I do not think that bitcoin can act as an object of pledge. In the case of investing money in the bank for deposit storage, this amount of money is not considered as a pledge. It is transferred to the bank for determining the time in operational control. The Bank has the right to use these funds to perform various banking operations and make profit. Part of this profit is then paid to the person who transferred his money to deposit storage.
Bitcoin, I think, can not be used for transfer to deposit storage in a bank or as a mortgaged property, because it is too volatile in price and banks will not be so risky.