Direct Bioprinting of Vessel-Like Tubular Microfluidic Channels

Although great progress has been made in biofabrication of tissue constructs over the past decades, the engineered constructs still have difficulty in biomimicking the functional thick tissues or organs, due to an inefficient media exchange rate [1]. Nonhomogeneous cell distribution and limited metabolic activities are often observed, since planted cells cannot get enough oxygen, growth factors and nutrients for their metabolic activities which are needed for maturation during perfusion. Microfluidic system integration has shown great potential to alleviate current limitations. Lee and his coworkers [2] have shown great difference in cell viability with or without an embedded microfluidic channel in hydrogel scaffolds. Ling et al. [3] demonstrated that microfluidic channels are capable of delivering sufficient nutrients to encapsulated cells, and higher cell viability resulted in the region closer to the microfluidic channel. In addition, microfluidic channel systems were not only able to provide media to maintain cell metabolic activities but also to delivered signals to guide cell activities. To date, several methods have been used in microfluidic fabrication, including soft lithograph [3–5], photo-patterning [6–8], laser-based technologies [9,10], molding [11–13], and bioprinting [2,14–16]. However, due to their intrinsic characteristics, each of the above-mentioned technologies has its advantages and disadvantages. Soft lithography is the most popular method in microfluidic channel fabrication due to its low cost, accuracy, and reproducibility. Using soft lithography technology, Ling et al. [3] fabricated microfluidic cell-laden agarose hydrogel, which resulted in a significant increase in cell viability during media perfusion compared to static controls. Cuchiara et al. [4] developed a soft lithography process to fabricate a poly(ethylene glycol) diacrylate hydrogel microfluidic network. With media perfusion, encapsulated mammalian cells maintained a high viability rate in bulk hydrogel. However, soft lithograph is not a viable option for fabrication of complex three-dimensional (3D) constructs due to its cumbersome procedures. Despite their superior accuracy and repeatability, photo-patterning and laser-based methods may not be suitable for fabricating thick tissue constructs because of their limited light-penetrating depths in precursor solution. Offra et al. [6] proposed a focal laser photoablation capable of generating microstructures in transparent hydrogels. Cell behavior was successfully guided by the microchannel pattern. Molding is an inexpensive and scalable method, but complex 3D geometry is difficult to achieve and postprocedures are required after fabrication. In Ref. [12], Nazhat et al. used a molding method to incorporate unidirectionally aligned soluble phosphate-based glass fibers into dense collagen scaffolds. The diameters of the achieved microfluidic channels were around 30–40 μm, and a significant increase in cell viability was observed in the hydrogel sheets. Despite the plethora of work in microfluidic channel fabrication using the traditional methods, only a few researchers have developed strategies for bioprinting of microfluidic channels, where bioprinting can be defined as computer-controlled layer-by-layer bioadditive process enabling printing living cells precisely per predefined patterns [1]. Cell encapsulated biomaterials can be directly patterned onto substrate without any pretreating steps (such as mold or mask preparation). It offers several advantages, including precise control [16,17], automated fabrication capability [18,19], and feasibility of achieving complex shapes [15]. Zhao et al. [20] recently presented a methodology in bioprinting of perfused straight microfluidic channel structures in thick hydrogel. They created a temporary structure to form the hollow cavity which was then removed by a postprocess. In this study, a novel bioprinting fabrication process is introduced, where vessel-like microfluidic channels can be directly printed in complex shapes without any need of pre/post processes. Microfluidic channels, in the form of hollow filaments, are directly printed by a pressure-assisted robotic system using hydrogels. We performed geometric characterization of microfluidic channels through studying multiple biomaterials and their dispensing rheology. Then, microfluidic channels were embedded in bulk hydrogel to evaluate their structural integrity and media perfusion capability. Further, we examined the media transportation capability of printed and embedded microfluidic channels by perfusing oxygenized cell culture media through patterned channels. A cell viability study was carried out to access the effect of perfusion on encapsulated cells.