
The Airshell Prototype
This paper by Alessandro Liuti, Sofia Colabella, and Alberto Pugnale, presents the construction of Airshell, a small timber gridshell prototype erected by employing a pneumatic formwork.
Shape-shifting Structured Lattices via Multimaterial 4D Printing
Shape-morphing structured materials have the ability to transform a range of applications. However, their design and fabrication remain challenging due to the difficulty of controlling the underlying metric tensor in space and time. Here, we exploit a combination of multiple materials, geometry, and 4-dimensional (4D) printing to create structured heterogeneous lattices that overcome this problem.
Our printable inks are composed of elastomeric matrices with tunable cross-link density and anisotropic filler that enable precise control of their elastic modulus (E) and coefficient of thermal expansion (α). The inks are printed in the form of lattices with curved bilayer ribs whose geometry is individually programmed to achieve local control over the metric tensor.
For independent control of extrinsic curvature, we created multiplexed bilayer ribs composed of 4 materials, which enables us to encode a wide range of 3-dimensional (3D) shape changes in response to temperature. As exemplars, we designed and printed planar lattices that morph into frequency-shifting antennae and a human face, demonstrating functionality and geometric complexity, respectively.
Our inverse geometric design and multimaterial 4D printing method can be readily extended to other stimuli-responsive materials and different 2-dimensional (2D) and 3D cell designs to create scalable, reversible, shape-shifting structures with unprecedented complexity.
Shape-morphing structured systems are increasingly seen in a range of applications from deployable systems and dynamic optics to soft robotics and frequency-shifting antennae, and they have led to numerous advances in their design and fabrication using various 3-dimensional (3D) and 4-dimensional (4D) printing techniques.
However, to truly unleash the potential of these methods, we need to be able to program arbitrary shapes in 3 dimensions (i.e., control the metric tensor at every point in space and time), thus defining how lengths and angles change everywhere. For thin sheets, with in-plane dimensions that are much larger than the thickness, this is mathematically equivalent to specifying the first and second fundamental forms of the middle surface.
These quadratic forms describe the relation between material points in the tangent plane and the embedding of the middle surface in 3 dimensions and thus, control both the intrinsic and extrinsic curvature of the resulting surface. From a physical perspective, arbitrary control of the shape of a sheet requires the design of material systems that can expand or contract in response to stimuli, such as temperature, humidity, pH, etc., with the capacity to generate and control large in-plane growth gradients combined with differential growth through the sheet thickness.
Such systems are difficult to achieve experimentally; hence, most current shape-shifting structures solutions rarely offer independent control of mean and Gaussian curvatures. We address this challenge by 4D printing a lattice design composed of multiple materials.
Beginning at the material level, we created printable inks based on a poly(dimethylsiloxane) (PDMS) matrix, an elastomeric thermoset that exhibits a large operating temperature window and a high thermal expansion coefficient. Although the inks are printed at room temperature, the broad range of polymerization temperatures for PDMS enables us to cure the resulting structures at much higher temperatures.
On cooling to room temperature, these cured matrices achieve maximal contraction, hence transforming into their deployed states. The same base elastomer is used in all inks to facilitate molecular bonding between adjacent ribs and layers. To create inks with reduced thermal expansivity, we fill the elastomer matrix with short glass fibers (20% wt/wt) that preferentially shear align along the print path.
Thin shape-shifting structures are often limited in their ability to morph into complex and doubly curved shapes. Such transformations require both large in-plane expansion or contraction gradients and control over extrinsic curvature, which are hard to achieve with single materials arranged in simple architectures.
We solve this problem by 4-dimensional printing of multiple materials in heterogeneous lattice designs. Our material system provides a platform that achieves in-plane growth and out-of-plane curvature control for 4-material bilayer ribs. The lattice design converts this into large growth gradients, which lead to complex, predictable 3-dimensional (3D) shape changes. We demonstrate this approach with a hemispherical antenna that shifts resonant frequency as it changes shape and a flat lattice that transforms into a 3D human face.
This paper by Alessandro Liuti, Sofia Colabella, and Alberto Pugnale, presents the construction of Airshell, a small timber gridshell prototype erected by employing a pneumatic formwork.
In this paper by Gregory Charles Quinn, Chris J K Williams, and Christoph Gengnagel, a detailed comparison is carried out between established as well as novel erection methods for strained grid shells by means of FE simulations and a 3D-scanned scaled physical model in order to evaluate key performance criteria such as bending stresses during erection and the distance between shell nodes and their spatial target geometry.
In this paper by Frederic Tayeb, Olivier Baverel, Jean-François Caron, Lionel du Peloux, ductility aspects of a light-weight composite gridshell are developed.
In this paper by Julian Lienhard, Holger Alpermann, Christoph Gengnagel and Jan Knippers structures that actively use bending as a self forming process are reviewed.
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