Laser printing is an orifice-free printing approach and has been investigated for the printing of two-dimensional patterns and simple three-dimensional (3D) constructs. To demonstrate the potential of laser printing as an effective bioprinting technique, both straight and Y-shaped tubes have been freeform printed using two different bioinks: 8% alginate solution and 2% alginate-based mouse fibroblast suspension. It has been demonstrated that 3D cellular tubes, including constructs with bifurcated overhang structures, can be adequately fabricated under optimal printing conditions. The post-printing cell viabilities immediately after printing as well as after 24 h incubation are above 60% for printed straight and Y-shaped fibroblast tubes. During fabrication, overhang and spanning structures can be printed using a dual-purpose crosslinking solution, which also functions as a support material. The advancement distance of gelation reaction front after a cycle time of the receiving platform downward motion should be estimated for experimental planning. The optimal downward movement step size of receiving platform should be chosen to be equal to the height of ungelled portion of a previously printed layer.

Three-dimensional (3D) bioprinting, as a freeform biomedical manufacturing approach, has been increasingly adopted for the fabrication of constructs analogous to living tissues. Generally, materials printed during 3D bioprinting are referred as bioinks, which may include living cells, extracellular matrix materials, cell media, and/or other additives. For 3D bioprinting to be an enabling tissue engineering approach, the bioink printability is a critical requirement as tissue constructs must be able to be printed and reproduce the complex micro-architecture of native tissues in vitro in sufficient resolution. The bioink printability is generally characterized in terms of the controllable formation of well-defined droplets/jets/filaments and/or the morphology and shape fidelity of deposited building blocks. This review presents a comprehensive overview of the studies of bioink printability during representative 3D bioprinting processes, including inkjet printing, laser printing, and micro-extrusion, with a focus on the understanding of the underlying physics during the formation of bioink-based features. A detailed discussion is conducted based on the typical time scales and dimensionless quantities for printability evaluation during bioprinting. For inkjet printing, the Z (the inverse of the Ohnesorge number), Weber, and capillary numbers have been employed for the construction of phase diagrams during the printing of Newtonian fluids, while the Weissenberg and Deborah numbers have been utilized during the printing of non-Newtonian bioinks. During laser printing of Newtonian solutions, the jettability can be characterized using the inverse of the Ohnesorge number, while Ohnesorge, elasto-capillary, and Weber numbers have been utilized to construct phase diagrams for typical non-Newtonian bioinks. For micro-extrusion, seven filament types have been identified including three types of well-defined filaments and four types of irregular filaments. During micro-extrusion, the Oldroyd number has been used to characterize the dimensions of the yielded areas of Herschel-Bulkley fluids. Non-ideal jetting behaviors are common during the droplet-based inkjet and laser printing processes due to the local nonuniformity and nonhomogeneity of cell-laden bioinks.

Matrix-assisted pulsed-laser evaporation direct-write (MAPLE DW) has been successfully implemented as a promising laser printing technology for various fabrication applications, in particular, three-dimensional bioprinting. Since most bioinks used in bioprinting are viscoelastic, it is of importance to understand the jetting dynamics during the laser printing of viscoelastic fluids in order to control and optimize the laser printing performance. In this study, MAPLE DW was implemented to study the jetting dynamics during the laser printing of representative viscoelastic alginate bioinks and evaluate the effects of operating conditions (e.g., laser fluence) and material properties (e.g., alginate concentration) on the jet formation performance. Through a time-resolved imaging approach, it is found that when the laser fluence increases or the alginate concentration decreases, the jetting behavior changes from no material transferring to well-defined jetting to well-defined jetting with an initial bulgy shape to jetting with a bulgy shape to pluming/splashing. For the desirable well-defined jetting regimes, as the laser fluence increases, the jet velocity and breakup length increase while the breakup time and primary droplet size decrease. As the alginate concentration increases, the jet velocity and breakup length decrease while the breakup time and primary droplet size increase. In addition, Ohnesorge, elasto-capillary, and Weber number based phase diagrams are presented to better appreciate the dependence of jetting regimes on the laser fluence and alginate concentration.

Laser-induced forward transfer-based laser printing has been being implemented as a promising orifice-free direct-write strategy for different printing applications. The printing quality during laser printing is largely affected by the jet and droplet formation process and subsequential impingement. The objective of this study is to investigate the impingement-based printing type and resulting printing quality during the laser printing of viscoelastic alginate solutions, which are representative inks for soft structure printing such as bioprinting. Three printing types are identified: droplet-impingement printing, jet-impingement printing with multiple breakups, and jet-impingement printing with a single breakup. Printing quality, in terms of printed droplet morphology and size, has been investigated as a function of alginate concentration, laser fluence, and direct-writing height based on a time-resolved imaging approach and microarrays of printed droplets. Of these, the best printing quality is achieved with single-breakup jet-impingement printing, followed by multiple-breakup jet-impingement printing, with droplet-impingement printing producing the lowest quality printing. The printing quality can be improved by using high-concentration alginate solutions. The increase of laser fluence may lead to a well-defined primary droplet for low-concentration alginate solutions; however, this can cause the droplet diameter to increase, which may not be desirable. The direct-writing height (i.e., ribbon coating-receiving substrate distance) also influences the print quality. For example, an increase in direct-writing height can cause the printing type to change from the ideal jet-impingement with a single breakup, to the jet-impingement with multiple breakups, and even the least desired droplet-impingement printing, with only slight variations in droplet diameter.

Three-dimensional (3D) laser-assisted orifice-free printing technique has emerged as a promising approach for the fabrication of tissue constructs. For the better adoption of this technique, this study has investigated the effects of operating conditions, including the laser fluence and receiving substrate velocity, on the printing quality in terms of whether printed features are well-defined or not. Four main morphologies of printed lines before gelation have been identified during line printing: isolated droplets, discontinuous segments, well-defined lines, and over-printed lines. The 2,125 mJ/cm 2 laser fluence and 100 mm/min substrate velocity conditions, corresponding to a 0.42 overlap ratio, have been identified as a combination of optimal printing conditions by using an 8% alginate solution under a 10 Hz laser repetition rate. Alginate straight and bifurcated Y-shaped tubes have been successfully printed by applying the identified optimal printing conditions.