The field of tissue engineering has made it possible to engineer organs that can replace damaged hearts or kidneys in the human body. The technology involves growing live cells in artificial scaffolds to create biological tissue. However, to assess the success of this process, scientists need a reliable method to monitor the movement and multiplication of the cells.
To address this need, researchers from the National Institute of Standards and Technology (NIST), the U.S Food and Drug Administration (FDA), and the National Institutes of Health (NIH) have developed a non-invasive technique to count live cells in a three-dimensional (3D) scaffold. This real-time method uses imaging to evaluate the viability and distribution of the cells within the scaffold, an essential capability for scientists who are working to create complex biological tissues from simple materials such as living cells.
The team began by creating a 3D scaffold system composed of a network of polymer molecules that can hold significant amounts of water, resulting in a hydrogel. They then embedded this hydrogel with a type of human white blood cell that has the ability to reproduce infinitely.
For researchers studying cell growth, the specific conditions in which the cells are grown can greatly impact their behavior. For example, if someone wants to examine the growth of bone cells instead of breast tissue, different environmental factors are necessary. Additionally, the scaffolds that contain the cells are made of various materials and serve different purposes.
As NIST biologist Carl Simon explained, “The scaffold serves as a micro-environment for the cells and holds everything in place. By adjusting the scaffold, researchers can direct cells to behave in a specific manner.”
To assess the viability of the cells in the 3D scaffold, the team utilized a non-invasive imaging technique called optical coherence tomography (OCT). This method is similar to an ultrasound test, except that it employs light waves instead of sound waves.
According to NIST physicist Greta Babakhanova, the first author of the paper, “To determine cell viability, we analyzed the optical signal produced by the motion of organelles within the cells.” The researchers detected organelle motion by shining light through the cells and identified live or viable cells by the movement of organelles, as evidenced by changes in the transmitted light.
The NIST team’s approach is noninvasive, avoiding the need for cutting or staining samples. It’s also label-free, meaning that cells don’t need to be marked with fluorescent “labels” to be visible. In contrast, previous techniques relied on contact with the samples, which can harm the cells and skew results. The new method also saves time, reducing measurement time from hours to minutes.
Another advantage of the technique is that it’s designed to analyze cells in a 3D environment, unlike earlier methods that were limited to 2D samples. As Babakhanova notes, “With this method, we can image a one-millimeter cube of hydrogel and see where the cells are located within the gel.” This is important because 2D approaches don’t accurately mimic the 3D microenvironment of cells in the body.
Moving forward, the researchers plan to apply their technique to investigate other aspects of tissue engineering, such as the structure of biofabricated tissue. According to Simon, “The OCT methods may be able to nondestructively measure specific structures that evolve as the tissues mature in real time as a measure of their readiness for implantation.”
Overall, the NIST team’s method addresses an unmet need in tissue engineering, enabling researchers to monitor the number and arrangement of cells in an artificial scaffold without having to destroy it.