New implantable chip to hook up bionic arm to brain

Creating
a more embodied experience for patients with artificial limbs 

By Dries Braeken

When someone loses an arm in a car or
industrial accident, they are left with a stump. But their brains’ primary motor
cortex still holds all the circuitry that used to drive that arm with its hand
and fingers. That circuitry is the brain’s image of that arm. It creates what many
patients experience as ghost itches and pain in a limb that is no longer there.
But it also keeps sending signals to move the hand and fingers. When patients inadvertently
want to shake someone’s hand or catch a ball that is thrown at them, the motor
cortex initiates burst of electricity. These are passed down through the spinal
cord, into the nerves that still serve the stump, and from there into the
remaining muscles, which can be seen twitching.

All that is needed to help these patients,
it seems, is to make a sufficiently sophisticated artificial arm with
electromotors and sensors and fuse it with the stump.

To do so, however, a major
multidisciplinary effort is required. It involves e.g. developing superlight
and strong mechanics, integrating electromotors that allow the same forces and
flexibility as human muscles, developing a biocompatible interface with the
stump, and translating between human-generated and artificial signals.

A first generation of such smart
prosthetics was designed to pick up the electrical signals from the remaining
muscles, using these to drive the electromotors in the artificial arm. But
patients equipped with such an artificial arm often experience it as a dead
weight, an object that not really belongs to them. And therefore, they’d often
rather not use it.

There are a number of reasons why they
wouldn’t.

For one, these prosthetics do not allow the
superfine finger control with which people interact with their environment and
which they have learned through decades of finetuning. Patients can pick up a
pen but they cannot write.

But even more important is the missing
feedback. The artificial limb is not able to signal to the brain if it is
touching a soft surface, picking up a burning coal, or applying just enough
pressure to hold that Starbucks coffee cup but not crush it.

To create a more natural prosthetics
experience, DARPA (the research agency of the USA army) set up the HAPTIX
program, shorthand for ‘Hand Proprioception and Touch Interfaces’. The goal of
the program is to create a prosthetic hand that moves and provides sensations just
like a natural hand. It wants to do so by interfacing directly with the nerves
instead of the muscles, to connect the biological and artificial circuitry
through a permanent, implanted link that sends and reads electrical signals in
two directions.

The nerves that run through our body are
bundled and shielded in nerve fascicles, resembling how e.g. control and
communication wiring in a building is bundled in a cable tray. Our arms and
hands are controlled by two such bundles, called the median and ulnar fascicle.
One promising technique that is developed and tested by HAPTIX is to integrate electrodes
in a collar that fits around the 5mm thick bundles. But the challenge is to read
and stimulate from the exact right nerves in the bundle without contacting
them, nerves moreover that move and slide inside the fascicle. So there is an
upper limit to the precision that can be reached with this method

A more precise approach might be to insert
electronics into the fascicle to contact the individual nerves. It’s a method
that carries more risk, needing precise microsurgery that places electrodes
inside the fascicle but doesn’t damage the actual nerves. Four years ago, a
first such electrode was implanted temporarily in a patient (https://actu.epfl.ch/news/amputee-feels-in-real-time-with-bionic-hand/).
The test was successful but showed the need to create compact implantables that
include much more intelligence and tightly-spaced contact points, i.e.
electrodes that stimulate and read the individual nerves.

That is where the recently unveiled imec
chip comes in.

Manufactured with the same silicon
technology that has been fine-tuned to make today’s advanced computer chips,
the new biochip is only 35um thick, thinner than an average hair. On its
surface are 64 electrodes that allow stimulation and recording of nerves, with
a possible extension to 128. Through a needle attached to the chip, the package
can be precisely lodged inside a nerve bundle so that the electrodes come into
close contact with individual nerves.

Imec’s experts made the chip in close
collaboration with their colleagues from the University of Florida, a main
contractor under the HAPTIX program. They selected imec because it offers two unique
advantages over other R&D facilities. Unlike most R&D labs involved in
chip technology, it also has the equipment and processes in place to do actual high-quality
manufacturing. And unlike most other manufacturing sites, it has the flexibility
to set up dedicated manufacturing processes for such innovative designs.

What was actually the biggest challenge was
not manufacturing the chip itself but rather integrating it with the package.
In recent years, imec has developed a series of comparable biochips, think of
neuroprobes with hundreds of electrodes along a long flexible shaft. So its
researchers knew how to design the prosthetics chip. But until now, they had
never created a hermetically sealed, biocompatible and flexible package to
serve as a long-term human implant.

To do so, the engineers sandwiched
nanolayers with superior barrier properties with very thin flexible polymer
layers. The final result is an ultrathin flexible electronic device with a
thickness comparable to that of a human hair and suitable for minimally
invasive implantation.

The first phase of the project has been
successfully concluded, and will now be followed by a testing phase in which
the prototype will be manufactured in larger volumes. The new prosthetics chip
will then be tested at the University of Florida and possibly also at other
labs, mainly to see how it behaves in a biological environment – if it remains
sealed and functioning for an extended period. In the meantime, researchers can
start looking how to convert all the signals from the nerves into useful
signals for the prosthetics. And in the other direction, they should figure out
which useful signals from the prosthetics’ sensors they can inject in the nerve
system and where precisely these signals should go.

These studies are a first step towards a sensory-enhanced
prosthetic. Although it will take another few years to develop a commercially
available arm, results like those of the University of Florida and imec show
that this type of prosthetic can be made, that patients will eventually get
prosthetics that feel more embodied, and that even the full bionic prosthetics
of science fiction movies may one day become a reality.