One of the hallmark pathologies of Parkinson’s disease is the “pill-rolling” tremor that
affects more than 75% of PD patients. These tremors interfere with a wide variety of daily tasks
involving fine motor control. The goal for our group was to design a user-friendly and
minimally invasive device that would be able to attenuate or eliminate these tremors. The design
our group arrived at takes inspiration from the destructive interference technology used in active
noise-canceling headphones and hopes to apply it to electrically attenuate the nerve signals
responsible for the tremors many Parkinson’s patients struggle with.
There are two parts to this approach. First, there must be a method to sense the action
potential traveling down the motor neurons towards the forearm muscles – this must be as
minimally invasive as possible to stick with the goal of having a non-intrusive and user-friendly
device. Secondly, there needs to be a way to cancel out the action potentials responsible for the
unwanted tremors. For the first part, an atomic magnetometer will be used. These chip-scale
sensors can sense the minuscule magnetic field fluctuations that accompany action potentials in
nerves from the surface of the skin [15]. In addition to this, they operate well at body
temperature and their small size makes them minimally intrusive.
Figure 2. The figure above demonstrates the location of the electronic neuromodulator and the
isolation of the nerve fiber after the action potential is detected by atomic magnetometer.
For the second part, electric neuromodulation – the application of an electric current
across the motor neuron bundle to block the action potential from traveling to the effector’s
muscles – will be used.
Figure 3. Chart showing effect of DC current applied across nerve fiber on the intensity
of action potential signal traveling through the application site .
Research done by Sassen and Zimmermann in 1973 demonstrated the feasibility of
canceling out action potentials by applying a direct current on the neuronal axon between the
source of the AP and the target site [23]. The electric current will need to be applied to the nerve
using an implant around the median nerve bundle in the forearm. This implant will consist of a
wireless charging antenna that attaches to the median nerve with an implant similar to the one
tested by Liu and Yu [16]. The implant will cancel nerve signals when the wireless charging
transmitter located above the implant on the skin surface is switched on. The implant will consist
of a device-to-nerve interface made of viscoplastic electronic materials [16] and a biocompatible
wireless charging antenna [24]. The implant will be fitted to the section of the median nerve
above the elbow via surgery and will be housed in the space between the bicep muscle and the
bone. The implanted wireless charging antenna will be actuated wirelessly by the primary coil in
the charging circuit positioned on the surface of the skin to provide the 59 microamps of current
at 0.4 volts needed to block the action potential from traveling further down the neuronal bundle
[23]. It was demonstrated that a biocompatible wireless charging antenna implant can be built at
the centimeter scale [24] and that morphing electronics enabling electric interface with neurons
can be built around 2 mm in width and with negligible thickness [16], so the implant for tremor
attenuation will likely be able to be built with a compact and minimally invasive footprint.
A computer chip will process the neuronal activity data collected by the atomic
magnetometer and sort out the signals associated with the tremors. The action potential
conduction velocity in the median nerve is between 49 and 64 m/s [25]. Assuming the
magnetometer and the ENM implant are spaced 10 cm apart, this gives between 1.5 and 2
milliseconds for the computer chip to make the computations and activate the ENM implant to
cancel out the signals. Active noise cancellation in headphones – an inspiration for the approach
outlined here – operates on a much shorter timeframe in the range of tens of microseconds due
to the higher speed of sound and the shorter distance between the headphone and the eardrum.
This gives a measure of confidence in the feasibility of the proposed approach. This approach
may allow for a compact and minimally invasive way to attenuate a major frustration in the lives
of people suffering from Parkinson’s disease.
Another method of electric neuromodulation uses kilo-hertz frequency alternating current.
This approach may have faster activation time and be more clinically feasible for the repeated
activation associated with the proposed design but requires more research