The forum considers all information to be of potential benefit to other users than just the OP and usually ask that the issue is discussed openly. Fire away!
If it's a supply then forget it. If it's a signal then apply it to a non-inverting op-amp arrangement with the appropriate gain (x12.5 to x30 variable).
Look up "energy harvesting". There are chips and circuits specifically made to source energy from such low voltage (low power?) sources.
Be aware that the input current is proportionally higher than the output current since energy must be conserved.
Example:
Output 5 V / 100 mA = 500 mW
Input 0.4 V / 1.25 A = 500 mW --- accounting for a non-ideal converter circuit input current will more on the 1.5 A side ...