Quote from "Wearing your apple watch" support document;
A great deal of care and research goes into choosing materials for all of our devices. In addition to ensuring that all materials adhere to existing regulations, we developed our own specification for Apple Watch that goes beyond those requirements.In fact, every material that touches your skin has gone through extensive evaluation in accordance with our specification. This includes:
Thousands of material composition test
More than a thousand prototypes worn for trial studies
Thousands of toxicological assessments
Consultations with board-certified dermatologists
A small number of people will experience reactions to certain materials. This can be due to allergies, environmental factors, extended exposure to irritants like soap or sweat, and other causes.