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Bioinks are substances made of living cells that can be used for 3D printing of complex tissue models. Bioinks are materials that mimic an extracellular matrix environment to support the adhesion, proliferation, and differentiation of living cells. Bioinks distinguish themselves from traditional biomaterials such as hydrogels, polymer networks, and foam scaffolds due to their ability to be deposited as filaments during an additive manufacturing process. Additionally, unlike traditional additive manufacturing materials such as thermoplastic polymers, ceramics, and metals which require the use of harsh solvents, cross-linking modalities and high temperatures to be printed, bioinks are processed under much milder conditions. These mild conditions are necessary to preserve compatibility with living cells, and prevent degradation of bioactive molecules and macroproteins. These bioinks are often adopted from existing hydrogel biomaterials and derived from natural polymers such as gelatins, alginates, fibrin, chitosan, and hyaluronic acids that are sensitive to their processing conditions.
Unlike the thermoplastics that are often utilized in traditional 3D printing, the chain entanglements and ionic interactions within the hydrogel-like  bioink rather than temperature dominate shape fidelity. The natural derivation of many bioinks often results in a high water content and sensitivity to harsh processing conditions. Therefore, bioink filaments are often deposited at or below human body temperature and under mild conditions to preserve bioink printability. Additional considerations must be taken into account when printing bioinks blended with a cell suspension due to the need to preserve cell viability.
Differences from traditional 3D printing materials
- Printed at a much lower temperature (37 °C or below)
- Mild cross-linking conditions
- Natural derivation
- Cell manipulatable
Bioink compositions and chemistries are often inspired and derived from existing hydrogel biomaterials. However, these hydrogel biomaterials were often developed to be easily pipetted and cast into well plates and other molds. Altering the composition of these hydrogels to permit filament formation is necessary for their translation as bioprintable materials. However, the unique properties of bioinks offer new challenges in characterizing material printability. Unlike traditional 3D printing materials such as thermoplastics that are essentially 'fixed' once they are printed, bioinks are a dynamic system due to their high water contains and often non-crystalline structure. The shape fidelity of the bioink after filament deposition must also be characterized. Finally, the printing pressure and nozzle diameter must be taken into account to minimize the shear stresses placed on the bioink and on any cells within the bioink during the printing process. Too high shear forces may damage or lyse cells, adversely affecting cell viability.
Important considerations in printability include:
- Uniformity in filament diameter
- Angles at the interaction of filaments
- "Bleeding" of filaments together at intersects
- Maintenance of shape fidelity after printing but before cross-linking
- Printing pressure and nozzle diameter
Alginate is a naturally derived biopolymer from the cell wall of brown algae that has been widely used as a biomaterial. Alginates are particularly suitable for bioprinting due to their mild cross-linking conditions via incorporation of divalent ions such as calcium. These materials have been adopted as bioinks through increasing their viscosity. Additionally, these alginate-based bioinks can be blended with other materials such as nanocellulose for application in tissues such as cartilage.
Gelatin has been widely utilized as a biomaterial for engineered tissues. The formation of gelatin scaffolds is dictated by the physical chain entanglements of the material which forms a gel at low temperatures. However, at physiological temperatures, the viscosity of gelatin drops significantly. Methacrylation of gelatin is a common approach for the fabrication of gelatin scaffolds that can be printed and maintain shape fidelity at physiological temperature.
Decellularized extracellular matrix based bioinks can be derived from nearly any mammalian tissue. However, often organs such as heart, muscle, cartilage, bone, and fat are decellularized, lyophilized, and pulverized, to created a soluble matrix that can then be formed into gels. These bioinks possess several advantages over other materials due to their derivation from mature tissue. These materials consist of a complex mixture of ECM structural and decorating proteins specific to their tissue origin. Therefore, dECM-derived bioinks are particularly tailored to provide tissue-specific cues to cells. Often these bioinks are cross-linked through thermal gelation or chemical cross-linking such as through the use of riboflavin.
Pluronics have been utilized in printing application due to their unique gelation properties. Below physiological temperatures, the pluronics exhibit low viscosity. However, at physiological temperatures, the pluronics form a gel. However, the formed gel is dominated by physical interactions. A more permanent pluronic-based network can be formed through the modification of the pluronic chain with acrylate groups that may be chemically cross-linked.
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