Building Affordable, Durable, Medium-Fidelity Ballistic Gel Phantoms for Ultrasound-Guided Nerve Block Training

So we leverage the latest technology such as computerated design and 3D printing to make custom durable ultrasound guided nerve block trainers with relatively low cost materials. In doing so, we explored questions relating to the durability and cost effectiveness of nerve block trainers while maximizing the fidelity and accessibility of these trainers. Currently, medical educators and simulation centers must either pay thousands of dollars for hands-on nerve block trainers or save money using perishable materials that spoil rapidly.

Our nerve block trainers use affordable materials that last for months to years, all while simulating anatomy more accurately than other low-cost simulators. Other attempts to create homemade nerve block trainers have generally involved perishable materials such as animal meat, which spoils quickly, or homemade gelatin, which grows mold within weeks, even with refrigeration. Our trainers last years with minimal maintenance and cost a fraction of the price of commercial trainers.

We're working to develop these nerve block trainers for a variety of other procedures within the operating room in an emergency room. Additionally, we're using infrared tracking in a variety of software options to record metrics about how physicians perform these procedures, so that we can analyze the results and try to correlate those to clinical performance. To prepare an ultrasound phantom, have a physician familiar with the nerve block procedure create a reference ultrasound image from a human volunteer, ensuring that the image has a view transverse to the applicable nerve or tissue plane for anesthetic injection.

Open the computerated design software, click on Solid, then Insert and Canvas to create a flat canvas from an ultrasound image with known dimensions. Select file, then click on XY plane. Drag the image into the X Y space, and scale it to match known unit length.

Next, click on Solid, followed by Create Sketch. Then click the XY plane and select the two-point rectangle tool to create a rectangular prism over the region of the image. Drag the rectangle over the area and refine it within the length or width boxes when closed.

Press Enter and click on Finish Sketch. Click on the rectangle, then select Solid and Extrude. Drag the rectangle to the desired height, refine the height box when closed, and press enter.

To create another sketch on top of the prism, click on top of the rectangular prism. Then click on Create Sketch, and draw the desired anatomy and inner edge of the encasing rectangular prism by clicking Create and Sketch, followed by modify toolboxes. Ensure the canvas is visible behind the prism.

If required, adjust the canvas through Display Settings. After creating the sketch of the cross section of the model, click on Finish Sketch. To create an internal prism from the sketch, click on the shape in the sketch, then choose Solid and Extrude.

Drag the shape back into the rectangular prism at the desired length. Select the operation to new body and click enter. To view this new object, click on Bodies.

Then click on the I symbol next to the new body's name to turn off the visibility of other objects. Now, click on Bodies and the I symbol next to everybody except the one being exported to export each body individually for 3D printing. Click on File, followed by Export.

Select the Type as stl file and press Enter. In a slicer software compatible with the 3D printer, open the STL file. Using the Place on face button, position the model on the print bed.

Ensure the bottom of the model is in contact with the print bed. Then under Print settings, select 0.2 millimeter speed. Under Filament, select Generic PLA, and under Printer, select the desired printer.

On the Supports menu, select Everywhere. Select Infill as 15%and select the Brim for print stability. Then click Slice now.

Export the G-code file to an SD card. Plug the card into the 3D printer and print the file using polylactic acid filament. Glue the 3D printed model to the bottom of a topless plexiglass container.

Then submerge the model in rapid curing silicone rubber per manufacturer guidelines. Once the silicone is set, remove the hard plastic model and plexiglass case, leaving a silicone mold of the desired tissue layer and container for basic gel porting. To begin, open the STL file for the simulated bones in a slicer software compatible with the 3D printer.

Then 3D print the imitation bone using acrylonitrile butadiene styrene filament. Submerge 80%acrylic and 20%wool yarn in a plastic cup filled with ultrasound gel. Place the cup in a 1 atmosphere pressure chamber.

Using a single stage vacuum pump, repeatedly build up and release the pressure in the chamber until all bubbles are removed from the ultrasound gel. Heat ballistic gel and dye in a convection oven with intermittent stirring until the liquid reaches 132 degrees Celsius. Add approximately 4.5 grams of finely granulated flour per kilogram of ballistic gel into the liquid ballistic gel and stir the mixture.

Place the gel in the oven for 20 minutes with intermittent stirring for even mixing. Add additional clear ballistic gel or dye as necessary to adjust the mixture's color to simulate human tissue. Insert the solid steel rods of varying diameters into the designated locations on the reusable silicone molds to create channels in ultrasound phantoms representing blood vessels.

Pour the colored ballistic gel with suspended flower particles into molds and allow it to cool. After cooling, remove the metal rods and final ballistic gel tissue layers from the molds. For the tissue layers with simulated vessels, dip one side of the tissue layer into the liquid ballistic gel and allow it to cool.

Then hold these tissue layers upright, and using a needle attached to a syringe, introduce simulated blood into each vessel. Now, using liquid ballistic gel, cover up the remaining vessel opening and seal off each fluid-filled vessel completely. To assemble the phantoms, coat each component in ultrasound gel.

Insert the coated phantom into ballistic gel prisms, placing 3D printed bones or yarn nerves properly. Then to seal the models, dip them on both sides into a pan filled with liquid ballistic gel. Using a heat gun, smooth the edges of the phantom, removing bubbles and imperfections.

Finally, pour ballistic gel over a sealed model covered with a thin layer of ultrasound gel to prevent annealing between the newly poured skin layer and the existing model.