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arxiv: 2604.13339 · v1 · submitted 2026-04-14 · ❄️ cond-mat.mtrl-sci · cond-mat.mes-hall

Uncovering the role of ionic doping in hydroxyapatite: The building blocks of tooth enamel and bones

Mahdi Tavakol , Jinke Chang , Cyril Besnard , Gabriel Landini , Richard M. Shelton , Jin-Chong Tan , Alexander M. Korsunsky This is my paper

Pith reviewed 2026-05-10 14:23 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci cond-mat.mes-hall
keywords hydroxyapatiteionic dopingmolecular dynamicsthermodynamic integrationchemical stabilitytooth enamelmagnesiumdental materials
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The pith

Magnesium ion doping at surface sites of hydroxyapatite increases its resistance to acid dissolution more than other ions tested.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper develops a molecular dynamics framework to examine how different ions affect the stability of hydroxyapatite, the main mineral in teeth and bones. It shows that doping works best when ions replace atoms on the material surface rather than in the interior. Thermodynamic integration calculations single out magnesium as the ion that most improves chemical stability against acidic attack, the main cause of modern tooth decay. This matters because hydroxyapatite must now withstand bacterial acids far more than mechanical crushing from food. The results give practical rules for making stronger HAp-based coatings, remineralization agents, and restorative materials.

Core claim

The authors establish that surface doping is the most viable route for ion incorporation into hydroxyapatite, and that thermodynamic integration within their steered molecular dynamics protocol identifies magnesium as the dopant that yields the greatest gain in chemical stability against dissolution, while mechanical compression tests confirm that the doped structures retain load-bearing capacity.

What carries the argument

A combined molecular dynamics framework that uses conventional MD, steered MD to probe surface exchange, thermodynamic integration to compute doping free energies, and uniaxial compression tests to assess mechanical response.

If this is right

  • Doping efforts should concentrate on surface sites rather than bulk substitution to maximize stability gains.
  • Magnesium emerges as the preferred dopant among those examined for resisting chemical attack.
  • The simulation protocol can screen additional ions or combinations to tailor hydroxyapatite for specific dental uses.
  • Improved acid resistance in doped hydroxyapatite could reduce the rate of enamel loss in caries-prone environments.
  • Mechanical strength remains intact under the doping conditions studied, supporting use in load-bearing applications.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same surface-doping preference may apply to other calcium-phosphate minerals in biological hard tissues.
  • Coatings for bone implants could be designed by selecting magnesium or similar ions to slow degradation in physiological fluids.
  • Remineralization products for early caries lesions might incorporate magnesium to stabilize newly formed hydroxyapatite layers.

Load-bearing premise

The classical force fields and simulation protocols accurately capture the real atomic-scale energies of ion doping and dissolution without experimental calibration or higher-level quantum validation.

What would settle it

An experiment that measures the dissolution rate of magnesium-doped hydroxyapatite crystals in dilute acid and finds no reduction compared with undoped crystals would falsify the claim that magnesium enhances chemical stability.

read the original abstract

Hydroxyapatite (HAp) is the primary mineral component of various mineralized tissues in the human body, including bone and teeth, where it performs critical roles of structural support and load transmission. In the context of dental health, the two most crucial properties of HAp are mechanical stability, which ensures resistance to forces, and chemical stability, which preserves surface integrity in acidic environments. During early stages of human evolution, e.g. when teeth were used to crush uncooked food, mechanical stability was of paramount importance. However, with changes in diet and lifestyle, the principal origins of tooth damage and loss shifted towards bacterially mediated chemical attack, known as tooth decay, or caries. To enhance the chemical stability, ion doping has emerged as a particularly significant approach, and it lies at the focus of the present study. A Molecular Dynamics (MD) framework was developed to investigate the effects of ion doping on the chemical and mechanical stability of HAp and to identify optimal doping candidates. The framework combines conventional MD with Steered Molecular Dynamics (SMD), Thermodynamic Integration (TI) and uniaxial compression test simulations to provide comprehensive insights into the doping process. The findings revealed surface atoms as the most viable candidates for doping, as demonstrated by SMD and conventional MD simulations. Notably, TI calculations have identified magnesium ions as a better candidate among the ions considered here for enhancing the chemical stability of HAp. The results presented in this study offer valuable guidelines for synthesizing HAp-based substituent materials with properties tailored to meet the demands of modern dental applications such as implant coatings, enamel remineralization agents and restorative materials.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 2 minor

Summary. The manuscript develops a molecular dynamics framework that combines conventional MD, steered MD (SMD), thermodynamic integration (TI), and uniaxial compression simulations to examine ionic doping in hydroxyapatite (HAp). It concludes that surface atoms are the preferred doping sites and that TI calculations identify magnesium ions as superior to other ions considered for improving the chemical stability of HAp, with the results intended to guide synthesis of HAp-based materials for dental applications.

Significance. If the reported TI ranking of dopant ions proves robust, the work could supply practical guidelines for selecting dopants that enhance acid resistance in enamel and bone substitutes. The integration of multiple simulation techniques (SMD for site identification, TI for free-energy differences, and mechanical testing) is a methodological strength that addresses both chemical and mechanical properties. However, the absence of any numerical free-energy values, error estimates, or validation against quantum or experimental data substantially limits the current impact and generalizability of the magnesium claim.

major comments (3)
  1. [Abstract] Abstract: The central claim that 'TI calculations have identified magnesium ions as a better candidate among the ions considered here for enhancing the chemical stability of HAp' is presented without any reported free-energy differences, standard errors, reference states, or explicit comparison among the ions (e.g., Mg versus Sr or other candidates), preventing assessment of the magnitude or statistical reliability of the stability gain.
  2. [Methods] Methods (simulation protocols): No details are supplied on the classical force-field parameters, partial charges, cutoff distances, or system sizes used for the TI calculations of ion substitution and dissolution energetics; without these, it is impossible to judge whether the classical potential can correctly rank Mg against Ca or other ions at the HAp surface.
  3. [Abstract] Abstract and results: The manuscript supplies no comparison of the computed doping energetics or dissolution barriers to existing DFT calculations or experimental solubility measurements on Mg-doped HAp, leaving the ordering of chemical stability unanchored to physical reality.
minor comments (2)
  1. [Abstract] The abstract would be strengthened by inclusion of at least one key quantitative result (e.g., a relative free-energy value or stability metric) to support the magnesium claim.
  2. Ensure that all acronyms (SMD, TI) are defined at first use and that the uniaxial compression protocol specifies strain rate and box dimensions for reproducibility.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their constructive and detailed comments on our manuscript. These have helped us identify areas where the presentation can be improved for clarity and completeness. We address each major comment point by point below, indicating the revisions we will make to the manuscript.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The central claim that 'TI calculations have identified magnesium ions as a better candidate among the ions considered here for enhancing the chemical stability of HAp' is presented without any reported free-energy differences, standard errors, reference states, or explicit comparison among the ions (e.g., Mg versus Sr or other candidates), preventing assessment of the magnitude or statistical reliability of the stability gain.

    Authors: We agree that the abstract would be strengthened by including quantitative details from the TI calculations. In the revised manuscript, we will update the abstract to report the key free-energy differences for magnesium relative to the other ions considered, along with associated standard errors and reference to the dissolution process. The full TI profiles, comparisons, and error estimates are already provided in the Results section with accompanying figures. revision: yes

  2. Referee: [Methods] Methods (simulation protocols): No details are supplied on the classical force-field parameters, partial charges, cutoff distances, or system sizes used for the TI calculations of ion substitution and dissolution energetics; without these, it is impossible to judge whether the classical potential can correctly rank Mg against Ca or other ions at the HAp surface.

    Authors: We acknowledge that additional methodological details would aid reproducibility and evaluation. In the revised manuscript, we will expand the Methods section to explicitly list the classical force-field parameters (including the specific potential for HAp and ions), partial charges, non-bonded cutoff distances, and system sizes employed in the TI calculations for ion substitution and dissolution. revision: yes

  3. Referee: [Abstract] Abstract and results: The manuscript supplies no comparison of the computed doping energetics or dissolution barriers to existing DFT calculations or experimental solubility measurements on Mg-doped HAp, leaving the ordering of chemical stability unanchored to physical reality.

    Authors: We agree that linking our results to prior work would enhance context. In the revised manuscript, we will add a discussion paragraph that qualitatively compares our computed stability trends for Mg doping to published experimental solubility data on Mg-doped hydroxyapatite and relevant DFT studies on ion substitution energetics in HAp, while noting the limitations of classical MD. A fully quantitative validation would require new calculations outside the present scope. revision: partial

Circularity Check

0 steps flagged

No circularity: TI free-energy results are independent computational outputs

full rationale

The paper's central claim rests on standard MD, SMD, and TI simulations that compute doping energetics and stability metrics from force-field dynamics. No equations, fitted parameters, or self-citations are shown that define the target stability ordering in terms of itself or reduce the Mg ranking to a quantity fitted from the same data. The derivation chain is self-contained: the simulations generate the reported free-energy differences rather than presupposing them. This is the normal non-circular case for physics-based modeling papers.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract provides no explicit free parameters, axioms, or new entities; all details on force fields, cutoffs, or system sizes are absent.

pith-pipeline@v0.9.0 · 5625 in / 1084 out tokens · 25685 ms · 2026-05-10T14:23:19.851306+00:00 · methodology

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

  1. An Efficient Approach for Calculating Free Energy in Molecular Dynamics: Demineralization of Hydroxyapatite as a Case Study

    cond-mat.mtrl-sci 2026-05 unverdicted novelty 5.0

    The authors propose an adaptive non-equilibrium work method for free energy profiles in MD that combines high- and low-quality simulations to cut computational cost while matching results from long reversible referenc...

Reference graph

Works this paper leans on

2 extracted references · 2 canonical work pages · cited by 1 Pith paper

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    Nanoscale Research Le[ers, 2012

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    work page 2012

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    =4𝜀[(𝜎𝑟!

    Safarzadeh, M., et al., Effect of sintering temperature on the morphology, crystallinity and mechanical proper4es of carbonated hydroxyapa4te (CHA). Ceramics Interna9onal, 2020. 46(17): p. 26784-26789. 27. Saroj, S. and U. Vijayalakshmi, Structural, morphological and biological assessment of magne4c hydroxyapa4te with superior hyperthermia poten4al for ort...

    work page 2020