Phytomediated Green Synthesis of Mimosa pudica Silver Nanoparticle–Alginate– Curcumin Hydrogels: Sustainable Fabrication, pH-Responsive Drug Release, Antibacterial Activity and In Vitro Cytocompatibility

Technical Summary: Phytomediated Green Synthesis of Mimosa pudica Silver Nanoparticle–Alginate–Curcumin Hydrogels

Problem Statement
Chronic wounds, including burns and diabetic ulcers, present a significant global healthcare burden due to prolonged inflammation, susceptibility to infection, and poor re-epithelialisation. While tissue engineering offers a transformative strategy using biocompatible scaffolds, ideal materials must simultaneously provide structural support, maintain moisture, and possess intrinsic antimicrobial activity. Although sodium alginate (ALG) is a widely used biopolymer for wound healing, and curcumin and silver nanoparticles (AgNPs) are known for their bioactive properties, their individual limitations persist. Curcumin suffers from poor aqueous solubility and low bioavailability, while conventional chemical synthesis of AgNPs often involves hazardous reagents. Furthermore, a systematic evaluation of a freeze-dried porous alginate scaffold that simultaneously incorporates green-synthesised AgNPs (using Mimosa pudica) and curcumin, specifically assessed for human keratinocyte viability and proliferation, had not been previously reported.

Methodology
The study employed a "green synthesis" approach to fabricate a novel ternary hydrogel scaffold.

1. Green Synthesis of AgNPs: Silver nanoparticles were synthesized using an aqueous extract of Mimosa pudica leaves (MPLE) as both the reducing and capping agent. The reaction between silver nitrate and MPLE resulted in the bioreduction of Ag⁺ to Ag⁰, confirmed by a color change from pale yellow to dark brown.
2. Scaffold Fabrication: The synthesized MPLE-AgNPs were dispersed in an alginate solution, followed by the addition of a curcumin stock solution. The resulting ternary composite was cast, freeze-dried, and crosslinked using calcium chloride (CaCl₂) to form a porous hydrogel scaffold. A binary control scaffold (Alginate/AgNPs without curcumin) was also prepared for comparison.
3. Characterization: The scaffolds underwent comprehensive physicochemical characterization:
   • Optical/Structural: UV-Visible spectroscopy, FTIR, XRD, and FESEM with EDAX were used to confirm nanoparticle formation, crystalline structure, functional group interactions, and morphology.
   • Physical Properties: Porosity, swelling behavior, and degradation rates were tested in PBS at pH 7.4 and pH 5.5.
   • Drug Release: In vitro release of curcumin was monitored to determine kinetics and pH-responsiveness.
   • Biological Evaluation: Antibacterial activity was assessed against E. coli, Streptococcus sp., and Bacillus cereus via agar diffusion. Cytocompatibility and proliferation were evaluated using HaCaT human keratinocyte cells via MTT assay (days 1, 3, and 5) and Hoechst 33342 fluorescence microscopy.

Key Results

• Physicochemical Properties: UV-Visible spectroscopy confirmed the formation of AgNPs with a Surface Plasmon Resonance (SPR) peak at 435 nm. FTIR analysis identified electrostatic coordination interactions between the alginate carboxylate groups and the AgNP surface. XRD confirmed the face-centred cubic (fcc) crystalline structure of the AgNPs within the amorphous alginate matrix. FESEM revealed spherical AgNPs (approx. 11.5 ± 2.3 nm) and a highly interconnected macroporous scaffold architecture suitable for cell infiltration. EDAX confirmed the successful integration of all components, with silver constituting 31.59 wt% of the final scaffold and chloride impurities effectively removed.
• Swelling and Release: The scaffold exhibited pH-responsive behavior, showing significantly higher swelling (419.05% at pH 5.5 vs. 342.85% at pH 7.4) and accelerated degradation under acidic conditions. Curcumin release followed a zero-order kinetic model with non-Fickian anomalous transport, indicating sustained release that is accelerated in acidic environments typical of infected wounds.
• Antibacterial Activity: Both scaffolds demonstrated antibacterial activity against Gram-positive and Gram-negative strains. The curcumin-loaded scaffold consistently showed larger zones of inhibition compared to the binary control (e.g., 10.5 mm vs. 9.7 mm against Streptococcus sp.), suggesting a synergistic effect between curcumin and AgNPs.
• Cytocompatibility: In vitro evaluation with HaCaT cells showed that both scaffolds were non-cytotoxic. Cell viability increased progressively over 5 days. Notably, the curcumin-loaded scaffold demonstrated significantly higher keratinocyte viability (96.15% vs. 93.98% for the binary scaffold at day 5) and denser cellular coverage in fluorescence microscopy, confirming the pro-proliferative contribution of curcumin.

Key Contributions and Significance
The primary contribution of this work is the development and systematic evaluation of a novel, all-natural bioactive hydrogel scaffold that integrates green-synthesised AgNPs and curcumin within a sodium alginate matrix. The study establishes that:

1. Synergistic Bioactivity: The combination of Mimosa pudica-derived AgNPs and curcumin creates a synergistic effect, enhancing both antibacterial efficacy and keratinocyte proliferation compared to binary systems.
2. Targeted Delivery: The pH-responsive nature of the scaffold allows for accelerated drug release and degradation in the acidic microenvironment of infected or chronic wounds, facilitating targeted therapy.
3. Sustainable Fabrication: The use of plant-mediated green synthesis eliminates hazardous chemical reagents, while the scaffold's composition (alginate, curcumin, plant-derived AgNPs) ensures biodegradability into non-toxic metabolites.

Conclusion
The authors conclude that the Alginate/AgNPs–Curcumin scaffold is a promising candidate for skin tissue engineering and wound healing applications. The material successfully combines antimicrobial protection with a supportive environment for cell growth, addressing the dual challenges of infection control and tissue regeneration. The study positions this scaffold as a viable, sustainable biomaterial for regenerative medicine, with future work recommended to include in vivo wound-healing assessments.