Hello, handwritten IOUs have legal effect as long as the meaning is true and accurate and does not violate the law. The specific requirements are as follows: 1. Write down the full names of borrowers and lenders; 2. Write clearly the loan amount (preferably including the amount in words and figures); 3. Write clearly the term of the loan, the specific time and method of repayment; 5. Write clearly the interest of the loan; 6. There should be the borrower's personal signature, handprint or handwritten signature.

Legal basis:

Article 668 of the Civil Code stipulates: "A loan contract shall be in written form, unless otherwise agreed between natural persons. The contents of the loan contract generally include the loan type, currency, purpose, amount, interest rate term, repayment method and other terms. "