Neural interfaces are currently the leading option to provide true sensory feedback from bionic limbs but they have significant drawbacks.
The Need for Neural Interfaces
It is challenging to use a bionic limb without sensory feedback, especially a bionic hand. Recognizing this, the bionics industry has placed an increasing emphasis on providing sensory feedback from their devices.
For the most part, this feedback has been rudimentary. For example, placing a pressure sensor in bionic fingertips and using the strength of vibrations in the prosthetic socket to convey grip force. Or tightening and releasing the skin in the upper arm to reflect the opening/closing of a hand, as shown in this video:
However, this isn’t true sensory feedback from the hand. That is, the brain isn’t directly experiencing the hand’s sensory input. It is experiencing a proxy of that input, which doesn’t fully satisfy the desire of most end-users for a more natural experience.
Another form of sensory feedback is achieved using Transcutaneous Electrical Nerve Stimulation (TENS). The idea here is to directly stimulate nerves in the residual limb by sending an electrical charge through the intervening skin and tissue.
In this case, the brain does experience sensations in its missing hand that abstractly represent sensory input from the bionic hand. For example, the contact of a bionic hand with an object might cause the user to feel a tingling sensation in the missing hand. This helps the user feel more connected to the bionic hand but the feedback itself isn’t very informative.
At some point in this journey, it occurred to scientists that they needed to have more direct and sophisticated communication with nerves, i.e. a neural interface.
Neural Interface Basics
An amputee’s nervous system remains intact above the point of amputation. That system is fully capable of conveying the same sensory information to the brain as it did previously. Scientists just need to figure out how to talk to it.
One method is to apply electrical stimulation to a nerve from a distance, as we described for TENS. But to have any kind of detailed conversation, one needs to directly interface with the nerve. If the right kind of stimulation is applied to a precise area of a nerve, it can trick the brain into experiencing something representative of what a bionic device’s sensors are experiencing. This is the fundamental concept behind a neural interface
So, how do you stimulate a nerve in this manner? Two popular methods are to surgically implant an electrode so that it either a) wraps around a nerve bundle (i.e. cuff model) or b) it transects the nerve bundle. Both options are shown in this diagram:
There has been some success with this approach. For example, this next video demonstrates one of the most effective examples we’ve seen of a neural interface used in combination with a bionic hand:
However, creating an effective neural interface is not without its challenges.
The Surgical Challenges of Creating Neural Interfaces
To begin with, surgically implanting electrodes to interface with nerves is not minor surgery, as it might seem from the preceding illustrations. Here is what an implant looks like in real life (Warning: Graphic Image).
The obvious drawbacks of such a procedure include:
- the cost of surgery;
- the pain of recovery;
- potential nerve damage;
- risk of infection;
Other drawbacks may not be so obvious:
- It is difficult to position an electrode for the precise stimulation of a nerve, meaning that stimulation intended to convey sensory feedback from a thumb may be interpreted as coming from another digit. To make matters worse, the brain seems incapable of adjusting to these inaccuracies.
- There is also a risk of scarring, which may eventually interfere with the ability to stimulate the nerve.
- A solution that relies on complicated surgery is not exactly scalable for the many millions of people around the world who might benefit from it.
Still, there are significant benefits to the solution.
The Benefits of a Neural Interface
The first and obvious benefit of a neural interface is that having sensory feedback significantly improves user control.
Another possible benefit is a reduction in phantom limb pain. There hasn’t been any conclusive scientific evidence of this yet, but several scientific studies indicate that increasing an amputee’s sense of prosthetic embodiment can reduce pain. This includes a 2018 study titled, “Motor Control and Sensory Feedback Enhance Prosthesis Embodiment and Reduce Phantom Pain After Long-Term Hand Amputation“.
Neural Interface Alternatives
One alternative to creating a neural interface using surgically implanted electrodes is Targeted Sensory Reinnervation (TSR).
TSR does involve surgery but does not implant electrodes into the patient’s residual limb, which eliminates the risk of scars forming between the electrode and the nerve.
Instead, it reinnervates a patch of skin with the sensory nerves from the missing portion of the limb such that touching the patch of skin is equivalent to touching the missing limb. This seems like a more natural method of communicating with the nerve compared to using direct electrical stimulation.
Which method will win out in the end? We have no idea!
How Amputees Should Use This Information
The whole point of our 8-part series on Surgical Techniques That Improve the Use of Bionic Limbs is to make amputees aware of their surgical options.
Some of these surgical options are so compelling that we think they will eventually be incorporated into standard amputation procedures.
Others have a more uncertain future.
From our vantage point, neural interfaces involving surgically implanted electrodes fall into this latter group. If TSR can provide an adequate sensory feedback link between bionic sensors and the human nervous system, it is hard to imagine patients opting for implanted electrodes. But that is a big if!
For a complete description of all current upper-limb technologies, devices, and research, see A Complete Guide to Bionic Arms & Hands.
For a complete description of all current lower-limb technologies, devices, and research, see A Complete Guide to Bionic Legs & Feet.