ABSTRACT
As a consequence, tissue engineers have turned their attention to the development of 3D technologies capable of hydrogel scaffold manipulation on the micron scale [35]. In this chapter, we review the most recent advances of 3D micropatterning technologies and discuss their application in the field of tissue engineering and regenerative medicine. Specifically, we discuss the continued development of 3D printing and photolithography as a means to three-dimensionally dictate biomaterial properties and cell behavior, and examine the progress thus far toward the development of more complex tissues and organs. 2.2 3D PrintingEarly 3D printing systems for tissue engineering were borrowed from the field of materials manufacturing and were composed of a print head mounted on X-Y rails, a building platform with an axial elevator, and a powder dispensing roller. Scaffolds were fabricated by laying down a powder across the building platform, and then directing the print head to dispense a liquid binder in designed two-dimensional patterns. After axial adjustment, another layer of powder was applied and bound in the areas where the binding liquid was deposited [13]. Using computer automated design software, a series of two-dimensional images were thereby translated into a 3D tissue engineering scaffold. In 2002, this type of 3D printing patterned a scaffold with controlled properties to better mimic the characteristics of both bone and cartilage [60]. Specifically, the top section of a scaffold was printed using poly(D,L,-lactideco-glycolide) (PLGA) and L-poly lactic acid (PLA) at 90% porosity with staggered macroscopic channels to facilitate chrondrocyte seeding and cartilage formation. The bottom portion of the same scaffold was printed into a cloverleaf shape using PLGA and calcium phosphate at 55% porosity to achieve strong mechanical properties and initiate bone growth. This 3D printing technology demonstrated control of porosity, material composition, physical structure, and mechanical properties within a 3D biomaterial scaffold, with the goal of fabricating multifaceted materials for
total joint replacement [60]. Excited by early success, engineers have since moved to 3D printing of hydrogel structures on the micron scale. Further, over the last 10 years the term 3D printing has evolved and split to encompass at least three valuable yet distinct technologies now being utilized in the tissue engineering community. These technologies, known as direct-write bioprinting, inkjet bioprinting, and biological laser printing will now each be discussed in turn, with an emphasis on recent successes in tissue engineering applications. 2.2.1 Direct-Write BioprintingDirect-write bioprinting involves the use of a three-dimensionally controlled actuator to extrude a liquid material, or bioink, through a dispensing pen in the form of a designed pattern (Figs. 2.1a,b) [10,43]. The pattern is subsequently cross-linked or gelled using chemical reagents or environmental factors. 3D structures are then built through an additive layer-by-layer process (Fig. 2.1c). Direct-write bioprinting generally prints a continuous line of material extruded from the dispensing pen and the resolution is highly dependent on the printing material, including its viscosity, as well as cross-linking or gelling conditions [10]. Some direct-write bioprinters are capable of printing 3D structures as small as 5 µm and as large as many millimeters [10,66]. Direct-write bioprinting was recently utilized to print 3D collagen scaffolds containing microfluidic channels [41]. (a)
(b) (c)
Figure 2.1 Direct-Write Bioprinting. (a) A schematic of a direct-write bioprinting apparatus. Biomaterials are dispensed by a pen onto a translatable stage to form a scaffold. Adapted from Lewis et al. [43]. (b) Close up image of a dispensing pen extruding the first layer of a hydrogel biomaterial onto a surface (arrow is leading edge). (c) After curing the first layer, additional layers are deposited to form 3D structures. Adapted from Chang et al. [10].
