Page 297 - IJB-9-1

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

contributes to alleviating clogging during printing and A B
shape fidelity after printing.
The ink experiences high shear stress during the
printing process and there is no shear stress after printing.
The ink’s behavior can be analyzed in terms of the resistance C D
to deformation depending on shear stress, called viscosity,
 ∂ 
u
derived from Newton’s law of viscosity, τ =   µ ∂   [27] .
y
Materials that have a constant viscosity µ independent
of the shear stress are called Newtonian materials, while
materials in which the viscosity differ in accordance with Figure 1. (A) Typical design of an electrical stimulation system with
the shear stress are called non-Newtonian materials . metal rod, (A) reproduced from Ref. under Creative Common License,
[28]
[11]
The viscosity of a shear-thickening material increases with (B) Three types of fluids in terms of viscosity, (C and D) Commonly
shear stress, while the viscosity of a shear-thinning material examined rheological properties with respect to time.
[29]
decreases with shear stress (Figure 1B). Shear-thinning
materials have a prominent advantage in extrusion-based diverse sizes and aspect ratios, ranging from nanometer
printing as the shear stress during the extrusion process scales to micrometer scales, spherical to rods or platelets.
decreases the viscosity, facilitating its flow out from the The aspect ratio is the largest characteristic length divided
nozzle. After extrusion, the material recovers its viscosity, by the opposite length; the aspect ratio of a cylinder is
resulting in high fidelity in the printed result. defined as L/D and that of a platelet is defined as D/L
(Figure 2B). The distance between particles, which
Similarly, the complex modulus quantifies the depends on the concentration of the filler greatly affects the
viscoelastic behavior, which describes both the viscous properties of an ink. The interparticle distance (denoted as
and elastic behaviors under deformation . Complex H in Figure 2B) typically has a value of several nanometers
[29]
modulus comprises the storage (G’) and loss modulus for an ink with a high concentration filler.
(G’’) . The storage modulus measures the stored energy, The properties of the ink rely on the interactive forces
[30]
which represents the material’s elastic behavior, while the between particles or between particles and the ink . The
[32]
loss modulus measures the energy dissipated through colloidal interactions are the dominant force when the
heat, which represents the material’s viscous behavior. concentration is high and the particles are sufficiently close.
The tan δ value is defined as the ratio of the loss modulus The Van der Waals force acts as an attractive force between
over storage modulus (G’’/G’), which indicates whether a particles that are closer to several nanometers. A high
[33]
[30]
material is elastic (tan δ < 1) or viscous (tan δ > 1) . The aspect ratio (>1,000) and low particle size contribute
[34]
modulus varies with conditions such as the shear stress or to an increase in the contact site and the Van der Waals
temperature. The complex modulus shows the behavior force . The attractive depletion force develops when the
[32]
of the material concerning the shear stress and time. filler is large and dispersed in the ink with non-adsorbing
Thixotropy represents the rheological property, in which small molecules . The electrostatic force is a repulsive
[35]
the viscosity of a material recovers over time . Figure 1C force when the fillers have a charge and the behavior
[31]
[33]
shows the common complex modulus of a thixotropic of fillers is determined by the correlation between three
material with respect to time, and Figure 1D shows the tan forces (Figure 2C).
δ value with applied shear stress. The thixotropic behavior
affects the hardening of the ink after printing and increases 3. Properties of ink with conductive fillers
the printing fidelity. for extrusion-based printing process
2.2. Understanding the property of a filler and the 3.1. Modifying rheological and electrical properties
forces between fillers with filler supplement

Biocompatible polymers are frequently used as the base The addition of fillers modifies the ink’s rheological
material for bioelectronic devices. To meet the myriad properties. The attractive force between particles
properties of natural tissue, the properties of polymers constructs a 3D structural linkage within the ink and
are modified by adding fillers (Figure 2A). The introduced upregulates the ink’s viscosity with the filler at low shear
fillers can upregulate the mechanical properties and provide stress. When shear stress is applied, the bond between
electrical conductivity. Fillers with high conductivity have particles “slips” to another particle . This “slip” modifies
[36]

Volume 9 Issue 1 (2023) 289 https://doi.org/10.18063/ijb.v9i1.643