Many thanks to all of you.
"the enforceability of this intended loan agreement is not terribly high on your list of concerns. " It is true but if I live longer, I want to get the money back, but my long term survival chances are slim.
This is what I understood from the above responses and request to correct me if I misunderstood:
1. a demand loan (loan that should be repaid whenever it is demanded) agreement can be oral, as it can be performed within 1 year.
2. between wife and husband, no interest needs to be paid for a loan or gift as far as IRS or government is concerned, and such loan or gift need not be reported in IRS.
3. I can give $15k gift to someone and, in the same year, I can pay his college tuition directly paid to college, and I need not report any of these matters to IRS.
4. Finally, better to have written loan agreement.