Tissue engineering: how to grow a bypass

In this interview with MEDICA-tradefair.com, Prof. Cornelia Blume talks about the right formula to cultivate vascular grafts in a rotating bioreactor, describes the measurement technique this requires and explains the role stem cells play in this process.

Prof. Blume, along with your team you have developed a bioreactor to cultivate vascular grafts. What does this device look like?

Prof. Cornelia Blume: The centerpiece of the reactor is a small tube, in which a one by eight centimeter sized vessel can be cultivated. The housing rotates to ensure an even distribution and growth of cells. Tubing and pump systems are connected to the housing, allowing us to simulate the body's cardiovascular blood flow within the vessel. We can run a second circuit outside the vascular graft, which simulates the tissue pressure in the human body.

So the reactor simulates different conditions found inside the human body. Which of these is particularly important to successfully engineer a vascular graft?

Blume: We start with a fetal circulatory system, where the vasculogenesis takes place at a blood pressure reading between 40-60 mm Hg. This is our estimation for the initial phase of the cultivation process. In the course of the cultivation process, we also want to test higher blood pressure values to make sure the implant can withstand these levels.

In a second step, we determine the so-called shear stress based on the blood pressure readings. This lets us determine the effect the existing pressure has on the cells. Many studies have shown that cells only combine to form tissue under a certain amount of shear stress.

Other parameters are also important for the cultivation process. We have integrated different sensor systems into the reactor to monitor them. The wireless control of these sensors ensures that we do not have to open the reactor during the cultivation process. This would disrupt the process. Among other things, we monitor the nutrient content based on glucose and lactate, the pH value and oxygen tension within the growing vessel. Lower oxygen tension benefits certain cells like the stem cells we work with and that come from a hypoxic niche of the body. We can also check the growth of the vessel by using ultrasound.

How does this work exactly?

Blume: The reactor's housing is made of polyetherimide, which is a synthetic material that ultrasound can easily penetrate. We can make the inside vascular structure and its surface very visible with a high-performance ultrasound machine. Yet we can also see the inside of the vessel, monitor the increase in wall thickness and determine whether there is consistent flow. We can illustrate the flow using Doppler mode, meaning we can also see a pulse wave just like we do with a human vessel and draw conclusions about the resistance in the artificial vessel.

What are the steps it would take to engineer a vascular graft in the bioreactor?

Blume: As a foundation, we use 3D printing to create a scaffold made of synthetic biomaterials such as biodegradable polymer. Thanks to the printing process that we have simultaneously developed in this facility, we can print a tube in the respective size. It is colonized with the body’s own cells in a secondary process. We plan to support their adhesion to the structure with fibrin. This is a protein that is produced naturally in the body and is also active in the coagulation process. Having said that, we will also test whether and to what extent we can print the cells directly on the basic scaffold structure using a bio-ink such as fibrin. This biohybrid structure will then be placed inside the reactor and subjected to the different cultivation conditions.

What types of cells do you use for the colonization?

Blume: We are presently working with so-called adipogenic adult stem cells. We isolate them from the fatty tissue of patients by using a mechanical, enzymatic procedure. The patients have undergone liposuction at the Hanover Medical School (MHH) and the fatty tissue we get is, therefore, ethically sourced. Some of these cells also change into perivascular cells. Now we want to find out whether we can convert enough of these cells into endothelial cells.

Another strategy might be to use immature endothelial cells. These can be found in the remnants of plasma cell donations in a blood bank for example. Plasma donations actually leave these types of cells behind in the filtration system. We could continue to cultivate them. It is less intricate to work with these progenitor cells than it is to work with adult stem cells. We would subsequently only use stem cells as so-called "homing cells". If we then use these "homing cell" and place them into the nutrient solution along with other cells, the "target cells", they express certain factors that convert target cells into endothelial cells.

How far along is your project and what are your next steps?

Blume: We have laid the technical foundation: the bioreactor with the monitoring systems is available, as is the 3D printer with the first copies of scaffolds based on different polymers and various types of cells. We were also able to indicate favorable cultivation conditions. Now we have to combine these components the right way and manage to cultivate the vascular graft in 3D.

To do this, we have secured DFG funding for the next three years. During this time, we want to create a vascular structure, which we want to test in large-scale animal testing together with our clinical partners, the project group of Prof. Mathias Wilhelmi of the Department of Cardiovascular and Thoracic Surgery at MHH. The next step would be to determine how well this type of structure can be used in animals and how it will continue to develop and mature inside the body.