Most materials around us have properties that are determined at extremely small scales. Often the atoms and molecules that make up these materials determine how they behave under load. While this already leads to a great variety of material properties, a lot more is possible. Mechanical metamaterials use structure to extend the available range of material properties. In this way, we can design material properties that are not found in nature. An example of this is materials with a negative Poisson’s ratio. When these materials are compressed, they will not expand in the direction perpendicular to the applied deformation, as we would expect from natural materials. Rather, they will contract. In general, mechanical metamaterials allow us to design materials with properties that are tailored to their intended solution. This provides more design freedom; instead of choosing from a list of available materials, the material properties themselves now become variables that can be designed. Additionally, this makes it possible to integrate multiple functions within a material. In this way, the material is no longer passive but can react based on applied forces, deformations, or changes in the environment. In practice, designing mechanical metamaterials has turned out to be difficult. While there are many examples of mechanical metamaterials with exceptional properties, their discovery has rarely been based on a rational and structured design process. The lack of such a design strategy makes the design of metamaterials with exactly the desired properties, at least for now, difficult and unreliable. This dissertation explores this design problem and presents a method to aid in the structured and rational design of mechanical metamaterials. This method is based on a pseudo-rigid body approach, borrowed from the field of compliant mechanisms. Following this approach, the metamaterial is modeled as a collection of rigid parts, which are connected by joints to which we assign a stiffness. This allows us to model both the deformation and the stiffness of the material while keeping the complexity of the models as low as possible. Because of the limited complexity of the models, this approach allows the designer to understand the effects of design decisions and adaptations. This enables directed and conscious changes to the design, of which the consequences are known beforehand. This is different from alternative methods where highly complex and time-consuming computer models are used to calculate the effects of changes. By using less complex models and making directed choices, new design iterations can be generated more quickly. Especially at the start of a design process, this is expected to quickly lead to new insights. These can then at a later stadium be refined using more detailed methods.@en

The SARS-CoV-2 pandemic resulted in shortages of production and test capacity of FFP2-respirators. Such facemasks are required to be worn by healthcare professionals when performing aerosol-generating procedures on COVID-19 patients. In response to the high demand and short supply, we designed three models of facemasks that are suitable for local production. As these facemasks should meet the requirements of an FFP2-certified facemask, the newly-designed facemasks were tested on the filtration efficiency of the filter material, inward leakage, and breathing resistance with custom-made experimental setups. In these tests, the facemasks were benchmarked against a commercial FFP2 facemask. The filtration efficiency of the facemask’s filter material was also tested with coronavirus-loaded aerosols under physiologically relevant conditions. This multidisciplinary effort resulted in the design and production of facemasks that meet the FFP2 requirements, and which can be produced at local production facilities.

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Auxetic behavior refers to lateral widening upon stretching or, in reverse, lateral shrinking upon compression. When an initially auxetic structure is actuated by compression or extension, it will not necessarily remain auxetic for larger deformations. In this paper, we investigate the auxetic range in the deformation of a periodic framework with one degree of freedom. We use geometric criteria to identify the interval where the deformation is auxetic and validate these theoretical findings with compression experiments on sample structures with (Formula presented.) unit cells.

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Introduction: The current COVID-19 pandemic has caused large shortages in personal protective equipment, leading to hospitals buying their supplies from alternative suppliers or even reusing single-use items. Equipment from these alternative sources first needs to be tested to ensure that they properly protect the clinicians that depend on them. This work demonstrates a test suite for protective face masks that can be realized rapidly and cost effectively, using mainly off-the-shelf as well as 3D printing components. Materials and Methods: The proposed test suite was designed and evaluated in order to assess its safety and proper functioning according to the criteria that are stated in the European standard norm EN149:2001+A1 7. These include a breathing resistance test, a CO₂ build-up test, and a penetration test. Measurements were performed for a variety of commercially available protective face masks for validation. Results: The results obtained with the rapidly deployable test suite agree with conventional test methods, demonstrating that this setup can be used to assess the filtering properties of protective masks when conventional equipment is not available. Discussion: The presented test suite can serve as a starting point for the rapid deployment of more testing facilities for respiratory protective equipment. This could greatly increase the testing capacity and ultimately improve the safety of healthcare workers battling the COVID-19 pandemic.

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In this paper, a pseudo-rigid body model is proposed for the analysis of a spatial mechanical metamaterial and its application is demonstrated. Using this model, the post-buckling behavior of the mechanical metamaterial can be determined without the need to consider the whole elastic structure, e.g., using finite-element procedures. This is done by analyzing a porous cylindrical mechanical metamaterial using a rigid body mechanism, consisting of rigid squares that are connected at their corners. Stiffness in this model comes from torsion springs placed at the connections between rigid parts. The theory of the model is presented and the results of two versions of this model are compared through experiments. One version describes the metamaterial in the free state, while the other, more extended, version includes clamped boundaries, matching the conditions of the experimental set-up. It is shown that the mechanical behavior of the spatial metamaterial is captured by the models and that the shape of the metamaterial in the deformed state can be obtained from the more extended model.@en

Dilational structures can change in size without changing their shape. Current dilational designs are only suitable for specific shapes or curvatures and often require parts of the structure to move perpendicular to the dilational surface, thereby occupying part of the enclosed volume. Here, we present a general method for creating dilational structures from arbitrary surfaces (2-manifolds with or without boundary), where all motions are tangent to the described surface. The method consists of triangulating the target curved surface and replacing each of the triangular faces by pantograph mechanisms according to a tiling algorithm that avoids collisions between neighboring pantographs. Following this algorithm, any surface can be made to mechanically dilate and could, theoretically, scale from the fully expanded configuration down to a single point. We illustrate the method with three examples of increasing complexity and varying Gaussian curvature.@en

Poisson’s ratio is one of the most studied material proper- ties that can be designed in mechanical metamaterials. However, in most studies so far, Poisson’s ratio is not constant for larger compressions. Only for structures in which ν = −1, structures with a constant Poisson’s ratio have been demonstrated. This paper studies the design of planar mechanical metamaterials with a constant Poisson’s ratio based on the pantograph, inversor, straight-line and parabolograph mechanisms. Using these classical mechanisms as building blocks, periodic mechanisms with 0 and 1 are proposed.

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Most currently existing mechanical metamaterial designs are based on Bravais lattices. These consist of parallelogram or parallelepiped unit cells, which are respectively translated along two or three independent vectors to fill the complete space. This approach is inherently unable to match curved surfaces like spheres, since these cannot be constructed only from parallel and perpendicular lines. In this paper, we introduce a generalized unit cell, based on the symmetry groups of the sphere. We use this approach to develop a spherical transformable origami-inspired metamaterial. We describe the motions of this new metamaterial, as well as experimental observations on a physical, 3D printed model.

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Dilational structures are one degree of freedom structures able to change their size without changing their global shape. In this paper, we present a method to create dilational shells with arbitrary curvature. For this, we designed triangular tiles, which can be placed on a triangulation of the desired surface. We present the method and illustrate it with the example of a dilational octahedron. Using this regular polygon, we demonstrate that the whole structure has a single degree of freedom, and that the maximum obtainable scaling factor is directly linked to the range of motion of the individual triangular tiles.

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