Now that there are many tablet computers, our goal is to create the world's smallest tablet!! We designed and structured a 9x12 pixels display made of DNA origami. Our tablet shows multiple pictures autonomously, corresponding to its environment.
To realize this idea, we combined two technology, DNA origami and Bistable system. Detailed information is written in latter section.
What is the Bistable System?
The bistable system enables us to encode two exclusive states, using a network of chemical reactions.
For example, we represent two states by materials A and B. The picture on the right side shows a reaction network implementing a bistable system using those two materials. In this network, A promotes the production of A itself and iB ("i" means inhibition), and B promotes B itself and iA. On the other hand, iA inhibits the production of A and iB inhibits production of B. There are also reactions that continuously degrade the number A, B, iA, and iB.
If the system is started with a larger amount of A than B, this network promotes more the production of A, which in turn will induce a decrease of B (Detailed information is written in Simulation|Simulation section). Afterwards, there remains only A, and vice versa if the initial conditions are opposite. Thus, thanks to the bistable system, we can make two states easily: "only A" or "only B". Any intermediate state cannot be stable and will go toward one of these two exclusive states.
We design the bistable system with DNA. 
- The Production of A,B,iA,iB
B and iA are also produced in the same way.
- The Inhibition of Production of A,B
The production of B is also inhibited in the same way by iB.
- The Decomposition of A,B,iA,iB (nothing means dNMPs a non-reactive monomer of DNA)
What is the DNA tablet ?
The DNA tablet shows two different pictures in response to its environmental changes. We observe those pictures by AFM.
There are four kinds of pixels:
- Θ is always on (it can always be observed).
- Φ is always off (it can not be observed).
- can be observed when there is a lot of A in the solution.
- can be observed when there is a lot of B in the solution.
These pixels are made of hammerhead-like structures. They stand on DNA origami and can be observed when they are hybridized. Please note that both and can not be observed at the same time because the bistable system only makes the two condition; "only A" or "only B".
These four kinds of pixels enable us to switch pictures (You can see the simulation here). The way of switching is very simple: you just need to switch the state of the bistable system. If you put enough B when you are observing the image A, all A will disappear, and B will be produced, which makes the image B appear. Then you can switch again to come back to image A just by putting enough A. Note that if you don't have the bistable system and you just add A (or B) strands when you want to see the image A (or B), the solution will end up with a concentrated mixed one of A and B strands, and the image on the surface will be blurred after a few changes of states. On the contrary the bistable keeps the picture clear and reversible in any time.
How to See
We use AFM to see the DNA tablet. Generally, AFM detects the difference between hybridized and single-stranded ends of the staple strands on the DNA origami surface. However, even if the single strands on the DNA tablet are hybridized, they do not produce enough contrast to be seen by AFM. Therefore, we applied hammerhead structures to the DNA tablet because the structures can be observed as pixels with higher AFM contrast. However, they cannot be dynamically updated by hybridization-dehybridization reactions. Therefore, we introduced open hammerheads (pseudo-hammerheads) stuructures to produce enough contrast and make themselves updatable.
When the strands produced by the bistable system are hybridized, they compose stable hammerhead structures and produce visible pixels.
DNA Tablet with a N-stable System: More Pictures
We are sure that the size of the library can be extended by using a n-stable system.
The figure above depicts a model of the tristable system. Each of A, B and C produces itself and inhibits the production of the others. Also, the number of the all products (A,B,C,iA,iB,iC) are degraded by the exonuclease. The decreasing processes are not shown in the picture. We can change the picture which the DNA tablet shows while this system realizes the condition of "only A" or "only B" or "only C" in the same way as the bistable system.
In a similar way, the DNA tablet can show many pictures by using an n-stable system. In this system, each of A1, A2, A3......A(n-1) and An produces itself and inhibits the production of all the others. This system can realize all conditions of "only Ak (k = 1,2,...n)".
We can represent such n-stable system by an n-sided polygon in order to make the diagram simpler. Mutual inhibitions between each two of A1, A2...and An are expressed by red lines. The degradation and self-production processes are not shown in the figure.
The characteristics of the DNA-tablet with n-stable system is that we can freely switch the pictures.
If you want to see the simulation of n-stable system, please click here.
DNA Tablet with an N-oscillator System: Play a Movie
The DNA tablet will also be able to show a short movie by using an n-oscillator system.
The figure above is the model of the trioscillator system. The difference between this system and the tristable one is that each of A, B and C inhibits the production of only one of the other DNA so that inhibitions are not mutual. In this system, the condition changes make a circuit of "only A", "only B" and "only C", and the DNA tablet switches its pictures automatically: if A is dominant, the inhibition of A to C becomes strong, then the inhibition of C to B gets weaker so that B increases. Then inhibition to A from B gets strong, and as a result B becomes dominant. In the same way, the circuit continues to go around. Here we don't have to input some specific strands to change the pictures.
The figures below are simplified models of the tristate oscillator and n-oscillator systems. The difference between these models and those of the tristable and n-stable systems is quite simple: the sides of the polygon are blue. The blue lines represent non-mutual inhibition while red ones mean mutual inhibition. For example, A inhibits all of the self-productions except B's, and B does so except C's, and so on. In this case, the change of the states is clockwise: A to B to C to A, or A1 to A2 to...to An to A1.
This system enables the DNA tablet to play a short movie. If we prepare slightly different images like film frames, and they change automatically, we can observe a movie on the surface of the tablet. If you want to look at the movie in the reverse direction, you just need to change the direction of the inhibition circuit.
If you want to see the simulation of oscillator system, please click here.