Demand for anything Elon Musk related is booming.

The near-billionaire’s brain chip startup, Neuralink, is valued at up to $42 billion in recent secondary market transactions, according to correspondence seen by Business Insider and a person familiar with the matter.

That’s a nearly five-fold increase from Neuralink’s last funding round last year, when the startup raised a $9 billion valuation, Semafor reported at the time.

According to private market research provider Caplight, transactions in recent months have ranged between $29 billion and $42 billion, and some current bidders have even attempted to buy shares at a valuation of nearly $60 billion.

Neuralink’s rising secondary market valuation reflects growing investor appetite for a piece of Elon Musk’s robotics and artificial intelligence empire. Following SpaceX’s successful initial public offering, private investors are increasingly willing to pay high premiums to access Musk’s remaining startups.

Because Neuralink is a private company, some investors purchase shares through secondary markets, where employees or early investors sell the shares. These valuations are estimates based on demand and not ongoing public market prices.

Neuralink did not respond to a request for comment.

Musk has earned a reputation for making private investors money, in part by combining his businesses. Investors in X and XAI were eventually able to get SpaceX shares through those acquisitions. One venture capitalist said several of his limited partners have been asking to buy Neuralink shares “at any price.”

However, there is hardly any consensus on where Neuralink should trade, and some investors are resistant to the pricing of these transactions.

Neuralink is one of Musk’s most futuristic companies. He builds brain implants designed to help paralyzed people control computers with their thoughts and eventually restore their sight. That plan is still early, as the chips have been implanted in 21 trial patients so far, the company announced earlier this year.

Neuralink’s official valuation and whether it is raising a new round of financing is unclear. The most recent publicly announced investment came in May, when Oman’s sovereign wealth fund revealed it had invested in Neuralink at an undisclosed valuation.

Musk’s wealth is increasing thanks to SpaceX’s IPO last month. SpaceX is currently valued at around $1.6 trillion in the public markets.