Degradation and hemostatic properties of polyphosphate coacervates

Sodium polyphosphate is a linear polymer formed from phosphate units linked together by sharing oxygen atoms. Addition of calcium to a solution of sodium polyphosphate results in phase separation and formation of a polyphosphate coacervate best described as a polymeric rich viscoelastic material. Polyphosphate coacervate is an interesting candidate as a biomaterial based on its ability to bind with different cations and to be loaded with drugs. Here, in vitro degradation and hemostatic properties of polyphosphate coacervates are comprehensively evaluated. We show that polyphosphate coacervates degrade and dissolve at a fast rate, losing half of their original mass in a week and transforming to mainly pyrophosphate after 4 weeks. This burst dissolution phase happens earlier for the coacervate prepared from very short chain polyphosphate but overall using longer polyphosphate chains does not increase the coacervate longevity significantly. Substitution of Ca with Sr or Ba does not affect the hydrolysis of coacervates but slows down their dissolution into the media. In a whole blood clotting assay, coacervates profoundly decrease the clotting time especially when very long chain polyphosphates are used. While coacervate chain length and divalent cation type were found to significantly affect prothrombin time and thromboplastin time compared to the control, no discernible trends were observed. Platelets adhere in large numbers to coacervates, especially those containing long chain polyphosphate, but the cell morphology observed suggests that they might not to be fully activated. Overall, the long chain polyphosphate coacervate holds a great potential as a resorbable hemostatic agent. Divalent cation additions to a sodium polyphosphate solution result in polyphosphate coacervates, or highly viscous gel-like materials, having great potential in bio-applications such as drug delivery and hemostasis. As these coacervates degrade in aqueous environments, we undertook a comprehensive evaluation to better understand the impact of polyphosphate chain length and divalent cation substitution on this hydrolytic response in order to better predict degradation behavior in the body. Furthermore, there is great interest in the role of polyphosphates in hemostasis following recent publications showing that platelets secrete polyphosphates upon thrombin stimulation. In this paper, we evaluate the hemostatic potential of polyphosphate coacervates as bulk constructs, demonstrating that indeed these materials hold great potential as a degradable hemostatic agent.