to be lower than 35 dyne cm−1 for good compatibility (Shastry et al. 2009). Also, it was reported that a contact angle of less than 50° is desired for achieving good printability. Low polar materials such as carnauba wax were used as a coating material over the hard‐panned sugar confectioneries that showed adverse effects on the accuracy and precision of printed images. Hence, hydrophilic materials are used for coating of substrate’s surface in order to improve the compatibility of the water‐based ink with the substrate (Liu and Zhang 2019). Thus, the material viscosity, ink density, and surface tension are significant properties that must be optimized for preventing the overflow and spreading of droplets from the nozzle. A successful 3D printed construct using binder jetting must possess adequate mechanical strength, minimal shrinkage and minimal bleeding or oozing out of liquid binder (Von et al. 2015a).
The binder concentration greatly influences the dimensional stability and precision of the printed sample. Other factors such as flowability and wettability also influence binder jetting (Godoi et al. 2016). Generally, materials with free‐flowing nature are more suitable for binder jetting that possess good spread ability and packing properties. Typically, the angle of repose of material should be low enough generally smaller than 30° (Diaz et al. 2017). Adequate wettability must be preferred for binder jetting as low wettability of materials affects the powder alignment that subsequently declines printability. On the other hand, higher wettability interferes with the binding of the layers that significantly affects the end quality of printed construct. The particle size and distribution affect the binding behaviour of the water‐based binder (Liu et al. 2017). As a common practice, the coarse particles are mixed with the fine powders for achieving an edible construct with compact packing. Materials with a moisture content of less than 6% are more suitable for binder jetting (Von et al. 2015b).
2.7.2 Classification of Binder Jetting
There is no distinct classification in binder jetting printing technology. Similar to inkjet printing binder jetting is used for the fabrication of complex porous internal structures. The printhead with binder reservoir can be operated either by thermal or piezoelectric mechanism (Shirazi et al. 2015). Further, the dispersion of the binder jetting is usually performed by DoD manner since the continuous dispersion would result in moistening of powder bed that makes the post‐drying to be difficult. The characteristic restructuring and amorphization of cellulose have been utilized for 3D printing applications. Holland et al. (2018) reported a study on the ball‐milled amorphous cellulose powder with xanthan gum for the creation of 2D structures. In view of 3D printing, the researchers play around with the hygroscopic and thermal properties of the ink for designing a 3D crystalline network that resembles the gluten network. Results showed that heat treatment after printing aids in the recrystallization of the cellulose powder printed with liquid binder. Different kinds of food structures with novel textures can be fabricated using this approach. Further, the use of food‐grade xanthan gum proved to be edible binder and its applications can be extended in printing of different food inks. The technology can only be applied for low viscous food inks that limits its applications in food printing (Nachal et al. 2019). This technology is well established for biomedical applications in the fabrication of functional drugs and pharmaceuticals. Advancements in binder jetting technology help in designing porous tablets for rapid dispersion when get contacted with a fluid medium (Prasad and Smyth 2016). However, more studies are required in exploring the binder jetting technology in the food sector. There is a great scope for binder jetting in the fabrication of soft materials. Future studies on understanding the fluid property and solvation chemistry of food inks would bridge up the void that exists with food applications of binder jetting technology.
2.8 Bio‐Printing
3D bioprinting is an interdisciplinary science that is closely related to medical science, material science, and mechanical engineering. 3D bioprinting deals with a manoeuvre of cell‐laden bio‐inks to fabricate living structures such as 3D tissue scaffolds and organs (Dey and Ozbolat 2020). Major driving needs for the in‐vitro culturing of the living structures are the existing huge demand for organ transplantation and tissue engineering (Matai et al. 2020). Traditional approaches of 2D culturing methods suffer from flaws during drug screening and medical examination studies. Approaches based on 3D printing helps in spatiotemporal directional manipulation of different kinds of cells and better prediction of organ models within an appropriate period (Armstrong and Stevens 2020). When applied to foods bioprinting provides a new dimension of research in the fabrication of cultured meat. Artificially cultured meat by in‐vitro methods remains to provide a promising solution for the rising protein demand for mankind. The application of bioprinting for the fabrication of edible meat has a great scope to be the food of the future that possesses both economic as well as environmental benefits (Tomiyama et al. 2020). Most of the 3D printing studies reported in literature utilize natural meat and tested for its printability. Not many studies have been reported on the development of artificially cultured in‐vitro meat. The rising awareness about the vegan diet explored the opportunities and feasibilities in the fabrication of meat analogues (Alexander et al. 2017). Hence, future research must be directed for a better understanding of the state‐of‐the-art of printability of cultured meat with special consideration in material selection, material response, safety, edibility, and acceptability of the artificial meat.
2.8.1 Working Principle, System Components, and Process Variables
3D bioprinting is an AM process that involves the fabrication of cells and biomaterials using a digital file to print organ‐like structures in a layer‐by‐layer manner. The process of 3D bioprinting involves a sequence of steps: data acquisition, material selection, bioprinting, and functionalization (Gu et al. 2019). In data acquisition, the information about the 3D models can be obtained using the scanning techniques such as computed tomography (CT), X‐ray imaging, and magnetic resonance imaging (MRI) (Ozbolat and Gudapati 2016). Then the scanned image can be reconstructed to extract the information, or the design can be modelled directly using CAD software. Thereafter the model is sliced into 2D cross‐sections using appropriate slicing software (Figure 2.13). The materials for bioprinting can be selected based on the requirements of printed structures and printing approaches used. The cell cultures, hydrogels, and growth factors are the common base materials used in the bioprinting process. Based on the printing approach used, the system components of the bioprinter would vary. The printed cells are then dispersed in solutions for maturation (Koch et al. 2013). Later the cultured organ is subjected to physical and chemical simulations to determine its targeted function. A similar approach is utilized for the culturing of artificial meat.
The mixture of cell‐matrix and nutrient mixture is supplied and printed from the printer cartridge in the form of layers. The selection of bio‐ink is crucial for successful printing in terms of biocompatibility, printability, and mechanical integrity (Gao et al. 2019). The characteristic feature of bioprinter is the controlled and precise deposition of the biomaterials. The major advantage of bioprinting is the ability to fabricate porous structures in the desired shape with specific functionalities (Ying et al. 2018). 3D bioprinting offers rapid and customized fabrication of scaffolds with higher precision. With recent advancements in technology, the system components of the bioprinter allow for accurate control over the distribution of pore size, pore volume, and interconnectivity of the pores with network formation. Researchers of the University of Missouri used this approach for the printing of a strip of edible porcine tissue (Nachal et al. 2019). A plant‐based cell‐laden hydrogel was utilized for the production of unique textured food using 3D printing (Park et al. 2020). The callus‐based food ink was prepared by blending callus dispersion with 4% alginate in different ratios of 1 : 2, 1 : 1, and 2 : 1 (w/w). The process involves the curing of the callus samples using Ca2+ ions that aids in the formation of rigid gel suitable for 3D printing (Figure 2.14). The samples of 1 : 2 and 1 : 1 ratio showed a proper shape fidelity while the sample with excess cell concentrations i.e., 2 : 1 lowered the printing resolution and resulted in deviation from intended dimensions.
