We introduce two essential building blocks with binary stiffness for mechanical digital machines. The large scale fully compliant mechanisms have rectilinear and rotational kinematics and use a new V-shaped negative stiffness structure to create two extreme states of stiffness by static balancing. The use of a mechanical bistable switch allows us to toggle between near-zero-stiffness and high-stiffness states, effectively turning off and on stiffness. A stiffness reduction of 98.8% and 99.9% is achieved for linear and rotary motion over a range of 13.3% (20mm) and 0.4rad (23∘) respectively. Stiffness states can be reversibly changed by toggling the mechanical switch, or irreversibly by actuating the main stage. These binary stiffness mechanisms could set the stage for a new type of mechanical logic, adaptive and programmable metamaterials and other types of digital mechanical devices. Practical mechanical digital machines and materials require miniaturized and easily micro-manufactured components. We have therefore carefully considered scalability by integrating all required structures into a planar and monolithic architecture. This allows miniaturization and fabrication with conventional surface-micro-machining and additive manufacturing such as photolithography, two-photon lithography and fused deposition modeling.

Stiffness in compliant mechanisms can be dramatically altered and even eliminated entirely by using static balancing. This requires elastic energy to be inserted before operation, which is most often done with an additional device or preloading assembly. Adding such devices contrasts starkly with primary motivations for using compliant mechanisms, such as part count reduction, increased precision, and miniaturization. However, statically balanced compliant mechanisms with a fully monolithic architecture are scarce. In this article, we introduce two novel statically balanced compliant mechanisms with linear and rotary kinematics that do not require preloading assembly, enabling miniaturization. Static balance is achieved by the principle of opposing constant force and extended to a rotational device by using opposing constant torque mechanisms for the first time. A constant force mechanism based on existing work is used and inspired a novel constant torque mechanism. A single-piece device is obtained by monolithically integrating a bistable switch for preloading, which allows static balance to be turned on and off. The linear device reduces stiffness by 98.5% over 10 mm, has significantly reduced device complexity and has doubled relative range of motion from 3.3% to 6.6% compared to the state of the art. The rotary device reduces stiffness by 90.5% over 0.35 rad.

A great deal of engineering effort is focused on changing mechanical material properties by creating microstructural architectures instead of modifying chemical composition. This results in meta-materials, which can exhibit properties not found in natural materials and can be tuned to the needs of the user. To change Poisson's ratio and Young's modulus, many current designs exploit mechanisms and hinges to obtain the desired behavior. However, this can lead to nonlinear material properties and anisotropy, especially for large strains. In this work, we propose a new material design that makes use of curved leaf springs in a planar lattice. First, analytical ideal springs are employed to establish sufficient conditions for linear elasticity, isotropy, and a zero Poisson's ratio. Additionally, Young's modulus is directly related to the spring stiffness. Second, a design method from the literature is employed to obtain a spring, closely matching the desired properties. Next, numerical simulations of larger lattices show that the expectations hold, and a feasible material design is presented with an in-plane Young's modulus error of only 2% and Poisson's ratio of 2.78×10^-3. These properties are isotropic and linear up to compressive and tensile strains of 0.12. The manufacturability and validity of the numerical model is shown by a prototype.