Imagine flying in a plane whose wings change like those of a bird, taking on different configurations during take-off, cruising and landing. Imagine being treated in a hospital where personalised prosthetics have the same movement as real muscles. Imagine wearing clothes which become warmer as the weather gets cooler, and vice versa. This may all sound like distant science fiction, but it’s closer to fact than you might think. Welcome to the four-dimensional future.

That fourth dimension is time. Today’s three-dimensional material world is made from passive, inanimate materials like brick, steel and glass. Four-dimensional structures are made from active, animated, so-called ‘smart materials’ which move autonomously – swelling, shrinking or bending in reaction to a stimulus – combined with passive materials. That allows them to move and change shape without robotics, electronics or engines: instead, they are triggered by changes in the material’s environment, such as heat, light, moisture, electricity, magnetism, physical forces or chemicals.

A naturally-occurring example of a 4D structure is the pinecone. Pinecones are covered with woody scales. The underside of each scale is made up of long, parallel, thick-walled cells which get longer when they get wet, and shorten when dry. Bonded to it on the other side is woody material with cells in a different configuration, which resists swelling when wet. This layered structure means that when it rains, the underside of the scale expands, and the top side doesn’t, which causes it to bend, closing the whole pinecone and keeping the seeds safe inside – held in store for a day with weather more suitable to pine tree propagation. In this example, the smart material is the moisture-swelling wood, but the 4D structure is the bi-layered scale.

The future – four-dimensional or otherwise – is steeped in uncertainty, magnified by the grand challenges which humanity is currently facing. The combination of dwindling material reserves, a changing climate, unsustainable energy consumption and demographic change makes it difficult to imagine what the future might hold. We will need to rely on a diverse arsenal of technologies if we are to overcome these challenges and create a habitable future for everyone on Earth.

From flint knapping to the blast furnace, mastery of materials and manufacturing methods has always been central to our survival as a species. The uncertainty of the future as it stands will require us to be adaptable, sensitive and responsive to the environment – just like four-dimensional materials.

The story of 4D materials begins in 1880, in the Parisian laboratory of Pierre and Jacques Curie. The Curie brothers discovered that when a quartz crystal is physically deformed, a small electric voltage appears across its length, and vice versa. Quartz represents the first smart material to be discovered which didn’t come from a plant or animal. It was soon used in electrical devices – for example, to detect noises underwater by turning the sound waves into electrical signals.

The distortion of a quartz crystal with the application of electrical voltage is a relatively small effect – the movement is much smaller than, say, the contraction of a bicep muscle. Smart materials capable of generating larger motions – the sort which would be useful for making 4D structures at the human scale – remained unknown for decades.

One early example was discovered by accident, by a scientist with a lighter. In the early 1960s, the US Naval Ordnance Laboratory was seeking an alloy which wouldn’t break when it was repeatedly bent out of shape. Researcher William J. Buehler came up with a 50:50 mixture of nickel and titanium which did the job well, and in 1961 he took it to a laboratory management meeting.

The sample was a folded strip of thin metal, bent alternately like an accordion. To demonstrate its resilience against breaking, Buehler bent and stretched it again and again. Later in the meeting, one of the associate technical directors, David S. Muzzey, decided to see what would happen if the strip was heated, using his pipe lighter. To the amazement of everybody present, the strip of metal stretched back out into its original shape! The room had witnessed the response of the first shape memory alloy; a metal which moves in response to heat into a different, ‘remembered’ shape.

This metal, and others like it, found niche applications in the aerospace, automotive, robotics and medical fields, used to make simple components like temperature-sensitive valves and switches. However, it was still just a smart material component in a device, not closely integrated into a 4D structure. For this, new design and manufacturing methods were needed.

These came in the form of 3D printing. Various processes for manufacturing objects additively were developed throughout the 1980s and 1990s, such as welding together powdered material in layers using lasers, or extruding molten plastic through a heated nozzle. 3D printers require precise computer instruction, and so were accompanied by rapid advances in 3D computer design and modelling, further helped along by online enthusiasts, expired patents and open-source software.

Meanwhile, new soft morphing materials were being produced by scientists in laboratories – such as shape memory polymers and moisture-swelling hydrogels – which were more suitable than shape memory alloys for use in early 3D printers. This coincidence of three elements – new morphing materials, a suitable manufacturing method and better computer modelling software – was what finally allowed structures to enter the fourth dimension.

The concept of ‘4D printing’ was introduced by MIT researcher Skylar Tibbits in a 2013 TED talk. Inspired by the pinecone, researchers in his lab, and many others since, have produced synthetic materials and structures which move and morph in response to water and other stimuli. In the last six years the field has exploded with activity, with research focussed on the same three areas that brought about the 4D concept – materials, hardware and software.

Today, the 4D engineer can choose from a growing palette of smart materials and stimuli. Synthetic cellulose composites and hydrogels mimic the pinecone to expand when wet, and shape-memory polymers can move with heat. The motion of electroactive polymers is triggered by an electric field, whereas liquid crystal elastomers respond to light. The list continues to grow.

Advances in 3D printing hardware have been accelerated by interest and investment from industry, academia and hobbyists. Today’s 3D printers can accommodate a vast range of materials – including metals, ceramics, gels and composites – and can produce objects made up of multiple materials. Extra features like controllable UV lights to ‘cure’ liquid polymers into solids or lasers to add cutting functionality are now being commercialised too.

Today’s computer design software is capable of modelling virtually any 3D shape you care to imagine. However, the design of 4D structures must also consider their movement, which involves more complex simulation. Four-dimensional programs such as Project Cyborg from engineering software giants Autodesk have been used by Tibbits’ team at MIT to design folding 4D objects, but the software is not yet commercially available.

The ultimate goal is to combine such modelling and simulation software with hardware which can monitor the shape-change of printed objects using sensors, and even apply machine learning to optimise the model. Furthermore, the ideal package would be able to handle a diverse library of smart materials and their multitude of stimuli. The academic success of Project Cyborg suggests that a commercial version of such a program might be imminent. With that final piece of the puzzle so tantalisingly close, today we find ourselves at the tipping-point for 4D technology.

A 4D material world could play a crucial part in solving the grand challenges facing humanity.

Take, for example, flatpack furniture. Today, you open a more or less two-dimensional box of components, and spend all morning assembling them into a 3D structure using hammers, an armoury of Allen keys and – if you’re anything like me – not a small amount of swearing at the instructions.

The 4D equivalent would arrive at your door in the same packaging. But as you pull back the cardboard and the light hits the item inside, the furniture would begin to unfurl, self-assembling into the finished product before your very eyes.

That sounds magical, but not world-changing given the existing prevalence of flatpack furniture. The real difference, however, is in the efficiency of shipping and global commerce when the same model is extended to other products – particularly those, unlike a flatpack wardrobe, that can’t readily be supplied in component form but could be supplied in a 4D compressed form - for example, shoes, electrical goods or even food.

Objects could even be programmed to self-disassemble at their end of life. One of the greatest hurdles in recycling is the difficulty of separating out the various materials at the end of an object’s usable lifetime. Most of these multi-material items end up in landfill. If wardrobes could disassemble into their constituent components – or even better, be made of a single material whose 4D properties arise from structural differences, like the all-wood pinecone – we would be closer to closing the loop.

The fact that 4D objects can be intrinsically multifunctional would further help to ease the increasing strain on material resources. For instance, shape-shifting polymer fabrics could be used to make 4D adaptive clothing which reacts to sweat – so could either serve as a thin, breathable outer layer or a warm, fluffy puffer jacket. Sportswear brand Nike has already brought such adaptive sportswear to market in their AeroReact range. A 4D future could prompt a move away from fast fashion and disposable culture to extend the utility of our possessions.

Four-dimensional thinking could help with our other major challenges, too. Just around the corner are 4D wearable assistive devices which augment personal mobility. Designed to provide support on the basis of a 3D scan of the patient’s own body, these would prolong independence for the elderly, or provide prosthetic assistance for the infirm – think Wallace and Gromit’s Wrong Trousers but worn subtly under clothing. In fact, engineers in Bristol have developed just such a garment called The Right Trousers. Whilst problems with power supply and usability remain to be solved, they predict such trousers could be available in 10 years, and smaller assistive devices like ankle or knee braces could be faster to commercialise.

And 4D materials can also address the pressing need to change our energy consumption in the face of climate change. Architects are already designing with temperature-sensitive shape-changing materials to automatically regulate how much the building is warmed by the Sun, to reduce the amount of artificial and energy-intensive heating and cooling required.

Looking further to the future, NASA engineers have suggested that aircraft whose wing-shapes move and morph according to the forces involved in the different stages of flight could have improved fuel efficiency and superior aerodynamics compared to current fixed-wing designs.

Smart materials could also help us mitigate the effects of climate change and other environmental changes: for example, smart foundations which adjust their stiffness according to changes in the moisture of the ground soil could protect vulnerable buildings in regions prone to flooding.

All this sounds wonderful, but there will be some serious questions to be answered along the way. We’ve seen the ethical implications of 3D printing rise to prominence in the public consciousness – the dissemination of files to make working 3D printed guns being an obvious example – and there will be just as much need for the ethics and regulation of 4D printing to be closely scrutinised.

While some 4D objects may keep materials and resources out of landfill for longer, their creation might require greater energy and raw materials than their 3D equivalent. And 4D objects will inevitably cost more, at least to begin with; so will only the rich be able to live the 4D lifestyle? An assessment of the net benefits by life-cycle and economic analysis is yet to be carried out.

Continued advances in the materials, hardware and software of 4D printing are a safe bet. But we may come up against hard technological limits when it comes to commercialisation: 4D printing may be slow or difficult to make work on a large scale. Either new processing methods – which do not rely on additive manufacturing – would need to be found for 4D objects, or 3D printing will have to get much faster for large 4D objects to be mass-producible.

Perhaps the greatest threat to the 4D future is the need for acceptance by the human user. Incorporating 4D materials into our lives will inevitably mean relinquishing some control over to what we are used to thinking of as inanimate objects. People can be fickle and individualistic in their preferences. For instance, having brighter light in indoor spaces might boost someone’s mood, even if that sacrifices the 4D building’s energy efficiency, or our monthly cycle may mean we want at times to be warmer or cooler than our 4D clothes would ordinarily regulate for. Will it be possible to automate buildings, objects or clothing in ways that will satisfy everyone, every day?

I believe the severity and imminence of the grand challenges humanity faces is justification enough to pursue any materials technology with the potential to do good in the world. After all, our welfare is and always has been intrinsically linked to the materiality of society. For instance, plastic is a wonder material with a myriad of uses, from food storage to medical supplies, that has improved life for all of us; it is only recently that it has become seen as a problem in need of fixing. If we are to improve welfare for future generations, it is vital that new materials and manufacturing methods be part of the conversation.

The irony of all this future-gazing is that what we seek – smart materials and 4D structures which intelligently adapt to their environment – have been right in front of us all along. The heads of sunflowers track the Sun across the sky, our muscles contract with the pulse of a nerve, the humble pinecone protects its seeds from the rain.

Before we came along, nature had created the ultimate sustainable material world; everything was recycled, all energy was balanced in a perpetual loop. Human influence has created a drastic imbalance: materials and energy are ploughed into the ground or burnt up to pollute the sky. Four-dimensional thinking allows today’s material scientists, engineers and garden-shed tinkerers to open their eyes to the ultimate adaptable, sustainable and responsible material world. We are engineering our way back to nature the smart way: into a 4D future.

Dr Anna Ploszajski is an award-winning materials scientist, engineer and communicator. By day, Anna is a Research Fellow at the Institute of Making, researching 4D printing and metamaterials. By night she communicates materials science on stage, on radio, on TV and on the page. In 2017 she was named Young Engineer of the Year by the Royal Academy of Engineering, and in 2018 won the Silver Medal from the Institute of Materials, Minerals and Mining.