UC San Diego

Bone can achieve high strength and toughness simultaneously through its composite structure and multiple structural hierarchies, which are revealed from the nanoscale to macroscale. While numerous studies have been published on hierarchical structure of bone, the nanostructure is still a topic of debate. To date, there are three main models of bone at the nanoscale. The first and most widely accepted is the staggered crystal model which states that hydroxyapatite (HA) crystals are generally confined to the gap regions in the collagen matrix. Another model states that mineral is primarily extrafibrillar, surrounding collagen fibrils as curved lamellae. Finally, we propose that bone is an interpenetrating phase composite with a continuous mineral phase.

This doctoral dissertation explores whether bone is an interpenetrating composite by isolating the component phases in bone through either deproteinization to remove the organic phase (collagen) or demineralization to remove the mineral phase (non-stoichiometric hydroxyapatite). In these studies, the most efficient chemical treatments that also preserve residual phases was found. Quality of the remaining phases was examined using techniques such as Raman spectroscopy, Fourier transform infrared spectroscopy, x-ray diffraction, and scanning electron microscopy. Mineral was found to form a continuous phase in bone from the deproteinization study, and demineralized bone was used to make a collagen scaffold with biomedical applications.

Next, the effect of geometrical arrangement of phases on mechanical properties was explored through 3D printing and finite element modeling. The geometrical arrangements tested include a discontinuous phase composite, a matrix inclusion composite with a stiff or soft frame, and an interpenetrating composite with a stiff or soft phase representing the mineral and protein phases of bone, respectively. A continuous stiff phase was found to increase composite stiffness. The soft phase functioned to redistribute stress. Size effects of 3D print features on mechanical properties of composites are also discussed, showing that imperfections in additive manufacturing have a large effect on mechanical properties.

Finally, drawing inspiration from two collagen-hydroxyapatite composites (porcupine fish spines and bone), synthetic scaffolds are designed and created using freeze casting. The effect of microstructure and hydration on mechanical properties is discussed for the topics, respectively.