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Goals and Design Specifications
Goals and Design Specifications
- User Feedback
In order for the model to be effective at training students, there must be feedback to inform the user if the syringe has been placed in the correct location (the femoral head-neck junction), has hit a different negative structure (nerve or vasculature), or is in tissue that would have neither a negative or positive response.
2. Use of Ultrasound
2. Use of Ultrasound
The material selected for the model must allow ultrasound to pass to a degree that provides an image acceptably similar to that seen when using ultrasound on a patient. Additionally, the included anatomical structures, for example, nerves, vasculature or bone, must image similarly to that seen in a clinical setting. This can be tested by using ultrasound on the model and comparing signal intensities to a known standard image.
3. Anatomical Accuracy
3. Anatomical Accuracy
The model must provide an acceptable level of accuracy to the relevant anatomical structures. This is to ensure that trainees learn the procedure using accurate and useful landmarks that will transfer to clinical practice. To accommodate variable anatomy from patient to patient, the model will be designed to align with an average anatomy. Anatomical accuracy will be measured through evaluation by clinicians with expertise on the relevant anatomy.
4. Durability
4. Durability
The model must have the ability to withstand more than one injection and still provide the same level of training. The material cannot leave puncture holes that would change the training effectiveness for students using the model following other users. This will be tested through material durability testing.
Proposed Solution
Proposed Solution
The block diagram on the left shows an overview of the proposed solution.
The model of the hip will contain the following components:
Femoral artery and vein (silicone tubing covered in conductive mesh)
Sciatic nerve (silicone rod covered in conductive mesh)
Top of femur and pelvic bone (3D printed ABS covered in mesh)
Joint Capsule (silicone coated in mesh and electrical tape)
All of these components will be connected to wires that will be attached to an Arduino. During insertion, the internal circuitry in the model would detect if the needle contacts any structures representing vessels, nerves, bones, etc. that should not be touched; in clinical practice, accidentally puncturing these structures could cause negative consequences, such as internal bleeding and nerve damage. The circuitry would also be able to detect if the user had reached the correct target at the end of insertion.
A syringe will be wired and will consist of a button sitting at the bottom that the plunger will hit and wires coming out a hole cut into the side (one wire runs to the needle tip and two to the button itself).
The last component of the HIP model is the GUI used by the trainer to record and track their performance. A serial communication between the GUI and the Arduino microcontroller will be established, and will output to a text console when the user strikes any of the 5 relevant structures (bone, vein, artery, nerve, and joint capsule).
Initial Uncertainties
Initial Uncertainties
Initial uncertainties include:
Execution of the circuit
Selection of an ultrasound-compatible material for the bulk material
Integration of the circuit into a model to provide the user feedback
The unknowns were prioritized and defined with the corresponding objective. It is assumed that the feedback element of the final model will be the most difficult task to complete in comparison to the other unknowns. Therefore, the primary goal of the prototype is to develop a circuit. Other goals include selecting an ultrasound compatible material and tubing to simulate vessels, analyzing the materials under ultrasound and displaying the exterior of the hip model.
Brainstorming and Prototype Testing
Brainstorming and Prototype Testing
Material Selection
Material Selection
Bulk Material
Bulk Material
The most sophisticated underlying engineering principles justifying the accuracy of the appearance under ultrasound and physical texture of our training model lie in the material science optimization of the bulk simulated tissue that will be implemented in our final design. With the guidance of a recent journal article published by the Society for Simulation in Healthcare [15], our team has decided to pursue the use of modified PVC as the main bulk material simulating the soft tissues contained within our model of the hip and pelvic region of a standard adult male.
Our bulk tissue design consists of 3 ingredients:
Plasticized PVC suspension supplied by M-F Manufacturing (Fort Worth, TX)
Mineral oil
Chalk powder
Our team reproduced the recipe for “Mixture 2” published in the [15] paper. The recipe consisted of mixing plasticized PVC suspension with mineral oil in an 11:1 ratio by volume, heating the mixture to 340 oF and allowing for the mixture to transition from a milky white and runny fluid to a viscous and translucent homogenous solution. At this point, 1 g of chalk powder per 150 mL of liquid volume is stirred in to create an opaque viscous suspension. Finally, the entire mixture can be poured into a mold of choice and cured at room temperature until hardened into a tough and flexible gel. Mixture 2 was chosen by our team because the researchers reported this polymer possessed qualitatively realistic appearance under ultrasound, while closely replicating the needle injection forces of an 18-ga needle as compared with cadaver tissue. The training model will use a 21-ga injection needle, meaning that additional testing will have to be performed to quantitatively validate that the injection forces corroborate with those of cadaver tissue despite the smaller needle diameter.
This recipe offers a great deal of flexibility in tuning the overall mechanical toughness and appearance of a gel under ultrasound.
Mineral oil is added to the PVC-plasticizer suspension in order to soften the final gel that is formed, as well as giving the surface of the final polymer an oily residue that helps it to easily pop out of whatever mold it is formed. The PVC/plasticizer/mineral oil volume ratios must be optimized experimentally, since there is no practical model to predict the bulk elastic properties of the polymer at room temperature. Moreover, chalk powder suspended in the gel reflect ultrasound waves, giving the simulated tissue a bright and shimmery appearance under ultrasound, like how real human tissues look. This also improves the contrast between the bulk tissue and suspended anatomical structures, like blood vessels, nerves, and bone. By simply modulating the amount of chalk powder added to the gel, our team will be able to fine tune the appearance of our ultrasound-guided hip injection model.
Anatomical Structures within the Hip
Anatomical Structures within the Hip
The main design considerations that factor into the materials used for the anatomical structures suspended within the PVC gel (blood vessels, nerves, bone, and joint capsule) arise because of the relatively high temperature of the liquid PVC gel (340 oF). Thankfully, high melting silicone rubber is sold over a variety of hardnesses, which will be used to construct the blood vessels, nerves, and joint capsule needed to give our training model anatomical accuracy. Additionally, simulated bones 3D printed from high melting ABS plastic filament will remain rigid while the PVC gel cures. Finally, the silver-coated nylon meshes that will be incorporated in our needle location sensing circuitry will also be able to withstand the high curing temperatures.
Prototype Tests
Prototype Tests
Ultrasound Test
Goal: Verify silicone tubing and 3D printed ABS cubes can be seen under ultrasound to show feasibility for V1 prototype.
Circuitry Test
Goal: Verify that the circuitry is working properly submerged within bulk material, and that all components are insulated from one another.
Graphical User Interface (GUI) Test
Goal: Verify that the user interface is able to properly interact with an Arduino.
Repairability Test
Goal: Verify that the effectiveness of the model will be maintained for subsequent users.
Procedure and Test Results from each test are below.
Procedure and Test Results from each test are below.
Ultrasound Test: Procedure
Ultrasound Test: Procedure
Goal: Verify silicone tubing and 3D printed ABS cubes can be seen under ultrasound to show feasibility for V1 prototype.
Materials:
Polymer materials to test
Silver-Nylon Mesh samples (2”x2”)
Wires
Tubing to embed
Rigid object to embed with mesh (3D printed ABS cubes)
Heating element (Propane burner or equivalent)
Glass or plastic container to heat polymer in
Mixing tool
Molds to pour in (tupperware or equivalent)
Methods:
Sample preparation (Take pictures along way for records)
Solder one wire to each end of silver-nylon mesh sample
Prepare polymer solutions per instructions on container
Place tubing and rigid object with mesh in mold so that the ends of the tubing and wires are outside of the mold and will be accessible after polymer has been poured into the mold.
Pour polymer solutions into molds
Wait for polymers to cure
Bend or cut mold to remove polymer samples
Ultrasound test
Setup ultrasound system per system documentation
Place each sample under the ultrasound probe, adjust gain to find when image has highest quality
Save images of each sample for analysis
Analysis
Compare saved images to actual physiology ultrasound image, note any clear differences
Note how clear the embedded structures (tubing and rigid object with mesh) are on each saved image.
Figure 3. The tubing (dark circle near the bottom) can clearly be distinguished from the rest of the bulk material.
Ultrasound Test: Results
Ultrasound Test: Results
The goal was to find and test an ultrasound compatible material to be used for the bulk of the model as well as tubing to be used as blood vessels. The material tested for the bulk was a PVC and mineral oil mixture with suspended chalk powder, and the material used for the blood vessels was silicone tubing. When testing a mold of the PVC containing a stretch of tubing under ultrasound, the tubing could clearly be distinguished from the bulk (Figure 3) which was considered a successful material combination. The images were compared to a human ultrasound (Figure 2). A clinical advisor was able to confirm that the image appeared sufficiently clear under ultrasound and resembled the human ultrasound image. This demonstrates that the material selection was able to function under ultrasound as desired for the device.
A test was done in order to assess how well the material is able to hold up to multiple injections (this was done by ultrasounding prior to any injections, then again after the material was punctured 20 times in the same area, and a final time following an additional 30 punctures in the same area), and it was found that after 20 injections the marks were slightly visible on ultrasound, and after 50 even more visible (Figure 4). It was also noted that with more injections, the material because softer and the needle moved through more easily.
The hip model was unable to be ultrasounded after using a heat gun due to extenuating circumstances.
Figure 4. The PVC material following 50 injections with a needle. The vertical white line towards the right side of the screen demonstrates the degradation of material following numerous punctures.
Circuitry Test: Procedure
Circuitry Test: Procedure
Goal: Verify that the circuitry is working properly submerged within bulk material, and that all components are insulated from one another.
Materials:
Multimeter
Completed hip model
Test Protocol:
To confirm properly working circuitry:
Attach the wires to the appropriate ports on the multimeter to perform a continuity test
Turn on the multimeter to the continuity test setting
Touch one of the probes to the wire coming out of the hip model attached to the bones
Insert the other probe into the bulk material until contact is made with the bone (note- if the structure is too far for the probe to reach, it may be necessary to attach a needle to the probe and touch the structure with that)
Take note of if the multimeter beeps (if it does it indicates the circuitry is working properly)
Repeat steps 3-5 with all submerged structures (femoral artery, femoral vein, sciatic nerve, joint capsule)
To confirm proper component isolation:
Attach the wires to the appropriate ports on the multimeter to perform a continuity test
Turn on the multimeter to the continuity test setting
Touch one of the probes to the wire coming out of the hip model attached to the bones
Insert the other probe into the bulk material until contact is made with the joint capsule (note- if the structure is too far for the probe to reach, it may be necessary to attach a needle to the probe and touch the structure with that)
Take note of if the multimeter beeps (if it does not, it indicates the components are properly insulated)
Repeat steps 4-5 with all other structures submerged near the bone
Repeat steps 3-6 with all other submerged structures (joint capsule, femoral artery, femoral vein, sciatic nerve)
Circuitry Test: Results
Circuitry Test: Results
Based on the circuitry test, all components are insulated from one another.
Graphical User Interface Test: Procedure
Graphical User Interface Test: Procedure
Goal: Verify that the user interface is able to properly interact with an Arduino.
Materials:
Computer running user interface
Arduino/ circuit
Fully constructed hip model
Test Protocol:
Open/ run user interface on computer
Hook up arduino to a simple circuit including one of the submerged structures from the hip model
Start trial on user interface by hitting “begin”
Probe one of the structures with the wire/ needle
Check that the results posted by the user interface are correct
Repeat steps 3-5 for all internal structures
Figure 5: Results of graphical user interface test
Graphical User Interface Test: Results
Graphical User Interface Test: Results
The core functionality of the Arduino code is to continuously sense which mesh the needle is in contact with, and then to correlate that mesh with a positive or negative outcome which will be communicated to the user. Mesh discrimination happens by having each mesh have a different voltage associated with it, when the needle reads a specific voltage, it can determine which mesh has been hit based on the voltage. Mesh voltages are predetermined using simple voltage divider circuits with two resistors. The resistors values are selected so each mesh has a voltage that can be distinguished from the other meshes and so resistors with common off the shelf resistances would be used.
The needle was injected multiple times into different locations and Figure 5 displays the output. Ultrasound was not able to be used during the test due to extenuating circumstances. Since the circuitry test was a success, it is most likely that the displayed output corresponds with the correct injection site.
Repairability Test: Procedure
Repairability Test: Procedure
Goal: Verify that the effectiveness of the model will be maintained for subsequent users.
Materials:
Fully constructed hip model
Heat gun (reaches >350 degrees Fahrenheit)
21-ga needle
Hair dryer
Test Protocol:
Look at fully constructed hip model under ultrasound before performing any injections.
Look at the surface of the hip model before performing any injections and note any surface variations.
Using the needle, perform 50 injections in the same area of the model. Repeat in another area.
Using a heat gun, set the temperature to 350 degrees Fahrenheit. If the surface does not appear to close, raise temperature of heat gun in 20 degree increments until the surface closes. Stop immediately if the surface appears to be burning.
Using the second site of injections, use a hairdryer for up to a minute to remelt the surface.
Look at the hip model under ultrasound and compare to previous results.
Look at the surface of the hip model after performing the injections and compare to previous results. Note any surface variations.
Repairability Test: Results
Repairability Test: Results
The ultrasound part of the test was unable to be performed due to extenuating circumstances. By looking at the surface of the model, the results show that heating the material to PVC's melting temperature, with a heat gun, allows the holes created by each injection to close. The holes were closed at the surface and there was minimal surface variation. As a result, the effectiveness of the model will be maintained for subsequent users. While other training models made from self-healing materials exist (such as those by Blue Phantom), our device is much more inexpensive, allowing hospitals to purchase multiple training models. The hair dryer did not reach a temperature hot enough to melt the surface and will not be used in repairing the device in the future.
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