BioRap – Artificial Blood Vessel System

Bio-artificial tissues from human cells are produced in the lab with the help of tissue engineering. However, to construct larger organs, blood vessel systems that are necessary for nutrient supply and removal of metabolic products are still missing.

In the project “BioRap” five Fraunhofer Institutes are developing biocompatible and elastic artificial blood vessels using novel methods. 3-D-print technology has been established in rapid prototyping and can produce work pieces directly according to any complex 3-D-model. This method is being combined with high-resolution multiphoton polymerization.

The goal of tissue engineering is the production of human tissues and organs in the lab. However, the production of larger tissue constructs is currently limited, because the nutrient supply via vascular structures – comparable to the body’s blood vessel system – is lacking.  Fraunhofer researchers have joined their expertise in biology, polymer research, machine construction, laser technique, computational simulations and material characterization  with the objective of manufacturing technical equivalents to the native blood vessel system.

Combination of 3-D-print and multiphoton polymerization

Three dimensional objects can be generated flexibly using 3-D-inkjet-printing. The printer applies the material layers according to a CAD file and each layer is than chemically crosslinked using UV radiation. The 3-D-printing technique generates sub-micrometer structures in reasonable production times, yet for the very fine structure of artificial capillaries the printing technology is still too imprecise. Therefore, the scientists combine this method with multiphoton polymerization: This technique generates short intensive laser pulses, that stimulate crosslinking of the precursor molecules in very well defined small volumes of the material. This reaction can be specifically directed so that the construction of finest structures is possible according to a three dimensional blue print. Depending on the chemical material composition the elasticity of the material can be adjusted according to natural blood vessels

Material biofunctionalization

For biologization of synthetic tube systems endothelial cells (the cells that cover natural blood vessels in the body) will be connected to artificial vessels. In the first step, biofunctionalization of the artificial material is achieved by chemically coupling of biopolymers (e.g. heparin), and specific cell anchor peptides to the polymer surface in order to promote endothelial cell adhesion. The chemical coupling of biofunctional components is also important with respect to future applications of the artificial blood vessels as medical vascular grafts, because then the formation of blood clots (thrombogenecity) has to be strictly prevented.

Hybrid resins from synthetic and biologic polymers

As an alternative to the biofunctionalization of the finished synthetic tubes by coating, a new class of materials is developed:  hybrid materials composed of crosslinkable synthetic and biological polymers are designed to constitute novel “bio-inks”: These are printable and cytocompatible resins with integrated cell adhesion anchor-groups for direct biomaterial processing using additive manufacturing techniques.

Bioreactor for biomimetic blood vessels

The generation of a functional endothelial lining is essential for mimicking the biofunctionality of blood vessels: The endothelial cell layer avoids blood clotting, acts as selective barrier in native blood vessels, and initiates neoangiogenesis by the formation of new capillaries that sprout from an existing blood vessel. Therefore, future application of vascularized tissue models as in vitro testing systems for pharmaceuticals or chemicals require an intact endothelial cell layer. An important step in the cultivation of functional endothelial cells is to imitate the conditions in the body. For this purpose Fraunhofer develops a special bioreactor system where artificial vessel systems that are populated with endothelial cells are dynamically cultivated.

Applications

Manufacturing of artificial blood vessel systems will contribute to the development of complex bio-artificial organs. These can be applied for in vitro testing of pharmaceuticals and chemicals and can thus help to reduce animal experiments. As a long-term objective such bioartifical tissues may also be used as implants and can then be connected directly to the patients’ blood circulation to be supplied with nutrients. In addition, the treatment of bypass patients with artificial small caliber vascular grafts is feasible using the new materials and freeform fabrication techniques.

Further information:

→ www.biorap.de
→ www.igb.fraunhofer.de/en/competences/interfacial-materials/biomaterials/biorap-vascular-structures.html

Production of tube structures in the micrometer to the centimeter range

By combining 3-D-inkjet-print technology (Fraunhofer IPA), multiphoton polymerization, and 3-D-laser curing (Fraunhofer ILT) it is possible for the first time to generate branched vessels in the centimeter range with diameters smaller than 1 mm. Under the direction of the Fraunhofer IAP, special inks are developed for use in the combined additive manufacturing process, that allow for  tailoring the elastic properties of the cured polymer tubes. The prototype machine for the combined manufacturing process is currently located at the Fraunhofer Institute for Manufacturing Engineering and Automation IPA in Stuttgart and permanently under development.

Biomimetic tube structures

Biomimetic branching geometries for the artificial blood vessel systems are designed at the Fraunhofer Institute for Mechanics of Materials IWM and computational fluid dynamic modeling is performed to optimize the wall shear rates as well as the branching angle to avoid flow dead zones and eddies. For the biologization of synthetic tube structures chemical modification of biomolecules such as heparin is performed at the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB to enable chemical coupling of  biofunctional coatings to the synthetic materials.

Microvascular endothelial cells adhere freely to such biofunctionalized materials. They proliferate in static cultures as well as in bioreactors under dynamic condition until they form confluent cell layers.

By integrating the growth factor VEGF in the heparin-based layer, the cells are stimulated to enhanced proliferation and metabolic activity. In the future it will be investigated whether sprouting of new natural capillary structures (neo-angiogenesis) can be achieved in porous vessels through the effect of VEGF.