Researchers 3D-print living cells with modified form of xolography

A modified form of Xolography enables 3D printing of living cells. This offers interesting possibilities, and may pave the way for 3D printing kidneys and muscle tissue.

This is research by TU/e researchers Lena Stoecker and Miguel Dias Castilho. Xolography is a new 3D printing technique that uses the crossing of light rays of two different wavelengths in a light-reactive liquid. The convergence of the light rays causes the liquid to transform into a detailed and solid 3D object. Thus, an object the size of a gummy bear can be printed in less than a minute.

Whereas many other light printing techniques are either slow and accurate or fast and inaccurate, Xolography actually combines speed and accuracy. In doing so, it can print up to a resolution of 20 µm. With Xolography, you print by 'curing' a liquid, attaching it. The printer determines the shape by dividing slices of the desired tissue into slices. It then projects this image slice by slice onto a light sheet, curing the material each time.

The printing process requires two types of light: UV light and visible light. The liquid reacts to exposure to UV light, which 'turns on' the material. Projecting images of the desired structure onto the liquid takes place with visible light. Using polymerisation, the liquid solidifies into its final shape.

The technique was developed by German chemist Stefan Hecht and physicist Martin Regehly. Within the spin-off Xolo, they have adapted the technique for various applications. Dias Castilho: "Four years ago, Xolo was looking for ways to further develop its technology for biomedical applications, while my team was looking for a disruptive technology that could potentially offer high resolution, fast production speeds and scalability - so a perfect marriage."

Stoecker and Dias Castilho have now succeeded in printing tissue with light. This makes them the first scientists in the world to use the technology to print living tissue. Hecht and Regehly are therefore following their research closely.

Tissue printing presented several challenges. "Among other things, the materials used must be biocompatible. Besides the hydrogels we developed for the process, we discovered that the photoinitiator system itself was not very cell-friendly and needed to be replaced. Working closely with the company, we developed and optimised the material formulations to ensure they were safe for biomedical applications," reports Dias Castilho.

'Scaffolds' made of hydrogel are printed, and then used in the lab as support structures to grow cells on. "To successfully grow tissue, we aim for the hydrogel scaffolds to contain features that mimic the natural environment of bone marrow cells, for example," Stoecker explains. "We were able to print detailed scaffolds with pores in the range of 100 μm-1 mm, which could ensure the supply of nutrients through the scaffold during cell culture. Small raised elements could be printed down to just 20 μm, in the size of a human cell."

Another challenge consisted of the different properties exhibited by natural tissues. Therefore, printing on a small scale is not enough to mimic natural tissue and exercise precise control over the behaviour of the cell. "Natural tissues exhibit different properties, for example, they are stiffer in one place and more flexible in another," Stoecker said. Many existing printing techniques are aimed at printing objects that are more homogeneous. The researchers managed to fully control the properties, allowing them to create stiffer and more flexible areas where needed.

The team has managed to field thermally responsive hydrogels, which allow the creation of 4D-printed structures. "These materials can change shape or properties over time in response to temperature changes, enabling more complex and functional tissue structures," reports Dias Castilho, "such as artificial muscles that can flex and stretch in response to subtle temperature changes." The researcher adds, "We have now shown that this technology has promising potential for healthcare. It allows us to create more realistic and better-functioning tissue models and implants."

The results of the study have been published by the researchers in Advanced Materials. The researchers expect that the insights from the study could advance the light-based fabrication of cell-loaded hydrogels with high resolution and programmable mechanical properties and shape. Stoecker added: "I am aware that our research still has a long way to go to reach people, but I like the idea that the techniques we are developing in the lab might one day contribute to improving health and someone's life."

Author: Wouter Hoeffnagel
Image: Bart van Overbeeke