Cambridge Polymer Group, Inc.

Testing, Consultation, and Instrumentation for Polymeric Materials

Cambridge Polymer Group, 56 Roland Street, Suite 310

Boston, MA 02129

7-17 Presentation (10/1/2010)

Hydrogels: There is always room for Jello®

Soft-solids and the evolution of medical device design

Gavin Braithwaite

Gels: the fourth phase of matter

• Solid – Permanent shape, fixed volume – Elastic recovery

• Liquid – Fixed volume, “conformable” shape – No recovery (viscous)

• Gas – Volume expands to fill container

• Gel – Properties between solids and liquids – Fixed volume and shape (like solids) while static

• Often turn liquid when agitated – Partially elastic, partially viscous (viscoelastic)

Cambridge Polymer Group 2 Innovation Briefs - Boston 2012

• Gels and hydrogels – Gel – gelatus: frozen, immobile

• Gels structure continuous solid supporting a discontinuous solvent – Solid is usually a crosslinked or associated network of molecules – Liquid is anything compatible with the network

• Chemistry of network is critical – solubility of the network “draws in” solvent to “fill” the network – must be balanced by a “restraining” force generated within the network

• Network can’t expand beyond the length of the chain

Gel microstructure

Cambridge Polymer Group 3 Innovation Briefs - Boston 2012

• Xerogel – Liquid is removed leaving air

• Insulators

• Organogel – Liquid is organic fluid – Network is hydrophobic polymer

• Silicone gels

• Hydrogel – Liquid is water – Network is hydrophilic polymers or particles

• Collagen (jello, cartilage) • Pectin (jam) • pHEMA (contact lenses) • Poly(acrylic acid) (diapers)

Soft solids

Cambridge Polymer Group 4 Innovation Briefs - Boston 2012

Characteristics of hydrogels

• High water content – Free water allows diffusion of solutes – Viscous damping of mechanical deformation – Density “matched” to water – Very low solids content

• Network structure – Can be static or dynamic

• Permanent crosslinks (contact lenses) • Thermally sensitive (jello) • “labile” (hair-gel)

– Contains and confines water in 3d shape – Provides elastic recovery – Provides support for attaching active ingredients

Cambridge Polymer Group 5 Innovation Briefs - Boston 2012

• Hydrogels are ubiquitous in the body – Mucus and tear films – Cartilage – Vitreous humor and cornea – Tendon

• Microstructure critical function

Soft-solids: Natures solution

Cambridge Polymer Group 6 Innovation Briefs - Boston 2012

Current uses of hydrogels in medicine

• Predominantly as a carrier or protector – Drug release

• e.g. drug eluting stents – “smart” gels

• e.g. enteric coatings (lower stomach targeted delivery) – Tissue guides

• Nerve regeneration guides – Vision

• Contact and intraocular lenses – Tissue bulking

• Soft solid supports tissue • Provides fluid motion • Flexible and conformable

– Cartilage replacement • Load-bearing • lubricious

Cambridge Polymer Group 7 Innovation Briefs - Boston 2012

Urinary incontinence

• Urinary Incontinence profound quality of life impact – 13-15 million people in US – 35%+ of adult women have some measure – Often results from pregnancy – Can also impact children during development

• Results from loss of muscle tone around sphincter – Usually due to distortion of the urethra – Current treatments conservative – Subset require surgical intervention

• Soft solid re-closes the sphincter at rest

Cambridge Polymer Group 8 Innovation Briefs - Boston 2012

Stroke mitigation

• Myocardial infarction occurs when part of heart dies – Due to blockage of blood flow to heart muscle – Can be survived by leads to chronic problems – Muscle wall looses strength and gradually deforms – Stretches the internal structure of the heart – Distorts valves – Ultimately leads to mitral regurgitation (blood flow reversal)

• Injection of a soft-solid in to the heart wall – Stiffens wall – Preserves geometry

Cambridge Polymer Group 9 Innovation Briefs - Boston 2012

Expanding the applications

• Soft tissue replacement and augmentation – Conceptually simple – Easy application – Limited demands on material

• Larger market – Aging demographics

• Demand mobility • Minimal surgery

– Load bearing applications • Cartilage mimic

Cambridge Polymer Group 10 Innovation Briefs - Boston 2012

Engineering Solutions

• Solution: Designs based on simple engineering models – Existing joints “simple”

• Hinge (knee) • Ball-and-socket (hip, shoulder)

– “easy” to design • Well understood materials • “simple” fixation • “easy” to validate

– Highly successful • Hips now > 20 years • Very low failure rates

Cambridge Polymer Group 11 Innovation Briefs - Boston 2012

What is wrong?

• Irreversible • Tissue preserving

– Support structures and bone removed • Not biomechanically compatible

– Bearing surface optimized for wear – Fixation methods require anchors

Cambridge Polymer Group 12 Innovation Briefs - Boston 2012

What is missing?

• Conformable lubricious surface – Native cartilage cushions, lubricates and spreads loads

• Naturally low friction • Reduced contact stresses • Conforms under loads

• Minimally invasive – Current procedures require tissue excision

• To allow the joint to operate • To allow easier access and operation • To allow fixation

• Biomechanically compatible – Current devices change way joint operates

• e.g. meniscus and ligaments removed

• Solution – Cartilage replacement – i.e. “hydrogels”

Cambridge Polymer Group 13 Innovation Briefs - Boston 2012

20%-65%* Loading Curve E

0

0.5

1

1.5

2

2.5

3

DP DP DP AG RSA3 AG RSA3 AG RSA3-PEG

AG RSA3-PEG

1FT 5FT 5FT

10-28 15-28 25-28 15-28 25-28 15-28 25-28 15-28 15-28 15%

Elas

tic M

odul

us [M

Pa]

Permeability

Cambridge Polymer Group 14 Innovation Briefs - Boston 2012

• Tunable microstructures

Compression loading of hydrogels

Cambridge Polymer Group 15 Innovation Briefs - Boston 2012

00.10.20.30.4

0.50.60.70.8

0 0.25 0.5 0.75 1 1.25 1.5Time [hours]

Stra

in [m

m/m

m]

5PEG2.5 0.5MPa 1 hr20% 5 FT 0.5 MPa 1 hr25-28 0.5 MPa 1 hr

• Viscoelastic response

Ultra-low friction

• High lubricity

Cambridge Polymer Group 16 Innovation Briefs - Boston 2012

DATA

0

0.1

0.2

0.3

0.4

0.5

0.6

0.007 0.021 0.035 0.049

Coe

ffici

ent o

f fric

tion,

[ ]

Contact stress [MPa]

PVA freeze-thaw gelCPG PVA gelCPG modified PVA gel

Cambridge Polymer Group 17 Innovation Briefs - Boston 2012

Thank you

Cambridge Polymer Group is a contract research laboratory specializing in polymers and their applications. We provide outsourced research and development, consultation and failure analysis as well as routine analytical testing and custom test and instrumentation design.

Cambridge Polymer Group, Inc. 56 Roland St., Suite 310 Boston, MA 02129 (617) 629-4400 http://www.campoly.com info@campoly.com