Novel film-driven stretcher for modulating ex vivo crystalline lens power

Technical Summary: Novel Film-Driven Stretcher for Modulating Ex Vivo Crystalline Lens Power

Problem Statement
Investigating the biomechanics and optical behavior of the crystalline lens during accommodation requires in vitro systems that accurately simulate ocular physiology. While lens-stretching apparatuses have existed for decades, conventional designs rely on four or eight rigid pulling arms. These systems are structurally complex, bulky, and often incompatible with standard laboratory equipment such as incubators, microscopes, and MRI scanners. Furthermore, multi-arm configurations can induce polygonal deformation of the lens, failing to replicate the uniform 360° radial traction exerted by the ciliary muscle in vivo. Previous attempts to use ring stretchers lacked flexible adjustment of zonular tension. There is a need for a compact, physiologically relevant, and easily integrable device that allows for precise control of lens stretching while maintaining lens transparency and native geometry.

Methodology
The study introduces a novel film-driven stretching system designed to address these limitations. The methodology encompasses three primary components:

1. Dissection Support Base: A custom 3D-printed base utilizes a Kimwipe tissue with a central aperture to anchor the ciliary body via hygroscopic adhesion. This setup stabilizes the anterior globe between inter-fitting rings (DS-inner and DS-outer), preserving the lens's native configuration and zonular tension during dissection.
2. Rubber Film Stretcher: The core innovation is a rubber film clamped between two rings (ST-inner and ST-outer). The film is adhered to the ciliary body and limbus of the porcine eye using Vetbond tissue adhesive. Two polyethylene fiber threads attached diametrically opposite on the outer ring transmit force.
3. Modified 6-Well Culture Plate System: The stretcher is housed in a modified 6-well plate containing M199 medium. The plate lid is equipped with a micrometer screw and an annular cylinder. Rotating the micrometer tightens the fiber threads, lifting the stretcher and inducing uniform radial expansion of the rubber film. This mechanism translates vertical micrometer displacement into 360° radial traction on the zonular fibers.
4. Pre-stretching Protocol: To establish a baseline of relaxed zonular tension, a pre-stretching protocol was developed using a larger double-ring device. This allows the ciliary ring to contract upon release, simulating the relaxed state of the lens before experimental stretching.
5. Optical Measurement: A modified Topcon LM-8 manual lensmeter was used to measure refractive power. To accommodate the high dioptric power of porcine lenses in culture medium, a plano-concave reference lens was placed at the bottom of the well, creating a combined optical system within the lensmeter's measurement range (-25 D to +25 D). A linear calibration equation was derived using biconvex reference lenses to convert lensmeter readings into true dioptric power.

Key Contributions

• Design Innovation: The development of a film-driven stretcher that replaces complex multi-arm assemblies with a simple elastic membrane, ensuring uniform 360° radial traction and avoiding polygonal lens deformation.
• Integration and Accessibility: The system is compact enough to fit within a standard 6-well culture plate, making it compatible with conventional laboratory equipment (incubators, microscopes) and facilitating rapid, reproducible preparation.
• Protocol Refinement: The establishment of a pre-stretching protocol to reduce baseline zonular tension, enabling investigation across the full physiological accommodative range.
• Measurement Adaptation: A modified optical setup using a lensmeter and reference lenses to accurately quantify dioptric changes in high-power ex vivo lenses.

Results
The system was validated using five porcine lenses. Key findings include:

• Refractive Power Change: Between the unstretched state (micrometer setting: 0 mm) and a stretching displacement of 1.5 mm, the mean change in dioptric power was 1.47 ± 0.18 D.
• Morphological Integrity: Under stretching, the porcine lens maintained a circular geometry while increasing in diameter, confirming the uniformity of the radial traction.
• Measurement Range: The study measured stretching up to 1.5 mm. Displacements beyond 2.0–2.5 mm resulted in markedly increased resistance, indicating the approach of the stretching limit and risk of irreversible damage.
• Calibration: A linear relationship ($P_{true} = 0.3933 \cdot P_{read} + 39.67$) was successfully established between lensmeter readings and true dioptric power in M199 medium.

Significance and Claims
The author claims that this film-driven stretcher offers a simpler, more physiologically relevant alternative to conventional multi-arm systems. By exploiting the uniform tension of an elastic membrane, the device mimics the natural radial traction of the ciliary muscle more closely than polygonal pulling methods. The compact nature of the setup allows for the integration of lens stretching experiments with diverse analytical techniques and standard laboratory environments. The author positions this tool as an enhanced experimental platform for investigating the biomechanics and optical behavior of the crystalline lens during accommodation, noting that the observed optical power change (1.47 D) is consistent with the specific displacement used (1.5 mm) and does not necessarily represent the maximum accommodative amplitude of the porcine lens, which previous literature suggests may be higher (approx. 3.5 D). The work is supported by a pending Chinese patent application covering the experimental approach.