Every time someone attempts to create an unbreakable password or send a private message, they must trust a number. That number is supposed to be one that a computer has generated completely at random. Unfortunately, no computer has been able to actually guarantee that this number is truly random. Until now.

To get truly random numbers, physicists at ETH Zurich in Switzerland turned to the quantum world. They can prove that the numbers their technique spits out really are random (with only a vanishingly small chance that they are wrong).

A number that’s proven to be random can help protect private data through encryption. Here’s why. Encryption works by turning private information into a scrambled code using a secret “key.” The key is built from a random number. If that number is truly random, then no one can unscramble the information without the key. However, if it’s not truly random, a hacker could figure out the pattern behind it and reconstruct the key. And that would expose the formerly masked information.

“When randomness is weak, the keys built from it become guessable. An attacker can shrink work that should take billions of years into something practical,” explains David Holtzman. In other words, a guessing game that could take longer than the age of the universe becomes a task that a computer might pull off in months or years.

Holtzman did not take part in the new work. Still, he knows about such things. He’s worked as a cryptographer for the U.S. National Security Agency and now runs a cybersecurity firm called Naoris Quantum Protocol.

The new study cites real cases where weak randomness let attackers steal private data — and offers a new solution.

Solving the bias problem

Regular computers haven’t been able to make truly random numbers because those machines are designed to be predictable. In a normal computer, the same starting conditions should always produce the same outcome. To create a “random” number, these devices use a formula and a starting number, called a “seed.” And while the outcome may look random, it’s really only sort of random.

Scientists call this pseudo-randomness. Why? If someone guesses the seed, they can rebuild the whole sequence of steps a computer takes to make a “random” number. This will break the code meant to securely mask data.

But a natural solution exists in the quantum world. The word “quantum” describes the very smallest things in our universe, such as tiny particles. In this realm of microscopic particles, physical processes are actually random. In fact, researchers have already used quantum devices to generate randomness. But so far, no device used to capture that randomness has been perfect. Small hardware flaws have kept the results from being perfectly random.

In the past, researchers have tried to build better, less-biased devices. The ETH Zurich team found a different way: a two-part system that works even if the devices are imperfect. The setup relies on running two experiments at once, in two different places, explains physicist Renato Renner. This kept the results of one from being able to directly affect the other.

Think of the split experiments like two rubber balls. The balls look the same. But each one has its own tiny surface imperfections. Those unique features give each ball its own slight bias, making it bounce one way more often than another. Because each ball was made separately, its imperfections have nothing to do with the other’s.

In the same way, ETH Zurich’s two devices each produced random numbers with its own bias. But because the devices were separated, the biases in their outputs were completely unrelated. Renner’s team collected results from its two experiments, then combined their findings. To do this, they used something called a randomness extractor.

That extractor doesn’t fix either experiment’s output to be perfectly random on its own. Instead, it looks at the two outputs together. Then it combines them using a mathematical formula. Because the flaws in each device’s random number are unrelated, combining them cancels those flaws out. The result is truly unpredictable.

For the ETH Zurich experiment, the team used two quantum devices 30 meters (about 100 feet) apart. This separation made sure no signal — even if it were traveling at the speed of light — could pass between them before each outcome was generated. It’s one guarantee that the output from one could not influence the other.

Each device measured the outcome of a quantum event, such as a photon passing through a light-splitting device. Running the results from both through the randomness extractor increased the randomness already present in each.

Renner’s group described how it did this on May 27 in Nature.

Passing the ‘Bell test’

Proving randomness is very hard, says Nicole Yunger Halpern, who did not take part in the new work. “We can certify a random-number generator only by proving that no possible attack can have corrupted it,” explains this quantum physicist. She works at the U.S. National Institute of Standards and Technology in Gaithersburg, Md.

To confirm that the output from its extractor was genuine, the ETH Zurich team used something known as a Bell test. This physics test proves a quantum result wasn’t faked or planned in advance.

Passing this test meant something important. The team didn’t have to trust that their devices were perfect. A device might have a hidden flaw or secretly been tampered with. Either way, it couldn’t fake the exact pattern found. This pattern could only be due to genuine, unpredictable quantum behavior.

Scientists call this type of proof “device-independent.” The certainty comes from the test itself. There is no need for blind faith in the equipment or the test setup.

In 2025, a team led by Minzhao Liu at JPMorgan Chase in New York City claimed it had generated numbers with “certified randomness.” However, its claim relied on one big premise. It assumed that no classical — that is, non-quantum — computer would ever be powerful enough to copy its results. (Otherwise, a hacker using an ordinary computer might eventually guess or predict that team’s random numbers.)

“Our result,” says Renner, “does not require any [limits] on the capabilities of someone trying to predict the randomness.” As such, he claims this is the first real-world showcase that natural quantum randomness can be turned into something perfectly random. Renner and a co-worker first proposed this idea in 2012. Now, he’s helped show it’s true.

With the help of 3-D printers, even hobbyists can make model rockets or gaming accessories out of seemingly nothing. What if that printer could make new body parts, such as an ear, lung or skin? This may sound like science fiction. But scientists are already using living cells to “ink” prototypes of these tissues. 

This 3-D bioprinting uses the same basic strategy now used to print toys, sculptures, metal pieces or even basketballs. Specialized printers just swap out plastic or metal for living cells.

Actually, there’s a bit more to it than that. In most cases, you can’t use the same devices to make tissues. And you can’t just spew cells onto a surface, layer by layer, and expect them to link up into working organs. The cells have to be mixed with some soft, gel-like materials to make them unite with their neighbors.

Although still experimental, some devices have already printed skin, cartilage and even early-stage blood vessels. In 2025, researchers in Australia used an experimental robot to bioprint ink made from a patient’s own skin cells directly onto her burn wounds.

It starts with the ink

In most 3-D printers, a resin, plastic filament or metal serves as the ink. Bioprinters instead use cells — and something to keep them alive.

“Bioinks are specialized soft gels that can have living cells mixed inside them,” explains Ryan Martin. He works on this tech at Virginia Commonwealth University, or VCU, in Richmond. Think of these inks like Jell-O: wobbly, soft materials that can hold their shape while protecting what’s inside.

Scientists make these hydrogels from materials such as alginate (AL-jin-ate) and collagen. Alginate comes from seaweed. Collagen is a protein found in skin and bones.

But bioinks contain more than just cells and hydrogel. That extra stuff — part of a complex matrix — helps the mix mimic the material that surrounds cells in the body, explains Ankita Pramanick. She’s a biomedical engineer at Utrecht University in the Netherlands.

What makes bioinks different from most other 3-D inks is that they host living cells. And those cells must survive the printing process, where they’ll get squeezed through nozzles and exposed to temperature changes. The matrix is designed with these challenges in mind.

After printing, the inked structures go into incubators, which are containers that maintain ideal conditions to help cells grow. There, the newly printed cells can mature, strengthen and grow together to form tissues — perhaps cartilage for joints or layered skin with blood vessels.

Printing living cells

Engineers have been developing several different ways to turn bioinks into tissue. Which one they choose depends on what they’re building.

Most times, a “bioink is pushed through a nozzle like toothpaste from a tube,” Martin explains. This process is known as extrusion. A machine controls how fast the ink comes out, building up a tissue layer by layer.

For example, to create skin tissue, you’d first print fibroblasts (FY-broh-blasts) — the cells that make up the inner layer of your skin. Later, you’d layer on keratinocytes (Kare-uh-TIN-oh-sites). These are the cells that make up our skin’s protective outer layer. Once printed, this combined structure matures within an incubator.

Extrusion works well for thicker, structured tissues. These include skin and cartilage. But it’s relatively slow. Squeezing cells through a nozzle can also stress them.

Inkjet bioprinting is faster and gentler. It sprays tiny droplets of bioink onto a surface in much the same way an office printer sprays ink onto paper. It works best with a very liquid matrix. That’s why it’s often used for thin cell layers, such as those that line the insides of blood vessels.

Another type of bioprinting creates entire structures at once instead of building them layer by layer. To start, a laser shines its light into a spinning tube of light-sensitive gel and cells. This solidifies them into the precise shape needed.

With this, scientists might print a small network of blood vessels all at once — complete with all its branches. It could take just seconds. And that’s important, because speed matters. The faster tissues are printed, the less time their cells spend outside their ideal environment.

What’s being printed

In 2022, a patient received a 3-D–printed ear made from their own cells. In late 2025, surgeons successfully implanted a bioprinted cornea — the clear outer layer of the eye — into someone who had been legally blind.

Skin grafting is perhaps the most advanced application. A skin graft is a type of transplant where healthy skin is used to cover missing or injured skin. A 2025 trial in Australia tested bioprinted skin on burn patients. In a news conference with her doctor, the first patient to receive such a graft showed it had successfully worked on her leg.

Bone is trickier to print. Unlike skin or cartilage, it contains hard minerals mixed with living cells. Scientists have used 3-D printing to create bone scaffolds — frameworks that encourage the body’s own bone cells to grow and fill in the gaps. But no one has yet bioprinted actual bone with living cells in it for people.

Complex, fully-functioning organs such as hearts and kidneys are still a work in progress. Tissues thicker than a few millimeters (about a tenth of an inch) face a critical problem: Shortly after printing, their cells quickly start dying from a lack of oxygen and nutrients.

The missing piece: blood vessels. These are needed for bringing oxygen to cells deep inside thick tissues and carrying away wastes. Without that oxygen and waste removal, cells will die. That’s why building blood vessels into tissues is the biggest hurdle standing in the way of fully printed organs.

The problem comes down to diffusion — the movement of molecules from areas of high concentrations to low concentrations. Oxygen can diffuse only a few millimeters into tissue. In thicker tissues, cells in the center would suffocate before oxygen could reach them. Imagine a big, densely populated city with no roads. Food trucks can’t bring in supplies. Garbage trucks can’t haul away waste.

Scientists have tried ways to avoid this problem. One method uses sacrificial inks. These serve as temporary placeholders. The inks are printed in the shape of vessels, then tissue is printed around them. Once the tissue solidifies, the sacrificial ink dissolves. In its place sit hollow channels through which blood can flow.

Scientists are also looking for a way to print blood vessels directly into tissue as it’s being built up.

Moving forward

In September 2025, researchers made a promising advance with a 3-D bioprinting system they’re calling GRACE. ”This technology enables the printing of blood vessels directly around [the printed tissue itself],” explains Pramanick at Utrecht. She works in the lab that’s developing GRACE.        

Here’s how it works. A computer-vision system “sees” where cells are located. It then asks artificial intelligence to figure out the best pattern of blood vessels to deliver oxygen-rich blood to all those cells. In seconds, bioprinting then creates pipe-shaped structures to begin delivering that blood.

Speed matters enormously here. Traditional methods might take hours to print vessel networks layer by layer. Because GRACE does it in seconds, cells spend little time without oxygen.

Full organs remain years away. Forming stable, treelike structures in different sizes — from big arteries down to the tiniest vessels known as capillaries — to seamlessly work with a patient’s blood-vessel system remains a big challenge, notes Martin at VCU. As bioinks and printing speeds improve, the prospect of integrating printed tissues with blood vessels into the body appears ever nearer. Today, the question is no longer if 3-D bioprinting can create replacement organs — it’s when.