4D printing: thermal energy shrinks printed objects

Advance 4D manufacturing technology and help prevent the spread of coronavirus -these goals inspired a project by the Fraunhofer Cluster of Excellence Programmable Materials, a collaboration of the Fraunhofer IAP, IWU, ITWM and IWM Institutes. Unlike 3D printing, 4D production features the added dimension of time. The technology uses shape memory polymers to print objects that can change their shape once at a later point in time. All it takes is their exposure to heat. Find out in this interview how the Fraunhofer Institutes combined their respective competencies to advance the technology and discover their process!

4D printing adds the dimension of time and shrinks printed objects. What objects did you print and what was your process?

Thorsten Pretsch: Our initial 4D printed objects were small rod-shaped samples. We then heated them up past their transition temperature and studied the subsequent shrinking effects. Once we achieved a high degree of control over the material behavior, we produced larger objects such as hands-free door openers, which can be shrunk onto a door handle.

You can control the shrinkage behavior via the chosen material, processing temperature and printing speed. Are there any deviations from the defined values ​​or can you adjust the material behavior with precision?

Pretsch: Our study closely examined two materials. The first one is a thermoplastic polyurethane (TPU), which we developed. The second material is bio-based polymer polylactic acid, or PLA, which is widely available on the market and commonly used in additive manufacturing. Our TPU material demonstrated very pronounced and easily replicable shrinkage behavior. We performed a series of tests where we studied the pressure parameters and how they affect the material behavior. We created four-centimeter sized rod-shaped samples made from both TPU and PLA and subsequently heated them above their respective transition temperatures. We were able to demonstrate shrinkage that varies depending on the choice of printing parameters. Some cases even exhibited curvatures. The heating process prompts the materials to release internal stresses that were previously introduced using additive manufacturing. This enables precision control of material behavior, meaning our research results are very replicable.

What happens inside the materials during the shrinkage process?

Pretsch: The printing process enables us to introduce preferred orientations into the materials, which differ depending on which printing parameters are selected. By cooling the polymers very quickly below their glass transition temperatures, we can store higher-order states in them. Reheating results in an entropy-driven shape recovery, i.e., the material assumes a state that is characterized by a higher degree of disorder. Polymers strive for maximum disorder; ultimately, the shrinkage effect can be explained by so-called entropy elasticity.

In the "door opener" application, the printed object is applied to a door handle and then heated above the polymer’s transition temperature - the material contracts. As the material cools down to room temperature, it hardens on the door handle. This achieves a great form fit - our assembly concept has thus proven sustainable.

The material can be recycled and reprocessed into filament. What happens to the material if it can no longer be used for 4D printing?

Pretsch: We have shown that the door opener detaches from the door handle without leaving any residue if heated above the transition temperature. The material can be mechanically recycled. Provided there is no degradation or contamination, it can be used for the same type of application or a different one. The latter is consistent with our mission to promote a cross-industry circular economy for functional polymer materials.

Does this mean the material can be reused in "typical" additive manufacturing?

Pretsch: Our TPU material can actually also be used to make objects that only respond slightly to heat, meaning they essentially exhibit no thermo-responsive material behavior. For example, one can use the material for a sports-related application and later utilize it in another industry sector for a different application. Additive manufacturing is a very versatile technology in this setting. But here again, reuse implies that the material is not too badly degraded or contaminated due to prior use.

What are the costs of 4D printing technology?

Pretsch: The prerequisites to make 4D printed objects differ only slightly from those needed in fused filament fabrication. We use a reduced nozzle temperature to liquefy the filament, which may result in some energy savings. There is also no need to heat the chamber. Another key benefit is that FFF printers are already widely available, making it a common and affordable type of technology many people can use.

What does the future hold for 4D manufacturing technologies?

Pretsch: Unfortunately, I don't have a crystal ball. However, the latest 4D printing market studies indicate the technology is expected to grow at an average annual growth rate (CAGR) of 30 to 40+ X% in the coming years. I personally believe in the future of additive manufacturing technologies. Needless to say, the Fraunhofer Society is always happy to assist you and develop innovative and sustainable polymeric materials and technologies that are specifically tailored to the application's requirements.

How could this benefit medical technology?

Pretsch: By creating a door opener that is shrunk onto a door handle, we wanted to do our part to prevent the spread of COVID-19. Our goal was to find a solution that protects people from surface transmission by not having to touch a door handle with their hands and using their elbow instead. I believe we were successful in this endeavor. If similar door handles – like the ones we used in our experiments - are applied, hospitals and nursing homes can soon reap the benefits of our technology.