Soft Solids Research

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1 Soft Solids Research Hydrogels for load-bearing applications Gavin Braithwaite Cambridge Polymer Group, 56 Roland Street, Suite 310 Boston, MA Cambridge Polymer Group, Inc. Testing, Consultation, and Instrumentation for Polymeric Materials 7-17 Presentation (10/1/2010)

2 Large joint degeneration Pain arising from large joints a serious societal burden Lower back >25% of total US adult population Hip >8% Knee >14% Osteoarthritis a serious societal burden Impact for over 45 population ~9% in hips 7-17% in knees Chronic issues cost society and impact general health and lifestyle Restricts mobility and ability to work 65% of people with restricted mobility report due to back pain 30% report due to lower limb Cost Spinal fusions cost > $17 Bn in 2004 Total Hip and knee > $22 Bn in 2004 Cambridge Polymer Group 2 Data from The Burden of Muscoskeletal Diseases in the United States

3 Motion preservation Engineering solution Replace the existing joint with a mechanical equivalent Hip: Ball-and-socket Knee: Hinge Spine: Pivot Simple mechanical design meets primary functional performance Total joint replacement now highly successful Hips can be expected to last 20 years Knees approaching this value Total Spinal disks promising As joint complexity goes up issues occur Cambridge Polymer Group 3

4 The down-side of the engineering solution Mimicking basic function can work well Hip very successful Long lasting Routine Mature designs Preserves biomechanics Knee less so Difficult kinematics Soft tissue critical to natural function Requires excision of surrounding material Partially compromises biomechanics Spine weaker still Removes cushioning role Limits kinematics to simple motion Progressively move away from original structure Cambridge Polymer Group 4

5 Total disc replacement as an example Pain sources complex in disc space Soft tissue Facet joints Intervertebral disc Non-invasive methods successful 80% of patients well addressed Longer term may not be final solution Most successful surgical strategies invasive Fusion gold standard removal of motion and excision of pain centers 320,000 surgeries annually in US, 70%+ success rates Total disc Up and coming, preserves motion, replace structure Cambridge Polymer Group 5

6 Early intervention may be key Degenerative cascade starts early (simplistic picture) Mid-teens dehydration beginning in nucleus Oxygen depleted and poorly vascularized Reduction of hydration results in depressurization Changes to disc height change biomechanics Disrupt annulus, and change loads on facets Pushes deterioration to adjacent segments Engineering solution Excise intervertebral disc and replace entire structure Invasive, removes substantial tissue, not biomechanically accurate Biomechanical solution Replace deteriorated nucleus before cascade has fully initiated Cambridge Polymer Group 6

7 The intervertebral disc The Intervertebral Disc (IVD) is a unique joint Poorly vascularized and immune privileged Complex structure ply-like periphery (Annulus Fibrosus) surrounds a jelly-like core (Nucleus Pulposus) Annulus Fibrosus highly oriented collagen sheets Fiber orientation in sheets ~65-60º from vertical Adjacent sheets alternate orientation 65-75% water 75-90% collagen (dry weight) 10% proteoglycans (dry weight) Nucleus Pulposus Jelly-like collagen hydrogel 75-90% water 25% collagen (dry weight) 20-60% proteoglycans (dry weight) Composite structure Cambridge Polymer Group 7

8 What is a hydrogel? Gels and hydrogels Gel gelatus: frozen, immobile Continuous solid supporting a discontinuous solvent Solid phase 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 Network is hydrophilic balanced by a restraining force generated by network Network can t expand beyond the length of the chain High confined water content provides unique properties Biocompatible, viscoelastic, porous... Natural hydrogels are ubiquitous cartilage, cornea, mucus, nucleus Cambridge Polymer Group 8

9 Biomedical hydrogels Hydrophilic polymers form network Polymeric backbone grown from monomers, oligomers or fully formed polymers Crosslinking During polymerization, post-polymerization Through functional groups, or addition of crosslinker Chemical or physical Vast range of performance specifications possible Environmental response (monomeric units) Swellability (crosslink density and monomers) Permeability (pore-size through crosslink density) Viscoelastic response (monomeric units and pore size) Biological interaction (chemistry of monomeric units) Degradability (presence of specific monomeric units) Gelation time (crosslinking mechanism) Injectability (size of initial monomers/oligomers) Cambridge Polymer Group 9

10 Hydrogels in medicine A snapshot of common chemistries 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 Many, many more Largely not capable of supporting high loads So how does the body do it? Cambridge Polymer Group 10

11 Hydrogels as hybrid materials Load-bearing requires complex functionality Hydrogels are rarely utilized alone by the body Cartilage Meniscus Spine Cambridge Polymer Group 11

12 Hydrogels for nucleus replacement Many forms of deterioration of IVD and spinal column appears to start with loss of nucleus properties Inadequate nutrition => loss of hydration Loss of hydration => reduction in disc height Reduction in disc height => changed biomechanics Early intervention implies restoring the nucleus Relies on relatively intact annulus Native nucleus was not load-bearing in itself Questions: What are the relevant performance specifications? How do you test such a device? Cambridge Polymer Group 12

13 The nucleus and the disc Nucleus 90% water (incompressible fluid) 25% of solids are network Osmotically active Modulus ~0.3 MPa Pressurized to MPa Engineering solution Provide load-bearing through nucleus device Loads end-plates incorrectly results in remodeling and subsidence Biomechanical solution Replace mechanical function of the nucleus Repressurize disc Load born by hoop stress in annulus Cambridge Polymer Group 13

14 Performance criteria Composite structure Relies on intact annulus Distribute loads to annulus Incompressible Fully conforms with cavity Deformable Viscoelastic Resists expulsion and fluid flow Annulus and end-plates partially permeable and may have some fissuring Surgical site introduces weakness Tear resistant and continuous structure Must be long-lived Fatigue resistant No particles Cambridge Polymer Group 14

15 Fatigue testing of nucleus replacements Nucleus function is complex Confined environment Fluid egress/ingress through end-plates High loads Moderately high deformations Motion of device during flexion/extension may involve abrasion Simple engineering testing not applicable Stress/strain neglects viscoelastic response Creep more representative But what strain and what confinement? Is strain-based fatigue sufficient? What about wear debris? Cambridge Polymer Group 15

16 ASTM F Standard Guide for Mechanical and Functional Characterization of Nucleus Devices Cambridge Polymer Group 16

17 Test configuration ASTM guide suggests use of a surrogate annulus Silicone rubber compressive stiffness similar to literature values First-pass attempt at confining system Neglects friction against nucleus Neglects end-plate interactions Neglects tensile modulus for pressurization Revised model for fatigue setups Composite annulus PVA or RTV & fiber reinforcements System pressurizes closer to the native disc Semi-permeable end-plates Allows diurnal cycle of fluid Annulus Nucleus cavity Porous end-plates Membrane 5 mm hole Cambridge Polymer Group 17

18 Simulating the annulus and plates Oriented fibers in annulus model closer approximation to mechanical behavior of true annulus Repeatable and long-lived Filter disks provide partial permeability Material Tensile Modulus (annulus) Annulus compressiveend-plate permeability Transverse/Longitudinal Modulus Healthy/diseased IVD 0.15 / 400 MPa > 5 MPa /10-16 m 4 /Ns RTV / 2.78 MPa 5-60 MPa m 4 /Ns RTV630 / PVA fiber ~34 / 230 MPa 9-20 MPa m 4 /Ns Tangent tensile modulus at 0.4 MPa stress [MPa] PVA/PVA RTV Human lumbar AF Cambridge Polymer Group 18

19 Dynamic Cycling Strain [mm/mm] Water alone Hydrogel Empty Disc Time [min] Strain [mm/mm] Strain Force Time [hr] Force [N] Cambridge Polymer Group 19

20 Soft-solids require novel designs and testing Hydrogels are closest to the natural solution Like the natural tissue, can only work in composite design Nucleus replacement, relies on annulus Too hard, results in end-plate failure Too soft, device failure (extrusion) Cartilage repair, needs reliable fixation method Hydrogels rarely have good tensile properties Discontinuities in designs poorly tolerated Wear is difficult to predict Tests have to take in to account the use of the material simple engineering tests no longer relevant stress-strain, fatigue crack, tensile etc Design new tests that mimic conditions likely to be encountered Cambridge Polymer Group 20

21 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 (617) info@campoly.com Cambridge Polymer Group 21