Abstract
The nanostructure of bone has been replicated using a polymer-induced liquid-precursor (PILP) mineralization process. This polymer-mediated crystallization process yields intrafibrillar mineralization of collagen with uniaxially-oriented hydroxyapatite crystals. The process-directing agent, an anionic polymer which we propose mimics the acidic non-collagenous proteins associated with bone formation, sequesters calcium and phosphate ions to form amorphous precursor droplets that can infiltrate the interstices of collagen fibrils. In search of a polymeric agent that produces the highest mineral content in the shortest time, we have studied the influence of various acidic polymers on the in vitro mineralization of collagen scaffolds via the PILP process. Among the polymers investigated were poly-L aspartic acid (PASP), poly-L-glutamic acid (PGLU), polyvinylphosphonic acid (PVPA), and polyacrylic acid (PAA). Our data indicate that PASP and the combination of PGLU/PASP formed stable mineralization solutions, and yielded nano-structured composites with the highest mineral content. Such studies contribute to our goal of preparing biomimetic bone graft substitutes with composition and structure that mimic bone.
Highlights
Bone is a composite material composed of collagen and hydroxyapatite (HA) [Ca10(PO4)6(OH)2], which has been highly studied due to its unique mechanical properties and the possibility of creating synthetic bone substitutes
We have proposed that non-collagenous proteins (NCPs) that are close to the mineralization front play a crucial role in the biomineralization process of bone by inducing such a precursor pathway [2,5,6,7,8,9,10,11]
Tay and Pashley utilized these polymers in combination to achieve intrafibrillar and interfibrillar mineralization of demineralized human dentin. They theorized that polyacrylic acid (PAA) serves as a stabilizer of amorphous nanoprecursors of the calcium phosphate phase, while polyvinylphosphonic acid (PVPA) facilitated mineralization by templating the binding to collagen [29,30]. This hypothesis is similar to ours with respect to nanoprecursors, except they have found that both polymer analogues are required to cause intrafibrillar mineralization in their system, while we find that poly-L-aspartic acid (PASP) alone is effective under our reaction conditions
Summary
Bone is a composite material composed of collagen and hydroxyapatite (HA) [Ca10(PO4)6(OH)2], which has been highly studied due to its unique mechanical properties and the possibility of creating synthetic bone substitutes. The structure of bone is arranged in hierarchical levels, described by Weiner and Wagner [1] as starting from the individual constituents of hydroxyapatite and collagen fibrils, to the nanostructure of an interpenetrating mineralized collagen network, to the lamellar microstructure of osteons, to the macrostructure of cancellous and cortical bone. This study focuses on the nanostructural level, which is comprised of collagen fibrils mineralized with intrafibrillar, [001]. One proposed process for recreating bone structure, called the polymer-induced liquid-precursor (PILP) process, focuses on the intrafibrillar mineralization of self-assembled collagen arrays, and the evidence suggests that the mechanism involves an amorphous, liquid-phase mineral precursor. We have proposed that non-collagenous proteins (NCPs) that are close to the mineralization front play a crucial role in the biomineralization process of bone by inducing such a precursor pathway [2,5,6,7,8,9,10,11]
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