In 1955 while addressing the National Academy of Sciences Richard Feynman stated "Scientific knowledge is a body of statements of varying degrees of certainty." As usual, Feynman's statement was spot on, and holds true decades later. In his famous "Plenty of Room at the Bottom" lecture Feynman talked about what we now call nanotechnology, and all the different applications. Here I am, half a century later, working "at the bottom" and living in a world of uncertainty. I hope to share some of the exciting discoveries at the nanoscale and explain how they apply to my passion of biotechnology- as well as the everyday world. Learn more about Nicholas Fahrenkopf
My posts are presented as opinion and commentary and do not represent the views of LabSpaces Productions, LLC, my employer, or my educational institution.
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In case you haven’t heard, memristors are a big deal (see NanoLetters, ACS Nano, and Nature). So what are they and why are researchers in academia and industry so interested? Are they going to change life as we know it?
Scientists, at least folks like me working at the intersection of biology and technology, are really interested in memristors because they are a fully electronic component that act (broadly speaking) like neurons. That is, they have the capacity to “remember” based on the current that flows through them. So, just like you might develop muscle memory from touching a hot pan (and hopefully learn to pull your hand away fast) memristors can learn. What caught my eye recently was this article that touted protein based memristors. That is, an electronic component that can mimic biological circuits, using biological molecules. Woah.
So let’s back up. What is a memristor? The typical spiel is that memristors are the fourth fundamental circuit element besides the resistor, capacitor, and inductor. They were theorized back in 1971 by Leon Chua of UC Berkeley. In the most basic electronic components you have charge, voltage, current, and magnetic flux. The description of a capacitor relates charge to voltage; a resistor is described as the relationship between voltage and current; an inductor as current and magnetic flux. Those are the first 3 fundamental elements. Chua theorized there should be a 4th element that closes the loop and can be described by the relation of magnetic flux and charge. Not until 2008 did anyone ever build or observe the memristor, when a group at HP Labs announced they had made a memristor from titanium dioxide.
A memristor is a two terminal device- you basically have two wires coming off of it. Like any wire or resistor when you apply a voltage between those two terminals you’ll get a characteristic current to flow. The current you get for a specific voltage depends on the resistance of the wire (its physical dimensions, and the intrinsic resistivity of the material). Typically, this resistance is constant, so if you double the voltage, you double your current. However, in a memristor the resistance is constant until you hit what is called a “set” voltage where the device changes its resistance. So you could slowly increase your voltage from zero and have a certain “high” resistance but once you hit 1.4V (for example) the current would jump as if you shorted out the circuit. Then, if you were to sweep the voltage back to zero you would notice that you now have a “low” resistance. You could even start sweeping back up and still have that “low” resistance until you hit (say) 1V which is the “reset” voltage. At that point the current drops and you’re back in the “high” resistance state. Overall, a memristor is a variable resistor; there are two resistance states that are switched based on the amount of current or charge that you flow through it. There is a lot of interest in these devices in the electronics industry because they can serve as non-volatile memory (or switches). The memristor will “remember” if it was in a high or low state even if you remove all voltages for a long time. In addition, memristors can take up much less space than other non-volatile memory and switches.
So, since 2008 reserachers have been doing scientific experiments to figure out how and why memristors do what they do at the molecular level. Engineers have been figuring out what materials can be used as electrodes and what materials can be used can be used as the material in between the electrodes. In fact, one group in India has claimed to have made a tube of blood act like a memristor (not entirely useful). Some groups have started trying to incorporate them in traditional nanoelectronics processing, while others have started to think that they could be used for bio-inspired computing. These things “learn”!
So back to this recent paper (and here’s a link to the actual study). The authors fabricated a wire out of gold with a nano-gap. They then functionalized the surfaces of the gap with a thiol based self-assembled monolayer, which then captured their protein. The idea was that their protein would act as the memristive layer. And it appears to have worked. From the summary:
“In summary, we have demonstrated for the first time protein-based nanodevices showing reproducible memristive behavior. … This work provides a direct proof that natural biomaterials, especially redox proteins, could be used to fabricate solid-state devices with transport junctions, which have potential applications in functional nanocircuits.”
The idea that you can use proteins as electronic devices- in this case a memristor- is intriguing, but I’m not convinced from this study alone. The protein they chose to use was ferritin. So, they filled a 12nm gap with a bunch of protein stuffed with an iron core. To really buy this, I would have liked to see:
a) A clear image of the nanogap, or more information on how they made the gap- 12nm is hard to make!
b) A protein without a metal core since there are so many materials (like iron oxide) that have been used before.
c) Selective functionalization in the gap. By using SAM chemistry you’re functionalizing the gap, and the sides of the gold wires, and who knows how that is contributing.
(Image is the graphical abstract of "Protein-Based Memristive Nanodevices" DOI: 10.1002/smll.201101494)
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