Shape-Memory for Certificates

3D-printed shape-memory polymers enable the production of QR codes, e.g. to identify goods at risk of plagiarism.

Programmable Materials

Mechanics and Transport on Request

Giving materials new functionalities? We are finding ways to do that in various projects in the field of »Programmable Materials«. What we find is that not only the basic material itself is changed, but also its internal structures, mechanics or even transport.

New manufacturing methods make it possible to specifically produce structures in the micrometer range. Additive processes such as 3D printing are used for this purpose. An engineer uses these methods not only to design the outer shape, but also to target the internal microstructures to give certain properties to a component. In cooperation with other Fraunhofer Institutes, we go one step further and define multiple states for such microstructures and apply external stimuli to switch between them.

At Fraunhofer Cluster of Excellence »Programmable Materials«, we develop mathematical procedures for optimizing structures and provide support to engineers in designing production processes and choosing the appropriate microstructures. Working with the Fraunhofer Institute for Mechanics of Materials IWM, we develop the microstructures for 3D-printing, which can change the internal stiffness or the surface shape as desired when subjected to external mechanical stress. Specifically, we have achieved extraordinary mechanical effects that do not exist in the naturally occurring material.

 

Auxetische Zelle im Verbund
© Fraunhofer ITWM
Auxetische Zelle im Verbund.

Programmable Mechanics and a Tool Kit Full of Structural Possibilities

Examples of such material behavior can be found in metamaterials made of pentamode structures or auxetic cells. A structure of pentamode cells gives solids properties similar to those of liquids. Auxetic materials are known to expand in tension.  

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 output 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.

Designstudie für ein Metamaterial, aufgebaut aus verschiedenen Elementarzellen
© Fraunhofer ITWM
Designstudie für ein Metamaterial, aufgebaut aus verschiedenen Elementarzellen
Strömung durch eine virtuelle Membranstruktur
© Fraunhofer ITWM
Strömung durch eine virtuelle Membranstruktur
Querstromfiltration durch eine Membran mit Oberflächenstrukturierung
© Fraunhofer ITWM
Querstromfiltration durch eine Membran mit Oberflächenstrukturierung
Strömung in geschichteten laserstrukturierten Membranen
© Fraunhofer ITWM
Strömung in geschichteten laserstrukturierten Membranen

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.