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This idea would eliminate the necessity of soap bars during a bath.
Imagine a spa/tub that has soap dispensers that allow the soap to saturate the water to the point that the water becomes your hygiene source. After bathing, you could push a button to drain the soapy water and simultaneously fresh, warm water is gently being poured into the spa/tub to rinse away the soap from your body; sort of like a low shower because your sitting down.
The spa/tub would be equipped with 4-5 dispensers for the soaps of your choice. This idea seems more feasible for a spa because of the many jets and tubes that are eqipped with it.
Of course there is more think about, such as cost of body wash, but I think the experience would be fun.