5 - 4D-printed stimuli-responsive hydrogels modeling and fabrication

Abstract

Among several materials that can be processed to present a shape-morphing effect (four-dimensional (4D) printing), hydrogels have gained their importance due to the ability to morph their shape reversibly and because it is possible to control and predict this behavior. Moreover, this type of abiotic material can be printed together with biotic ones, such as cells in what is generally designated as bioprinting. To induce the shape-morphing property associated with the 4D effect, several trigger stimuli such as humidity, temperature, pH, ionic strength and concentration, electric field, and light can be applied to induce these changes. The tailoring of the shape change can be defined by inducing different swelling and crosslinking degrees in different parts of the printed hydrogel. Consequently, it is possible to model the final part/component to define where and how the shape change will occur and in which direction. 4D-printed hydrogels can be prepared with only one precursor hydrogel formulation or in multi-material mode. Light-based and extrusion-based techniques are the processing technologies most often referred to as fabricating these constructs. This chapter presents an overview of the state of the art of 4D-printed hydrogels and their applications from a materials science point of view, focusing on the formulation composition and chemistry-related events behind the shape transition behavior. The text is organized into different sections that intend to discuss the most common formulation design strategies and fabrication techniques to establish a comparison between materials according to distinct insights quickly. The reversibility of the shape change and degree of recovery will also be discussed.