1. Introduction

The term “additive manufacturing (AM)”, which is often termed 3D printing and rapid prototyping, covers several quite different manufacturing technologies that enable the creation of objects to be fabricated on demand^[1][2] [1,2]. The basic concept of AM is a controlled process whereby a product is built up from a digital design (usually a Computer-Aided Design (CAD) file)^[3][4] [3,4]. While printing the object, the 3D model is sliced into layers, by the software, and then printed one at a time (layer by layer)^[5] [5]. AM has emerged as one of the most disruptive manufacturing technologies and, consequently, has significant implications for companies and industries^[6] [6]. The adoption of these technologies comprises four successive stages: rapid prototyping, rapid tooling, digital manufacturing, and home fabrication^[5] [5]. Each stage corresponds to a different degree of involvement of 3D printing in the manufacturing process. By using a business model, Rayna and Striukova^[5] [5] have concluded that rapid prototyping and rapid tooling had a limited impact, but direct manufacturing and home fabrication can be highly disruptive^[5] [5].

The advantages of AM, in general, are low waste, increased precision, time-saving, and high efficiency^[7] [7]. Table S1 collects a series of benefits and shortcomings alongside potential economic effects. AM is a platform that enables the manufacture of complex structures from digital design data immediately, without special tools and equipment, by providing new opportunities for freedom of design^[8] [8]. Rapid prototyping is one of the main benefits of 3D printing^[9] [9]. AM offers the capacity to build objects with any shape and dimension as well as the ability to control chemical, physical, mechanical properties, and an internal structure (microstructure) by modifying their composition^[10][11] [10,11].

AM is a mass-customisation and personalisation-enabling technology^[6] [6]. Due to limits in production speed among other technological bottlenecks, 3D printing is ideal for mass customisation but is not yet capable of mass production^[6][12] [6,12]. AM enables private and commercial users to design and create their products simply and quickly, especially for low volume, customised, high-value products^[8][13] [8,13]. Most equipment used in AM can be used to produce a variety of different parts at nearly any location by potentially allowing decentralisation and localisation of production for companies^[6] [6].

Therefore, AM offers opportunities for advancement in remote areas with low economic profiles by bridging the gap between these areas and the next market, and by supplying these areas with the necessary objects to improve quality of life^[13] [13]. AM significantly lowers labour demands as well as financial and energy resource inputs into manufacturing processes, which leads to a decrease in production costs and CO₂ emissions^[13] [13]. The layer-by-layer nature of production, specifically the ability to lower product infill, greatly lowers resource demands and process-related waste^[13] [13]. Conventionally, one-of-a-kind manufacturing is associated with surplus material being wasted^[8] [8]. However, with AM technologies, highly customised products can be manufactured more sustainably^[1] [1]. By using 3D printing, it could be possible to develop a local material recycling and manufacturing loop, which translates into reductions in landfill and emissions, and leads to an increase in local employment through recycling centres as well as value creation^[14] [14].

In its early days, AM was used for rapid prototyping and then tooling and these application areas continue to be used^[1] [1]. There is an abundance of evidence that suggests AM will be promising in the following areas^[15][16] [15,16]: (1) customised healthcare products for improving health and quality of life. There are several reports on amenable 3D printing technologies for pharmaceutical manufacturing^[17][18] [17,18], which are applicable to different drug development phases (early-phase screening, testing, manufacturing, and dispensing)^[19] [19]. Human medicine, as well as veterinary medicine, can benefit from technological advances in 3D printing^{[20][21][22][23][24]} [20–24]. 3D printing is important to dentistry and can be used to print personalised braces for patients^[25][26] [25,26]. Surgeons have also reported the application of 3D printing in their respective subspecialties^[27][28][29] [27–29]. (2) Other evidence that shows AM is promising includes reduced environmental impact by increasing manufacturing sustainability. In the construction industry, many experiments have been conducted to explore the full potential of 3D printing as a core method to build more sustainable and environmentally-friendly buildings, or to produce construction components^[30] [30]. Considerable research is ongoing to use wood and forest products as 3D printing feedstock to impart texture to the surface of the printed products and meet the demand for sustainability from consumers^[12] [12]. (3) The third piece of evidence is supply chain simplification to increase efficiency and responsiveness. The various AM technologies that enable the development of fashion products like apparel and jewelry are closely associated with low-volume manufacturing and mass customization^[31] [31].

Additive manufacturing has been labelled a “disruptive technology” because it will fundamentally affect many processes in production, supply chain design, logistics, product life-cycle planning, and consumer behavior [8]. Due to the performance improvements of AM technologies, they are seeing increased usage in direct manufacturing rather than being classified as a ‘technique for prototyping’^[10] [10]. For example, 3D printers are being utilised in analytical laboratories in a wide array of applications^[32] [32]. Moreover, the societal impact study from a technical perspective shows further research is necessary in life-cycle energy consumption evaluation and potential occupation hazard assessment for additive manufacturing^[15] [15].

One of the more challenging and complex areas of AM are in the emerging field of gastronomy, or in other words, “3D Food printing” [4]. The ability to selectively deposit material within a 3D volume and, hence, gradate the composition offers the possibility of controlled production of complex structures for altering texture, taste, and morphology in food products. Manipulation of microstructures by regulating the mixing and selective deposition of materials can allow regulation of fracture, breakdown, or dissolution mechanics during product use, which gives the possibility of a range of functional and novel foods.

In the past few years, several reviews highlighted different aspects of 3D Food printing^[33] [33]. For example, an overview of current research in printable food formulations^[34][35] [34,35], the review of 3D printing techniques applied to design food materials^[36][37] [36,37], the potential legal challenges and environmental implications of 3D Food printing^[38] [38], a review on existing food 3D printers alongside their advantages/disadvantages^[39] [39], and a review on 3D model design, model building, and model slicing in 3D Food printing^[40] [40]. However, the field of printable food materials has been growing, and, therefore, there is a need to collate and categorise published reports and to consolidate these developments. As such, we can have a better understanding of the accomplishments to date, and potential areas for future studies.

2. Additive Manufacturing: Material and Methods

Various 3D printing processes have been reported to date^[41] [41]. A summarised timeline of the history of AM can be found in Jakus (2019)^[42] [42]. The American Society of Testing and Materials (ASTM F2792-12a) have categorised the existing technologies under the following seven headings: (1) Vat photo-polymerization, (2) material extrusion, (3) directed energy deposition, (4) powder bed fusion, (5) binder jetting, (6) material jetting, and (7) sheet lamination^[41][43] [41,43]. These can be further categorised by the characteristics of the printed medium (liquid-, solid-, and powder-based materials) and, by the process, used to fuse matter on a molecular scale (thermal, UV-light, laser, or electron beam)^[13] [13]. These are described briefly in Table S2. To differentiate themselves, various manufacturers often use different acronyms for describing the same process, and, therefore, similar techniques might have different names^[43] [43]. These have all been previously compared and reviewed comprehensively^{[17][44][45][46]} [17,44–46], and many of these techniques have been individually reviewed in high detail including powder-based electron beam additive manufacturing (EBAM) technology^[47] [47], and the Fused Filament Fabrication (FFF) process, also termed, Fused Deposition Modelling (FDM)^[48][49] [48,49].

Materials have long been a barrier to the broader uptake of AM in the manufacturing industry^[50] [50]. There are several comprehensive reviews of 3D printing techniques in terms of the materials utilised among them^{[9][45][51][52][53][54][55]} [9,45,51–55]. The material selection depends on the process and the physical state of the material^[13] [13] as well as on the phase transitions or chemical reactions to bind the layers together^[56][57] [56,57]. The basic materials in 3D printing include a wide range of plastics such as commonly used acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), and nylon. A handful of other materials, for example, cellulose plus its derivatives^[58] [58] and hydrogels^[59] [59], are also well established in AM and well accepted in some manufacturing applications. However, not many have reached the stage of full commercialisation.

Further progress in 3D printing will require the concurrent development of pioneering 3D printing methods in addition to novel feedstocks of material suitable for the devised processing methods^[51] [51].