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# Active Mixing Nozzle for Multimaterial and Multiscale Three-Dimensional PrintingOPEN ACCESS

[+] Author and Article Information
Hongbo Lan

Qindao Engineering Research
Center for 3D Printing,
Qingdao Technological University,
Qingdao 266033, Shandong, China
e-mail: hblan99@126.com

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received June 10, 2017; final manuscript received August 26, 2017; published online September 27, 2017. Assoc. Editor: Yayue Pan.

J. Micro Nano-Manuf 5(4), 040904 (Sep 27, 2017) (10 pages) Paper No: JMNM-17-1026; doi: 10.1115/1.4037831 History: Received June 10, 2017; Revised August 26, 2017

## Abstract

Multiscale and multimaterial three-dimensional (3D) printing is new frontier in additive manufacturing (AM). It has shown great potential to implement the simultaneous and full control for fabricated object including external geometry, internal architecture, functional surface, material composition and ratio as well as gradient distribution, feature size ranging from nano-, micro-, to macro-scale, embedded components and electrocircuit, etc. Furthermore, it has the ability to construct the heterogeneous and hierarchical structured object with tailored properties and multiple functionalities which cannot be achieved through the existing technologies. That paves the way and may result in great breakthrough in various applications, e.g., functional tissue and organ, functionally graded (FG) material/structure, wearable devices, soft robot, functionally embedded electronics, metamaterial, multifunctionality product, etc. However, very few of the established AM processes have now the capability to implement the multimaterial and multiscale 3D printing. This paper presented a single nozzle-based multiscale and multimaterial 3D printing process by integrating the electrohydrodynamic jet printing and the active mixing multimaterial nozzle. The proposed AM technology has the capability to create multifunctional heterogeneously structured objects with control of the macroscale external geometry and microscale internal structures as well as functional surface features, particularly, the potential to dynamically mix, grade, and vary the ratios of different materials. An active mixing nozzle, as a core functional component of the 3D printer, is systematically investigated by combining with the theoretical analysis, numerical simulation, and experimental verification. The study aims at exploring a feasible solution to implement the multiscale and multimaterial 3D printing at low cost.

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## Introduction

There is increasing demands for the ability to construct multifunctional heterogeneously structured objects with control of the macroscale external geometry and microscale internal structures as well as functional surface features [110]. For example, researchers have begun to design biologically inspired robots (e.g., soft robotics) with soft or partially soft bodies, integrated sensors, electronics and functions and complex shapes [6,8], which have the potential to be more robust and adaptable, and safer for human interaction, than traditional rigid robots. To build a functional wearable device including its electronic components, it is necessary to seamlessly transition from the flexible material that moves with the wearer's joints to the rigid material that possesses the electronic components. It would also need to embed electrical circuitry with multiple inks of varying conductivity and resistivity, precisely switching between them [11]. However, key challenges in the design and manufacture of such soft robots and wearable devices involve the complex multimaterial and multiscale fabrication processes, as well as the interfacing of soft and rigid components. It is critical difficult to construct such heterogeneous hierarchically structured objects by existing manufacturing technologies except adopting the emerging multimaterial and multiscale additive manufacturing (AM) technologies.

The emerging multimaterial and multiscale three-dimensional (3D) printing technique has shown great potential to implement the simultaneous and full control for fabricated object including external geometry, internal architecture, functional surface, material composition and ratio as well as gradient distribution, feature size ranging from nano-, micro-, to macro-scale, embedded components and electrocircuit, etc. It is able to construct the heterogeneous and hierarchical structured object with tailored properties and multiple functionalities which cannot be achieved through the existing technologies. Such technology has been considered as a revolutionary technology and next-generation manufacturing tool which can really fulfill the “creating material” and “creating life,” especially subvert traditional product design and manufacturing scheme.

As additive manufacturing technology is quickly evolving from producing primarily single-material, homogenous objects to producing heterogeneous and hierarchical structured objects with multifunctionality and tailored property, there is also a growing need for the multimaterial and multiscale 3D printing [1217]. However, very few of the established additive manufacturing processes have the capability to implement the multimaterial and multiscale 3D printing. We proposed a novel multimaterial and multiscale 3D printing process which combined the electrohydrodynamic jet printing (as shown in Fig. 1) and the active mixing multimaterial nozzle technologies. Figure 2 demonstrated the basic principle of the multimaterial and multiscale 3D printing [18]. That offers a promising solution to implement the multiscale and multimaterial 3D printing at low cost. An active mixing nozzle is considered as one of the core functional components of the multimaterial and multiscale 3D printer. The focus of this paper is to perform a systematic investigation for the active mixing multimaterial printhead by combining with the theoretical analysis, numerical simulation, and experimental verification.

## Modeling of Active Mixing Nozzle

Figure 2 shows the structural diagram of the active mixing nozzle proposed. It consists of material A and B inlets, a compressed air inlet, cleaning fluid inlets, waste exports, a feed compartment, a mixing chamber, a conductive nozzle, and a mixing agitator. The mixing agitator includes a motor, screw impeller, and end cover.

Assuming the impeller rotates clockwise at a low-speed, the diagram of the stress on the fluid in an active mixing nozzle is shown in Fig. 3. The force acting on the fluid, which is the volume force F, can be expressed as

Display Formula

(1)$F=F1−f+f1−f2$

where F1 is the mechanical force provided by the impeller; f is the resistance of the liquid on the impeller, f1 represents the friction given by parts of the fluid in the impeller to the outer fluid of the impeller, and f2 represents the resultant force of the inertial force and the friction force given by the wall to the fluid.

The rotating impeller can generate the centrifugal force and Coriolis force, which greatly influence viscous flow. Rotation causes turbulence flow and affects the stability of the boundary layer. Assuming the fluid in the active mixing nozzle is incompressible turbulent fluid, the physical equations that describe the mixing process consist of the flow control equation, standard k–ε equation, and diffusion equation.

###### Fluid Control Equation.

Under the condition of constant temperature, for turbulent fluid with no changes in density and viscosity, the fluid continuity equation and momentum equation in vector form established under a rectangular coordinate system are, respectively, Display Formula

(2)$∂ρ∂t+∇⋅(ρu)=0$
Display Formula
(3)$ρ∂u∂t+ρ(u⋅∇)u=∇⋅[−pI+(μ+μT)(∇u+(∇u)T)−23(μ+μT)(∇⋅u)I−23ρkI]+F$

where u is the velocity vector of the fluid, ρ denotes the density of the fluid, μ is the dynamic viscosity of the fluid, F is the volume forces acting on the fluid, t is time, p is the pressure of the flow field, and k is the turbulence kinetic energy. The viscosity coefficient of the turbulence can be represented by the turbulence stress Display Formula

(4)$μT=ρCμk2ε$

where Cμ is a calculating constant.

In a turbulent motion with outer power impetus, the mechanical power provided by the motor is generally the only driving force. Compared to the driving forces, these forces including the friction force, shear force, and inertial force in the flow field are too small, which can even be neglected to simplify the calculation. As a result, the volume force acting on the fluid can be replaced by the resultant forces of the mechanical force F1 and the resistance force given by the fluid to the impeller f, as in Display Formula

(5)$F=F1−f$

The resistance force given by the fluid to the impeller f is influenced by many factors, including the relative velocity of the impeller in the fluid, the contact area of the impeller and the fluid s, and the properties of the fluid (density, viscosity, etc.). Its computation formula is Display Formula

(6)$f=12cρ|v|2s$

where the resistance coefficient c is proportional to the viscosity and ρ is the density of the fluid.

###### Standard k–ε Equation.

For the incompressible fluid without considering the source term, the transport equation for turbulent kinetic energy k and dissipated energy ε based on the k–ε turbulence model can be expressed as Display Formula

(7)$ρ∂k∂t+ρ(u⋅∇)k=∇⋅[(μ+μTσk)∇k]+pk−ρε$
Display Formula
(8)$ρ∂k∂t+ρ(u⋅∇)ε=∇⋅[(μ+μTσk)∇ε]+Cε1εkpk−Cε2ρε2k$

where pk is the generation item of turbulent kinetic energy kDisplay Formula

(9)$pk=μT[∇u:(∇u+(∇u)T)−23(∇⋅u)2]−23ρk∇⋅u$

ε is the dissipation rate of the turbulence, and the parameters of the turbulence model are

$Cε1=1.44, Cε2=1.92, Cμ=0.09, σk=1.3, σε=1$

For the fluid in the active mixing nozzle, the Reynolds number can be expressed as Display Formula

(10)$Re=|u|dμ$

where |u| is the velocity of the fluid, d is the diameter of the active mixing nozzle, and μ is the dynamic viscosity of the fluid.

###### Diffusion Equation.

Assuming that when two liquids mix, the properties (such as the density and the viscosity) of the liquids do not change, and the diffusion of more diluted material satisfies the law of conservation of mass Display Formula

(11)$∇⋅(−Di∇ci)+u⋅∇ci=Ri$

where ci is the concentration of materials, Di is the diffusion coefficient, u is the distribution of the velocity vectors in the velocity field, and Ri is a source term for the response to describe the change in the concentration of flow field.

Based on the aforementioned analysis, we can see that the flow of fluid in an active mixing nozzle with a mechanical impeller is a turbulent flow. These factors that may influence the mixing performance and efficiency involve the rotational speed of the impeller, the size of the impeller (diameter), and the viscosity of the fluid, among others. Furthermore, it is important to determine suitable process parameters for mixing material to achieve better blending performance and higher efficiency.

## Numerical Simulation

###### Calculation Methods.

It was assumed that the fluid was incompressible Newtonian fluid with mutual seepage and dissolution, and that the flow was in a turbulence state. A tracer was introduced to facilitate observing the mixing effect of the fluid. The tracer was a tiny point with a clear color that would not affect the mixing of the fluid. The tracer was released through Gaussian pulse excitation.

The blending process of the tracer is a dynamic process that changes with time. The initial concentration of the tracer was set to 1, and other areas were set to 0. Mixing time was determined by observing the dispersion degree of the liquid in the tracer.

To achieve high-efficiency computation, we adopted the frozen rotor method to simulate the mixing of the fluid. Frozen rotor means the impeller and rotor are stationary. The fluid in the rotating field is presumed to be fixed in a rotating coordinate system. The effect of rotation is realized by introducing centrifugal force and Coriolis force.

###### Numerical Simulation Model.

Figure 4 is a simplified sketch of the mixing chamber of the active mixing nozzle. The numerical simulation used a two-dimensional model. Figures 5 and 6 show the structure of the mixing chamber and the location of the tracer, respectively. The main parameters were as follows: the diameter of the mixing chamber was 40 mm, the interior diameter of the mixing chamber was 36 mm, the diameter of the impeller was 32 mm, and the rotating direction was clockwise.

A free triangular mesh was adopted, the size of the mesh cells used mesh refinement. Eventually, a complete mesh consisting of 14,252 domain units and 880 boundary units was generated.

###### Numerical Simulation of Mixing Process.

Assuming the fluid is not affected by friction, gravity, surface tension, and other factors, the temperature remains constant in the mixing process. The mixing medium was liquid resin with a density of 1080 kg/m3 and a dynamic viscosity of 20 cps. The rotational speed of the impeller was set to 20 r/min.

###### Analysis of the Structure of the Flow Field.

Figures 7(a) and 7(b) are diagrams of the velocity field and pressure field of the mixing chamber, respectively. Judging from the figure, if the distance between the center of rotation and the field is greater, the speed of the flow field will be higher, and the volume force on the fluid will be greater. If the speed of the flow field in the tip of the rotating impeller is at the highest level, it will face the largest volume force. Therefore, in the process of dynamic mixing, when adding other materials in the active mixing nozzle, the material should be added to the area where it can be swept by the tip of the active mixing nozzle's blade to the greatest extent to ensure that this material and the material in the nozzle are mixed fully and efficiently.

###### Analysis of the Mixing Process.

Figure 8 shows the tracer's mixing results in the mixing chamber at 0.9 s, 3.3 s, 7.2 s, 9.3 s, 12.9 s, and 18.3 s. Analyzing the figure reveals the following: when t = 0.9 s, the diffusion and mixing of the tracer are relatively slow. This is because the flow velocity and volume force of the tracer's releasing position are relatively small, which is not conducive to mixing the fluid. When t = 3.3 s, the distribution area of the tracer in the mixing chamber becomes larger. This is because the tracer comes into the high-speed area of the mixing chamber and mixes in all directions. As shown in Fig. 8(d), it only takes a few seconds (t = 9.3 s) for the tracer to go throughout almost the entire mixing chamber. It takes a long time for the tracer to go from crossing the mixing chamber (t = 9.3 s) to fully mixing with the fluid (t = 18.3 s). This is because the slowest diffusion area is located near the center of the rotational impeller. Moreover, it takes a long time for the tracer and the fluid to fully mix. Therefore, when adding materials to the mixing chamber, the loading port should be located in the front area of the blade of the impeller.

Figure 9 shows the response curve of the tracer's concentration with the changes in mixing time. Based on the figure, we can find that mixing efficiency is low at the beginning of the tracer's releasing phase. However, the mixing efficiency greatly increases later. When t > 7.2 s, the normalized response curve begins to flatten and mixing efficiency declines. When t = 12.9 s, the mixing degree of the tracer in the liquid is close to 95%. When t = 18.3 s, the tracer achieves complete mixing with the liquid. Thus, the active mixing nozzle can achieve highly efficient and even mixing among various materials in a relatively short period of time.

###### Diameter of the Impeller.

The diameter of the impeller is a key factor influencing the performance of the active mixing nozzle. The chosen mixing medium was liquid resin with a density of 1080 kg/m3 and a viscosity of 20 cps. The rotational speed of the impeller was set to 20 r/min. Figure 10 shows the mixing effect of the fluid at t = 6 s with impeller diameters of 8 mm, 12 mm, 16 mm, 24 mm, 32 mm, and 34 mm. Figure 11 shows the curve of mixing time changes according to the diameter of the impeller.

The results show that when the diameter of the impeller d is less than 32 mm, the mixing efficiency of the fluid will increase with the increase in diameter. When the diameter is greater than 32 mm, mixing efficiency will decrease. The reasons for this are as follows. Under the condition of the same rotational speed, the size of the impeller's diameter determines the size of the volume force input by the impeller machine. If the diameter is smaller, the volume force input by the impeller machine will be smaller, causing the speed of the fluid in the mixing chamber to become slower and even decreasing the mixing effect. With an increase in the impeller's diameter, the volume force input by the impeller machine will increase, and the fluid velocity will enlarge, improving mixing efficiency. However, when the diameter is greater than 32 mm, the space between the edge of the impeller and the interior wall of the mixing chamber will decrease, and the flow of fluid will be restricted. This is not conducive to the diffusion of the tracer in the fluid and can even cause uneven mixing with low efficiency. The mixing effect will also be affected. Based on the simulation results, mixing efficiency is higher when the diameter of the impeller is 28–32 mm for the given conditions.

###### Rotational Speed of the Impeller.

The rotational speed of the impeller is another important factor that influences the efficiency of an active mixing nozzle. The rotational speed of the impeller depends on the kinetic energy input of the motor and directly affects the diffusion effect of the fluid in the mixing nozzle. If impeller's rotational speed is optimal, it will not only reduce the motor's power consumption but will also have a good flow state and improve mixing efficiency.

The chosen mixing medium was liquid resin with a density of 1080 kg/m3 and a viscosity of 20 cps. The diameter of the impeller was set to 32 mm. Figure 12 shows the mixing effect of the fluid at t = 6 s with impeller rotational speeds of 5 r/min, 10 r/min, 20 r/min, 40 r/min, 50 r/min, and 60 r/min. Figure 13 shows the curve of mixing time changes according to the rotational speed of the impeller.

The results show that when the impeller's rotational speed is 5 r/min, the flow of the tracer in the fluid is very slow and mixing efficiency is low. With increased rotational speed, the flow speed of the tracer in the fluid increases and mixing efficiency is enhanced. The reasons for this are as follows. The rotational speed of the impeller determines the scale of the volume force in the mixing chamber. If the impeller's rotational speed is higher, the volume force in the mixing chamber will be greater, and the flow speed will higher. This is more conducive to diffusing the fluid and enhancing mixing efficiency. However, when the impeller's rotational speed exceeds 40 r/min, the mixing time will be further shortened, but the change will be slow. This is because many factors—not just the speed of the impeller—determine the mixing efficiency of the fluid. Therefore, considering power consumption and other factors, a faster speed is not necessarily better. The recommended optimal rotational speed is approximately 40 r/min.

###### Fluid Viscosity.

The mixing performance of an active mixing nozzle is not only associated with the process parameters of the nozzle itself but is also related to the physical properties of the mixing medium, especially the viscosity of the mixed fluid. This is because the viscosity of the mixed fluid directly determines the diffusion properties of the fluid.

The chosen mixing medium was liquid resin with a density of 1080 kg/m3 and a viscosity of 20 cps. The diameter of the impeller was 32 mm, and the rotational speed was 30 r/min. Figure 14 shows the mixing effect of the fluid at t = 6 s with fluid viscosities of 10 cps, 30 cps, 50 cps, 60 cps, 80 cps, and 100 cps. Figure 15 shows the curve of the mixing time changes according to viscosity.

The results show that when the viscosity is 10 cps, the mixing effect is better and mixing efficiency is higher. With increased viscosity, mixing efficiency will be reduced and mixing time will increase significantly. Comparing Figs. 14(c) and 14(d), we find that the mixing effect of the tracer in the fluid is better with a viscosity of 50 cps. When the viscosity is 80 cps, though the tracer disperses among the four blades, the distribution of the tracer is concentrated. The reasons are as follows. Viscosity determines the liquidity and diffusion of the fluid. The greater the viscosity of the fluid, the lower its liquidity; this is not conducive to spreading the fluid, and mixing time will increase. Therefore, suitable mixing times should be selected for fluids with different viscosities in order to achieve optimal mixing effect and mixing efficiency.

## Experiments

###### Experimental Equipment.

Figure 16 shows the experimental equipment (setup) developed by our group. The diameter of the mixing chamber is 36 mm, and the impeller's diameter is 32 mm. The printed materials are cured by ultraviolet (UV) light-emitting diodes.

The effect of the active mixing nozzle on mixing materials can be intuitively verified by examining printed gradient-colored models. The experimental material was liquid photosensitive resin; the specific performance of this material is shown in Table 1.

The used materials were white resin and blue resin (a UV curable polymer). They were provided to the active mixing nozzle in turn according to the volume ratios of the resin materials shown in Table 2. Based on the simulation results, the impeller's rotational speed was set to 30 r/min, and the stirring time was set as 22 s (mixing time was obtained when viscosity was 50 cps). A stainless steel nozzle with an inner diameter of 160 μm is used. The printing process parameters used are as follows: applied voltage of 2.1–2.6 kV; printing speed of 20–40 mm/s; back pressure of 30–50 kPa; standoff height of 0.5 mm. The printed models are shown in Fig. 17; (a) is a cube model and (b) is a model of a cylinder.

The experimental results demonstrate that the models had five layers in total. The color (grayscale) of each layer was relatively uniform. The entire model presented a gradient color (grayscale). The experiment indicated that the active mixing nozzle can achieve uniform mixing between two kinds of resin materials in due time with high mixing efficiency.

###### Variable Stiffness Models.

The effect of an active mixing nozzle on mixing materials and its ability to realize integrated printing on multimaterial were intuitively verified by measuring the hardness of variable stiffness models. This experiment adopted a flexible resin as the matrix and white resin as the additive (white resin can adjust the hardness of flexible resin). Table 3 shows the performance parameters of these two kinds of materials.

These two kinds of resin materials were added to the active mixing nozzle in turn following the volume proportions given in Table 4. Based on the simulation results, the rotational speed of the impeller was set to 30 r/min, and the stirring time was set as 25 s (mixing time was obtained when viscosity was 80 cps). A stainless steel nozzle with an inner diameter of 160 μm is used. The printing process parameters used are as follows: applied voltage of 2.1–2.6 kV; printing speed of 20–40 mm/s; back pressure of 30–50 kPa; standoff height of 0.5 mm. Figure 18 shows the printed models of variable stiffness.

The hardness of the variable stiffness models in Fig. 18 were measured using a Shore durometer. The model with the same ratio was measured five times in different positions (the distance of each testing point was at least 6 mm). Table 5 shows the hardness values of the variable stiffness models. Table 5 shows that the hardness distribution of models printed by the same ratio of materials was uniform. This indicates that the active mixing nozzle can achieve the balanced mixing of two kinds of resin materials within 25 s with high mixing efficiency. This also verifies the reliability of the simulation results indicating that “the viscosity of the fluid will influence the mixing performance.” With an increase in the proportion of white hard resin in the resin materials, the hardness of models printed by the active mixing nozzle also increased. A resin plate with gradient changes in hardness was ultimately obtained. This means that the active mixing nozzle can implement the integrated manufacturing of multiple different materials.

###### Microscale Models.

Figure 19 shows the printed modes with microscale features. Different line width patterns have been printed using the same needle with a diameter of 60 μm, as shown in Fig. 20. The printing process parameters used are as follows: applied voltage of 1.5–2 kV; printing speed of 80–140 mm/s; back pressure of 5–30 kPa; standoff height of 0.3 mm.

###### Multiscale Model.

Figure 21 shows a frustum of a right circular cone with macro- and micro-multiscale features printed. The following process parameters are used to produce macroscale features: a stainless steel nozzle with an inner diameter of 200 μm; layer thickness of 50 μm; applied voltage of 2.8–3.2 kV (continuous jet mode); back pressure of 30–50 kPa; standoff height of 0.5 mm; printing speed of 20–40 mm/s. The process parameters printing microstructures are: a nozzle with an inner diameter of 100 μm; 1.8–2.2 kV pulsed direct current voltage; standoff height of 0.3 mm; back pressure of 5–30 kPa; printing speed of 50–80 mm/s. The results show that the proposed process has high potential to implement the multiscale 3D printing.

## Conclusions

Multiscale and multimaterial 3D printing has the great potential to accelerate innovation—designers have the capability to create objects that have been previously impossible or very difficult to fabricate. Multimaterial AM has become a powerful fabrication tool that adds on two extra dimensions in the available design space. Recent advances in additive manufacturing are enabling a shift from a geometry-centric to a material-centric design practice. In particular, the integrated design of “structure-material-performance-process” was introduced. Multimaterial 3D printing has been considered as next-generation AM technology. This paper presented an active mixing nozzle for implementing a multiscale and multimaterial 3D printing process proposed. The active mixing nozzle has shown great potential to fulfill multimaterial additive manufacturing with high efficiency and low cost. That made it possible to 3D print more complex and gradient architectures, thus allowing for better quality manufacturing of soft robotics, wearable devices, functionally graded materials/structures, tissue organs, and other embedded electronics as well as structure electronics.

## Funding Data

• The National Natural Science Foundation of China (Grant Nos. 51775288 and 51375250).

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## References

Derby, B. , 2012, “ Printing and Prototyping of Tissues and Scaffolds,” Science, 338(6109), pp. 921–926. [PubMed]
Lewis, J. A. , and Ahn, B. Y. , 2015, “ Device Fabrication: Three-Dimensional Printed Electronics,” Nature, 518(7537), pp. 42–43. [PubMed]
Vaezi, M. , Chianrabutra, S. , Mellor, B. , and Yang, S. , 2013, “ Multiple Material Additive Manufacturing Part 1: A Review,” Virtual Phys. Prototyping, 8(1), pp. 19–50.
Kokkinis, D. , Schaffner, M. , and Studart, A. R. , 2015, “ Multimaterial Magnetically Assisted 3D Printing of Composite Materials,” Nat. Commun., 6, p. 8643. [PubMed]
Wegst, U. G. K. , Bai, H. , Saiz, E. , Tomsia, A. P. , and Ritchie, R. O. , 2014, “ Bioinspired Structural Materials,” Nat. Mater., 14(1), pp. 23–36. [PubMed]
Wehner, M. , Truby, R. L. , Fitzgerald, D. J. , Mosadegh, B. , Whitesides, G. M. , Lewis, J. A. , and Wood, R. J. , 2016, “ An Integrated Design and Fabrication Strategy for Entirely Soft Autonomous Robots,” Nature, 536(7617), pp. 451–455. [PubMed]
Oxman, N. , 2011, “ Variable Property Rapid Prototyping,” Virtual Phys. Prototyping, 6(1), pp. 3–31.
Bartlett, N. W. , Tolley, M. T. , Overvelde, J. T. B. , Weaver, J. C. , Mosadegh, B. , Bertoldi, K. , Whitesides, G. M. , and Wood, R. J. , 2015, “ A 3D-Printed, Functionally Graded Soft Robot Powered by Combustion,” Science, 349(6244), pp. 161–165. [PubMed]
Park, J. U. , Hardy, M. , Kang, S. J. , Barton, K. , Adair, K. , Mukhopadhyay, D. K. , Lee, C. Y. , Strano, M. S. , Alleyne, A. G. , Georgiadis, J. G. , Ferreira, P. M. , and Rogers, J. A. , 2007, “ High-Resolution Electrohydrodynamic Jet Printing,” Nat. Mater., 6(10), pp. 782–789. [PubMed]
Rahman, T. , Renaud, L. , Heo, D. , Renn, M. , and Panat, R. , 2015, “ Aerosol Based Direct-Write Micro-Additive Fabrication Method for Sub-mm 3D Metal-Dielectric Structures,” J. Micromech. Microeng., 25(10), p. 107002.
Burrows, L. , 2015, “ New Frontiers in 3D Printing,” Wyss Institute, Boston, MA, accessed July 16, 2017,
Chimate, C. , and Koc, B. , 2014, “ Pressure Assisted Multi-Syringe Single Nozzle Deposition System for Manufacturing of Heterogeneous Tissue Scaffolds,” Int. J. Adv. Manuf. Technol., 75(1–4), pp. 317–330.
Hohmann, J. K. , Renner, M. , Waller, E. H. , and Freymann, G. , 2015, “ Three-Dimensional μ-Printing: An Enabling Technology,” Adv. Opt. Mater., 3(11), pp. 1488–1507.
Oropallo, W. , and Piegl, L. A. , 2016, “ Ten Challenges in 3D Printing,” Eng. Comput., 32(1), pp. 135–148.
Hardin, J. M. , Ober, T. J. , Valentine, A. D. , and Lewis, J. A. , 2015, “ Microfluidic Printheads for Multimaterial 3D Printing of Viscoelastic Inks,” Adv. Mater., 27(21), pp. 3279–3284. [PubMed]
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Lan, H. , 2016, “ Apparatus and Method for Multi-Material and Multi-Scale 3D Printing Using Single Nozzle,” Qingdao Technological University, Qingdao, China, Patent No. WO 2017071388 A1.

## Figures

Fig. 1

Schematic of electrohydrodynamic jet printing [9]

Fig. 2

Schematic of multimaterial and multiscale 3D printing

Fig. 3

Force and structure of flow field in the mixing chamber: (a) XY plane and (b) YZ plane

Fig. 4

Schematic of the active mixing nozzle

Fig. 5

Simplified structure of the mixing chamber

Fig. 6

Diagram of the location of tracer

Fig. 7

Velocity field and pressure field of the mixing chamber: (a) velocity field and (b) pressure field

Fig. 8

Mixing process of tracer in the mixing chamber

Fig. 9

Response curve of tracer's concentration

Fig. 10

Mixing effect of the fluid at t = 6 s with different impeller diameters: (a) d = 8 mm, (b) d = 12 mm, (c) d = 16 mm, (d) d = 24 mm, (e) d = 32 mm, and (f) d = 34 mm

Fig. 11

Mixing time changes with diameter of impeller

Fig. 12

Mixing effect of the fluid with different rotational speeds of the impeller: (a) n = 5 r/min, (b) n = 10 r/min, (c) n = 20 r/min, (d) n = 40 r/min, (e) n = 50 r/min, and (f) n = 60 r/min

Fig. 13

Mixing time changes with rotational speed of the impeller

Fig. 14

Mixing effect of the fluid with different fluid viscosities: (a) μ = 10 cps, (b) μ = 30 cps, (c) μ = 50 cps, (d) μ = 60 cps, (e) μ = 80 cps, and (f) μ = 100 cps

Fig. 15

Mixing time changes with the viscosity of the fluid

Fig. 16

Experimental equipment

Fig. 17

Fig. 18

Variable stiffness model (scale bar 5 mm)

Fig. 19

Microscale models: (a) uniform grid and (b) nonuniform grid

Fig. 20

Different line width patterns printed using the same needle with a diameter of 60 μm (a) 25 μm, (b) 40 μm, (c) 60 μm, and (d) 110 μm

Fig. 21

Multiscale component printed

## Tables

Table 1 Property parameters for resin materials
Table 2 Material proportioning
Table 3 Property parameters for resin materials
Table 4 Material proportioning
Table 5 Hardness of the models

## Discussions

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