Silica-gel microspheres are the critical elements in a new type of fiberoptic sensor and an emerging flat-panel display (FPD) technology. Materials scientist Edward Pope, adjunct professor in the department of materials science, University of Utah (Salt Lake City, UT), has pioneered the production of micro spheres using sol-gel fabrication techniques. When doped with an application-specific dye, the spheres can either be mounted on the end of optical fibers and used as sensors or arranged in lines and coaxed into fluorescing for FPD applications.
The process uses sol-gel techniques to produce silica spheres ranging from a few microns to 1 mm in diameter. Two immiscible fluids, tetraethoxysilane and hydro chloric acid, are mixed together at room temperature, and the diameter of the resultant spheres is controlled by stirring speed. During the production process, the spheres can be doped with a variety of organic or inorganic fluorescent dyes. The dye chosen is application-dependent. The silica spheres are highly porous, with an average pore diameter of 16 Å, making them excellent hosts for dopants. The pores are generally too small to allow the dye molecules to diffuse out of the spheres, yet large enough that significant dopant surface area is exposed.
Applications proposed for micro spheres range from raw materials for production of high-purity glass to controlled-release agents for medical applications. Pope is currently developing microsphere-tipped fiberoptic sensors and FPDs through his company Matech (Westlake Village, CA).
Microsphere fiberoptic sensors
Certain organic and inorganic dyes display absorption-spectrum shifts when subjected to variation in pH or temperature, or in the presence of certain elements and compounds. For example, fluorescein dye undergoes an absorption-spectrum shift when it is immersed in liquids of varying pH. These spectral shifts constitute the fundamental principle behind the microsphere fiberoptic sensor.
Microspheres roughly 200 µm in diameter are doped with application-specific dyes, then mounted on the tip of a multimode optical fiber. This is then placed in the material to be measured, and the absorption spectrum of the dopant is detected by a spectrophotometer. Laboratory-generated spectra can be used to obtain absolute measurement values from the recorded spectral shift.
Doped-silica-gel films have been applied to single-mode fibers for use as sensors, taking advantage of evanescent-wave interactions. Such systems are very sensitive but re quire costly instrumentation to cope with the low signal strength. The micro sphere sensor does not operate on the evanescent-wave principle but rather works with direct signals. It has the advantage of low manufacturing cost and high signal strength, opening up the possibility of disposable versions for certain medical and chemical-sensing applications.
Pope has demonstrated the feasibility of using these sensors to monitor pH, temperature, and chemical concentrations. In particular, the sensors would be well suited to environmental monitoring such as detection of heavy metals in hard-to-reach locations and in medical applications such as noninvasive biomedical diagnostics.
Flat-panel displays
A second application for the technology involves FPDs with optically active microspheres functioning as pixel elements. Microspheres can be doped with fluorescent dyes that luminesce in the red, green, and blue, then arranged in a raster on a screen. A monochromatic liquid-crystal (LC) array is used as the spatial light modulator for an ultraviolet (UV) source that excites the dopant in the spheres. Red, blue, or green emitted radiation creates an image. Matech has produced an operational prototype of this type of system.
Conventional liquid-crystal displays (LCDs) use filter arrays to produce color output and are consequently plagued by low throughput. Color in micro sphere FPD systems is produced radiatively by the individual pixels. Such systems can be optimized to yield a brightness in crease 50% to 100% higher than current LCD levels. The viewing-angle dependence of LCDs is also eliminated by use of microsphere arrays. Output from the screen is unpolarized, as compared to the polarized output from LCDs. More important, the micro spheres radiate over a hemispherical surface, resulting in a viewing angle comparable to that of conventional cathode-ray tubes.
Matech's current work in FPD systems includes optimization of LC polymers and polarizers for UV operation and development of alignment methods for large arrays of microspheres. For example, HDTV displays require more than 1 million pixel elements, requiring some sort of rapid sequential positioning during the manufacturing phase.
Additional applications for micro sphere technology are being pursued. Pope has patented the technology and is seeking development partners or companies interested in licensing the processes.