Demonstration of Femtosecond Laser Induced Refractive Index Change in Silk-Fibroin Hydrogels

Materials Science Optics Life Sciences Physics Bioengineering Ophthalmology Medicine Biochemistry
femtosecond laser refractive index change silk fibroin hydrogel vision correction biocompatibility refractive errors corneal implants

Demonstration of Femtosecond Laser Induced Refractive Index Change in Silk-Fibroin Hydrogels: A New Hope for Future Ophthalmic Biocompatible Implants

In today’s era of advanced biotechnology and rapid developments in the biomedical field, refractive correction techniques have become a global research hotspot in ophthalmology. However, current correction technologies, such as mechanical reshaping of the cornea and the use of commercially available intraocular lens (IOL) materials, still face challenges such as insufficient precision and poor biocompatibility. As such, the scientific community is increasingly focusing on a novel, non-destructive refractive correction technique called Laser Induced Refractive Index Change (LIRIC). Against this backdrop, a research team from the University of Rochester, in collaboration with the Instituto de Óptica “Daza de Valdés,” conducted a groundbreaking study exploring how LIRIC techniques can induce localized refractive index changes in silk fibroin (hydrogel) materials. This study paves the way for designing novel ophthalmic implants.

The research paper titled “Demonstration of Femtosecond Laser Induced Refractive Index Change in Silk-Fibroin Hydrogels” was co-authored by Quazi Rushnan Islam, Rocio Gutierrez-Contreras, Susana Marcos, and Wayne H. Knox and published in the Biomedical Optics Express (2025, Vol. 16, No. 2, pp. 657-668). The study demonstrates significant refractive index changes in silk fibroin hydrogels under femtosecond laser treatment, highlighting its potential in designing custom refractive correctors and biocompatible corneal implants.

Background and Research Objectives

Globally, vision impairments like myopia, hyperopia, astigmatism, and presbyopia have become significant health challenges. Studies show that most individuals over age 45 are affected by presbyopia, and uncorrected refractive errors have been identified as a leading cause of blindness. Therefore, the development of high-efficiency, biocompatible correction technologies is urgent.

LIRIC is a groundbreaking laser-based method that relies on multiphoton absorption to create localized refractive index changes in materials without causing damage. This technique has already succeeded in commercial ophthalmic hydrogels, contact lenses, IOLs, and corneal tissues. However, the application of LIRIC to ophthalmic implants has been restricted by the limitations of existing materials. Silk fibroin, due to its biocompatibility, optical transparency, and versatility, stands out as a promising new platform for LIRIC applications.

The primary objective of this study was to evaluate the feasibility and efficacy of LIRIC-induced refractive index changes in silk fibroin materials. The findings explore its potential as a cornerstone for developing corneal implants and biodegradable contact lenses for refractive correction.

Research Methodology

The study comprised the following key steps: Material Preparation, Femtosecond Laser Micromachining Experiments, Microscopy and Interferometry Imaging, and Experimental Results Analysis.

1. Preparation of Silk-Fibroin Hydrogels

Silk fibroin was derived from Bombyx mori cocoons collected under standardized conditions in Murcia, Spain. The preparation followed an adapted version of a prior method [15], involving a 40-minute degumming process, drying, dissolution, dialysis, and centrifugation to remove impurities, ultimately yielding a 6%-7% (w/v) silk fibroin solution.

To enhance optical properties, the team copolymerized the silk fibroin solution with HEMA (2-Hydroxyethyl Methacrylate) and EGDMA (Ethylene Glycol Dimethacrylate), resulting in silk fibroin hydrogel membranes with approximately 100 µm thickness in the hydrated state.

2. Femtosecond Laser Micromachining

Two custom femtosecond laser micromachining setups were employed:

  1. 400 nm, 80 MHz System

    • Light source: Mode-locked Ti:Sapphire laser producing 100 fs pulses.
    • Writing parameters: 5-25 mm/s scanning speed, 39 mW average power.

  2. 517 nm, 8.3 MHz System

    • Light source: Ytterbium-doped fiber laser generating 120 fs pulses frequency-doubled to 517 nm.
    • Writing parameters: 20-200 mm/s scanning speed, up to 980 mW average power.

Phase bars were written on dry and hydrated silk hydrogel samples, with varying laser parameters, followed by microscopy and phase-shift characterization.

3. Microscopy and Interferometry

Bright Field Microscopy and Mach-Zehnder Interferometry were used to evaluate the morphology and refractive changes in the treated regions. A wedge test determined the sign of the phase changes, vital for understanding the nature of the induced refractive index shifts.

Results and Analysis

1. Confirmation of Negative Refractive Index Change

The study observed negative refractive index changes in silk fibroin hydrogels, consistent with previous findings using commercial hydrogel materials for LIRIC. These changes likely resulted from localized increases in water content in the laser-treated regions.

2. Performance Differences Between Laser Systems

  • 400 nm System: Successfully induced full-wave phase changes in dehydrated silk fibroin at relatively low power (39 mW). However, no measurable changes were observed in hydrated samples.
  • 517 nm System: Achieved phase shifts ranging from -0.25 waves to -0.6 waves in hydrated silk fibroin, albeit requiring higher average power (hundreds of mW).

These findings indicate that the 400 nm system is more efficient for processing dehydrated samples, while the 517 nm system is suitable for hydrated environments, aligning with real-world corneal applications.

3. Feasibility and Material Stability

The silk fibroin hydrogels maintained phase stability even under high-pressure, high-temperature conditions (e.g., 121°C autoclaving). This robustness ensures compatibility with standard manufacturing and sterilization processes, vital for commercial applications.

Conclusions and Significance

This study successfully demonstrates the feasibility of inducing localized refractive index changes in silk fibroin hydrogels using femtosecond laser micromachining. The key conclusions include:

  1. Biocompatibility and Optical Transparency: Silk fibroin is an exceptional candidate for novel ophthalmic implants.
  2. Non-Invasive, Adjustable Treatments: LIRIC offers real-time, adjustable refractive correction options.
  3. Environmental Impact: The potential for biodegradable contact lenses addresses microplastics pollution concerns from discarded hydrogel lenses.

Research Highlights

  1. Innovative Technology: Combines LIRIC with silk fibroin materials to introduce non-invasive optical correction tools.
  2. Broad Applications: Opens doors for addressing presbyopia and complex refractive errors while promoting greener, sustainable material alternatives.
  3. Experimental Reliability: Demonstrates stable phase changes across various environmental conditions, supporting future commercialization and surgical applications.

Summary

This interdisciplinary study extends femtosecond laser technology into biocompatible material applications, breathing new life into ophthalmic medical solutions and sustainable material research. Future investigations will focus on optimizing writing parameters, enhancing laser system efficiency, and elucidating the mechanisms behind the refractive index changes. This cutting-edge approach not only advances refractive correction but also holds promise for broader biomedical applications like drug delivery waveguides and tissue engineering.