Page 298 - IJB-9-1

International Journal of Bioprinting 3D printable conductive composite inks for biocompatible electrodes

the distance between particles and breaks down the linkage, Where σ is the conductivity of the ink, V is the
leading to low viscosity. In addition, viscous interactions, volume fraction of filler, V is the volume fraction of
c
which represent the friction between particles and the filler at the percolation threshold, and σ and t values are
0
surrounding matrix, overwhelm the colloidal interactions. determined analytically. The equation is only valid when
Fillers with a high aspect ratio have a larger surface area, the concentration is greater than the percolation threshold
which results in high friction and viscosity compared to concentration. Figure 3 shows a typical conductivity
the fillers with a low aspect ratio . After printing, the increase of ink with conductive filler. The conductivity
[37]
particles reform their bonds and retain their viscosity. rapidly increases around the percolation threshold and
The alteration of the rheological property enhances the saturates beyond that. Therefore, the percolation threshold
shear-thinning behavior of the material and yields high is an important criterion in the selection of filler and their
printability and fidelity in the printed product . concentrations.
[38]
The conductivity of the ink increases proportionally The rheology and conductivity of the ink differ greatly
with the concentration of the conductive fillers. High in proportion with concentration, particle size , and
[44]
conductivity contributes to low-energy dissipation and aspect ratio of the filler. For example, when using a filler
[34]
reduces the leakage current and tissue damage . Electrons with a high aspect ratio, the percolation threshold will be
[22]
and ions move with an electrical field, causing an electrical low due to the large contact surface. The ink will show low
conduction in conductive ink. The conductivity of metals printability due to the highly attractive forces between
or carbon originates from mobile electrons, which is the fillers. Therefore, a relatively low concentration of
known as electronic conductivity. When electrolytes are filler should be chosen for the ink to be easily extruded,
included in the material, the charges can be transferred with conductivity reaching the saturation value (over the
through ions, which is known as ionic conductivity . percolation threshold). In addition, a high concentration
[39]
The electronic conductivity originates from the network of fillers is known to decrease cell viability . Hence, one
[45]
formed by the conductive fillers with the interactive should decide the required conductivity of the bioelectronic
forces. In a high concentration of filler, direct contact device and the printing process and then choose the
between fillers works as a conductive path . However, appropriate filler and ink concentration considering the
[40]
it is known that direct contact is not the main root of cell viability.
conductivity. Meanwhile, in the part, where fillers do not
directly contact with each other, electrons pass through 3.2. Various methods for preparing conductive ink
the polymer by the quantum tunneling effect, hopping The printability and reproducibility depend not only on
over the non-conductive polymer . The main charge the property and concentration of the filler but also on
[41]
transport mechanism is known to be electron tunneling the ink’s dispersion state. Uneven dispersion of the ink
in a percolated network . Percolation theory implies that leads to nozzle clogging and low-quality printing results.
[42]
the rigid network of the conductive path is formed over The fillers inside the polymer tend to aggregate due to the
percolation threshold concentration. The conductivity is interactive forces between particles . This aggregation
[46]
calculated using the empirical equation ; increases the minimum distance between the agglomerate,
[43]
−
  = 0 (VV c ) t thus increasing the percolation threshold concentration
and lowering the conductivity of the ink at the same
concentration [47,48] . In addition, Brownian interactions
A B

C

Figure 2. (A) Conductive fillers are blended in polymer. Properties of
fillers (B) and forces between fillers (C). Figure 3. The conductivity increases with concentration.

Volume 9 Issue 1 (2023) 290 https://doi.org/10.18063/ijb.v9i1.643