A 2005 master’s thesis from Montana State University investigated the design and performance validation of a miniature endoscopic confocal microscope. This technology could lead to dramatic advancements in the biomedicine by providing substantially improved inspection, analysis, and diagnosis through in-situ imaging. Before its development, medical professionals would have to take samples from the native source and image on a microscope. Time and capital are expended to transfer the tissue and analysis at another location. Confocal endo-microscopy (CEM) is the in situ scanning of tissue with a confocal scanning microscope with illumination delivered through an optical fiber.
Key Parameters and Requirements
The principal objective of the thesis was to design a miniature confocal microprobe for instantaneous tissue scanning. The researcher limited target dimensions of the probe to maxima of 2.0 mm diameter and 10.0 mm length. System NA was 0.33, with 4x magnification. Optimally, the scan would occur at two wavelengths, 488 and 515 nm to implement fluorescent dye imaging. The diffractive optical element (DOE) needed to be attached to lens using an optical adhesive, UV16. The study cited beneficial properties of UV16, including refractive index (1.517), ultra-low viscosity (120-150 cps) and less than 2% shrinkage during curing. The essential characteristic was the viscosity, which enabled an ultra-thin adhesive layer between the lens and DOE. UV16 epoxy is very resistant to both solvents, acetone and isopropanol. The adhered assembly could be submerged in acetone in a cleaning step to remove excess process residue.
The research successfully achieved its desired results, with lens dimensions of 4.0 mm length and 2.0 mm diameter (1.6 mm diameter without packaging). UV16 epoxy proved to be robust and the proper adhesive choice.
Another application of UV16 was to assemble the refractive lenses and the DOE. Assembly utilized high accuracy stages to precisely align the DOE with the lens. The research team again used UV16 for the adhesive. After completing the lens assembly, the end of the tube was sealed with UV16 to create surface contact with the tube without a gap.
The author listed properties of UV16 that lead to improvements to the application. The lenses need to be perfectly aligned without getting scratched during assembly. Introducing transparent film between the lenses before coating with epoxy would avoid undesired wicking into the lenses due to the low viscosity of UV16, while simultaneously preventing scratching during insertion of the lenses into the housing. Coating the entire assembly with UV16 is the best way to ensure sufficient adhesion without scratching while minimizing the thickness of the epoxy layer through its very low viscosity.