Consistent quasistatic and acoustic elasticity determination of poly-L-lactide-based rapid-prototyped tissue engineering scaffolds Krzysztof W. Luczynski, 1 Tomasz Brynk, 2 Barbara Ostrowska, 2 Wojciech Swieszkowski, 2 Roland Reihsner, 1 Christian Hellmich 1 1 Institute for Mechanics of Materials and Structures, Vienna University of Technology, Karlsplatz 13/E202, 1040 Vienna, Austria 2 Faculty of Materials Science and Engineering, Warsaw University of Technology, Politechniki Square 1, 00-661 Warsaw, Poland Received 2 February 2012; revised 31 May 2012; accepted 15 June 2012 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.a.34316 Abstract: This paper is concerned with reliable and physically sound elasticity determination of rapid-prototyped tissue en- gineering scaffolds made of poly-L-lactide (PLLA), with and without small portions of tricalcium phosphate (TCP) inclu- sions. At the level of overall scaffolds, that is, that of several millimeters, multiple uniaxial loading–unloading (quasistatic) tests were performed, giving access to the scaffolds’ Young’s moduli, through stress–strain characteristics during unload- ing. In addition, acoustic tests with 0.05 MHz frequency deliv- ered an independent access to elastic properties, in terms of the normal components of the scaffolds’ stiffness tensors. The latter strongly correlate, in a linear fashion, with the Young’s moduli from the unloading tests, revealing porosity independence of Poisson’s ratio. The magnitude of the latter is in full agreement with literature data on polymers. Both of these facts underline that both ultrasound tests and quasi- static unloading tests reliably provide the elastic properties of tissue engineering scaffolds. V C 2012 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 00A:000–000, 2012. Key Words: elasticity, quasistatic unloading tests, ultrasound tests, PLLA scaffold, elastic energy How to cite this article: Luczynski KW, Brynk T, Ostrowska B, Swieszkowski W, Reihsner R, Hellmich C. 2012. Consistent quasistatic and acoustic elasticity determination of poly-L-lactide-based rapid-prototyped tissue engineering scaffolds. J Biomed Mater Res Part A 2012:00A:000–000. INTRODUCTION Development of scaffolds reliably replacing bone and carti- lage has remained a key challenge in tissue engineering. Such a scaffold has to be not only biocompatible but also needs to carry sufficient load, and nevertheless, it has to enable tissue ingrowth. 1–3 The last requirement leads to usage of porous scaffolds, with pore sizes of one to several hundred micro- meters. 1,4 With increasing porosity, the load-bearing ability decreases, but on the other hand, bone ingrowth can occur more easily. 1,3,4 Besides the load-bearing capacity of such scaffolds, their elasticity is of importance, since it is the elas- ticity that predominantly drives the stress distribution in organ-implant systems, 1,5 and this distribution should devi- ate as little as possible from the natural state (without implant). Therefore, mechanical tests aiming at elasticity determination belong to the standard testing protocol for bio- materials. Besides classical mechanical testing (where the material sample is quasistatically deformed), ultrasound test- ing (where acoustic waves are sent through the material) is a very interesting alternative concerning elasticity testing. 6–10 Although elasticity determination from ultrasound is fairly well agreed upon (the normal and shear stiffness compo- nents are gained from longitudinal and transverse waves sent through the specimens 11 ), evaluation of the mechanical tests is done by a variety of methods, which we here shortly sketch for the uniaxial testing case: in force-per-area versus dis- placement-over-sample-height diagrams recorded during sample loading, the elastic modulus (also called Young’s mod- ulus) was derived from the slope of these diagrams’ ‘‘most linear portion,’’ 12 or their ‘‘steepest portion,’’ 13 or their ‘‘lon- gest linear portion,’’ 14 or their ‘‘initial linear region,’’ 15,16 or from ‘‘the tangent at the origin’’ of the aforementioned stress– strain diagrams. 17 If the sample truly behaved elastically only, then all these methods should lead to the same result. However, the problem is that, in most cases, biomaterials undergo elastic as well as plastic deformations when they are quasistatically loaded in a mechanical testing machine. Much of this plasticity occurs around the load transfer platens, where, due to friction and not perfectly plane sample surfa- ces, local stress concentrations occur, which lead to plastic material behavior. As a whole, then, the material sample behaves softer than it would do in a purely elastic state. Correspondence to: C. Hellmich; e-mail: christian.hellmich@tuwien.ac.at Contract grant sponsor: Seventh Framework Program of the European Commission (FP7), theme FP7-2008-SME-1, within project BIO-CT- EXPLOIT; contract grant number: 232164 V C 2012 WILEY PERIODICALS, INC. 1