Hydroxyapatite nanoparticle loaded collagen fiber composites: Microarchitecture and nanoindentation study Andrei Stanishevsky, 1 Shafiul Chowdhury, 1 Peserai Chinoda, 2 Vinoy Thomas 1 1 Department of Physics, University of Alabama at Birmingham, Birmingham, Alabama 35294 2 Department of Biomedical Engineering, University of Alabama at Birmingham, Birmingham, Alabama 35294 Received 11 October 2005; revised 18 April 2007; accepted 19 July 2007 Published online 27 November 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.a.31657 Abstract: Hydroxyapatite (HA) nanoparticle—collagen composite materials with various HA/collagen weight ratios were prepared from HA/collagen dispersions using the solution deposition and electrospinning with static or rotating collectors. The composites with nanoparticle HA to collagen weight ratio of 80:20 can be easily prepared in the solution deposition approach, whereas in the electro- spun fibrous composites it was possible to reach a maxi- mum HA/collagen weight ratio of 30:70 while maintaining a good fibrous structure. The structure, surface morphol- ogy, and nanoindentation properties of these nanoparticle HA/collagen composites with different microarchitectures were investigated. The values from 0.2 GPa to 20 GPa for nanoindentation Young’s modulus and from 25 MPa to 500 MPa for hardness, were obtained depending on the fabrication technique, composition, and microarchitecture of the composites. It was observed that the nanoindenta- tion Young’s modulus and hardness of the HA/collagen composite materials seem to achieve maximum values for 45–60% HA content by weight. Ó 2007 Wiley Periodicals, Inc. J Biomed Mater Res 86A: 873–882, 2008 Key words: hydroxyapatite; nanoparticles; collagen; elec- trospinning; nanoindentation INTRODUCTION The growing need for resorbable bone substitutes 1 and implant fixation devices 2,3 require the develop- ment of materials that are both resorbable and bioac- tive, 4 while structurally and mechanically similar to bone and stable for certain periods of time. Many natural (collagen, starch, chitin, and chitosan) and synthetic (e.g., polyglycolide (PGA), polylactide (PLA)) resorbable polymers have been used to repair cartilage and bone tissues. However, the mechanical strength, toughness, and elastic modulus of these polymers are lower than those of natural hard tis- sues. Attempts to mimic the hard tissue properties and architecture caused explosive growth of research on polymer/ceramic composite materials combining resorbability and bioactivity with improved mechan- ical properties. 5–9 Bone is a complex hierarchically structured mate- rial where up to 70% of its dry weight is an inor- ganic phase that consists mostly of nonstoichiometric hydroxyapatite (HA) with low crystallinity, and the rest is mostly collagen. The mechanical properties of bone depend on its internal architecture at all levels of hierarchy. 10 At the lowest level of hierarchy, bone matrix consists of mineralized collagen fibrils. While the toughness of bone results from its complex hier- archical microstructure, at the level of the mineral- ized collagen fibrils, the soft matrix between the hard but brittle mineralized HA particles plays a crucial role in bone fracture toughness. 11 The HA nanocrystals are usually plate-like shaped with a lateral size in the range of 15–200 nm, and are only 2–8 nm thick. They grow in direct contact with colla- gen fibers with the c-axis of crystals aligned along the longitudinal axis of collagen fibrils. The mineral HA component embedded into the collagen fibrils increases their stiffness, but decreases their fracture strength. These mineralized collagen fibrils are the elementary units of the complex bone structure, and the amount of mineral particles and their arrange- ment within the fibrils critically affect the mechanical performance of the whole system. One can suggest that a synthetic nanocrystalline HA/natural collagen fibrous composite could poten- tially be chemically and structurally the closest mate- rial to resemble a low level of the bone microarchi- tecture. A number of research groups have prepared Correspondence to: A. Stanishevsky; e-mail: astan@uab. edu Contract grant sponsor: National Science Foundation; contract grant number: CMS-0555778 Ó 2007 Wiley Periodicals, Inc.