Reported by the Heart of the Machine.
Edited by Rome Rome
Forest Neurotech and the Butterfly Network have collaborated to build a brain-computer interface capable of sub-millimeter precision manipulation, which will use ultrasound to stimulate and record brain activity compared to traditional electrical signals.
In 2023, brain-computer interface (BCI) technology is still experiencing a year of rapid development.
There are already some achievements that can decode brain signals to control external devices, and there are also achievements that can change brain signals through external devices. Some BCI development companies, such as Neuralink, Paradromics, and Synchron, are hopeful of entering human clinical trials.
In May of this year, Neuralink received approval from the U.S. Food and Drug Administration (FDA) for human clinical trials. In September, Neuralink announced its first human trial of a brain-computer interface, and the related developments received a lot of attention. Bloomberg later reported that Neuralink plans to perform 11 surgeries on clinical volunteers implanted with brain-computer interface devices next year.
In December 2022, Musk showed off an experimental monkey that can type by mind at the launch of Neuralink.
But recently, researchers from Forest Neurotech and the Butterfly Network announced that they are collaborating to develop a brain-computer interface device with a completely different medium: ultrasound.
In October, Forest Neurotech and Butterfly Network announced a joint $20 million investment to advance research into ultrasound-based brain-computer interfaces. Forest Neurotech will license technology from Butterfly Network's ultrasound chip to develop a minimally invasive ultrasound BCI device for academic research.
Most BCI systems use electrical signals as a medium to measure brain activity, and some electrical signals are also used to stimulate the brain. The device released by Forest will use ultrasound to interact with the brain instead of the usual electrical signals.
The underlying idea behind this work is based on the principle that direct focused ultrasound (FUS) can alter the action potential of neurons, the ionic currents that brain cells use to communicate with each other. By measuring changes in local blood flow through the Doppler effect, ultrasound can also be used to estimate neural activity in areas of the brain, known as functional ultrasound imaging (FUSI).
The Utah Array is a traditional implantable brain-computer interface that can cause a strong rejection response inside the brain.
William Biederman, chief technology officer at Forest Neurotech, said Butterfly's technology will allow his team to build a brain-computer interface that enables sub-millimeter precision manipulation that, when implanted in the user's skull, will use ultrasound to stimulate and record brain activity.
Why was ultrasound chosen as a medium for BCI?Compared with other nerve stimulation and imaging techniques, focused ultrasound can flexibly target specific areas of the brain from outside the skull, while the spatial range of electrical signal stimulation is limited. To stimulate the deeper areas of the brain with an electrical current, a craniotomy is performed in which electrodes are placed near the relevant area. In addition to implanting electrodes at depth, electrodes can be attached to the scalp to record electrical signals in the brain, but this method can only record activity in areas of the brain close to the surface. While ultrasound has not yet been able to penetrate the skull to record neural activity in the depths, it has been possible to do so by removing a portion of the skull and placing the device on the surface of the brain.
Ultrasound hardware developed by Butterfly integrates audio generation, control, and recording capabilities for large clinical ultrasound systems into a single chip-sized device. Based on these ultrasound chips, the Forest program utilizes directed, focused ultrasound waves to stimulate the brain and uses fusi to measure neural activity.
Schematic diagram of the ultrasonic chip of the butterfly network, sourced:
FUSI is a technique that estimates changes in neural activity by measuring changes in the blood. Like all cells, neurons need a blood supply. When nerve activity increases, blood flow also increases. The principle of the fusi technique is to measure blood flow by emitting ultrasound waves to brain areas where neurons are active and recording the waves that come back. When the sound waves are reflected back from the flowing blood, the vibrating frequency of the returned sound waves is different from the sound waves when they are emitted. The fusi technique indirectly estimates the electrochemical response of neurons by estimating changes in blood flow based on the phenomenon of Doppler shifts.
Mikhail Shapiro, a consultant at Forest Neurotech and a professor at the California Institute of Technology, said: "Using functional ultrasound to generate images of blood looking at nerve activity, this method has worked very well, even exceeding all expectations when it was first proposed. 」
Ultrasound can both record and control nerve activity. Although the fact that ultrasound can control nerve activity has been known since at least the 50s of the 20th century, scientists are still unsure how ultrasound causes neurons to fire. Recently, scientists have made new progress in an experiment using ultrasound to separate brain tissue from rodents. Experiments have shown that the opening of high-frequency sound waves appears to open specific calcium ion channels, but the exact mechanism of their occurrence remains a mystery.
In order to perform BCI stimulation and recording simultaneously, the system developed by Forest requires multiple Butterfly ultrasound chips implanted in the user's skull. While ultrasound stimulation can effectively penetrate the bone, fusi recording technology is not yet able to fully do so. As the sound passes through the skull, it is weakened, plus attenuation at the output and return, which makes a completely non-invasive ultrasound recording system unachievable. As a result, Forest plans to place the sensor inside the skull, flush with the dura mater, the protective membrane that covers the brain.
The partnership between Forest Neurotech and the Butterfly Network is part of the Butterfly Network Butterfly Garden program. The program is designed to provide technical support to medical device teams. Founded in 2011, Butterfly Network is an innovative digital health company with a mission to make high-quality medical imaging accessible to everyone, making ultrasound imaging the most accessible, easy to operate, globally accessible, and intelligently connected.
Forest Neurotech is part of Convergent Research, a nonprofit organization that focuses solely on technology development than manufacturing and marketing clinical devices. We have no plans to submit an application to the FDA during the lifetime of our non-profit research institution. Biederman, Chief Technology Officer at Forest Neurotech, said, "We are working hard to advance the technology of ultrasound brain-computer interfaces, as well as new discoveries in basic science. Once this technology has made more recent advances, Forest will make it available to academia and other industrial research institutions to drive product development and scientific and technological advancement.
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