Family Offices Slow Direct Deals in September but Back Mistral, Brain Implants and Geothermal

The decline in deal count did not stop family offices from participating in startup mega-rounds. Hillspire, the personal investment firm of former Google CEO Eric Schmidt, participated in a 3 billion euro, or about $3.4 billion, Series D for Mistral. Mistral is a three-year-old French startup that builds a wide range of artificial intelligence models, including chatbots and coding assistants, as well as freely available open-weight models.

Duquesne Family Office, the firm of hedge fund titan Stanley Druckenmiller, participated in a $500 million Series A for Fab2 and a $250 million Series D for Precision Neuroscience. Fab2 manufactures small semiconductor factories. Precision Neuroscience develops brain implants that record neural signals to let users control digital devices.

Clean energy startups represented five of the month's transactions. Centaurus Capital, the firm of John Arnold, and John Doerr's namesake family office led a $135 million Series B for Mazama Energy, a geothermal energy startup. Bill Gates' private venture capital firm also participated in the round. Arnold, who made his fortune in hedge funds, and Doerr, who made his in venture capital, have backed multiple clean energy startups this year.

Mazama Energy drills into extremely hot rock and then circulates water through engineered cracks to bring heat back to the surface to generate carbon-free electricity. According to the three-year-old company, the technology enables Mazama to generate more energy from fewer wells and reduce the cost of power.

In 2025, Mazama drilled into rock at its Oregon pilot site that reached 629 degrees Fahrenheit, which the company says is the hottest engineered geothermal system on record. Mazama CEO Sriram Vasantharajan said the proof of concept showed a clear path to commercialization and was key to attracting top investors.

"When you talk about family offices or even any other investor, what they look for is a demonstrated track record. It's one thing to create the tech platform, but what Mazama has done is actually applied it," Vasantharajan said.

Investing in cutting-edge technology often comes with long investment horizons. Vasantharajan said that is not the case with geothermal energy. The Oregon site is already capable of producing 10 gigawatts of energy.

"The heat beneath our feet exists. Its temperature is proven," he said. "It is not a scientific problem that we're tackling. It is an engineering challenge."

Editor's Summary Family offices made fewer direct investments in September, with 42 deals versus 52 a month earlier, but still joined large funding rounds for Mistral, Precision Neuroscience and Mazama Energy. The activity shows family offices remain willing to place long-term bets on AI, semiconductors, brain implants and clean energy even as overall direct deal counts slow.