"We are looking at alternative materials, but so far our membrane fabrication concept works only on Ni-based alloys," Debashis Mukherji told nanotechweb.org. "To get around the problem, we've decided to try and coat the structure with a biocompatible material."

A porous membrane was made at the Technical University Braunschweig in Germany, transferred to the Laser Center Leoben, Austria, for plasma coating and then sent back to Germany for characterization at the Hahn-Meitner Institute, Berlin.

The team chose a high-energy coating technique dubbed pulsed laser deposition to encourage material deep inside the membrane's pores, which measure approximately 200 nm in width and have a depth of around 250 µm. The method involves firing a high-power, infrared (1064 nm) laser beam periodically onto a target to evaporate the substance and develop a dense plasma plume.

Diamond-like carbon (DLC) and titanium were selected as coating candidates as they are known to be highly biocompatible. In each case, the membrane was moved through the coating plume at a relative speed of 5.4 cm/s to encourage uniform coverage.

Using focused ion beam tomography, the researchers were able to view the treated structure in three dimensions. "It took around 3–4 hours to process 178 cross-sections," said Mukherji. "We used a low ion current of 50 pA for slicing to give a smooth surface for high quality SEM imaging."

The images together with spectroscopy data reveal that the plasma plume has penetrated all of the membrane's pores. Both the titanium and DLC-treated samples show similar coating behaviour with a thickness of around 50 nm, although the titanium coverage is less uniform.

"Next, we need to test how adherent the coatings are in hazardous environments, particularly in contact with the body fluid," commented Mukherji. "Once an optimal coating is developed, the following step would be to work with biomedical specialists to trial the coated substrate for tissue growth."

3D animation
A "fly through" video showing the membrane in detail can be downloaded here (file size 5.29MB , VLC media player recommended).

The researchers presented their work in Nanotechnology.