A 3D volume sample of live cells in a gel captured using optical coherence tomography. The volume is approximately 1mm by 1mm by 1mm; the white objects in the volume sample are the live cells. Image: G. Babakhanova/NIST.
A 3D volume sample of live cells in a gel captured using optical coherence tomography. The volume is approximately 1mm by 1mm by 1mm; the white objects in the volume sample are the live cells. Image: G. Babakhanova/NIST.

In the burgeoning field of tissue engineering, live cells are grown in artificial scaffolds to form biological tissue. But to evaluate how successfully the cells develop into tissue, researchers need a reliable method for monitoring the cells as they move and multiply.

Now, researchers at the US National Institute of Standards and Technology (NIST), US Food and Drug Administration (FDA) and US National Institutes of Health (NIH) have developed a non-invasive method for counting the live cells in a three-dimensional (3D) scaffold. This real-time technique can image millimeter-scale regions to assess the viability of the cells and how they are distributed within the scaffold – an important capability for researchers who manufacture complex biological tissues from simple materials such as living cells. The researchers report their findings in a paper in the Journal of Biomedical Materials Research Part A.

The researchers began by creating a 3D scaffold system made from a network of polymer molecules that can hold large amounts of water, forming a type of material known as a hydrogel. Next, they embedded a type of human white blood cell that can reproduce endlessly into this 3D hydrogel.

Cells can be very sensitive to the environment in which they’re grown: if a researcher wants to study the growth of bone cells instead of breast tissue, they need to be cultured under different conditions. Moreover, the scaffolds that house the cells are also made from different materials and can serve a variety of purposes.

“The scaffold holds things in place, and it provides a micro-environment for whatever you want from cells,” said NIST biologist Carl Simon. “You could tune the scaffold to direct cells to behave in a certain way.”

The team used a non-invasive imaging technique called optical coherence tomography (OCT), which is like an ultrasound test, except instead of sound waves it uses light waves.  “To determine if a cell is alive, we analyzed the optical signal created due to the motion of the organelles inside the cells,” explained NIST physicist Greta Babakhanova, first author of the paper.

The researchers detected organelle motion by shining light through the cells. They classified cells as live or viable when the organelles were moving, indicated by changes in the transmitted light.

The NIST method is non-invasive, and there’s no cutting or staining of samples. The method is also label-free: cells do not need fluorescent molecules known as ‘labels’ to be attached to them to be seen. Previous methods required constant contact with the samples, which can be destructive and costly and affect the results. The new technique also reduces the time researchers spend on their measurements from hours to minutes.

The method differs from previous methods that rely on flat, two-dimensional (2D) samples. “The drawback of the existing techniques is that you can measure a certain number of cells, but you don’t know where they’re located,” said Babakhanova. “With this method we can image a 1mm cube of hydrogel and see where the cells are located within the gel.” She added that another reason why 2D approaches don’t work quite as well is because they don’t closely mimic the 3D microenvironment that cells experience in the body.

As a next step, the researchers are looking at using this technique to study other properties, such as the structure of biofabricated tissue. “The OCT methods may be able to non-destructively measure specific structures that evolve as the tissues mature in real time as a measure of their readiness for implantation,” said Simon.

In the meantime, the method already meets an unmet need in tissue engineering, with its ability to monitor the number and arrangement of cells in an artificial scaffold without having to disassemble and destroy it.

This story is adapted from material from NIST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.