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The whole field of biosensing has been enhanced by the development of optical fibers(l). They provide a means of activating the sensors and of carrying the sensor's signal back to the detector. The small dimensions and flexibility of the fibers allow the development of miniaturized sensor packages capable of introduction into a patient's vascular system. Several in vitro systems are already competing in high risk operations with standard laboratory procedures to provide more timely and accurate analysis of the patient's condition. During the next decade real time sensing in vivo together with various endoscopic and developing laser surgery techniques will provide doctors and patients with many new and delicate alternative procedures to common surgery. Not only will trauma be minimized, but with better and quicker knowledge of the patient's blood gas levels and other body conditions, doctors will be able to efficiently correct for unforeseen difficulties while controlling the known problems effectively. To achieve these wonders extremely small sensing areas will be available so that the sensor systems must be highly efficient. The optical fibers must activate all or at least most of the sensor's area and must be able to gather as much of the return signal from the sensor as possible. This places some strong requirements on the optical properties of the fibers. The need for flexible, yet sturdy, devices places requirements on the mechanical properties of the fibers. For the preferred silica based fibers, the fact that body fluids are primarily water place additional demands on the fibers' mechanical properties. Obviously the sensing system being in contact or close proximity with the body or its fluids must satisfy certain biocompatibility and sterilization requirements. This paper further describes these requirements and presents details about a new optical fiber which meets these requirements and provides some additional very useful properties. This new fiber which has a high numerical aperture, a hard cladding, and a pure silica core is called High NA, HCS* fiber.
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