Printing the invisible

By Assoc. Prof. Dr. Chong Wu Yi

We are all familiar with 3D printing. A typical desktop 3D printer builds an object layer by layer by pushing melted plastic through a small nozzle.

You may have seen one printing a toy, a phone stand or a keychain. The process is straightforward: the nozzle moves across the surface, depositing material layer by layer until an object takes shape. But what happens when we want to make something thousands of times smaller?

This is where conventional 3D printing reaches its limits. No matter how precisely the machine moves, the physical size of its nozzle limits the smallest details it can create. Imagine trying to paint a picture the width of a human hair using a thick paintbrush. You can move the brush as carefully as possible, but still, you cannot create details smaller than the brush itself.

So, rather than simply making the nozzle smaller, researchers are taking a completely different approach: removing the physical nozzle and using light as the tool.

At Photonics Research Centre, Universiti Malaya, researchers are using light and a highly precise positioning stage to create structures at the micro- and nanoscale, structures so small that they are invisible to the naked eye.

The idea may sound simple, but achieving this level of precision requires remarkable control over light. Ordinary laser beams can heat the surrounding material, much like a tiny blowtorch. At the microscopic scale, even a small amount of unwanted heat can damage the very fine features we are trying to create.

This is why the use of ultrafast lasers is important. These types of lasers produce a very short pulse width, on a timescale known as a femtosecond. So, what is so special about femtosecond lasers compared to other lasers? Imagine trying to cut a piece of meat with two different knives. A blunt knife requires more effort, pressing down very hard, crushing the meat, and the cut may spread beyond the intended area. A sharp knife cuts through effortlessly. It slices exactly where you want it to, requiring minimal force and leaving the surrounding area perfectly intact.

But creating a fine, small mark is only part of the challenge. How can we actually build a 3D object from scratch using an ultrafast laser?

One way to achieve this is through a technique called two-photon polymerization. Imagine a small container filled with a special liquid resin that can turn into solid plastic when exposed to light. If an ordinary laser beam passes through the resin, it can cause the material to harden along the beam’s path, creating a solid line. For making extremely small structures, however, this line is too large.

Two-photon polymerization works differently with an ultrafast laser. The resin is specially designed so that one photon of light is not enough to make it solid. Instead, two photons must arrive at the same spot at almost the same time. This requires a very high concentration of light, which is why ultrafast lasers, with their extremely short pulses, are commonly used. The intense light is focused into a tiny point, allowing the resin to remain liquid along most of the laser beam’s path and become solid only where the two photons act together. By moving this tiny focal point, researchers can create very small and detailed structures inside the resin.

It is somewhat like having a pen that only writes at the exact point where its light is focused. By moving this tiny focal point through the liquid resin using a highly precise positioning system, researchers can “draw” a 3D structure point by point. Instead of stacking relatively large layers like a conventional 3D printer, this technique allows researchers to form tiny, detailed structures inside the material using light.

Why does it matter if these structures are too small for our eyes to see? Because many of the technologies of tomorrow will depend on precisely controlling matter at very small scales.

In medicine, for example, microscopic structures could potentially be designed for targeted drug delivery, helping transport medicines more precisely to specific locations in the body. Similar microscopic structures can also provide scaffolding to guide cells as they grow, opening possibilities for tissue engineering and nerve repair.

This technology could also transform miniature optical devices. Smartphones, virtual-reality headsets and medical instruments increasingly require smaller and more sophisticated optical components. Microfabrication could allow tiny lenses and other optical structures to be produced directly on optical fibres, supporting compact imaging systems and future medical devices.

There are also possibilities beyond medicine and optics. Microscopic security codes could potentially be created inside or beneath the surface of banknotes, medical implants and valuable products. To the naked eye, the object would appear unchanged, while specialised equipment could reveal its hidden microscopic “fingerprint”, providing an additional layer of protection against counterfeiting.

As more technologies begin to use light to process and transmit information, the demand for smaller and more precise optical components will continue to grow. Micro- and nanoscale laser fabrication could therefore become an important technology for building the optical systems of the future.

The objects we create may be invisible to the human eye, but their potential impact could reach far beyond what we can see. From a desktop printer that pushes plastic through a nozzle to a laser that can create structures smaller than a human hair, we are moving towards a new way of manufacturing, using light to create incredibly small structures that could become essential to the technologies of tomorrow.