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Neuralink’s headline-making first human implant encountered a serious hardware-positioning problem: many of the flexible threads connecting the implant to the brain moved out of position. Reports in May 2024 estimated that approximately 85% of the threads in participant Noland Arbaugh’s implant had retracted.
That did not mean the entire brain implant came loose or was removed. Neuralink said the implant remained in place, while the displaced threads reduced the number of electrodes making useful recordings. The company compensated with software and decoding changes, then implanted its second PRIME Study participant, Alex, in July 2024.
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What happened to Neuralink’s first implant?
Noland Arbaugh, who became paralyzed after a diving accident, received Neuralink’s PRIME Study implant in January 2024. The device was designed to record neural signals associated with intended movement and translate them into computer commands.
The implant used 64 flexible threads carrying a total of 1,024 electrodes. Each thread is thinner than a human hair and is inserted into brain tissue by Neuralink’s surgical robot. The threads connect the brain’s electrical activity to the implant, which wirelessly sends data to a computer.
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Neuralink’s May 8, 2024 update said that a number of threads had retracted from the brain. Reporting based on Arbaugh’s account and Wall Street Journal information estimated that roughly 85% of the threads had become displaced. The 85% number was not presented as an official, detailed Neuralink measurement of every electrode.
Important distinction: “85% of the threads retracted” is not the same as “85% of the electrodes failed.” Each thread carries multiple electrodes, and Neuralink described the result as a reduction in effective electrodes rather than publishing an exact electrode-by-electrode failure percentage.
Retraction did not mean the implant detached
The term retraction describes threads moving back from their intended positions in brain tissue. It does not describe the Link implant itself falling out, detaching, or being surgically removed.
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Neuralink reported that Arbaugh’s bits-per-second, or BPS, performance initially declined after the thread movement. BPS is a measure of how quickly and accurately the system can translate neural activity into computer actions. The company said it improved performance by:
- making the recording algorithm more sensitive to signals from groups of neurons;
- improving cursor-decoding techniques; and
- modifying the user interface.
After those changes, Neuralink said Arbaugh’s BPS performance recovered and exceeded his initial result. That recovery showed that software could offset at least some loss of signal quality, but it did not remove the underlying surgical and mechanical concern.
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Why the second participant mattered
The second implant was intended to test whether changes to the operation and thread placement could reduce the chance of retraction. At the time of the May 2024 report, Neuralink planned to reduce brain movement during surgery and minimize the gap between the implant and the brain’s surface.
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Reporting also said the threads for the second participant would be placed approximately 8 millimeters deep, compared with roughly 3–5 millimeters for Arbaugh. That depth detail came from reporting rather than from a complete public Neuralink surgical specification.
Neuralink subsequently confirmed that the second participant, identified as Alex, received the Link at Barrow Neurological Institute in July 2024. The company said Alex was discharged the next day and had a smooth recovery.
What Neuralink reported about Alex
In an August 21, 2024 update, Neuralink said Alex controlled a computer cursor within five minutes of connecting to the system. The company also said he used the implant to play Counter-Strike and began working with computer-aided-design software.
At that point, Neuralink reported no observed thread retraction in Alex’s implant. That was an encouraging early result, not proof that the design had permanently eliminated the risk. Thread position and signal performance require continued monitoring over time, especially in a small investigational study.
| Issue | First participant: Noland Arbaugh | Second participant: Alex |
|---|---|---|
| Implant timing | January 2024 | July 2024 |
| Reported thread problem | Substantial retraction; reports estimated about 85% | No observed retraction in Neuralink’s August update |
| Reported response | Software, decoding, and interface changes | Surgical and placement changes intended to reduce risk |
| Early reported use | Cursor control and computer interaction | Cursor control, Counter-Strike, and CAD software |
Was the second implant approved for general use?
No. The regulatory authorization allowed Neuralink to proceed with another investigational clinical-trial implant. It was not commercial approval, product clearance, or permission to sell the brain-computer interface as a generally available medical device.
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The relevant regulatory framework is the FDA’s Investigational Device Exemption, or IDE. An IDE permits an investigational device to be used in a clinical study so researchers can collect safety and effectiveness data. That status is separate from FDA marketing approval or clearance.
Why the original headline can be confusing
The original report was published on May 20, 2024, when the second participant had not yet been implanted. Its wording therefore said Neuralink was “to implant” a second human. Neuralink later confirmed that the procedure took place in July 2024.
The original headline also used a 75% figure before being corrected to 85%. Some old links and forum posts may retain “75%” in their URL slug, but the corrected reporting refers to an estimated 85% thread retraction.
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The first implant exposed a weakness in the system’s physical interface with the brain. A brain-computer interface can have sophisticated decoding software, but its performance still depends on electrodes staying close enough to the right neural populations.
At the same time, Arbaugh’s recovery in BPS performance demonstrated an important benefit of software adaptation. Neuralink did not simply lose 85% of the implant’s usefulness; it adjusted the decoder to extract more information from the remaining signals. That is a meaningful engineering result, but it should not be confused with a demonstration that thread retraction is harmless.
The second participant was an early test of whether changes to the operation and placement could prevent the same problem. Alex’s initially stable implant was positive news, while the small number of participants and limited follow-up meant that longer-term safety and reliability remained unresolved.
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FAQ
Did 85% of Neuralink’s electrodes fail?
No. The widely reported estimate referred to approximately 85% of the flexible threads becoming displaced. Each thread contains multiple electrodes, and Neuralink did not publish an exact percentage of failed electrodes.
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No. The implant itself remained in place. The reported problem was that many of the electrode-bearing threads retracted from the brain, reducing the number of effective neural recordings.
Who was Neuralink’s second human participant?
Neuralink identified the second PRIME Study participant as Alex. The company said Alex received the Link at Barrow Neurological Institute in July 2024.
Was Neuralink’s second implant commercially approved by the FDA?
No. The authorization concerned another investigational clinical-trial implant under the FDA’s Investigational Device Exemption framework. It was not commercial approval or clearance of the device.
The Bottom Line
Neuralink’s first human implant suffered substantial retraction of its flexible brain threads, with outside reporting estimating that about 85% had moved out of position. The implant remained in place, and software changes restored the first participant’s reported cursor performance. Neuralink then implanted Alex in July 2024 using surgical and placement changes intended to reduce the risk, initially reporting no thread retraction. That result was promising, but it was still an early investigational finding rather than proof of long-term reliability.
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