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Here is a type of polyurethane foam with roughly the correct density that gives a Young’s modulus in the 10-30 MPa range: https://makerstock.com/collections/foam/products/cnc-and-modeling-foam-rigid-polyurethane-foam-high-density-8lb-ft3
Material | Young Modulus (basically how much it deforms under stress) | Stress-Strain Curve | Ultimate Strength |
Intervertebral Disk | 30 MPA / 0.03 GPa | ||
Rubber (Small Strain) | 10-100 MPa | ||
0.517-62.1 MPa | 0.138 - 165 MPa (tensile strength, ultimate) | ||
150-520 MPa | 10.3-18 MPa (tensile strength, ultimate) | ||
Polyethylene | |||
UHMW Polyethylene (used in actual disc replacements) | 760 MPa | ||
Polyurethane Rubber | 6 MPa | 25 MPa | |
EVA (Ethylene Vinyl Acetate) (this is what shoe soles are made of!) | Pure EVA, 0.3 wt%, 2 wt% stress-strain curve, Young’s modulus, elongation | ||
0.16g/cm^3 polyurethane foam | 15 MPa | ||
Memory Foam (a type of polyurethane foam) | |||
Notes for substitute intervertebral disc material:
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Note: Material for head weight
Material | Height | Density | Radius | Outer Diameter | Shape | Weight (not accounting for hole) | Price |
Brass | 0.303 to 0.315 lb/in^3 | 1.5 | Cylinder | 3 lb | |||
Stainless Stain | 1.48 in. | 0.27 to 0.29 lb/in^3 | 1.5 in. | 3 in. | Cylinder | 3lb | $86.70 for 6 in |
Low-Carbon Steel |
Vertebrae
https://www.thingiverse.com/thing:4801717
Proportion Head to Vertebrae:
Piece | Real Human | Prototype |
Head Weight | 10 lbs | 3 lbs |
Vertebrae Weight | 44.1 grams | |
Head Weight | Vertebrae Weight | |
Real Human | 10 lbs | 44.1 grams |
Prototype | 3 lbs | 20 grams |
Determination of Proportion:
50% scale for the vertebrae, 30% scale for the head (maximum size for each permissible by dimensions of payload)
11/17/2024 Work Session:
We took off the supports on the initial 3d prints for the spine models. They turned out decent - print quality was low but ball-socket joints work. The spine model could be fully assembled. Next print is being printed so we have another version, but it is using tree supports to see if that is easier. The file was saved and will be printed when someone gets to the metropolis.
Spine model:
- 3d printed vertebrae (C1-C7) connected via ball and socket joints
- 6 foam disks made of high density polyurethane foam (has similar Young’s modulus as an intervertebral disk)
- We need to decide how the disks are going to be cut
- Primary tool - laser cutter. Check if the foam can be cut (regulations), and if possible, does the laser burn the foam when cutting.
- Secondary tool - waterjet. May involve drying out the foam (ensure properties are not modified) after cutting.
- Possibly use cricket?
- Force sensor between C1 and C2, C6 and C7
- Foam disks connect vertebrae via glue
- superglue
- Force sensor layer is between foam disks (total thickness should be the same - cut normal foam disk into half)
Neck model (what surrounds the neck)
- Use dragon skin (https://www.smooth-on.com/products/dragon-skin-10-medium/)
- Will surround the spine model (during curing process)
- Mixing
- Pre-mix Part B thoroughly. After dispensing required amounts of Parts A and B into mixing container (1A:1B by volume or weight), mix thoroughly for 3 minutes making sure that you scrape the sides and bottom of the mixing container several times.
- Molding
- After pouring the pre-mixed substance into the mold ( likely going to be the neck brace without the foam ), we’re going to leave the substance to conform around the spine/rod system
- Curing
- Because of worries of outgassing when applying a vacuum to the neck/brace/spine system, we’re going to leave the Dragonskin out to remove any air bubbles
- We are not going to worry about using a vacuum
- Using a vacuum will cause issues with the foam disks and for the 3d printed vertebrae
Brace
- Plastic outer layer with foam layer on the inside
- PETG for outer layer plastic: 3D print the shape that we need
- CAD file is in rocket team drive
- Identify a foam for the inner layer
- Polyethylene foam (most commonly used for soft braces)