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Introduction

I've recently stumbled upon David Eagleman's TED talk on the concept of sensory substitution and addition. Being the most inspiring thing I've seen on the internet for the last few months I've decided to give it a try and mix it with some of my coding/electronics projects.

I've settled on using electrical impulses over electrode pads to feed sensory data onto a patch of skin, instead of vibro-motors as it is more energy efficient. That being said, when planning on running electrical current through me 24/7, safety obviously comes first.

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I am targeting the lower back or the side of the hips as these parts have the least contact with the environment and won't make you look goofy. In the long run this would probably have to run on dry skin to avoid the "wrinkling effect", caused when staying in water for too long.

Questions

Main unknowns I want to sort out are:

  1. What is the range of perceivable voltage and current that can be safely fed through the skin without causing any damage to the nerves or any other part of the body?

  2. Is there a risk of permanent damage or reduction of sensitivity of nerves when exposed to an electrical current for a prolonged time? Any way to circumvent this problem?

  3. What "resolution" or input density can I hope to achieve, before signal becomes "blurry" because of cross-talk?

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  • $\begingroup$ Welcome to Bio.SE! Interesting question. Are you planning on using the electrode grid on dry skin, on the skin using an electrolytic solution/gel or are you targeting the tongue (aka BrainPort). Your questions hinge on impedance, that's why I ask. As of now the question is too broad to my opinion. $\endgroup$ – AliceD Mar 28 '15 at 0:25
  • $\begingroup$ Thanks for the support! Added some additional info in the description. $\endgroup$ – Alexander Ivanov Mar 28 '15 at 10:58
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Nice question(s). I will go through them in sequence.

  1. Safe current limits. With regard to electrical stimuli it is current density that determines safety limits. Therefore, in medical devices, it is not a matter of what is a safe maximum current level, but what is the maximum safe current level given the surface of the electrode? And in terms of skin stimulation, it is not the safety limit that is the bottle neck, but the maximum tolerable current density, which is around 0.5 mA/cm2 (Prausnitz, 1996). Higher stimuli result in pain sensations. This is one of the reasons why the BrainPort device is used on the tongue; the moist surface reduces impedance and hence the currrent needed to elicit a tactile response.

  2. Permanent damage. As said, the tolerable sensation limit is more of an issue (a must-read in this regard is Kaczmareck et al., 1991).

  3. Acuity. The best-possible tactile "resolution" is generally expressed in terms of tactile acuity. Tactile acuity varies across body sites, and unfortunately it is about the worst on the thigh (45 mm) and the back (40 mm). A much cited picture on tactile acuities is the following from Weinstein (1968):

2-point discrimination thresholds

These acuities represent the distance in which 2 simultaneously presented, static pressure points need to be spaced to be just noticeable (the so called two-point discrimination threshold). Although your proposed electric stimuli are different from pressure stimuli, these reported values in the figure (40 - 45 mm) give an impression on the inter-electrode distance needed. From my own work I can confirm the ~40 mm using vibratory stimuli. The unfavorable acuity of the back and external bodily skin in general was also one of the rasons to put the electrode grid of the BrainPort on the tongue, which has excellent tactile acuity.

References
- Kaczmareck et al. IEEE Trans Biomed Eng 1991;38:1-16
- Prausnitz, Adv Drug Delivery Rev 1996;18:395-425
- Weinstein S. 1968. In: Kenshalo. The Skin Senses:195-222. Springfield, IL: Thomas

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  • $\begingroup$ Thanks for the answers and references. The chart is especially eye-opening and would help placing the "display" somewhere more appropriate. I am approaching this whole matter with the hope that the brain can (learn to) subconsciously recognize information more finely than the perceived tactile acuity, which can then be a measure for base precision to be built upon. $\endgroup$ – Alexander Ivanov Mar 28 '15 at 16:13
  • $\begingroup$ While, for practical reasons, the tongue might not be suited for sensory addition, I am now considering other mucous membranes such as the inner part of the cheeks as viable surfaces which are more sensitive and require less power to stimulate. $\endgroup$ – Alexander Ivanov Mar 28 '15 at 16:22

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