Page 206 - IJB-9-5

International Journal of Bioprinting Using droplet jetting for bioprinting

Table 2. Summary of splashing parameter for dry or wet solid surface in literature
K = A. Oh . We b Boundary condition Comment References
a
c
A a b
1 −0.37 1 Dry surface with different K is dependent on roughness. Cossali et al. [100] suggested K = 649 + 3.76 R ND -0.63 , [98]
c
c
roughness condition where R is the nondimensional roughness defined as R /D.
ND a
1 −0.4 1 Wetted surface with K = 658, this splashing parameter is independent of roughness due to the [101]
c
different roughness existence of liquid film as explained by Mundo et al. [100]
1 −0.4 1 Wetted surface with K = 2100 + 5880 δ , where δ is the nondimensional film thickness defined [99]
1.44
c
different roughness as h/D, with h being the thickness of liquid layer. The splashing parameter is
experimented within 0.1 < δ < 1.0
1.17
1 −0.17 0.59 Thin liquid film covering a K = 63 = Oh. Re ≈ √We, where δ = 0.1 [102]
c
solid surface
conditions, such as varying velocity, impact angle, surface aerosol jet printing, lies in the evaporation dynamics of pico-
roughness, and liquid film thickness. The value is relevant to nanoliter droplets. The evaporation dynamics of the ink
to inkjet bioprinting during the prewetting phase when influences the pattern of deposition, therefore determining
depositing materials of the first few layers. These scenarios the functionality of ink. A sessile drop’s evaporation is
are summarized in Table 2. complex, which is influenced by a number of variables,
including the ink properties (e.g., concentration, additives,
In bioprinting, cell compatibility is a major particle morphology) [104] , substrate properties (e.g.,
consideration when choosing the appropriate printing surface roughness, rigidity, permeability, hydrophobicity,
technology and formulating bioink for printing. For texture) [105-108] , and the ambient conditions (e.g.,
instance, in droplet-based printing, the splashing of humidity and temperature). Higher evaporation kinetics
droplet onto substrate has direct impact on cell viability are associated with higher printing resolution, which is
in cell-laden droplet. Ng et al. demonstrated the inverse determined by smaller droplet size. Other factors that
[60]
relationship between droplet impact velocity and cell influence print resolution include wettability of surface [109]
viability. A decrease in droplet impact velocity of cell-laden and suppression of coffee ring effect [110] . Whereas it
droplets increases cell viability when jetted against solid was found that the droplet would lose volume owing to
surface such as glass slide or petri dishes. The group used penetration of liquid into porous and permeable substrate,
HP D300e Digital Dispenser to deposit cell-laden droplet which occurs faster than evaporation [111] . Substrate
with varying cell concentrations. The HP D300e, a thermal wettability increases the infiltration rate, which further
inkjet (TIJ) system, works by vaporizing a small amount suppresses the coffee ring effect by dominating convective
of fluid through rapid heating and subsequent generation flow [111,112] . In inkjet bioprinting, this relation between print
of a gas bubble that expands to eject precise amounts of resolution and droplet size causes challenge for printing
fluid. When jetting cells are suspended in a fluid, a higher sensitive materials, such as cell-laden ink. In bioprinting,
concentration of cells reduces the splashing of pool from the evaporation dynamics of nanoliter droplets negatively
subsequent droplets. affect cell viability of the cell-laden bioink. There is a trade-
Nooranidoost et al. [103] investigated the impact of a off between achieving higher printing resolution and
[60]
cell-laden droplet on a surface to understand the effect reducing cell viability due to droplet evaporation .
of the droplet impingement and droplet spreading on cell 3.3. Heterogeneous droplet patterning of substrate
viability. The authors determined that the accumulation In recent years, researchers are also looking into the droplet
of stress within the droplet upon impacts deformed cells impingement behavior on surface with heterogeneous
within the droplet. Additionally, this simulation study wettability [114,115] . Heterogeneous wettability is created by
demonstrated how enhancing bioink viscoelasticity chemical alteration and physical engineering, giving rise
enhances cell survival by lowering cell deformation during to nonuniformity in adhesion forces along the surface [116] .
the impact phase. In droplet-based printing methods like Unlike dropping on surface with homogenous wettability,
inkjet bioprinting, it is advised to employ strategies that the droplets that fall on surface with heterogeneous
lessen cell deformation during and after the jetting and wettability can experience a resultant lateral force that
impacting phase of cell-laden droplets. pushes them sideway because of the asymmetrical force
Another process-specific consideration for droplet- distribution around the droplet caused by the different
jetting printing processes, such as inkjet printing and surface wettability [117] . Generally, the droplets can

Volume 9 Issue 5 (2023) 198 https://doi.org/10.18063/ijb.758