Neuroprosthetics

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Neuroprosthetics

• Week 6
• Design Issues

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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

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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 bodys natural senses

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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

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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

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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

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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

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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

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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

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Power

• 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

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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

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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
  arms 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

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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

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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 days use
• Control of a hook or hand provides even more
  problems

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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

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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

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Bladder

• 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.

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Next Week

• Cochlea Implants