Neuralink, Elon Musk's brain chip startup, is being valued as high as $42 billion in recent secondary market transactions, according to correspondence seen by Business Insider and a person familiar with the matter. That is an almost five-time increase from Neuralink's last funding round last year, when the startup raised at a $9 billion valuation. According to private markets research provider Caplight, transactions in recent months have ranged from $29 billion to $42 billion, with some current bidders even attempting to buy shares at an almost $60 billion valuation. Neuralink's soaring secondary-market valuation reflects investors' growing appetite for a piece of Elon Musk's AI and robotics empire. Because Neuralink is a private company, some investors buy shares via secondary markets, where stock is sold by employees or early investors. Neuralink builds brain implants designed to help people with paralysis control computers with their thoughts, and eventually restore their eyesight, with the chips implanted in 21 trial patients so far. Neuralink's official valuation and whether it's raising a new funding round aren't clear.