Dongbo Wang1, Jing Ye1, Steve Hudson1, Vivek Prahbu1, Keana Scott2, Sheng Lin-Gibson1

1 NIST Polymers Division, Gaithersburg, Maryland

2 NIST Surface and Microanalysis Science Division, Gaithersburg, Maryland

Collagen is the primary structural protein found in connective tissues, making up more than 25 % of the whole-body protein content. It is also the primary biological molecule responsible for the organization of mineralized tissue. Bone and teeth are surprisingly complex organic/inorganic hybrid structures mainly composed of collagen and the calcium phosphate mineral - hydroxyapatite. These tissues are hierarchically organized from the nanometer to the meter length scales. At the shorter (sub-micron) length scales, the collagen/mineral interaction is of primary interest to those studying the dynamic process of biomineralization. Traditionally, collagen mineralization was thought to occur by calcium phosphate nucleation and growth processes from dissolved ions. Recently, these ideas have been challenged and the new concept of “non-classical” mineralization has emerged. In biological tissues it is now believed that the mineralization process starts with the formation of less than 10 nm reactive amorphous calcium phosphate (ACP) clusters which infiltrate collagen and then transforms into the final crystalline phase.

We begin to test this new “non-classical” understanding of collagen mineralization by studying the interaction between collagen matrices and colloidally stable/non-reactive gold nanoparticles as models for ACP. We dynamically measure changes in collagen matrix mechanical properties after introduction of gold nanoparticles using Quartz Crystal Microbalance with Dissipation monitoring (QCM-D). The QCM-D data show a particle size dependent interaction, where 2 nm particles strongly interact with the collagen matrix and cause stiffening in the bulk. Larger particles (3 nm – 40 nm) only interact to the surface of the collagen matrix and do penetrate into the bulk. We observe the same interaction dynamics with both positively and negatively charged nanoparticles. These results have been confirmed through imaging by AFM and FIB/SEM. However, the 2 nm particles, which are comparable in dimension to ACP likely do not fully infiltrate into the tightly formed collagen fibular structures. Therefore size alone cannot fully explain the ability of ACP clusters to form mineralized collagen. Continuing work focuses on using QCM-D to investigate stabilized calcium phosphate nanoparticles and their interactions with collagen matrices.