SAN FRANCISCO — Big drug companies and biotechnology start-ups have forged ­alliances in recent years to share the risks of new ventures, and now that trend is spreading to nonprofit hospitals as government funding for research and expansion projects becomes harder to secure.

In the latest such deal, Boston Children’s Hospital, the nation’s largest pediatric research medical center, said Tuesday that it has struck a partnership with a California diagnostics equipment maker to form a company to develop tests for pediatric diseases and interpret results.