Programmable shape changing and mechanics

Motivation

 

Methods for optimizing the topology of the outer form of components are firmly established in industrial practice. For the past few years, the importance of the so-called free material optimization and multi-scale methods has been increasing. Due to the rapid development of additive production technologies such as 3D printing, a designing process using them has in the meantime become possible, which optimizes the outer form parallel to the local internal (micro)structure of the material. This leads to materials whose internal structure fulfills the required system functions. To distinguish these mechanical metamaterials from normal mechanical structures, the applied load or the externally received signal does not dissolve the internal structure, but perceives the metamaterial as a quasi-homogeneous material. Under predefined loads, these materials show complex reactions and can therefore respond as a technical system.

Such programmable system materials that allow variation and switching of the mechanical properties by an external trigger (e.g. external load, temperature change) are preferable in many applications. It is practical to use adaptive materials particularly when there are severe spatial limitations such as in space travel or in case of special requirements as regards the individuality of a product.

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Research topic 

 

The development process of programmable materials requires a paradigm change, because the internal structure of the material and its properties must be adapted specifically to the application and not vice-versa. In order to design programmable materials, the internal structure of the materials must be predicted and produced at various locations in the material such that the different areas show certain elastic properties in combination, some of which are not available in nature, such as a negative Poisson’s ratio or an adjustable E-module. In many cases, the optimum design depends on the load history, which must thus be included in the optimization.

In the  research topic of programmable deformation and mechanics, the cluster is researching the development of programmable materials based on mechanical unit cells. This includes the search for functional elementary structures, a complete understanding of their programming as well as the optimization of the production processes of unit cells.

Any time the development of mechanical metamaterials is begun, there is always the question of how a unit cell should look so that it can fulfill specific functions. The researchers of the topic initially start with the mechanism, e.g. torsion or contraction, which then implies a function. With the help of simulations or thought experiments, the researchers then consider which unit cell can transport the desired mechanism optimally. Another function of the topic is to design the search for such correlations in as systematic and automated a manner as possible. A challenge faced here is that it makes a lot of difference whether a single unit cell performs a function or whether a function is possible only by combining several or even different cells.

In addition to the research of Beul structures that can run through the body like a wave, the work on the topic of concrete metamaterials also extends to the wettability of surfaces. The objective here is to use adaptive wettability through microstructured surfaces to move fluids along a specific transport route.

A high-resolution 3D printer is used to print the unit cells on a polymer substrate. The cells should then unfold under an external force such that they expose a surface with greatly altered wettability. One possibility tested by the cluster is to design the structures in such a manner that a mechanical force deforms them such that previously hidden tips protrude out of the unit cell. The tips then minimize the contact area to the liquid, thus minimizing the adhesion forces of the surface against the cohesive forces in the liquid. The periodically applied structures could then be switched from hydrophilic to hydrophobic and ideally back by an external stimulus.

For applications, the researchers plan to structure the unit cells in the form of a gradient, e.g. with different wall thicknesses, so that the material could switch from hydrophilic to hydrophobic at different times in a chain. The liquids are then transported from one side to the other and a transport route is thus established. Obvious applications for the same would be for instance Lab-on-a-chip or filter systems.

The researchers were successful indesigning both of these conditions of the unit cells according to the requirements. The opened-out as well as the folded-in conditions were printed using a Nanoscrive 3D printer and partly coated. The reference measurements regarding the wettability of both the conditions resulted in a difference of the contact angle of approx. 50°, where the opened structures led to a super hydrophobic condition already known from the lotus blossom.

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Challenges and perspective

 

The biggest challenge of the research topic of programmable deformation and mechanics is to find unit cells that perform the desired functions, with minimum possible effort. Even the next step, i.e. the comparison of simulation and reality in order to derive the material and its production process from a cell still presents the cluster with difficulties. Even if functional materials were found, their effects are often still relatively weak and must be reinforced.

The scaling of the materials also poses a challenge. The high resolution of modern additive methods in the three-digit nanometer range makes programmable structures possible. However, they require a correspondingly long time to produce macroscopic components in the centimeter range. The production of a surface itself on which a drop of water fits requires 10 whole days using the current method.

The opening of the closed structure is still not completely controllable in the unit cell structures for the adaptive wettability itself, because the carrier medium fails under load and the desired change thus does not happen in the unit cells. Stretchable polyethylene is said to be the current carrier medium, which is coated with an Indium-Tin-Oxide layer. As it turned out, the fragility of the coating is also transferred to polyethylene so that a thinner and more flexible coat provides a remedy in the next step. In addition, the current material combination is not as yet hydrophilic enough, which is to be dissolved by another separate coating of the closed structures.

For the next step, the participants of the  research topic would like to use Shape-memory polymers as substrate. These perform a mechanical conversion in the shape at specific temperatures and thus enable thermal actuators instead of mechanical actuators. Their inherent temperature programming, their reversible form and high elasticity make them interesting for application in programmable materials in several ways. They would be predestined as applications for the cooling of microchips among other things, e.g. if a coolant is introduced from a certain temperature onwards or ventilation flaps open.

Moreover, polymer-based structures should be converted into ceramic materials by means of pyrolysis, because they mainly allow extended temperatures. The developed mechanical mechanisms should then be optimized on different scales and applied in macroscopic components.

Figure 1: On the left: Microdosing system as a demonstrator for a system consisting of a mechanical metamaterial.

Status: 28.06.2019

 

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Publications

 

Referred journals

Schönfeld, Dennis; Chalissery, Dilip; Wenz, Franziska; Specht, Marius; Eberl, Chris; Pretsch, Thorsten; Actuating shape memory polymer for thermoresponsive soft robotic gripper and programmable materials, Molecules 26/3 (2021) Art. 522, 20 S. Link
Beteiligte Institute des Clusters: IAP, IWM

Fischer, Sarah C. L.; Hillen, Leonie; Eberl, Chris; Mechanical metamaterials on the way from laboratory scale to industrial applications: Challenges for characterization and scalability, Materials 13/16 (2020) Art. 3605, 16 S. Link
Beteiligte Institute des Clusters: IZFP, IWM

Specht, Marius; Berwind, Matthew; Eberl, Chris; Adaptive wettability of a programmable metasurface, Adaptive wettability of a programmable metasurface, Advanced Engineering Materials 23/2 (2020) Art. 2001037, 6 S. Link
Beteiligte Institute des Clusters: IWM

Weisheit, Linda; Wenz, Franziska; Lichti, Tobias; Eckert, Medardus; Baumann, Sascha; Hübner, Christof; Eberl, Christoph; Andrä, Heiko, Domänenübergreifende Workflows zur effizienten Entwicklung  Programmierbarer Materialien, ZWF Zeitschrift für wirtschaftlichen Fabrikbetrieb 115/7–8 (2020) 470-475 Link
Beteiligte Institute des Clusters: IWU, IWM, ITWM, ICT

 

Non-referred journals

Chalissery, Dilip; Pretsch, Thorsten; Staub, Sarah; Kasack, Katharina; Andrä, Heiko; 3D-Druck von QR-Codes mit Formgedächtniseigenschaften, Der Druckspiegel 11-12 (2019) 34-37 Link
Beteiligte Institute des Clusters: IAP, ITWM, IZI BB

 

Lectures

Wenz, Franziska, IWM
Developing programmable materials based on metamaterial unit cells
International Conference on Nonlinear Solid Mechanics
Rom, Italien; 16.09.2019 - 19.06.2019

Eberl, Chris, IWM
Programmable (meta) materials as a framework to enable adaptivity and longevity in technical materials and systems
Gordon Research Conference on Multifunctional Materials and Structures GRC 2020
Ventura, CA, USA; 19.01.2020 - 24.01.2020

Kappe, Konstantin
Metallic Metamaterial with bistable behavior
DDMC2020, Fraunhofer Direct Digital Manufacturing Conference
online, Deutschland; 23.06.2020 - 23.06.2020

Lichti, Tobias
Multiscale modelling and optimization of complex unit-cell based materials for large deformations
Materials Science and Engineering Congress MSE 2020
Digital Conference, Deutschland; 22.09.2020 - 25.09.2020

Schönfeld, Dennis
Synthesis of a shape memory polymer for soft robotics
Materials Science and Engineering Congress MSE 2020
Digital Conference, Deutschland; 22.09.2020 bis 25.09.2020

Wenz, Franziska, IWM
Developing programmable materials based on metamaterial unit cells
Materials Science and Engineering Congress MSE 2020
Digital Conference, Deutschland; 22.09.2020 - 25.09.2020

Pretsch, Thorsten
Programmierbare Polymere
Fraunhofer Symposium Netzwert 2019 »MOMENTUM«
München, Deutschland; 26.02.2019 - 27.02.2019

 

Students’ theses (Bachelor, Master, Diploma)

Classification of mechanical metamaterials and inverse unit cell design (M)

Visualization of fourth order tensor fields (M)

Neuartige Formgedächtnispolymer-Materialien als thermische Stellglieder (M)

 

Dissertations

Matthew Berwind
Albert-Ludwigs-Universität Freiburg im Breisgau
Materials design: the influence of structure, size and composition on material properties

Wissenschaftliche Preise und Auszeichnungen
Posterpreis WerkstoffWoche Dresden 2019
an PM1 für »Versuche zum Impact-Verhalten programmierbarer Dämpfungssysteme«  ID 5
Dresden, Deutschland; 20.09.2019

 

Posters

Specht, Marius
Developing programmable adaptive wettability
Gordon Research Conference on Multifunctional Materials and Structures GRC 2020
Ventura, CA, USA, 19.01.2020 bis 24.01.2020

Wenz, Franziska
A programmable mechanical metamaterial with designed strain-dependent poisson's ratio
Gordon Research Conference on Multifunctional Materials and Structures GRC 2020
Ventura, CA, USA, 19.01.2020 bis 24.01.2020

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