TABLE II.

Biomechanical factors and summary of their significances for pre, during, and post bioprinting processes.

3D bioprinting	Biomechanical factor	Summary of significances	References
Pre bioprinting	Osmotic pressure	• Cells absorb water in a hypotonic solution, resulting in cell burst.	75, 76, 80, and 81
		• Cells lose water in a hypertonic solution, resulting in cell shrinkage.
		• Cells can be protected from osmosis by using physiological buffer as a solvent, gel coating on cells, and adding chemically inert osmotic regulators.
	Viscosity	• Higher viscosity increases bioink preparation time required to achieve homogenously distributed cells/biological reagents.	82 and 83
		• Higher shear stress levels are generated when preparing more viscous bioinks, which may cause the rapture of cellular membrane and cell death.
		• Viscosity affects the injectability of a bioink.
	Injectability	• It ensures that prepared bioink can be injected/extruded in an even flow without clogging.	84
		• Regulating the bioink viscosity or the needle size can adjust the injectability.
During Bioprinting	Flow pattern	• Bioinks normally exhibit non-Newtonian flow, those exhibiting shear thinning are preferred for bioprinting.	51 and 62
		• Bioinks with certain level of yield stress (yield-pseudoplastic) can improve the printing fidelity and preserve the stability of 3D structures.
	Viscosity	• Bioinks with viscosity ranges 30 mPa/s to 6 × 107 mPa/s are compatible for 3D bioprinting.	41 and 85–87
		• Higher viscosity better supports the bioprinted structure but may restrict cellular functions; lower viscosity provides a cell friendly environment but limits printability.
		• Viscosity alone cannot capture the entire behavior of a bioink; a high viscosity does not guarantee a high printing fidelity.
		• Viscosity can be well controlled by regulating hydrogel concentration, cell density, additives, temperature, and pre-cross-linking.
	Viscoelasticity	• Storage and loss moduli are used to report viscoelasticity.	32 and 88–91
		• Bioinks with higher storage modulus show more solid-like behavior, which supports the structural stability, but may cause impairments like clogging and discontinuous filaments.
		• Bioinks with higher loss moduli can be easily manipulated, but face challenges in forming 3D structures.
		• Certain ranges of loss tangent values have been reported to support printability, but these ranges are not universal and are bioink dependent.
		• Viscoelasticity can be regulated following the same strategies used for viscosity.
	Time dependency vs. Time independency	• Time dependency of bioinks is normally identified via time sweep, given a certain shear strain or frequency.	92 and 93
		• Rheological properties of bioinks are altered during a timescale when they exhibit time-dependent behavior.
	Surface tension	• Bioink is expected to have surface tensions that can allow their detachment from the surface of the needle tip, while enabling them to resist the surface tension-driven droplet formation.	51, 94, and 95
		• Surface tension is closely related to contact angle of printed filament in the first layer; a large contact angle preserves the shape fidelity, while a smaller angle helps to anchor the layer.
		• Contact angle can be adjusted by adding the second material into the bioink or changing the wettability of the bioprinting substrate by coating.
	Flow rate	• Flow rate significantly influences the diameter of printed filaments.	96 and 97
		• Flow rate is determined by the flow behavior of the bioink, and the bioprinting control parameters including printing pressure, needle size, and temperature.
	Process-induced mechanical forces	• Mechanical forces, including hydrostatic pressure, shear stress, and extensional stress, can induce cell damage, where shear and extensional stresses are more destructive. Lower stress levels may reduce cell damages and maintain higher cell viability.	47 and 98
		• The bioprinting process-induced mechanical forces can be controlled by the rheological properties of bioinks, the printing pressure, and the needle size.
	In-situ cross-linking	• Bioinks can be solidified using temperature control, atomized cross-linking, agent medium cross-linking, and light cross-linking.	13 and 56
		• In-situ cross-linking makes it possible for low viscosity bioink printing, but due to the dynamic cross-linking process, the printing fidelity is normally compromised.
Post Bioprinting	Post-print (secondary) cross-linking	• Normally applied to increase the stability of bioprinted constructs.	99
		• Mechanical properties of printed constructs can be well adjusted during post cross-linking.
	Stiffness	• It is the extent to which an object resists deformation in response to an applied force. Elastic modulus is normally used to report the stiffness of printed constructs.	48 and 100–103
		• Can be regulated by hydrogel type, concentration, cross-linking, cell-hydrogel interactions, porosity, and degradation.
		• Can significantly affect various cell functions, including cell differentiation, migration, angiogenesis, contractile function (e.g., cardiomyocytes), and intercellular connectivity.
		• Bioprinted structures with stiffness approaching that of the native tissue are preferred.
	Viscoelasticity	• Can be affected by hydrogel type and concentration, cross-linking, cell-hydrogel interactions, porosity, and degradation.	104 and 105
		• It determines the structural stability and integrity, while it affects the functions of cells such as cell spreading, proliferation, and differentiation.
	Poroelasticity	• Stress relaxation can be used to identify poroelasticity of a bioink.	106
		• Characteristics, including shear modulus and diffusivity, are used to describe poroelasticity.
		• Poroelasticity is a function of hydrogel type and concentration, cross-linking, porosity, and degradation.
		• It determines the diffusivity of a bioprinted structure which is highly important for the metabolism of the encapsulated cells.
	Degradation and ECM remodeling	• Degradation could result in enhanced intercellular connectivity, facilitated ECM secretion and remodeling, fusion/assembly of cellular structures, angiogenesis, and cell migration.	87, 107, and 108
		• Excessive degradation could result in the deterioration and collapse of printed architectures.
		• Can significantly alter retention/release of therapeutics in bioprinted scaffolds.
		• An ideal degradation rate should match the ability of cells to secrete ECM proteins to replace the degraded materials.
		• Degradation byproducts should be nontoxic and easily cleared from the structure.