With the approval of the director, any bank may at any time, through action of its board of directors and without requiring any action of its stockholders, issue and sell its capital notes or debentures. Such capital notes or debentures shall be subordinate and subject to the claims of depositors and may be subordinated and subjected to the claims of other creditors. Before any such capital notes or debentures are retired or paid by the bank, any existing deficiency of its capital, disregarding the notes or debentures to be retired, must be paid in, in cash, to the end that the sound capital assets shall at least equal the capital or capital stock of the bank. Such capital notes or debentures shall in no case be subject to any assessment. The holders of such capital notes or debentures shall not be held individually responsible as such holders for any debts, contracts, or engagements of such bank and shall not be held liable for assessments to restore impairments in the capital of such bank.