At the age of four to five, children learn to tie their own shoes. Velcro shoes and rubber boots give way to the first sneaker. Around the same time, they discover a love of asking questions. From “Why is the banana crooked?” to “Why does the sun rise every morning?” they notice things that seem completely self-evident to us.
The bright green, new sneaker is their first shoe with laces; only later do they learn that shoes come in a variety of styles — from patent leather to hiking boots. They might even pose the question: “Can you also tie your shoes with laces made of glass?” In their eyes everything is extraordinary, and everything is possible.
This connection might seem absurd at first. We know shoelaces as thin, flexible cords made of fabric that might be black and thin or white and wide or come in any variety of other colors. But we have never heard of shoelaces made of glass. Because we know glass from drinking glasses and windows, we think of it as hard and fragile.
However, in principle, it is actually possible to thread or knot glass through the eyelets of shoes! That is, if you use flexible optical glass fibers that conduct images and light.
By heating glass rods, it is possible to draw glass fibers that are about as thick as a human hair. The speed of the drawing process determines the fiber thickness, which in turn determines the flexibility of the fiber. The thinner the fiber diameter, the smaller the bending radii that can be achieved. In this respect, fibers made of special glass are even more pliable and flexible than plastic fibers.
The breaking loop test is used to test the flexibility of glass fibers. This is performed by placing a flexible glass fiber in a loop, which is pulled tighter until the fiber breaks. This test has shown that glass fibers with a thickness of 50 micrometers can withstand a bending radius of about five millimeters. So, threading it through the eyelets of shoes is not a problem because a bending radius of about 10 millimeters would be needed. However, tying a knot might be tricky. Although it is possible to tie a knot in optical fibers, tying it tightly would not be possible due to the pulling power.
While fabric laces are somewhat more suitable for footwear than glass, there are many areas where glass fibers come out ahead. Unlike shoelaces, optical fibers can conduct light and even transmit images.
Thus, optical fibers are the optimal solution in both medical and industrial applications where light or images must be brought to hard-to-reach places through a tight passage. For example, flexible endoscopes incorporating glass fibers can be moved smoothly and flexibly through the human body and guided to their place of use thanks to their flexibility. Because these fibers take up little space they enable thinner endoscopes that help make procedures even less invasive.
Light guides can do more than just transport white light from end to end to illuminate objects or tissue with pinpoint accuracy. Light scattering effects in all colors can also be achieved. The glass fibers then sparkle along their entire length and are undoubtedly an eye-catcher for any child. So, we can’t blame four-year-olds for wanting colorful, glowing shoelaces made of glass.