Introduction

Kinetic architecture can be intended as a response to technological, economic and social changes and its role is extended to the creation of architectural artefacts which are able to adapt according to various human needs and environmental conditions [1]. Speculative projects have vastly promoted the potential of adaptable spaces, but the construction sector has generally rejected to embrace and diffuse this approach. The introduction of kinetic devices in the building sector is considered complex and it requires the use of mechanical interfaces which are hardly manageable by architects and contractors. This research suggests a possible direction towards the diffusion of soft-kinetic architectural systems with the use of a special category of structures, the auxetics, that have an intrinsic capacity to be expanded and compressed due to their structure. The paper describes how this special class of structures can be materialized through 3D printing to generate a class of architectural systems with inherent kinetic properties. Computational design methods, behavioral simulations and development of prototypes are investigated and discussed.

Auxetic structures are known among scientists for about a century, but only in the last decades they gained more attention. One of the earliest publications on the topic was in 1987,  in the journal Science by R.S. Lakes, but the term auxetic started being used not earlier than 1991. Auxetics are materials that have a negative Poisson's ratio (v), which is defined as the ratio of the lateral contractile strain to the longitudinal tensile strain for a material undergoing tension in the longitudinal direction [2]. Generally, materials have positive Poisson’s ratios, that is, stretching is expected to make a material thinner and compressing results in bulge. However, this common knowledge has been challenged by the auxetic structures which exhibit the very unusual property of becoming wider when stretched and narrower when compressed [3]. This occurs due to their hinge-like structures, which flex outwards when stretched. Their auxetic properties are not given merely by their material composition, but rather by their organization in specific-shaped repetitive modules which directly determines the behaviour, thus opening interesting perspectives for its computational development. A large number of auxetic polymers have been developed in the form of foam, fiber, or composite materials and many others can be found in nature. Auxetic structures can be found both at the macroscopic (our field of study) and at the microstructural level, and in some cases even at the mesoscopic and molecular levels [2] .

Currently, applications of auxetic materials are used in the biomedical industry in the creation of filters for chemical processes (auxetic foam, honeycomb), in the generation of auxetic fibers (crash helmets, body armour, sports clothing), as well as in the production of panels with defined characteristics like energy/vibrations absorption but still able to bear high compressive forces. Nevertheless, due to their complex structure, auxetics are difficult to be produced with traditional processes and their application is restrained to high-tech fields. While in medical and chemical sectors the auxetics are mainly used for their capacity to change porosity, in engineering and in sports applications they are mainly used for their static properties.

This research takes into consideration two properties of the auxetics that are strictly connected to the implementation of kinetic movable architectural interfaces. The first one, derives from the relation between Poisson’s Ratio v and shear modulus G and represents the fact that when v decreases to -1, G becomes very high, so that the material becomes difficult to shear but easy to deform volumetrically [4]. The second characteristic, fundamental for this research, is the tendency of auxetic planar structures to create a synclastic curvature when subjected to an out-of-plane bending moment, a behavior opposite to the one of conventional elastic materials which display anticlastic curvatures when bended. Among the multiple typologies of auxetic patterns that have been studied in the last decades, our investigation focuses on the chiral lattice, a structural network composed of rings connected by tangent ligaments [5]. The relation between the geometrical configuration and the physical behaviour of chiral lattice has been pointed out and analyzed in various studies [5,6,7,8]. In this research these behaviours are simulated with the use of parametric software. After the aforementioned characteristics have been exploited, the prototypical fabrication can be initiated, generating a prestressed structure, producible “on plane” with a FDM (Fused Deposition Modelling) printer. Afterwards, the structure would be subdued to an out-of-plane bending moment is capable to create a synclastic self-bearing shell, afterwards revertible to the original planar configuration. Following the definition of H. Engel, the final system could be defined as a form-active structure [9].

Methodological Procedures

The research is organized in three main parts: parametric design, prototyping, and system integration of the auxetics.In the first part, a design methodology is developed through parametric modelling to define the best auxetic structure (among the 2D typologies of re-entrant structures classified by [4,10]) capable to perform a synclastic curvature. In this phase the embedded kinematic and differential behavior along axis are simulated through the use of particle spring systems like Kangaroo for Grasshopper and Finite Element Method like Abaqus. This method permits to preview different configurations and investigate several design options. The research aims at exploring and defining the pattern which exhibits the most enhanced synclastic curvature, implementing it in the successive phases of the project.

In the second part the auxetic structure is prototyped through additive manufacturing, avoiding traditional material processing in the fabrication of auxetics that usually consists of a multi-step process involving the complex design of molds and heat compression passages. To investigate the auxetic properties in relation to different materials a double FDM extruder is employed. This allows for the use of two different filaments within the same printing session, leaving wider possibilities for the creation of objects with non-static behaviours. Rigid materials like ABS or Nylon are used for the resistant parts whereas the joints will be printed in Flexmark (TreeD Filaments) to permit movement. Importance is given to the research of the materials with specifications which allow their use during the same printing session. In order to mediate between transversal rigidity and out-of-plane curving, the two types of materials are modulated in quantity and disposition. In this phase the correspondence between the digital simulation and the actual behavior of the printed patterns is tested, a re-iterating process of correction of the simulation model and of the printed prototype is set. A final material setup is done with the application of Layfoom filament (Filabot) which demonstrates a unique material property called Viscoelasticity, capable of both viscous and elastic characteristics when it is reshaped or deformed. The specific characteristics that the designed object needs are satisfied by changing the material of the extrusion during the same printing session, thus creating a structure with kinetic characteristics that are not caused by the union of multiple parts with specific behaviours joined after the production but that are embedded in the produced object itself. In order to obtain a final product capable to react autonomously to the environment, we are taking into consideration the possibility of utilize smart 3D printable materials in the creation of the joints or the rods. The capacity of smart materials (Electro-active Polymers in this case) to deform in specific conditions can be exploited to make the structure grow when needed.

Results

A method to design auxetic patterns is developed through parametric tools: this involves the definition of the most suitable technique to simulate the expanding kinematics of these structures. Once this method has been developed, basic structures are implemented with variable parameters to achieve a-periodic patterns able to generate differential responses in behaviour. In this sense, the developed method will allow the exploration of several patterns, the tuning of their performance, and the prediction of their behaviors. Different material configurations and printing techniques are tested and analyzed. Combination of materials are studied in order to favour the specific ability of these patterns to expand: different flex materials are studied, as well as electro-active polymers (EAP). A matrix of printed results is analyzed according to their resistance. Finally the patterns will be applied to the design experiment of an architectural cladding system able to move and expand to specific configurations through electric stimulations.

Discussion

The exploratory character of the research has the intention to stimulate discussion on the possible applications of auxetics for architecture. Considering the unexplored field, the paper attempts at finding a method to design and control the characteristics of these structures. This research suggests auxetics patterns are ideal for kinetic architecture and applications are provided, moreover attempting at describing possible fabrication procedures. Auxetics are surely a promising field of research for self-computing kinetic architecture, and the paper attempts at defining development perspectives in the construction sector.