**4. The applications of EFD jet deposition 3D printing**

In this part, four typical printed structures were used to present the applications of EFD jet deposition 3D printing in multi-scale and multi material 3D printing: (1) lines and dots (onedimensional structure); (2) high aspect ratio micro-scale "wall" structure (two-dimensional structure); (3) high resolution tissue engineering scaffold (3D structure); (4) 3D structure electronics (multi-scale and multi-material heterogeneous 3D structure).

#### **4.1. Lines and dots (one-dimensional structure)**

The droplet can be precisely deposited at the designated position by controlling the process of droplet jetting and the movement of stage. A dot array with the resolution of 50-60 μm and the different dot spacing has been printed to show the capability of drop-on-demand printing in the pulsed cone-jet mode, as shown in **Figure 9(a)**. The dots' spacing decreases correspondingly with the decreasing of stage moving speed. Using the continuous cone-jet mode, a line

**Figure 9.** Dot and line arrays.

pattern has been successfully printed on a glass slide, as shown in **Figure 9(b)**. The average line width in this pattern is about 40 μm, and the line pitch is about 150 μm. Due to the ability of depositing materials directly as desired patterns on the substrate with a simple fabrication process and high efficiency, the proposed printing method can be adapted for applications in thin-film transistors, optical elements, organic light-emitting diodes, photonics crystals, and DNA microarrays.

By using the conductive nanosilver paste, the proposed printing method can be applied for fabrication of metal-mesh patterns used in various electronics such as flexible displays, solar cells, touch panels, etc. **Figure 10** shows the optical images of the printed metal-mesh patterns. The line width and spacing of the metal mesh in **Figure 10(a)** are 20 and 250 μm, respectively. And the line width and spacing in **Figure 10(b)** are 10 and 150 μm, respectively. The line resolution of the printed metal-mesh patterns is less than 20 μm, which is almost invisible to the naked eye. It indicates that the proposed technology is promising to fabricate an invisible fine transparent electrode with good electricity and optical properties, which can be widely applied to electronic devices without any cosmetic issues due to the appearance of metal pattern.

#### **4.2. The micro-scale "wall" structure (two-dimensional structure)**

using four different types of materials, including the photosensitive resin, nanosilver conductive paste, polycaprolactone (PCL), and conductive silver adhesive, as shown in **Figure 8**.

The materials in the experiment have different fluidic properties and the viscosity. The viscosity of the photosensitive resin is 800 cP while the viscosity of nanosilver conductive paste is 5000 cP. The heating system was used for molding of PCL because of its solid-state at RT. The nozzle with an inner diameter of 250 μm is adopted to print objects with a line width of 10 μm, where the reduction ratio in dimensions between the nozzle and the printed line reaches 25:1. The viscosity of the conductive silver adhesives at ambient temperature is 8000 cP, the inner diameter of the nozzle is 250 μm, the line width of the object to be printed is 200 μm, and the print patterns must have good morphology. The results showed that the EFD jet 3D printing is suitable for almost any materials, compared to existing 3D printing technology. It is unexamined for its potential to provide high-resolution (that is, ~10um) patterning of materials

In this part, four typical printed structures were used to present the applications of EFD jet deposition 3D printing in multi-scale and multi material 3D printing: (1) lines and dots (onedimensional structure); (2) high aspect ratio micro-scale "wall" structure (two-dimensional structure); (3) high resolution tissue engineering scaffold (3D structure); (4) 3D structure elec-

The droplet can be precisely deposited at the designated position by controlling the process of droplet jetting and the movement of stage. A dot array with the resolution of 50-60 μm and the different dot spacing has been printed to show the capability of drop-on-demand printing in the pulsed cone-jet mode, as shown in **Figure 9(a)**. The dots' spacing decreases correspondingly with the decreasing of stage moving speed. Using the continuous cone-jet mode, a line

**4. The applications of EFD jet deposition 3D printing**

tronics (multi-scale and multi-material heterogeneous 3D structure).

**4.1. Lines and dots (one-dimensional structure)**

with ultra-high viscosity.

32 3D Printing

**Figure 9.** Dot and line arrays.

The EFD jet deposition 3D printing technology is mainly used for liquid printing materials, it also can be used for printing molten polymer materials by changing the nozzle structure. The material feeding unit is integrated into the printhead to shorten the distance between material feeding unit and the nozzle, because solid state printing material is difficult to be delivered to the nozzle through pipeline. The double heating module is used to heat both nozzle and feeding unit. The purpose of heating feeding unit is to keep the printed material in the melting state with certain fluidity, and that of heating nozzle is to ensure the quality and precision of the printing process.

Instead of utilizing polymer solutions as the printing material, the molten EFD jet 3D printing employs molten polymers as the printing materials. Due to the printing material is PCL with a melting point of about 60°C. A heating module with a heating temperature of 80°C is utilized to melt the solid polymer into flowing melts. Moreover, the molten polymer solidifies very quickly that benefits for the layered manufacturing of high aspect ratio structures. **Figure 11**

**Figure 10.** Metal-mesh patterns with (a) line width of 20 μm and spacing of 250 μm; (b) line width of 10 μm and spacing of 150 μm.

**Figure 11.** A cylinder structure with diameter of 20 mm, height of 550 μm, and wall thickness of 20 μm.

shows a micro-scale high aspect ratio wall structured cylinder with diameter of 20 mm, the wall thickness of 20 μm, and the height of 550 μm (continuous stacking of 20 layers). Compared to the traditional EHD printing, The experimental results show the molten EFD jet 3D printing offers a promising approach to produce the micro/nanostructures with ultra-high aspect ratio at low cost and high throughput.

**4.4. 3D structural electronic (layered heterogeneous structure)**

**Figure 13.** A tissue engineering scaffold: (a) overall view; (b) top view; and (c) microview.

of 3D structure electronics.

**Figure 14.** A 3D structure electronic device.

3D structure electronic is a typical multi-material structure product, which is widely used in many fields, such as aerospace, national defense, biological medicine and so on. However, how to realize the manufacturing of 3D structure electronic products with high efficiency and low cost is a very huge challenging problem. The EFD jet deposition 3D printing with multinozzle can provide an effective method for the manufacturing 3D electronic. In this case, two kinds of printing materials, photosensitive resin (viscosity 800 mPa s) and conductive silver (viscosity 8000 mPa s), are used. The photosensitive resin is used to construct main body of the 3D structure electronics, while the conductive silver paste is used to print interconnecting wires. The processing parameters of photosensitive resin are printed with voltage 3.2 kV, gas pressure 50 kPa, and moving speed 30 mm/s, while the printing process parameters of conductive silver paste are voltage 2.0 kV, gas pressure 20 kPa, and moving speed 3 mm/s. As shown in **Figure 14**, the printed structure made of photosensitive resin is a circular platform with a diameter of 7.5 mm at the bottom surface, a diameter of 5.5 mm at the top surface, and a height of 4 mm. There is a printed conductive wire made of conductive silver paste on the top surface of the round platform. Conductive silver paste can be cured at room temperature without heating and other post-processing, which will never damage printed main body

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#### **4.3. Tissue engineering scaffold (3D structure)**

A typical macro/micro-scale tissue engineering scaffold is a 3D porous structure for transporting nourishment and excreting metabolites for cell growth. A desirable scaffold is characterized by controllable porosity, pore size, and pore distribution, which can provide cells with sufficient oxygen and nutrient supply. It is challenging for the current manufacturing technologies to the fully controlled orderly morphology and accuracy as requested. Polycaprolactone (PCL) with good bio-degradable performance serves as the printing material of the biological scaffold, a molten EFD jet 3D printing was employed to fabricate 3D scaffold, as shown in **Figure 12**. The printing process parameters are set as follows: voltage 2.8kV, pressure 20 kPa, moving speed 5 mm/s, and heating temperature of 80°C. The overall size of the scaffold is 4 mm x 4 mm, the line width is 60 μm, the period is 300 m, and the height is 300 μm, shown in **Figure 13**. The experimental results confirm that the EFD jet deposition 3D printing possesses a very prominent ability for the macro/micro scale printing.

**Figure 12.** The macro view of a tissue engineering scaffold.

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**Figure 13.** A tissue engineering scaffold: (a) overall view; (b) top view; and (c) microview.

#### **4.4. 3D structural electronic (layered heterogeneous structure)**

shows a micro-scale high aspect ratio wall structured cylinder with diameter of 20 mm, the wall thickness of 20 μm, and the height of 550 μm (continuous stacking of 20 layers). Compared to the traditional EHD printing, The experimental results show the molten EFD jet 3D printing offers a promising approach to produce the micro/nanostructures with ultra-high aspect ratio

**Figure 11.** A cylinder structure with diameter of 20 mm, height of 550 μm, and wall thickness of 20 μm.

A typical macro/micro-scale tissue engineering scaffold is a 3D porous structure for transporting nourishment and excreting metabolites for cell growth. A desirable scaffold is characterized by controllable porosity, pore size, and pore distribution, which can provide cells with sufficient oxygen and nutrient supply. It is challenging for the current manufacturing technologies to the fully controlled orderly morphology and accuracy as requested. Polycaprolactone (PCL) with good bio-degradable performance serves as the printing material of the biological scaffold, a molten EFD jet 3D printing was employed to fabricate 3D scaffold, as shown in **Figure 12**. The printing process parameters are set as follows: voltage 2.8kV, pressure 20 kPa, moving speed 5 mm/s, and heating temperature of 80°C. The overall size of the scaffold is 4 mm x 4 mm, the line width is 60 μm, the period is 300 m, and the height is 300 μm, shown in **Figure 13**. The experimental results confirm that the EFD jet deposition 3D printing possesses

at low cost and high throughput.

34 3D Printing

**4.3. Tissue engineering scaffold (3D structure)**

a very prominent ability for the macro/micro scale printing.

**Figure 12.** The macro view of a tissue engineering scaffold.

3D structure electronic is a typical multi-material structure product, which is widely used in many fields, such as aerospace, national defense, biological medicine and so on. However, how to realize the manufacturing of 3D structure electronic products with high efficiency and low cost is a very huge challenging problem. The EFD jet deposition 3D printing with multinozzle can provide an effective method for the manufacturing 3D electronic. In this case, two kinds of printing materials, photosensitive resin (viscosity 800 mPa s) and conductive silver (viscosity 8000 mPa s), are used. The photosensitive resin is used to construct main body of the 3D structure electronics, while the conductive silver paste is used to print interconnecting wires. The processing parameters of photosensitive resin are printed with voltage 3.2 kV, gas pressure 50 kPa, and moving speed 30 mm/s, while the printing process parameters of conductive silver paste are voltage 2.0 kV, gas pressure 20 kPa, and moving speed 3 mm/s.

As shown in **Figure 14**, the printed structure made of photosensitive resin is a circular platform with a diameter of 7.5 mm at the bottom surface, a diameter of 5.5 mm at the top surface, and a height of 4 mm. There is a printed conductive wire made of conductive silver paste on the top surface of the round platform. Conductive silver paste can be cured at room temperature without heating and other post-processing, which will never damage printed main body of 3D structure electronics.

**Figure 14.** A 3D structure electronic device.

Therefore, combining the capability of printing variable materials and the use of multi-nozzle technology, the EFD jet deposition 3D printing presents a very prominent advantage and great potential in the multi-scale and multi-material 3D printing. This technology provides a new solution for the integrated printing of heterogeneous multi-material, multi-scale (macro/ micro-scale) structure.

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