Neuroprosthetics Week 6 Design Issues

Neuroprosthetic Devices Three basic types Three basic types 1. Use neural signals as an input, possibly to control a mechanical device 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 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 3. Those that have neural inputs and outputs – bypassing spinal cord damage

Devices with neural input Operation based directly or indirectly on neural activity Operation based directly or indirectly on neural activity Measurements interpreted to create control signals for an external device Measurements interpreted to create control signals for an external device Feedback is usually either: Feedback is usually either: Confined to the neuroprosthetic device itself, or Confined to the neuroprosthetic device itself, or Is via the body’s natural senses Is via the body’s natural senses

Devices with neural output Stimulation arises from an external trigger Stimulation arises from an external trigger Signal is processed and tailored for transmission to a nerve Signal is processed and tailored for transmission to a nerve Neuromuscular stimulation involves the transmission of a signal, via electrodes, to a muscle Neuromuscular stimulation involves the transmission of a signal, via electrodes, to a muscle

Devices with neural input and output Device modifies a nerve signal to achieve a desired goal Device modifies a nerve signal to achieve a desired goal Challenges of accurate input interpretation and appropriate interpretable output are significant 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 Such devices tend not to have significant material or power requirements – no aerial for example

Signal Measurement Measure signal from neural inputs over time without medical intervention Measure signal from neural inputs over time without medical intervention Must not be subject to an inhibitory body response or side effects Must not be subject to an inhibitory body response or side effects Any surface electrodes must be placed to avoid abrasion or local pain Any surface electrodes must be placed to avoid abrasion or local pain Development needed for body state measurement Development needed for body state measurement Reliability is key Reliability is key

Signal Interpretation Network (Neural) is nonlinear and complex Network (Neural) is nonlinear and complex Even with ANNs, internal signals are not meaningful Even with ANNs, internal signals are not meaningful Correlation between measured neural signal and intended action is a challenge Correlation between measured neural signal and intended action is a challenge Often signals have no apparent or simple meaning Often signals have no apparent or simple meaning Artificial devices are usually first order approximations to complex nonlinear relationships Artificial devices are usually first order approximations to complex nonlinear relationships

Intent Interpretation Inputs indicate state of the body Inputs indicate state of the body Body tissues will respond to any signals input Body tissues will respond to any signals input Feedback mechanism is in place Feedback mechanism is in place If a specific signal is input, what will the body likely do in response? If a specific signal is input, what will the body likely do in response? Whole body focus necessary Whole body focus necessary

Action Performance Device must be able to adequately perform an action Device must be able to adequately perform an action Power and material type are critical Power and material type are critical Prosthetic limbs – sufficient power required to lift and manipulate objects Prosthetic limbs – sufficient power required to lift and manipulate objects Battery lifetime is important – minor surgery for replacement Battery lifetime is important – minor surgery for replacement

Power Inductive power supply is an alternative to a battery – less needs to be implanted 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: Induced emf in implanted coil, related to external exciting coil current is: E = -nAdB/dt E = -nAdB/dt A is area of receiver coil A is area of receiver coil dB/dt = KIw dB/dt = KIw I = amplitude of exciting current I = amplitude of exciting current w = frequency of exciting current w = frequency of exciting current K is a constant K is a constant

Signal Generation & Transmission Inserting a neural input into the complex network – understanding of other possible results than just desired action 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 E.g. muscle fatigue and cramps can occur Long term approach for reliable and robust signal transmission onto/into nerves Long term approach for reliable and robust signal transmission onto/into nerves Devices must be supported – so not adversely affected by movement Devices must be supported – so not adversely affected by movement Devices must not cause irritation or other problems Devices must not cause irritation or other problems

Prosthetic Arms Loss of a limb is a traumatic experience whether through disease or accident 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 Limbs are the primary way in which humans interact with the world Amputees have an expectation of a prosthetic arm’s performance 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!! 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 So huge challenge both technical and social

State-of-the-art An ideal prosthetic arm would replace all natural functions: An ideal prosthetic arm would replace all natural functions: Strong, gentle, fast, high precision, stiffness range, sense heat and pressure Strong, gentle, fast, high precision, stiffness range, sense heat and pressure Must look and move as a natural limb Must look and move as a natural limb Arm must respond correctly to neural signals Arm must respond correctly to neural signals Most common, commercially available – Boston Arm, Utah Arm Most common, commercially available – Boston Arm, Utah Arm

Commercial Arms Performance of commercial arms is well short of original arms Performance of commercial arms is well short of original arms Boston elbow can lift only 9lbs Boston elbow can lift only 9lbs Speed varies with orientation to gravity – Utah arm flexes in 1.1 secs Speed varies with orientation to gravity – Utah arm flexes in 1.1 secs Utah has powered free swing in walking Utah has powered free swing in walking Switching from walking to normal mode Switching from walking to normal mode Removable, rechargeable batteries – do not provide enough energy for one day’s use Removable, rechargeable batteries – do not provide enough energy for one day’s use Control of a hook or hand provides even more problems Control of a hook or hand provides even more problems

Prosthetic Arm - Signals Commercial arms use myoelectric activity measured using surface electrodes Commercial arms use myoelectric activity measured using surface electrodes Electrodes typically are mounted in the limb socket – sit on stump surface Electrodes typically are mounted in the limb socket – sit on stump surface Superimposed muscle signals are recorded Superimposed muscle signals are recorded Other inputs include cables and switches to convert other body movement actions Other inputs include cables and switches to convert other body movement actions Surface electrodes now starting to be used Surface electrodes now starting to be used But this is complex + suffers from skin irritation and extraneous signal components But this is complex + suffers from skin irritation and extraneous signal components

Cochlea Implants Signals from microphone transmitted to implanted electrodes in the auditory nerve Signals from microphone transmitted to implanted electrodes in the auditory nerve DSP used to extract features DSP used to extract features No claim that hearing is restored No claim that hearing is restored User can usually recognise speech – assisted with lip reading etc User can usually recognise speech – assisted with lip reading etc Prefer no visible signs of implant Prefer no visible signs of implant Power, size, SP problems Power, size, SP problems

Bladder Loss of bladder control has social implications as well. Loss of bladder control has social implications as well. Regaining bladder control allows an individual to be part of society again Regaining bladder control allows an individual to be part of society again Vocare (commercial) system provides an external unit which controls bladder emptying Vocare (commercial) system provides an external unit which controls bladder emptying Signals are transmitted to an implanted receiver – activates appropriate nerves Signals are transmitted to an implanted receiver – activates appropriate nerves Future – smaller, less obtrusive and allow user overall control without external trigger. Future – smaller, less obtrusive and allow user overall control without external trigger.

Next Week Cochlea Implants Cochlea Implants