A nerve can be visualised as a communications path transporting information via neurons from the body to the brain and vice versa. Individual neurons transfer stimulus spikes called action potentials (AP) along neuron bodies (axons) and across neural interfaces (synapses). Our focus is to model a communications channel along the human peripheral nervous system using multiple stimulated neurons to exploit the higher level of voltage pulse (compound action potential or CAP) that can be generated.
A data transmission system requires a data source, transmission channel and a receiver. In our model, shown schematically in Fig 1, an external source converts data into coded ultrasound pulses for transmission to a nanodevice array embedded under the skin. The array converts the ultrasound to electrical pulses using piezoelectric nanowires and stimulates groups of neurons (fascicles) in an underlying peripheral nerve. The stimulation creates a corresponding stream of CAPs along the nerve. A receiver at the distant end must detect these CAPs and decode them as instructions for functions like programmed drug delivery or prosthesis control.
Fig. 1 Block diagram of end-to-end path from the external ultrasound source to the embedded stimulus array and along the peripheral nerve to the embedded receiver.
The CAP pulses are modulated using on-off keying (OOK) to generate a stream of logical “ones” and “zeros”. The maximum achievable range between transmitter and receiver depends critically on the number and diameters of the activated neurons (contributing to the CAP amplitude) and the level of background neural noise.
The maximum bit rate is 200 bit/s, a limit imposed by the refractory period (reset time) of neurons following activation. This bit rate can be sustained for between 100 mm and 200 mm, as shown in Fig 2. The bit rate must then be reduced to compensate for the reduction in the amplitude of the CAP and the corresponding increase in the signal-to-noise ratio (SNR).
Fig. 2 OOK bit rate for a CAP generated by stimulating 4000 neurons with a mean diameter of 9.5 µm and a background noise level that varies from 5 µV to 10 µV.
The modelled neural CAP communications channel is serial, unidirectional, low bit rate and therefore suitable for asynchronous transmission, where the sender and receiver have separate clocks. Modulated data packets can provide simple one-byte commands to an embedded receiver to trigger drug release, as shown in Fig. 3, or initiate a specific pattern of neural stimulation.
Fig. 3 CAP coded commands transmitted to an embedded drug delivery system. Drug reservoirs can be addressed and activated separately or in combinations.
Publication Title: Capacity Analysis of a Peripheral Nerve using Modulated Compound Action Potential Pulses
Authors: Michael Donohoe, Brendan Jennings and Sasitharan Balasubramaniam.
Journal: IEEE Transactions on Communications, vol. 67, no. 1, 2019.