Biologically degradable polymers can be electrically spun into a type of fleece that can be used as a scaffold and growth matrix for living cells. This fleece can be used to replace diseased or damaged tissue. In the field of cardiovascular medicine, the fleece can be used to produce new heart valves and tissue.
Fibres produced using a method known as electrospinning are ultrathin, and hence an ideal scaffold for living cells. The electrospinning method has been around for several decades, but it has only recently been optimised for applications in biotechnology and medicine. The fibres are produced, amongst other things, from polylactide, a polymer that consists of long chains of lactic acid molecules. The fibres are only a few nanometres in diameter and act as a kind of feel-good environment for the cells in which they divide and form their own extracellular matrix.
Fibre production is not easy to achieve and is associated with several challenges. The developers of suitable growth substrates therefore have to find ways to attract the cells they are interested in and entice them to settle permanently in the cell-free structure and form new cell- and eventually tissue structures. In addition, the synthetic scaffold needs to gradually degrade inside the patient's body as the new cell structure grows and supports itself.
A team led by chemist Dr. Svenja Hinderer at the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB in Stuttgart has addressed these challenges and, together with partners from the Tübingen University Hospital and the University of California in Los Angeles (UCLA), has developed a particular variant of electrospinning in which synthetic and biodegradable polymers are spun into fibres using an electrical charge. "We built the whole system ourselves. This initially involved developing cell-free heart valves that grow with the patient. We were able to benefit from work already done in the development of scaffold structures for tracheas," said Hinderer.
The group of researchers has modified the electrospinning method so that special proteins can be integrated into the spun tissue, and serve as kind of attractants for the cells. "We use proteoglycans such as decorin to which cells are able to adhere well. Work with tracheal cells has already shown that cell adhesion improves considerably when decorin is integrated into the scaffold structure," said Hinderer, going on to list a range of other advantages of proteoglycans: "They have an inflammatory effect and also store water. This moves us towards something resembling an extracellular matrix. This fits in well with our philosophy of giving cells as natural an environment as possible."
In the field of cardiovascular medicine, the team uses proteoglycans for several applications, including the production of an injectable hyaluronic acid-based gel. "Injected into the infarction area, the proteoglycans will help to attract and bind cells that are able to form functional heart tissue," said Hinderer. Another application involves spinning the proteins into a three-dimensional planar structure that looks a bit like a wound dressing. "The material is placed on diseased areas on the heart in the form of so-called heart patches to stimulate the formation of new tissue.
Manufacturing heart valves using electrospinning turned out to be by far the biggest challenge. The mechanical demands on heart valves are completely different from those made on heart tissue. The method therefore had to be adapted. "The degradation of fibres takes longer than the degradation of cardiac patches that dissolve relatively quickly. In addition, the material needs to be able to move like a natural heart valve and withstand the same shear and tensile forces as a real heart valve, be able to withstand changing blood pressure at the same time as being both flexible and stable. Only then are valves able to withstand functional strain," said Hinderer.
Hinderer and her colleagues now have a broad range of different applications in mind. "For example, we want to find out how old heart valves differ from young ones, which is why we are currently studying age-related tissue alterations of the heart. We hope that this will give us the information we need to adapt the different scaffolds to the relevant application." In addition to decorin, the researchers are testing a number of other proteoglycans for their suitability for electrospinning and to find out whether they support cell colonisation and tissue regeneration.
The new tissue substitute with proteoglycans was presented to the expert community at Medtec held in Stuttgart in April 2015. However, it is still too early to say when the material will be ready for market and subsequently find its way into medical care. As it is a cell-free implant that only comes into contact with cells after it has been implanted into the body, Hinderer knows that there is likely to be a time advantage, at least as far as the approval process is concerned. "As our product is a cell-free substrate, it is classified as a medical device rather than an ATMP, i.e. an advanced therapy medicinal product, which would involve a far more complex process," said Hinderer.
Svenja Hinderera, Jan Seifert, Miriam Votteler, Nian Shen, Johannes Rheinlaender, Tilman E. Schäffer, Katja Schenke-Layland. Engineering of a bio-functionalized hybrid off-the-shelf heart valve. Biomaterials 2014 Feb;35(7):2130-9. doi: 10.1016/j.biomaterials.2013.10.080