Micro- and Bio-Rapid Prototyping Using Drop-on-Demand 3D ...Micro- and Bio-Rapid Prototyping Using Drop-on-Demand 3D Printing Jerry Fuh* Department of Mechanical Engineering, National

Micro- and Bio-Rapid Prototyping Using Drop-on-Demand 3DPrinting

Jerry Fuh*Department of Mechanical Engineering, National University of Singapore, Singapore, Singapore

Abstract

Rapid prototyping or referred as layered manufacturing has been widely used in fabricating 3Dcomponents from CAD data. It has evolved from liquid, plastic, powder to metal-based systems inthe past two decades. Recently, 3D printing using polymeric material has caught much attention inmany industry applications including in micro- and bio-fabrication due to its advantage of layermanufacturing. In this chapter, drop-on-demand (DoD) printing will be specially discussed. Amongthose, the novel electrohydrodynamic jet printing (E-jetting) and piezo-actuated micro-dispensingmethods will be described in details. Their applications to biomedical engineering – e.g., PCLscaffolding, HA bioactive multilayered coating, and cell printing – will be presented to demonstrateits potentials.

Introduction

Rapid prototyping (RP) is a solid free-form fabrication technique which creates products usingadditive manufacturing technology. This technique is different from traditional manufacturingmethods of subtractive manufacturing using CNC machine tools. Based on the concept of materialaddition, physical objects are fabricated by adding materials layer by layer. Computer-aided design(CAD) is usually used in the RP system to create a 3D model of the object in the first place. Thesoftware of the RP system then converts the 3D model generated from the CAD drawing intoa format compatible with the system. An example would be the STL format that is also adopted inthis project. The 3D model is then converted into 2D data usually by slicing and printed out layer bylayer into a solid physical object. In this manner, RP technology is able to build complicated shape orgeometric features without the use of tools or molds. This flexible method allows more effectivecommunication between design andmanufacturing and greatly reduces the time required for productdevelopment.

RP is the name generally given to the various additive processes. Besides the advantage ofenvironmental benignity, additive manufacturing process is also a low-cost production method forreducing the material wastage, especially for the specialty polymers and precious metals. RPsometimes also referred as additive manufacturing (AM) technologies have attracted considerableattention because they have the potential to greatly reduce ecological footprints as well as the energyconsumed in manufacturing. Inkjet printing is one of the most successful additive manufacturing

*Email: [email protected]

Handbook of Manufacturing Engineering and TechnologyDOI 10.1007/978-1-4471-4976-7_79-1# Springer-Verlag London 2013

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technologies. It develops at a rapid pace and has been expanded from conventional graphic printingto various new applications, such as organ printing, displays, integrated circuits (ICs), opticaldevices, MEMS, and drug delivery. Accordingly, the dispensed liquids have been expanded fromthe conventional pigmented ink (or standard dye-based ink) to polymers, gels, cell ink, or othermaterials which often have higher viscosities or even contain large particles or cells.

Drop-on-Demand (DoD) Inkjet Printing

The traditional inkjet printer designed for graphic printing is unable to fulfill the new challenges, oneof which is to dispense fluids of very high viscosities. For most of the commercial inkjet printheads,only liquids with viscosities lower than 20 cps can be consistently dispensed. Fluids with evenhigher viscosities have to be diluted before printing or warmed up during the printing, which willadversely affect the properties of the liquids. Another challenge is raised by nozzle clogging. Fluidscontaining particles, or cells, can easily block the nozzle orifice, resulting in time-consuming nozzlecleaning or even damage of the entire conventional printhead.

Drop-on-demand 3D inkjet printing is an additive or RP manufacturing process, a data-drivenprocess that patterns directly onto the substrate with ejected droplets. It is capable of precisedeposition of picoliter volumes (down to 2 pL, 15 mm in diameter) of liquids into 3D layer-by-layer structures at high speed (up to 60 kHz (Kyocera 2009)) and accuracy (<5 mm) on a targetsurface, even onto nonplanar surface. Due to its advantages in high resolution, automation, low cost,noncontact, flexible, environmental benignity, and ease of material handling, the application of DoDinkjet printing technology has been expanded from conventional graphic printing to new areas, suchas organ printing, displays, integrated circuits (ICs), solar cells, memory devices, optical devices,MEMS, and drug delivery.

Introduction to Inkjet PrintingInkjet printing is a contact-free dot-matrix printing technique in which an image is created bydirectly jetting ink droplets onto specific locations on a substrate. The concept of inkjet printing cantrace its history to the nineteenth century, and the inkjet printing technology was first developed inthe early 1950s. Inkjet printers that are capable of reproducing digital images generated bycomputers were developed in the late 1970s, mainly by Hewlett-Packard, Epson, and Canon. Thebooming of the personal computer industry in 1980s has led to a substantial growth of the printermarket, and nowadays personal printer is present in almost every office and home. Inkjet printingtechnology is developing at a rapid pace. It has been expanded from conventional graphic printing tovarious applications, such as organ printing, displays, integrated circuits (ICs), optical devices,MEMS, and drug delivery.

Classification of Inkjet Printing TechniquesInkjet printing technology has been developed in a wide variety of ways. In Fig. 1, the inkjet treestructure shows a layout for most of the better-known inkjet printing techniques and some of thecorresponding adopters. As can be seen, there are two categories of inkjet printing technology:continuous inkjet printing and drop-on-demand inkjet printing.

Continuous Inkjet Printing The earliest inkjet devices operated in a continuous mode. The ideawas first patented by Lord Kelvin in 1867, and the first commercial model was introduced bySiemens in 1951. In this technique, a continuous jet of the liquid ink is formed by applying pressure


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to the ink chamber with a small orifice at one end. A fluid jet is inherently unstable and will break upinto droplets, which is entirely a consequence of the surface tension effects. This phenomenon wasfirstly noted by Savart in 1833 and described mathematically by Lord Rayleigh (1878). If surfacetension force is the only force acting on the free surface of the jet, it will break up into droplets ofvarying size and velocity; when a periodic perturbation of an appropriate frequency is applied to theliquid, typically using a piezoelectric transducer, the jet will break up into droplets of uniform sizeand velocity.

The droplets separate from the jet in the presence of a properly controlled electrostatic field whichgenerated by an electrode that surrounds the region where break-off occurs. As a result, an electriccharge can be induced on the drops selectively. Subsequently, when the droplets pass throughanother electric field, the charged droplets are directed to their desired location on the substrate toform an image; those uncharged droplets will drift into a catcher for recirculation. Continuous inkjetcan be classified into binary deflection or multilevel deflection according to the drop deflectionmethodology, as can be seen in Fig. 1.

The major advantage of continuous inkjet printing is that it can generate ink droplets with veryhigh velocity, which can reach to 50 m/s. This feature allows for the usage of a relatively longdistance between printhead and substrate. It also allows for rapid droplet formation rate, also knownas high-speed printing. Another advantage of continuous inkjet is no waste of ink, due to dropletrecycling. Furthermore, since the jet is always in use, nozzle clogging can be avoided in continuousinkjet. Therefore, volatile solvents such as alcohol and ketone can be employed to promote drying ofdroplets onto the substrate.

Fig. 1 Layout of the different inkjet printing technologies


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The major disadvantage of continuous inkjet is that the ink to be used must be electricallyconducting to ensure that ink droplets can be charged and directed to the desired location. Further-more, due to ink recycling process, ink can be contaminated.

Drop-on-Demand Inkjet Printing Drop-on-demand inkjet systems were developed in the 1970s,when different actuation principles were utilized (Heinzl and Hertz 1985). In this technique, inkdroplets are produced only when they are required. According to the mechanism used during thedroplet formation process, DoD inkjet can be categorized into four major types: thermal mode,piezoelectric mode, electrostatic mode, and acoustic mode. Most of the DoD systems in the marketare using the thermal or the piezoelectric modes. Nevertheless, no matter which mode is used, thebasic principles of all these different inkjet methods are similar: a transducer, normallya piezoelectric element or a thermal heater, generates a pressure pulse into the ink and forcesa droplet out of the orifice, as schematically shown in Fig. 2. The only difference lies in that theway how this pressure pulse is generated.

Electrohydrodynamic Jet Printing (E-Jetting)Built based on the layered manufacturing principle, electrohydrodynamic technology, which ismainly consisted of electrospinning (E-spinning) (Formhals 1934) and electrospraying(E-spraying) (Jaworek and Sobczyk 2008), is a novel method to produce fibers or particles inmicro-/nanoscale, utilizing the principle of the dynamics of electrically charged fluids. E-spinningprocess has attracted rising interest as an effective method to fabricate micro-/nanofibers. DuringE-spinning, the charged jet of polymer solution is deposited onto the collector under the influence ofa high-voltage electric field. The feasibility of generating nanofibers with diameters under 100 nmhas been demonstrated using E-spinning method. Representative applications include optical sensorfabrication (Wang et al. 2002), drug delivery (Liao et al. 2006), and tissue engineering (Phamet al. 2006). To further expand its applications, it is important to achieve the high controllability offibers.

However, the resultant fibers from conventional E-spinning process are disordered, attributing tothe chaotic whipping of liquid jet over the long distance between nozzle and collector. ThusE-spinning process is limited for the applications to the requirements of fiber patterning. To alignfibers is challenging to achieve for the E-spinning process. Researchers have made an effort tomodify the process using a mandrel as the fiber collector to achieve aligned fibers, but fullycontrolled fiber orientation is still yet achieved. Sun et al. (2006) have also demonstrated the ability

Fig. 2 Schematic of the DoD inkjet printing process


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of the near-field electrospinning process to produce aligned fibers with diameter ranging from 100 to500 nm. However, the throughout is limited, and fibers are discontinuous since the needle tip has todip again to get solutions after dispensing each time. Furthermore, a few works on improved near-field electrospinning have been done to evaluate the substrate effect dispensing patterns (Changet al. 2008a) and process parameter optimization (Thirumalpathy and Leroy 2011).

The E-jetting system is an alternative electrohydrodynamic jet printing system which applies highvoltage on the fluid to achieve higher resolution for printing. Figure 3 presents the schematic of theE-jetting system. As shown, the E-jetting system mainly consists of a pneumatic system, a high-voltage amplifier, an in-house made printhead, and an XYZ stage with a controller. During theE-jetting process, sufficient solution is added into the reservoir which is fixed with a stainless steelblunt needle. The supply of solution is controlled by the precise pneumatic system with a constantpressure, and a high DC voltage is applied between the needle and the substrate to generate fiberscontinuously. At the beginning, there is a conical meniscus of the solution formed at the tip of thenozzle. After charging with high voltage, the conical meniscus is drawn and Taylor cone is formed.Following that, the liquid is stretched out by the electrostatic forces.

Various phenomenons of droplets, jet, and multi-jet will form during electrohydrodynamicprinting process, depending on the different processing parameters applied. To achieve the fiberprinting and orientation, only the process with single jet is studied in this work. Before jet bending bythe whipping instability as the electrospinning process (Doshi and Reneker 1995), the fiber isdeposited continuously on the substrate, and a complex fiber pattern can be precisely realizedwith the user-defined path created by the XYZ stage.

Advantages and Disadvantages of Inkjet PrintingAs a rapid prototyping technique, inkjet is an additive manufacturing process. It ejects droplets onlywhen required and hence reduces the material wastage. This implies a lower cost for the applicationsthat requires expensive materials. In the meantime, environmental impact of the technique is reduceddue to reduction of material wastage and less usage of solvents, which is sponsored for conservationand the realization of a sustainable society. As compared to the traditional photolithography-basedpatterning process, which consists of many subprocesses and leads to long processing time and highcost, inkjet printing is much more compact. It avoids all those complicated subprocesses.

Fig. 3 Schematic view of the E-jetting system


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Furthermore, it also saves the cost for lithography masks as well as the huge work for storing thehundreds of masks.

Inkjet printing is a data-driven direct-write process that can directly transfer computer-aideddesign (CAD) into device patterns, which can greatly save the process time and accommodatecustomization. Inkjet printing offers the advantage of noncontact between the nozzle and thesubstrate; thus there is no mechanical wear on the printed sample. The possibility of cross-contamination is also reduced to a minimum.

To conclude, inkjet printing offers advantages in low cost, compact, automation, noncontact,environmental benignity, and ease of material handling. It is a highly flexible technology that is ableto accurately deposit small volumes of materials in almost any pattern. There are two maindisadvantages in inkjet printing. Firstly, the anisotropic nature of the inkjet process, due to theintrinsic pinhole nature of the deposited ink, results in the uneven surface roughness of the printedfeatures (Chang et al. 1999). Furthermore, this film nonuniformity can also be produced by theinevitable “coffee stain” effect, which arises from interaction of multiple effects of the solvent dryingprocess (Singh et al. 2010; Deegan et al. 1997; Ikegawa and Azuma 2004). This disadvantage in filmuniformity does not exist in planar processing currently used in industry. Secondly, the resolution ofinkjet method is limited by the dispensed droplet size. The minimum diameter of an inkjet droplet fora state-of-the-art inkjet printhead is around 10 mm. The resulting printing resolution is enough fordocument printing use, but for nanotechnology use and many other industrial applications, suchresolution is not good enough, and a more precise inkjet device needs to be developed. Furthermore,the attainable feature size of a component fabricated by inkjet printing is also affected by how thedroplet interacts with the substrate. Thus the impact behavior of the droplets onto a substrate alsoneeds to be carefully studied.

Applications of DoD Printing Technique to Bioengineering

The DoD printing is proven to be a promising micro-fabrication process for micro- and bio-RPapplications, which owns high resolution and particularly good controllability of fiber orientation. Itis relatively easy and cost-effective to set up. The technique shows the feasibility of fabricatingcomplicated 3D microstructures with ultrafine fibers, showing its potential applications in themanufacturing of optical devices, sensors, drug delivery, tissue scaffolding, etc.

Fabrication of Scaffolds Using E-Jetted Oriented FibersExperimental SetupAn experimental setup of the E-jetting system is demonstrated in Fig. 4. Positive pressure rangingfrom 0 to 6 bar is provided by the PC-controlled 5-channel pneumatic system, maintaining thesufficient solution supplement. As Food and Drug Administration approved biocompatible material,polycaprolactone (PCL) is used as demonstration for fiber printing. The high-voltage amplifier,which is the critical component of the whole system, is applied between a nozzle and substrate, togenerate the high DC voltage (�3.25 kV � + 3.25 kV) for solution charging. The in-house madeprinthead is adaptable to different nozzles with an inner diameter varies from 80 to 510 mm. Siliconsubstrate is placed under the nozzle with a very close gap (less than 2 mm). To orient the fibersduring E-jetting, a precise XYZ stage is applied. It is controlled by the software and controller viaTTL signals to move with the predefined path.


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Ejected Scaffolds for Tissue RegenerationWith good controllability of fiber orientation, E-jetting technique is applied to create three-dimensional (3D) scaffold. For conventional E-spinning process, the resultant fibers are randomlydeposited without capability to define the pore size. Different from that, it is the significantadvantage of E-jetting technique to construct 3D structures with controlled pore size. It is requiredfor scaffolds to own pores ranging from 100 to 500 mm for tissue growth and prosthesis fixationenhancement. However, the smaller pores less than 300 usually present low porosity, limiting thecell ingrowth and nutrient penetration. In this study, the scaffolds with various given pore sizes of500, 400, and 300 mm are fabricated (Fig. 5), consisting of layer-by-layer-deposited uniform fibers.

As illustrated from the zoomed picture of the scaffold with pore size of 500 mm, the diameter offibers is less than 20 mm, while the gap between the adjacent fibers is 500 mm, meaning thatslenderness ratio is 25:1. It is challenging for such fine fibers to support themselves without bendingin the long-span structure. While for the scaffold with decreased pores of 400 mm, uniform fiberswith diameter of 20 mm are also achieved. With further reduction of pore size down to 300 mm, it isillustrated from the zoomed pictures that the E-jetted fibers are uniform, providing good intercon-nectivity of the 3D structures. With oriented ultrafine fibers, the scaffolds own desired pore size andconnectivity, exhibiting superiority for tissue engineering application.

Fig. 5 E-jetted 3D scaffold structure with various pore sizes: (a) pore size ¼ 500 mm, (b) pore size ¼ 400 mm, (c) poresize ¼ 300 mm

Fig. 4 Experimental setup of the E-jetting system


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DoD Printing of Multilayer Bioactive CoatingDepending on the fluid properties of dispensing materials and requirements for specific applications,various micro-dispensers are selected by different researchers. Walter et al. (2005) utilized bothpiezo-based and solenoid valve-based liquid dispensers to print miniature biological assays. Toconstruct a 3D tissue, Chang et al. (2008b) used four types of nozzles, i.e., solenoid-actuated nozzle,piezoelectric glass capillary nozzle, pneumatic syringe nozzle, and spray nozzle to deposit specifiedhydrogels with different viscosities.

Experimental SetupIn a proposed DoD micro-dispensing system (Sun et al. 2012), a solenoid-actuating micro-valve isused for the biocoating fabrication, which functions based on the control commands sent froma PC. The solenoid-actuated micro-valve dispensing system consists of a micro-valve and its driverand a pneumatic controller.

Figure 6a shows a schematic diagram of the solenoid micro-valve. This micro-valve operatesthrough a magnetic-piston system to open or close the valve. The opening duration is guaranteed byfirst 24 V spike voltage to activate the valve and then 3 V to hold the open status (shown in Fig. 6b).The total time period including spike and hold status is defined as operational on time (OOT), whichis the key parameter to control the droplet dispensing volume. Figure 6c shows the assembled micro-dispenser with a reservoir, a filter screen, a Teflon tubing, the micro-valve, and a nozzle.

Figure 7 illustrates the proposed dispensing system from both front view and the isometric sideview including an XYZ precision stage from Aerotech (http://www.aerotech.com/), a stage control-ler, two dispensing units and their drivers, a pneumatic controller, and a curing controller. The twodispensing units are attached on the Z-axis stage with a mounting structure. The substrate plate ismounted on the X stage which rides on the Y stage. When the stage reaches the given position, thestage controller will output TTL trigger signals to activate the corresponding dispensing unit driver,and then the micro-dispenser will complete droplet ejections. The curing controller is to adjustheating temperature, thus accelerating the solidification of deposited droplets on the substrate, withlimits within 60 �C. The pneumatic controller provides independent positive outlets for differentdispensers. When the valve is open, this pressure pushes the liquid along the chamber towards thenozzle. With the proper pressure and OOT, the liquid can form a droplet at the nozzle tip.

Fig. 6 Schematic diagram of the solenoid-actuated micro-valve dispenser


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http://www.aerotech.com/

Surface MorphologyAfter fabrication process, the surface morphology of the coating and cross-sectional area is observedusing scanning electron microscope (SEM). The accelerating voltage and beam current are set at15 kVand 12 mA, respectively. Elemental analysis is obtained by energy-dispersive X-ray spectros-copy (EDS) coupled with the built-in EDS detector of SEM. The results demonstrate the elementaldistribution of a specific area in weight percentage.

Figure 8a and b shows the surface morphology observation of pure hydroxyapatite (HA) coatingand 4-layer multilayer HA/collagen (Col) coating structure, respectively. This structure is mostlyconsisted of materials which possess the chemical affinity to the natural body tissue, such ashydroxyapatite (HA) and collagen. This composite coating is expected to induce a continuoustransition from tissue to implant surface, hence promoting bone regeneration. As observed in Fig. 8,there is a significant difference on the surface morphology between the two types of coating. InFig. 8a, the surface of pure HA coating looks relatively “rough.” For the HA/Col coating, thecollagen is being physically absorbed by the HA layer, which dampens and smoothens out thetopmost coating surface as Fig. 8b. This characteristic may tend to improve osteointegration with thebone as compared to a smoother surface (Nakashima et al. 1997). The 4-layer HA/Col structure was

Fig. 7 Experimental setup of the micro-dispensing system

Fig. 8 SEM photos for HA and 4-layer HA/Col coating


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fabricated based on the concept of the multilayer coating structure. As shown in Fig. 9, the structurepresents a uniform coverage by the dispensing materials on the titanium surface, and through theelemental distribution analysis by EDS, C and Ca/P appear at the desired positions according to thedesigned layered coating structures.

DoD Cell PrintingTo date, different types of cells have been printed successfully, and their viability has been verified(Chen et al. 2006, 2008c; Xu et al. 2004, 2005, 2006; Barron et al. 2005; Nakamura et al. 2005;Saunders et al. 2008; Takahashi and Tomikawa 2003). One of the comprehensive studies was carriedout by Saunders et al. (2008), who used a commercial desktop printer to dispense human fibroblastcells, for the investigation of the relationship between cell survivability and the inkjet printingparameters. Their study supported previous claims (Xu et al. 2004, 2005, 2006; Nakamuraet al. 2005) that cell survivability was not significantly affected by the printing process since cellsurvival rates only fell from 98% to 94% in their case, when the excitation pulse was increased from40 to 80 V. However, in their study, the printing process was all carried out using a modifiedcommercial printer, thus limiting their experiments to a fixed nozzle diameter (60 mm) and a smallrange of the drop velocities (lower than 1.0 m/s). These limitations may be of importance, becausethe shear stresses, which are expected to be the main factor in the killing of cells during the printingprocess, are proportional to the velocity gradients within the nozzle.

Experimental SetupAn experimental DoD cell printing system developed (Li 2010) will be used as an example toillustrate its feasibility. Printing experiments were carried out by using the self-developed squeezemode piezoelectric inkjet printing system. The setup is comprised of a compressor, a pressureregulator, a reservoir, a piezo-actuated printhead, a piezo driver, an Arrisun-5 lamp, and a PhotronFASTCAM SA1 camera (high-speed video camera), as shown in Fig. 10. The present printheaddesign is a great improvement over conventional printheads, as it allows for the use of interchange-able nozzles, for the same piezoelectric transducer. The interchangeable nozzle design allows one toeasily clean or change the clogged or damaged nozzle.

The inkjet process is highly periodic. Fig. 11a shows the droplet formation process in timesequence. Drop velocity can be calculated by dividing the spacing between two droplets by theirtime difference. Fig. 11b shows images of a few cells inside the nozzle. Liquid used was 1.0 % (w/v)aqueous solution of sodium alginate. Drop velocity is 0.74 m/s.

For the study of cell survival rates, L929 rat fibroblast cell suspensions were printed throughorifices of three different diameters (119, 81, and 36 mm) onto well plates (Costars) which contained

Fig. 9 Elemental distribution of 4-layer HA/Col coating


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the live-dead assay solution. Preparation of the live-dead assay solution will be introduced later. Theelectric pulses which used to drive the piezoelectric transducer were in the range of 52–140 V. Eachsample was printed for approximately 20 s with a driving frequency of 1.5 kHz for the printhead.Prior to the printing process, a 15 ml cell suspension was deposited into a well plate in the sameenvironment as the printing system, to act as a control.

Fig. 11 Images taken by using a high-speed video camera: (a) formation of a 160 mm droplet from a 119 mm nozzle,taken at a frame rate of 8,000 fps, time between frames of 125 ms; (b) cell motion in the nozzle with diameter of 119 mm

Fig. 10 Schematic showing the DoD setup for cell printing experiment


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Survivability TestsA LIVE/DEAD Viability/Cytotoxicity Kit (L3224, Molecular Probes, Invitrogen) was used toassess the survivability of the cells after the printing. The frozen vials containing the assay werethawed and centrifuged briefly before use. 20 ml of the supplied 2 mM EthD-1 solution and 5 ml ofthe supplied 4 mM calcein AM solution were added into 10 ml of 1� DMEM solution and mixedthoroughly, which gave an approximately 4 mM EthD-1 and 2 mM calcein AM working solution.Cells were directly dispensed into well plates which each contained 100 ml of the assay mixture, thenincubated for 30 min. For each printing condition, cells were dispensed into five separate Petridishes, to study the variation of survival rates. Controls were taken directly from the cell ink beforeprinting and put on a set of separate Petri dishes, undergoing the same environment and procedure.The stained samples were then partly transferred onto microscope slides and observed undera fluorescence microscope. Six images were captured from each Petri dish for cell counting. Cellsthat remained alive after the printing were stained green, and the damaged cells were stained red. Thenumbers of alive and dead cells for each sample were tallied with respect to that of the control whichwas taken prior to the printing.

Figure 12 shows the effects of excitation pulse amplitude on the mean cell survival rate, for the36 mm nozzle with excitation pulse amplitude from 60 to 130 V, at a frequency of 1.5 kHz and withrising and falling times of 3 ms. It is shown that the survival rate falls from 95 % to 78 % as theexcitation pulse is increased from 60 to 130 V, and the lowest survival rate of 76 % is observed whenthe highest voltage is approached. The excitation pulse amplitude represents the power for thepiezoelectric actuator to dispense the droplets, and this power directly affects the droplet velocityand thereby the shear stress in the liquid. In Fig. 13, the mean cell survival rates against excitationpulse amplitude for all of the three different orifices are drawn together to compare the effects ofdifferent orifice sizes on cell survivability. Samples printed through orifices with the diameter of36, 81, and 119 mm, with excitation pulse amplitude from 52 to 140 V, at frequency of 1.5 kHz, withrising and falling times of 3 ms. Comparing the three different trend lines in Fig. 13, we conclude thatit is not the strength of the electric field which directly affects cell survival rate; rather it is the fluidshear stress.

Fig. 12 Mean cell survival rate with respect to excitation pulse amplitude for the samples printed through the 36 mmorifice. Error bars show the standard error from five replicates


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Summary

Compared with the conventional electrospinning process, the E-jetting technique provides goodcontrollability of fiber orientation in a precise manner, demonstrating high feasibility of buildingcomplicated 3D structure with fine fibers for various applications in manufacturing of sensors,optical devices, and tissue engineering. The cell printing study has demonstrated that the piezoelec-tric DoD printing is able to successfully deliver L929 rat fibroblast cells through nozzles as small as36 mm. There was no significant cell death when dispensing the cells through the 81 mm and the119 mm nozzle, with the mean survival rates only reducing from 98 % to 85 %. This is in goodagreement with the existing study reported (Saunders et al. 2008), in which a commercial printer wasused to print human fibroblast cells.

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Fig. 13 Graph showing the mean cell survival rate against excitation pulse amplitude. Each cell survival rate data wasthe average value from five replicates


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http://global.kyocera.com/news/2009/0903_kdos.html

Index Terms:

Additive manufacturing (AM) 1–2Bioactive coating 8Cell survival rate 12Continuous inkjet printing 4DoD Cell printing 10Drop-on-demand 3D inkjet printing 2Ejected scaffold 7Electrohydrodynamic jet printing (E-jetting) 4E-spinning 4Fiber orientation 5–6Hydroxyapatite (HA) coating 9Micro-dispensing 9Micro-valve dispenser 8Nanofibers 4Osteointegration 9Piezo-actuated printhead 10Polycaprolactone (PCL) 6Rapid prototyping (RP) 1, 5Surface morphology 9Survivability test 12Tissue regeneration 7


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Micro- and Bio-Rapid Prototyping Using Drop-on-Demand 3D ...Micro- and Bio-Rapid Prototyping Using Drop-on-Demand 3D Printing Jerry Fuh* Department of Mechanical Engineering, National