Page 283 - Advances in Biomechanics and Tissue Regeneration

14.4 3D PRINTING OF PERSONALIZED SILICONE IMPLANT 281

14.4.5 3-D Printing of Silicone for Healthcare
14.4.5.1 Technology and Challenge
The selection of 3-D printing technologies to process elastomer materials depends on the chemical reactivity and
rheological properties of the material. Two major technologies are used to print silicone materials: UV light technology
[81] and liquid deposition modeling (LDM) [82], using UV and thermal curing, respectively. Nevertheless, in health-
care applications, the use of UV-cured silicone rubber can present toxicity due to the presence of unreacted photoini-
tiator leakage that has strong cytotoxicity [83]. Therefore, the use of thermally cured silicone rubber together with LDM
is the most advantageous approach, provided that the rheological properties are adapted.
The LDM 3-D printing technology uses rheological properties of the material to flow it through a nozzle (predeposi-
tion) and maintain the shape of the 3-D object (postdeposition) up to the end of the cross-linking reaction. In the pre-
deposition step, the flow of silicone through a nozzle is easier if the formulation presents a shear thinning effect and an
adequate yield stress value regarding the extrusion system (mechanic, pneumatic, or screw-based). In the postdeposi-
tion part, the thixotropic behavior must be minimized to ensure the layer stability when the next layer will be depos-
ited. If the thixotropic time is low enough, the yield stress character will be recovered, and the 3-D object shape fidelity
will be ensured. Thus, the value of yield stress is related to the capacity of the material to build simple or complex
geometries as shown in the next section.
The progression of rheological properties of thermally cured silicone is related to the chemical composition and the
kinetics of the phase transition (from liquid to solid). Two kinds of chemical reactions can be used: polycondensation
within monocomponent silicone and polyaddition within bicomponent silicone.

14.4.5.2 Mono-Component Silicone
RTV-1 silicone rubbers are monocomponent products that are free flowing or paste- like in consistency. They react
with atmospheric moisture to form flexible rubbers (RTV-1¼room temperature vulcanizing, 1-component) [84].By
virtue of their outstanding properties, these silicone rubbers are ideal for many sealing, bonding, and coating
applications.
During the manufacturing process, terminal OH groups of the polysiloxane react with the cross-linking agent, gen-
erating curable products. The reaction itself takes place on exposure to atmospheric moisture and is accompanied by
the liberation of hydrolysis products. This reaction, which is also referred to as vulcanization, starts with the formation
of a solid skin at the surface of the rubber and continues gradually toward the inside.
In 3-D printing, these materials are really interesting since they can attain phase transition within tens of minutes. In
these conditions, first layers of silicone are quickly cured and acquire strong mechanical properties that then help to
keep the shape of the 3-D object. However, release of volatile or soluble parts of these materials often occurs that are
toxic and hinder their use for healthcare applications.

14.4.5.3 Bi-Component Silicone
For RTV-2 or LSR silicone, the chemical reaction consists of an addition curing reaction leading to the binding of
Si-H groups to vinyl groups. Salts or platinum, palladium, or rhodium complexes may serve as catalysts [84].If
platinum-olefin complexes are used, curing will take place at room temperature. Platinum complexes containing
nitrogen are used to trigger addition reaction at elevated temperatures (e.g., Pt complexes with pyridine, benzonitrile,
or benzotriazole).
In 3-D printing, the kinetic of hydrosilylation can be managed with the use of inhibitors to keep the flow properties
of silicone constant throughout the whole additive manufacturing process. However, contrary to RTV-1, a specific
attention must be given to yield stress character when using bicomponent silicone in 3-D printing. Indeed, as the pro-
gression of rheological properties is slow to maintain the flow properties during printing, bicomponent silicone with
low yield stress value cannot be printed (Fig. 14.8) into complex geometries presenting overhang structures, important
mass/area ratio or bridges. In this case, silicone formulation has to be adapted with the addition of yield stress mod-
ulating agents such as polyethylene glycol (PEG), which reacts with silica (contained in silicone) to form a more stable
macromolecular network [85].

14.4.6 Rheological Properties of Printable Silicone

14.4.6.1 Rheological Testing and Parameters
The determination of silicone rheological behavior for 3-D printing can be performed using stress- or strain-
controlled rheometers. With respect to the aforementioned rheological properties pertinent to the 3-D printing process,

II. MECHANOBIOLOGY AND TISSUE REGENERATION