Most of us are used to hearing about 3D printing in factories or in makerspaces, not inside the tiniest units of the human body. Yet that is exactly what a team in Slovenia has done by using a finely-tuned laser to build solid three-dimensional objects inside living human cells.
The work, led by physicist Maruša Mur at the Jožef Stefan Institute together with colleagues at the University of Ljubljana and CENN Nanocenter, shows that custom shaped microstructures can be printed directly inside living HeLa cells using a laser technique known as two-photon polymerization.
The researchers report that many cells survive the procedure and keep dividing, suggesting a new way to engineer cell interiors in a controlled way.
Why putting solid objects inside cells was so hard
Human cells are tiny, roughly twenty micrometers across, about one fifth the width of a human hair. Inside that cramped space sit the nucleus, mitochondria, and many other components, all swimming in cytoplasm, the fluid interior of the cell.
Trying to park a solid object in that crowded room without breaking something has been a long-standing problem.
Until now, most methods either brought in small molecules or genetic material, or relied on the cell itself to swallow particles from the outside. When immune cells engulf foreign material, they wrap it in a membrane bubble, which keeps it locked away from the rest of the cytoplasm.
Techniques that punch temporary holes in the cell membrane can deliver cargo, yet free-standing solid structures tend to cause serious damage or end up trapped in compartments.
How the laser-based 3D printing method works
The Slovenian team approached the problem in a different way by building the object inside the cell rather than trying to push it in from outside. First they microinjected a droplet of a light sensitive resin called IP S into individual HeLa cells using ultra-fine glass needles.
The resin was chosen because it is relatively well tolerated while still liquid, becomes non toxic once hardened, and slowly dissolves if it is not fully solidified.
Once the droplet was in place, the scientists focused an ultrafast infrared laser into it through a high precision microscope.
Two photon polymerization means that the resin hardens only at the exact point where the laser light is most intense, where two packets of light are absorbed almost at the same time. By steering that focus through the droplet layer by layer, they could trace out complex three-dimensional shapes while leaving the surrounding cell interior mostly untouched.
To show what is possible, the team printed a hollow sphere, lattice-like scaffolds, barcodes for tracking cells, tiny optical components, and even a ten micrometer long model of an elephant.
Imaging confirmed that these solid structures sat freely in the cytoplasm and that the nucleus often bent and shifted to make room for them. At the end of the day, the cell looked like it had been furnished with custom-built microscopic furniture.
How cells react to tiny structures inside them
A natural question follows. If you build a rigid object inside a living cell, does the cell survive? The answer is mixed but informative.
After twenty four hours, about half of the cells that received an injected droplet and a printed structure were no longer viable, a rate similar to cells that only experienced the needle injection without any printing. That pattern suggests that the main damage comes from piercing the membrane, not from the light or the hardened resin.
For the most part, the surviving cells kept their usual appearance and behavior. Time lapse videos showed them moving, dividing, and passing the printed object on to one of the daughter cells during mitosis.
When the embedded structure was small, cell division looked almost normal. Larger objects, roughly five micrometers or more, delayed division by around an hour or sometimes a few hours, which hints at a direct physical tug on the internal machinery.
The nucleus itself did not escape untouched. Confocal and fluorescence microscopy revealed that this control center of the cell deformed to wrap around bulkier printed shapes. In effect, the object becomes a built in obstacle that the cell has to work around, providing a way for researchers to probe how forces and shapes inside the cell affect its life cycle.
What this could mean for future medicine and research
The study is still in its early stage, and right now each cell must be injected by hand, which is slow and risky. Even so, experts see it as a powerful proof of concept.
Previous research on two-photon polymerization focused on printing micro optics, mechanical parts, or three-dimensional scaffolds outside cells to guide how they grow and move.
By moving the same idea inside the cytoplasm, scientists can imagine new types of tools. Barcodes printed inside cells could let labs track billions of individual cells through a tissue, a bit like labeling every item in a warehouse.
Tiny lasers or diffraction gratings could act as built-in light sources and sensors, while spring-like structures might push or pull on organelles to test how cells respond to stress from the inside.
In the long run, this approach could help design cells that perform new functions or deliver drugs in smarter ways, although that remains a distant goal and not something patients will see in the clinic soon.
As Maruša Mur put it, “Our method provides a new tool to manipulate living cells from the inside,” and that tool will likely raise as many questions as it answers.
The main study has been published in Advanced Materials.
