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

                                                   14.2 BIOPRINTING

              Three-dimensional printing is a manufacturing process in which materials are assembled in a predetermined manner
           by a process under computer control to create complex 3-D objects. The initial use of 3-D printing in the medical fields
           was for patient-specific anatomical models for reconstructive surgeries. This was followed by patient-specific implant
           designs such as structures for replacing cranium parts, as such replacements are generally the result of damage that
           is unique to each patient [5]. Another example of its use is in the manufacture of vascularized tissue, bioprinting [6].
              Three-dimensional bioprinting is an important tool for creating complex tissues; it enables a breakthrough in the
           field of tissue engineering. With this tool, it is possible to manufacture 3-D structures with specific (multi)materials that
           have a good compatibility (both biologically and anatomically) with the human body (e.g., hydrogels) via a model in
           predefined dimensions [7]. Once printed, the material will be placed in an in vitro culture environment (prevascular-
           ization in vitro) to transform it into tissue that can subsequently be used for transplants or for partial replacement of
           tissues (cardiac patches, ligaments, cartilage or bone segments, etc.). The success of artificial tissues and their integra-
           tion is directly related to their ability to be vascularized. Therefore, the structuring of hydrogels or other materials to
           allow a better vascularization is very important, and the bioprinting technique is particularly appropriate for that [8].
           The other potential option is to use the host body as a bioreactor for the maturation of the tissue in vivo (in situ tissue
           engineering), but this is not suitable for all cases.
              Recently, Kolesky et al. reported an important work describing thick, vascularized human tissues with program-
           mable cellular heterogeneity that are capable of long-term (more than 6weeks) perfusion on-chip fabricated by multi-
           material 3-D bioprinting [7]. With their results of in situ development of human mesenchymal stem cells (hMSCs)
           within tissues containing a pervasive, perfusable, endothelialized vascular network, they demonstrated that the
           3-D tissue microenvironments enable the exploration of emergent biological phenomena. Moreover, their 3-D tissue
           manufacturing platform opens new avenues of fabrication and investigation of human tissues for both ex vivo and
           in vivo applications [7]. Bioprinted tissue constructs have also been widely used for disease modeling [9]. The fabri-
           cation of disease models using the bioprinted technique is recently commercialized, especially for cancer drug
           screening [3].



           14.2.1 Bioprinting Techniques

              Bioprinting technology is an important fabrication methodology for producing cell-laden scaffolds, cells in aggre-
           gated forms to obtain tissues and organs. In this section, we provide a brief overview of the bioprinting techniques
           used for developing artificial tissue constructs for transplantation. Up to date, bioprinting techniques can be mainly
           divided into three categories, namely, laser-, inkjet-, or extrusion-based bioprinting.
              In laser-based bioprinting, a laser pulse guides an individual cell from a source to a substrate, as shown in
           Fig. 14.1A. The cells, suspended in a solution, are transferred from a donor slide to a collector slide by the laser pulse.
           The transfer happens due to the bubble formation by the laser, which forces cells to move toward the collector sub-
           strate. This technique allow precise deposition of cells in relatively small 3-D structures [10, 11]. However, it is limited
           for construction of milliscale and beyond structures. Researchers have demonstrated the feasibility of cell deposition
           by using this bioprinting technique. One example of this outstanding tool for the generation of multicellular 3-D con-
           structs mimicking tissue functions was the study realized by Koch et al. They demonstrated the 3-D arrangement of
           viable cells by laser-assisted bioprinting as multicellular grafts analogous to native archetype and the formation of
           tissue by these cells. For that, fibroblasts and keratinocytes were embedded in collagen and printed in 3-D as a dem-
           onstration for skin tissue [12].
              The inkjet-based bioprinting is very similar to inkjet printers used in offices and with personal computers. How-
           ever, in this case, the ink is a “bioink” made of hydrogel and cells that is printed in the form of droplets, Fig. 14.1B [13,
           14]. Inkjet bioprinters are relatively cheap and can work under ambient conditions without strict requirements. It has
           been commonly used for creation of blood vessel-like channels capable of transporting oxygen and nutrients. Chris-
           tensen et al. presented a work in which they used inkjet bioprinting technique for the fabrication of vessel-like channels
           with bifurcations printed in sodium alginate only and mouse fibroblast containing alginate bioinks. The postprinting
           fibroblast cell viability of printed cellular tubes was found to be above 90% even after a 24h incubation, considering the
           control effect [15].
              In extrusion-based bioprinting, a deposition system (based on air pressure or piston-based extrusion) dispenses
           the bioink under the control of a computer, resulting in a precise deposition of encapsulated cells in cylindrical fila-
           ments, Fig. 14.1C. This technique is based on a combination of fluid dispensing system (pneumatic or mechanical) and



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