As I assume you already know the possibility to place a form control button in the layout especially for the PDF export format (see also PDF signature field), so that a signature field is created in the PDF document and the signature process is carried out via the functionality of the used PDF viewer, right?
If so, and it still doesn’t solve your requirement, then leave some more inspiring details about
… signature can be captured via touch screen…
so that we can better estimate and plan to think about to include this requirement in the pool of future functions.