July/August_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 0 2 1 Fig. 2 — A schematic showing inkjet printing: (a) bubbles with ink can be produced using a thermal actuator (b) or piezoelectric actuator (c) [1] . (b) (a) (b) (a) (c) mechanical strength. The complex forms of producing scaffolds may fa- cilitate the growth of cellular and tis- sue components. Another method uses solution-based deposition to pro- cess scaffolds by extruding the solution without heating the material. This ap- proach is associated with the presence of solvent in the deposited structure [12] (Fig. 1). In the case of hard tissue engineer- ing, the filament can be made from bio- degradable β-Ca 3 (PO 4 ) 2 /poly(D,L-lactide) and β-Ca 3 (PO 4 ) 2 /poly(ε-caprolactone), which are perspective biocomposite implant prototypes for bone substitu- tion [13] . Their predetermined complex shapes which possesses appropriate mechanical properties (grid or Kelvin structure) [14] , as well as polymer ma- trix, allow modifying the surface by low temperature O 2 plasma treatment and by wet-chemistry derived hydroxyapa- tite layer [13] . Instead of a conventional bioresorbable TCP phase, mix-cationic polyphosphates can be used as a filler for such implants [15] . INKJET PRINTING Bioprinting started with inkjet printers as a method of printing text and images onto a hydrogel substrate or culture dish using narrow orifices under computer control. A hydrogel pre-polymer solution with encapsulat- ed cells (called a bio-ink) is stored in the ink cartridge. The cartridge is then con- nected to a printer head and acts as the bio-ink source while the printing is elec- tronically regulated (Fig. 2). Based on the droplet actuation mechanism, the printing methods can be subdivided into thermal and piezo- electric actuator methods. In the ther- mal method, ink droplets are expelled by heating in a micro-heater so that an inflated bubble forces the ink out of the orifice onto the substrate. The lo- cal temperature can easily increase up to hundreds of degrees in just a few mi- croseconds to generate pulse pressure. With a piezoelectric inkjet printer, forc- es generated by a piezoelectric actua- tor cause bioink droplets to be expelled from the printing head [4] . The main pros of inkjet printers are their cost- effectiveness and enhanced print speed. Clogging issues, heterogeneous droplet sizes, less directionality, and poor cell encapsulation are some of the disadvantages of this approach. How- ever, because the printheads are based on micro-electromechanical system (MEMS) devices, there is a relatively neg- ligible deformation created by either thermal or the piezo-electric actuation at the opening of the nozzle. Conse- quently, printheads cannot squeeze out high viscous materials and do not apply to bio-inks that have higher cell densi- ty. Another limitation of inkjet printing is called the settling effect. When the cartridges are filled with bio-inks, they are well mixed. But, the cells can start to settle in the cartridge, increasing the viscosity of the bio-ink and clogging the printer head. The requirement for a bio-ink to be inkjet printed is low vis- cosity; this requirement inhibits usage of inkjet printing for many biomedical applications [17] . SELECTIVE LASER SINTERING Selective laser sintering (SLS) or melting (SLM) is an additive manu- facturing technique in which powder serves as the starting material. A high power laser is the source of energy that fuses small particles into a larger mass of 3D shape. The powder bed is scanned by means of the laser beam [18] . The post scanning of each section leads to the powder bed being lowered; the process repeats itself to finish com- pleting every part of the powder bed (Fig. 3). Fig. 1 — Schematic of 3D printing using extrusion/FDM technique. Pressure can be made by rollers or by screw (a); a solution based method emolliates the polymer using appropriate solvent instead of high temperature (b) [16] .