First introduced in 2013, it has since received great interest within material science showing potential for application within the fields of soft robotics, defence and manufacturing, among others. Fabricating 4D structures for use in tissue engineering and drug delivery systems provides a promising prospective technology for future generations and hence this review will focus on biomedical applications. A development of 3D printing called 4D food printing enables the personalization of fresh food items. Smart design and smart materials, which allow 4D printed things to change shape or function, are the main differences between 4D and 3D printing. 4D printing can achieve self-assembly, self-repair and multi-functional purposes, which provides time-dependent, predictable, reprogrammable properties.
4D printed food provides new ideas for the design of interactive food and enhances the interaction between diners and food materials. Another advantage of 4D printing over 3D printing is that the flavour, nutrition or colour of the printed product can be released when the user eats it rather than during storage. In contrast, these sensory or nutritional properties of 3D printed products decrease during storage.
Drying 3D printed food is not recommended, because drying causes unwanted and irregular deformation, reducing the usefulness of 3D printing. 4D printing is useful for controlling structure changes during drying, which is used to produce some healthy snacks. The curvature of potato chips, for example, contributes to their crispier mouth feel. The combination of 4D printing and drying results in crisp-textured products.
As printing technology advances and becomes more popular, it can be integrated into the manufacturing process to achieve automatic production. The concept of “flat packaging” is very compatible with 4D food printing. Swedish furniture company pioneered the concept of “flat packaging”, which allows people to assemble 2D furniture into 3D furniture in their own homes. This concept is well suited to deformed food prepared using 4D printing, which reduces transportation and storage costs. Fused Deposition Modelling (FDM), stereolithography, direct ink writing, inkjet, digital light processing and select laser melting are the most common printer types used in 3D food printing. FDM is currently the most widely used technology for 4D food printing, owing to its high universality, low cost and ease of operation. FDM is classified into Cartesian, Delta, Polar and Scara displacement platform configurations, with the Cartesian configuration being the most widely used.
The idea of four-dimensional printing is primarily reliant on five elements: three-dimensional printers or other related machinery, stimulus-responsive materials, stimuli, interaction mechanisms and mathematical modelling. Food produced using four-dimensional printing will be more individualized and have distinctive flavours developed specifically for it. The stimulus-induced changes in the 4D printed food samples can be achieved by using the printing ink in different combinations of food materials based on the structure and specific food formula. In the recent past, a large number of food materials have been developed by using printing technology. Food materials used in food ink production include soybean, chocolate, starch fruit and vegetables, meat, food hydrocolloids, etc. The efficacy of their printing depends significantly on the moisture content of the food ingredient employed as food ink. In order to preserve the nutritional content and usable qualities of food components, they can be used in dried and powdered form.
The research scale of 4D food printing in recent years is rapidly expanding and natural food gels suitable for printing are gradually being developed. But there are still relatively few foodstuff systems that can be used for 4D food printing, due to the fact that natural foodstuff systems generally cannot be printed directly. However, an essential condition for the rapid development of 4D food printing technology is that it has a wide variety of printing materials and good printing characteristics. Food printing can alter the nutritional content of food based on the need of consumers. It can be achieved by the introduction of healthy ingredients such as plant chemicals, cellulose and high-quality proteins and the reduction of adverse substances such as allergens and anti-nutritional factors. The nutritional value of printed food will increase as ink and tissue are replaced with food-grade materials in 4D biotechnological printing.
At present, 4D food printing experiment is mainly based on soy protein isolate, starch and hydrogels and is realized through material properties, internal structural design and spatial arrangement of recombined food materials. The stimulation factors responsible for causing alteration of 3D printed products include pH, water absorption, microwave and temperature, resulting in deformation, modification of colour, flavour and nutritional value.
It is noteworthy that 4D food printing is still at research stage. There are opportunities for enhancement in the link between internal structure and stimulation, design of the printer, printing software, printing materials as well as evaluation of the properties of the printed foods. The process of 4D printing gives the production of food a wider view of complicated geometry and shapes in a comparatively shorter period. 4D printing in the food industry is still quite some time away. However, it is definitely an area to watch. As much as it is a novelty, it may become something very important to us in the future.