He came up with a compound called acetylsalicylic acid (ASA) which appeared to share the therapeutic properties of other salicylates, without causing stomach irritation.

BBC: Aspirin: A century of good news

The discovery of aspirin was inspired by Felix Hoffman's attempt to find a drug to ease his father's arthritis without causing the severe stomach irritation associated with the standard anti-arthritis drug of the time, sodium salicylate.

BBC: Aspirin: A century of good news

The device was worn for up to 24 hours without loss of function or skin irritation.

BBC: Health