The Design of 4D‐Printed Hygromorphs: State‐of‐the‐Art and Future Challenges

Abstract

In recent years, 4D printing has allowed the rapid development of new concepts of multifunctional/adaptive structures. The 4D printing technology makes it possible to generate new shapes and/or property-changing capabilities by combining smart materials, multiphysics stimuli, and additive manufacturing. Hygromorphs constitute a specific class of new smart materials where their properties and morphing capabilities are dependent on the surrounding humidity, which drives actuation. Although multiple efforts have been made to fabricate hygromorph demonstrators, a comprehensive design process to produce hygromorphs by multiple 4D printing techniques is not yet available. The broad aim of this review and concept paper is to i) highlight existing scientific and technology gaps in the field of 4D-printed hygromorphs, ii) identify tools existing in other research fields for filling those gaps, and iii) discuss a series of guidelines for tackling future challenges and opportunities to develop 4D-printed composite hygromorph materials and related manufacturing processes. Accordingly, this review describes the materials and additive manufacturing techniques used for hygromorph composite fabrication. Moreover, the relevant parameters that control actuation, the models selection and performance, the design methods and the actuation measurements for customized 4D-printed hygromorph materials, are discussed.

4D‐Printed Soft and Stretchable Self‐Folding Cuff Electrodes for Small‐Nerve Interfacing

“…[ 40 ] The use of active materials, which deform under certain conditions, is commonly referred to as 4D printing. [ 41–43 ] Thus, we call the self‐folding electrodes “4D electrodes”. Our devices consist of a bilayer structure, comprising of a non‐swelling flexible material and a highly swelling hydrogel on its backside as the substrate.…”

Human–Material Interaction Enabled by Fused Filament Fabrication 4D Printing

“…By harnessing the power of humidity, these products can offer enhanced user experiences, improved sustainability, and a broader range of applications in various industries. [165] The main focus of 4DP is to integrate a specific morphing function by controlling the material properties and architecture. Le Duigou et al [166] looked into the possibility of customizing continuous flax-fiber (CFF) 4DP for hygromorph biocomposite (HBC), opening up possibilities for creating environmentally friendly metamaterials that could change shape in a sequential manner.…”

“…By harnessing the power of humidity, these products can offer enhanced user experiences, improved sustainability, and a broader range of applications in various industries. [ 165 ]…”

“…[ 166 ] However, there are substantial challenges associated with material limitations, such as printability, responsiveness, and mechanical properties, which hinder the achievement of reliable and repeatable humidity‐responsive actuation. [ 165,172 ] The promising results of this study present new opportunities for leveraging 4DP and natural resources to develop functional humidity‐responsive smart structures. [ 173 ] By overcoming material limitations through the codesign approach and incorporating cellulose‐filled filaments, these structures showcase their potential for practical applications as end‐user products.…”

Recent Advances in the Additive Manufacturing of Stimuli‐Responsive Soft Polymers

“…Stimuli-responsive materials are programmable materials, which exhibit the potential to change their geometries from temporary states to original states in response to an external stimulation such as moisture, light, heat, pH, electric field, and magnetic field. [83][84][85][86] These materials exhibit a variety of features such as self-folding, self-assembly, and self-adaptability. [87] In addition, self-healing properties can be easily induced in soft responsible materials.…”