MME2259a September 30, 2011 1 Engineering Contraception Design Toolkit for the Intrauterine...
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Transcript of MME2259a September 30, 2011 1 Engineering Contraception Design Toolkit for the Intrauterine...
1MME2259a September 30, 2011
Engineering ContraceptionDesign Toolkit for the
Intrauterine Contraceptive Device
Steve Nazar, MSc.Nazar Associates Inc.
For Dr. Paul Kurowski’s Engineering Design Class MME2259a
September 30, 2011
2MME2259a September 30, 2011
Global Significance of Birth ControlThomas Malthus, 1798: “Population grows exponentially, but resources are finite.”
Population projections
3MME2259a September 30, 2011
(Modified from U. Michigan course noteshttp://www.globalchange.umich.edu/globalchange2/current/lectures/human_pop/human_pop.html)
“Demographic Transition” happens ONLY because of birth
control!
4MME2259a September 30, 2011
Modes of Birth Control
• Diaphragms (barrier method)
• Condoms (barrier method)
• Foams and creams (spermicides)
• The Pill (daily hormonal management)
• Norplant, Depo-Provera (slow release hormone implants)
• Surgical ligations, male or female (irreversible)
• Intrauterine Contraceptive Devices:• Copper-bearing, and• Hormonal slow release
7MME2259a September 30, 2011
How an IUD is used
Uterus should remain sterile through IUD insertion.Tee frame must unfold promptly after insertionTee frame must fold “upward” for removal – 10 years later
cervix
Issues
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IUD Advantages
• Very low cost
• Immediately effective
• Reversible
• Non-hormonal (recent exception – Mirena™)
• Zero maintenance up to 10 years
• No male cooperation needed
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IUD Disadvantages
• No protection from venereal diseases
• Pregnancies not completely suppressed
• Moderately high rates of excessive bleeding, cramping, expulsion
• Incidents (extremely rare) of uterine perforations.
13MME2259a September 30, 2011
Anatomy of a Tatum Tee
Staked copper tubes
Copper wire0.010 inch dia
Wire anchor hole
Ball tip with string hole
Loose wire end
“armpit”
Low Density Polyethylene frame
knot
Oriented high density polyethylene monofilament suture
Starter tag, over-wound
1.25 inches 0.0625 inches
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Insertion Tube and Package
Plunger
Insertion tubeDepth marker
IUD
Barrier package for sterility
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How an IUD is used
“Ouch factor” –
• Cervix may have small diameter• Instructions say to pull cervix straight with a “single-toothed tenaculum”
16MME2259a September 30, 2011
Functions Required of an IUD• must flex for insertion and later removal -- Have stiffness to be neither too irritating nor too easily expelled -- Retain strength of frame & string 10 years or more
• present 380 mm2 of copper surface
• frame made of equivalent material to “DuPont LDPE 20”
-- must be visible on X-rays – so is also BaSO4 pigmented
• minimize “ouch factor” on insertion and removal• minimum diameter for insertion or removal • invert tee for removal with less than about 2 N force• no sharp edges
17MME2259a September 30, 2011
Three problems:
• Frame material needed generic specification AND substitution
• Armpit radius specification needed range
• Barium sulfate dispersion needed specification
18MME2259a September 30, 2011
The Generic Material Specification
• Specification by key properties is needed, NOT tradename & grade
• Properties have been “grandfathered” i.e. based on past successful IUD use -- NOT theory or measurement.
Thus two problems arise:
• Du Pont will not sell into any long-term human implant market. Alternative resins must be qualified soon!
• We aren’t sure what ranges to use for key functional specifications.
20MME2259a September 30, 2011
Key Material Properties• Availability!• Stiffness in “correct” range • Toughness retention to 10 years• Springback (creep resistance)• Absence of toxic reactions• Undamaged by gamma or e-beam sterilization• Injection moulding grade
Unknown:
• Oxidative stability (use stabilizers?)• Fatigue resistance?• Creep crack resistance?
21MME2259a September 30, 2011
Model needed (1)Stiffness matching
• A different polymer may have different stiffness
• Beam stiffness formula:
Stiffness is proportional to polymer modulus X (section depth)3
…very approximately!
• So we need a computational model for stiffness adjustment
22MME2259a September 30, 2011
Computational Model = FEA
• Define the material stiffnesses
• Define the geometry of the 3D assembly
• Mesh the assembly’s parts
• Assign loads and constraints
• Run simulation “engine”
23MME2259a September 30, 2011
Second Problem:Radiusing
• A re-entrant corner is a “stress riser.” Local strains rise rapidly when arms are bent.
• Tatum’s original design did not specify an armpit radius.
• In removal, optimum radius is unclear -- too small a radius encourages cracking, -- too large a radius increases arms’ stiffness
• In loading, optimum radius also unclear -- interacts with loading devices, as shown by the UWO student team
24MME2259a September 30, 2011
Loading vs Removal Deformation
Insertion: risks compressive damage
Removal: risks tensile damageWorst stretch zone
Worst crush zone
(images are schematics - not real)
25MME2259a September 30, 2011
1. Arranged collection of 80 used IUDs2. Measured strengths of frames and strings3. Examined armpits by scanning electron microscopy
Bolivia, England, Kenya, Philippines, Vietnam - thanks to Marie Stopes International & Dr. Paul O’Brien, London
Evidence for Role of Radiusing
26MME2259a September 30, 2011
Compressive damageand then
tensile fracture
Specimen B4, left armpit at 35X
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Many cycles, but -
Oxidative Damage?
Stress-Cracking?
Fatigue?
1200X magnification of right armpit of B4
30MME2259a September 30, 2011
Aged Frame Strengths
Figure 5: Break strengths of New and Used IUDs (tested in "arms-up" geometry)
20
25
30
35
40
45
0 2 4 6 8 10 12 14 16 18
age in utero , years
bre
ak
str
en
gth
, N
ew
ton
s
UK
Bolivian
Kenyan
Philippina
Vietnamese
New tee strengths n = 19 frames
mean = 38.9 Nstd dev = 1.7 N
31MME2259a September 30, 2011
Insertion Tube and Package
Plunger
Insertion tubeDepth marker
IUD
Barrier package for sterility
32MME2259a September 30, 2011
Compressive Damage?Consider Tee loaded for insertion:
• Highly deformed into insertion tube• Wide variety of methods of loading
36MME2259a September 30, 2011
Model Needed (2)Radiusing and Loading Devices
--To design the junction & loading device for less crush damage during insertion folding
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Third problem: barium sulfate dispersion
• Barium sulfate has very high surface energy• LDPE has moderately low surface energy• So barium sulfate will not “wet” with LDPE melt.
Droplet of molten LDPE
Bed of hot barium sulfate powder
38MME2259a September 30, 2011
The barium sulfate dispersion problem:experimental examination
Razor-cut frame
Moulding void
Free surface
Moulded surface
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The barium sulfate dispersion problem
Cut surface
Free surface Of void
41MME2259a September 30, 2011
The barium sulfate dispersion problem
Cut surface
Mould contact surface
45MME2259a September 30, 2011
Barium sulfate dispersion by X-ray microscopy
Marked-up scan of X-ray- Can count particles by size category
46MME2259a September 30, 2011
Model Needed (3)Dispersion
1. Given unfilled material strength, estimate critical particle size as a function of stress
2. Estimate volumes in bent IUD at each stress bracket
3. Measure the particle size distribution per unit volume.
4. Calculate the probability of exceeding critical size in each stress bracket.
5. Sum over stress brackets to get failure probability for a given dispersion quality.
6. Test and calibrate the model against measured strength, filled vs unfilled.
47MME2259a September 30, 2011
The Western team:2009-2010
Helen BrennickPete McIntosh
Andrea Sylvester
…supervised by Dr. Kurowski
48MME2259a September 30, 2011
The 4th Year Practicum 2009-2010
Objectives:
• Design and build a simple arm flex tester
• Measure arm stiffness
• Model IUD frame arms-up and arms down by FEA
• Check model predictions against measurement.
49MME2259a September 30, 2011
Dimensioned IUD dwg – included in WHO specification
• Starter tag over-wind is very difficult to model
• Problems arose from use of student licenses
50MME2259a September 30, 2011
Arm stiffness instrument
• Inexpensive and rugged
• Slightly difficult to read
52MME2259a September 30, 2011
Material data for SW nonlinear(from UWO team)
Example data from actual frame material – not corrected yet to “true” stress, strain
56MME2259a September 30, 2011
Model conclusions
• Suspicions reinforced, re loading devices vs damage
• But radiusing issue difficult to model quantitatively– Large deformations cause mesh failure – resolved with first order elements– Nonlinear material input caused program failure – unknown reasons
Removal Insertion
stem
stem
57MME2259a September 30, 2011
A modern toolkit needed for old problems:
• adapt design to alternatie materials of different stiffness, yield
• estimate radiusing specification
• estimate fracture risk versus dispersion quality