Yes. Under Texas law, you can sign your advance directives digitally or electronically. Digital or electronic signatures include scans, uploaded images, computer-generated images, and others that are intended to serve as a signature.
However, there are extensive requirements in Texas regarding the technology you use for this purpose. Among other requirements, the electronic or digital signature must be unique to you, verifiable, and solely under your control. You can find a full list of these requirements in the Texas statutes.
These state laws are likely to change as time passes and technology evolves. Further, it may be difficult to know whether your signature complies with the technological standards set forth in the statutes. If you have any doubt about the validity of your electronic signature, it may be safer to sign your advance directives on a hard copy.