BIOMATERIALS FOR BIOPRINTING - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 126

percentage of viable cells can be determined ( Klouda et al., 2009 ). MTS proliferative assay can be
performed on alternate days to assess cytocompatibility and to determine the metabolic activity of
the cells ( Park et al., 2009 ).
6.2.2 SHEAR THINNING AND THIXOTROPY
Non-Newtonian behavior, which shows the viscosity of the biomaterials decreasing as shear rate in-
creases, is one of the fundamental requirements for a bioprintable material. During dispensing, the
shear force reorganizes the random polymer chains into an aligned conformation that reduces hydrogel
viscosity during the process ( Yucel et al., 2009 ). The effect of shear thinning varies among biomaterials
with high molecular weight. This property is important for bioprinting as it enables easy dispensing of
the fluid when pressure is applied, and allows the fluid to return to its gel state by removing its stress.
However, the hydrogel must be given time to return to its viscous state. The degree of this behavior
can be determined through a hysteresis loop test ( Barbucci et al., 2008 ). During this test, the sample
is subjected to increasing and decreasing shear. By determining the area between the two curves, the
degree of thixotropy can be known.
6.2.3 YIELD STRESS
Yield stress is the instantaneous pressure required to initiate the flow of material. For bioprinting,
this property affects the required minimum stress applied on the material before it is dispensed This
property is important as the stress applied on the gel will in turn affect the shear stress induced
on the cells during printing. Physically cross-linked soft polymers form a fragile network when at
rest. When shear force above the yield stress is applied, the network breaks. It slowly forms back
when shear force is detached. Such stress is responsible for delaying the flow and collapse of the
gel structure. Yield stress also helps keep the cells homogenous in the hydrogel reservoir ( Malda
et al., 2013 ). To determine the yield stress required for these materials, an extrapolation of the flow
curve at low shear rate to zero shear rate can be done ( Malana et al., 2012 ). However, researchers
have suggested the use of the Bingham equation (τ = τB + m B g ) and modified Bingham equation
(τ = τMB + m MB g + C g 2 ) at low shear rates to determine these values, as doing so minimizes errors
during extrapolation of the flow curve.
6.2.4 WATER CONTENT
The amount of water in hydrogel determines the adsorption rate and the diffusion of the solutes through
the material. Hydrogel's water content must be controlled; if it is too high, cell proliferation rate will
deteriorate ( Ogawa et al., 2010 ). Water can be bound to the gel by two interactions. First, the polar
hydrophilic groups are hydrated and the network of polymer swells. Second, hydrophobic groups in
the gels are exposed and water molecules are bound to the hydrophobic groups ( Hoffman, 2002 ). Ad-
ditional water is absorbed as free water, which fills the space such as macropores and voids formed be-
tween the chains. Hydrogel's water content can easily be determined by either measuring the percent-
age change in the mass between the hydrogel and its dry form or determining the gel's light absorbance
( Huglin and Yip, 1987; Nagaoka et al., 1990 ).