These issues involved the device's lithium battery and the potential for the implant's tiny wires to migrate to other areas of the brain. Questions over whether and how the device can be removed without damaging brain tissue were also put forward, according to the seven sources.
In November last year, Musk predicted that the company would secure a go signal from the regulator to conduct human trials by spring.
"I could have a Neuralink device implanted right now, and you wouldn't even know," the SpaceX, Tesla and Twitter CEO said during the Nov. 30 presentation. He predicted the approval at that same presentation, which did not quite come true. Musk made headlines after saying he was so confident in the device's safety that he would be willing to implant it in his own children.
Kip Ludwig, former program director of neural engineering at the National Institutes of Health, remarked that Musk's claims and impatience pose a critical test for the FDA in balancing demands for speedy review with the diligence that safety and efficacy call for.
"Everybody in the industry was saying: 'Oh my God, they're going to run straight into a brick wall,'" Ludwig said of Musk's application. "Neuralink doesn't appear to have the mindset and experience that's needed to get this to market anytime soon."
Owen Faris, principal deputy director of the FDA's Office of Product Evaluation and Quality, emphasized the agency's high standards in vetting all products even as it aims to expedite reviews. "Innovation and safety are not an either-or scenario," he said.
Even before the FDA turned down Neuralink's application for human trials, the company had been beset by various issues.
Late last year, Reuters reported that the federal government – through the U.S. Department of Agriculture (USDA) – was investigating the company's treatment of its research animals. The probe was launched amid growing employee concern that the company's rushing of experiments caused additional suffering and deaths of pigs, sheep and monkeys. (Related: Elon Musk accused of "mutilating and killing monkeys" in gruesome Neuralink experiments.)
Another Reuters report revealed that the Department of Transportation (DOT) launched a separate investigation. This time, the probe scrutinized whether Neuralink illegally transported dangerous pathogens – via the chips removed from monkey brains – without proper containment measures. The DOT told Reuters that its investigation is ongoing as of press time.
Putting investigations by federal agencies aside, Neuralink's acceptance among the population remains low as evidenced by a Pew Research poll.
Of the more than 10,000 American adults surveyed by Pew, around 78 percent said they would not avail of a Neuralink brain chip even if the product was readily available. In contrast, only 13 percent of respondents said brain implants are good for society.
Moreover, 56 percent of respondents believed technology would be actively bad for society. Another 57 percent expressed worry that the widespread use of Neuralink implants and other brain devices could potentially widen the gap between high and low-income Americans.
One of the seven sources Reuters talked to remarked that Musk operates Neuralink to operate like Tesla, which brought several electric vehicles to market relatively quickly.
"He can't appreciate that this is not a car," they said. "This is a person's brain. This is not a toy."
Visit ElonMuskWatch.com for more stories about Neuralink brain implants.
Watch this video that discusses monkeys dying after being implanted with Neuralink devices.
This video is from The Prisoner channel on Brighteon.com.