Quantification of embedded H2O in soft contact lenses by NIR spectroscopy

Importance of the Topic

The hydration level of soft contact lenses critically influences user comfort, visual acuity, and lens performance. Traditional methods to measure water content often require extensive sample handling or destructive analysis. Near-infrared (NIR) spectroscopy offers a rapid, non-destructive alternative that preserves lens integrity and avoids solvent use.

Study Objectives and Overview

This study aimed to develop and validate an NIR-based method to quantify embedded water in soft contact lenses. Four lens groups with nominal water contents of 24, 38, 48, and 58% were analyzed to build a predictive multivariate model.

Methodology and Instrumentation

Sample Preparation:

• Lenses were gently patted dry between wipes and placed on a gold diffuse reflector.
• Transflection mode was used to minimize contact damage.

Instrumentation:

• NIRS XDS RapidContent Analyzer
• Iris Adapter
• Liquid Sample Kit for RCA

Data Acquisition and Modeling:

• Spectra collected for 1 minute per sample (16 seconds scan per spectrum, five spectra per lens).
• Wavelength range: 1372–1648 nm, standard normal variate pretreatment (SNV).
• Partial least squares regression with three factors in Vision software.

Main Results and Discussion

NIR spectra processed with SNV revealed distinct absorption features correlating with water content. The calibration model achieved a standard error of calibration (SEC) of 1.69% and an R2 of 0.988. Self-validation yielded a standard error of prediction (SEP) of 1.36%. Experimentation with additional air-drying times demonstrated that prolonged exposure can lead to underestimation of water content, emphasizing the need for consistent sample handling.

Benefits and Practical Applications

Advantages of the NIR approach include:

• Non-destructive analysis with no sample preparation solvents.
• Rapid measurement allowing high throughput.
• Preservation of intact lenses for further testing or use.
• Elimination of analytical waste.

Future Trends and Opportunities

Potential developments include:

• Use of transmitters with larger reflector gaps to prevent lens deformation.
• Integration into manufacturing lines for real-time quality control.
• Extension of methodology to other hydrogel-based products.
• Advancements in NIR imaging for spatial water mapping.

Conclusion

NIR transflection spectroscopy coupled with PLS regression is a reliable and efficient tool for quantifying embedded water in soft contact lenses. Ensuring uniform sample preparation and measurement timing is crucial to maintain accuracy. With further refinement in hardware and data processing, this approach has the potential for broad application in industrial quality control.