Mini Review
Tailoring of the rheological properties of bioinks to improve bioprinting and bioassembly for tissue replacement

https://doi.org/10.1016/j.bbagen.2020.129782Get rights and content

Highlights

  • Most of bioinks are hydrogel based.

  • Printability is closely related with rheological conditions of the bioink.

  • Cell behavior depends on physicochemical properties of the bioink.

Abstract

Background

Tissue replacement is among the most important challenges in biotechnology worldwide.

Scope of review

We aim to highlight the importance of the intricate feedback between rheological properties and materials science and cell biological parameters in order to obtain an efficient bioink design, supported by various practical examples.

Major conclusions

Viscoelastic properties of bioink formulas, rheological properties, injection speed and printing nozzle diameter must be considered in bioink design. These properties are related to cell behavior and the survival rate during and after printing. Mechanosensing can strongly influence epigenetics to modify the final cell phenotype, which can affect the replacement tissue.

General significance

In tissue engineering, biotechnologists must consider the biophysical properties and biological conditions of the materials used, as well as the material delivery mode (in a case or tissue) and maturation mode (curing or biomass), to ensure the development off appropriate materials mimicking the native tissue.

Graphical abstract

Overview of biofabrication by bioprinting and bioassembly. A) Bioprinting. Bioink hydrogel based. Bioactive molecules include substances from culture media or serum, such as cytokines and hormones. Polymer network(s) could be made from natural polymers, including extracellular matrix or synthetic polymers; copolymers are often used. B) Bioassembly. Building block formation is based on cell aggregation, nucleation and compaction. Then, the self-assembly depends on printability and building block stability.

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Introduction

The replacement of damaged tissues is among the most important health science needs worldwide. Organ and tissue damage caused by trauma and pathologies leads to the frequent need to employ therapeutic strategies to support patient homeostasis. In remaining tissues, the damaged section is often isolated by fibrosis or poor remodeling, given the limited healing mechanisms that humans have developed as a consequence of our evolutive condition. Fortunately, surgical techniques and tissue engineering enable the support of tissue function and tissue replacement in many medical contexts; although organ replacement continues to depend on homologous donor strategies.

In tissue engineering, biomaterials for tissue replacement are prepared using various strategies; biofabrication must be performed with consideration of the characteristics of the tissue to be replaced, such as its composition and location. Bioprinting and bioassembly technologies can be classified according to the scale of the minimum manufacturing unit used. Bioprinting is performed at the molecular level via the manipulation of materials such as cells, synthetic polymers, and biomolecules in bioink; bioassembly involves preformed manufacturing units containing cells (e.g., tissue explants, cell sheets, spheroids, and microspheres) [1]. With both technologies, the proper selection of elementary units with suitable physicochemical properties is critical [2].

Groll et al. [3] defined bioink as “a formulation with cells suitable for processing by an automated biofabrication technology that may also contain biologically active components and biomaterials.” The selection of appropriate materials (including cells) for bioink development, in terms of improving the traditional biofabrication window, remains challenging, given the need to achieve printability and shape fidelity with cell survival. Several studies have suggested that suitable rheological properties of materials can help to increase bioink printability while maintaining cell viability and function [4,5]; printing-related factors such as the temperature and extrusion pressure must also be considered [6].

The use of “building block” units in bioassembly must be undertaken with specific considerations to produce macrotissues with specific designs, biological functions, and biomechanical properties. Such considerations include the reproducibility of the manufacturing method, the ability to produce large numbers of homogenously sized units, and the production of units of appropriate size and rigidity for handling by an automated system with the retention of shape and biological integrity [7]. In this review, we focus on the interrelationships between rheometric parameters and materials science and cell biology with the purpose that tissue engineers and other researchers interested in bioprinting and bioassembly can choose the appropriate strategy for tissue replacement. We emphasize recent works involving biofabrication with the aim to provide the frequent strategy for bioink design, as well as more fine assessment tools that improve printability and maturation of the deposited bioink.

Section snippets

Bioprinting and bioassembly

Bioinks can be matrix based (cell free), cell based (scaffold free), or cell and scaffold based [8]. The raw material used for ink preparation is usually a soluble fraction of a matrix capable of forming a scaffold that will be populated by cells [9]. Hydrogels are often used for bioink formulation, as their physicochemical properties confer bioink printability and cell-friendly conditions mimicking the native extracellular matrix (ECM) environment. Various natural, synthetic, and hybrid

Shearing stress and printability

Bioassembly methods involving microtissue recovery and delivery differ considerably from bioprinting procedures, in which a mixture of independent materials, including living cells, must be integrated into the bioink, resulting in numerous physical, electrostatic, and biological interactions (Fig. 1A).

The biophysical properties of the material are known to affect cell–matrix interaction, modifying focal adhesion and cell activity [17]. Viscosity, a measure of the resistance of a material to

Assessment beyond viscosity

Besides viscosity, the knowledge of the rheological properties of the materials employed in bioink preparation could help to predict bioink behavior during printing [29]. The forces involved in extrusion-based bioprinting include shear strain of the bioink, which is the deformation derived from the pressure of the bioprinter, and which leads to shear thinning because of reduced bioink viscosity. Along with shear thinning, shear stress results in gradient changes in the bioink that modify its

Elastic behavior of bioinks

Bioink behavior can also be assessed practically by examining rheological characteristics related to elastic properties (G'), flow conditions (G"), and the mathematical function of their relationship (Tan δ, which represents the viscous portion in relation to the elastic portionTanδ=GG; Fig. 2A) [40]. These concepts are more aligned with practical understanding employed in daily life. For example, measurement of the flow properties of alginate and alginate/gelatin-based bioinks using a

Cell behavior depends on the physicochemical properties of bioink components

Cell function impairment and cell damage characteristics, such as pyknosis (chromatin condensation) and karyolysis (nucleus destruction due to necrosis or apoptosis) have been observed during and after bioprinting. In the optimization of printing parameters, bioink viscosity is a crucial cue: high viscosity is needed for better printability, but more pressure is required to print viscous inks, increasing nozzle-wall shear stress and clogging, and thereby increasing cell deformation and reducing

The mechanical memory of the system dictates cell integration into the printed matrix

Mechanical memory involves the cellular ability to respond to the physical conditions of the scaffold. An understanding of how cells (do not) adapt to different mechanical responses is crucial for the perfection of bioprinting techniques, as the mechanical properties of the printed matrix could determine cell differentiation and behavior in a bioink. For example, primary rat-lung fibroblasts cultured on a rigid polydimethylsiloxane surface (100 kPa, mimicking a pathological substrate)

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank Jennifer Piehl PhD. from Write Science Right for English correction and editorial review. This research did not receive any specific grant from any funding agency in the public, commercial, or not-for-profit sector.

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