Daniel C. McCarthy, News Editor
The atomic force microscope (AFM) has become an increasingly common research and development tool for contact lens manufacturers. Rather than requiring that lenses be examined in a vacuum (as in scanning electron microscopy) or in artificial environments, the atomic force microscope allows lens manufacturers to perform three-dimensional analysis of a lens' surface morphology in saline and other solutions, or in ambient air. One lens manufacturer selected the Dimension 3100 atomic force microscope from Digital Instruments Inc. for these reasons, as well as for its ability to scan the cantilever and the integrated microscope tip, rather than the sample.
Digital Instruments' atomic force microscope incorporates a dichroic mirror in the microscope's head that directs the laser light to track movement of the cantilever and tip as they scan the sample and then align the device's tip to scan those features. Courtesy of Digital Instruments.
A spokesman for the lens manufacturer said his company had searched specifically for a tip-scanning microscope. The alternative is sample scanning technology, which sometimes requires that the lenses be cut into small samples for examination. "Tip scanning doesn't give as high resolution, but it allows analysis of arbitrary samples, not limited by size or dimension, and it allows us to examine submerged materials," the lens manufacturer said.
According to Michael Serry and David Grigg, scientists at Digital Instruments, the Dimension 3100's tip scanning ability is facilitated by a special optical arrangement that directs the laser light to track movement of the microscope's cantilever and tip as they scan the sample surface. The optical arrangement in the microscope head uses a dichroic mirror to provide a top-down view of both the sample surface and the AFM cantilever while also allowing the focused laser light to pass through to the cantilever for deflection detection.
This technique allows the users of the Dimension 3100 to identify features of interest on the sample and to align the tip to scan those features. The addition of an antireflective coating on the fluid cell of the atomic force microscope reduces interference fringes in the instrument's images.
The contact lens manufacturer added that the Dimension 3100 also has proved to be a more robust instrument than others that were considered. "Digital Instruments makes a very well-engineered instrument. It works every time you start it up. Here I have 20 to 50 samples thrown at me every week. I can't afford to have it fail on me."