By now, most people are familiar with the concept of 3D printing. The technology is wellestablished across every manufacturing sector, from home appliances to aerospace to construction. Within the medical devices industry, 3D printing (otherwise known as additive manufacturing) is widely used to create surgical tools, anatomical models and medical implants. A lesser-known technology is 4D printing. If 2D printing refers to printing flat pages and 3D printing adds the extra dimension of volume, then 4D printing brings in the fourth dimension – time. If this sounds implausibly sci-fi, it doesn’t have to. It simply means that the constructs printed aren’t static. Unlike 3D printed objects, they exhibit dynamic features, which are designed to undergo pre-programmed changes in particular contexts.
“4D printing is a relatively new technology in additive manufacturing,” says Professor Mohammad Mirzaali, an assistant professor at the Faculty of Mechanical Engineering at the Delft University of Technology. “It refers to creating 3D printed objects capable of changing their properties, function or shape over time in response to external stimuli.”
As he notes, these dynamic transformations can be one-way – meaning the object changes its shape without referring back to its original configuration – or two-way, meaning the shape change can be reversed. Materials with these properties are known as shape memory materials or SMMs. And while they have been around for decades, it is only recently that scientists have started to approach them through the lens of additive manufacturing. And mastering the associated techniques could pave the way for a host of new applications.
A new wave of possibilities
“If you look into medical applications, the first that comes to mind would be for heart stents,” says Professor Eujin Pei, associate dean of the College of Engineering, Design and Physical Sciences at Brunel Design School, who is also the research group leader for 4D printing. “This has been shown by the Australian research agency CSIRO, which has manufactured self-expanding heart stents using a nickel-titanium alloy called nitinol. Through the use of shape memory materials, the stents expand and allow more blood to flow through.”
These devices could one day disrupt the global market for stents, worth a staggering $16bn. Not only can they be customised on-site – ensuring that every patient has the stent that best suits their requirements – but they can offer better conformity to blood vessels and improved recovery times. Because nitinol has ‘superelastic’ properties, exhibiting structural changes when stressed or heated, the device expands in the body to keep the vascular structures open.
Another potential application, says Pei, would be 4D printed leg braces. “Let’s say you have an injury and you need to wear a brace,” he explains. “That brace could be controlled not through mechanical approaches, but through shape memory materials.”
And the possibilities don’t stop there. As Mirzaali points out, 4D printing is being actively explored across a wide range of adaptable and personalised medical devices. “This includes drug delivery systems that release medications over time or under specific physiological conditions,” he says. “It also includes biodegradable microneedles, which are designed to deliver treatments in controlled doses and safely dissolve after use. In soft robotics, 4D printed materials could lead to the development of minimally invasive surgical tools that adapt to various anatomical structures during procedures. Meanwhile, wearable medical devices that respond to environmental changes could be used to monitor and maintain health parameters.”
One further example lies within the field of regenerative medicine. In theory, 4D printing could be combined with 3D bioprinting (printing living cells) to create advanced tissue scaffolds. These could be implanted into the patient at the site of injury or disease, supporting new cell growth and gradually dissolving as new tissue forms. Compared to 3D printed scaffolds, these structures would be better suited to the dynamism of living tissue.
“Such scaffolds could adjust their stiffness or shape dynamically in response to the healing process, eventually conforming to the target organ’s form and functionality,” says Mirzaali’s colleague, Professor Amir Zadpoor, chaired professor of biomaterials and tissue biomechanics at Delft University of Technology. “Eventually, 4D printed scaffolds could reduce or eliminate the need for donor organs.”
Barriers to entry
While the list of possibilities is long and tantalising, we’re not there yet. Indeed, 4D printing is a young technology, which has yet to make much of a realworld impact. The term itself dates to February 2013, when MIT scientist Skylar Tibbits debuted the concept during a TED talk. During his talk, Tibbits presented some strand-like structures that, when immersed in water, morphed into complex polygons. These were the earliest true examples of 4D printed objects.
Since then, research has continued apace on 4D printed technologies and materials. As Zadpoor explains, the success of the process relies on three key pillars: stimuli-responsive materials, types of stimuli and structural design. Scientists have made significant progress in all three areas, with a particular focus on biomedical applications.
“Several research groups worldwide are focused on developing biocompatible smart biomaterials,” he says. “One significant advancement has been using smart hydrogels, which swell or contract when exposed to moisture or pH, guiding their shape deformation. Regarding structural designs, artistic principles such as origami and kirigami have been adapted to biomedical engineering, inspiring 4D printed structures capable of complex transformations.”
It’s no wonder that the market for 4D printing is growing rapidly. According to Precedence Research, the global market is expected to rise to almost $3bn by 2032, from just $137m in 2022. That represents a CAGR of more than 36%. To date, though, most of the work being conducted remains at the experimental stage. Researchers are learning more about the mechanical behaviour of 4D printed parts and how the results might change in response to different printing parameters. They are testing out different types of materials, often in conjunction with computer simulations. But while ideas and innovations abound, there are many roadblocks standing in the way of widespread commercial application.
“One of the main barriers would be that the 4D printed parts still don’t have a proper way of being certified as safe for use,” says Pei. “Particularly for medical applications, you need to get FDA approval or equivalent, which is very stringent. I think there needs to be a much clearer means for getting certified and fit for use, with the right types of testing and certification processes in place.”
In order to pass muster with the regulators, manufacturers would have to show that any device was biocompatible, stable and reliable under physiological conditions. That’s not an easy ask for shapeshifting materials of this kind. “Key obstacles include managing the complex behaviours of materials over time, particularly when in direct contact with physiological solutions or living materials,” notes Mirzaali. “It can be challenging to achieve complex, reliable shape transformations. And ensuring the predictability of responses to external stimuli, especially in vivo, is a significant bottleneck.”
After all, 4D printing is a complex field, which requires a deep understanding of the relationship between material properties and shape changes. That, in turn, requires a multidisciplinary approach, spanning materials science, biology, design, modelling and engineering. “This complexity can hinder progress, making the design and fabrication process time-intensive and costly,” says Zadpoor. “What’s more, current computational modelling and 3D printing techniques require better advancements to control material behaviour, enabling more accurate predictions of how 4D printed structures will function within biological systems.”
Playing catch-up
None of that is to say that 4D printing is a write-off; simply that these are early days, and our technologies are still playing catch-up with our ambitions. Mirzaali hopes we will see advancements in computational tools, material sciences and multi-material printing technologies, with a view to ironing out some of the challenges. “Collaborative efforts between researchers, clinicians, and regulatory bodies will also be crucial to streamline development and ensure the safe and effective implementation of 4D printing in clinical settings,” he adds.
In their own lab, Mirzaali and Zadpoor are working on the three foundational pillars of 4D printing (materials, stimuli and design), with a focus on advanced computational modelling and 4D bioprinting. They are also exploring the miniaturisation of 4D printed structures, which would operate at the micron and submicron scales (70 times smaller than the width of a human hair). At a more fundamental level, they are interested in the physics behind the 4D printing process: for instance, how can defects in the material be harnessed to create shape-morphing structures? “This approach transforms potential drawbacks into functional advantages,” says Mirzaali.
Pei, meanwhile, is partnering with industry to understand what types of applications might be derived from his printing technologies. He is also working with standardisation bodies, to try to find a way to harmonise the language that’s being used to communicate 4D printing. “4D printing can be quite hard to visualise because you can’t just draw one shape – you need your technical drawing and the computer models to describe multiple shapes in different phases,” he points out. “It’s like drawing a moving figure. So trying to communicate your designs in technical terms can be very difficult.”
Despite the current limitations, Pei, Mirzaali and Zadpoor remain optimistic about what’s to come. Pei thinks that in the near-term future, we will see the world’s first 4D printed parts being certified as safe for use. He also believes that university courses will start to incorporate 4D printing into their curricula.
Zadpoor expects that, over the next decade, we will start to see widespread adoption of 4D printing across various biomedical fields. “That’s particularly true in orthopaedic applications,” he says. “Devices such as self-adjusting casts or shape-adapting splints could become standard, offering more comfort and functionality than current static solutions.”
Of course, there can often be a gulf between research and commercialisation. The kind of designs that are concocted in a lab – or, indeed, dreamed up in a TED talk – are not necessarily going to revolutionise medicine any time soon. That said, the 3D printing boom has more than lived up to expectations. There’s no reason why, further down the line, 4D printing should not do the same.
