Comparison of Synthetic vs. Biogenic Polymeric Process-Directing Agents for Intrafibrillar Mineralization of Collagen.

Neha Saxena,Vanessa Guarnizo,Joshua Mizels,Maegan A. Cremer,Douglas E. Rodriguez,Laurie B. Gower

doi:10.3390/polym14040775

Abstract

With the aging population, there is a growing need for mineralized tissue restoration and synthetic bone substitutes. Previous studies have shown that a polymer-induced liquid-precursor (PILP) process can successfully mineralize collagen substrates to achieve compositions found in native bone and dentin. This process also leads to intrafibrillar apatitic crystals with their [001] axes aligned roughly parallel to the long axis of the collagen fibril, emulating the nanostructural organization found in native bone and dentin. When demineralized bovine bone was remineralized via the PILP process using osteopontin (OPN), the samples were able to activate mouse marrow-derived osteoclasts to similar levels to those of native bone, suggesting a means for fabricating bioactive bone substitutes that could trigger remodeling through the native bone multicellular unit (BMU). In order to determine if OPN derived from bovine milk could be a cost-effective process-directing agent, the mineralization of type I collagen scaffolds using this protein was compared to the benchmark polypeptide of polyaspartic acid (sodium salt; pAsp). In this set of experiments, we found that OPN led to much faster and more uniform mineralization when compared with pAsp, making it a cheaper and commercially attractive alternative for mineralized tissue restorations.

Highlights

Researchers have been exploring ways to mineralize type I collagen scaffolds with hydroxyapatite, both to better understand biological mineralization processes and to create biologically resorbable bone substitutes
The function of OPN in mineralized tissues is debated, in vitro studies have shown that OPN inhibits hydroxyapatite formation in solution [29], but we have found that this inhibitory activity can promote intrafibrillar mineralization of collagen scaffolds [30]
Note that all of these values are artificially higher by about 13.5% due to the argon gas used for Thermogravimetric Analysis (TGA), which does not fully combust the organics

Summary

Introduction

Researchers have been exploring ways to mineralize type I collagen scaffolds with hydroxyapatite, both to better understand biological mineralization processes and to create biologically resorbable bone substitutes. Previous work in our group has shown that the polymer-induced liquid-precursor (PILP) process can effectively mineralize collagen substrates up to and beyond levels of native bone (65 wt.% mineral) [1,2,3,4,5,6]. The mechanism of intrafibrillar mineralization remains controversial, Niu et al [10] showed strong evidence that the Gibbs–Donnan effect (balancing of electroneutrality and osmotic equilibrium) provides the driving force for ion accumulation within the fibrils, while our group proposed that capillarity may play a role in the transport of the liquid-like phase deep into the interstices of the fibrils [1]. The key point for this paper is that simple anionic polypeptides (such as polyaspartic acid sodium salt, pAsp) can mimic the purported function of the highly charged, noncollagenous proteins (NCPs) found in mineralized tissues, which have long been thought to play a pivotal role in collagen mineralization in vivo [1,16]

Methods

Results

Discussion

Conclusion