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Research Papers

Study of Layer Formation During Droplet-Based Three-Dimensional Printing of Gel Structures

[+] Author and Article Information
Kyle Christensen

Department of Mechanical and
Aerospace Engineering,
University of Florida,
Gainesville, FL 32611

Yong Huang

Department of Mechanical and
Aerospace Engineering,
University of Florida,
Gainesville, FL 32611
e-mail: yongh@ufl.edu

1Corresponding author.

Manuscript received March 5, 2017; final manuscript received May 8, 2017; published online July 14, 2017. Assoc. Editor: Zhijian J. Pei.

J. Manuf. Sci. Eng 139(9), 091009 (Jul 14, 2017) (8 pages) Paper No: MANU-17-1132; doi: 10.1115/1.4036785 History: Received March 05, 2017; Revised May 08, 2017

Additive manufacturing, also known as three-dimensional (3D) printing, is an approach in which a structure may be fabricated layer by layer. For 3D inkjet printing, droplets are ejected from a nozzle, and each layer is formed droplet by droplet. Inkjet printing has been widely applied for the fabrication of 3D biological gel structures, but the knowledge of the microscale interactions between printed droplets is still largely elusive. This study aims to elucidate the layer formation mechanism in terms of the formation of single lines and layers comprised of adjacent lines during drop-on-demand inkjet printing of alginate using high speed imaging and particle image velocimetry. Inkjet droplets are found to impact, spread, and coalesce within a fluid region at the deposition site, forming coherent printed lines within a layer. The effects of printing conditions on the behavior of droplets during layer formation are discussed and modeled based on gelation dynamics, and recommendations are presented to enable controllable and reliable fabrication of gel structures. The effects of gelation on droplet impact dynamics are found to be negligible during alginate printing, and interfaces are found to form between printed lines within a layer depending on printing conditions, printing path orientation, and gelation dynamics.

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Figures

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Fig. 1

(a) Typical inkjet printing setup, (b) experimental scenario A for the study of the layer formation process during the fabrication of single layers, and (c) experimental scenario B for the fabrication of single layers which are more easily imaged (used herein for illustration purposes only)

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Fig. 2

Schematic of the high speed imaging setup with an inset showing a printed line (outlined by a dashed line) containing microbeads for PIV analysis

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Fig. 3

Illustration of a typical layer formation process

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Fig. 4

(a)–(c) Schematic representations of front and top views and velocity magnitude maps (m/s) at successive time points during the impact and coalescence of a droplet within a printed line. The primary printed line region is indicated by the dashed line (scale bars: 100 μm).

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Fig. 5

(a) Average velocity as a function of time during the impact and coalescence of a droplet within a printed line (inset: PIV map of velocity magnitude; scale bar: 100 μm) and (b) time evolution of velocity with insets representing simplified interpretations of the vector fields

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Fig. 6

Schematic of diffusion process during printing

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Fig. 7

(a) A single-layer alginate sheet printed into a Laponite bath (experimental scenario B) to allow for clear visualization of interfaces between adjacent lines (inset: a magnified view illustrating key printing conditions for the deposition of printed lines L1–L4), and (b) identification of an interface and the coalescence distance L between two adjacent lines. Unspecified scale bars are 200 μm.

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Fig. 8

Global and top view illustrations of the deposition and gelation of two adjacent printed lines and the formation of an interface. (a) Droplets are deposited along a printed line, where they coalesce within a fluid region, (b) the printhead feeds to begin printing the adjacent line, while gelation trails behind the deposition site, and (c) as deposition of the adjacent line continues, the previously printed line becomes fully gelled, and coalescence is prevented, forming an interface. Parameters shown: printhead travel speed vprint, feed distance dfeed, printhead feed speed vfeed, and coalescence distance L.

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Fig. 9

Predicted gelation front location G(t) through the layer thickness as a function of time

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