Introduction For those of you who wear glasses, you should have experienced the frustration on a scorching summer day. Can you recall the annoyance of sweat trickling down your temples, fogging up your lenses and obscuring your vision? During physical activities, your glasses constantly slip down your nose, interrupting your focus and hindering your performance. Besides, have you noticed how glasses can sometimes make your eyes appear smaller and less expressive? Unfortunately, myopia, or short-sightedness, is a common problem among teenagers. In Hong Kong, the rate of myopia is about 18% in 6-year-old, and 62% in 12-year-old students [1]. While glasses are a common solution, if you're tired of being restricted by glasses, contact lenses are an alternative option. The History of Contact Lenses The prototype of contact lenses has been around for centuries. Leonardo da Vinci first conceived the idea of wearing a huge glass hemisphere filled with water in front of the eyes to correct eyesight in 1508 [2]. It was not until 1887 that we are technologically capable of creating the first glass lenses in direct contact with the cornea [3]. Unfortunately, these glass lenses were very uncomfortable and inconvenient to wear, and could only be worn for a few hours [4]. From the 1930s, the development of polymer chemistry provided a new option for producing contact lenses [3, 4]. Poly(methyl methacrylate) (PMMA) was the first polymer used to make thinner contact lenses with better clarity, flexibility, and lighter weight. Despite the huge improvement in comfort level, PMMA lenses were still rigid and considered uncomfortable to wear [4, 5]. Other than rigidity, contact lenses made of PMMA have a more serious drawback: The oxygen can barely pass through the lenses [4, 5]. Low oxygen transmissibility can cause complications like corneal swelling, corneal neovascularization (the formation of new blood vessels into the transparent cornea) and loss of corneal transparency [6]. Therefore it is important to choose a material with a high oxygen transmissibility [7]. Contact Lenses Today Soft contact lenses made of hydrogel are the most common types of contact lenses used today [4]. Hydrogel consists of a hydrophilic (water-loving) but insoluble cross-linked polymer network (see Figure 1). Due to the highly electronegative atoms such as oxygen atoms in the polymer, water molecules can be trapped in the network to create a soft, flexible jelly-like structure by forming hydrogen bonds with the electronegative atoms in the structure. As a result, the material can absorb as much as 85 to 90 percent of water by weight [3]. Poly(2-hydroxyethyl methacrylate) (pHEMA) was found suitable to make soft hydrogel lenses due to its higher oxygen transmissibility [4]. To further enhance the oxygen transmissibility, co-polymers were created by adding other monomers to the hydrogel mix to modify the properties of the material. But still, the lenses could not be worn for extended periods [4]. Further efforts led to the development of silicone hydrogel contact lenses [4]. Silicones, By Jane Yang 楊靜悠 Getting Rid of Glasses: A Beginner's Guide to Contact Lenses 擺脫眼鏡：隱形眼鏡新手指南
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