User:Dtbooth/3D bioprinting

Extrusion-Based Methods
Extrusion-based printing is a very common technique within the field of 3D printing which entails extruding, or forcing, a continuous stream of melted solid material or viscous liquid through a sort of orifice, often a nozzle or syringe. When it comes to extrusion based bioprinting, there are three main type of extrusion. These are pneumatic driven, piston driven, and screw driven. Each extrusion methods have their own advantages and disadvantages. Pneumatic extrusion used pressurized air to force liquid bioink through a depositing agent. The air used to move the bioink must be free of contaminants. Air filters are commonly used to sterilize the air before it is used. Piston driven extrusion utilizes a piston connected to a guide screw. The linear motion of the piston squeezes material out of the nozzle. Screw driven extrusion uses an auger screw to extrude material. The rotational motion forces the material down and out of the nozzle. Screw driven devices allow for the use of higher viscosity materials and provide more volumetric control. Once printed, many materials require a crosslinking step to achieve the desired mechanical properties for the construct, which can be achieved for example with the treatment of chemical agents or photo-crosslinkers.

Direct extrusion is one of the most common extrusion-based bioprinting techniques, wherein the pressurized force directs the bioink to flow out of the nozzle, and directly print the scaffold without any necessary casting [28]. The bioink itself for this approach can be a blend of polymer hydrogels, naturally derived materials such as collagen, and live cells suspended in the solution [28]. In this manner, scaffolds can be cultured post-print and not need further treatment for cellular seeding. Some focus in the use of direct printing techniques is based upon the use of coaxial nozzle assemblies, or coaxial extrusion. The coaxial nozzle setup enables the simultaneous extrusion of multiple material bioinks, capable of making multi-layered scaffolds in a single extrusion step [30]. The development of tubular structures has found the layered extrusion achieved via these techniques desirable for the radial variability in material characterization that it can offer, as the coaxial nozzle provides an inner and outer tube for bioink flow [30]. Indirect extrusion techniques for bioprinting rather require the printing of a base material of cell-laden hydrogels, but unlike direct extrusion contains a sacrificial hydrogel that can be trivially removed post-printing through thermal or chemical extraction [29]. The remaining resin solidifies and becomes the desired 3D-printed construct.

Other Printing Methods
Droplet-based bioprinting is a technique in which the bioink blend of cells and/or hydrogels are placed in droplets in precise positions. Most common amongst this approach are thermal and piezoelectric-drop-on-demand techniques [31]. Thermal technologies use short duration signals to heat the bioink, inducing the formation of small bubbles which are ejected. Piezoelectric bioprinting has short duration current rather applied to a piezoelectric actuator, which induces mechanical a vibration capable of ejecting a small globule of bioink through the nozzle. A significant aspect of the study of droplet-based approaches to bioprinting is accounting for mechanical and thermal stress cells within the bioink experience near the nozzle-tip as they are extruded.

Laser-based bioprinting can be distinguished between two major classes in general, those being based upon either cell transfer technologies or photo-polymerization. In cell transfer laser printing, a laser stimulates the interface between energy-absorbing material (e.g. gold, titanium, etc.) and the bioink, which contains a sacrificial material. This sacrificial ‘donor layer’ vaporizes under the laser’s irradiation, forming a bubble from the bioink layer which gets deposited from a jet [32]. Photo-polymerization techniques rather use photoinitiated reactions to solidify the ink, moving the beam path of a laser to induce the formation of a desired construct. Certain laser frequencies paired with photopolymerization reactions can be carried out without damaging cells laden into the material.

[28] Datta, Pallab, Bugra Ayan, and Ibrahim T. Ozbolat. "Bioprinting for vascular and vascularized tissue biofabrication." Acta biomaterialia 51 (2017): 1-20.

[29]T.J. Hinton, Q. Jallerat, R.N. Palchesko, J.H. Park, M.S. Grodzicki, H.-J. Shue, M.H. Ramadan, A.R. Hudson, A.W. Feinberg, “Three-dimensional printing of complex biological structures by freeform reversible embedding of suspended hydrogels” Sci. Adv., 1 (2015), 10.1126/sciadv.1500758

[30] Gupta, P., & Mandal, B. B. (2021). Tissue‐Engineered Vascular Grafts: Emerging Trends and Technologies. Advanced Functional Materials, 2100027

[31]A. Munaz, R.K. Vadivelu, J. St John, M. Barton, H. Kamble, N.-T. Nguyen “Three-dimensional printing of biological matters” J. Sci. Adv. Mater. Devices, 1 (2016), pp. 1-17, [32]R. Devillard, E. Pagès, M.M. Correa, V. Kériquel, M. Rémy, J. Kalisky, M. Ali, B. Guillotin, F. Guillemot “Chapter 9 – cell patterning by laser-assisted bioprinting” Methods Cell Biol. (2014), pp. 159-174, 10.1016/B978-0-12-416742-1.00009-3