Programmable Materials

These materials consist of three-dimensional arrays of cubic cells. Each cell exhibits a non-linear mechanical behavior and multiple stable states. Such cells are also called unit cells and are developed, printed, and tested at Fraunhofer IWM.

There are many different ways to design these cells. One important type is auxetic cells, which expand under tension in a direction orthogonal to this. This means that they become thicker when stretched and thinner when compressed.

Algorithm for Unit Cells

The arrangement of thousands of cells into a unit offers even more options for the design of programmable materials. For this purpose, we develop software that generates a possible selection and arrangement of cells depending on the requirements. In this software we use methods from numerical optimization. The optimization goal is to adapt a given component to a desired deformation or loading behavior.

We developed a graphical user interface for the design of programmable materials made from these unit cells – similar to the CAD software used in architecture. In addition, we provide a database where all necessary information about unit cells can be found. At the end of the optimization process, the calculated structures are directly forwarded as input for 3D printing.

Programmable Transport: Clean Filters with Smart Materials

Together with the Fraunhofer Institute for Applied Polymer Research IAP, we are developing filter membranes. In this case, the focus is on the use of programmable materials that can change their properties as a result of external stimuli, in particular, in the area of effective filter cleaning.

These membranes are made from thermally activated shape memory polymers, with or without a porous structure, that can change form at the time of cleaning and make the process more effective. Shape memory polymers are polymers that seem able to »remember« their previous form. The project also studies membranes with additional surface structuring for applications with cross-flow filtration. Such structuring can delay the fouling process during the filtration phase, for example, by keeping away bacteria from the membrane. Another kind is the chemo-selective membranes, where permeability can change depending on the presence of certain chemicals. This effect is used to block pollutants. In all cases, we assist project partners with simulations to support their development efforts.

Adaptive Filtration Using Membrane Structure

The »Programmable Materials in Science and Engineering (ProMiSE)« project is a collaborative project with other Fraunhofer Institutes, with a research focus on new programmable materials, specifically, »programmable porosity«.

As possible trigger mechanisms, we are exploring piezoelectrical and thermo-mechanical effects. The aim is to achieve a deformation of the pore geometry on the micro-scale depending on these effects and thus to change the material permeability (porosity). This ability can be used in adaptive filtration, for example, in water treatment or chemical processes.

The modeling and simulation of the piezoelectrical effects poses a challenge. These methods describe the changes in electrical polarization and show the presence of electrical current in solids under conditions of elastic deformation. The expansion and orientation of the polymer must be mechanically modeled on a continuum scale. We then compare the effects of differently structured pore geometries. Project partner Fraunhofer Institute for Applied Optics and Precision Engineering IOF, produced the necessary membrane geometries using laser irradiation. The required adaptive filtration is achieved through deliberate deformation.

Together with other institutes of the Fraunhofer Society, we are working on making programmable materials usable for medical technology. In the »ProFi – Programmable Multistable Finger« project, we are developing a finger-like joint structure that is made entirely from a single material, without additional screws or mechanical components. The finger can then assume four stable deformation states, for example, stretched or bent.

The basis for this are so-called bistable unit cells. These are tiny, repeating components in the material that can assume two stable states – similar to a snap mechanism that jumps between two positions and remains there without expending energy. We also use such metastructures in other projects. With our »ProgMatCode« software for structure optimization, we design these unit cells to meet the mechanical requirements of a prosthesis – for example, in terms of force, movement sequences, or stability. This enables us to achieve functions that previously could only be realized with complex mechanical joints.

Division of tasks in the project: Bistable structures for prostheses

• Fraunhofer LBF developed the basic structure of the joint based on a metamaterial that was originally designed for elbow replacements. It allows movement around only one axis while blocking lateral rotation, similar to a natural joint. To ensure that this structure also works on a smaller scale, the researchers adapted the geometry and optimized it using numerical simulations such as FEM analyses. This enabled them to achieve stable bending of up to 90 degrees while reducing material stresses.
• The Fraunhofer IWM developed the corresponding bistable unit cells – the central element for programmable behavior. These elastic beam structures have two stable states and can switch between them depending on the external impulse.
• Finally, the finger was additively manufactured at Fraunhofer IAP – using 3D printing and as a completely assembly-free component. The shape can be individually customized and modeled realistically without the need for subsequent screwing or assembly.
• We at Fraunhofer ITWM contributed our expertise in mathematical modeling and optimization: We use our software solution “ProgMatCode” to analyze and optimize the microstructures digitally.

This approach demonstrates the potential of programmable materials: through the targeted design of microstructures, we can create functions that natural materials do not possess, thereby opening up new avenues for lightweight, customizable, and low-maintenance medical technology.