They are not a replacement, but an extension for natural ones. There are two main approaches to creating them--tips and forms. Tips are made of lightweight plastic plates that are 'shaped. They are glued on the end of the natural one and it is then applied over the entire base. Tips are now available in many different colors and designs, ranging from simple colors such as yellow or blue to flamboyant designs such as animal prints and mixed metallic colors. Forms are fitted over the natural and then an artificial is molded out of material and the form is removed and then properly shaped and buffed to a shine.
There are several popular techniques that can be used to create longer, better-looking enhancements. One popular material commonly called 'Polymethyl methacrylate' is a mixture of a polymer powder and a liquid monomer. The mixture starts to harden in 30-40 seconds after application and continues to cure to final hardness typically within fifteen minutes. Powder and liquid ones can be removed by using a variety of solvents (usually acetone is used). They are the strongest and last the longest of all the overlays. But, if they aren't done well, they can look thick and artificial. This type usually needs to be filled in every two to three weeks. Another material, commonly called 'UV Top Coat' hardens under ultraviolet light. Depending on brand these can show a broader variety of quality and properties (flexibility, strength, etc.) but may be more expensive. They generally cannot be removed by organic solvents, but only be removed by filing (or left to grow out with the natural one). There are, however, a variety of soak off UV which can be quickly and easily removed using acetone.