Scientists Achieve Precision 3D Printing Inside Living Human Cells

Human cells are extremely small and tightly packed – at about 20 micrometers across, roughly one-fifth the width of a human hair, each cell contains a dense mix of proteins, organelles, and molecular machinery. Being able to place tiny structures inside this space could allow scientists to track cells over time, measure chemical changes, or study how cells respond to physical forces. Doing this has been difficult. Most cells cannot take in solid objects larger than about one micrometer. Immune cells can engulf foreign material, but this traps it inside membrane-bound compartments rather than releasing it into the cytoplasm, where it could interact freely with the cell. Other methods, such as microinjection or temporarily opening the cell membrane, work well for delivering molecules, but they have not been used to place solid, free-standing structures directly inside the cell. Building functional structures inside living cells Printing structures inside a living cell introduces a unique set of constraints though. The laser and the printing materials must function in a space smaller than the objects being created, all while avoiding toxic effects and preserving the cell’s internal structure. Researchers in Slovenia have now shown that this can be done. In a study published in Advanced Materials, the team demonstrated that custom polymer microstructures can be fabricated directly inside living human cells using a laser-based method known as two-photon polymerization. The results point to a new way of building functional structures within cells, effectively turning the cellular interior into a site for precise biofabrication. The process starts by placing the printing material inside the cell. Using ultra-fine glass needles, the researchers injected tiny droplets of a commercial photoresist, known as IP-S, into HeLa cells, a common human cell line. The material was carefully chosen to be compatible with living cells, remain non-toxic once hardened, and dissolve away if it was not fully solidified, Nanowerk reports. After injection, each droplet, measuring about 10 to 15 micrometers across, was exposed to an ultrafast laser through a high-precision microscope. The laser triggered polymerization only at its focal point, enabling accurate three-dimensional shaping inside the cell. By scanning the focus through the material layer by layer at high speed, the team was able to build solid microstructures while leaving the surrounding cellular environment intact. Cells adapt to printed structures within their interior To demonstrate the method, the team printed a range of tiny shapes, including a 10-micrometer elephant, laboratory logos, hollow spheres, and lattice-like structures. Imaging confirmed that these objects were located inside the cell membrane, with cell nuclei visibly shifting shape to make room for the printed material. The impact on cell survival was comparable to other invasive techniques. After 24 hours, about 55 percent of cells containing printed structures were no longer viable, similar to the roughly 50 percent mortality seen when the cell membrane was punctured without printing. Cells that retained printed structures generally behaved normally. They maintained typical shape and continued to divide, and time-lapse imaging showed the printed objects being passed on to daughter cells during mitosis. Larger structures, however, had a measurable effect: objects bigger than 5 micrometers delayed cell division by at least an hour, suggesting that foreign bodies inside the cell can subtly alter its behavior. At present, the method relies on individual cell injection, limiting it to small-scale use, though better materials and delivery techniques could improve scalability. Even so, the ability to print solid, precisely shaped objects inside the cytoplasm opens new possibilities, from applying controlled mechanical forces to enabling localized sensing or drug release.