5 ILCS 70/1.15

"Written" and "in writing" may include printing, electronic, and any other mode of representing words and letters; but when the written signature of any person is required by law on any official or public writing or bond, required by law, it shall be (1) the proper handwriting of such person or, in case he is unable to write, his proper mark or (2) an electronic signature as defined in the Uniform Electronic Transactions Act, except as otherwise provided by law.