Sustainable feedstocks for 4D printing: biodegradable polymers and natural resources

Additive manufacturing is an emerging technology supported by ‘Industry 4.0’. When combined with a stimulus-responsive behavior, incorporating the fourth dimension of time, it results in a manufacturing technique known as four-dimensional (4D) printing. 4D-printed entities have the distinctive characteristic of property transformation, under the influence of a drafted stimulus, which can be cleverly engineered for the desired application. The conventional stimulus-driven smart printing inks deployed in 4D printing are non-biodegradable polymers that pose a challenge to sustainability. Hence, it is imperative to appraise the utilization of sustainable raw materials, composed of natural and synthetic biodegradable polymers, as feedstocks for 4D printing. Natural resources have fluctuating properties, which make them receptive toward intelligent engineering. This review is an effort toward the implementation of sustainable feedstocks as printing inks for 4D printing, for eventual environmental benignity. It covers several sustainable raw materials 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 review can serve as a reference for designers and engineers wishing to use sustainable inks for 4D printing, thereby boosting the momentum needed to consolidate this next-generation technology in line with the UN Sustainable Development Goals.