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Advanced Materials series: Could 4D printing change how we design rolling stock and infrastructure?

4D printing is the process of giving 3D structures the ability to change shape overtime.

Latest update: March 2019

While 4D printing is still in its R&D stage, prototypes have already been developed in industries such as aviation and fashion. The biomedical industry has also started looking at this technology to develop the next generation of medications as well as to revolutionise the surgeon methodology for orthopaedic diagnostic and treatment. A project conducted by researchers at City, University of Hong Kong, is developing a 4D printer for ceramic materials where the final product, a robust and intricate ceramic structure can remember the original shape and resist deformation due to mechanical forces, temperature or electromagnetic field.

What is 4D printing?

4D printing uses similar techniques to 3D printing to create three-dimensional objects through computer programmed deposition of materials in successive layers. What differentiates 4D from 3D printing is its ability to add a dimension of transformation over time to the printed product due to the properties of the material used. The final product reacts to stimuli within the environment, such as humidity and temperature, giving it shape morphing capabilities. There has been an increase in research in 4D printing in disciplines such as bioengineering and materials science.

Image by R4d3dp (Own work) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons.

Is 4D printing already in use?

 

What R&D is underway and what uncertainties remain?

Scientists at Wollongong University in Australia have developed a hydrogel compatible with 3D printers that generate quick, reversible and mechanically reliable changes in response to changes in water temperature. The hydrogel demonstrates high toughness and researchers have used it to print a valve that closes when exposed to hot water and opens once temperature drops.
A team at the Georgia Institute of Technology is using 3D printers to create small objects which expand when exposed to heat, which could potentially be useful for space structures. Utilising this technology, the systems would be lighter, stronger and easily collapsible for storage purposes, making them ideal for space travel.
MIT’s Self Assembly Lab has carried out research on 4D printed water pipes that can adapt to water flow and ground changes by expanding or contracting depending on the environmental conditions. The Lawrence Livermore National Laboratory has developed a composite silicone material that is flexible, stretchable, possesses shape memory behaviour and is completely reversible.
A project led by a group of researchers from the City University of Hong Kong is developing the world’s first 4D printer for ceramic materials. The printer - still in its development stage - can generate robust and extremely intricate ceramic structures. The malleable materials obtained can remember their original shape and resist deformations due to mechanical forces, temperature or electromagnetic field.
4D printing shares similar challenges to 3D printing; it is time consuming and not currently suited for large scale manufacturing despite technological advances over the last few years. Moreover, since deformations can be applied and performed more often, the material will degrade over time, so improvements are very much required in long term durability.
For some shape memory polymers, once the shape has been made by subjecting it to an environmental condition, the structure is fixed. Whether a printed product has reversibility depends on the choice of material and the process undertaken.

How will 4D printing impact the rail industry?

  • The applications of this technology are not currently well researched, and limited activity has happened in this area. 
  • Expansion of the printed object could be of use in infrastructure design, pre-fabricated structures can be expanded for installation on site for example benches and canopies. 
  • Self-healing 4D printed products could influence areas that are subject to wear, such as passenger seats or wheelsets. 
  • Materials that change shape could be used in braking systems, where components can alter in shape to prevent overheating. The contact area of braking components could increase in times of emergency to increase the rate of deceleration.

What should the rail industry do?

  • The industry should maintain an interest in 4D printing, following the developments of this new technology.
  • Research should be undertaken to identify potential application for the rail industry as new materials are developed with reversibility.
  • Applications for irreversable materials could also be explored too e.g. failsafe devices.

 

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