With great power comes great responsibility - A PSU Discussion.

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Ignoring the safety concerns, you'd of course need a heat bed designed to operate off the mains voltage. A 120V or 230V heat bed would be tougher to fabricate than one that operates on 12V.

We can calculate the required heater resistance using R = (V * V)/P, where V is the voltage applied and P is the desired power dissipated as heat. For a nominal 130 watt 12V heat bed, this comes up with R equal to about 1.1 ohms. Such a heat bed is readily achievable in a circuit board layout by applying the input voltage across a trace designed with a particular copper thickness, trace width, and trace length.

For 230V mains and the same 130W heat dissipation, the same formula shows R would be around 400 ohms. This would be extremely hard to achieve with just a circuit board trace, meaning the bed would have to be formed from a resistive material or multiple resistive components.

Those interested can play with the necessary trace characteristics using any of several online circuit trace resistance calculators.