Bio-ink

Structural bio inks are used to create the framework of the desired print using materials like alginate, decellularized ECM, gelatins, and more. From the choice of material you are able to control mechanical properties, shape and size, and cell viability. These factors make this type one of the more basic but still one of the most important aspects to a Bio-printed design.

Sacrificial bio inks are materials that will be used to support during printing and then will be removed from the print to create channels or empty regions within the outside structure. Channels and open spaces are massively important to allow for cellular migration and nutrient transportation lending them useful if trying to design a vascular network. These materials need to have specific properties dependent on the surrounding material that needs to stay such as water solubility, degradation under certain temperatures, or natural rapid degradation. Non Crosslinked gelatins and pluronics are examples of potential sacrificial material.

Functional bio inks are some of the more complicated forms of ink, these are used to guide cellular growth, development, and differentiation. This can be done in the form of integrating growth factors, biological cues, and physical cues such as surface texture and shape. These materials could be described as the most important as they are the biggest factor in developing a functional tissue as well as structural related function.

Bio printed structures can be extremely fragile and flimsy due to intricate structures and overhangs in the early period after printing, these support structures give them the chance to get out of that phase. Once the construct is self supportive, these can be removed. In other situations, such as introducing the construct to a bioreactor after printing these structures can be used to allow for easy interface with systems used to develop the tissue at a faster rate.

Alginate is a naturally derived biopolymer from the cell wall of brown seaweed that has been widely used in biomedicine because of its biocompatibility, low cytotoxicity, mild gelation process and low cost. 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.[7] Additionally, these alginate-based bioinks can be blended with other materials such as nanocellulose for application in tissues such as cartilage.[8]

Since fast gelation leads to good printability, bioprinting mainly utilizes alginate, modified alginate alone or alginate blended with other biomaterials. Alginate has become the most widely used natural polymer for bioprinting and is most likely the most common material of choice for in vivo studies.

Gellan gum is a hydrophilic and high-molecular weight anionic polysaccharide produced by bacteria. It is very similar to alginate and can form a hydrogel at low temperatures. It is even approved for use in food by the United States Food and Drug Administration (FDA). Gellan gum is mainly used as a gelling agent and stabilizer. However, it is almost never used alone for bioprinting purposes.[1]

Agarose is a polysaccharide extracted from marine algae and red seaweed. It is commonly used in electrophoresis applications as well as tissue engineering for its gelling properties. The melting and gelling temperatures of agarose can be modified chemically, which in turn makes its printability better. Having a bio-ink that can be modified to fit a specific need and condition is ideal.

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.[9]

Collagen is the main protein in the extracellular matrix of mammalian cells. Because of this collagen possesses tissue-matching physicochemical properties and biocompatibility. On top of this, collagen has already been used in biomedical applications. Some studies that collagen has been used in are engineered skin tissue, muscle tissue and even bone tissue.[1]

Pluronics have been utilized in printing application due to their unique gelation properties.[10] 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.[11]

Polyethylene glycol (PEG) is a synthetic polymer synthesized by ethylene oxide polymerization. It is a favorable synthetic material because of its tailorable but typically strong mechanical properties.[1] PEG advantages also include, non-cytotoxicity and non-immunogenicity. However, PEG is bioinert and needs to be combined with other biologically active hydrogels.

Decellularized extracellular matrix based bioinks can be derived from nearly any mammalian tissue. Organs such as heart, muscle, cartilage, bone, and fat are decellularized, lyophilized, and pulverized, to create a soluble matrix that can then be formed into gels.[12] These bioinks possess several advantages over other materials due to their derivation from mature tissue. They consist of a complex mixture of ECM structural and decorating proteins specific to their tissue origin, and 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.[13] Different additives, e.g. GelMA, alginate, have been used to improve the printability of decellularized ECM.[14]