Biodegradable Elastomeric Scaffolds for Tissue Engineering
Ana Paula P
In the past years the search for suitable materials for use in tissue engineering represents a major area of study. Most attention has been given to polymers based on lactic acid and glycolic acid, however these materials have the disadvantage of being rather stiff and brittle. In addition, the rate of hydrolysis of these polymers can be quite high, with high degrees of swelling at late stages of the degradation. We have recently reported on the properties of poly(ester carbonate)s based on the slowly degrading poly(trimethylene carbonate) (poly(TMC)). TMC is copolymerized with ?-caprolactone (CL) or with D,L-lactide (DLLA) to prepare processable, hydrophobic elastomers with suitable mechanical properties and little or no crystallinity. By copolymerization, the degradation rate of poly(TMC) can also be tuned. In vitro degradation studies conducted on these polymers show that the nature of the comonomer and the composition are of influence on the degradation rate. DLLA copolymers with 50 or 80 mol% of DLLA lose their tensile strength in less than 5 months and undergo total degradation in 11 months. For CL copolymers a slow and gradual decrease of molecular weight was observed during the same time period. This is accompanied by a small deterioration of the mechanical performance. In both cases the higher the comonomer content the higher the observed hydrolysis rate. Presently we are investigating the use of scaffolds based on TMC elastomers for long- and short-term applications. Poly(TMC-CL) (10:90 mol%) is processed into porous two-ply tubes by means of salt leaching (inner layer) and fiber winding (outer layer) techniques. These grafts seeded with Schwann cells will be used as nerve guides for the bridging of large peripheral nerve defects. Such a graft is designed to maintain its shape and mechanical performance for 1 year. Poly(TMC) and poly(TMC-DLLA) (50:50 mol%) are totally amorphous and very flexible, they are excellent candidates for scaffolds for tissue engineering of blood vessels, heart tissue and skin substitutes. Porous structures are being prepared by the combination of phase separation techniques and porosifying agents.
Illucidation of the reaction mechanisms of silane coupling agent between silica and rubber
L.A.E.M. Reuvekamp; J.W. ten Brinke; P.J. van Swaaij J.W.M. Noordermeer
Adding a bi-functional coupling agent enhances filler-matrix compatibility. The most commonly used coupling agent is bis(3-triethoxysilylpropyl) tetrasulphide, known as TESPT, Si-69 or A1289. Its dual function encompasses a better dispersion of the filler during mixing and a chemical coupling between filler surface and polymer. The reactions that take place between the TESPT and the silica surface (primary reaction) and the rubber matrix (secondary) were studied.
Primary reaction The primary reaction was studied by using HPLC and dynamic mechanical measurements, from which kinetic parameters were calculated. There are indications that ZnO and/or stearic acid reacts as catalysts for the reaction. Mechanical testing revealed the mechanism of the secondary reaction of the coupling agent with the rubber matrix, silica-rubber bonding vs. the action as a curing agent.
Materials Science and Technology of Polymers
Marcin Debowski, University of Twente, Faculty of Chemical Technology
Morphology of Filled Elastomers via Atomic Force Microscopy (AFM). Fine.particle precipitated silica has been used as an active, reinforcing filler for many years, however, its wide application in one of the most obvious and promising areas . the tires production, has been limited for a long time due to a number of disadvantageous properties including the strong filler.filler and weak rubber.filler interactions. These problems could be circumvented by using specifically designed organosilanes as coupling agents. The silica filler treated with such reagents can be significantly better dispersed in the rubber matrix. The dispersion of various fillers in rubber has been determined predominantly by optical and electron microscopy. More recently atomic force microscopy (AFM) was also applied to the visualization of rubber morphologies and to studies of silica and carbon black micro.and nanodispersions. Outstanding flexibility and usefulness of this modern technique was utilized many times to solve or elucidate a wide range of phenomenon related to rubber multicomponent systems. The most obvious and already mentioned application is the determination of filler particles distribution. Using AFM phase.imaging one can clearly distinguish between the filler particles and the surrounding rubber matrix. This provided insight can be very helpful and supportive for explanation of many macroscopic, namely mechanical relations. For numerous rubber systems a distinctive morphology has been recorded. In this lecture the representative examples of differences in silica distribution for various unvulcanised rubber compositions will be shown. Different elastomers and different types of silica, both modified with silane coupling agents and these based on raw, unmodified silica filler were used. Various morphological features characteristic for multicomponent elastomeric systems will be also presented.
Reduction of zinc oxide levels in rubber compounds
G. Heideman, J.W.M. Noordermeer - University of Twente
B. van Baarle - TNO Industrie
Zinc oxide is generally known as the best activator for sulfur vulcanisation. It reduces the vulcanisation time and improves rubber properties. Although zinc is considered one of the least harmful of the heavy metals, soluble zinc compounds are classified as ecotoxic to aquatic organisms. Release of zinc into the environment from rubber occurs during production in the scrap, during service through wear, especially wear of tyres, during disposal or recycling of rubber products, for instance through leaching in land-fill sites. The increasing concern regarding the potential environmental and health effects of release of zinc compounds into the environment led to several proposals for reduction of zinc in rubber compounding. Complete elimination would be rather ambitious and would involve a fundamental change of the current technology of rubber vulcanisation. A possibility to eliminate zinc oxide is by replacing it by a Multifunctional Additive (MFA).
Thermoplastische elastomeren : invloed uniforme segment lengte
Dr. R.J. Gaymans, Universiteit Twente
Eigenschappen van TPE's hangen direct af van de fasenscheiding. Bij Polyurethaan en polyester type TPE's is de fasenscheiding door kristallisatie. Kistallisatie van segmenten in een keten hangt sterk af van de struktuur- regelmaat. Indien de lengte van de segmenten uniform is dan is een veel snellere kristallisatie te verwachten. Als de segmentlengte afneemt dan is de kristallizatie weer lastiger. Systemen die zeer goed uitkristallizeren zijn aramiden (Kevlar, Twaron). We hebben gevonden dat zelfs zeer korte aramide units van maar 1,8 nm lang, ingebouwd in een keten, uitermate snel uitkristalliseren en tevens een stabiele kristallijne structuur vormen.
Hierdoor is de fasenscheiding snel en compleet en zijn de eigenschappen weinig van temperatuur afhankelijk. Copolymeren bestaande uit deze aramide units en polyether segmenten zijn eenvoudig te synthetiseren, goed in de smelt verwerkbaar en hebben een bijzonder elastische gedrag. De materialen hebben dan ook goede eigenschappen voor thermoplastische elastomeer en elastische vezel toepassingen. Door toepassing van de korte aramide segmenten kon de vloeistof-vloeistof ontmenging voorkomen worden, waardoor tot een zeer hoog moleculair gewicht van het ether segment (9000 g/mol) er geen "melt phasing" optrad. Tevens vindt kristallisatie van de uniforme units al plaats bij aramide concentraties van 3%. Hierdoor zijn o.a. zeer zachte en bijzonder elastische materialen te maken. De uniforme lameldikte van de kristallijne fase maakt het ook mogelijk polymeer-eigenschappen afhankelijk van de kristalliniteit te bestuderen zonder dat een correctie nodig is voor lameldikte of warmte behandeling. De gevonden relaties zijn dan ook verrassend eenvoudig.