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Polycarbonate-urethane provides toughness for implanted valve

A new German project shows the growing importance of polycarbonate-urethane as a medical implant material.

A new German project shows the growing importance of polycarbonate-urethane as a medical implant material.

Engineers at the Fraunhofer Institute for Manufacturing Engineering and Automation IPA in Stuttgart are developing an automated production facility that can make venous valve prostheses from polycarbonate-urethane (PCU) plastic. The project is sponsored by the German Federal Ministry of Economics and Technology BMWI.

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The finished venous valve is highly durable.  Source: Helmholtz-Institute of Biomedical Engineering of RWTH Aachen
The device targets a medical condition called chronic venous in-sufficiency (CVI), which is caused by malfunction of the venous valves in human legs. If the venous valve does not close properly, blood will collect in the legs. This leads to edemas, and can cause open ulcers.

It's usually treated with anti-inflammatory and other drugs.

Efforts to develop a valve to treat the condition is being conducted by Fraunhofer, a leading global contract research organization, with four industrial partners and Helmholtz-Institute for Biomedical Engineering of RWTH Aachen University.

Built drop-by-drop

They are using a 3D droplet dispensing tool that enables researchers to apply a polymer on freeform surfaces, and at the same time combine various grades of Shore hardnesses. "3D droplet dispensing technology is an additive procedure that allows three-dimensional geometries to be created layer by layer using a polymer", says Oliver Schwarz, a group manager for Faunhofer.

The scientists say they chose PCU because of it strength and flexibility. It can also be sewn into surrounding tissue and be fabricated in very thin layers. That's a requirement for replacement of wafer-thin atrioventricular valves.

"By using PCU in combination with our 3D dispensing kinematics, we can achieve seamless transitions within the material between six different grades of elasticity and hardness - without any breaking points whatsoever. This technique mirrors the design of highly stressed structures in nature. It can't be done using injection molding," says Schwarz.

To start the process, PCU dissolved in a solvent and deposited drop-by-drop onto a venous valve prosthetic mold using the dispensing tool. The system is accurate to within 25 micrometers, and can deliver up to 100 droplets per second, each with a volume of 2 to 60 nanoliters.

A six-axis kinematic system then positions a piezo feeder above the mold. Once it is fully coated with droplets, the mold is bathed in nitrogen, causing the solvent to evaporate. More layers are applied until the structure is thick enough to be peeled  from the mold. They are implanted into the veins of the leg with a catheter passed through the skin.

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