Spindle-like-aggregating behavior of hydroxyapatite nanorods in polyacrylic acid aqueous system

Significance 

Hydroxyapatite is one of the primary components of hard tissues in human body, accounting for 60% of the bone and 96% of the enamel. Enamel, which is mainly made up of calcium phosphate, is the hardest human tissue. It exhibits excellent mechanical properties desirable for various biological functions. Under proteases and structural proteins regulation, the enamel is mainly derived from the highly ordered mineralization of sub-enamel crystals into oriented nano- hydroxyapatite crystals bundles. Consequently, bone tissue, which is the ordered composites of hydroxyapatite nanoplates and collagen fibrils, also possesses the remarkable bone regenerative ability and mechanical strength properties.

Considering the critical role of ordered mineral structuring in the mineralization of hard body tissues, biomimetic mineralization has drawn significant research attention for studying the aggregation behavior of nano-hydroxyapatite. It also underpins the development of an effective and controllable method for studying ordered aggregation. The assembly of hydroxyapatite to form rod-like hydroxyapatite and spindle-like aggregates has been demonstrated in both osteogenesis and biomimetic mineralization processes. To this end, a thorough understanding of the spindle-like aggregation behavior of hydroxyapatite nanorods and the underlying biomimetic mineralization mechanism is of great importance.

On this account, a team of researchers from the Wuhan University of Technology: Mr. Zhenhao Sun, Mr. Jia Chen, Mr. Ziyou Ding, Mr. Yiran Fan and led by Professor Yingchao Han who is also affiliated with Foshan Xianhu Laboratory of the Advanced Energy Science and Technology Guangdong Laboratory, investigated the spindle-like aggregation behavior of hydroxyapatite nanorods by dispersing the hydroxyapatite ultrasonically in a polyacrylic acid aqueous system. The main objective of the study was to provide more and deep insights into the aggregation behaviors and associated mechanisms. Their research work is published in the journal, Colloids and Surfaces A: Physicochemical and Engineering Aspects.

In their approach, the polyacrylic acid utilized here had a low molecular weight of about 1800, desirable to influence the aggregation of the hydroxyapatite nanorods in water conditions. The particle size distribution, surface charge, potential energy, phase composition and aggregation morphology of the nano-hydroxyapatite parameters were characterized by varying the hydroxyapatite molar ratio and by using different techniques like X-ray diffraction. In addition, the underlying mechanism underpinning the nanorods aggregation processes into spindle-like aggregates was explored based on the extended Derjaguin–Landau–Verwey–Overbeek (DLVO) theory.

The research team found that the hydroxyapatite nanorods formed spindle-like primary aggregations with an increase in the hydroxyapatite molar ratio and aspect ratio and a decrease in the diameter. The most preferred polyacrylic acid adsorption on the side surface of hydroxyapatite nanorods generated steric repulsion effects that were opposed to the van der Waals interaction in the radial interaction. Therefore, compared with the aggregation and precipitation of hydroxyapatite without polyacrylic acid, the produced primary aggregates exhibited sufficient repulsion to keep the nanorods suspended and dispersed in a water with remarkable stability. This was attributed to the ability of combined electric double layer repulsion and steric repulsion to overcome the effects of the van der Waals interaction forces.

In summary, the study reported the successful adjustment of the spindle-like behavior of the hydroxyapatite nanorods by simply controlling the content of polyacrylic acid. The study provided an important and valuable insights into the underlying mechanisms responsible for the aggregation behaviors of spindle-like hydroxyapatite nanorods based on the Derjaguin–Landau–Verwey–Overbeek theory as well as the steric stabilization mode. In a statement to Advances in Engineering, Professor Yingchao Han, the lead and corresponding author said that their findings would be of great benefit for mimicking ordered mineral structuring associated with hard tissue mineralization processes.

Reference

Sun, Z., Chen, J., Ding, Z., Fan, Y., & Han, Y. (2022). Spindle-like-aggregating behavior of hydroxyapatite nanorods in polyacrylic acid aqueous systemColloids and Surfaces A: Physicochemical and Engineering Aspects, 634, 127933.

Go To Colloids and Surfaces A: Physicochemical and Engineering Aspects

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