Vestaron makes an eco-friendly pesticide derived from spider venom. Bagaveev uses 3-D printers to make rocket engines for nanosatellites. Transatomic Power is developing a next-generation reactor that runs on nuclear waste.

They all have one thing in common: money from Silicon Valley venture capitalists.

After years of shying away from science, engineering and clean-technology start-ups, investors are beginning to take an interest in them again, raising hopes among entrepreneurs in those areas that a long slump is finally over. But these start-ups face intense pressure to prove that their science can turn a profit more quickly than hot tech companies like Snapchat and Uber.

In August, the Founders Fund, which has backed social networks like Facebook and Yammer and the streaming-music service Spotify, announced a $2 million investment in Transatomic Power of Cambridge, Mass. Days earlier, Y Combinator, known for aiding web and mobile-app start-ups like the social news site Reddit and the game maker Omgpop, took part in a $1.5 million early investment in Helion Energy, which is developing an engine powered by nuclear fusion.

And last month, Google said it was buying Lift Labs, a San Francisco biotechnology start-up that makes a high-tech spoon for people with hand tremors.