Fracture strength and adhesive strength of hydroxyapatite-filled polycaprolactone Shing-Chung Wong Æ Avinash Baji Received: 16 March 2006 / Accepted: 27 March 2007 / Published online: 1 August 2007 Ó Springer Science+Business Media, LLC 2007 Abstract Fracture toughness and tear strength of hydroxyapatite (HAP)-filled poly(e-caprolactone) (PCL) with increasing HAP concentration were studied. The toughness was assessed in terms of essential work of fracture (EWF). Adhesive strength between HAP and PCL interfaces was evaluated using T-peel testing. The adhesion between the two components was found to be relatively strong. Double edge notched tension (DENT) and trousers test specimens were used for the EWF tests. The effect of HAP phase in PCL on the fracture and tearing toughness was investigated. The results obtained from the EWF tests for the HAP-filled PCL complied with the validity criteria of the EWF concept, namely, (1) geometric similarity for all ligament lengths; (2) fully yielded ligament and (3) plane-stress fracture condition. Values for specific essential work of fracture (w e ) and specific plastic work of fracture (bw p ) were found to decrease with increase in HAP con- centration. The testing procedure showed promise in quantifying the tearing resistance and rising R-curve behavior common in natural materials and it can be ex- tended to other biomaterials that exhibit post-yield defor- mation. A quantitative assessment based on fracture mechanics of the adhesive strength between the bioactive interfaces plays an important role for continued develop- ment of tissue replacement and tissue regeneration mate- rials. Introduction Biodegradable polymers reinforced by bioactive ceramics such as hydroxyapatite (HAP) have been widely studied for potential applications in tissue engineering [1–4]. An in- depth understanding of bioactive composites [3] offers attractive potential for tissue replacement and tissue regeneration. Most efforts concentrated on processing routes for tissue scaffolds such as fused deposition [5, 6], computer-aided manufacturing [7, 8] and other freeform fabrication techniques [9, 10]. Little is understood on how to design tough and strong scaffolds that emulate interac- tively the mechanical behavior of human bones. All scaf- folds need to be mechanically robust like bones, yet many are weak and brittle, fracturing under impact or cyclic fa- tigue loading easily. Studies on toughness characterization of cortical [11–15] bones were done in the last few decades but the high toughness values of natural materials (bovine femoral cortical bone: longitudinal K Ic ~ 3.2 MPaÖm[11] and transverse K Ic ~ 6.56 MPaÖm[12], human cortical bone K c ~ 2 MPaÖm[14]) were not obtained from bi- omimetic equivalents (monolithic HAP ~ 0.6 MPaÖm and HAP/nylon ~ 0.8 MPaÖm[16]). Natural bones show a rising R-curve behavior, K R , which suggests toughening mechanisms operate in the crack wake as cracks propagate. Most tissue engineering studies on mechanical properties only characterize the tensile strength and stiffness [17–19], with little reference to fracture mechanics, which was widely employed for the study of natural and human bones [11–16]. It is important for researchers to gain an under- standing of the micro/nano structures of natural materials and translate them into the design of bone analogue materials, or to optimize the properties of scaffold tissues via other materials design strategies. At present, an in- depth understanding of the design principles for tough S.-C. Wong (&)  A. Baji Department of Mechanical Engineering, The University of Akron, Akron, OH 44325-3903, USA e-mail: swong@uakron.edu 123 J Mater Sci: Mater Med (2008) 19:929–936 DOI 10.1007/s10856-007-3016-7