MME2259a November 9, 2012 1 Engineering Contraception The TCu380A Intrauterine Contraceptive Device...

1MME2259a November 9, 2012

Engineering Contraception

The TCu380A

Intrauterine Contraceptive Device

Steve Nazar, MSc.Nazar Associates Inc.

[email protected]

For Dr. Paul Kurowski’s Engineering Design Class MME2259a

November 9, 2012


Global Significance of Birth ControlThomas Malthus, 1798: “Population grows exponentially, but resources are finite.”

Population projections


(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!


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


Mirena (TM)

Slow-release levonorgestrel


TCu380A:

“Tatum Tee”; or“Copper T”; or

various trade names


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


IUD Advantages

• Very low cost

• Immediately effective

• Reversible

• Non-hormonal (recent exception – Mirena™)

• Zero maintenance up to 10 years

• No male cooperation needed


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.


The Tatum Tee

Original monograph by Howard Tatum, MD, PhD. 1974


Geneva, 2007


Bangkok Conference, January 2010


Bangkok Attendees, January 2010


Outcome:

Free, online at http://whqlibdoc.who.int/publications/2011/9789241500999_eng.pdf

The new manufacturing and global purchasing standard


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


Insertion Tube and Package

Plunger

Insertion tubeDepth marker

IUD

Barrier package for sterility


Three problems:

• Frame material needed generic specification AND substitution

• Armpit radius specification needed range

• Barium sulfate dispersion quality needed specification


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.


Uncertainty of Single-Source

!


Key Material Properties

• Availability!

• Stiffness in “correct” range

• Toughness retention to 10 years

• Springback (creep resistance)

• No toxic reactions…

• …& lots more…


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)


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!


Typical Polymer Stress vs Strain Curve

ElasticInelastic


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


Three problems:

• Frame material needed generic specification AND substitution

• Armpit radius specification needed range

• Barium sulfate dispersion quality needed specification


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


Deformation: Loading vs Removal

Insertion: risks compressive damageRemoval: risks tensile damage

Worst stretch zone

Worst crush zone

“arms down”“arms up”


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


Compressive damageand then

tensile fracture

Specimen B4, left armpit at 35X


Compressivedamage first

Specimen B4, low magnificaitonright armpit


Many cycles, but -

Oxidative Damage?

Stress-Cracking?

Fatigue?

1200X magnification of right armpit of B4


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


Compressive Damage?Consider Tee loaded for insertion:

• Highly deformed into insertion tube• Wide variety of methods of loading


…loading from inserted end:


…loading from inserted end:


…loading from external end


Modeling Needed for

• Stiffness adjustment

• Radiusing

• Loading Devices


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


The Western team:2009-2010

Helen BrennickPete McIntosh

Andrea Sylvester

…supervised by Dr. Kurowski


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.


Dimensioned IUD dwg – included in WHO specification

• Starter tag over-wind is very difficult to model

• Problems arose from use of student licenses


Arm stiffness instrument

• Inexpensive and rugged

• Slightly difficult to read


Material data for SW nonlinear(from UWO team)

Example data from actual frame material – not corrected yet to “true” stress, strain


From UWO team presentation


Load cases: loading methods


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


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

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