4D printing of natural materials: A review

Abstract

4D printing involves the creation of dynamic structures that can change their properties, shape, or functionality over time in response to specific external stimuli like temperature, pH, humidity, and light. By combining the capacity of 3D printing to produce complex architectures with the realm of smart materials, 4D printing enables the design and manufacturing of objects capable of interacting with their environment. Despite its potential, the field of 4D printing is predominantly reliant on synthetic responsive materials, with limited advancements towards more eco-friendly alternatives. Additionally, while many 4D structures draw inspiration from nature, the integration of bio-based or biodegradable materials remains relatively uncommon. Aiming to draw the 4D printing community's attention to the need for sustainable smart materials, this review highlights the use of natural materials in 4D printing as alternatives to conventional synthetic ones. First, an introduction to 4D printing technology and its foundational elements is provided. Next, the natural materials that have been printed are detailed, including wood, natural fibers (flax and ramie), cellulose, lignin, plant oils (limonene, linalool, and soybean oil), starch, alginate, agarose, silk, chitosan, keratin, gelatin, pollen, proteins (bovine serum albumin), and xanthan gum, along with their biocomposites rheological, mechanical, and responsive properties. Finally, current applications, challenges, and future perspectives for green materials in 4D printing are discussed, contributing to the advancement of this emergent research field.