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

              Polydimethylsiloxane (PDMS) belongs to a group of polymeric organosilicon compounds. It has been widely used
           to fabricate microfluidic systems due to its rheological properties, and it is a low-cost material [49]. Additionally,
           PDMS elastomer is used in a wide range of biomaterial applications including cell culture substrates, flexible electron-
           ics, and medical devices [50]. Ozbolat et al. have recently demonstrated that the 3-D printed PDMS samples possess
           improved mechanical and cell adhesion properties compared with traditionally manufactured samples using casting
           process. Three-dimensional printing of PDMS not only enables the generation of 3-D models of tissues and organs but
           also brings a new concept in surface engineering for cell adhesion studies [51].



                           14.4 3D PRINTING OF PERSONALIZED SILICONE IMPLANT

              The development of personalized implant has become a necessity for some applications that require high
           degree of anatomical conformation. To overcome these problems, 3-D printing is one of the processes that will
           enable the manufacture of custom implants. The combination between the computer tomography (CT) scan from
           patient and the computer-aided design (CAD) can be employed to generate the personalized implants. The result-
           ing implant can be then produced with high degree of fidelity without requiring expensive molds. As explained in
           the previous paragraph, many materials have been used to develop 3-D printed medical implants, and most of
           them are water-soluble materials. Another kind of polymers, elastomers, and specifically silicone has received a
           great interest in the last few years for developing 3-D printed personalized implants. Even if silicone has been
           widely used to develop soft implants, most of these implants are produced using injection molding process that
           limit the application for the manufacturing of custom-made implants [52]. This can be explained by the difficulty
           of 3-D printing of silicone, as the technique has limitations in handling viscous liquids just before the curing limit.
           In this part, we will focus our discussion on the printing of silicone-based materials to develop personalized
           implants.


           14.4.1 Soft 3-D Implant Printing: Example of Silicone

              Elastomers have been widely used for biomedical applications especially in tissue engineering and to develop med-
           ical devices mainly due to their mechanical properties (highly elastic with low Young’s modulus) close to natural tis-
           sues. Indeed, elastomers also referred as « Rubbers » are a special class of polymers that are very elastic and composed
           of a cross-linked network with a glass transition below room temperature, which allow them to bend and flex within
           the body temperature. Despite of their mechanical properties that can mimic tissues, elastomers are easy to sterilize
           and easy to process using molding or liquid injection molding. Among elastomers, the main medical grades that have
           been successfully produced and commercialized are of silicone-, polyurethane-, and polyvinyl chloride-based
           materials [53].
              In this class of materials, silicone is the most widely used for biomedical applications. Silicone, also known as «
           PDMS: polydimethylsiloxane » (Fig. 14.2), has been used in the medical field for more than 70years. These materials
           represent a good candidate for implantable devices thanks to its low reactivity and relatively low immune response. Its
           strong SidOdSi (siloxane) backbone and the presence of methyl CH 3 confer high chemical stability and outstanding
           flexibility (high tear strength and high elasticity). Moreover, silicone-based materials are biocompatible and bio-inert
           and have high stability in rather abrasive physiological environment [54, 55].
              Below are the main properties that contributed to the success of silicone-based materials in the medical field:
           - Thermal stability
           - Chemical stability
           - Electrical insulation
           - High gas permeability
           - Mechanical properties close to natural tissues (highly stretchable and high compliance with soft tissue)








           FIG. 14.2  Chemical formula of PDMS.



                                          II. MECHANOBIOLOGY AND TISSUE REGENERATION
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