3D Printing of Foods. C. Anandharamakrishnan

Figure 2.8 3D printed wheat starch hydrogels.

      Source: From Maniglia et al. (2020a) / With permission of Elsevier.

Recently the effect of dry heat treatment (DHT) on the 3D printing of cassava starch hydrogels has been reported (Maniglia et al. 2020b). The effect of pre‐processing and the post‐stability of printed cassava hydrogels were analyzed. The starch was chemically modified with prolonged exposure to DHT (4 hours at 130 °C) that resulted in higher carbonyl content and larger granule size. Thus, DHT was proved to have a significant implication on rheological properties that in turn aids in printability. Further, it was reported that the longest storage period increases the firmness of hydrogel preserves the structural integrity of printed 3D constructs. Similar results were obtained for DHT of wheat starches (Figure 2.8) (Maniglia et al. 2020a). During DHT, the molecular depolymerization was evident with a reduction in starch crystallinity. Thus, studies on starch‐based hydrogels would extend the possibilities of fabrication of novel soft foods with altered textures. Hence, more research studies on the impact of pre‐treatments on the chemical modification of macro components of food systems would explore the range of potential opportunities in 3D printing. In another study, the fabrication of bio‐scaffold with hybrid gels of gelatin and alginate (1 : 2) at 7% resulted in a stable 3D printed matrix with the highest hardness and adhesiveness (Kuo et al. 2021). Subsequent post‐processing freeze‐drying of 3D printed hydrogels enhanced the mechanical properties and shelf‐life of the product. Also, the microstructural analysis revealed the porous structure of the 3D printed scaffolds that has a great scope for encapsulation of bioactive compounds such as probiotics, enzymes, vitamins, minerals, enzymes, and antioxidants (Pérez‐Luna and González‐Reynoso 2018). However, only a few studies reported on the scope of integration of 3D printing and encapsulation. More research works are required in improving design modification of 3D printers suitable for micro‐ and nano‐encapsulation; software integration with a high power source; material characterization and method for improving the bioavailability of micronutrients.

2.5 Selective Sintering

Selective sintering is the process of binding the powdered materials together to form a solid 3D construct using a laser or hot air as the power source. The movement of the 3D printer is controlled automatically by directing the axis movements at pre‐defined points in space according to the 3D model. The power source is applied to the powdered bed that selectively fuses the powdered particles in a layer‐by‐layer manner for fabrication of desired 3D structure (Diaz et al. 2016). After scanning of each cross‐section of structure, a new layer of powder is dropped and is sintered using the power source that leads to a 3D structure (Figure 2.9). Once the printing process is completed, the formed 3D structure is post‐processed for the removal of unfused powder thereby ensuring the smooth finishing quality of the 3D prints (Diaz et al. 2017). The sintering process is mostly applied for metals and ceramics 3D printing. However, for food applications, studies are being put forth for the development of sugar‐based confectionery 3D printed products (Fuh et al. 2015). The advantage of this technique when applied to foods is the use of the unfused powdered particles for the next cycle of printing and the feasibility for the fabrication of the complex internal designs. Compare to the extrusion technology, the selective sintering process allows for the construction of overhanging 3D structures with greater resolution (Liu and Zhang 2019). However, the sintering process is limited to powdered material such as sugar, starch, and fat. This limitation can be overviewed by the design freedom, high productivity, and throughput of the sintering technology.

Figure 2.9 Schematic diagram of selective laser printing.

2.5.1 Working Principle, System Components, and Process Variables

      As stated earlier, selective sintering is based on the powder bed fusion principle that refers to the selective consolidation of solid powdered particles in a specific area according to 3D design into a finished 3D printed construct using a thermal source. The use of a light source and the subsequent increase in printing temperature allows fabricating 3D constructs through melting and fusion of the powdered particles in a layer‐by‐layer manner (Vithani et al. 2019). One major advantage of this technology is the localized phase transition of powder particles that allows the reuse of the unfused feedstock materials. The basic system components of a selective sintering system include a build platform, a thermal power source, Galvano mirrors, powdered feedstock reservoir, mechanical roller, and powdered material vat (Ma et al. 2018). The printing process starts with the rising of the build platform to its uppermost position where a fresh layer of the powdered feed material is spread across the platform and flattened by the roller for uniform dispersion (Liu et al. 2017). Here the print head consists of an inbuilt thermal power source that scans across the powder and sinters it by following a pattern of 3D design. After the formation of each layer, the build platform is lowered to provide enough space for the formation of the next layer. Meanwhile, the reservoir platform ascends and spreads the next layer of material to form a new layer. Thus, the above process continues until the completion of whole 3D structure. After completion of 3D printing process, the system is allowed to cool that assists in the removal of excess un‐sintered material from the fabricated 3D object. The surface finishing of the 3D printed construct can be improved by appropriate post‐processing such as coating and polishing (Awad et al. 2020). These post‐processing methods not only improve the appearance but also it enhances the mechanical properties like hardness and tensile strength.

      Various process variables that must be considered during the sintering process are power source type (laser or thermal heaters), beam diameter, beam power, and scanning speed (Liu and Zhang 2019). The material phase change is associated with the complex interaction of powdered feed material with the light beam. Here the strength of interaction greatly depends on the energy density of the power source (Gu et al. 2012). Hence, the optimization of these process variables is crucial for attaining higher precision and resolution of 3D constructs. The energy density of the laser beam can be adjusted by varying the scan speed and laser power. A higher laser energy density can be obtained by a longer interaction time that results in the fabrication of denser 3D constructs with higher mechanical stability. On the other hand, a lesser interaction time produces a laser beam with a lower energy density that results in a porous brittle 3D construct (Amorim et al. 2014). Other process variables such as printing temperature, powdered bed thickness, space/gap between the print platform and laser head, and laser spot diameter also impact the stability of the printed structures. Apart from process variables, material properties such as particle size, bulk density, wettability, crystallinity, flowability, compressibility, glass transition temperature, melting, and solidification behaviour influence the printability of powdered materials (Yang et al. 2017). The powder bed fusion technology is useful in the fabrication of multi‐material structures that requires a more detailed understanding of the chemical transitions and interactions at the molecular level of feed material and binder component. This opens an array of research opportunities in fabrication of 3D constructs using sintering technology especially in system design, material science, and processing.

2.5.2 Classification of Selective Sintering System

Based on the operation, the powder bed fusion technology has been categorized as selective laser sintering (SLS), selective laser melting (SLM), electron beam melting (EBM), and multi‐jet fusion (MJF) (Awad et al. 2020). Thermoplastics, polymers, metals, and alloy powders are common feed materials employed in the sintering process. The above‐mentioned printing technologies differ based on the type of feed