LinkedIn Corp. co-founder Reid Hoffman is leading a $12 million funding round for Sanmai Technologies and joining the board of the startup, which aims to use ultrasound technology and artificial intelligence to improve human health.Hoffman’s investment is part of an explosion of interest by tech billionaires in funding the frontiers of brain science. Earlier this year, Coinbase Inc. co-founder Fred Ehrsam unveiled a company building a take-home ultrasound helmet to treat diseases and aid wellbei...