Understanding droplet jetting on varying substrate for biological applications

Abstract

In the inkjet printing process, the droplet experience two phases, namely the jetting and the impacting phases. In this review article, we aim to understand the physics of a jetted ink, which begins during the droplet formation process. Following which, we highlight the different impacts during which the droplet lands on varying substrates such as solid, liquid, and less commonly known viscoelastic material. Next, the article states important process-specific considerations in determining the success of inkjet bioprinted constructs. Techniques to reduce cell deformation throughout the inkjet printing process are highlighted. Modifying postimpact events, such as spreading, evaporation, and absorption, improves cell viability of printed droplet. Last, applications that leverage on the advantage of pixelation in inkjet printing technology have been shown for drug screening and cell-material interaction studies. It is noteworthy that inkjet bioprinting technology has been integrated with other processing technologies to improve the structural integrity and biofunctionality of bioprinted construct.

Keywords: Additive manufacturing; Bioprinting; Functional material; Hydrogel; Inkjet printing; Material jetting.

Figure 1.

The process of material jetting can be studied at two phases, i.e., jetting and impacting.

Figure 2.

Schematic for droplet formation from inkjet with respect to viscoelasticity of the fluid. (a) and (b) are the ejection and stretching of the droplets which is common to all fluids. (c) to (i) is the breakup of droplet from Newtonian fluid. (j) to (m) is non-Newtonian fluid with intermediate viscoelasticity. (n) to (q) is non-Newtonian fluid with high viscoelasticity. Reprinted with permission from ^[49]. Copyright (2010), The Society of Rheology.

Figure 3.

Droplet–substrate interaction classified based on the penetrative and nonpenetrative impact. The penetrative impact on non-Newtonian materials such as PEGDA^[56] and gelatin^[57]; penetrative impact on Newtonian material such as cell media^[58] and water^[57]. Nonpenetrative impact on viscoelastic substrate such as PDMS^[59], and rigid materials such as glass slides and plasticware^[60]. Figures are reproduced under the terms of the Creative Commons Attribution 4.0 International License.

Figure 4.

Result of droplet impact on liquid surface.

Figure 5.

Heterogeneous wettability substrate (A) which influenced the morphology and resolution of ink-jetted droplets (Reprinted with permission from^[109]. Copyright (2014) American Chemical Society) and (B) bounce trajectory of droplet impact on penetrative substrate (Reprinted with permission from^[113]. Copyright (2016) American Chemical Society).

Figure 6.

Inkjet printing of cells. (A) Viability of human dermal fibroblast printed through profiling cell proliferation over 7 days^[60]. Reproduced under the terms of the Creative Commons Attribution 4.0 International License. (B) Heterogeneous patterning of cell population within region of interest forming color variation similar to home-based inkjet printers^[58]. Reproduced under the terms of the Creative Commons Attribution 4.0 International License. (C) Layers of cells patterned with vertical variation^[124]. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.