Page 284 - Advances in Biomechanics and Tissue Regeneration

282 14. USING 3-D PRINTING AND BIOPRINTING TECHNOLOGIES FOR PERSONALIZED IMPLANTS

FIG. 14.8 Silicone 3-D printing with LSR (left) versus LSR+PEG (right). Pictures were kindly provided by 3dFAB, University Lyon 1.

three specific tests can be performed to ensure the printability of silicone: shear thinning effect, yield stress character,
and thixotropic behavior (Fig. 14.10).
Shear thinning is a characteristic phenomenon of some non-Newtonian fluids in which the fluid viscosity decreases
with increasing shear stress (the opposite of shear thickening). In 3-D printing, a shear thinning effect is required to
decrease the viscosity of silicone while flowing through the nozzle, that is, where wall shear rate is the higher. In this
case, the consequent wall shear stress will be sufficiently low to flow highly viscous silicone through nozzle. Exper-
imentally, the shear thinning effect can be documented through the access to stress rate test in flow mode.
Yield stress character describes the capacity of materials to keep its shape under a predetermined pressure. It is the
most important rheological property in 3-D printing that impacts predeposition and postdeposition behavior. If the
yield stress character of the material is too low, the flowing through nozzle is easy, but maintaining 3-D printed silicone
complex shape is impossible. If the yield stress character of the material is too high, the 3-D printed silicone complex
shape will be easy to maintain, but the flowing through nozzle will be highly challenging. Therefore, an adequate yield
stress value must be found to flow material regarding deposition system and to keep the shape of 3-D object. The yield
stress can be measured through stress-controlled rheometer using stress ramp test in flow mode.
As a typical example, LSR and PEG mixing in different ratio can be used to control silicone yield stress. A large range
of yield stresses allow users to print 3-D objects, but the value of yield stress is related to the complexity of geometry
(Fig. 14.9). A human ear is a complex geometry with high overhang and mass/area ratio: a yield stress around 1500Pa
will be then required.

FIG. 14.9 Accessible 3-D printed objects for different values of yield stress with silicone and PEG mix. Graphics were kindly provided by 3dFAB,
University Lyon 1.

II. MECHANOBIOLOGY AND TISSUE REGENERATION