Four Strategies used in Extrusion-Based 3D Cell Printing


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The crisis of global organ shortage has grown steadily in recent years; therefore, the reconstruction of human organs using tissue engineering techniques has become one of the ultimate goals of the medical industry. One of the new strategies is the idea of organ printing, which is based on 3D cell printing technology and could utilize a patient’s own cells to fabricate living organ equivalents.

There are three commonly used types of 3D bioprinting technology, namely inkjet, laser-assisted, and extrusion-based methods. Comparing these techniques, it is extrusion-based 3D cell printing that is capable of applying most of the fluidic biomaterials, print large 3D constructs, and incorporate a high density of cells. Moreover, extrusion-based bioprintingis capable of building structures with controlled, tunable mechanical strength. Although it is considered the most promising technology for biofabrication of tissues, it cannot fabricate such complex constructs without carefully designed bioink materials.

The primary role of bioink is to guide the positioning of cells and act as the controllable “building blocks” to build complex structures. An ideal bioink for the recapitulation of living structures must be versatile. It mustmatch several desirable features, including (1) good rheological properties to reduce the risk of cell damage caused by shear stress during extrusion;(2) excellent printability to dispense the structure with the designed shape; (3) tunable and cell-friendly gelation kinetics to provide good shape fidelity; (4) sufficient mechanical properties to maintain the printed structures for a long period; and (5) cytocompatibility to benefit cell survival and functions.

Today, there are only a few materials which can provide versatility (e.g., peptide-based bioinks), despite the wide range of natural and synthetic bioinks available on the market.

Whilst there are still performance challenges with many available bioink materials, exploring advanced extrusion-based cell printing methods is another feasible direction to achieve the goal of organ printing. Since bioink gelation processes are generally thermo-, pH-, photo-, and enzyme-sensitive, providing assisting external environments during the fabrication process has been widely investigated to address the limitations in cell printing.

In the followings, we discuss four printing approaches being investigated to support the complexity required for organ printing.

  • Bath-Assisted Approach is a commonly used approach for low viscosity bioinks. It refers to dispensing bioinks into an additional reservoir (bath), providing either physical support to avoid gravity-induced collapse or crosslinking reagents. Often the bath is composed of either a gelatin slurry or a carbomer hydrogel.
  • Aerosol Spraying is a great option to induce gelation of bioinks with quick gelation times. As the name suggests, crosslinking reagents can be supplied in the form of aerosols. Compared with an aqueous bath, spraying aerosolized crosslinking reagents prevents suspension of printed structure within the liquid medium. It is important that crosslinking occurs quickly so the bioink can be printed as firm gel filaments to ensure printing resolution and enable complex structure fabrication. Bioinks such as alginate or Biogelx’s peptide-based bioinks can be considered for aerosol treatment as they gel almost instantaneously when treated with calcium ions.
  • Thermally Controlled Printing is a common practice to print with collagen, gelatin, agarose, and Pluronic. This technique combines heating or cooling apparatuses into print heads to expose the bioink to different temperatures during the printing process to trigger gelation. Unfortunately, the process typically requires several (sometimes dozens) of minutes to form a stable gel,which is too slow to provide sufficient shape fidelity for complex 3D constructs.
  • The Combination of Multiple Nozzles enables the simultaneous dispensing of diverse bioinks and cells in a single fabrication process. Due to the wide range of applicable materials, this approach can achieve various unmet needs, such as strong mechanical stability of printed constructs, acceptable bioink printability, and biomimetic tissue constructs. This technique can work with both natural and synthetic bioink materials.

 

 Source: Gao et al. (2019) Recent Strategies in Extrusion-Based Three-Dimensional Cell Printing toward Organ Biofabrication, ACS Biomater. Sci. Eng.

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