Note: This text is a work in progress. If you find any inaccuracies or material that needs to be included, please contact us.
Concept development
Along with EMG, we are considering the use of several other technologies which may offer reduced cost and increased reliability.
Electrical Impedance Tomography (EIT)
Impedance tomography is a method of getting a cross-sectional image of the body very safely and cheaply. All that is needed is a ring of electrodes, often as few as 16, contacting the skin, a source of constant current, and an analog to digital converter to sense voltages on the electrodes. A small alternating current is passed through two of the electrodes and the voltage is recorded at all of the others. This process is then repeated very rapidly while changing the electrode pair the current passes through. The resulting data is then used to solve a non-linear system of equations, producing an image of the tissues between the electrodes. Different tissues resist electric current to different degrees, and so muscle, bone, and blood can all be differentiated with this method. Further details of the process can be found in chapter 26 of the web edition of Bioelectromagnetism. You can also look at some sample images and videos of EIT data. Possible analyses of the data might include changes in the cross-sectional area of a contracted muscle or changes in the impedance of a contracted muscle.
Advantages of EIT:
- It generates electricity for measurement, and so is not as susceptible to noise as EMG.
- The redundancy of multiple sensors might make it less sensitive to noise and contact positioning than EMG.
- It may require substantially less analog amplification and filtering than EMG.
Disadvantages of EIT:
- It probably requires an array of many sensors, which in turn will need multiplexing and lots of labor-intensive freeform wiring.
- It will need some computing power to handle the data stream, but this may be less than expected because a full and accurate image is not needed by a prosthetic control system.
- It will require a constant supply of power, at least a few milliamps, which will use battery life.
- To our knowledge, this techology is untested in prosthetics and is still mostly in the academic realm.
Acoustic/Mechanical Myography (AMG/MMG)
Muscles actually make sounds when contracting, which you can test by putting your thumbs in your ears and making a fist. The low rumbling sound you hear is your muscle fibers resonating as they contract. It turns out that this resonance (at between 20 and 30 Hz) can be picked up by a subsonic microphone and easily analyzed to detect muscle movements. The power (loudness) of the sound correlates well with the force on the muscle. The phenomenon is described in this article, and its use in prosthetics is covered by US patent 4571750, which will expire in February 2006, but also by US patent 4748987, which will not expire until June of 2008. We are attempting to get in touch with the inventor, Daniel T. Barry to discuss intellectual property issues.
Advantages of MMG:
- Muscle sounds can be monitored by cheap microphones, even through a prosthetic sock, without many of the concerns of fitting an electrode.
- The signal is more powerful than EMG, and less susceptible to electrical interference, so only sound shielding is needed.
- The signals can be processed by ubiquitous and cheap digital audio hardware like DSPs.
Disadvantages of MMG:
- It detects actual muscle effort, not intent, so fatigue will tend to reduce the response of the prosthesis.
- It is susceptible to loud environmental noises. This effect can probably be mitigated by active noise control, the same technology used in some consumer headphones.
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