It’s a simple question, what’s so special about nichrome (nickel chromium alloy) wire that makes it suitable as a heating element?
The important factors are its high electrical resistivity, its high temperature corrosion resistance and high melting temperature of 1400 degrees c.
Nichrome wire is irreplaceable in situations such as hair dryers and spaceheaters as other metals will quickly oxidise and degrade at elevated temperatures. However, at the relatively low temperatures (<300C) used for reprap heated beds other materials such as common copper could be used provided some precautions are taken.
The high conductivity of copper could be overcome by using a very long length of wire of a very small cross sectional area. Also being a near pure element in use with a melting point of just over 1000 degrees C it will have a stable microstructure during use. That is providing there is a good thermaly conducive path to a heated bed and so its not allowed to glow “red hot” at any point. The problem of corrosion could also be eliminated provided the copper is removed from oxygen, such as when covered by fire cement or polyimide tape.
So why go to all this trouble when nichrome wire is so cheap and widely available? Well there is one thing that nichrome wire isn’t; printable. Making a heated bed with nichrome wire will always be a labor intensive and possibly dangerous part of making a reprap, more so if mains power is used.
However, now that great strides are being made with producing single sided PCBs on mendels there is the possibility of “printing” a heater bed element.
Imaging a blank PCB that is slightly smaller than the print bed. Then print on a square spiral or zigzag patten with each trace as thin as possible to create a single, tens of meter long ‘wire’. Etch the PCB, cover with a layer of polyimide tape and attach to you print bed material of choice. Then you would have a thin evenly heating heated print bed that is, in a way, printable.