Confocal Raman analysis of a transdermal nicotine patch by a DXR2 Raman Microscope

Significance of the topic

Transdermal patches offer a controlled, non-invasive route for delivering active pharmaceutical ingredients through the skin into systemic circulation. Understanding the layered structure and chemical distribution within these patches is essential for quality control, formulation optimization, and regulatory compliance in pharmaceutical manufacturing. Confocal Raman microscopy provides a powerful, non-destructive approach to probe multilayer polymer systems and map the distribution of drugs and excipients with high spatial resolution.

Objectives and Study Overview

This study investigates the layer composition, thickness, and drug distribution in a commercial transdermal nicotine patch using confocal Raman analysis. The main goals were to:

• Identify and characterize each polymeric layer in the patch architecture
• Locate and confirm the presence of nicotine within the reservoir layer
• Measure individual layer thicknesses without destroying the sample

Methodology and Instrumentation

Sample preparation involved mounting the patch on a gold-coated microscope slide, with the backing layer toward the objective and the release liner at the base. Two complementary Raman profiling methods were applied:

• Z-profiling (line depth profiling): 532 nm laser, 5 mW power, 50× objective, 25 µm pinhole, 5 µm step size over 220 µm depth (45 spectra)
• X-Z area profiling: same laser and optics, mapping a 120 µm×245 µm cross-section with 350 spectral points

Used Instrumentation

• Thermo Scientific™ DXR™2 Raman Microscope
• Thermo Scientific™ OMNIC™ for Dispersive Raman Software Suite
• OMNIC™ Atlμs™ for mapping and visualization
• OMNIC Specta Software with High-Resolution Raman Polymer Library

Main Results and Discussion

Raman depth profiling revealed six distinct layers with thicknesses ranging from ~15 µm to ~75 µm. Contour maps displayed intensity variations corresponding to polymer-specific Raman peaks. Layer assignments were as follows:

• Layer 1 (Backing): Poly(ethylene terephthalate) (PET)
• Layer 2 and 4: Microporous polyethylene (PE) used for controlled release
• Layer 3 (Reservoir): Ethylene-vinyl acetate (EVA) copolymer containing nicotine
• Layer 5 (Adhesive): Polyisobutylene (PIB) blended with PET
• Layer 6 (Release liner): PET

Multi-component spectral searching identified S-(-)-nicotine within the EVA reservoir, with a composite spectral contribution of ~36%. X-Z correlation maps confirmed uniform layer thicknesses and localized the nicotine/EVA mixture precisely in layer 3. The approach avoided the need for chemical extraction or physical sectioning, preserving sample integrity.

Benefits and Practical Applications

Confocal Raman profiling of transdermal patches delivers:

• Non-destructive, depth-resolved chemical imaging of multilayer systems
• Rapid identification of both polymer excipients and active drug compounds
• Quantitative estimates of layer thickness for quality assurance
• Reduction in sample preparation time compared to GC-MS, LC-MS, or electron microscopy techniques

These advantages support robust product development, batch release testing, and troubleshooting of drug delivery performance.

Future trends and applications

Advances in Raman instrumentation, such as higher sensitivity detectors and faster scanning systems, will further enhance throughput and spatial resolution. Coupling Raman mapping with complementary techniques (e.g., infrared imaging or mass spectrometry imaging) can yield richer multidimensional chemical insights. Expanded applications may include real-time monitoring of drug release kinetics, in-situ analysis during manufacturing, and evaluation of novel nanostructured transdermal formulations.

Conclusion

Confocal Raman microscopy using the DXR2 instrument and OMNIC software suite provided a rapid, non-destructive method to characterize the layered architecture and drug content of a commercial nicotine patch. The technique enabled precise layer identification, thickness measurement, and localization of nicotine within the EVA reservoir. Its ease of use and minimal sample preparation make it a valuable tool for pharmaceutical quality control and formulation research.

References

• Guillory P., Deschaines T., Henson P. Confocal Raman microscopy analysis of multilayer polymer films, Thermo Scientific Application Note 51718, 2008