Sustainable feedstocks for 4D printing: a review of biodegradable polymers and natural resources for stimuli-responsive manufacturing

Additive manufacturing (AM), alternatively known as 3D printing is an emerging technology supported by Industry 4.0. When combined with a stimulus-responsive behavior, decorated with the fourth dimension of time, it results in a manufacturing technique known as 4D printing. Although 4D printing technique is currently in its infancy stage, it has attracted exponential rise in attention in last 5 years. 4D printed entities have a distinctive characteristic of property transformation, under the influence of a drafted stimulus, which can be cleverly engineered for the desired application. Currently, 4D printed constructs have been implemented in renewables, textiles, electronics, biomedicals, agriculture, aerospace, purification, etc. and is aggressively growing. The conventional stimuli-driven smart printing inks deployed in 4D printing are non-biodegradable polymers that pose a defiance to sustainability. Hence, it is imperative to appraise the utilization of sustainable raw-materials, comprising of natural and synthetic biodegradable polymers, such as polylactic acid, polyvinyl alcohol, polycaprolactone, etc. as feedstocks for 4D printing. Natural resources, such as carbon, starch, cellulose, alginate, chitosan, collagen, etc. have fluctuating properties, that fortunately make them receptive toward intelligent engineering. This review is an effort towards the implementation of sustainable feedstocks as printing inks for 4D printing, for eventual environmental benignity. It incorporates several sustainable raw materials used for 4D printing and the strategies to use them in conjunction with conventional inks, in order to bring down the volume of non-biodegradables. This article would serve as a reference for designers and engineers wishing to practice sustainable inks for 4D printing, thereby boosting the momentum needed to consolidate this next-generation technology in-line with the sustainable development goals.