Clinical context: stabilizing the biodegradation of Mg-Ag alloys

Magnesium (Mg) alloys are preferred for biodegradable implants due to their low density and biocompatibility. However, their rapid and unpredictable post-implantation degradation limits their clinical efficacy. While the addition of silver (Ag) confers antibacterial properties, the challenge remains to isolate the surface to slow down corrosion while promoting osseointegration. Hydroxyapatite (HAP) is commonly used as a coating for its osteoconductive capabilities, but it lacks intrinsic anti-inflammatory and antibacterial activity to secure the tissue-implant interface during the initial healing phase.

Objectives and hypotheses: the innovation of the HAP-Capsaicin nanocomposite

This study examines the fabrication of a hybrid nanocomposite coating combining hydroxyapatite with capsaicin (CAP) on a Mg-Ag alloy via the electrophoretic deposition (EPD) process. The objective is to measure the precise impact of voltage (up to 160 V) and deposition time on the microstructure, surface hardness, and corrosion kinetics. The authors test the hypothesis that the incorporation of capsaicin — a bioactive molecule with antioxidant and anti-inflammatory properties — can improve cytocompatibility and regulate the release of Mg2+ ions by creating a denser barrier, without altering the structural stability of the hydroxyapatite matrix.

Study methodology

This in vitro experimental study focused on the fabrication and characterization of nanocomposite coatings combining hydroxyapatite (HAP) and capsaicin (CAP) on a magnesium-silver (Mg-Ag) alloy. The objective was to evaluate the impact of fabrication parameters on surface properties and biological response.

The experimental protocol was structured around the following steps and methods:

• Electrophoretic Deposition (EPD): The nanocomposite layers were deposited by varying two critical parameters: the deposition voltage (reaching a maximum of 160 V) and the immersion time.
• Thermal stabilization: Annealing was applied after deposition to ensure partial retention of the surface functionalities of capsaicin (C₁₈H₂₇NO₃).
• Microstructural and chemical analyses: The morphology of the deposits was examined by microscopic observation. The surface chemical composition was validated by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS).
• Mechanical and electrochemical tests: Hardness was measured by Vickers microhardness (HV). Degradation resistance was evaluated by electrochemical polarization tests, monitoring pH variations, and measuring the kinetics of Mg²⁺ ion release (expressed in ppm h^-1).
• Biological evaluation: Cytocompatibility and cell proliferation were quantified via MTT assays on the various coated samples.

The experimental groups were compared to conventional HAP coatings without capsaicin to isolate the effect of the active agent on the overall performance of the implant.

Performance analysis of the HAP-CAP coating

The results of this study mark an advancement in the management of magnesium alloy (Mg-Ag) degradation. The integration of capsaicin within a hydroxyapatite (HAP) coating via electrophoretic deposition (EPD) addresses the major challenge of biodegradable implants: corrosion control. With Mg2+ ion release reduced to 5.2 ppm h-1 and a hardness reaching 141 HV at 160 V, this composite stabilizes the implant-tissue interface. Clinically, this increased coating density slows down hydrogen production and pH variations, creating a micro-environment more favorable for initial osseointegration.

The contribution of capsaicin is fundamental here. Unlike conventional approaches using polymer additives for mechanical strength, capsaicin improves biological performance (cell growth confirmed by MTT test) while preserving the structural stability of HAP. This synergy offers a dual functionality: the osteoconduction of hydroxyapatite coupled with the intrinsic anti-inflammatory and antibacterial potential of capsaicin, which HAP alone cannot guarantee.

Limits and perspective

Although promising, this in vitro study has inherent limitations due to its design. Electrochemical polarization and cell proliferation tests do not perfectly simulate the biomechanical stresses and physiological fluid flows of a real implant site. Furthermore, the hardness profile showed significant fluctuations at high voltages (160 V), suggesting a possible heterogeneity of the deposited layer that could influence the long-term fatigue of the material. The partial retention of capsaicin functionalities after annealing, although confirmed by XPS, also warrants a quantitative evaluation of its sustained release kinetics.

Implications for implant surgery

For the practitioner, these data suggest that optimizing deposition parameters (time and voltage) allows for precise modulation of implant resorption. The use of Mg-Ag alloys coated with HAP-CAP could reduce post-operative complications related to excessively rapid degradation, while providing "integrated" antibacterial protection directly on the implant surface. This type of multifunctional coating represents a robust strategy to secure the clinical use of absorbable biomaterials in oral and orthopedic surgery.

Summary of key results

This experimental study demonstrates that the electrophoretic deposition (EPD) of a HAP-Capsaicin nanocomposite on Mg-Ag alloy optimizes implant stability. At a voltage of 160 V, the layer reaches a maximum hardness of 141 HV and reduces the release of Mg²⁺ ions to only 5.2 ppm h⁻¹. This denser coating significantly improves cytocompatibility (MTT test) and stabilizes the pH, offering effective protection against the rapid degradation of magnesium.

Specifically, for the practitioner:

• Increased durability: The addition of capsaicin is not limited to antibacterial properties; it strengthens the hydroxyapatite structure, allowing for the consideration of biodegradable implants that are more mechanically stable during the critical healing phase.
• Favorable microenvironment: By limiting pH fluctuation and ionic release, this technology reduces the risks of early peri-implant inflammation, a critical point for the success of magnesium alloys.
• Boosted biocompatibility: The presence of capsaicin actively promotes cell proliferation, potentially accelerating osseointegration compared to conventional HAP.

Technical lexicon of the study

Electrophoretic Deposition (EPD): Electrophoretic deposition process used to apply the composite coating onto the Mg-Ag alloy. The control of voltage (up to 160 V) and duration directly influences the thickness, density, and hardness of the protective layer.

Hydroxyapatite (HAP): Bioactive ceramic with a chemical composition similar to the mineral phase of human bone. It is used here as a coating matrix for its osteoconductive properties, promoting carbonated apatite precipitation and cellular adhesion.

Capsaicin (CAP): Organic compound (C18H27NO3) integrated into the HAP coating for its anti-inflammatory, antibacterial, and antioxidant properties. Its incorporation aims to modulate the peri-implant inflammatory response and promote cellular response during tissue repair.

Mg-Ag Alloy: Biodegradable metallic substrate composed of magnesium and silver. It combines the high strength and low density of magnesium with the intrinsic antibacterial properties of silver, although its rapid degradation requires a coating to stabilize the material-tissue interface.

MTT Test: Colorimetric assay used to measure cell viability and proliferation. In this study, it confirms that HAP-CAP coatings improve the cytocompatibility of the implant compared to untreated surfaces.

Nanocomposite: Multi-phase material where one of the phases has nanometric dimensions. Here, the integration of capsaicin with hydroxyapatite forms a hybrid coating capable of simultaneously regulating corrosion and tissue regeneration.

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