Which procedure involves forcing chemical solutions into unbroken skin?

The procedure that involves forcing chemical solutions into unbroken skin is iontophoresis. This technique utilizes a mild electrical current to enhance the penetration of various substances, such as serums and medications, into the skin. The application of the current essentially pushes the positively or negatively charged ions of the solution into the skin, improving the effectiveness of the treatment.

In contrast, cataphoresis is similar but specifically refers to the process of using electric current to introduce positively charged ions into the skin. Although it is related to iontophoresis, it is more specialized in its application.

Microdermabrasion focuses on mechanically exfoliating the outermost layer of skin using abrasive particles, which does not involve the use of chemical solutions. Similarly, exfoliation, as a general term, refers to the removal of dead skin cells and can be achieved through physical, chemical, or mechanical means, but does not imply the introduction of solutions into the skin.

Iontophoresis stands out as the correct answer because it specifically involves the application of electric current to introduce chemical solutions into unbroken skin, an important concept in esthetics and skincare treatments.