Polysaccharide-modified silicone interfaces

Silicone elastomers are a class of polymers widely used in commerce because of their exceptional properties. The repertoire of different silicone products that silicone manufacturers have developed is astounding and silicones have already become an integral part of people`s lives. Their application area ranges from anti-graffiti coatings over hair conditioners to biomaterials, for example, pacemakers, drainage implants, catheters or drug delivery systems. Despite this broad utilization there are still some difficulties that are particularly apparent when silicones come into contact with bacteria, cells or proteins and when they are implanted for a long time. Catheters, for example, always expose the patient to the risk of infection because any bacteria adsorbed from skin contact are carried into the body, where they can lead to infections. This problem can be addressed by modifying the surface of silicones with substances that inhibit bacterial growth. Another disadvantage of silicone implants is their lack of lubricity. Lubricious materials are advantageous for both the insertion and removal of medical devices from the human body, to reduce both pain for the patient and tissue damage. Due to this reason, the materials have to be coated with polymers that have a great affinity to adsorb water and, thereby, assume a jellylike state. Polysaccharides are able to take up very high amounts of water. Therefore, they represent highly interesting materials for the modification of silicone elastomers. In this research project our attention is focused on two hypotheses: (i) polysaccharides will render a silicone elastomer lubricous and permanently water wettable and (ii) selected carbohydrate-modified silicones will have a hydrophilic/hydrophobic balance suitable to suppress bacteria adhesion while optionally supporting the growth of mammalian cells. The innovative aspect and one of the most important challenges is the combination of silicone elastomers with polysaccharides. Silicones are insoluble in water and have water-repellent properties, whereas polysaccharides as mentioned above - can swell and absorb high amounts of water. The main focus of this project will therefore be given to the development of robust methods to combine silicones and polysaccharides. We will investigate two different pathways: one resulting in a permanent linkage between the silicones and the polysaccharide, and another with a weaker, reversible bonding situation. The latter will open the possibility to exchange polysaccharides on the surface and since polysaccharides differ greatly in their physical properties, this will allow one to deliberately design the physical properties of the materials. This topic has not been described in literature so far. Any gain in knowledge will be relevant for the further development of silicone-based materials. After finishing this project, we will be able to better understand the influence of the polysaccharide on the water-repelling properties of silicones and their influence to biological performance.

Silicones are one of the most important materials in our everyday life and are used in sealants, cooking utensils, medicine, and for thermal and electric insulation to name just a few. Commonly, silicones are water repellant and despite their popularity and importance in many fields there is a paucity of studies for green and environmental friendly modification approaches to increase the affinity of silicone materials for the absorption of water and the generation of lubricous surfaces which are advantageous in many application areas. The aim of the FWF Schrödinger project was the exploitation of a simple, green, and straightforward approach to bind hydrophilic (water loving) compounds to hydrophobic (water repellent) silicone chains to obtain the water affinity mentioned above and to investigate the properties of these novel compounds. Performed studies showed that sugars represent appropriate hydrophilic compounds for the above-mentioned task. A simple synthetic approach towards their permanent attachment onto hydrophobic silicone chains under mild reaction conditions and in non-toxic solvents was developed. It was discovered that after the attachment of sugar, silicones can absorb water and swell when immersed in aqueous solutions. In addition, the reaction products showed a rather unusual behavior for silicone materials. To give an example: the sugar-modified silicones were able to respond to external stimuli, e.g., to the application of shear or force by transforming from a liquid into a rubber. In more detail: the reaction products are liquid at rest but form a rubber when stirred at high stirring rates or when compressed rapidly. This fluid to rubber transition (not just fluid to more viscous fluid, or fluid to solid) is extremely rare and highly interesting for applications in which the mobility of a fluid but at the same time the energy dissipating properties of a rubber are required. Shock absorbers, e.g., in liquid armor for personal protective applications are one example. Factors with which the liquid to rubber transition can be tailored were investigated. It was found that in addition to temperature and the sugar/silicone ratio, solvents can be added to adjust the shear rate or force needed to trigger the liquid to rubber transition. These outcomes allow to adjust the energy dissipating properties of the polymers in a programmed manner and to generate polymers that either absorb all applied energy or to recover parts of it and thus to perfectly adapt the polymers for the desired application.