Good biocompatibility and elastomeric mechanical properties contribute to the success of thermoplastic polyurethanes (TPU) in a range of medical applications such as dialysis tubing and pace maker casings. Increasingly TPUs are being studied for use in vascular grafts as a replacement for other currently used plastics (ePTFE, PET). While avoiding biodegradation was considered for many years optimal for implants, it has more recently been used in a controllable fashion to stimulate tissue regeneration. Ester linkages undergo slow hydrolysis with a rate highly dependent on conditions and the structure of adjacent moieties.

We have focused on aliphatic isocyanates and ester based chain extenders to give TPUs with mechanical performance (modulus, tensile strength, percent elongation) comparable to that of native blood vessels. Materials with promising performance were electrospun and collected as thin-wall narrow tubes. Electrospun prostheses were implanted into rats to replace the infrarenal aorta. After six months, the grafts were explanted and found to be compliant in all cases. Growth of new endothelial tissue was found to be greater in the degradable grafts. Further studies in larger animals will help elucidate therapeutic application of biodegradable TPUs.

S. Baudis, S. C. Ligon, K. Seidler, G. Weigel, C. Grasl, H. Bergmeister, H. Schima, R. Liska „Hard-Block Degradable Thermoplastic Urethane-Elastomers for Electrospun Vascular Prostheses“ J. Polym. Sci. A.: 2012, 50, 1272–1280

Photopolymers exhibiting biocompatibility and biodegradability are promising materials for the application in the field of Tissue Engineering. The possibility of structuring compounds via Additive Manufacturing Technologies (AMT) such as Microstereolithography, Digital Light Processing (DLP) or Two-photon Induced Photopolymerization (TPIP) enables the fabrication of constructs with complex geometries and high resolutions of about 10µm - or even 200nm in the case of TPIP - in order to mimic the cellular structures of natural materials such as bone.

State of the art compounds for these applications are (meth)acrylate-based. However, methacrylates exhibit low photoreactivity and acrylates are prone to side-reactions with proteins inside the human body giving explanation for the high irritation and sometimes toxicity. Furthermore, the formation of poly(acrylic acid) upon hydrolytic degradation provokes a local decrease of the pH value and can cause the formation of non-excretable precipitate in the presence of calcium ions. Hence, these materials are less suitable for biomedical applications.

Taking into account these adverse effects, new monomers with different polymerizable groups such as vinyl esters, vinyl carbonates and vinyl carbamates have been synthesized [1-3]. Therefore, various alcohols and amines with differing functionality, molecular weight and hydrophilicity were converted into the respective vinyl ester, -carbonate or -carbamate by one step syntheses with good yields. These new monomers were found to be significantly less cytotoxic than acrylates. Moreover, polymers derived from these classes of monomers give water-soluble and biocompatible poly(vinyl alcohol) as degradation product. Photo-Differential Scanning Calorimetry studies of the new monomer classes showed that their reactivity is between acrylates and methacrylates. Mechanical properties such as Young’s modulus and hardness were determined by nanoindentation experiments and were found to be similar to their (meth)acrylate counterparts. Additionally, basic studies towards the suitability of these new materials for biomedical applications were accomplished by measuring their cytotoxicity towards osteoblast-like cells and by conducting cell culture tests. To evaluate the degradation behavior, studies with model substances as well as the actual polymers were conducted under accelerated conditions. It was found that especially hydrophilic vinyl ester-based materials were readily degrading and exhibited significantly higher degradation rates than poly(lactic acid).

Finally, 3D structures were fabricated using AMT techniques and some of these parts made from selected compositions were successfully tested by in vivo-studies in New Zealand white rabbits.

[1] C. Heller, M. Schwentenwein, G. Russmüller, F. Varga, J. Stampfl, R. Liska:
"Vinyl Esters: Low Cytotoxicity Monomers for the Fabrication of Biocompatible 3D Scaffolds by Lithography Based Additive Manufacturing";
Journal of Polymer Science Part A: Polymer Chemistry, 47 (2009), 6941 - 6954.

[2] C. Heller, M. Schwentenwein, G. Russmüller, T. Koch, D. Moser, C. Schopper, F. Varga, J. Stampfl, R. Liska:
"Vinylcarbonates and Vinylcarbamates: Biocompatible Monomers for Radical Photopolymerization";
Journal of Polymer Science Part A: Polymer Chemistry, 49 (2011), 3; 650 - 661.

[3] Liska, Robert; Stampfl, Juergen; Varga, Franz; Gruber, Heinrich; Baudis, Stefan; Heller, Christian</span>; Schuster, Monika; Bergmeister, Helga; Weigel, Guenter; Dworak, Claudia; PCT Int. Appl. (2009), WO 2009065162 A2 20090528