofluid on the Specific Absorption Rate (SAR) in the Microwave Range
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsReview report
I have gone through the manuscript titled “Influence of the polarizing magnetic field and volume fraction of nanoparticles in a ferrofluid on the specific absorption rate (SAR) in the microwave range” by Malaescu et al.
This is an interesting and important study that investigates the magnetic loss properties of a ferrofluid under the influence of both an alternating microwave field and a static, polarizing magnetic field. The work on establishing limits for efficient heating and proposing new composite-system equations for SAR/HReff is noteworthy. However, the manuscript needs to be strengthened in many aspects and hence, a major review is suggested with respect to the following points.
1) The paper's novelty hinges on treating the ferrofluid as a composite system and proposing "new equations" (Eqs. 1-3). A brief section should be added to explicitly compare the results (e.g., values) from the authors' proposed equations against those calculated using the classical formula, which typically assumes all loss is concentrated in the magnetic solid phase for hyperthermia. This comparison is essential to demonstrate the magnitude of the improvement offered by the new composite-system approach.
2) The abstract and introduction mention diverse applications like MRI, mobile phones, and electromagnetic shielding. Given the detailed focus on SAR/ILP, which is most relevant to hyperthermia, the authors should consider streamlining the discussion.
3) The manuscript effectively uses the FMR phenomenon. However, a more explicit discussion is needed to confirm that Neel and Brownian relaxation losses are negligible in the microwave range compared to the FMR loss for the given particle size (10.47 nm).
4) Why are the proposed equations (1-3) considered "new"? What is the historical context of existing SAR/HReff models for composite ferrofluid systems, and how do the proposed equations fundamentally improve upon them?
5) The specific heat and density values are essential for the quantitative determination of HReff and SAR. Suggestion: Explicitly state the numerical values (and their source/reference) for the specific heat capacities of kerosene and magnetite used in Equations (1) and (3) in the Introduction or Methods section.
6) The dilution ratio used to prepare samples A1, A3, and A5 is 2/3. Question: Is this the ratio of or volume of ferrofluid to volume of kerosene or volume of new sample to volume of previous sample? Please clarify the precise meaning of the ratio to ensure reproducibility of the sample concentrations.
7) What is the specific surfactant used to coat the magnetite nanoparticles and stabilize them in kerosene?
8) The authors should provide a Transmission Electron Microscopy (TEM) image and the resulting physical core diameter distribution to complement the magnetic data, which is typical for Magnetochemistry publications.
9) A fixed nonmagnetic layer thickness of ~ 2 nm is used in the magnetic volume fraction calculation (Eq. 7). What is the source or experimental justification (e.g., TGA data, dynamic light scattering) for assuming this constant value across all samples?
10) The polarizing field is described as perpendicular to the coaxial cell axis (transverse geometry). Does this transverse geometry choice, as opposed to a parallel geometry, affect the standard Kittel-type resonance condition (Eq. 8)? Please briefly comment on the choice of geometry and any known effects on FMR in ferrofluids.
11) What is the corresponding RF power applied to the coaxial line that generates H0 = 5A/m inside the sample cell? This detail is crucial for assessing the uniformity and measurement environment.
12) The effective anisotropy constant decreases significantly with dilution. Please provide a more detailed physical explanation for this dependence. Does this decrease primarily indicate a reduction in dipolar magnetic interactions (chains/clusters) as the inter-particle distance increases?
13) For certain frequency ranges, the SAR increases H with while the ILP decreases with H. Can the authors explicitly explain this counter-intuitive behavior?
14) The SAR and ILP values drop for fields H > Hmax (Figure 7). Is this decrease primarily due to the magnetic moment being fully polarized (saturation of susceptibility) or the FMR peak being shifted to a frequency higher than the measurement range?
15) What is the specific role or theoretical justification for plotting SAR (dominated by FMR) against the anisotropy parameter (sigma)?
16) What is the physical mechanism (e.g., strong clustering, formation of magnetic chains, or increased damping) that causes the SAR/ILP to plateau at high volume fractions (Figure 8)?
17) Please detail the measurement setup. What kind of thermometer (e.g., fiber optic) was used?
18) How was the frequency of 4 GHz optimized?
19) What is the physical significance of minimum volume fraction?
20) The first two paragraphs of the Conclusion largely restate the methodology and characterization results. The authors should rewrite the conclusion to focus on a powerful summary of the key quantitative findings and their implications.
21) The references should be checked for consistent formatting according to MDPI guidelines.
Author Response
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Reviewer 2 Report
Comments and Suggestions for AuthorsMy suggestions are in the attached file.
Comments for author File: Comments.pdf
Author Response
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Reviewer 3 Report
Comments and Suggestions for AuthorsMagnetic hyperthermia is a new method of treating cancer of difficult–to-treat areas by injecting biocompatible non-toxic magnetic nanoparticles into the tumor, then heating them with an external electromagnetic field, leading to an increase in the temperature of tumor cells to +42 ° C and their subsequent death.
Traditional methods of cancer treatment are chemotherapy and radiation therapy, but the attention of scientists and doctors is focused on finding alternative methods that would be less dangerous to the patient's health. Thus, the hyperthermia method is considered as an alternative to the treatment of the last stages of cancer or an addition to traditional treatment, in which individual organs or parts of an organ affected by a pathological process are exposed to high temperature. The therapeutic effect of hyperthermia is due to the difference in the response to heat exposure between two types of tissues– healthy and tumor. Unlike healthy tissue, the affected one has large intercellular spaces, a high density of blood vessels and poorly developed lymph nodes. The thermal effect on such tissue leads to damage and further cell death (apoptosis), while healthy cells remain unaffected. The therapeutic effect of hyperthermia is limited by the temperature range from +38 ° C to 46 ° C. At a temperature of +38 ° C, active proliferation of tumor cells is observed, at +39 ° C, viability decreases, and at temperatures above +43 ° C, their death is observed. Hyperthermia can also reduce the size of the tumor to operable states.
- in the case of the use of magnetic nanoparticles in the magnetic hyperthermia method, the Brezovich criterion must be observed, which is obviously not fulfilled for the frequency and amplitude ranges under consideration. Would such an impact be harmful to health?
2. The authors consider superparamagnetic particles. in this case, how do they assess the effect of the blocking temperature on the heating properties? how do the Curie temperature and the blocking temperature relate for the particles under consideration? Moreover, is there a dependence of the blocking and Curie temperatures on the average diameter of the nanoparticles?
3. It is advisable for the authors to cite other works that consider alternative formulations for use in the magnetic hyperthermia method, for example, 10.1007/s10971-020-05237-8
4. It is advisable to add a part of the nanoparticle cooling curve in Fig. 11.
5. It is also advisable to compare the heating of the particles under consideration with the best samples, for example, on the contrary, superparamagnetic particles can generate significant amounts of heat at low magnitudes of the magnetic field. Thus, the best samples of ferrofluids are characterized by a SAR of ~ 209 W/g in fields with an amplitude of 14 kA/m and a frequency of 300 kHz, while for a ferromagnetic magnetite sample under the same conditions, the SAR does not exceed 75 W/g
Author Response
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Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors have provided suitable responses to the review comments and have made adequate corrections and revisions to the manuscript. I am happy to recommend the manuscript for publication.
Author Response
Thank you very much for your relevant observations and suggestions.
Reviewer 2 Report
Comments and Suggestions for AuthorsAuthors have taken into account some of my remarks. However, the main pretension is the same as in the previous review. Authors declare that the main motivation of their work is in biomedical applications of magnetic nanoparticles, and, in part, magnetic hyperthermia. However there are strict physiologically determined restrictions on the parameters of the applied magnetic field: the frequency must be in the frames of 1-1.5MHz; the field strength in the frames of 15-20 kA/m . At the same time the main results of authors correspond to the frequency of several GHz; the field strength - several tens, even more than 100 kA/m. This is for beyond of the physiological limits for the field. Authors must either comments this contradiction or to change the application motivation of this work.
Author Response
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Reviewer 3 Report
Comments and Suggestions for Authorsall my comments have been addressed
Round 3
Reviewer 2 Report
Comments and Suggestions for Authors1) I have to repeat that authors discuss magnetic hyperthermia (MH) as one of the main motivations of this work. However, considered (strength and frequency) of the field, are far beyond physiologically determined frames of MH. Thus the results can not be used for development of scientific background of the MH therapy. This point of the work must be reconsidered.
2) Surprisingly why authors call eq. (13) as the classical one. First, this equation is absent in the cited work of R.Rosensweig. Secondly, this equation seems physically incorrect, because the particle density is multiplied to the specific heat capacity of the surrounding liquid.
3) Authors claim that the equations (1-3) are new. However they are usual equations for the energy conservation under the assumption that temperature of the particles equals to the liquid temperature. Note that in the condition of the heat generation inside the particles this claim is not obvious and must be justified.
The general conclusion. The revision is necessary
Author Response
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Round 4
Reviewer 2 Report
Comments and Suggestions for Authors1) Authors ignore without any comments and explanations that the field strength and frequency are far beyond biologically determined frames acceptable for magnetic hyperthermia. Therefore, oppositely the authors statement that development of scientific background of magnetic hyperthermia is the main motivation of their work, the obtained results can not be applied tho this therapy process.
2) Unlike the authors statement, eq. (14) has not been used in the classical work [41] of R.Rosensweig.
3) Eqs. (13-15) suppose that temperature of the particles in ferrofluid equals to temperature if the carrier liquid; temperature of the liquid is homogeneous inside and does not depend on the distance from a particle. But in the condition of the particles heating it can be so only approximately. This statement must be justified based on the estimates of the rate of the particles heating and the heat transfer inside the liquid; any justification of that is absent.
5) Equation (4) for p_m is valid only if the particle magnetisation linearly depends on the magnetic field H. However, authors present their results in the range H>100kA/m, where, following Fig.1, the linear law can not be used.
4)Equation (15) allows to estimate change of the mean temperature T inside the ferrofluid at the known SAR (under assumption that this temperature is homogeneous inside the system, what is not obvious). However equations, used for calculations of SAR from the internal remagnetisation mechanisms inside the particles, are absent in the paper. Therefore, this is impossible to understand if the calculations are physically adequate.
I can not recommend this paper for publication in this form.
Taking into account that the proposed main motivation of this work does not correspond to its content; the absence of justification the main energetic equations; absence of the main equation of the particle remagnetisation, I recommend to reject this paper.
Comments on the Quality of English LanguageEnglish is suitable
Author Response
Thank you for your suggestions.
