key: cord-0327915-8j1idrgx
authors: Jiang, Liangkui; Huang, Yanhua; Zhang, Xiao; Qin, Hantang
title: Electrohydrodynamic inkjet printing of Polydimethylsiloxane (PDMS)
date: 2020-12-31
journal: Procedia Manufacturing
DOI: 10.1016/j.promfg.2020.05.024
sha: 7e9721a8e8f8f825cac35c775cb2f2b390ca96e1
doc_id: 327915
cord_uid: 8j1idrgx

Abstract The current fabrication methods for micro and nano scale Polydimethylsiloxane (PDMS) devices either require high environmental conditions or sophisticated procedures at high cost. Electrohydrodynamic inkjet (EHD) printing is a potential method to effectively fabricate micro scale PDMS devices because EHD printing is environment-friendly, low-cost, compatible to various inks, and most importantly, high resolution. However, little research has been conducted to study EHD printing of PDMS-based ink. In this paper, EHD printing of PDMS has been investigated. The effects of several critical parameters on the printing process were studied. The dimension of the printed patterns and the printing frequency were preciously controlled by the voltage parameters. The patterns with a network structure was demonstrated. The results indicated that the voltage amplitude and the pulse frequency could both control the dimensions of droplets and the printing frequency. As future work, we plan to develop a simulation tool to predict printing quality of PDMS and optimize printing process to fabricated 3D PDMS structures in micro scale.

Polydimethylsiloxane (PDMS) is one kind of polymer organic silicide, which consist of the chain structure with different degree of polymerization [1] . PDMS is widely used as the biomaterials in many fields, such as biology [2, 3] , medical science [4] , microfluidics [5] [6] [7] , because of its good biocompatibility [8] , optical transparency [9] [10] [11] [12] , high structural flexibility [13, 14] , and low cost [15, 16] . In terms of investigation on the biology at the cellular level, PDMS is a proper material adopted to create a viable environment and culture for generation and manipulation of the cells, because it is nontoxic and compatible for various kinds of cells [2, 3, 6] . The structure based on PDMS has a proper length scale, which is suitable for the cell scale and the precise control of microenvironment depending on the study objective. Many research has been conducted to create a cell culture based on PDMS. For example, Yeh et al. developed a two-step method to form and increase a PDMS crosslinking and studied cell behavior under different PDMS substrate stiffness [17] . The results indicated the response of the cells to the changes in the mechanics of the PDMS substrate. Islam et al. adopted micro reactive ion etching (micro-RIE) to manufacture nanotextured polydimethylsiloxane (PDMS) substrate, which could help cells grow faster [18] .

The fabrication of PDMS devices plays an essential role in biology researches because the properties of the PDMS substrate have a significant effect on the attachment, growth, manipulation of the cells [19] . For example, the stiffness of PDMS mold impacts on the cytocompatibility of cells [17] . Softer PDMS substrate presents higher cytocompatibility, whereas the stiff substrate repels the cells [8] . The elasticity of PDMS also has a significant influence on adhesion and proliferation of cells [20, 21] . Besides, the structure of the PDMS substrate has a predominant effect on the shape of the tissue [22] . The cells would grow and reproduce along with the

Polydimethylsiloxane (PDMS) is one kind of polymer organic silicide, which consist of the chain structure with different degree of polymerization [1] . PDMS is widely used as the biomaterials in many fields, such as biology [2, 3] , medical science [4] , microfluidics [5] [6] [7] , because of its good biocompatibility [8] , optical transparency [9] [10] [11] [12] , high structural flexibility [13, 14] , and low cost [15, 16] . In terms of investigation on the biology at the cellular level, PDMS is a proper material adopted to create a viable environment and culture for generation and manipulation of the cells, because it is nontoxic and compatible for various kinds of cells [2, 3, 6] . The structure based on PDMS has a proper length scale, which is suitable for the cell scale and the precise control of microenvironment depending on the study objective. Many research has been conducted to create a cell culture based on PDMS. For example, Yeh et al. developed a two-step method to form and increase a PDMS crosslinking and studied cell behavior under different PDMS substrate stiffness [17] . The results indicated the response of the cells to the changes in the mechanics of the PDMS substrate. Islam et al. adopted micro reactive ion etching (micro-RIE) to manufacture nanotextured polydimethylsiloxane (PDMS) substrate, which could help cells grow faster [18] .

The fabrication of PDMS devices plays an essential role in biology researches because the properties of the PDMS substrate have a significant effect on the attachment, growth, manipulation of the cells [19] . For example, the stiffness of PDMS mold impacts on the cytocompatibility of cells [17] . Softer PDMS substrate presents higher cytocompatibility, whereas the stiff substrate repels the cells [8] . The elasticity of PDMS also has a significant influence on adhesion and proliferation of cells [20, 21] . Besides, the structure of the PDMS substrate has a predominant effect on the shape of the tissue [22] . The cells would grow and reproduce along with the

Polydimethylsiloxane (PDMS) is one kind of polymer organic silicide, which consist of the chain structure with different degree of polymerization [1] . PDMS is widely used as the biomaterials in many fields, such as biology [2, 3] , medical science [4] , microfluidics [5] [6] [7] , because of its good biocompatibility [8] , optical transparency [9] [10] [11] [12] , high structural flexibility [13, 14] , and low cost [15, 16] . In terms of investigation on the biology at the cellular level, PDMS is a proper material adopted to create a viable environment and culture for generation and manipulation of the cells, because it is nontoxic and compatible for various kinds of cells [2, 3, 6] . The structure based on PDMS has a proper length scale, which is suitable for the cell scale and the precise control of microenvironment depending on the study objective. Many research has been conducted to create a cell culture based on PDMS. For example, Yeh et al. developed a two-step method to form and increase a PDMS crosslinking and studied cell behavior under different PDMS substrate stiffness [17] . The results indicated the response of the cells to the changes in the mechanics of the PDMS substrate. Islam et al. adopted micro reactive ion etching (micro-RIE) to manufacture nanotextured polydimethylsiloxane (PDMS) substrate, which could help cells grow faster [18] .

The fabrication of PDMS devices plays an essential role in biology researches because the properties of the PDMS substrate have a significant effect on the attachment, growth, manipulation of the cells [19] . For example, the stiffness of PDMS mold impacts on the cytocompatibility of cells [17] . Softer PDMS substrate presents higher cytocompatibility, whereas the stiff substrate repels the cells [8] . The elasticity of PDMS also has a significant influence on adhesion and proliferation of cells [20, 21] . Besides, the structure of the PDMS substrate has a predominant effect on the shape of the tissue [22] . The cells would grow and reproduce along with the 48th SME North American Manufacturing Research Conference, NAMRC 48 (Cancelled due to structure of the mold such that the shape of the tissue is closely related to the shape of the mold structure. Researchers have proposed some methods to fabricate the PDMS substrates. Jo et al. proposed a technique to fabricate 3D micro channels in PDMS, which fabricate a positive and a negative PDMS layer by using corresponding negative and positive molds [11] . The channel would form by matching these two layers together. Leclerc et al. fabricated a 3-Dimensional microfluidic structure of PDMS for cell culture [23] . They fabricate two mutual matched layers of PDMS based on two corresponding mold masters. The results indicated that the cells are successfully attached, spread, and grow in this device. Peng et al. [24] presented a method for fabricating the multiple channels with controllable diameter and spacing in PDMS chips by using mutually matched molds. Isiksacan et al. used laser ablation to fabricate the microfluidic device [25] . They fabricated the PDMS layer with a designed channel via laser ablation. Based on this layer, they manufactured the positive and negative layers of PDMS.

However, currently, the fabrication of micro scale PDMS structure in aforementioned literature all required manufacturing either negative or positive molds [26] . The fabrication of such mold masters must be conducted in a cleanroom without dust, as well as many other requirements, such as microfabrication tools, long-time fabrication, complex procedures, well training, which all could result in high-cost and long duration of fabrication. Besides, the surface of the PDMS device, which has a significant effect on the attachment and growth of cells, might be destroyed when the PDMS layer is peeled from the mold master [27] . Therefore, it is necessary to find an alternative method with low-cost, easy setup, less environment dependence, and higher efficiency. Electrohydrodynamic inkjet printing (EHD printing) is a potential printing technique with high resolution, where the ink is forced out of a micro nozzle by the electrical field and form a Tylor cone, where the fine filament is produced from the cone. EHD printing has excellent abilities, including low cost [28] , low environment requirement, environment-friendly [29] , and reasonable material compatibility [30] . EHD printing has a similar experiment setup with Electrospinning, but they have a large difference from both mechanisms and applications [27, 31] . For electrospinning, when the proper voltage is added, the materials are generated at the tip of the nozzle to form a continuous fiber. In the EHD printing of our study, a pulsed voltage is added between the nozzle and the substrate, and the ink is forced to form a droplet in a more stable way working at Tylor-cone mode. Each pulse will generate one droplet, and the droplet would periodically drop on to the substrate as separate dots printed. The EHD printing process is periodic and the printed pattern is made of many droplets. EHD printing has many advantages. The selection of the ink is very wide, including mental nanoparticle, polymer, ceramic, solution, suspension liquid, and colloid. The width of the printed line could be much smaller than the diameter of the nozzle so that this method could achieve a high resolution [31] . Therefore, these advantages make EHD printing a potential method of fabricating PDMS substrate for cell culture.

However, few studies focus on the EHD printing of PDMS. The printing process and the effects of printing parameters on printing qualities are still unclear. In this study, we prepared the ink based on PDMS, studied the effects of printing parameters on the printing qualities, and optimized the printing parameters to printing a 2.5D structure using EHD printing.

The ink, based on PDMS (SYLGARD 184), was prepared in this study. Basement and cure components were dissolved in pure Toluene with a 25% mass fraction, respectively. The ratio of the basement to the cure component was 10:1.

The EHD printing system, as shown in Fig. 1 , consists of a three-axis stage, a syringe, a nozzle, a signal generator, a voltage amplifier, a glass substrate (75 mm× 20 mm× 1mm), and a camera. The voltage was generated by a waveform generator and added between the nozzle and the substrate through a voltage amplifier. The charges would move from inside to the surface of the ink, leading to the ink forced by the electrical force. With the increasing voltage, Tylor cone formed at the tip of the nozzle and then the droplets generated from the tip of the nozzle. The three-axis stage could provide x, y, z movement for printing. The motion range of the stage was 160 mm × 160 mm × 250 mm. The precision of the stage was 50 nm. 

The basement and the cure component were dissolved in the Toluene with a 25% mass fraction, respectively. The reason is that Toluene has a lower viscosity and better volatility than PDMS. Using Toluene as the solvent could improve the printability of ink. Also, the PDMS line would be smaller because of the high volatility of Toluene. Three concentration of Toluene, 25%, 33%, 50% are tested, and 25% have better printing performance. Before the experiment, these two solutions were vibrated by a vortex mixer and a sonic vibrator for 3 min. After that, the two solutions were mixed and vibrated for 5 min. In this printing process, the onset voltage was determined first, leading to finding a proper voltage for a stable printing process. Two parameters were adopted to control the printing process: voltage, voltage frequency, and duration time. Voltage amplitude was related to the intensity of the electrical field. A small voltage would result in no droplets produced from the nozzle, and a large voltage could lead to an unstable printing process. Voltage frequency was expected to control the printing frequency. Duration time was expected to control the size of the droplet. The other parameters were fixed: the offset distance between the nozzle and the substrate was 30 µm, the diameter of the nozzle was 25 µm. After printing, the glass slide was heated on a heating plate with 200℃ for 1 hour to make PDMS cured completely. The printing results were observed directly by a high-resolution microscope. The printing system was used to printing lines continuously. Designed patterns were also directly observed by a high-resolution microscope. In EHD printing, the voltage and the duration time were significant parameters for the EHD printing process, especially for determination of the start of the printing. With the increase of the voltage, the meniscus formed at the tip of the nozzle and then deformed into the Tylor cone. Finally, the droplet would generate from Tylor cone and fell to the substrate. Duration time was expected to control the size of the droplet. With the same voltage frequency, longer duration time meant the longer time for the ink forced by the electrical force, leading to the larger size of the droplet. Fig. 2 The threshold voltage for a start formatting the droplet with respect to the voltage frequency and the duration time.

As can be seen in Fig. 2 , for each duration time, the larger voltage frequency had a higher threshold voltage. At a very low frequency, the threshold voltage is around 500V, which is very close to the DC voltage. With the increase of the frequency, the threshold voltage increased as well. That was because, in a cycle, the charges needed some time moving from the inside droplet to the surface of the droplet. Higher frequency meant a shorter time for movement of the droplet in every cycle so that higher voltage was needed to apply enough electrical force to the charges. At the same voltage frequency, the higher duration time resulted in a larger threshold voltage. The reason is that the charges needed a period of the time to accumulate at the tip of the nozzle. With higher duration time, the droplet had more time to accumulate at the tip of the nozzle. The lower threshold voltage could achieve the requirement of the droplet formation. Besides, voltage frequency had more significant effects on the threshold voltage at a higher duty cycle. The reason for this behavior was that with a lower duty cycle, the charges had less time moving from the inside to the surface of the droplet. The effects of the frequency played a more critical role in this situation.

In order to study the effects of the voltage on the diameter of the droplets and the ejection frequency, the other parameters were fixed: the offset distance was 30 µm, the diameter of the nozzle was 25 µm, the duty cycle was 25%, and the printing speed was 20 mm /s. The results of the printed droplet and the relationship of the diameter of the droplet and the printing frequency with respect to the voltage showed in Fig. 3 . From the right figure in Fig. 3 , it was clearly observed that the size of the droplet decreased with the increase of the pulse voltage. The distribution of the droplet, however, became denser. As can be seen in Fig. 3 (left) , with the increase of the voltage, the diameter of the droplet decrease, but the printing frequency increased. The explanation for this behavior is that at a different voltage, the number of the pulse cycle for formatting a droplet was different. With a higher voltage, fewer pulse cycles could produce a droplet. So, printing frequency increased, and the size of the droplet decreased because of less time for the charge accumulation.

In order to study the effects of the frequency on printing behavior, the other variable was fixed: the offset distance was 30 µm, the diameter of the nozzle was 25 µm, the duty cycle was 25%, the printing speed was 15 mm/s. Fig. 4 presented the diameter of the droplet and the printing frequency with respect to the voltage frequency, and the printed droplet at different voltage frequencies. From Fig. 4 (right) , it was found that the size of the droplet would decrease with the increasing frequency. The distance between two close droplets decreased. When the frequency was very high, the printing frequency could not be stable, and there were many small satellite droplets close to the primary droplets. From Fig. 4 (left) , it could be found that with the increase of the voltage frequency, the diameter of the droplet decreased, and the printing frequency increased. The reason for this phenomenon was that the increase of the voltage frequency resulted in less time for the charges accumulating at the tip of the nozzle in every cycle. Although at the same voltage amplitude, the produced droplets were smaller.

Fig . 5 presented the printed results. The left letters were "SU" for state university, and the right pattern was a Chinese knot pattern. From Fig. 5 (right) , the clear network structure and the multiple parallel lines could be seen, the width of the lines was controlled in a relatively small range. However, there were some larger droplet points in the figure of the Chinese knot, which was because, at the end of the printing, the stage had stopped moving, but there was still a voltage between the nozzle and the substrate. Therefore, more than one droplet would generate at this position, leading to a collection of the droplets. 

In this study, the printing process of PDMS-based ink was studied, and the effects of several critical parameters on the printing process were characterized for the best printing quality. By designing the pulse voltage, pulse frequency, the dimension of the droplet and the printing frequency could be preciously controlled. Increasing the pulse voltage and the frequency could both increase the printing frequency and decrease the size of the droplet. The relationship between the printing frequency and the pulse frequency was close to the linear, which indicated that the printing frequency could be controlled by pulse frequency. Through controlling the parameters, a clear pattern could be obtained, and the network structure and the parallel lines could be clearly observed. As future work, we plan to develop a simulation tool to predict printing quality of PDMS and optimize printing process to fabricated 3D PDMS structures in micro scale.

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The research is supported by the startup accounts of Dr. Qin and the Undergraduate Research Assistant Program from the Department of Industrial and Manufacturing Systems Engineering (IMSE_URA) at Iowa State University. We also thank the URAs, Kimmo and Vandi Hartanto, for assisting the image analysis, and the graduate student, Ms. Shalin Unnikandam Veettil, for collaborating with us on this project.

The authors declare that they have no other known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.