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Journal Club for September 2019: Hydrogel 3D printing with the capacitor edge effect

tongqing.lu's picture

Hydrogel 3D printing with the capacitor edge effect

 

Recent decades have seen intense developments of hydrogel applications for cell cultures, tissue engineering, soft robotics, and ionic devices. Advanced fabrication techniques for hydrogel structures are being developed to meet user-specified requirements. Existing hydrogel 3D printing techniques place substantial constraints on the physical and chemical properties of hydrogel precursors as well as the printed hydrogel structures. We propose a novel method for patterning liquids with a resolution of 100 μm by using the capacitor edge effect. This technique is applicable to a wide variety of hydrogels, overcoming the limitations of existing techniques. We demonstrate printed hydrogel structures including a hydrogel scaffold, a hydrogel composite that responds sensitively to temperature, and an ionic high-integrity hydrogel display device. The proposed technique offers great opportunities in rapid prototyping hydrogel devices using multiple compositions and complex geometries.

https://advances.sciencemag.org/content/5/3/eaau8769

 

Pattern liquid using electrostatic force

Existing technique: Electrowetting 

Electrowetting means to change the wettability of droplets on the substrate by changing the voltage between droplets and the insulating substrate, that is, changing the contact angle to make the droplets deform and displace (Fig. 1a). Electrowetting has found many applications in lab-on-a-chip [1-4], micro-lenses [5-7], fiber optics [8,9] and display technology [10,11]. In particular, electrowetting can be harnessed to pattern liquid. For example, Chiang et al. developed an electro-microfluidic device that effectively assembles liquid patterns on a chip [12]. The liquid droplets on the chip are controlled  to dispense, mix, transport, position, and pattern by electrowetting (Fig. 1b,1c).

 

Fig. 1. (a) Schematic of electrowetting. When a voltage is applied across the liquid, the contact angle changes due to electrostatic force. (b) Patterned liquid by electrowetting, top view. (c) Cross-sectional view [12].

Proposed technique: PLEEC (Patterning liquid using edge effect of capacitor) 

Fig. 2. Principle of PLEEC. An asymmetric capacitor is separated by a dielectric layer.

Here, we propose a novel method of patterning liquids with the capacitor edge effect (PLEEC) [13]. Edge effect of capacitor refers to the distortion of electric field line at the edge of capacitor, and a small part of electric field exists in outer space of capacitor. For a symmetric capacitor, i.e., where the upper and lower electrodes are of equal size, the edge effect is very weak. However, for an asymmetric capacitor, the edge effect can be greatly strengthened. 

The proposed PLEEC panel consists of five layers (Fig. 2). A pair of electrodes made of silver adhesives is separated by a dielectric layer (polyimide film). The upper electrode has a smaller size than the lower electrode. The three layers form an asymmetric capacitor. At the bottom is the substrate made of insulating material (acrylate film). The top layer (Teflon film) acts as an insulating cover to separate the liquid on the top from the upper electrode. The top layer is chosen to be hydrophobic so that any liquid on the top tends to flow away in the absence of an electric field. Upon applying an electric field, the edge effect generates electrostatic force to trap the liquid on top of the hydrophobic layer. We find that the asymmetric design greatly amplifies the edge effect of the capacitor and thus can trap the liquid firmly in competition with its surface energy. The change of Helmholtz free energy of the PLEEC system with and without liquid is calculated using finite element methods. In the optimal design for the asymmetric capacitor, the size of the upper electrode is roughly half of the lower electrode’s size.

 

Fig. 3 (a) Asymmetric capacitors with different shapes. The lower electrodes have double the widths of the upper electrodes. When the voltage is on, the liquid is trapped within the patterned region of the lower electrodes. (b) Liquid pattern in the shape of an angry bird and letters “X,” “J,” “T,” and “U”. (c) Liquid patterns of nine natural numbers by independently controlling line pixels. (d) Changeable liquid patterns in the same PLEEC panel by independently controlling 10 × 10 pixels. (e) Liquid patterns of four representative hydrogel precursors: UV/heat/Ions polymerizable materials and four functional materials: temperature sensitive, biocompatible, ionically conductive, and molding materials.

 

Hydrogel 3D printing

Existing technique: Light-based printing 

DLP (Digital Light Projection), SLA (Stereo Lithography Apparatus)

In DLP, hydrogel structures are fabricated through a pull-out procedure from a hydrogel precursor with the aid of photo-patterned crosslinking[14-16]. In SLA, the precursors are selectively photopolymerized by a laser layer by layer[16]. These two printing methods enable high-speed processing of hydrogels with very high resolution, which ranges from 10 µm to 100 µm [15,17]. However, they are limited to patterning with photopolymerizable hydrogel precursors [16]. One way to increase photoinitiator (PI) content is to mix poorly water-soluble PIs with hydrogel precursors through agitation, heating, or using organic solvents [18]; the other way is to transform poorly water-soluble PIs into highly water-dispersible PI nanoparticles by surface modification [17]. 

 

Existing technique: Ink-based 3D printing

DIW (Direct Ink Writing)

DIW deposits hydrogel precursors through positioned ejection from a movable print head. The resolution of DIW is usually lower than that of DLP and SLA, which ranges from 100 μm to 1 mm; nevertheless, DIW provides higher degrees of flexibility in choosing different types of hydrogels and can print multiple hydrogels simultaneously. However, the hydrogel precursor is water-like and difficult to deposit. To increase the viscosity of the precursor, nanoclays must be added or pre crosslinking must be done before deposition. Moreover, the velocity of the extruded precursors and the velocity of nozzle movement need to be well controlled to match the viscosity of the precursor. The mechanical properties of DIW-printed objects are usually greatly affected by the processing mentioned above.

 

Fig. 4. Existing hydrogel 3D printing technologies mainly fall into two categories: Light-based printing and Ink-based printing. For Light-based printing, the hydrogel precursor should be made to be photo-polymerizable. For Ink-based printing, the hydrogel should be mixed with nano-clays to tune its viscosity.

 

Proposed technique:  Hydrogel 3D printing with PLEEC

The technique of patterning liquid by using electrowetting cannot be used for hydrogel 3D printing, because all the patterning manipulation are constrained in the narrow region between two electrodes. We now explore a new hydrogel 3D printing principle by using the PLEEC technique [13]. The new printing method has the potential to eliminate the restrictions of material properties and complex requirements for hydrogel precursors in existing techniques. The printing process is described in Fig. 5.

 

Fig. 5. Hydrogel 3D printing process with PLEEC. (a), (b) Patterning process. When liquids flow over the designed electrode, the liquid patterns are trapped by the electric field. (c) Polymerization process. The curing platform moves down to contact the liquid pattern, and the hydrogel solution is polymerized by UV light. (d) Resetting process. The curing platform moves upward together with the newly formed hydrogel layer.

 

We designed a hydrogel 3D printing system using the PLEEC technique. The system consists of seven parts: a mechanical module, a PLEEC panel, a solution-adding unit, a curing platform, a curing unit, a power supply, and a control module (Fig. 6). 

 

Fig. 6. (a) System schematic. The system consists of seven parts: a mechanical module, a PLEEC panel, a solution-adding unit, a curing platform, a curing unit, a power supply, and a control module. (b) Our in-house printing system.

 

We printed several hydrogel structures using our in-house printing system.

 

Fig. 7 Printed hydrogel structures using the PLEEC system. (a) A scaffold-structured hydrogel lattice. (b), (c) PAAM and PNIPAM hydrogel composites. When the polymerized hydrogel composite is placed in hot water, the PNIPAM hydrogel tends to shrink so that fingers roll up. (d) A stretchable LED belt. LEDs work well when the belt is stretched to double its length and suffers 100 loading cycles. (e) A soft display device. Each LED can be lit independently. (Photographer name: Jikun Wang; Photographer institution: Xi’an Jiaotong University)

 

Discussions

We proposed a novel design of a PLEEC panel to generate complex liquid patterns. The PLEEC technique is capable of trapping a wider variety of liquid solutions and offers potential opportunities for massive-scale liquid manipulation, flexible displays, transfer printing, and hydrogel 3D printing.

We used the PLEEC technique to set up a 3D printing system for additively manufacturing hydrogel structures and demonstrated the printed hydrogel lattice, hydrogel composite, and hydrogel display device. This technology has several advantages compared to existing methods for hydrogel 3D printing, including a wide suitability of hydrogel materials, multiple material printing and convenient operation. 

The precision of our printing technique can be further improved if a dielectric layer with higher permittivity is used or if the apparatus is immersed in an environment with higher electrical breakdown strength. The activation voltage can also be markedly decreased if a more advanced technique is used to fabricate a much thinner layer. If the pixel size can be further decreased to micrometer scale or smaller, this printing technique has great potential to print very complex and precise hydrogel structures such as artificial tissues, soft metamaterials, soft electronics, and soft robotics.

 

References

[1] Cho S K , Moon H , Kim C J . Creating, transporting, cutting, and merging liquid droplets by electrowetting-based actuation for digital microfluidic circuits[J]. Journal of Microelectromechanical Systems, 2003, 12(1):70-80.

[2] Paik P, Pamula V K, Fair R B. Rapid droplet mixers for digital microfluidic systems[J]. Lab on A Chip, 2004, 3(4):253-259.

[3] Paik P, Pamula V K, Pollack M G, et al. Electrowetting-based droplet mixers for microfluidic systems[J]. Lab on A Chip, 2003, 3(1):28-33.

[4] D. Tian, Q. Chen, F.-Q. Nie, J. Xu, Y. Song, L. Jiang, Patterned wettability transition byphotoelectric cooperative and anisotropic wetting for liquid reprography. Adv. Mater. 21,3744–3749 (2009).

[5] Berge B, Peseux J. Variable focal lens controlled by an external voltage: An application of electrowetting[J]. European Physical Journal E, 2000, 3(2):159-163.

[6] Yang S, Krupenkin T N, Mach P, et al. Tunable and Latchable Liquid Microlens with Photopolymerizable Components[J]. Advanced Materials, 2010, 15(11):940-943.

[7] Kuiper S, Hendriks B H W. Variable-focus liquid lens for miniature cameras[J]. Applied Physics Letters, 2004, 85(7):1128-1130.

[8] Acharya B R, Krupenkin T, Ramachandran S, et al. Tunable optical fiber devices based on broadband long-period gratings and pumped microfluidics[J]. Applied Physics Letters, 2003, 83(24):4912-4914.

[9] Mach P, Krupenkin T, Yang S, et al. Dynamic tuning of optical waveguides with electrowetting pumps and recirculating fluid channels[J]. Applied Physics Letters, 2002, 81(2):202-204.

[10] Hayes R A, Feenstra B J. Video-speed electronic paper based on electrowetting. [J]. Nature, 2003, 425(6956):383-385.

[11] Roques-Carmes T, Hayes R A, Feenstra B J, et al. Liquid behavior inside a reflective display pixel based on electrowetting[J]. Journal of Applied Physics, 2004, 95(8):4389-4396.

[12] Chiang M Y, Hsu Y W, Hsieh H Y, et al. Constructing 3D heterogeneous hydrogels from electrically manipulated prepolymer droplets and crosslinked microgels[J]. Science Advances, 2016, 2(10): e1600964-e1600964.

[13] Wang J K, Lu T Q, Yang M, et al. Hydrogel 3D printing with the capacitor edge effect[J]. Science advances, 2019, 5(3).

[14] Billiet T, Vandenhaute M, Schelfhout J, et al. A review of trends and limitations in hydrogel-rapid prototyping for tissue engineering[J]. Biomaterials, 2012, 33(26):6020---6041.

[15] Truby R L, Lewis J A. Printing soft matter in three dimensions[J]. Nature.

[16] Chan V, Zorlutuna P, Jeong J H, et al. Three-dimensional photopatterning of hydrogels using stereolithography for long-term cell encapsulation[J]. Lab on a Chip, 2010, 10(16):2062.

[17] Pawar A A, Saada G, Cooperstein I, et al. High-performance 3D printing of hydrogels by water-dispersible photoinitiator nanoparticles[J]. Science Advances, 2016, 2(4): e1501381-e1501381.

[18] Fairbanks B D, Schwartz M P, Bowman C N, et al. Photoinitiated polymerization of PEG-diacrylate with lithium phenyl-2,4,6-trimethylbenzoylphosphinate: polymerization rate and cytocompatibility[J]. Biomaterials, 2009, 30(35):6702-6707.

 

Comments

706430871@qq.com's picture

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? 

tongqing.lu's picture

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.

Jiawei Yang's picture

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

tongqing.lu's picture

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

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

Zheng Jia's picture

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

tongqing.lu's picture

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

 

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

Ruobing Bai's picture

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

tongqing.lu's picture

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

Cai Shengqiang's picture

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 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

tongqing.lu's picture

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

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