A Matrix for Tissue

12/13/2010

Three-dimensional scaffolds on which cells can reside and grow into tissue or organs are in great demand in regenerative medicine. For the purpose of building such structures, materials scientists of the University of Würzburg have successfully developed new fibers with special characteristics.

The fibers have to satisfy high requirements: For use in the human body, they must be fully degradable – not too fast, but not too slowly either. Only specific cells are supposed to settle on them, then interconnect and develop into complex structures. Other substances, such as proteins or blood cells, are expected to stay away from them.

The fibers in question are extremely thin polymer threads, which can be woven into nets or three-dimensional structures. Professor Jürgen Groll conducts research on such materials with intended application in medicine. Since August this year, he has been the head of the Department for Functional Materials in Medicine and Dentistry of the University of Würzburg. Now, he presents to the public a very promising new development. His work is reported in the current issue of the renowned journal Nature Materials.

Extremely thin fibers grow in the electric field

"We have succeeded in developing a method for producing such fibers in one single step," says Groll. To generate ultra-fine polymer fibers has been possible before. The respective method is called "electrospinning". The underlying principle: An electric field is applied to a liquid, producing thin "jets" of the material. The fibers generated in this process are extremely thin – maximally ten nanometers, which is one hundred-thousandth of a millimeter.

Groll and his colleagues have now advanced this method in a significant way by developing a special macromolecule. If this molecule is added to the liquid from which the fibers are produced, the surface of the generated fibers changes radically. "The molecule transforms the typically 'hydrophobic' fibers into 'hydrophilic' fibers, meaning that they acquire an affinity for water," says Groll. This prevents the adhesion of undesired proteins to the fiber surface.

The uncontrolled accumulation of proteins on polymer fibers is a dreaded effect in regenerative medicine. It usually occurs rapidly after a material is introduced to the body. "The proteins quickly denature on the hydrophobic surfaces," says Groll. Thus, there is the risk that the immune system is activated and wound healing is affected – such are the undesired side effects of an implant. "Therefore it is very important to prevent the adhesion of such proteins," the polymer chemist explains.

A scaffold for body cells

Some adhesion processes, however, are very welcome: Specific body cells are expected to attach to the fiber structures, then interconnect and grow into a compact structure. In this way, health professionals can assist the body in closing large wounds more quickly, for instance. In the laboratory, scientists are working on the task to produce new tissues or even new organs with the help of these fibers. For this purpose, they use the polymer fibers to fabricate three-dimensional scaffolds in the required shape, onto which the desired cells are subsequently seeded – e.g. liver cells for producing a new liver or cartilage cells for replacing damaged joint surfaces.

The benefit of such implants is obvious: Since the new organ has been developed from the respective patient's own cells, there is no risk of rejection after the implantation. This dispenses with the need for a drug therapy, which is still required at the moment after transplantations with foreign tissue. Furthermore, the fibers are fully degraded after just a few months.

New organs grow in the laboratory

"Depending on which cells are intended to attach to the fibers, the respective bioactive peptides are provided on the fiber surface," says Groll. These peptides ensure that precisely the cells required in the respective case are attracted.

The method developed by Groll and his colleagues makes it possible to produce fibers and fiber structures in a much quicker way than before and to equip them with various characteristics. Groll is convinced that – with this method – it will become feasible in the near future to design structures in the laboratory that sustain the growth of complex tissues.

“Degradable polyester scaffolds with controlled surface chemistry combining minimal protein adsorption with specific bioactivation”, Dirk Grafahrend, Karl-Heinz Heffels, Meike V. Beer, Peter Gasteier, Martin Möller, Gabriele Boehm, Paul D. Dalton and Jürgen Groll. Nature Materials, DOI: 10.1038/NMAT2904