Monitoring of Carbonated Hydroxyapatite Growth on Modified Polycrystalline CVD-Diamond Coatings on Titanium Substrates

At a Glance

Metadata	Details
Publication Date	2024-01-06
Journal	Crystals
Authors	Rocco Carcione, Valeria Guglielmotti, Francesco Mura, Silvia Orlanducci, Emanuela Tamburri
Institutions	Sapienza University of Rome, National Agency for New Technologies, Energy and Sustainable Economic Development
Citations	2
Analysis	Full AI Review Included

Executive Summary

This research demonstrates a novel strategy for enhancing the bioactivity of polycrystalline CVD-diamond (PCD) coatings on titanium (Ti) substrates through controlled surface chemistry modification, optimizing them for hard tissue engineering applications.

Core Achievement: Oxidative annealing of PCD films significantly accelerates the nucleation and growth of bone-like Carbonated Hydroxyapatite (C-HA) from Simulated Body Fluid (SBF).
Surface Modification: Thermal treatment (350 °C in air) successfully introduced singly bonded C-O polar groups (increasing surface oxygen content from 1.5% to 10%), which act as nucleation sites for C-HA.
Structural Improvement: Annealing reduced amorphous carbon (sp²) content and improved the crystalline quality of the PCD layer (I_DIA/I_α-C ratio increased from 0.14 to 0.57).
Enhanced Bioactivity: The modified diamond (D_A) formed a continuous, thick C-HA coating with needle-like morphology, a critical feature for inducing osteogenic activity.
Biomimetic Composition: After 20 days in SBF, the C-HA deposited on D_A was confirmed as B-type C-HA, exhibiting a Ca/P ratio of 2.11 and a carbonate weight percent of 5.5%, closely matching the composition of natural human bone mineral.
Value Proposition: This study provides a scalable method for tailoring CVD-diamond properties, making the annealed diamond/Ti system a highly valuable bioactive platform for implantable prostheses and scaffolds.

Technical Specifications

Parameter	Value	Unit	Context
Ti Substrate Thickness	0.5	mm	Polycrystalline sheet
CVD Gas Mixture (CH₄/H₂)	1.25:100	Ratio	Hot Filament CVD (HF-CVD) synthesis
Filament Temperature	2130 ± 10	°C	HF-CVD synthesis
Chamber Pressure	36 ± 1	torr	HF-CVD synthesis
CVD Duration	3	hours	PCD film synthesis
Oxidative Annealing Temp	350	°C	Surface modification (D_A sample)
Oxidative Annealing Duration	20	min	Surface modification (D_A sample)
Surface Roughness (Ra)	~350	nm	Pristine (D) and Annealed (D_A) PCD films
Diamond Volume Fraction (D_A)	96	%	Post-annealing (improved from 92% for D)
Amorphous Carbon Reduction (I_DIA/I_α-C)	0.14 to 0.57	Ratio	D to D_A (Significant reduction)
Surface Oxygen Content (D)	1.5	%	XPS quantitative analysis
Surface Oxygen Content (D_A)	10	%	XPS quantitative analysis
C-O Bond Concentration (D_A)	9.5	%	XPS C 1s high-resolution
Final C-HA Carbonate Content	5.5	wt%	D_A after 20 days SBF immersion
Final C-HA Ca/P Ratio (D_A)	2.11	Ratio	EDX analysis (Target: human bone mineral ~2.0)
C-HA Structural Quality	B-type	Classification	Raman spectroscopy analysis

Key Methodologies

The synthesis and modification process involved four main stages: substrate preparation, PCD deposition, surface functionalization, and C-HA precipitation monitoring.

Titanium Substrate Preparation:
- 0.5 mm thick polycrystalline Ti sheets (1 cm x 1 cm) were electropolished at -30 °C using a 20 V potential for 15 minutes (solution: HClO₄, methanol, n-butanol).
- Substrates were seeded via 15-minute sonication in a solution containing 50 mg nanodiamond powder in 20 mL ethanol to encourage homogeneous nucleation.
PCD Film Synthesis (HF-CVD):
- A 1.25:100 CH₄/H₂ gas mixture was activated by a Joule-heated Tantalum (Ta) filament.
- Synthesis was performed for 3 hours, maintaining filament temperature at 2130 ± 10 °C and chamber pressure at 36 ± 1 torr.
Surface Functionalization (Oxidative Annealing):
- PCD films (D_A samples) were annealed in air at 350 °C for 20 minutes.
- This process selectively introduced C-O moieties, increasing surface oxygen content and eliminating amorphous carbon/sp² phases at grain boundaries.
Carbonated Hydroxyapatite (C-HA) Precipitation:
- Samples (D_A, pristine D, and bare Ti) were immersed in 1.5x Simulated Body Fluid (SBF) at 37 °C, buffered to pH 7.40 using 1 M HCl.
- Immersion was monitored over a 20-day period, with fresh SBF provided daily.
Characterization:
- Raman Spectroscopy: Used to monitor C-HA growth kinetics (I₉₆₀ and FWHM₉₆₀) and calculate carbonate content (%W(CO₃^-)).
- XPS: Used to quantify surface elemental composition and confirm the presence of C-O polar groups post-annealing.
- SEM/EDX: Used to analyze morphology (needle-like texture) and determine the final elemental composition (Ca/P ratio).

Commercial Applications

The ability to synthesize highly bioactive, structurally sound diamond coatings on titanium opens pathways for advanced medical device manufacturing and tissue engineering.

Application Area	Specific Product/Function	Relevance to Research Findings
Hard Tissue Engineering	Implantable Prostheses (Hip, Knee, Dental)	Modified PCD acts as a superior scaffold, promoting rapid and strong osteointegration by forming biomimetic C-HA coatings faster than bare Ti.
Orthopedic Scaffolds	3D Printed Ti Scaffolds with Diamond Coating	The continuous, needle-like C-HA morphology achieved on D_A is fundamental for inducing osteogenic differentiation of bone-forming cells.
Biomaterials Science	Corrosion-Resistant Bioactive Surfaces	CVD diamond provides chemical inertness and corrosion resistance against body fluids, while the C-HA layer ensures biological compatibility and bonding.
Conductive Bio-Platforms	Advanced Biosensors and Electrodes	While focused on C-HA, the underlying Ti-doped CVD diamond platform is noted in the literature for its electroconductivity, useful for biosensing (e.g., dopamine detection) and memristive devices.
Regenerative Medicine	Bone-Implant Interface Enhancement	The controlled surface oxidation method provides a reliable, scalable industrial process for manufacturing highly functionalized bioactive surfaces.

View Original Abstract

Production of diamond coatings on titanium substrates has demonstrated as a promising strategy for applications ranging from biosensing to hard tissue engineering. The present study focuses on monitoring the nucleation and growth of bone-like carbonated-hydroxyapatite (C-HA) on polycrystalline diamond (PCD) synthetized on titanium substrate by means of a hot filament chemical vapor deposition (HF-CVD) method. The surface terminations of diamond coatings were selectively modified by oxidative treatments. The process of the C-HA deposition, accomplished by precipitation from simulated body fluid (SBF), was monitored from 3 to 20 days by Raman spectroscopy analysis. The coupling of morphological and structural investigations suggests that the modulation of the PCD surface chemistry enhances the bioactivity of the produced materials, allowing for the formation of continuous C-HA coatings with needle-like texture and chemical composition typical of those of the bone mineral. Specifically, after 20 days of immersion in SBF the calculated carbonate weight percent and the Ca/P ratio are 5.5% and 2.1, respectively. Based on these results, this study brings a novelty in tailoring the CVD-diamond properties for advanced biomedical and technological applications.

Tech Support

Original Source

DOI: https://doi.org/10.3390/cryst14010066

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