The Quantification of Injectability by Mechanical Testing

By measuring the force, we can ascertain whether a material is injectable and how changing the needle and syringe size, or the formulation processing and chemistry can affect the injectabilty. This technique is fast and simple. Can be used with a variety of needle and syringe geometries, and requires little data processing to obtain the key force values.

This method is particularly applicable for injectable biomaterials, such as hydrogels and cements, the sustained drug delivery and tissue engineering applications. To set up the mechanical tester, attach flat compression testing plantains to the mechanical tester and manually equip the tester with the load cell with the maximum load of 200 Newtons. Use the manual control buttons to separate the plantains to allow approximately 30 centimeters of space for the needle, syringe and plunger.

To create a testing protocol, open the test wizard in the device software and set the test type to uniaxial compression. Set the measured force preload value at which the testing will begin and adjust the speed to preload to five millimeters per minute, to set the speed at which the crosshead will move down until it encounters the preload. Set the loading to displacement control and select an appropriate test speed.

Then set an upper force limit at which to stop the test. To set up the clamping system, attach two sets of clamps with grips large enough to securely ensconce the selected syringe to two stands. Place the grips between the crosshead and the base plate with enough space below the grips for the syringe and needle and line up the centers of the grips with the center of the crosshead.

Use an empty syringe to help with the alignment. Then check that the clamps are firmly secured so that there is no movement in the clamps when a downward force is applied and place a dish onto the bottom plate to collect the extruded material. To perform an injectability experiment, insert the syringe into the clamp grips and close the grips so that the syringe is held in place but can move up and down without resistance with the syringe and plunger perpendicular to the crosshead.

Use the buttons to lower the top plate just above the plunger and click zero force to zero the measured force. Then click run to run the testing protocol. At the end of the test, raise the plates to a sufficient height, such that the syringe can be removed and repeat the test with the next sample.

After each trial, save the data in a format such that a table of force and displacement values can be generated and plot the results from each trial with the displacement on the X-axis and the force on the Y-axis. When all of the trials have been plotted, determine the maximum and plateau forces from each graph. A typical force displacement curve consists of three sections.

An initial gradient, as the plunger overcomes the friction from the barrel and the material has accelerated a force maximum. And the plateau as the material is extruded at a steady state. For viscous samples passing through a more narrow orifice, the force needed to inject the sample at constant speed is greater than the force required to overcome the friction in the barrel and to accelerate the material.

Therefore, no distinct peak is observed. For highly viscous samples or very narrow needles, the force required to extrude the material may be so great that the syringe buckles and fails. Often with very little extrusion of the material.

If the material being injected contains particles or is beginning to set, filter pressing or bulk setting may occur leading to an incomplete injection. It is essential to make sure that the clumps are firmly secured so that the syringe fits snugly, is centered and is perpendicular to the crosshead. A correlation can be used to relate the measured force of the ease of injection.

Alternatively, Rheology can be used to understand how the process of injection affects the material.