What's next after 3D-printing? It's 4D-printing!

The one-way shape effect

Dr. Thorsten Pretsch has been working at the Fraunhofer IAP for 8 years and is responsible for the research division "Synthesis and Polymer Technology".

Here, his working group conducts research on shape memory polymers. This field of work has expanded in recent years, with one of the group's strengths lying in function-integrated additive manufacturing. This enables the production of stimuli-responsive objects.

In additive manufacturing, the working group specialises in the FFF process (Fused Filament Fabrication). This requires the use of a polymer that can be thermoplastically processed and has suitable thermal and mechanical properties. The team begins at the molecular level by synthesising the functional polymers themselves.

IV: "We are very pleased to be able to interview a pioneer in 4D-printing. Thank you very much for your time! Can you let us know about any experience you have already gained in the subject and the choice of materials for it?"

Dr. Thorsten Pretsch: "Thank you very much, I am also pleased to be able to give you an insight into our work. We have focused on thermoplastic polyurethanes, so-called TPU materials, which are processed via extrusion so that the result is a filament which is available for additive manufacturing.

In the course of additive manufacturing, we carry out a functional integration. We talk about this when it is possible to produce printed objects that react to changes in their environment. These are able to adapt their shape in a predefined way through an external stimulus such as an increase in temperature. In doing so, we have to clearly distinguish between two effects: There is the so-called one-way shape memory effect and the two-way shape memory effect."

IV: "How can the one-way shape memory effect be explained?"

Dr. Pretsch: "Today it is possible to print thermo-responsive objects. These can change their shape once by heating. In 2021, we published parts of our work (https://onlinelibrary.wiley.co...). We have gradually developed the materials and technology for additive manufacturing so that it finally became possible to make heat-induced shape recovery controllable."

IV: "Can you give us an example of this? Are there already applications using this process?"

Dr. Pretsch: "Yes, the Fraunhofer Institute for Industrial Mathematics ITWM has developed a design for a heat-shrinkable door opener from our newly developed material to counteract the spread of smear infections. This was a particularly timely and appropriate development when the infectious disease Corona was spreading rapidly.

By heating the 4D-printed object above the switching temperature of the material it can be shrunk onto a door handle. In order to also take sustainability aspects into account, a lightweight construction was chosen for the design of the door opener. After shrinking, the door handle can be operated with the elbow so that contact with the hand is no longer necessary. This can contribute to counteracting the spread of smear infections. By reheating the door opener above the switching temperature of the polymer using a hot air dryer, it is possible to remove the shrunk-on door opener from the door handle without leaving any residue. Here we use a material concept from the Fraunhofer Institute for Machine Tools and Forming Technology IWU for switchable mechanical stiffness. In this way, the "design for end-of-use" and the "design for recycling" could be practically incorporated. Finally, another partner, the Fraunhofer Institute for Mechanics of Materials IWM, carried out practical tribological tests on the door openers."

Sustainability is written in capital letters in 4D-printing

IV: "Can 4D-printing have an impact on the all-important issue of sustainability?"

Dr. Pretsch: "We mechanically recycled our TPU material after 4D-printing and then processed it again in 4D-printing. The samples produced in this way had slightly worse shrinkage behaviour than in the first cycle (63% shrinkage in the first cycle, 57% in the second cycle), which is a very promising working result in our eyes. Provided that there is no significant ageing in the material during the use phase, it can be used several times, for example in different application contexts.

Our medium to long-term vision is to pave the way into a circular economy for highly functional shape memory polymers. Materials should be reused. They are also understood as valuable materials in 4D-printing."

IV: "In-Vision" has reported on this topic before in the context of "Design for Disassembly" in an interview with Professor Jürgen Stampf. How can this area of application now be developed further?"

Dr. Thorsten Pretsch: "With Design for Disassembly", Professor Stampf is addressing a very important topic. Here I see the central challenge in making companies aware, as early as the conception stage of tomorrow's products, to consider functionality in the context of repairing or disassembling individual components at the end of their service life. After all, it is the use of highly functional materials that makes the difference when they are skilfully used and "awakened from hibernation" by a suitable stimulus. This opens up significant advantages in the recovery of individual components.

We would like to further develop our technologies together with companies in new application contexts and also disseminate our results in the maker community so that an ever-growing circle of people can benefit from them.

In the course of our studies we have already shown with polylactide (PLA), a simple, bio-based material frequently used in 3D printing, that it is also possible to achieve shrinkage effects with it (Highly Shrinkable Objects as Obtained from 4D-Printing - Chalissery - 2022 - Macromolecular Materials and Engineering - Wiley Online Library). However, the functional properties of PLA do not come close to those of our TPU materials. The publication of our results was the starting point to also carry out additive manufacturing of objects with two-way shape memory effects."

The two-way shape memory effect

IV: "We now know about the one-way shape memory effect, but how can we now think of the two-way shape memory effect?"

Dr. Pretsch: "The approach pursues the idea of producing "programmable" materials that are capable of thermo-reversibly changing their shape. We always speak of a two-way shape memory effect when the manufactured printed object repeatedly adapts its shape through the raising or lowering of the ambient temperature. We call this switching back and forth between two shapes 'thermal actuation'.

After 4D printing, an actuation-capable object is thus available. For this, we programme the information on thermo-responsiveness into the polymer structure. The material behaviour is then dependent on the ambient temperature. As long as a critical temperature is not exceeded, actuation then takes place."

IV: "Are there already applications that illustrate this effect?"

Dr. Pretsch: "We built grippers in the course of researching the two-way shape memory effect. These could grip a small glass vial as the inner diameter of the gripper was designed to be larger than the outer diameter of the vial. The circular gripper was heated to the upper switching temperature, whereupon it shrank onto the vial so that it could then transport it. Lowering the temperature then caused the vial to be released again (https://onlinelibrary.wiley.com/doi/pdf/10.1002/mame.202200214).

In another example, we used a lever mechanism to actuate the wings of an artificial butterfly. The technology has potential applications as a sunshade or privacy screen, for example.

At the moment, we are working on laying the foundations for making thermal actuation as useful as possible in new areas of application."

The future of 4D-printing

IV: "How do you see the developments in 4D-printing? What will 4D-printing focus on?"

Dr. Pretsch: "Looking at the progress in 4D-printing, we are still at the beginning. More specifically, we are trying to figure out what can work in which context, but if we look at the shrinkable door opener, with the materials available in the market, objects like this can already be produced using standard printers.

With additive manufacturing, we have a technology that allows almost unlimited scope for the production of objects with potential for use in a wide range of industries.

For the coming years we expect that spaces for completely new developments will open up. In particular, it is a matter of making programmable material behaviour controllable, a task that we also face every day in the Fraunhofer Cluster of Excellence Programmable Materials, CPM. Programmable materials adapt to changing conditions. You do not need a permanent energy supply, no continuous monitoring, because the functionality is in the component. It is ultimately determined by the molecular design and the macroscopic structure.

Depending on which source materials are selected and how their ratio to each other is determined, one can influence the functionality of printed objects. It is always important to us that we also design at the molecular level in order to derive the greatest possible benefit from this in the application."

Recycling in 4D-Printing

IV: Which project is closest to your heart at the moment?

Dr. Pretsch: "My current favourite project is the research of printed objects with multiple stimuli-responsiveness. This involves the development of materials that are capable of responding with a change in shape to different stimuli, a behaviour which could soon further increase the application potential.

Of course, the topic of recycling is one of the most challenging, but probably also particularly appealing for that very reason. In the field of technology, there are still many opportunities for development.

With reference to recycling, we are particularly focussed on mono-material systems. Function-integrated additive manufacturing is a promising technology for the future."

IV: Thank you Mr. Pretsch for the illustration of the 4D printing process. We are looking forward to the progress in this field and are convinced that we can contribute to it with our particularly reliable devices. It is always exciting to see how quickly new developments emerge.

If you would like to learn which of our products can support you in your applications, please do not hesitate to contact us!