By Christina Farr and Malathi Nayak SAN FRANCISCO (Reuters) – A group of ex-gaming industry executives say they can use their design chops to solve a major health challenge: Sick patients neglecting to take their medication and costing employers and insurance providers billions of dollars. Jason Oberfest began thinking about applying game design tricks to complex medical problems in 2011, while at mobile game company ngmoco. Oberfest built the app to engage users in their health, but he maintained some of the most viral aspects of mobile games, such as gifts, and a feature to see how friends are faring in their treatment. An analysis in the Annals of Internal Medicine found that Americans are failing to comply with prescriptions and it is costing the U.S. health system between $100 billion and $289 billion annually.