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In a paper entitled “A Real-time Rheological Measurement for Biopolymer 3D Printing Process,” a group of researchers develops a method of measuring the rheological properties of solutions for 3D bioprinting. Rheology is the study of the flow of matter, and the flow rate of 3D printing materials is extremely important to the final object, particularly in bioprinting. The typical method for measuring rheological properties is the use of a rheometer before the materials are dispensed.

“However, the rheological properties of biopolymers are time-dependent,” the researchers state. “Therefore, inaccurate parameters may be used under certain processing conditions. It also affects the precise control of flow rate, especially in the case of biopolymers with rapid gelation.”

In the paper, the researchers present a system for the real-time measurement of rheological properties during the dispensing process, rather than before it. The system is a combination of a user interface for setting up measurement and a vision measurement system. An image processing method is applied to measure the volume of dispensed fluid.

“The combination of pressure data in a syringe acquired from a pressure sensor and measured volume flow rates is used to construct a pressure-dependent fluid flow rate curve as a function of time,” the researchers explain. “The rheological properties of fluid materials are then determined by a numerical analysis procedure. The results from the numerical analysis provide a time-dependent power law index (N) of fluids. This index can be used as the flow control parameters of dispensing processes.”

For their experiments, the researchers used both a polymer and a biopolymer: a PVA solution and a PVA solution mixed with chitosan, respectively. The measurement setup included a fluid dispensing system, a system for the measurement of syringe pressure, a compressed air pressure regulator system, and a vision measurement system.

The rheological properties of the solutions gradually evolved after being mixed and put in the syringe during the dispensing process within 50 minutes. The researchers discovered that the increase in syringe pressure increases the fluid droplet volume.

“For different time stamps and recorded applied pressures, the measured flow rates were used to construct pressure-dependent fluid flow rate curves every 10 min for 6 experiments in the case of PVA (within 60 min),” the researchers state. “…According to the pressure-dependent fluid flow rate curve, the fluid flows of both PVA and PVA/CS are significantly sensitive to time changes. With these results, it is clear that the real-time rheological properties measurement system is critically needed to identify fluid flow behavior at a specific time during the fluid dispensing process.”

A time-dependent power law index of the fluids was determined as N, the value of which changed significantly according to the rheological properties of the fluids.

“They are comparable to the actual behavior of dispensed fluids in which they become more viscous (increasing n) after they are mixed and injected from the syringe,” the researchers conclude. “The resulting N can be used to perform automatic fluid flow control in future research.”

This research is very exciting as it could be used to vastly improve the quality of 3D printed parts in the future. This system could let you know more precisely what is being deposited or in real time syringe pressure or other variables could, in the future, be adjusted in order to get more accurate deposition.

Authors of the paper include Anchyza Yokpradit, Teerawat Tongloy, Supranee Kaewpirom and Siridech Boonsang.

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