Evaluation of Hydrous Lenses Using SPM

Importance of the topic

Hydrous contact lenses rely on finely tuned surface properties to deliver optimal comfort, tear interaction, and visual performance. Detailed understanding of surface morphology, mechanical behavior, frictional characteristics, and electrical potential distribution under near-use conditions is critical for lens design and quality control.

Objectives and study overview

The aim of this application note was to conduct a comprehensive comparison of two types of hydrous contact lenses—ionic (Sample A) and non-ionic (Sample B)—using scanning probe microscopy (SPM) in environments that mimic real-world usage. Key objectives included:

• Visualization of surface topography and phase contrast in artificial tears
• Quantification of elastic modulus via force-curve analysis
• Determination of friction coefficient under lateral force microscopy (LFM)
• Mapping of surface potential distribution through Kelvin force microscopy (KFM)

Methodology and instrumentation

All measurements were performed with a Shimadzu SPM-9700 equipped for liquid-phase imaging. Two lens samples (A: ionic, B: non-ionic) were immersed in artificial tear solution and also measured in air. Cantilevers and operational modes included:

• Phase and force-curve imaging with BL-AC40 probes (0.1 N/m, 25 kHz)
• LFM friction measurements using OMCL-RC800 probes (0.05 N/m)
• KFM potential mapping with EFM probes (2.8 N/m, 75 kHz)

Main results and discussion

Surface shape and phase images revealed a smoother morphology for Sample A, whereas Sample B showed line-patterned roughness. Force-curve analysis indicated the elastic modulus of Sample B (~0.5 MPa) was an order of magnitude greater than that of Sample A (~0.04 MPa). Friction measurements yielded coefficients μ≈0.14 for Sample A and μ≈0.12 for Sample B, reflecting slightly lower friction in the non-ionic lens. KFM mapping demonstrated a more uniform surface potential distribution on Sample A compared to heterogeneous patches on Sample B.

Benefits and practical applications of the method

SPM offers high-resolution, quantitative multiparametric evaluation of hydrophilic polymer surfaces in liquid environments. This enables:

• Optimization of surface treatments to enhance wearer comfort and wettability
• In-depth quality control during lens manufacturing
• Correlation of microscopic surface features with macroscopic performance

Future trends and potential applications

Advances in SPM probe design and integration with complementary techniques (e.g., Raman, infrared nanospectroscopy) will expand capabilities for in situ chemical mapping. Development of high-throughput liquid SPM platforms could facilitate rapid screening of novel lens materials and surface coatings.

Conclusion

This study demonstrated the utility of the Shimadzu SPM-9700 for comprehensive surface characterization of hydrous contact lenses under near-use conditions. The technique distinguished ionic and non-ionic materials by topography, mechanical stiffness, friction, and electrical potential, underscoring SPM’s value in lens development and QC.

Reference

• Shimadzu Corporation. Evaluation of Hydrous Lenses Using SPM. Application News No. S08, May 2013.