lecture21

lecture21 - 3.051J/20.340J Lecture 21: Biomaterials for...

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1 3.051 J / 20 .340 J Lecture 21: Biomaterials for Organ Replacement Therapies for Organ Replacement 1. Transplantation Replacement of tissue or organ from human or animal donor Allograft —human donor (e.g., kidney, liver, heart) Xenograft —animal donor (e.g., porcine aortic valves) Adv : complete recovery of lost function for patient lifetime Disads: possibility of rejection—attack by immune system side effects of immunosuppressive drugs (e.g., steroids) limited donor pool 2. Autograft Donor is also recipient Examples: skin grafts, nerve grafts, breast reconstructions, saphenous (calf) vein for coronary or peripheral artery bypass (~300,000/yr in U.S.) Adv : complete recovery of lost function for patient lifetime virtually no danger of rejection Disads: limited self-donor tissue available trauma/scarring at removal site
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2 3.051 J / 20 .340 J 3. Regenerated Tissues/Organs Cells grown on a scaffold device (synthetic or collagen-based, often resorbable) provide restored function (e.g., skin and cartilage) Adv: no donor/self-donor tissue limitations function restored for patient lifetime (in principle) Disads: biological complexities of complete organ regeneration unsolved possible immune response, depending on cell source 4. Permanent Implants Prosthetic devices manufactured from synthetic materials (e.g., hip prostheses, >200,000/yr in U.S., many designs) Acetabular cup: metal with UHMWPE (2M Daltons) liner Ti, Al 2 O 3 PMMA cement: adhesion, shock absorption ( Alt: metal bead or HAp coatings: bone ingrowth) stainless steel Femoral stem: CoCr, Ti, Femoral head: CoCr, Femur
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3 3.051 J / 20 .340 J Advs: no donor/self-donor tissue limitations cannot by “rejected” by classical complement mechanisms Disads: “full” organ function not restored e.g., orthopedic replacements: - loss of bone marrow (origin of blood stem cells) - no regenerative ability - reduced range of mobility often must be replaced - chronic inflammation e.g., PE wear debris immune response bone breakdown - mechanical failure e.g., cement loosening other long-term side effects stress-shielding : modulus mismatch between stem & femur load imbalance on surrounding bone osteoporosis (bone resorption > deposition) increased likelihood of re-fracture Clearly, mechanical properties play a critical role in materials choice!
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4 3.051 J / 20 .340 J Mechanical Properties of Interest in Biomaterials Applications: Stiffness Strength Toughness We need methods to quantify Hardness these material qualities. Fatigue (especially cyclic) Fracture strength Wear resistance Let’s define some terms in the context of a simple uniaxial load experiment: F=Applied Force A o = Area (recoverable) L o δ F) (∝ “elastic” deformation fracture “plastic” deformation (irrecoverable)
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5 3.051 J / 20 .340 J E σ = F/A (N/m 2 =Pa) UTS YS necking
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lecture21 - 3.051J/20.340J Lecture 21: Biomaterials for...

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