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Hydrogels for Medical Applications Past, Present and Future
Gavin Braithwaite
Cambridge Polymer Group, Boston, MA
Biological hydrogels
• Hydrogels are everywhere in nature• Can be dynamic or permanent• Diverse usage but mostly water
Cambridge Polymer Group (2)Polymer and Plastics in Medical Devices, SF 2013Images: http://web.mechse.illinois.edu/research/ewoldt/research.html; www.scq.ubc.ca; www.naturalheightgrowth.com
What are hydrogels?
• Ferry– “…a substantially diluted system which exhibits no stead-state flow.”– Chemistry of the polymer, locked in place by “crosslinks” and supported
by water• Stabilization (crosslinks)
– Electrostatic (clays)– Associative (mucins and consumer gels)– Hydrogen bonded “crystals” (gelatin and PVA)– Chemical crosslinks (contact lenses)
Cambridge Polymer Group (3)Polymer and Plastics in Medical Devices, SF 2013Images: http://quick-dish.tablespoon.com
Chemically identical
Physically different
Why are they so important?
• Hydrogels are– Water filled (>80% in most cases)– Permeable– Viscoelastic– Lubricious – Hydrophilic– Chemically diverse– Environmentally sensitive– Allow solute transport– Multi-functional
Cambridge Polymer Group (4)Polymer and Plastics in Medical Devices, SF 2013Images: Bodugoz-Senturk
Production of hydrogels
• Controlled by chemistry of chain and method of crosslinking
• Chemical crosslinker
• Self-assembly– Dynamic and labile gels generally– Usually pronounced yield stress– Triggered by temperature or environment
Cambridge Polymer Group (5)Polymer and Plastics in Medical Devices, SF 2013Image:http://origin-ars.els-cdn.com/content/image/1-s2.0-S1748013209000905-gr2.jpg
Common chemistries
Cambridge Polymer Group (6)Polymer and Plastics in Medical Devices, SF 2013
• A snapshot of common biomedical systems– Poly(HEMA): poly(Hydroxyethyl methacrylate)
• Contact lenses, dressings, drug release– PEG: poly(ethylene glycol)
• Injectables, drug release, scaffolds– PVA: poly(vinyl alcohol)
• Contact lenses, nerve guides cartilage, wound dressings, reconstructive
– PVP: poly(vinyl pyrrolidone)• Wound dressings
– Poly(acrylamide)• Water “solidifiers”, tissue bulking, soil conditioner
• Other common hydrogels– Starch, agarose, pectin, gelatin, and many others
Past & current uses
• Industrial– Bulkhead seals– Waste clean-up
• Consumer– Hair-gel– Cosmetics
• Medical– Contact lenses– Drug release – Nerve guides– Coatings– Tissue bulking– Nucleus replacement
Cambridge Polymer Group (7)Polymer and Plastics in Medical Devices, SF 2013Images: www.mide.com; www.chemicalengineering.byu.edu
Where to next
• Current usage largely non-load bearing (even nucleus)– Drug release, scaffolds, guides, tissue bulking
• The body makes extensive use of hydrogels in the joint– Meniscus– Cartilage– Labrum– Intervertebral disc
• These joints readily compromised– Osteoarthritis
• 27M people in US– Trauma
• >800,000 meniscus surgeries annually• Accelerates degeneration• Associated with lesions
Cambridge Polymer Group (8)Polymer and Plastics in Medical Devices, SF 2013Image: http://soni2006.hubpages.com/hub/Advice-for-patients-suffering-from-osteoarthritis-both-knees
The trouble with the knee
• The human knee is a “simple” joint– Cylinder on flat– Contact forces 3-5x body weight– ~10 MPa normally
• 30 MPa (stair) up to 50 MPa (lunge)– Kinematics
• Rotation over 20°• Flexion up to 50°• Displacement 3-4 cm
– Velocities up to 0.6 m/s– Coefficient of friction <0.04
• Soft-tissues allow it to function– Also represent its weakness
Cambridge Polymer Group (9)Polymer and Plastics in Medical Devices, SF 2013Image: www.blenderartists.org, stemcelldoc.wordpress.com
How to (re)build a knee
• Soft-tissues have complex structure– Work as hybrid structures
• Cartilage– Properties change from surface to bulk to
transition to boney substrate– Provides unique shear response
• Meniscus– Cartilagenous structure supported by
radial hoops
Cambridge Polymer Group (10)Polymer and Plastics in Medical Devices, SF 2013Image: kneejointsurgery
Engineered (and testing) soft-tissue repair
• Problem:– Low friction– High load– Load distribution– Displacement/shear tolerant– Soft-tissue preserving
Cambridge Polymer Group (11)Polymer and Plastics in Medical Devices, SF 2013Image: kneejointsurgery
Cartilage Meniscus
Water content [%] 68-85 60-70
Collagen % 10-20 15-25
Proteoglycan [%] 5-10 1-2
Young’s modulus [MPa] 5-10
Tensile modulus [MPa] ~10 100-200
UTS [MPa] 2/5
Aggregate modulus [MPa] ~0.7 ~0.4
Permeability x 1015 [m4/N.s] ~4 ~1
Friction Coeff [ ] 0.1 0.08
Thickness [mm] 0.5-5 0-10
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PVAc PVA
Cartilage mimicking hydrogel
• Cartilage properties in part due to PGs• Mimic structure with hydrolyzed PVAc hydrogel with dangling chains
Cambridge Polymer Group (12)Polymer and Plastics in Medical Devices, SF 2013
Lubricity and Friction
• Low friction is a defining feature of cartilaginous joints– Impacted by disease level– Diseased synovial fluid loses lubricity
Cambridge Polymer Group (13)Polymer and Plastics in Medical Devices, SF 2013Image: Neu et al. 2010
0.000
0.100
0.200
0.300
0.400
0.500
0.600
0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140Contact Pressure [MPa]
CO
F
LB on CoCr in DILB on CoCr in BSnon-LB on CoCr in DInon-LB on CoCr in BS
Lubricity and Friction
• Correct structure generates superb lubricity
Cambridge Polymer Group (14)Polymer and Plastics in Medical Devices, SF 2013
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 gelCartilage-cartilage
Reinforcement
Cambridge Polymer Group (15)Polymer and Plastics in Medical Devices, SF 2013Image: sciencedirect.com
Fiber wound
Simple compression (strain/strain)
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Stre
ss [M
Pa]
Strain [mm/mm]
no reinforcement PVA monofilament wound around outside
Mechanical properties
Cambridge Polymer Group (16)Polymer and Plastics in Medical Devices, SF 2013
Issues
• Use of polymers in soft-solids raises its own problems• Permeability
– Elutables major concern– Reaction biproducts– Access for media
• Mechanical testing complex– Viscoelastic solid implies rates critical
• Environment important– Humidity, salinity, pH
• Wear– How does one measure wear?
Cambridge Polymer Group (17)Polymer and Plastics in Medical Devices, SF 2013
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 310Boston, MA 02129(617) 629-4400http://[email protected]
Cambridge Polymer Group (18)Polymer and Plastics in Medical Devices, SF 2013