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Neuroprosthetics . Week 6 Design Issues. Neuroprosthetic Devices. Three basic types 1. Use neural signals as an input, possibly to control a mechanical device 2. Based on some action, transmit a signal to a nerve for a desired response

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Week 6

Design Issues

Neuroprosthetic devices
Neuroprosthetic Devices

  • Three basic types

  • 1. Use neural signals as an input, possibly to control a mechanical device

  • 2. Based on some action, transmit a signal to a nerve for a desired response

  • 3. Those that have neural inputs and outputs – bypassing spinal cord damage

Devices with neural input
Devices with neural input

  • Operation based directly or indirectly on neural activity

  • Measurements interpreted to create control signals for an external device

  • Feedback is usually either:

  • Confined to the neuroprosthetic device itself, or

  • Is via the body’s natural senses

Devices with neural output
Devices with neural output

  • Stimulation arises from an external trigger

  • Signal is processed and tailored for transmission to a nerve

  • Neuromuscular stimulation involves the transmission of a signal , via electrodes, to a muscle

Devices with neural input and output
Devices with neural input and output

  • Device modifies a nerve signal to achieve a desired goal

  • Challenges of accurate input interpretation and appropriate interpretable output are significant

  • Such devices tend not to have significant material or power requirements – no aerial for example

Signal measurement
Signal Measurement

  • Measure signal from neural inputs over time without medical intervention

  • Must not be subject to an inhibitory body response or side effects

  • Any surface electrodes must be placed to avoid abrasion or local pain

  • Development needed for body state measurement

  • Reliability is key

Signal interpretation
Signal Interpretation

  • Network (Neural) is nonlinear and complex

  • Even with ANNs, internal signals are not meaningful

  • Correlation between measured neural signal and intended action is a challenge

  • Often signals have no apparent or simple meaning

  • Artificial devices are usually first order approximations to complex nonlinear relationships

Intent interpretation
Intent Interpretation

  • Inputs indicate state of the body

  • Body tissues will respond to any signals input

  • Feedback mechanism is in place

  • If a specific signal is input, what will the body likely do in response?

  • Whole body focus necessary

Action performance
Action Performance

  • Device must be able to adequately perform an action

  • Power and material type are critical

  • Prosthetic limbs – sufficient power required to lift and manipulate objects

  • Battery lifetime is important – minor surgery for replacement


  • Inductive power supply is an alternative to a battery – less needs to be implanted

  • Induced emf in implanted coil, related to external exciting coil current is:

  • E = -nAdB/dt

  • A is area of receiver coil

  • dB/dt = KIw

  • I = amplitude of exciting current

  • w = frequency of exciting current

  • K is a constant

Signal generation transmission
Signal Generation & Transmission

  • Inserting a neural input into the complex network – understanding of other possible results than just desired action

  • E.g. muscle fatigue and cramps can occur

  • Long term approach for reliable and robust signal transmission onto/into nerves

  • Devices must be supported – so not adversely affected by movement

  • Devices must not cause irritation or other problems

Prosthetic arms
Prosthetic Arms

  • Loss of a limb is a traumatic experience whether through disease or accident

  • Limbs are the primary way in which humans interact with the world

  • Amputees have an expectation of a prosthetic arm’s performance

  • Many amputees expect their prosthetic arm will be much more powerful than the original – due partly to science fiction!!

  • So huge challenge both technical and social

State of the art

  • An ideal prosthetic arm would replace all natural functions:

  • Strong, gentle, fast, high precision, stiffness range, sense heat and pressure

  • Must look and move as a natural limb

  • Arm must respond correctly to neural signals

  • Most common, commercially available – Boston Arm, Utah Arm

Commercial arms
Commercial Arms

  • Performance of commercial arms is well short of original arms

  • Boston elbow can lift only 9lbs

  • Speed varies with orientation to gravity – Utah arm flexes in 1.1 secs

  • Utah has powered free swing in walking

  • Switching from walking to normal mode

  • Removable, rechargeable batteries – do not provide enough energy for one day’s use

  • Control of a hook or hand provides even more problems

Prosthetic arm signals
Prosthetic Arm - Signals

  • Commercial arms use myoelectric activity measured using surface electrodes

  • Electrodes typically are mounted in the limb socket – sit on stump surface

  • Superimposed muscle signals are recorded

  • Other inputs include cables and switches to convert other body movement actions

  • Surface electrodes now starting to be used

  • But this is complex + suffers from skin irritation and extraneous signal components

Cochlea implants
Cochlea Implants

  • Signals from microphone transmitted to implanted electrodes in the auditory nerve

  • DSP used to extract features

  • No claim that hearing is restored

  • User can usually recognise speech – assisted with lip reading etc

  • Prefer no visible signs of implant

  • Power, size, SP problems


  • Loss of bladder control has social implications as well.

  • Regaining bladder control allows an individual to be part of society again

  • Vocare (commercial) system provides an external unit which controls bladder emptying

  • Signals are transmitted to an implanted receiver – activates appropriate nerves

  • Future – smaller, less obtrusive and allow user overall control without external trigger.

Next week
Next Week

  • Cochlea Implants