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.

[email protected]

For Dr. Paul Kurowski’s Engineering Design Class MME2259a

September 30, 2011


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


Outcome:


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


How an IUD is used

“Ouch factor” –

• Cervix may have small diameter• Instructions say to pull cervix straight with a “single-toothed tenaculum”


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


Three problems:

• Frame material needed generic specification AND substitution

• Armpit radius specification needed range

• Barium sulfate dispersion needed specification


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.


Uncertainty of Single-Source


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?


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


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”


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


Loading vs Removal Deformation

Insertion: risks compressive damage

Removal: risks tensile damageWorst stretch zone

Worst crush zone

(images are schematics - not real)


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


Cyclic tensile events

Specimen B4 at 120X, left armpit


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


Insertion Tube and Package

Plunger

Insertion tubeDepth marker

IUD

Barrier package for sterility


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


Model Needed (2)Radiusing and Loading Devices

--To design the junction & loading device for less crush damage during insertion folding


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


The barium sulfate dispersion problem:experimental examination

Razor-cut frame

Moulding void

Free surface

Moulded surface


The barium sulfate dispersion problem

Cut surface

Free surface Of void





Cut surface

Mould contact surface




Barium sulfate dispersion: X-ray microscopy for particle density


Barium sulfate dispersion: X-ray microscopy


Barium sulfate dispersion by X-ray microscopy

Marked-up scan of X-ray- Can count particles by size category


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.


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


Arm stiffness instrument


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


Elastoplastic material model, assumed 20 MPa yield point


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


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


These problems are common engineering

problems

• Armpit problem can be generalized to any gusset

• Critical-particle-size problem can be generalized to any filled polymer system

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MME2259a September 30, 2011 1 Engineering Contraception Design Toolkit for the Intrauterine...

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