Introduction

 Over the past few decades, there has been much  interest in the structure and properties of mineralized biological tissues like bone and dentin (a structurally simpler analogue of bone that makes up the bulk of the human tooth); in particular, considerable research have been focused on their mechanical properties and into how they fracture.  An understanding of these properties is of great importance from the perspective of developing a realistic framework for life prediction, particularly in light of the effect that microstructural modifications from aging, disease, remodeling, etc., can have in degrading the tissue.  Central to these issues is the fracture toughness of these materials, which characterizes their resistance to incipient cracking and fracture, and the microstructural mechanisms that are the source of such resistance.  Understanding such properties in the context of the inherent hierarchical complexity of the microstructure of these tissues (Fig. 1) is of obvious importance.  However, surprisingly, such questions have largely remained unanswered and to a large extent, even uninvestigated.
 
 

Fig. 1:  Hierarchy of the microstructure of two common mineralized tissues, human teeth and bone are shown here.  Though very different at higher scales of organization, fundamentally, both tissues are comprised of collagen fibers.
 
 

    Evidence based on microscopic observations has indicated that a number of mechanisms that could “toughen” mineralized tissues are active.  Indeed, it has been observed that microcracks and crack bridges form during the fracture of both materials.  Typically microcracks preferentially form at the peritubular cuffs within the inelastic zone surrounding a macroscopic crack in (particularly human) dentin and around osteons, due to osteon-matrix interface debonding or osteon pull-out, in bone.  Crack bridging, conversely, have been suggested to occur by uncracked ligaments and/or collagen fibers in both dentin and bone.  Fig. 2 shows some typical examples of such mechanisms.

Fig. 2:  Scanning electron micrographs of (a) crack bridging by collagen fibers in human cortical bone, and (b) microcracking (indicated by white arrows) at tubule sites in dentin. (c) Evidence of uncracked-ligament bridging (indicated by white arrows) shown for a crack in 61-year old female cortical bone in an optical micrograph (center) and in x-ray computed tomographic reconstructions of through-thickness slices.  The horizontal black arrow in each case indicates the direction of nominal crack growth.  (Tomography was performed at the Stanford Synchrotron Radiation Laboratory, SSRL, Menlo Park, CA)
 
 

    Although, there are differences in the architecture of various mineralized tissues, at the nano-scale, they are fundamentally quite similar with a network (almost matte) of collagen fibers forming the basis of the microstructure.  Consequently, this study aims at furthering our understanding of the macroscopic fracture behavior in the context of the underlying collagen-based nano-structure.  Such research is believed to be critical to the development of a micromechanical fracture mechanics based framework for understanding the problem of fracture and fatigue failure in mineralized tissue.
 
 

Current Researchers

R. K. Nalla

J. H. Kinney

J. J. Kruzic

R. O. Ritchie
 
 

Recent Publications

R. K. Nalla, J. J. Kruzic, J. H. Kinney, R. O. Ritchie, "Mechanistic Aspects of Fracture and R-Curve Behavior in Human Cortical Bone ", Biomaterials, 2004. 

V. Imbeni, R.K. Nalla, C. Bosi, J.H. Kinney, and R.O. Ritchie, “On the in vitro fracture toughness of human dentin”, J. Biomed. Mater. Res., 2003; 66A:1-9.

R.K. Nalla, V. Imbeni, J.H. Kinney, M. Staninec, S.J. Marshall and R.O. Ritchie, “On the in vitro fatigue behavior of human dentin with implications for life prediction”, J. Biomed. Mater. Res., 2003; 66A:10-20.

R.K. Nalla, J.H. Kinney, and R.O. Ritchie, “On the fracture of human dentin- Is it stress- or strain-controlled?”, J. Biomed. Mater. Res., 2003. 67A: 484–495.

R.K. Nalla, J.H. Kinney, and R.O. Ritchie, “Effect of orientation on the in vitro fracture toughness of dentin: The role of toughening mechanisms”, Biomater., 2003; 24:3955-3968.

R.K. Nalla, J.H. Kinney, and R.O. Ritchie, “Mechanistic Fracture Criteria for the Failure of Human Cortical Bone”, Nature Mater., 2003; 2:164-168.

R.K. Nalla, J.H. Kinney, S.J. Marshall and R.O. Ritchie, “On the In Vitro Fatigue Behavior of Human Dentin: Effect of Mean Stress”, J. Dent. Res., 2004;  83(3):211-215.

J.J. Kruzic, R.K. Nalla, J.H. Kinney and R.O. Ritchie, “Crack blunting, crack bridging and resistance-curve fracture mechanics in dentin: Effect of hydration”, Biomater., 2003; 24:5209-5221.

V. Imbeni, R.K. Nalla, J.H. Kinney, M. Staninec, S.J. Marshall, G.W. Marshall and R.O. Ritchie, “Stress/Life Cyclic Fatigue Behavior of Human Dentin”, in: Proc. IADR/AADR/CADR meeting (J. Dent. Res.), San Diego, CA, March 2002. [View poster]

R.K. Nalla, V. Imbeni, J.H. Kinney, S.J. Marshall and R.O. Ritchie, “On the development of life prediction methodologies for the failure of human teeth”, submitted to: Proc. Symposium on Materials Lifetime Science and Engineering, Eds.: P.K. Liaw et al.), TMS, San Diego, CA, March 2003. [View paper] [View presentation]

R.K. Nalla, V. Imbeni, J.H. Kinney, M. Staninec, S.J. Marshall, G.W. Marshall and R.O. Ritchie, “On the Development of a Fracture Mechanics-Based Approach to Failure Prediction in Human Dentin”, in: Proc. IADR/AADR/CADR meeting, San Antonio, TX, March 2003. [View poster]

R. O. Ritchie, C. L. Muhlstein, and R. K. Nalla, “On the Fatigue and Fracture of 'Nano' and 'Bio' Materials”, in: Proceedings of the International Conference on Advanced Technology in Experimental Mechanics 2003 (ATEM'03), H. Kimura, ed., 2003. 

A.E. Porter, R.K. Nalla, J.H. Kinney and R.O. Ritchie, “Changes in mineralization with aging-induced transparency in human root dentin”, in: The 2004 Gordon Research Conference on Biomineralization, New London, NH, August 2004.  [View Poster]