eSignature is a term that refers to applying some sort of signature to an electronic document.
One illustration of eSignature is when a physcian reviews and approves an electronic document - perhaps a progress note that has been scanned and linked to a patient encounter.
There are several common requirements:
1. an audit trail must be retained that provides access details regarding changes to the document, including the signature. Usually this audit trail will provide user and date / time stamps for each action taken with the document. For example, when a physician affixes his / her signature to the document, the physician's details and date / time are recorded.
2. the original document needs to be altered in both a visual and a physical manner. Often the signature is a text string, "Approved and digitally signed by John Smith, MD on June 18, 2010 at 1:07 PM ET." and the text is over-layed at a selected location on the document image; thus, visually the document has been altered. Next, a new document is created that "burns' the signature into the image, permanently changing the image with the signature.
3. the workflow associated with applying the signature varies but in general follows: HIM analyst identifies who needs to approve / sign the document and where on the document the signature needs to be placed. Next, the document is routed to those that need to review & sign. The people that need to do the reviewing / signing are presented with options (approve / sign, reject) and make their selection. Those that have been approved for signature are then signed and a new document created with the signature. Finally, references to the unsigned-document are translated so that only the new, signed document is returned to users.
In terms of integrating the eSignature capability among multiple systems, usually an EMR for the user interface and a DM system for managing the documents, I recommend that you rely on the EMR's workflow to manage user interaction. The key reason for this approach is that the EMR provides a consistent UI for the most critical user - the doctor. There are other reasons too (such as the EMR typically implements a role-based security model and it usually has a chart deficiency module to control the workflow), but not having to train the doctors is essential.