Quantification of embedded H2O in soft contact lenses by NIR spectroscopy

Significance of the Topic

The water content of soft contact lenses is a critical parameter influencing comfort, oxygen permeability and lens performance. Conventional methods for moisture determination can be time-consuming, destructive or require sample preparation. Near-infrared spectroscopy (NIRS) offers a non-destructive, rapid approach to quantify embedded water in delicate polymer materials without altering or disposing of the sample.

Study Objectives and Overview

This application note describes the development of a quantitative NIR method to measure the percentage of embedded water in soft contact lenses. Four lens sets with nominal water contents of 24 %, 38 %, 48 % and 58 % (three samples each) were analyzed. The goals were to acquire transflection spectra, build a partial least squares (PLS) regression model correlating spectral features to reference moisture values, and evaluate prediction performance and factors affecting accuracy.

Methodology

Spectra were collected in transflection mode using lenses gently patted dry and placed on a gold diffuse reflector. Each spectrum was recorded in 16 seconds, with total sample handling and scanning limited to about one minute. Standard normal variate (SNV) preprocessing was applied to reduce scattering and path-length variability. A PLS regression model was constructed using 12 calibration spectra (three per moisture level, five replicates each) over the 1372–1648 nm region with three latent factors.

Instrumentation

• NIRS XDS RapidContent Analyzer (model 2.9211.110)
• Iris Adapter (model 6.7425.000)
• Liquid Sample Kit for RCA (model 6.7400.010)

Main Results and Discussion

The PLS model achieved a standard error of calibration (SEC) of 1.69 % and a coefficient of determination (R2) of 0.9879. Self-validation (two repeat measurements per product) yielded a standard error of prediction (SEP) of 1.36 %. Scatter plots showed close agreement between predicted and laboratory moisture values. An additional study of air-drying time demonstrated that extended exposure before measurement led to systematic underestimation of water content, highlighting the need for consistent sample handling times.

Benefits and Practical Applications

• No sample preparation or reagents required, preserving lens integrity
• Rapid analysis (under one minute per sample) and minimal waste generation
• Non-destructive measurement allowing further use of lenses
• High predictive accuracy suitable for quality control and formulation screening

Future Trends and Opportunities

Expansion of the method could include testing larger sample sets for external validation, development of portable NIR probe configurations for in-line manufacturing monitoring, and design of reflectors with optimized gap spacing to prevent lens deformation. Integration with advanced multivariate algorithms and spectral libraries may further enhance robustness across diverse lens materials.

Conclusion

Near-infrared transflection spectroscopy combined with PLS regression provides a reliable, non-destructive technique for quantifying embedded water in soft contact lenses. The method delivers rapid results with low prediction error and preserves sample integrity, making it attractive for routine quality control and research applications.

References

Application note: Quantification of embedded H2O in soft contact lenses by NIR spectroscopy