Porang Flour-Based Hydrogel as a Vitreous Substitute: Rheological, Optical, and Structural Evaluation for Biomedical Applications

Technical Summary: Porang Flour-Based Hydrogel as a Vitreous Substitute

Problem Statement
The vitreous humor is a critical, transparent colloid responsible for maintaining the structural stability and optical clarity of the eyeball. Damage to this fluid, often caused by conditions such as diabetes or trauma, necessitates vitreoretinal surgery where the native fluid is replaced. Current standard substitutes—including sulfur hexafluoride gas, silicone oil, and saline buffers—suffer from significant drawbacks: high cost, limited availability, and poor long-term biocompatibility. Furthermore, these materials often exhibit issues such as emulsification, lower refractive indices compared to native fluid, and low density, which forces patients to maintain specific post-operative positions. There is a distinct need for a low-cost, biocompatible, and optically suitable alternative that closely mimics the physical properties of native vitreous humor.

Methodology
This study investigates the synthesis and characterization of a bio-based hydrogel derived from porang flour (Amorphophallus muelleri Blume), a natural resource abundant in Indonesia rich in glucomannan (15–64% dry weight). The research followed a multi-step process:

1. Purification: Porang flour was purified using a maceration method involving soaking in 70% ethanol for 7 days to reduce oxalic acid and increase glucomannan content.
2. Synthesis: The purified flour was dissolved in deionized (DI) water at a 1:40 ratio. To determine the optimal formulation, the mixture was stirred using a magnetic stirrer (300 rpm) for varying durations: 6, 12, 18, and 24 hours.
3. Characterization: The resulting hydrogels were evaluated against the physical standards of native human vitreous humor (pH 7.0–7.5, viscosity 0.3–2 Pa·s, refractive index 1.3345–1.3348, density 1.0053–1.008 g/cm³). Tests included:
   • Physical Properties: pH, static viscosity, density, and refractive index.
   • Rheology: Shear rate viscosity analysis and viscoelastic properties (Storage Modulus $G'$ and Loss Modulus $G''$).
   • Optical: UV-Vis spectroscopy to identify chromophore groups and assess transparency.

Key Results
The study compared four hydrogel formulations (Hydrogel I–IV) based on stirring time against native vitreous humor (VH) standards:

• Optimal Formulation: The hydrogel stirred for 18 hours (Hydrogel II) demonstrated the closest alignment with native vitreous properties. It exhibited a static viscosity of 0.516 Pa·s, a pH of 7.2, and an apparent viscosity of 8.1 Pa·s under specific shear conditions.
• Rheological Behavior: The 18-hour sample showed a Storage Modulus ($G'$) of 2.602 Pa and a Loss Modulus ($G''$) of 1.0087 Pa. However, rheological analysis indicated that in the plateau frequency range (0.1–10 rad/s), the $G'$ was lower than the $G''$, suggesting the gel exhibited predominantly melting behavior rather than solid-like gel behavior.
• Optical and Physical Discrepancies: While the viscosity and pH of the 18-hour sample were suitable, other metrics deviated from the target. The refractive index was 1.3356 (slightly above the 1.3345–1.3348 range), and the density was 0.976 g/cm³ (significantly lower than the 1.0053–1.008 g/cm³ target).
• Shorter Stirring Times: Samples stirred for 6 and 12 hours showed excessively high viscosities (up to 56.6 Pa·s) and did not meet the rheological requirements for a vitreous substitute.
• UV-Vis Analysis: While spectroscopy confirmed the absence of chromophore groups in the 250–300 nm range, the study's conclusion states that the formulation did not meet the transparency criteria based on the combined analysis of UV-Vis characterization, refractive index, and density.

Significance and Claims
The paper posits that porang flour-based glucomannan hydrogel represents a promising, low-cost, and biocompatible candidate for vitreous humor substitution. The 18-hour formulation successfully replicated the viscosity and pH of native vitreous humor, addressing the need for a material that avoids the high costs and biocompatibility issues of synthetic alternatives like silicone oil.

However, the authors maintain a modest stance regarding the current readiness of the material. The study explicitly concludes that while the 18-hour stirring time yields properties "close" to natural vitreous in terms of viscosity and pH, the formulation did not fully meet the transparency criteria based on the combined analysis of UV-Vis characterization, refractive index, and density. Consequently, the authors state that "more experiments are needed to verify this complicated flow topology" and to refine the formulation to fully satisfy all optical and physical requirements for clinical application. The work serves as a foundational evaluation of porang-derived hydrogels, highlighting their potential while acknowledging the specific physical gaps that remain to be bridged.