4D printing, a new dimension for polymers

4D: a matter of time

Evoking 4D in the same breath as 3D printing may seem odd, but it is nothing of the sort. The term has been around since 2013 and we owe it to Skylar Tibbits, founder of the MIT Self-assembly Lab. While giving a lecture, he was the first to state that 3D printing technology could be given a fourth dimension, explaining that a new feature could be added to a 3D printing material to enable it to evolve over time without human intervention (3D printing + time). Einstein told us that the fourth dimension is time. 4D printing should therefore be understood as meaning the possibility of printing an object which could, after a predefined time, change shape due to the effect of an external factor such as light, heat, vibration, humidity, etc.

Printed soles that automatically adapt to the terrain are one of the promises of 4D printing

Fewer, but particularly innovative polymers

Although 3D printing has a plethora of materials at its disposal, including a large number of polymers, the same cannot be said for 4D printing. The reason for this is simple: the technology is new and still in its infancy. However, it is expected to evolve very quickly.

Currently, there are three families of polymers that could be compatible with 4D printing. The first of these is what is known as shape memory polymers. They are already well-known materials whose characteristics have been modified to include a dynamic memory effect. Once stimulated (heat, electricity, light, etc.), these polymers are able to change from one shape to another. For example, upon being heated, the polyurethane/polycaprolactone/styrene-butadiene-styrene composite is able to return to its original shape after deformation. At the University of Rochester in the USA, there is a great deal of interest in these materials which have many applications. Recently, researchers have succeeded in developing a composite (whose composition we obviously know nothing about!) able to return to its original shape upon being subjected to a temperature of 35° C. This is quite an achievement, as previously these materials had to be heated to much higher temperatures in order to return to their original shape.

What applications might be possible? At this point in time, potential applications include suture thread that can be adjusted to the scar with a simple pressure of the finger, or sewing thread for clothes that would then become self-adjusting when in contact with the skin, or even, a long time from now, artificial skin that could also be printed.

A polymer thread that can be stretched with a simple pressure of the finger would make it possible to have clothes that always fit perfectly

Liquid crystal elastomers are also attracting a lot of attention from researchers. They too have been around for a few years and their operating principle is quite simple. These polymers contain heat-sensitive liquid crystals. They can take on almost any shape by simply orienting them correctly and then heating them. In Leeds in the UK, an attempt is being made to develop a polymer of this type with auxetic properties. In other words, this means that it swells when stretched. This property could find applications in energy-absorbing materials such as insoles or in the medical field for making artificial tendons. All that remains is to make them printable.

A drug in the form of a printed polymer patch could release its active ingredient only when the patient’s body temperature rises

Hydrogel polymers are the third avenue explored by research because they have the particularity of swelling on contact with water or another liquid. These polymers, which are often biocompatible, are of interest primarily to the medical sector, more particularly in the field of dressings. Making these polymers printable would make it possible to manufacture custom-made second skins for burn victims, for example. Another possibility would involve 4D printed medicine that could release its active principle based on the patient's body temperature.

MIT is looking for the right formula to design the polymer(s) and thus create a drug delivery device that will act only at the onset of fever.

Plastics once again at the heart of a new revolution

The research laboratories working on the subject are convinced that there will be many applications, particularly in the medical field, as well as in the construction, industrial and fashion industries. In Hanover, Germany, a team is attempting to develop a polymer to be used to manufacture cochlear implants that would adapt perfectly to the shape of the inner ear. Elsewhere, at MIT, researchers are working on a carbon fibre composite that could be used to design race cars and whose shape could change with heat to improve the vehicles’ aerodynamics. Shoe soles that could adapt to the terrain underfoot could also be a possibility in the future.

Of course, polymers will remain at the forefront of this revolution because these futuristic materials will be composites. It is the combination of polymers with other materials (sometimes natural materials such as wood cellulose) that gives objects their ability to change shape or react to a stimulus. The printers required for this special task still have to be designed. It should be recalled that additive manufacturing is intended for custom or small series production. However, it may go beyond that as 4D printed object’s reactions to stimuli can be different to those of the same object produced through molding. This has been shown by researchers in Singapore who have succeeded in making the opening of a flower made from an acrylic and epoxy-based hydrogel twice as reactive when printed.

This 4D printed flower opens and closes by itself when in contact with water

When will we see these new developments?

Soon, but not right away. Unfortunately, there are still many hurdles to overcome both in terms of materials and machinery. The materials still raise many questions, particularly with regard to their ability to remain “smart” over time. How long will they be able to fulfil the functions for which they were designed? The answer is a complex one and is still unclear, but all experts agree that, once fully developed, 4D printing will have a significant impact on industry, much in the same way as 3D printing did in its day.