Upload
janis-may
View
219
Download
3
Embed Size (px)
Citation preview
1MME2259a November 9, 2012
Engineering Contraception
The TCu380A
Intrauterine Contraceptive Device
Steve Nazar, MSc.Nazar Associates Inc.
For Dr. Paul Kurowski’s Engineering Design Class MME2259a
November 9, 2012
2MME2259a November 9, 2012
Global Significance of Birth ControlThomas Malthus, 1798: “Population grows exponentially, but resources are finite.”
Population projections
3MME2259a November 9, 2012
(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 November 9, 2012
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
5MME2259a November 9, 2012
Mirena (TM)
Slow-release levonorgestrel
6MME2259a November 9, 2012
TCu380A:
“Tatum Tee”; or“Copper T”; or
various trade names
7MME2259a November 9, 2012
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
8MME2259a November 9, 2012
IUD Advantages
• Very low cost
• Immediately effective
• Reversible
• Non-hormonal (recent exception – Mirena™)
• Zero maintenance up to 10 years
• No male cooperation needed
9MME2259a November 9, 2012
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.
10MME2259a November 9, 2012
The Tatum Tee
Original monograph by Howard Tatum, MD, PhD. 1974
11MME2259a November 9, 2012
Geneva, 2007
12MME2259a November 9, 2012
Bangkok Conference, January 2010
13MME2259a November 9, 2012
Bangkok Attendees, January 2010
14MME2259a November 9, 2012
Outcome:
Free, online at http://whqlibdoc.who.int/publications/2011/9789241500999_eng.pdf
The new manufacturing and global purchasing standard
15MME2259a November 9, 2012
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
16MME2259a November 9, 2012
Insertion Tube and Package
Plunger
Insertion tubeDepth marker
IUD
Barrier package for sterility
17MME2259a November 9, 2012
Three problems:
• Frame material needed generic specification AND substitution
• Armpit radius specification needed range
• Barium sulfate dispersion quality needed specification
18MME2259a November 9, 2012
Generic Material Specification
• Specification by key properties is needed, NOT tradename & grade
-- Properties had 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.
19MME2259a November 9, 2012
Uncertainty of Single-Source
!
20MME2259a November 9, 2012
Key Material Properties
• Availability!
• Stiffness in “correct” range
• Toughness retention to 10 years
• Springback (creep resistance)
• No toxic reactions…
• …& lots more…
21MME2259a November 9, 2012
Some dimensions are fixed, but the armpit could be varied
• Copper area and location cannot be changed• Over-all length and width cannot be changed
(image from Solidworks model)
22MME2259a November 9, 2012
Model needed -for Stiffness Matching
• A different polymer may have a different stiffness – so how do we adjust the design to compensate?
Beam stiffness formula:
“ Stiffness is proportional to modulus X (section depth)3 ”
But this formula fails for polyethylene!
23MME2259a November 9, 2012
Typical Polymer Stress vs Strain Curve
ElasticInelastic
24MME2259a November 9, 2012
Why model?
• Experiments with machined steel mould would be expensive!
• We needed a computational model for stiffness adjustment
• But for the WHO specification, we made educated guesses based on past practice
25MME2259a November 9, 2012
Three problems:
• Frame material needed generic specification AND substitution
• Armpit radius specification needed range
• Barium sulfate dispersion quality needed specification
26MME2259a November 9, 2012
Radiusing Problem
• 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 insertion, optimum radius also unclear -- interacts with loading devices, as shown by the UWO student team
27MME2259a November 9, 2012
Deformation: Loading vs Removal
Insertion: risks compressive damageRemoval: risks tensile damage
Worst stretch zone
Worst crush zone
“arms down”“arms up”
28MME2259a November 9, 2012
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
29MME2259a November 9, 2012
Compressive damageand then
tensile fracture
Specimen B4, left armpit at 35X
30MME2259a November 9, 2012
Compressivedamage first
Specimen B4, low magnificaitonright armpit
31MME2259a November 9, 2012
Many cycles, but -
Oxidative Damage?
Stress-Cracking?
Fatigue?
1200X magnification of right armpit of B4
32MME2259a November 9, 2012
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
33MME2259a November 9, 2012
Compressive Damage?Consider Tee loaded for insertion:
• Highly deformed into insertion tube• Wide variety of methods of loading
34MME2259a November 9, 2012
…loading from inserted end:
35MME2259a November 9, 2012
…loading from inserted end:
36MME2259a November 9, 2012
…loading from external end
37MME2259a November 9, 2012
Modeling Needed for
• Stiffness adjustment
• Radiusing
• Loading Devices
38MME2259a November 9, 2012
Computational Model = “Finite Element Analysis”: FEA
• Model the geometry of the 3D part in (e.g.) Solidworks™
• Measure and input the material stiffness curve (both tension and compression)
• “Mesh” the parts: divide into small elements• Assign loads (“where you push”) and constraints (“where
the part is held”)
• Run nonlinear simulation software
39MME2259a November 9, 2012
The Western team:2009-2010
Helen BrennickPete McIntosh
Andrea Sylvester
…supervised by Dr. Kurowski
40MME2259a November 9, 2012
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.
41MME2259a November 9, 2012
Dimensioned IUD dwg – included in WHO specification
• Starter tag over-wind is very difficult to model
• Problems arose from use of student licenses
42MME2259a November 9, 2012
Arm stiffness instrument
• Inexpensive and rugged
• Slightly difficult to read
43MME2259a November 9, 2012
Material data for SW nonlinear(from UWO team)
Example data from actual frame material – not corrected yet to “true” stress, strain
44MME2259a November 9, 2012
From UWO team presentation
45MME2259a November 9, 2012
Load cases: loading methods
46MME2259a November 9, 2012
Model conclusions
• Suspicions reinforced, re loading devices causing damage
• But radiusing issue difficult to model quantitatively –
– Large deformations cause failure – (later resolved with 1st order elements)– Nonlinear material input caused program failure – unknown reasons
Removal Insertion
stem
stem
47MME2259a November 9, 2012
FEA analysis -- a toolkit for IUD design:
• Adapt design to alternative materials of different stiffness or yield
• Estimate a radiusing specification
• Design or select the least damaging loading devices