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from the Information-is-only-skin-deep dept.
A memristor (memory resistor) is a hypothetical circuit element that, in principle, would make up the fourth basic circuit element joining the resistor, capacitor, and inductor. One of the more interesting properties of an ideal memristor is that there exists a non-liner relationship between the applied voltage and current which gives rise to non-volatile memory behavior. This has resulted in a lot of exciting research in the semiconductor industry for new and improved memory chips.
The hallmark of a memristor is that the non-linear relationship between the electric flux and charge gives rise to a voltage-current plot that exhibits a pinched hysteresis behavior, namely that it looks like a frequency-dependent Lissajous figure that always crosses the plot at the origin. If one takes a step back from solid state devices and defines memristors in terms of this voltage/current behavior, there are a number of biologic-based systems that qualify, including human skin. If skin is a memristor, does that mean that it acts like non-volatile memory? In a new paper published in Nature's open-access journal Scientific Reports, Pabst et al show that this is indeed the case. They applied direct current voltage pulses to various parts of the human skin and show that analog information can be stored for at least three minutes.
DOI: https://doi.org/10.1038/s41598-019-55749-9
Paper Abstract
Much is already understood about the anatomical and physiological mechanisms behind the linear, electrical properties of biological tissues. Studying the non-linear electrical properties, however, opens up for the influence from other processes that are driven by the electric field or movement of charges. An electrical measurement that is affected by the applied electrical stimulus is non-linear and reveals the non-linear electrical properties of the underlying (biological) tissue; if it is done with an alternating current (AC) stimulus, the corresponding voltage current plot may exhibit a pinched hysteresis loop which is the fingerprint of a memristor. It has been shown that human skin and other biological tissues are memristors. Here we performed non-linear electrical measurements on human skin with applied direct current (DC) voltage pulses. By doing so, we found that human skin exhibits non-volatile memory and that analogue information can actually be stored inside the skin at least for three minutes. As demonstrated before, human skin actually contains two different memristor types, one that originates from the sweat ducts and one that is based on thermal changes of the surrounding tissue, the stratum corneum; and information storage is possible in both. Finally, assuming that different physiological conditions of the skin can explain the variations in current responses that we observed among the subjects, it follows that non-linear recordings with DC pulses may find use in sensor applications.
(Score: 2) by Rupert Pupnick on Thursday December 19 2019, @01:22AM (4 children)
The fourth basic circuit element from a theoretical point of view is a negative resistor: a circuit element that delivers power in proportion to an applied voltage or current.
Also, as khallow points out, being non linear also disqualifies it from consideration as a basic circuit element.
Wonder how much else they got wrong.
(Score: 0) by Anonymous Coward on Thursday December 19 2019, @02:12PM
https://www.onelargeprawn.co.za/wp-content/uploads/2017/04/EverythingBanner.png [onelargeprawn.co.za]
(Score: 3, Interesting) by hubie on Thursday December 19 2019, @05:17PM (2 children)
I don't understand your linearity requirement for something to be a basic circuit element.
Memristors as the fourth basic two-terminal circuit element comes directly out of circuit theory. If you take the four fundamental quantities (charge (q), current (i), voltage (v), and flux (f)), you can form six one-to-one relationships: i-q, i-v, i-f, q-v, and q-f. Of those six, five have well established relationships: charge is the time integral of current, flux is the time integral of voltage, the resistor is defined by the i-v relationship, the inductor from i-f, and the capacitor from q-v. The only one left is q-f. If you consider axiomatic completeness (which doesn't mean Nature does, but elegant mathematics does) this leads you to postulate a two-terminal device defined by the q-f relationship and you end up with the memristor. The characteristics of, and unique applications for memristors were described in a very nice paper [ieee.org] by Leon Chua in 1971.
It turns out that Chua is the third author on this human skin paper. I should be so fortunate almost 50 years after writing a seminal paper on a topic to be still doing active research on that topic. But I still need to write that seminal paper first before I start my 50-year clock.
(Score: 3, Interesting) by Rupert Pupnick on Thursday December 19 2019, @06:17PM (1 child)
Hi hubie, I'm starting from the assumption that if the original three basic circuit elements are linear, the fourth should be as well. I admit to having a narrower perspective on this as an engineer with a circuit design background, but linear time invariant systems are the fundamental building blocks for almost all modern electronic technology. You could make the argument that most modern electronics today are not linear in that they are digital, but the starting points, e.g. the development of the transistor, all strove originally to emulate linear behavior.
My view of how to define the four basic elements is based strictly on the phase relationship between voltage and current in a steady state sinusoidal excitation. You can think of the three commonly accepted elements as sitting on the unit circle at zero degrees (resistor), and +/- 90 degrees (inductor or capacitor). In my mind I see an empty slot at -180 degrees which corresponds to the negative resistor.
I admit that your four state variable approach is more comprehensive, particularly in that it allows for the description of the state of the energy storage elements (inductors and capacitors) that my concept ignores. Anyway, I never thought about q-f, so thanks for writing, and sorry about the snark.
(Score: 3, Informative) by hubie on Thursday December 19 2019, @08:07PM
No problem at all. As somewhat of a hobby I like to scan the open access literature for potentially interesting articles to summarize here (I'm a science journalist at heart, but I'm a lazy one at that, so I don't do it all that often). It gives me a way to learn something new, and in this case I did't know anything about memristors. I went back to Chua's original 1971 paper (I'm lucky to work at a place where I have institutional access to a lot of journals) and I found the paper to be very easy to understand, and with my background in high energy particle physics, Chua's Standard Model-like postulate of "what would it take to fill this hole in the theory" really appealed to me.