Here's 3D printing at the smallest scales

This story is about 3d printing. However, we are not talking about printing toys or phone cases, but rather entire sensors that are smaller than a grain of sand or even devices that can mimic the sensitivity of human skin. That’s the promise of 3D printing at the micro and nano scale, and a review article published in Microsystems & Nanoengineering takes us on a tour of just how far this field has come, and where it’s heading.

The paper, titled “3D printing of micro-nano devices and their applications”, authored by a team of researchers from Beijing University of Posts and Telecommunications, the University of Electronic Science and Technology of China (among others), explores the technologies, materials, and applications that are transforming the way we build the tiniest machines imaginable.

Micro- and nano-devices are the invisible workhorses of modern life. They’re found in smartphones, where tiny sensors and accelerometers that detect movement, medical devices (for instance, microneedles that deliver drugs painlessly), wearables such as flexible electronics that monitor heart rate or hydration, and optical systems, including microscopic lenses used in communication and imaging.

Traditionally, making such devices has been extremely expensive and slow, relying on methods borrowed from semiconductor manufacturing. Think of it as sculpting marble with a chisel: effective, but wasteful and rigid. 3D printing, by contrast, builds structures layer by layer, like a digital Lego set, allowing intricate designs and faster, cheaper customization.

The toolkit of tiny printing

As the authors explain, there isn’t just one kind of 3D printing. Instead, there are at least seven major families of techniques, each with its own strengths. Binder Jetting involves spraying glue onto powders to make solid objects. Directed Energy Deposition (DED) uses lasers or electron beams to melt and deposit metals. Material Extrusion (FDM/FFF), the most familiar consumer method, involves melting plastic and forcing it through a nozzle. Then, there's Material Jetting, which is similar to inkjet printers but with photopolymers or conductive inks, Powder Bed Fusion (SLS/SLM/EBM), that is, fusing powders with lasers or beams (which is ideal for metals). Furthermore, Sheet Lamination consists in layering thin sheets of material and bonding them. Finally, Vat Photopolymerization (SLA/DLP/2PP) uses light to cure liquid polymers into complex 3D structures, with resolutions down to 100 nanometers (about 1,000 times thinner than a human hair).

Some of the newer breakthroughs, like two-photon polymerization, can literally print at the nanoscale, enabling objects that look more like delicate spiderwebs under a microscope. One of the most exciting areas highlighted in the paper is 3D-printed electronics. Instead of etching flat circuits on silicon wafers, researchers can now print flexible, stretchable, and even biocompatible devices. For instance, this enables creating wearable sensors and patches that track your sweat chemistry, glucose, or alcohol levels in real time. Researchers are already printing “electronic skin” that mimics natural touch sensitivity. Also, 3d printing could be used for medical implants made of biocompatible polymers, where circuits and sensors to be printed directly into materials safe for the body. Other applications include energy devices using tiny 3D-printed batteries and supercapacitors to power sensors without needing bulky external batteries, and, finally, MEMS actuators, that is, microscopic machines that convert electrical signals into movement such as soft robots, microgrippers, or tiny pumps. 3D printing makes their production faster and more adaptable.

One particularly clever example? Printing frog-leg–shaped dielectric layers that improve sensor performance. Another team built mushroom-shaped electrodes that stick to skin for muscle monitoring. It’s a marriage of biology-inspired design and futuristic fabrication.

But what about the materials?

A major challenge in shrinking 3D printing has always been materials. Metals, ceramics, polymers, and even biological tissues are now being adapted for micro-scale printing. Some highlights include conductive inks like MXenes and graphene composites for printing circuits, piezoelectric polymers that generate electricity when bent or stretched, hydrogels that can self-heal and interface with biological tissue, and biomaterials that allow for printing scaffolds and tissues in biomedical research.

This mix of mechanical strength, flexibility, and electrical conductivity is what enables devices like flexible e-skin or ultra-small medical sensors.

However, the authors also point out the numerous current limitations. One is resolution trade-offs: some printers can print big but not tiny, others can print tiny but very slowly. There are also material constraints, because not all desired materials can be 3D-printed yet. Then, there is the challenge of durability and repeatability, that is, ensuring devices last in real-world conditions is still a challenge. Regardless, 3D printing has already changed how we make furniture, houses, and prosthetics. Now it’s starting to reshape the invisible layer of technology that powers our phones, monitors our health, and could one day seamlessly merge electronics with biology.

If you want to learn more, the original article titled "3D printing of micro-nano devices and their applications" on Microsystems & Nanoengineering at https://doi.org/10.1038/s41378-024-00812-3.