Tex. Health and Safety Code § 166.011

(a) For an advance directive in which a signature by a declarant, witness, or notary public is required or used, the declarant, witness, or notary public may sign the directive or a written revocation of the directive using:

(1) a digital signature that:

(A) uses an algorithm approved by the department;

(B) is unique to the person using it;

(C) is capable of verification;

(D) is under the sole control of the person using it;

(E) is linked to data in a manner that invalidates the digital signature if the data is changed;

(F) persists with the document and not by association in separate files; and

(G) is bound to a digital certificate; or

(2) an electronic signature that:

(A) is capable of verification;

(B) is under the sole control of the person using it;

(C) is linked to data in a manner that invalidates the electronic signature if the data is changed; and

(D) persists with the document and not by association in separate files.

(b) In approving an algorithm for purposes of Subsection (a)(1)(A), the department may consider an algorithm approved by the National Institute of Standards and Technology.

(c) The executive commissioner by rule shall modify the advance directive forms required under this chapter as necessary to provide for the use of a digital or electronic signature that complies with the requirements of this section.