Neuroprosthetics

1. Neuroprosthetics Week 6 Design Issues

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

17. 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

18. 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.

19. Next Week • Cochlea Implants