Dear Shengqinag,

Thank you for your interesting question.

According to our observation and other researcher’s work, the electric field would affect the distribution of tiny particles inside the droplet, which is relevant to a phenomenon called dielectrophoresis (ref.12). Whether this effect can work on ions or living cells still remains an interesting topic. More advanced techniques are needed to explore this problem.

Thank you!

Jikun Wang

Dear Zheng,

Very good questions! These are exactly the problems we meet in our experiments.

The printing speed of the PLEEC method is determined by the time of liquid patterning and the time of polymerization.

In this work, for the hydrogel polymerization process, we increase the content of photoinitiators in the hydrogel precursor so its curing time is about 100s. However, there are already some researches about producing high-efficient photoinitiators for hydrogels, which can reduce the curing time to merely 6s (ref. 17). Those works on photoinitiators are definitely compatible with our method.

For the liquid patterning process, it always takes us 10-20s to complete, because a high guiding speed of liquid may give the droplet more power to escape from the electric force. To shorten this time period, we need to focus on how to tune the hydrophobicity of the top surface and how to increase the electric force on liquid.

The pixel space between pixels is about 1/5 of the pixel size so that the liquid can link together between two adjacent pixels. If wider, two single droplets would be formed on the panel; if narrower, the panel would be easy to suffer electric breakdown. We choose this space according to our experimental observations. Behind those phenomena there may exist some interesting problems about electric field distribution and fluid dynamics.

Thank you!

Jikun Wang

Dear Jiawei,

Thank you for your questions.

In this work, we use dimensional analysis to estimate the precision of the PLEEC panel. The capture of a droplet is the competitive effect between the surface energy of the droplet and the electric field energy in the space. For a droplet with radius a, its surface energy is in the scale of γa2. When this droplet occupies a space in the electric field, the change of the total electric energy is in the scale of ε0E2a3. By comparing these two formulas, we estimate that the critical length scale of the liquid that can be trapped is a~γ/ε0E.

Thank you!

Jikun Wang

Dear Shengqiang,

Thanks for the interesting question! Actually we didn't pay attention to the effect of ions by electrowetting at all. We dont know if electrochemical reactions happen or not. What we can say is that the printed ionic hydrogel successfully lit the light. Thank you for pointing this out. We will carefully think about it in future.

Best

Tongqing

Dear Ruobing,

Thank you for the insightful thoughts. It's really a brilliant idea! I have read your paper before. I feel this optical printing idea is potentially doable.

I will read something about the photochemistry in solution. Hope I can get back to you soon for further discussions.

Best

Tongqing

Dear Zheng,

Thanks for your questions.

I cannot remember the printing speed precisely but I do have checked that the printing speed of our current method is of the same level compared to other popular technique when we print a structure with the same size.

I will ask my student to get involved to answer your questions.

Best

Tongqing

Hi Tongqing,

Congratulations for the very nice work!

Several really quick questions: if the liquid contains ions, how the electrowetting process will be affected. How significantly will the applied electric field change the distribution of ions? will the electrowetting/printing process be accompanied by any electrochemical reactions?

Thanks,

shengqiang

Dear Tongqing,

Congratulations on this very nice work. It is motivating to see such a combination of existing technology into new research directions. I'd like to brainstorm some future directions that may seem crazy but potentially doable.

This work reminds me our earlier paper, optomechanics of soft materials, where one uses optical force (rising from radiation pressure, or Maxwell stress) to change the shape of an ultra-soft material. If Maxwell stress can now serve as a shape-morphing technique for hydrogel 3D printing, it is conceivable that a well controlled optical force can do so as well, with an even higher resolution thanks to the optical wavelength. Of course, one has to be concerned about the high laser power and the potential heating, especially on the substrate, in this case.

Fortunately, optical shape morphing can go beyond using radiation pressure, in a more efficient way. One can introduce photochemistry to induce shape change of certain molecules, leading to deformation of the material without heating. Same as electrowetting, this technique was first studied (and is probably still mostly studied) in solution. However, using it to achieve shape morphing and patterning dates back to last century, such as optically recorded holography. See a review paper by Ikeda.

I hope this brainstorming can bring some further thoughts and discussions.

Best regards,

Ruobing

Dear Tongqing,

Congratulations on the great work! I found the discussion above is really inspiring and informative. I have two more questions below and would like to ask for your insights:

1. I am curious about the printing speed. As reflected by Fig. 5, the PLEEC technique proceeds via sequential patterning, polymerization, and resetting, by which one layer of the final product is printed. That is, we need to fully polymerize each layer of the material before printing the next layer. Since polymerization of hydrogels may take minutes or hours, the total time for printing a structure of complex geometry seems to be long (the total time = the time needed to polymerize one layer of hydrogel * the number of layers). To this end, I wonder how long it takes to print a real structure by PLEEC.

2. The PLEEC printer is constructed by patterning an array of unsymmetric layers (electrode pixels). I wonder how the spacing between neighboring pixels affect the printing performance and how did you choose the spacing when designing the PLEEC printer.

Many thanks!
Zheng

Dear Jiawei,

Thanks for your comments. These are really good questions.

1. The liquid should not be too viscous, otherwise it cannot flow easily on the hydrophobic cover surface. I dont think there are other limitations for the choice of liquid.

2. Just consider the limiting case where the top electrode is gone. In this case, the electric field is zero and there is no effect of electrostatic force.

3. For the unusua people who have been working with dielectric elastomers, high voltage was never an issue. But for practical application, it could be.

4. In our case, the expansion of the DE layer is very small, which can be neglected.

5. Great point! It is exactly what one student in my group is working on right now! We also notice that the PLEEC technique enables flexibility of the printing substrate. By a carefully designed/or even controlled substrate, the printed strucutre can be more complex. Actually, on this point we are also inspired from one of Xuanhe's old work:

Qiming Wang et al. Dynamic electrostatic lithography: Multiscale on-demand patterning on large-area curved surfaces, Advacned Materials, 24, 1947, (2012)

6. The size of the asymmetric layer (basically on the order of the thickness) determines the thickness of the printed liquid.

7. The resolution of the electrode is just one factor. The decisive factor is the competetion between surface energy and electrostatic force. We may miss the discussion of this part in the post, but we have discussed in detail in our paper.

8. We think hydrogel 3D printing is just one possible application of this PLEEC technique. We are further improving the printing method. On the other hand, we are trying to explore more possibilites in massive liquid manipulation for lab on a chip, and transfer printing. I'd like to hear suggestions from you guys!

Thank you!

Tongqing

Hi Canhui, thanks for your interests and comments.

Your first concern is: the hydrogel pattern depends on the electrode pattern.

The solution is that we can make electrode pixels, as shown in Fig. 3c, 3d. Currently we cannot do large arrays of pixel due to the difficulty of complex electric circuit. But as I know, for the people who work on integrated ciucuit, it is not a big issue.

Your second concern is：the accumulated thickness.

It is tricky that in our printing process, each newly polymerized layer of hydrogel material was attached to the previously printed hydrogel structure on the top platform (See Fig. 5d). So when we finish printing one layer, the printing area becomes vacant for the next layer. The thickness can be arbitrarily large without limitation. For example, in the printed strucutre in Fig. 7e, the thickness of the printed strcuture is on the order of centimeter while each layer is about 0.1 millimeter.

Dear Tongqing,

This work is really interesting! I have several questions regrading to the mechanism of the PLEEC.

Can the liquid be any liquid?

Can you explain what happens when the top electrode is much smaller than the bottom one, and why the performance is not good?

The PLEEC requires high volatge, is this a drawback compared to the other methods?

Under voltage, the DE layer would expand, which may cause delamination of layers?

Can you design a morphable substrate that allows changes shape of liquid on demand?

How do you control the thickness of the printed liquid?

Is the resolution of liquid determined by the resolution of the electrode in your design?

What is the future direction of this technology?

Thank you!

Jiawei

Thank you, tongqing, for sharing such an interesting and inspiring idea, as well as the brief review of various 3D printing technique of hydrogels. I appreciate that the PLEEC strategy does not rely on hydrogel precursors of specific physical physical properties such as rheology and shear thinning. But I have concerns here: the pattern of asymmetirc capacitor (similar to the printing plate in typography) is fixed. Does it mean one has to make new pattern of asymmetirc capacitor for new pattern of printed hydrogels? Also, what's the thickest sample the PLEEC printing strategy can achieve? Is it possible to print a "3D" structure using this technique?