This method can answer key questions in the neonatology field, such as how much cardiac output is generated during chest compressions. The main advantage of this technique is that it enables the creation of a flexible heart model without the loss of anatomical realism. Demonstrating the procedure with me today will be Jasper Sterk, a technician from our laboratory.
To acquire image segments of a representative neonatal heart, first open an appropriate processing software program. Import a neonatal thoracic magnetic resonance image, or MRI, formatted as a Digital Imaging and Communications in Medicine file. Using the Editing Masks tool, select the area of the heart muscle on each MRI slice where the heart is present and create a new sketch layer.
Separately segment the two atria and the two ventricles in the same manner and use the Calculate 3D tool to render the muscle and chambers into separate 3D representations. Then use the Stereolithography Plus tool to export the segments as five stereolithography files using the optimal resolution for each file. For processing and printing of the neonatal heart mold pieces, load the atria and ventricle valve mold files into a computer-aided design software program.
Use the original MRI to determine the positions of the aortic, pulmonary, mitral, and tricuspid valves. Using the insert part function, drag the valve files for the positive and negative mold halves for each valve to their respective positions in the loaded set of atria and ventricles into the current file. Click the location on the surface of the atrium or ventricle to indicate the position of placement.
Under the Features tab, select Extrude Boss Base to extrude the bases of the positive and negative valves so that the valves protrude in their respective chambers, and merge the valve parts as appropriate. Add the pulmonary and aortic valve files to their respective ventricle locations, and open the Sketch tab and select the Circle tool. Then open the Feature tab and use the Sweep Boss Base function to sketch two arching five-millimeter diameter cylinders from the tops of the valves until both circular cylinder surfaces reach the horizontal position.
After merging the valve parts to their respective ventricles and arteries, select the Circle tool again and use the Extrude Boss Base function to extrude five-millimeter diameter vertical cylinders from the base of each of the four chambers until the cylinders are 40 millimeters in length and protrude into their respective chambers. Next, open the Sketch tab and select the Sketch Circle tool. Then, to create different depth indentations, open the Features tab and use the Cut Extrude function to sketch semicircles on top of the cylinders to add differential notches to the cylinders.
To subtract the shapes from the chambers and arteries, right-click to select the solid body of the chamber and artery of interest and select the Combine function to allow the Subtract setting to be selected. After saving the chambers and arteries, import the heart muscle model. Then start a new sketch and hold down the Shift key to select all of the cylinder base sketches.
To offset the six cylinder base sketches by two millimeters, open the Sketch tab and select Convert Entities. Open the Features tab and use the Extrude Boss Base function to merge the arching cylinders with the heart muscle model. To model a cube from the base of the six cylinders down, select Reference Geometry and open the Sketch tab and select the Square tool.
Then sketch a square with a length and width four millimeters wider than the widest part of the heart muscle model. Use the Extrude Boss Base function to extrude the square downwards with a thickness of eight millimeters. Merge the extruded square to the base of the six cylinders using the Merge Parts function.
Add four-millimeter cubes to each of the four corners of the base in the same manner. Using the square base as a sketch, extrude the base to cover the entire heart model and subtract all of the other parts from the cover. Use the Reference Geometry function to generate a reference plane to the height of interest and select Insert, Molds, and Split to split the top part of the leftover rectangle at the widest part of the heart model.
Split the leftover rectangle again at the most convenient mold release position in the same manner, but in the vertical direction. Then use a jetting printer and rigid, rubber-like photopolymer materials to print the mold parts. It is essential to use rubber-like material for the inner mold so that these molds can later be removed without breaking or damaging the model.
For cold injection molding and finishing of the mold parts, first spray all of the surfaces of all of the printed parts except the valves with a release agent, and wipe the pieces clean with tissue paper. After air-drying for 15 minutes, close the base of the mold and two side panels and place the mold on top of two spacers so the base of the mold is not in direct contact with the table surface. Insert a silicone cartridge into a manual dispensing gun and use the gun to add five milliliters of silicone to a measuring cup.
Mix the silicone with a mixing tool and use the mixing tool to apply a generous amount of liquid silicone to the negative and positive sides of the right atria and ventricle valves. We coat the valve to ensure their functionality. Connect the two chambers at the right valve angle and push the chambers on to their respective cylinders in the base mold.
Then attach the pulmonary and aortic arch cylinders in a similar manner. Allow the silicone to solidify for two minutes. Attach the top part of the mold and a static mixer to the cartridge, squeezing the cartridge until all of the silicone has been dispensed, and then releasing the pressure.
Adjust the entire mold onto the two spacers and insert the gun into the eight-millimeter injection molding socket. Squeeze with low pressure over the course of three minutes until all of the air vents show signs of silicone overflow. Then insert a metal spacer into the crack between the top and lower parts of the mold to open the top of the mold.
Remove the side parts of the mold one at a time in the same manner, taking care not to puncture the heart wall, and use a scalpel to pierce any bubbles in the silicone. Use a toothpick to fill any bubble holes with silicone and allow the model to cure for another 30 minutes. When the model is ready, firmly enclose the heart model with one hand and use compressed air to blow the model off of the base of the mold, leaving the six inner molds in the heart model.
Use a syringe of water to fill and pressurize the left and right ventricles to release the inner molds. Then use a Magill forceps to pull out the inner two mold parts without compressing the valve segment. This 3D model printing method can also be applied to other internal organs, such as the lungs, or to bone structures, such as the ribs.
Using very flexible inner mold materials for the 3D printing allows the creation and release of complex organic structures that would be destroyed during the removal of the inner chambers if stiffer printing materials were used. A high resolution of the intricate 3D printed model parts is essential for the generation of small organic components, such as those used in the heart model system. After development, this method paved the way for researchers to explore neonatal patient physiology.
After watching this video, you should have a good understanding of how to create an anatomically realistic neonatal heart with four chambers and four valves in a single casting cycle. Don't forget that working with sodium hydroxide can be extremely hazardous. Please do wear hand and eye protection during this procedure.
