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Research on the spreading characteristics of biodegradable ethyl cyanoacrylate droplet of a piezoelectric inkjet
Journal Pre-proof Research on the Spreading Characteristics of Biodegradable Ethyl Cyanoacrylate Droplet of a Piezoelectric Inkjet Kai Li (Conceptualization) (Formal analysis) (Validation) (Writing original draft), Weishan Chen (Methodology) (Writing - review and editing), Junkao Liu (Writing - review and editing) (Supervision), Hengyu Li (Writing - original draft) (Formal analysis), Naiming Qi (Writing - review and editing), Yingxiang Liu (Formal analysis) (Writing - review and editing) (Supervision)
PII:
S0924-4247(19)31400-1
DOI:
https://doi.org/10.1016/j.sna.2019.111810
Reference:
SNA 111810
To appear in:
Sensors and Actuators: A. Physical
Received Date:
5 August 2019
Revised Date:
9 December 2019
Accepted Date:
23 December 2019
Please cite this article as: Li K, Chen W, Liu J, Li H, Qi N, Liu Y, Research on the Spreading Characteristics of Biodegradable Ethyl Cyanoacrylate Droplet of a Piezoelectric Inkjet, Sensors and Actuators: A. Physical (2019), doi: https://doi.org/10.1016/j.sna.2019.111810
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Equation Chapter 1 Section 1Research on the Spreading Characteristics of Biodegradable Ethyl Cyanoacrylate Droplet of a Piezoelectric Inkjet
Kai Li1, Weishan Chen1, Junkao Liu1, Hengyu Li1, Naiming Qi2, Yingxiang Liu1* 1
State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin 150001, Heilongjiang Province, China
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School of Astronautics, Harbin Institute of Technology, Harbin 150001, Heilongjiang Province, China
Highlights
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*Corresponding author: [email protected]
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1. A method of printing biodegradable structure by piezoelectric inkjet is proposed. 2. The controlling methods for obtaining better printing performance are discussed. 3. An ethyl cyanoacrylate biodegradable structure is successfully printed.
Abstract—Piezoelectric inkjet has been widely applied in printing field thanks to its advantages of ejecting small droplet precisely. The method of printing biodegradable structure for tissue support or connection by piezoelectric inkjet is proposed in this work, which will contribute to wound repair in vivo and avoid secondary
trauma caused by removing non-biodegradable support. A piezoelectric inkjet with simple structure is designed for printing biodegradable ethyl cyanoacrylate solution. Basic studies of printing biodegradable ethyl cyanoacrylate structure are carried out. The spreading characteristics of ethyl cyanoacrylates droplets before solidification under different conditions are studied by simulation analyses, and the methods for stabilizing the spreading performance and avoiding failure spreading are obtained. The spreading characteristics of the droplets before and after solidification under different conditions are analyzed by experiments, and the corresponding shrinkage ratios are obtained. Based on the obtained shrinkage ratios (between 70% and 75% when spreading without satellite droplets) and spreading dimensions before solidification (obtained by simulation analyses), the
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final formed size of the ethyl cyanoacrylate droplet can be estimated to determine the appropriate printing distance in continuous printing. A simple structure is printed by the piezoelectric inkjet to verify the feasibility of the proposed method of the printing ethyl cyanoacrylate biodegradable structure for medical care.
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Keywords—piezoelectric inkjet, biodegradable structure, droplet spreading characteristic, shrinkage rate
1. Introduction
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The 3D printing technology has been developed rapidly in the engineering field, as it has the advantages of breaking through the limitation of traditional machining process and realizing effective machining of
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special-shaped structures. At present, the 3D printing technologies used in engineering field mainly contain hot-melt deposition printing [1-4] and selective laser firing printing [5-8]. The hot-melt deposition printing is generally used for printing non-metallic materials such as polylactic acid (PLA) and acrylonitrile butadiene
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styrene (ABS) plastics. It has the advantages of low cost, strong reproducibility and easy operation. Tao et al. printed PLA composite filament by hot-melt deposition printing process [9]. Gaisford et al. printed tablets with different geometries by the method of melt depositing printing, and discussed the influences of geometry on drug release characteristics of printed tablets [10]. Connal et al. printed pH-responsive and functional polymers by hot
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melt deposition printing process, and studied the influences of thermal processing conditions on the quality of the printed objects [11]. Smith et al. developed a new hot melt deposition process for manufacturing thin-walled drug-free capsules, and the feasibility of the developed process was verified by experiments [12]. Masood et al. fabricated new metal/polymer composites by melt deposition process, and found that selective laser sintering printing was mainly used for printing metal materials such as nickel-based super-alloy, stainless steel, tool steel, titanium alloy, aluminum alloy. It has the advantages of high material utilization and printing precision [13]. Roy et al. printed metal nanoparticles by using selective laser sintering system and improved the
system optics which could ensure high printing resolution at micron level and realize the sintering process of large area metal nanoparticles [14]. Kong et al. printed 316L stainless steel structure, which had strong elongation, corrosion resistance, biocompatibility and could be used as medical implants in biomedicine field, by selective laser melting technology [15]. It can be seen that the hot-melt deposition printing and selective laser firing printing are mainly used to realize the manufacturing of engineering parts. Compared with hot-melt deposition printing and selective laser burning printing, the 3D printing technology based on the piezoelectric actuation has the advantages of high precision, fast response, simple structure, no electromagnetic interference, green manufacturing, etc. It can realize low cost, non-contact printing on demand
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and full utilization of raw materials theoretically. Kim et al. printed mammalian cells with high accuracy at room temperature based on piezoelectric inkjet technology, and the effects of nozzle diameter and cell liquid concentration on cell number and cell activity were studied [16]. Koo et al. proposed a method of 3D printing cells based on the assistance of piezoelectric actuation, which realized high-precision printing of in situ cell and
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enhanced the cell viability [17]. Uddin et al. printed 5-fluorouracil, curcumin and cisplatin on metal microneedles uniformly and accurately based on piezoelectric inkjet technology, and realized no material loss in the printing
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process [18]. Lorber et al. proposed the method of printing retinal ganglion cells and neuroglia of adult rats by piezoelectric inkjet technology [19]. Scoutaris et al. printed drug on vascular stents for drug delivery based on
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piezoelectric actuation, and proved the feasibility of the method using biological evaluation [20]. Lin et al. proposed the method of printing precisely controlled multiple cell patterning based on piezoelectric inkjet technology. By using this method, the biocompatible cell could be printed on the substrate in a large range of
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several hundred microns to provide a possibility for matrix-assisted laser desorption/ionization mass spectrometry analysis [21, 22]. He et al. successfully printed uniform sodium alginate micro-particles by using piezoelectric actuation printing system [23]. Sun et al. used a bi-piezoelectric inkjet to distribute the mixed reagent of polymerase chain to the oil droplets to carry out quantitative polymerase chain reaction analysis [24]. It can be
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seen that the 3D printing technology based on piezoelectric actuation can achieve accurate printing of biomedical-related fluids at ambient temperature. As the ethyl cyanoacrylate solution is a kind of glue which has the advantages of biodegradability and quickly
curing (catalyzed by water) at room temperature, the methods of stitching bio-tissue by ejecting ethyl cyanoacrylate solution droplet based on piezoelectric inkjet have been studied by researchers. Saska et al. printed ethyl cyanoacrylate glue to realize the bonding and fixation of bones based on piezoelectric inkjet technology [25]. Boehm et al. printed medical adhesives and sealants to realize the bonding of biological tissues by using
piezoelectric inkjet [26]. Barbosa et al. printed ethyl acrylate to achieve the fixation of free gingival transplantation based on piezoelectric inkjet technology [27]. By printing biodegradable glue at wound area, the secondary trauma caused by taking out stitches after healing can be avoided. For some cases the injured tissue need to be fastened or supported by printed biodegradable structures, while, the current studies mainly focus on the surface-to-surface gluing of tissues. In this work, the preliminary researches of printing biodegradable ethyl cyanoacrylate structure for medical treatment based on piezoelectric inkjet are carried out. In order to achieve the accuracy of printing biodegradable structure on demand, it is necessary to control the spreading characteristics of ejected droplet. The influences of
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physical properties of ejected ethyl cyanoacrylate solution and contact angle of target surface on the spreading characteristics are discussed by simulations. The injection and spreading statuses of ejected ethyl cyanoacrylate droplet before and after solidification under different conditions are studied by experiments. The amplitude ranges of pulse voltage for achieving single droplet injection and the shrinkage rates are obtained. Based on the obtained
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shrinkage rates and spreading results, the final forming size of the ejected droplet can be estimated. A simple ethyl cyanoacrylate structure is printed to prove the validity of printing biodegradable ethyl cyanoacrylate structure for
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medical treatment based on piezoelectric inkjet.
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2. Spreading Characteristics before Solidification
A piezoelectric inkjet used for printing biodegradable ethyl cyanoacrylate droplet is designed as shown in Fig. 1. Vibrations are created when the pulse voltages are applied on the piezoelectric vibrator, then, the droplets are
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ejected out by the pressure waves which are produced by the vibrations. When the ejected droplet moves to the target surface, the spreading process begins. In this work, the spreading characteristics of the ejected ethyl cyanoacrylate droplet before solidification are mainly studied by direct coupling simulation analysis which is carried out by using the finite element analysis software FLUENT, this simulation method have been proved to be
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effective in our previous work [28]. Before the direct coupling simulation analysis, the pressure boundary conditions at the nozzle part should be obtained through acoustic structural coupling analyses. Since this work mainly focuses on the dynamic characteristics of ejected droplet, only the ejecting and spreading characteristics of droplets are discussed here, and more details of acoustic structure coupling analysis can be found from reference [29].
Figure 1. The schematic diagram of the piezoelectric inkjet. In order to achieve the stability of the designed piezoelectric inkjet in printing biodegradable ethyl cyanoacrylate solution with different physical properties, the influences of main physical parameter on the spreading characteristics are analyzed. The spreading characteristics mainly include the spreading status, contact
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pressure, response characteristics and final spreading results. The characteristics when droplets moving and spreading onto the target surface are studied in this work by setting pressure boundary conditions obtained by acoustic structural coupling analyses at the nozzle part. The analyses follow the principle of single variable. When analyzing the influences of selected factor on the spreading
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characteristics, the values of other parameters remain unchanged and set based on Table 1. The material of the piezoelectric ceramic used in this work is PZT-5H, square wave is selected as the excitation signal on the
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piezoelectric vibrator. For comparison discussion of the droplet spreading statuses, the times when the spreading area of droplet reaches the maximum value are selected.
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Table 1
The values of variable parameters set in simulations. Surface tension (N/m)
Density (kg/m3)
Contact angle (°)
0.001
0.072
1000
90
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Dynamic viscosity (mPa·s)
2.1 The influences of dynamic viscosity
The maximum spreading area of the droplet occurs when the velocity of the droplet decreases to zero for the first time after contacting the target surface. To avoid the interference between adjacent droplets, the maximum spreading diameter should be taken as the reference value to determine the printing distance. Besides, the
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maximum contact pressure between droplet and target surface also occurs when the spreading area reaches the maximum, as the reverse acceleration of the droplet reaches the maximum at this time. To ensure the printing stability when droplets are ejected on tissues with low contact stiffness, the maximum contact pressure under different conditions should be obtained as well. When the amplitude of the applied square wave is 65 V and the ethyl cyanoacrylate solution with different viscosities is ejected. It can be seen that the number of satellite droplets at the tail of the main droplet decreases with the increase of the viscosity. The separation trend between the center and the periphery of the droplet
gradually weakens. The reason is that, with the increase of the viscosity, the volume and velocity of the droplet gradually decrease, and the inertia force of the periphery part of the droplet decreases. Then, the separation trend weakens under the action of the internal viscous force. The contact pressure curves of ejected droplet with different viscosities are shown in Fig. 2(b), where the abscissa axis represents the distance from the droplet center, the coordinate values at the abscissa axis where the corresponding contact pressures are zero can represent the radiuses of the droplets. It can be seen that, the maximum contact pressure occurs at the center of the droplet. The contact pressure at the center area is little affected by the viscosity, while the contact pressure at the periphery area decreases with the increase of the viscosity. Therefore, when the viscosity of the ejected fluid decreases, it is
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necessary to ensure that the contact stiffness of the target surface (biological tissue) is greater than the maximum
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contact pressure, so as to reduce the impact of the spreading process on the printing resolution.
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Figure 2. The final spreading characteristics of droplet: (a) The spreading statuses of droplets when ejecting fluid
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with different viscosities, (b) The contact pressure between droplet and target surface. The response characteristics of the spreading process of droplet with different viscosities are shown in Fig. 3, where the contact time represents the time when the droplet first contacts with the target surface; the stable time represents the time when the droplet formation is finished; the fluctuation time represents the time interval
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between contact time and stable time. The fluctuation time of the droplet formation determines the minimum time interval of ejection. It can be seen that, the contact time increases with the increase of the viscosity, which is mainly due to the decrease of the velocity of droplet. The fluctuation time of droplet decreases gradually with the increase of the viscosity, the reason is that with the increase of fluid viscosity, the energy loss produced by the high viscosity droplet is increased in the process of droplet fluctuation. In addition, with the increase of viscosity, the contact time of droplet increases gradually, but the fluctuation time of droplet decreases, which makes the stabilization time keep nearly stable. Therefore, when the viscosity of ejected fluid is relatively low, the amplitude of the pulse voltage should be weakened to reduce the effect of large fluctuation time.
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Figure 3. The response characteristics of the spreading process when droplet with different viscosities.
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Figure 4. The final spreading characteristics of droplet: (a) The final spreading diameter of droplet with different
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viscosities, (b) The final spreading height of droplet with different viscosities.
The final spreading diameter and height of the droplet with different viscosities are shown in Fig. 4. The results
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indicate that due to the decreasing of droplet volume caused by the increased fluid viscosity, the final spreading diameter and height all decrease. Thus, when solution with relatively low viscosity is ejected, the excitation
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intensity should be reduced appropriately to improve the injection accuracy. When fluid with relatively high viscosity is ejected, the distance between the neighboring droplets and the height feed of the printing head all should be increased to avoid the interference between the droplets. 2.2 The influences of surface tension
When the amplitude of the applied square wave is 25 V and solution with different surface tensions is ejected,
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the spreading statuses of the droplet are shown in Fig. 5(a). The contact pressure curves between droplets and target surface are shown in Fig. 5(b). It can be seen that with the increase of the surface tension, the contact pressure of both the central part and the peripheral part of the droplet all increase gradually. The reason is that along with the increasing of the surface tension, the contraction force of the droplet increases. Thus, the amplitude of the pulse voltage should be decreased to reduce the impact of droplet collision and spreading process on the system when the surface tension of the injected droplet increases.
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Figure 5. The final spreading characteristics of droplet: (a) The spreading statuses of droplets when fluid with different surface tensions, (b) The contact pressure between droplet and target surface.
When ethyl cyanoacrylate solution with different surface tensions is ejected, the response characteristics of the
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spreading process of droplets are shown in Fig. 6. It can be seen that, as the exit velocity of the droplet decreases with the increase of the surface tension, the contact time decreases with the increase of the surface tension. The
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fluctuation time decreases with the increase of the surface tension, which is due to that the surface energy loss of the droplet increases with the increase of the surface tension during the spreading process. With the increase of
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surface tension, the increasement of contact time is greater than that of fluctuation time, which makes the stabilization time increase. Therefore, when fluid with large surface tension is ejected, the excitation should be
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adjusted (reducing the duty ratio, etc.) to stabilize the response speed of the spreading process.
Figure 6. The response characteristics of the spreading process when droplet with different surface tensions. The final spreading results of the ejected droplet with different surface tensions are shown in Fig. 7. We can see
that, the final spreading diameter and the height of droplet all decrease gradually with the increase of surface tension. This is due to the volume and the contraction force of the drop all decrease with the increase of the surface tension. Thus, the distance between adjacent droplets should be increased to reduce the interference between droplets when fluid with low surface tension is ejected.
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Figure 7. The final spreading characteristics of droplet: (a) The final spreading diameter of droplet with different
2.3 The influences of surface density
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surface tensions, (b) The final spreading height of droplet with different surface tensions.
Based on the analysis results, it is found that the spreading statuses and contact pressure of the droplet change slightly when the density of the ejected fluid changes. Thus, only the influences of density on the spreading
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response characteristics and final spreading results are given here.
As we can see from Fig. 8, when the amplitude of the applied square wave is 65 V, with the increase of density,
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the fluctuation time of droplet increases gradually, while the contact time keeps almost unchanged, which makes the stabilization time rise with the increase of the density. Thus, when the density of the ejected fluid increases, the
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spreading process.
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excitation signal should be adjusted (such as reducing the duty ratio) to stabilize the response speed of the
Figure 8. The response characteristics of the spreading process of droplet with different densities.
When the density of the ejected fluid changes, the results of final spreading diameter and height of droplets are
shown in Fig. 9. It can be seen that with the increase of density, the spreading diameter of droplet increases gradually, while the spreading height of droplet keeps nearly unchanged. Thus, the printing distance between the droplets should be increased to avoid the interference when the density of the ejected fluid increases.
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Figure 9. The final spreading characteristics of droplet: (a) The final spreading diameter of droplet with different
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densities, (b) The final spreading height of droplet with different densities. 2.4 The influences of contact angle
The contact angle between the droplet and the target surface (different biological tissues) is affected by the properties of the target surface. When the value of the contact angle between droplet and target surface is different
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and the amplitude of the applied square wave is 25 V, the spreading statuses of droplet are shown in Fig. 10(a). The change curves of the contact pressure, when contact angle changes, are shown in Fig. 10(b). It can be seen
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that, the contact pressure decreases gradually with the increase of the contact angle. The reason is that, when the contact angle increases, the horizontal component of the surface tension, which along the inner normal direction
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of the droplet surface, increases gradually. Thus, when the contact angle decreases, the amplitude of the pulse
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voltage should be decreased to reduce the effect of droplet spreading process.
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Figure 10. The spreading characteristics of droplet: (a) The spreading statuses of droplets under different contact angles, (b) The contact pressure between droplet and target surface with different contact angles. The final spreading diameter and height of the droplet with different contact angles are shown in Fig. 11, respectively. It can be seen that, with the increase of contact angle, the final spreading diameter of droplet decreases gradually, while the spreading height increases gradually. The reason is that the change of contact angle
does not affect the volume of the ejected droplet, while the spreading diameter of droplet decreases with the increase of the horizontal component of surface tension, resulting in the increasing of the spreading height of droplet. Thus, when the contact angle increases, the height feed of the nozzle should be increased to avoid the
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interference between nozzle and droplets.
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Figure 11. The final spreading characteristics of droplet: (a) The final spreading diameter of droplet with different
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contact angles, (b) The final spreading height of droplet with different contact angles.
3. Final spreading characteristics
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Ethyl cyanoacrylate droplets can realize addition polymerization and solidify at room temperature under the catalysis of water, and the spreading size of droplet changes due to the solidification process. Based on the
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simulation analyses of droplet spreading characteristics before solidification, the spreading dimensions of unsolidified droplet under different conditions are obtained. Combining with the shrinkage rate of droplet during the solidification process, the final forming size of ejected ethyl cyanoacrylate droplet can be predicted and the
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adjustment method of printing distance to meet the requirement of efficient continuous printing can be obtained. Thus, the shrinkage rates and solidification characteristics of printed ethyl cyanoacrylate droplets under different conditions are studied by experiments.
Based on the designed structure as shown in Fig. 1, the prototype is manufactured and experiments are carried
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out. The commercial ethyl α-cyanoacrylate solution (with density as 1060 kg/m3) is used in this work. It can be see from our previous research results that the satellite droplets would be produced when high voltage amplitude is applied. While, the satellite droplet can influence the printing accuracy for printing and reduce the printing quality. In this work, when the voltage amplitude is less than 40 V, the satellite droplets can be eliminated. When pulse voltage with amplitude as 30 V is applied, the statues of ejected droplet are shown in Fig. 12, where the green arrows represent the moving direction of droplet. In order to observe the whole motion characteristics of
droplets after being ejected out conveniently, the ejecting direction of the piezoelectric inkjet is arranged vertically
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and the glass sheets, where the droplet spreading process occurs, are removed.
Figure 12. The statuses of ejected droplet when applied pulse voltage with amplitude as 30 V. The green arrows
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represent the moving direction of droplet, the droplets appear white under the illumination of an external light source and are marked by red circles.
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As can be seen from the Fig. 12, there is no satellite droplet existing at the tail of the ejected droplet, and the droplet is elongated due to the effect of two-phase flow during the moving process. When the droplet moves to its
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highest position, the velocity reduced to zero and the shape is similar to a sphere. Due to assembly and processing errors, the nozzle's normal direction is not absolutely vertical, there is an error between the droplet drop position
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and the nozzle.
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Figure 13. The statuses of ejected droplet when applied pulse voltage with amplitude as 50 V. The green arrows
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represent the moving direction of droplet, the satellite droplets are indicated by yellow arrows. The droplets appear white under the illumination of an external light source and are marked by red ellipses.
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When the amplitude of the pulse voltage increases to 50 V, the statues of ejected droplet are shown in Fig. 13, it can be seen that satellite droplets are formed at the tail of the main droplet. As the velocities of the satellite
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droplets are less than that of the main droplets, the maximum motion heights of the satellite droplets are lower. Besides, the falling positions of the satellite droplets are different from that of the main droplets due to assembly
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and machining errors. Therefore, in order to meet the requirement of high precision printing, single droplet injection mode should be selected.
To obtain the shrinkage ratio of ethyl cyanoacrylate droplet, the spreading statuses when applied pulse voltage with amplitude as 10 V are obtained as shown in Fig. 14, where the red line represents the spreading boundary before solidification and the blue line represents the spreading boundary after solidification. It can be seen that,
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under the same condition, the boundary line of the droplet before solidification is basically round and similar, but the shape of the droplet after solidification is irregular and different. Based on Fig. 14, the spreading diameters before/after solidification and the shrinkage ratios under same
conditions (when applied pulse voltage with amplitude as 10 V) are obtained as shown in Fig. 15. It can be seen that, under the same condition the spreading diameters of the droplets before and after solidification fluctuate slightly which proves that the designed piezoelectric inkjet is basically stable for repeated printing. We can see that when the voltage amplitude is 10 V, the shrinkage rate of the ethyl cyanoacrylate droplet is about 72.5%.
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Figure 14. The repeat experiment results of the spreading statuses of droplets when applied pulse voltage with amplitude as 10V. The red line represents the spreading boundary before solidification and the blue line represents
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the spreading boundary after solidification.
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Figure 15. The spreading diameters before/after solidification and the shrinkage ratios when applied pulse voltage with amplitude as 10V.
The spreading characteristics of ethyl cyanoacrylate droplet injected under different voltage amplitudes are shown in Fig. 16(a). It can be seen that with the increase of voltage amplitude, the spreading diameters of droplets before and after solidification all increase gradually. When the voltage amplitude is larger than 40V, the satellite
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droplets will be produced and spread around the main droplets, which is consistent with the previous results obtained by injection statuses analyses. The droplet diameters before and after solidification and the shrinkage ratios when applied pulse voltages with different amplitudes are obtained, as shown in Fig. 16(b). It can be seen that, with the increase of voltage amplitude, the shrinkage rate of the ejected ethyl cyanoacrylate droplet increase slightly, while, for single droplet spreading process (voltage amplitude less than 50 V), the shrinkage rate of single droplet injection is basically between 70% and 75%. The printing distance between droplets in continuous printing
mode can be estimated in advance by combining the simulation results of spreading size before solidification and the obtained shrinkage ratios.
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Figure 16. The final spreading characteristics of droplet: (a) The spreading statuses of droplets when applied pulse voltage with different amplitudes, (b) The spreading diameters before/after solidification and the shrinkage ratios when applied pulse voltage with different amplitudes.
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In order to verify the feasibility of printing biodegradable ethyl cyanoacrylate structure based on the designed piezoelectric inkjet, a simple structure is printed, as shown in Fig. 17. By testing, we find that the printed simple
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structure has certain strength, while the surface quality is poor. Thus, further research is needed to improve the
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surface quality.
Figure 17. The printed biodegradable ethyl cyanoacrylate simple structure based on the designed piezoelectric inkjet.
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4. Conclusion
The spreading characteristics of ethyl cyanoacrylate droplet ejected by the designed piezoelectric inkjet were
simulated and tested in this work. Based on the spreading results of droplet before solidification by simulations, the methods for stabilizing the spreading performance were obtained. When the viscosity of ejected fluid decreased, or the surface tension of droplet increased or the contact angle decreased, the amplitude of the pulse voltage should be decreased to reduce the impact of droplet collision and spreading process on the printing accuracy. When ethyl cyanoacrylate solution with relatively low viscosity or large surface tension or large density
was ejected, the excitation should be adjusted (reducing the duty ratio, etc.) to stabilize the response speed of the spreading process. The methods for avoiding failure spreading of droplet were obtained. When ethyl cyanoacrylate solution with relatively high viscosity or low surface tension or high density was ejected, or the contact angle increased, the distance between the neighboring droplets and the height feed of the printing head all should be increased to avoid the interference between the droplets. For improve the printing accuracy of the ethyl cyanoacrylate structure, the piezoelectric inkjet should work in the mode of ejecting single droplet without satellite droplets which could be achieved by reducing the voltage amplitude (in this work, the voltage amplitude should less than 50 V). Experimental results showed that the
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shrinkage rates of the printed ethyl cyanoacrylate droplet were between 70% and 75% when the piezoelectric inkjet worked in single droplet ejecting mode. The final formed size of the ethyl cyanoacrylate droplet could be estimated based on the obtained shrinkage ratios and the spreading dimensions of droplet before solidification. The feasibility of printing ethyl cyanoacrylate biodegradable structure for medical care by designed piezoelectric
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inkjet was proved to be feasible by experiments.
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Author Contributions Section
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Declaration of interests
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Kai Li: Conceptualization, Formal analysis, Validation, Writing-Original draft preparation Weishan Chen: Methodology, Writing - Review & Editing Junkao Liu: Writing - Review & Editing, Supervision Hengyu Li: Writing-Original draft preparation, Formal analysis Naiming Qi: Writing - Review & Editing, Yingxiang Liu: Formal analysis, Writing-Review & Editing, Supervision
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgments
This work was supported in part by the National Natural Science Foundation of China (No. 51905134, No.
51622502).
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[19] Lorber B, Hsiao W, Hutchings I and Martin K, Adult rat retinal ganglion cells and glia can be printed by piezoelectric inkjet printing, Biofabrication, 6 (1) (2014).
[20] Scoutaris N, Chai F, Maurel B, Sobocinski J, Zhao M, Moffat J, Craig D, Martel B., Blanchemain N. and Douroumis D., Development and Biological Evaluation of Inkjet Printed Drug Coatings on Intravascular
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Stent, Mol. Pharmaceutics, 13 (1) (2016) 125-133.
[21] Zhang J, Chen F, He Z, Ma Y, Uchiyama K and Lin J, A novel approach for precisely controlled multiple cell
Analyst, 141 (10) (2016) 2940-2947.
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patterning in microfluidic chips by inkjet printing and the detection of drug metabolism and diffusion,
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[22] Korenaga A, Chen F, Li H, Uchiyama K and Lin J, Inkjet automated single cells and matrices printing system for matrix assisted laser desorption/ionization mass spectrometry, Talanta, 162 (2017) 474-478. [23] Gao Q, He Y, Fu J., Qiu J and Jin Y, Fabrication of shape controllable alginate microparticles based on
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drop-on-demand jetting, J. Sol-Gel Sci. Techn., 77 (3) (2016) 610-619. [24] Sun Y, Zhou X and Yu Y, A novel picoliter droplet array for parallel real-time polymerase chain reaction based on double-inkjet printing, Lab. Chip, 14 (18) (2014) 3603-3610. [25] Saska S, Hochuli-Vieira E, Minarelli-Gaspar A, Gabrielli M, Capela M and Gabrielli M, Fixation of
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autogenous bone grafts with ethyl-cyanoacrylate glue or titanium screws in the calvaria of rabbits, Int. J. Oral Max. Surg., 38 (2) (2009) 180-186.
[26] Boehm R, Gittard S, Byrne J, Doraiswamy A, Wilker J, Dunaway T, Crombez R, Shen W, Lee Y and Narayan R, Piezoelectric Inkjet Printing of Medical Adhesives and Sealants, Jom, 62 (7) (2010) 56-60.
[27] Barbosa F, Correa D., Zenobio E, Costa F and Shibli J, Dimensional changes between free gingival grafts fixed with ethyl cyanoacrylate and silk sutures, J. Int. Acad. Periodontol., 11 (2) (2009) 170-176.
[28] Li K, Liu J, Yang R, Chen W, Zhang L and Liu Y, A Trace Redundant Lubrication Piezoelectric Microjet for Bearing System, IEEE/ASME T. Mech., 23 (5) (2018) 2263-2272. [29] Li K, Liu J, Li H, Qi N and Chen W, Research on the Acoustic-Structure Coupling Characteristics of a
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Piezoelectric Micro-Jet Used for Lubricating, IEEE Access, 6 (2018) 72691-72697.
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Author Biographies
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Kai Li was born in Shandong, China, in 1988. He received the B.E. degree from the School of Mechanical Engineering, Shandong University of Technology, in 2012. He received the M.E. and Ph.D. degrees in mechatronics engineering from the School of Mechatronics Engineering at Harbin Institute of Technology, China, in 2014 and 2018, respectively. He is currently a lecturer at the Harbin Institute of Technology, China. His research interests include piezoelectric micro-jet and microfluidics, etc. Email：[email protected]
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Weishan Chen was born in Hebei, China, in 1965. He received his B.E. and the M.E. degrees in precision instrumentation engineering, and the Ph.D. degree in Mechatronics engineering from Harbin Institute of Technology, China, in 1986, 1989, and 1997, respectively. Since 1999, he has been a professor with the School of Mechatronics Engineering at the Harbin Institute of Technology. His research interests include ultrasonic driving, smart materials and structures, bio-robotics. Email: [email protected] Junkao Liu was born in Hebei, China, in 1973. He received his B.E. degree in mechanical engineering and Ph.D. degree in mechatronics engineering from the School of Mechatronics Engineering, Harbin Institute of Technology, China, in 1995 and 2001, respectively. Since 2011, he has been a professor with the School of Mechatronics Engineering at the Harbin Institute of Technology. His research interests include ultrasonic driving, biomimetic robots and simulations of parallel mechanisms with multi-degree of freedom. Email: [email protected]; [email protected]
Hengyu Li was born in Jilin province, China, in 1991. He received the B.S. degree and M.E. degree in Mechatronic Engineering from Changchun University of Technology, Changchun, China, in 2015 and 2018. He is currently a Ph.D. candidate in Mechatronic Engineering at Harbin Institute of Technology, China. His research interests include the precision piezoelectric actuators and piezoelectric micro-jet technology. Email: [email protected]
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Naiming Qi was born in China, in 1962. He received his B.E. degree in hydraulic technology, M.E. degree in fluid transmission and control and Ph.D. degree in precision instrument and mechanical engineering at Harbin Institute of Technology, China, in 1984, 1990 and 2001, respectively. He joined the School of Astronautics at the Harbin Institute of Technology in 1990, where he has been a professor since 2002. His research interests include automatic docking and testing technology of aircraft electromechanical integration, space control and intelligent docking technology, UAV motion planning and advanced recovery technology, etc. Email: [email protected]
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Yingxiang Liu was born in Hebei Province, China, in 1982. He received his B.E. degree in mechanical engineering, M.E. and Ph.D. degrees in mechatronics engineering from the School of Mechatronics Engineering at Harbin Institute of Technology, China, in 2005, 2007 and 2011, respectively. He joined the School of Mechatronics Engineering at the Harbin Institute of Technology in 2011, where he has been a professor since December 2013, and is also a member of the State Key Laboratory of Robotics and System. He is the vice director of the Department of Mechatronic Control and Automation. He was a Visiting Scholar at the Mechanical Engineering Department, University of California, Berkeley, from August 2013 to August 2014. His research interests include piezoelectric actuating, ultrasonic motors, piezoelectric actuators, precision actuating, piezoelectric micro jets, bionic robots, fish robots and soft robots. He has served as an Associate Editor of IEEE Transactions on Industrial Electronics, an Associate Editor of IEEE Access and Guest Editor of Applied Sciences. Email: [email protected]
PII:
S0924-4247(19)31400-1
DOI:
https://doi.org/10.1016/j.sna.2019.111810
Reference:
SNA 111810
To appear in:
Sensors and Actuators: A. Physical
Received Date:
5 August 2019
Revised Date:
9 December 2019
Accepted Date:
23 December 2019
Please cite this article as: Li K, Chen W, Liu J, Li H, Qi N, Liu Y, Research on the Spreading Characteristics of Biodegradable Ethyl Cyanoacrylate Droplet of a Piezoelectric Inkjet, Sensors and Actuators: A. Physical (2019), doi: https://doi.org/10.1016/j.sna.2019.111810
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Equation Chapter 1 Section 1Research on the Spreading Characteristics of Biodegradable Ethyl Cyanoacrylate Droplet of a Piezoelectric Inkjet
Kai Li1, Weishan Chen1, Junkao Liu1, Hengyu Li1, Naiming Qi2, Yingxiang Liu1* 1
State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin 150001, Heilongjiang Province, China
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School of Astronautics, Harbin Institute of Technology, Harbin 150001, Heilongjiang Province, China
Highlights
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*Corresponding author: [email protected]
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1. A method of printing biodegradable structure by piezoelectric inkjet is proposed. 2. The controlling methods for obtaining better printing performance are discussed. 3. An ethyl cyanoacrylate biodegradable structure is successfully printed.
Abstract—Piezoelectric inkjet has been widely applied in printing field thanks to its advantages of ejecting small droplet precisely. The method of printing biodegradable structure for tissue support or connection by piezoelectric inkjet is proposed in this work, which will contribute to wound repair in vivo and avoid secondary
trauma caused by removing non-biodegradable support. A piezoelectric inkjet with simple structure is designed for printing biodegradable ethyl cyanoacrylate solution. Basic studies of printing biodegradable ethyl cyanoacrylate structure are carried out. The spreading characteristics of ethyl cyanoacrylates droplets before solidification under different conditions are studied by simulation analyses, and the methods for stabilizing the spreading performance and avoiding failure spreading are obtained. The spreading characteristics of the droplets before and after solidification under different conditions are analyzed by experiments, and the corresponding shrinkage ratios are obtained. Based on the obtained shrinkage ratios (between 70% and 75% when spreading without satellite droplets) and spreading dimensions before solidification (obtained by simulation analyses), the
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final formed size of the ethyl cyanoacrylate droplet can be estimated to determine the appropriate printing distance in continuous printing. A simple structure is printed by the piezoelectric inkjet to verify the feasibility of the proposed method of the printing ethyl cyanoacrylate biodegradable structure for medical care.
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Keywords—piezoelectric inkjet, biodegradable structure, droplet spreading characteristic, shrinkage rate
1. Introduction
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The 3D printing technology has been developed rapidly in the engineering field, as it has the advantages of breaking through the limitation of traditional machining process and realizing effective machining of
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special-shaped structures. At present, the 3D printing technologies used in engineering field mainly contain hot-melt deposition printing [1-4] and selective laser firing printing [5-8]. The hot-melt deposition printing is generally used for printing non-metallic materials such as polylactic acid (PLA) and acrylonitrile butadiene
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styrene (ABS) plastics. It has the advantages of low cost, strong reproducibility and easy operation. Tao et al. printed PLA composite filament by hot-melt deposition printing process [9]. Gaisford et al. printed tablets with different geometries by the method of melt depositing printing, and discussed the influences of geometry on drug release characteristics of printed tablets [10]. Connal et al. printed pH-responsive and functional polymers by hot
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melt deposition printing process, and studied the influences of thermal processing conditions on the quality of the printed objects [11]. Smith et al. developed a new hot melt deposition process for manufacturing thin-walled drug-free capsules, and the feasibility of the developed process was verified by experiments [12]. Masood et al. fabricated new metal/polymer composites by melt deposition process, and found that selective laser sintering printing was mainly used for printing metal materials such as nickel-based super-alloy, stainless steel, tool steel, titanium alloy, aluminum alloy. It has the advantages of high material utilization and printing precision [13]. Roy et al. printed metal nanoparticles by using selective laser sintering system and improved the
system optics which could ensure high printing resolution at micron level and realize the sintering process of large area metal nanoparticles [14]. Kong et al. printed 316L stainless steel structure, which had strong elongation, corrosion resistance, biocompatibility and could be used as medical implants in biomedicine field, by selective laser melting technology [15]. It can be seen that the hot-melt deposition printing and selective laser firing printing are mainly used to realize the manufacturing of engineering parts. Compared with hot-melt deposition printing and selective laser burning printing, the 3D printing technology based on the piezoelectric actuation has the advantages of high precision, fast response, simple structure, no electromagnetic interference, green manufacturing, etc. It can realize low cost, non-contact printing on demand
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and full utilization of raw materials theoretically. Kim et al. printed mammalian cells with high accuracy at room temperature based on piezoelectric inkjet technology, and the effects of nozzle diameter and cell liquid concentration on cell number and cell activity were studied [16]. Koo et al. proposed a method of 3D printing cells based on the assistance of piezoelectric actuation, which realized high-precision printing of in situ cell and
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enhanced the cell viability [17]. Uddin et al. printed 5-fluorouracil, curcumin and cisplatin on metal microneedles uniformly and accurately based on piezoelectric inkjet technology, and realized no material loss in the printing
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process [18]. Lorber et al. proposed the method of printing retinal ganglion cells and neuroglia of adult rats by piezoelectric inkjet technology [19]. Scoutaris et al. printed drug on vascular stents for drug delivery based on
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piezoelectric actuation, and proved the feasibility of the method using biological evaluation [20]. Lin et al. proposed the method of printing precisely controlled multiple cell patterning based on piezoelectric inkjet technology. By using this method, the biocompatible cell could be printed on the substrate in a large range of
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several hundred microns to provide a possibility for matrix-assisted laser desorption/ionization mass spectrometry analysis [21, 22]. He et al. successfully printed uniform sodium alginate micro-particles by using piezoelectric actuation printing system [23]. Sun et al. used a bi-piezoelectric inkjet to distribute the mixed reagent of polymerase chain to the oil droplets to carry out quantitative polymerase chain reaction analysis [24]. It can be
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seen that the 3D printing technology based on piezoelectric actuation can achieve accurate printing of biomedical-related fluids at ambient temperature. As the ethyl cyanoacrylate solution is a kind of glue which has the advantages of biodegradability and quickly
curing (catalyzed by water) at room temperature, the methods of stitching bio-tissue by ejecting ethyl cyanoacrylate solution droplet based on piezoelectric inkjet have been studied by researchers. Saska et al. printed ethyl cyanoacrylate glue to realize the bonding and fixation of bones based on piezoelectric inkjet technology [25]. Boehm et al. printed medical adhesives and sealants to realize the bonding of biological tissues by using
piezoelectric inkjet [26]. Barbosa et al. printed ethyl acrylate to achieve the fixation of free gingival transplantation based on piezoelectric inkjet technology [27]. By printing biodegradable glue at wound area, the secondary trauma caused by taking out stitches after healing can be avoided. For some cases the injured tissue need to be fastened or supported by printed biodegradable structures, while, the current studies mainly focus on the surface-to-surface gluing of tissues. In this work, the preliminary researches of printing biodegradable ethyl cyanoacrylate structure for medical treatment based on piezoelectric inkjet are carried out. In order to achieve the accuracy of printing biodegradable structure on demand, it is necessary to control the spreading characteristics of ejected droplet. The influences of
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physical properties of ejected ethyl cyanoacrylate solution and contact angle of target surface on the spreading characteristics are discussed by simulations. The injection and spreading statuses of ejected ethyl cyanoacrylate droplet before and after solidification under different conditions are studied by experiments. The amplitude ranges of pulse voltage for achieving single droplet injection and the shrinkage rates are obtained. Based on the obtained
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shrinkage rates and spreading results, the final forming size of the ejected droplet can be estimated. A simple ethyl cyanoacrylate structure is printed to prove the validity of printing biodegradable ethyl cyanoacrylate structure for
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medical treatment based on piezoelectric inkjet.
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2. Spreading Characteristics before Solidification
A piezoelectric inkjet used for printing biodegradable ethyl cyanoacrylate droplet is designed as shown in Fig. 1. Vibrations are created when the pulse voltages are applied on the piezoelectric vibrator, then, the droplets are
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ejected out by the pressure waves which are produced by the vibrations. When the ejected droplet moves to the target surface, the spreading process begins. In this work, the spreading characteristics of the ejected ethyl cyanoacrylate droplet before solidification are mainly studied by direct coupling simulation analysis which is carried out by using the finite element analysis software FLUENT, this simulation method have been proved to be
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effective in our previous work [28]. Before the direct coupling simulation analysis, the pressure boundary conditions at the nozzle part should be obtained through acoustic structural coupling analyses. Since this work mainly focuses on the dynamic characteristics of ejected droplet, only the ejecting and spreading characteristics of droplets are discussed here, and more details of acoustic structure coupling analysis can be found from reference [29].
Figure 1. The schematic diagram of the piezoelectric inkjet. In order to achieve the stability of the designed piezoelectric inkjet in printing biodegradable ethyl cyanoacrylate solution with different physical properties, the influences of main physical parameter on the spreading characteristics are analyzed. The spreading characteristics mainly include the spreading status, contact
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pressure, response characteristics and final spreading results. The characteristics when droplets moving and spreading onto the target surface are studied in this work by setting pressure boundary conditions obtained by acoustic structural coupling analyses at the nozzle part. The analyses follow the principle of single variable. When analyzing the influences of selected factor on the spreading
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characteristics, the values of other parameters remain unchanged and set based on Table 1. The material of the piezoelectric ceramic used in this work is PZT-5H, square wave is selected as the excitation signal on the
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piezoelectric vibrator. For comparison discussion of the droplet spreading statuses, the times when the spreading area of droplet reaches the maximum value are selected.
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Table 1
The values of variable parameters set in simulations. Surface tension (N/m)
Density (kg/m3)
Contact angle (°)
0.001
0.072
1000
90
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Dynamic viscosity (mPa·s)
2.1 The influences of dynamic viscosity
The maximum spreading area of the droplet occurs when the velocity of the droplet decreases to zero for the first time after contacting the target surface. To avoid the interference between adjacent droplets, the maximum spreading diameter should be taken as the reference value to determine the printing distance. Besides, the
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maximum contact pressure between droplet and target surface also occurs when the spreading area reaches the maximum, as the reverse acceleration of the droplet reaches the maximum at this time. To ensure the printing stability when droplets are ejected on tissues with low contact stiffness, the maximum contact pressure under different conditions should be obtained as well. When the amplitude of the applied square wave is 65 V and the ethyl cyanoacrylate solution with different viscosities is ejected. It can be seen that the number of satellite droplets at the tail of the main droplet decreases with the increase of the viscosity. The separation trend between the center and the periphery of the droplet
gradually weakens. The reason is that, with the increase of the viscosity, the volume and velocity of the droplet gradually decrease, and the inertia force of the periphery part of the droplet decreases. Then, the separation trend weakens under the action of the internal viscous force. The contact pressure curves of ejected droplet with different viscosities are shown in Fig. 2(b), where the abscissa axis represents the distance from the droplet center, the coordinate values at the abscissa axis where the corresponding contact pressures are zero can represent the radiuses of the droplets. It can be seen that, the maximum contact pressure occurs at the center of the droplet. The contact pressure at the center area is little affected by the viscosity, while the contact pressure at the periphery area decreases with the increase of the viscosity. Therefore, when the viscosity of the ejected fluid decreases, it is
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necessary to ensure that the contact stiffness of the target surface (biological tissue) is greater than the maximum
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contact pressure, so as to reduce the impact of the spreading process on the printing resolution.
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Figure 2. The final spreading characteristics of droplet: (a) The spreading statuses of droplets when ejecting fluid
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with different viscosities, (b) The contact pressure between droplet and target surface. The response characteristics of the spreading process of droplet with different viscosities are shown in Fig. 3, where the contact time represents the time when the droplet first contacts with the target surface; the stable time represents the time when the droplet formation is finished; the fluctuation time represents the time interval
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between contact time and stable time. The fluctuation time of the droplet formation determines the minimum time interval of ejection. It can be seen that, the contact time increases with the increase of the viscosity, which is mainly due to the decrease of the velocity of droplet. The fluctuation time of droplet decreases gradually with the increase of the viscosity, the reason is that with the increase of fluid viscosity, the energy loss produced by the high viscosity droplet is increased in the process of droplet fluctuation. In addition, with the increase of viscosity, the contact time of droplet increases gradually, but the fluctuation time of droplet decreases, which makes the stabilization time keep nearly stable. Therefore, when the viscosity of ejected fluid is relatively low, the amplitude of the pulse voltage should be weakened to reduce the effect of large fluctuation time.
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Figure 3. The response characteristics of the spreading process when droplet with different viscosities.
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Figure 4. The final spreading characteristics of droplet: (a) The final spreading diameter of droplet with different
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viscosities, (b) The final spreading height of droplet with different viscosities.
The final spreading diameter and height of the droplet with different viscosities are shown in Fig. 4. The results
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indicate that due to the decreasing of droplet volume caused by the increased fluid viscosity, the final spreading diameter and height all decrease. Thus, when solution with relatively low viscosity is ejected, the excitation
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intensity should be reduced appropriately to improve the injection accuracy. When fluid with relatively high viscosity is ejected, the distance between the neighboring droplets and the height feed of the printing head all should be increased to avoid the interference between the droplets. 2.2 The influences of surface tension
When the amplitude of the applied square wave is 25 V and solution with different surface tensions is ejected,
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the spreading statuses of the droplet are shown in Fig. 5(a). The contact pressure curves between droplets and target surface are shown in Fig. 5(b). It can be seen that with the increase of the surface tension, the contact pressure of both the central part and the peripheral part of the droplet all increase gradually. The reason is that along with the increasing of the surface tension, the contraction force of the droplet increases. Thus, the amplitude of the pulse voltage should be decreased to reduce the impact of droplet collision and spreading process on the system when the surface tension of the injected droplet increases.
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Figure 5. The final spreading characteristics of droplet: (a) The spreading statuses of droplets when fluid with different surface tensions, (b) The contact pressure between droplet and target surface.
When ethyl cyanoacrylate solution with different surface tensions is ejected, the response characteristics of the
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spreading process of droplets are shown in Fig. 6. It can be seen that, as the exit velocity of the droplet decreases with the increase of the surface tension, the contact time decreases with the increase of the surface tension. The
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fluctuation time decreases with the increase of the surface tension, which is due to that the surface energy loss of the droplet increases with the increase of the surface tension during the spreading process. With the increase of
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surface tension, the increasement of contact time is greater than that of fluctuation time, which makes the stabilization time increase. Therefore, when fluid with large surface tension is ejected, the excitation should be
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adjusted (reducing the duty ratio, etc.) to stabilize the response speed of the spreading process.
Figure 6. The response characteristics of the spreading process when droplet with different surface tensions. The final spreading results of the ejected droplet with different surface tensions are shown in Fig. 7. We can see
that, the final spreading diameter and the height of droplet all decrease gradually with the increase of surface tension. This is due to the volume and the contraction force of the drop all decrease with the increase of the surface tension. Thus, the distance between adjacent droplets should be increased to reduce the interference between droplets when fluid with low surface tension is ejected.
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Figure 7. The final spreading characteristics of droplet: (a) The final spreading diameter of droplet with different
2.3 The influences of surface density
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surface tensions, (b) The final spreading height of droplet with different surface tensions.
Based on the analysis results, it is found that the spreading statuses and contact pressure of the droplet change slightly when the density of the ejected fluid changes. Thus, only the influences of density on the spreading
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response characteristics and final spreading results are given here.
As we can see from Fig. 8, when the amplitude of the applied square wave is 65 V, with the increase of density,
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the fluctuation time of droplet increases gradually, while the contact time keeps almost unchanged, which makes the stabilization time rise with the increase of the density. Thus, when the density of the ejected fluid increases, the
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spreading process.
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excitation signal should be adjusted (such as reducing the duty ratio) to stabilize the response speed of the
Figure 8. The response characteristics of the spreading process of droplet with different densities.
When the density of the ejected fluid changes, the results of final spreading diameter and height of droplets are
shown in Fig. 9. It can be seen that with the increase of density, the spreading diameter of droplet increases gradually, while the spreading height of droplet keeps nearly unchanged. Thus, the printing distance between the droplets should be increased to avoid the interference when the density of the ejected fluid increases.
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Figure 9. The final spreading characteristics of droplet: (a) The final spreading diameter of droplet with different
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densities, (b) The final spreading height of droplet with different densities. 2.4 The influences of contact angle
The contact angle between the droplet and the target surface (different biological tissues) is affected by the properties of the target surface. When the value of the contact angle between droplet and target surface is different
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and the amplitude of the applied square wave is 25 V, the spreading statuses of droplet are shown in Fig. 10(a). The change curves of the contact pressure, when contact angle changes, are shown in Fig. 10(b). It can be seen
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that, the contact pressure decreases gradually with the increase of the contact angle. The reason is that, when the contact angle increases, the horizontal component of the surface tension, which along the inner normal direction
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of the droplet surface, increases gradually. Thus, when the contact angle decreases, the amplitude of the pulse
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voltage should be decreased to reduce the effect of droplet spreading process.
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Figure 10. The spreading characteristics of droplet: (a) The spreading statuses of droplets under different contact angles, (b) The contact pressure between droplet and target surface with different contact angles. The final spreading diameter and height of the droplet with different contact angles are shown in Fig. 11, respectively. It can be seen that, with the increase of contact angle, the final spreading diameter of droplet decreases gradually, while the spreading height increases gradually. The reason is that the change of contact angle
does not affect the volume of the ejected droplet, while the spreading diameter of droplet decreases with the increase of the horizontal component of surface tension, resulting in the increasing of the spreading height of droplet. Thus, when the contact angle increases, the height feed of the nozzle should be increased to avoid the
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interference between nozzle and droplets.
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Figure 11. The final spreading characteristics of droplet: (a) The final spreading diameter of droplet with different
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contact angles, (b) The final spreading height of droplet with different contact angles.
3. Final spreading characteristics
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Ethyl cyanoacrylate droplets can realize addition polymerization and solidify at room temperature under the catalysis of water, and the spreading size of droplet changes due to the solidification process. Based on the
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simulation analyses of droplet spreading characteristics before solidification, the spreading dimensions of unsolidified droplet under different conditions are obtained. Combining with the shrinkage rate of droplet during the solidification process, the final forming size of ejected ethyl cyanoacrylate droplet can be predicted and the
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adjustment method of printing distance to meet the requirement of efficient continuous printing can be obtained. Thus, the shrinkage rates and solidification characteristics of printed ethyl cyanoacrylate droplets under different conditions are studied by experiments.
Based on the designed structure as shown in Fig. 1, the prototype is manufactured and experiments are carried
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out. The commercial ethyl α-cyanoacrylate solution (with density as 1060 kg/m3) is used in this work. It can be see from our previous research results that the satellite droplets would be produced when high voltage amplitude is applied. While, the satellite droplet can influence the printing accuracy for printing and reduce the printing quality. In this work, when the voltage amplitude is less than 40 V, the satellite droplets can be eliminated. When pulse voltage with amplitude as 30 V is applied, the statues of ejected droplet are shown in Fig. 12, where the green arrows represent the moving direction of droplet. In order to observe the whole motion characteristics of
droplets after being ejected out conveniently, the ejecting direction of the piezoelectric inkjet is arranged vertically
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and the glass sheets, where the droplet spreading process occurs, are removed.
Figure 12. The statuses of ejected droplet when applied pulse voltage with amplitude as 30 V. The green arrows
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represent the moving direction of droplet, the droplets appear white under the illumination of an external light source and are marked by red circles.
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As can be seen from the Fig. 12, there is no satellite droplet existing at the tail of the ejected droplet, and the droplet is elongated due to the effect of two-phase flow during the moving process. When the droplet moves to its
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highest position, the velocity reduced to zero and the shape is similar to a sphere. Due to assembly and processing errors, the nozzle's normal direction is not absolutely vertical, there is an error between the droplet drop position
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and the nozzle.
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Figure 13. The statuses of ejected droplet when applied pulse voltage with amplitude as 50 V. The green arrows
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represent the moving direction of droplet, the satellite droplets are indicated by yellow arrows. The droplets appear white under the illumination of an external light source and are marked by red ellipses.
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When the amplitude of the pulse voltage increases to 50 V, the statues of ejected droplet are shown in Fig. 13, it can be seen that satellite droplets are formed at the tail of the main droplet. As the velocities of the satellite
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droplets are less than that of the main droplets, the maximum motion heights of the satellite droplets are lower. Besides, the falling positions of the satellite droplets are different from that of the main droplets due to assembly
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and machining errors. Therefore, in order to meet the requirement of high precision printing, single droplet injection mode should be selected.
To obtain the shrinkage ratio of ethyl cyanoacrylate droplet, the spreading statuses when applied pulse voltage with amplitude as 10 V are obtained as shown in Fig. 14, where the red line represents the spreading boundary before solidification and the blue line represents the spreading boundary after solidification. It can be seen that,
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under the same condition, the boundary line of the droplet before solidification is basically round and similar, but the shape of the droplet after solidification is irregular and different. Based on Fig. 14, the spreading diameters before/after solidification and the shrinkage ratios under same
conditions (when applied pulse voltage with amplitude as 10 V) are obtained as shown in Fig. 15. It can be seen that, under the same condition the spreading diameters of the droplets before and after solidification fluctuate slightly which proves that the designed piezoelectric inkjet is basically stable for repeated printing. We can see that when the voltage amplitude is 10 V, the shrinkage rate of the ethyl cyanoacrylate droplet is about 72.5%.
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Figure 14. The repeat experiment results of the spreading statuses of droplets when applied pulse voltage with amplitude as 10V. The red line represents the spreading boundary before solidification and the blue line represents
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the spreading boundary after solidification.
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Figure 15. The spreading diameters before/after solidification and the shrinkage ratios when applied pulse voltage with amplitude as 10V.
The spreading characteristics of ethyl cyanoacrylate droplet injected under different voltage amplitudes are shown in Fig. 16(a). It can be seen that with the increase of voltage amplitude, the spreading diameters of droplets before and after solidification all increase gradually. When the voltage amplitude is larger than 40V, the satellite
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droplets will be produced and spread around the main droplets, which is consistent with the previous results obtained by injection statuses analyses. The droplet diameters before and after solidification and the shrinkage ratios when applied pulse voltages with different amplitudes are obtained, as shown in Fig. 16(b). It can be seen that, with the increase of voltage amplitude, the shrinkage rate of the ejected ethyl cyanoacrylate droplet increase slightly, while, for single droplet spreading process (voltage amplitude less than 50 V), the shrinkage rate of single droplet injection is basically between 70% and 75%. The printing distance between droplets in continuous printing
mode can be estimated in advance by combining the simulation results of spreading size before solidification and the obtained shrinkage ratios.
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Figure 16. The final spreading characteristics of droplet: (a) The spreading statuses of droplets when applied pulse voltage with different amplitudes, (b) The spreading diameters before/after solidification and the shrinkage ratios when applied pulse voltage with different amplitudes.
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In order to verify the feasibility of printing biodegradable ethyl cyanoacrylate structure based on the designed piezoelectric inkjet, a simple structure is printed, as shown in Fig. 17. By testing, we find that the printed simple
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structure has certain strength, while the surface quality is poor. Thus, further research is needed to improve the
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surface quality.
Figure 17. The printed biodegradable ethyl cyanoacrylate simple structure based on the designed piezoelectric inkjet.
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4. Conclusion
The spreading characteristics of ethyl cyanoacrylate droplet ejected by the designed piezoelectric inkjet were
simulated and tested in this work. Based on the spreading results of droplet before solidification by simulations, the methods for stabilizing the spreading performance were obtained. When the viscosity of ejected fluid decreased, or the surface tension of droplet increased or the contact angle decreased, the amplitude of the pulse voltage should be decreased to reduce the impact of droplet collision and spreading process on the printing accuracy. When ethyl cyanoacrylate solution with relatively low viscosity or large surface tension or large density
was ejected, the excitation should be adjusted (reducing the duty ratio, etc.) to stabilize the response speed of the spreading process. The methods for avoiding failure spreading of droplet were obtained. When ethyl cyanoacrylate solution with relatively high viscosity or low surface tension or high density was ejected, or the contact angle increased, the distance between the neighboring droplets and the height feed of the printing head all should be increased to avoid the interference between the droplets. For improve the printing accuracy of the ethyl cyanoacrylate structure, the piezoelectric inkjet should work in the mode of ejecting single droplet without satellite droplets which could be achieved by reducing the voltage amplitude (in this work, the voltage amplitude should less than 50 V). Experimental results showed that the
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shrinkage rates of the printed ethyl cyanoacrylate droplet were between 70% and 75% when the piezoelectric inkjet worked in single droplet ejecting mode. The final formed size of the ethyl cyanoacrylate droplet could be estimated based on the obtained shrinkage ratios and the spreading dimensions of droplet before solidification. The feasibility of printing ethyl cyanoacrylate biodegradable structure for medical care by designed piezoelectric
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inkjet was proved to be feasible by experiments.
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Author Contributions Section
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Declaration of interests
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Kai Li: Conceptualization, Formal analysis, Validation, Writing-Original draft preparation Weishan Chen: Methodology, Writing - Review & Editing Junkao Liu: Writing - Review & Editing, Supervision Hengyu Li: Writing-Original draft preparation, Formal analysis Naiming Qi: Writing - Review & Editing, Yingxiang Liu: Formal analysis, Writing-Review & Editing, Supervision
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgments
This work was supported in part by the National Natural Science Foundation of China (No. 51905134, No.
51622502).
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Author Biographies
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Kai Li was born in Shandong, China, in 1988. He received the B.E. degree from the School of Mechanical Engineering, Shandong University of Technology, in 2012. He received the M.E. and Ph.D. degrees in mechatronics engineering from the School of Mechatronics Engineering at Harbin Institute of Technology, China, in 2014 and 2018, respectively. He is currently a lecturer at the Harbin Institute of Technology, China. His research interests include piezoelectric micro-jet and microfluidics, etc. Email：[email protected]
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Weishan Chen was born in Hebei, China, in 1965. He received his B.E. and the M.E. degrees in precision instrumentation engineering, and the Ph.D. degree in Mechatronics engineering from Harbin Institute of Technology, China, in 1986, 1989, and 1997, respectively. Since 1999, he has been a professor with the School of Mechatronics Engineering at the Harbin Institute of Technology. His research interests include ultrasonic driving, smart materials and structures, bio-robotics. Email: [email protected] Junkao Liu was born in Hebei, China, in 1973. He received his B.E. degree in mechanical engineering and Ph.D. degree in mechatronics engineering from the School of Mechatronics Engineering, Harbin Institute of Technology, China, in 1995 and 2001, respectively. Since 2011, he has been a professor with the School of Mechatronics Engineering at the Harbin Institute of Technology. His research interests include ultrasonic driving, biomimetic robots and simulations of parallel mechanisms with multi-degree of freedom. Email: [email protected]; [email protected]
Hengyu Li was born in Jilin province, China, in 1991. He received the B.S. degree and M.E. degree in Mechatronic Engineering from Changchun University of Technology, Changchun, China, in 2015 and 2018. He is currently a Ph.D. candidate in Mechatronic Engineering at Harbin Institute of Technology, China. His research interests include the precision piezoelectric actuators and piezoelectric micro-jet technology. Email: [email protected]
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Naiming Qi was born in China, in 1962. He received his B.E. degree in hydraulic technology, M.E. degree in fluid transmission and control and Ph.D. degree in precision instrument and mechanical engineering at Harbin Institute of Technology, China, in 1984, 1990 and 2001, respectively. He joined the School of Astronautics at the Harbin Institute of Technology in 1990, where he has been a professor since 2002. His research interests include automatic docking and testing technology of aircraft electromechanical integration, space control and intelligent docking technology, UAV motion planning and advanced recovery technology, etc. Email: [email protected]
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Yingxiang Liu was born in Hebei Province, China, in 1982. He received his B.E. degree in mechanical engineering, M.E. and Ph.D. degrees in mechatronics engineering from the School of Mechatronics Engineering at Harbin Institute of Technology, China, in 2005, 2007 and 2011, respectively. He joined the School of Mechatronics Engineering at the Harbin Institute of Technology in 2011, where he has been a professor since December 2013, and is also a member of the State Key Laboratory of Robotics and System. He is the vice director of the Department of Mechatronic Control and Automation. He was a Visiting Scholar at the Mechanical Engineering Department, University of California, Berkeley, from August 2013 to August 2014. His research interests include piezoelectric actuating, ultrasonic motors, piezoelectric actuators, precision actuating, piezoelectric micro jets, bionic robots, fish robots and soft robots. He has served as an Associate Editor of IEEE Transactions on Industrial Electronics, an Associate Editor of IEEE Access and Guest Editor of Applied Sciences. Email: [email protected]