Phase Scientific International (PSI), the Hong Kong start-up that edged out established mainland Chinese peers to win US approval for a Covid-19 rapid antigen test (RAT) kit, owes its success to its US roots and experience in the field, according to its founder. The Bill and Melinda Gates Foundation-backed biotech company last July became the first in the mainland China, Hong Kong, Macau and Taiwan region to win approval from the US Food and Drugs Administration for emergency use for its Covid-19 kit. Ricky Chiu Yin-to, a Hongkonger who moved to the US to pursue a PhD in bioengineering at…