Imagine the flatpack wardrobe of the future. Like today, it will be delivered to your door as a two-dimensional cardboard-encased slab. Unlike today, assembly won’t involve spending all afternoon sweating at the effort, cursing at the instructions and putting your back out as you attempt to shove bits of chipboard together.
Instead, as you pull back the packaging, and light hits the components inside, it will begin to morph and grow, self-assembling into your new wardrobe in less than the time it takes to make a cup of tea. This is the future of furniture. This is 4D.
The Future is smart
The wardrobe of the future will be possible with smart materials. These are substances which move in response to a stimulus (think pinecones which open and close with wet or dry weather, or sunflowers whose faces move to track the sun across the afternoon sky). When smart materials are 3D printed, they can lend the 4th dimension – time – to additive manufacturing.
Smart materials have these morphing properties not thanks to clever robotics or fancy electronics, but because of their intrinsic material properties. The idea of combining ‘active’ smart materials with other ‘passive’ components in simple self-assembling structures is one of the hottest topics in materials engineering today.
The history of 4D printing is a short one. The term was first popularised by Skylar Tibbits in 2013 , and his Self-Assembly Lab at MIT is still the world-leading institution for 4D research. The Lab has captured the imagination of researchers in both academia and industry around the world, and momentum is rapidly growing in this new and exciting field.
Video 1: 4D Printing: Truncated Octahedron by MIT Self-Assembly Lab.
The hardware of 4D printing is largely the same as the 3D variety. The most popular technique, and that which will be most familiar to hobbyist makers is the Fused Deposition Modelling (FDM) method, where the liquid material is extruded through a robot-guided nozzle to build up a 3D object layer by layer. This method is limited to low melting point materials like thermoplastics such as poly(lactic acid) (PLA) and acrylonitrile butadiene styrene (ABS), or solvent-containing liquids, which can be extruded from a syringe and dried on the print bed.
There is also Selective Laser Sintering (SLS), which fuses layers in a bed of powdered material using guided lasers, and can handle polymers like nylon as well as some metals like titanium. A similar process, stereolithography (SLA) uses lasers to photopolymerise resin in a liquid bath. In inkjet printing, the material is sprayed out in a jet layer by layer to make up the pre-programmed shape.
As well as material limitations, 3D printing hardware also imposes geometrical limitations on 3D printed objects. The FDM and inkjet printing techniques can only produce shapes without severe angles of overhanging parts since a layer must be able to be placed on the layer below it.
Since SLA and SLS create shapes from a bed of powder or resin chamber, the powder or resin is able to act as the material support, and so overhanging shapes are possible with these methods. However, since 4D prints move after being printed, 4D printing can be a great way of getting around the geometrical limitations of current hardware.
Video 2: 4D Printing: Cube Self-Folding Strand by MIT Self-Assembly Lab.
Printing in four dimensions
The basic premise of 4D printing is to use these manufacturing techniques together with computer-aided design (CAD) tools to model and create structures made from a single material or combinations of active and passive materials which move in a pre-programmed way to a given stimulus.
Recently, the term has been used to include other reactive outcomes, such as colour-change, self-healing or biological activity like tissue maturation, but this article limits the discussion to the shape-morphing variety .
Perhaps the simplest 4D material is the shape memory polymer. These are plastics which move in response to heat. The permanent ‘remembered’ shape is set when the material is above its melting temperature (for semi-crystalline polymers) or above its glass transition temperature (for amorphous polymers).
Upon cooling, the polymer is moved into its temporary shape, but when heated back above the transition temperature, the material morphs back into its permanent ‘remembered’ shape.
Other stimuli for shape memory polymers are light such as for methacrylate-based resins . Magnetism can be used as a direct stimulus for magnetic materials like poly(lactic acid)/Fe3O4 nanoparticle composite , or an alternating magnetic field can be used to remotely heat the polymer and trigger its thermal shape-memory response .
Video 3: 4D Printing Process: Self-Folding Surfaces by MIT Self-Assembly Lab.
With many of these examples, the material must be set in the ‘remembered’ shape manually. However, with some materials and methods, it is possible to use the strain which is produced during the extrusion process of FDM without an additional shaping step. For example, in materials like PLA, the internal strain caused by 3D printing is released when the material is heated above its glass transition temperature .
Another clever technique is to use conventional inkjet printing to place black patterns onto homogenously pre-strained polystyrene, exploiting the so-called photothermal effect . Upon irradiation with light, the black ink absorbs more thermal energy and preferentially heats up the polystyrene material, releasing the strain only in those areas to allow the structure to fold in a pre-programmed sequence. Sequential movement can also come from materials which have more than one transition temperature, for example, Nafion .
However, shape memory polymers can only perform one-way shape change. For reversible, two-way shape change, we need to turn our attention to liquid crystal elastomers. These are rubbery, stretchy materials which contain liquid crystal molecules . The molecules are aligned during extrusion, and this alignment is fixed by cross-linking the polymer to stiffen the material and join the molecules together.
When heated, the material contracts along the direction in which the liquid crystal molecules are aligned and expand in the perpendicular directions, and when cooled the material returns to its original dimensions. Similar effects can occur under direct light  or photothermal  stimuli.
The expanding library of shape-memory materials is providing materials scientists and engineers with a much wider choice of stimuli, transition states and materials properties. However, the greater capability of design can be realised when they are combined into structures with other materials or blended together with other materials into composites.
For example, Tibbits and co-workers combined a water-responsive hydrogel with rigid plastic bars to create bending hinges , a movement which was later made reversible by others when they added a thermally-responsive shape memory polymer . By creating a gradient in material composition or modulating the cross-linking density of hydrogels by selective UV curing, composite sheets could be produced which have differential shrinkage and elastic properties, so that they twist when heated .
Embedding glassy shape-memory polymer fibres into a rubbery matrix can create a shape-memory composite . Modifying parameters such as fibre orientation and fibre volume fraction can produce patterns with multiple step changes at different temperatures . The printing parameters themselves can be used to influence strain to create bending shape-memory polymer  and liquid crystal elastomer  bilayers.
Finally, a form of moisture-responsive shape-change is possible by incorporating materials which undergo de-solvation when immersed, for example, origami structures which are patterned using photo-cross-linking were left with some uncured oligomers in certain regions . Once immersed, the free oligomers underwent de-solvation, causing a volume shrinkage in that area and folding of the structure . In all such multi-material or composite approaches, there can be materials compatibility challenges, particularly in attempting to mechanically combine several shape-morphing materials.
Video 4: 4D Printing: Self-Folding Surface Cube by MIT Self-Assembly Lab.
Despite the rapid uptake of 4D printing by academic and industrial research, there remains much work to be done before these applications may be realised. We need consolidated efforts in hardware improvements, improvements in materials, as well as better design and modelling tools in order to really reap 4D’s potential.
For the hardware, better multi-material capabilities, improved resolution, and faster prototyping would significantly widen the playing field for researchers. Materials could be improved in several areas such as increasing the speed of motion, the stability of the materials and structures and enhancing their ability to handle many movement cycles.
Finally, improvements in modelling and design tools, which incorporate stimuli and multi-material compatibility would dramatically increase the throughput of designs and assist researchers before the physical prototyping stage.
The potential applications for 4D printing go way beyond self-assembling flatpack furniture. So far, soft robots , , pharmaceuticals , tissue engineering , textiles , biomedical devices  and architecture  are recurring themes in the literature. In the future, 4D printing could touch many more areas of our everyday lives.
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