Elon Musk's brain implant startup Neuralink, which was valued at close to $2 billion in a private fundraising round two years ago, is now worth around $5 billion based on privately executed stock trades described to Reuters by five sources with knowledge of the matter.

Some purchases by bullish investors boosted the valuation in recent months, ahead of Neuralink's May 25 announcement that U.S. regulators had approved a human trial on its brain chip, the sources said.

Experts have said it could take several years for Neuralink to secure commercial use clearance. Kip Ludwig, former program director for neural engineering at the U.S. National Institutes of Health (NIH), said he "optimistically" expected Neuralink to take at least 10 more years to commercialize its brain implant. The company also faces other challenges that include federal probes into its handling of animal research.

Following the trial's approval, however, Neuralink shares were marketed privately to investors in recent days at a $7 billion valuation, equivalent to $55 per share, according to an email seen by Reuters. Reuters could not establish whether the seller found buyers for that price. The email cited the U.S. Food and Drug Administration's (FDA) approval of the clinical trial as grounds for the deal being "sweeter."