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Some “short term” medical applications of degradable polymeric biomaterials

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Biodegradable polymers used in medical devices approved by the FDA for use in

humans

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Mechanisms leading to degradation of polymers

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Mechanisms for polymer degradation:

Top: breaking crosslinking bonds by absorbing water. Most prevalent in amorphous

polymers with low molecular weight.

Middle: cleavage of side chains leading to

formation of hydrophilic groups

Bottom: Scission of “mers” in polymer chain

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Surface degradation (left) results in thinning of the material, but no change in macroscopic properties (curve

2).

In bulk degradation (right), dimensions do not change, but macroscopic properties deteriorate (curve 1)

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Cleavage via hydrolysis

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Polymer Oxidation

Initiation steps in oxidation may be homolysis (top) or heterolysis (bottom)

Examples of chemical domains that are susceptible to oxidative degradation. * denotes sites of

homolysis or heterolysis

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Example of metal-catalyzed brittle fracture of a polymer. Corrosion of metal occurs first, releasing metal

ions that act as catalysts for oxidation of polymer.

Below: environmental stress cracking of a polymer in a biological

environment. Deep cracks are perpendicular to the tensile loading

Characteristics of poly(ether urethanes) that cracked in vivo

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Effect of metal ion oxidation potential on properties of poly(ether urethane)

Effect of ether content on poly(ether urethane) on

susceptibility to metal ion-induced oxidation

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Initiation of oxidation pathways by metal ions

Formation of metal ion from metal.

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Schematic of the interface of a passivating alloy surface in contact with a biological environment

Modular junction taper connection of a total hip arthroplasty showing corrosion of the taper connections. Macrograph of deposits of CrPO4 corrosion particle products on the rim of a modular Co-Cr

femoral head.

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Metal Degradative concerns• High release of ionic metallic debris

– Toxicity: Metal-on-metal bearings are not recommended for patients with poorly functioning kidneys because metal ions excreted through the kidneys can build up in the blood.

– osteolysis and implant loosening in total hip patients with metal-on-metal bearings may be associated with hypersensitivity to metallic debris

– Surface replacement with metal on metal is a new technology that has gained a great deal of recent interest. Hip surface replacement preserves more bone in the patient than conventional hip replacement. This has the potential of being a first-line treatment of end-stage arthritis in younger, active patients.

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Metal on Polyethylene Bearings• The adverse effects of oxidation during

radiation sterilization– Polyethylene components, like most medical devices,

are sterilized by exposure to gamma radiation. The radiation, while penetrating through the component, has sufficient energy to break the chains that form the molecular backbone of the polymer. If the radiation exposure is performed while the component is exposed to air, the broken ends can react with oxygen, causing harmful changes, including a decrease in molecular weight, a dramatic loss of ductility, and a decrease in strength. The combined effect may make the polyethylene markedly more susceptible to wear.

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Approaches to minimize degradation of PE

• Placing polyethylene joint replacement components into sealed packages that contain either a vacuum or an inert gas, such as nitrogen or argon, instead of air.

• Replacing radiation altogether, instead exposing polyethylene components to ethylene oxide or gas plasma, neither of which imparts sufficient energy to cause oxidation.

• Increasing dose of radiation to promote crosslinking of polymer chains– early results show a dramatic decrease in wear of between 30

and 96 percent in total hip replacements over that seen with conventional polyethylene.

– Disadvantage: increased crosslinking makes material more brittle

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• Corrosion occurs due to: --the natural tendency of metals to give up electrons. --electrons are given up by an oxidation reaction. --these electrons then are part of a reduction reaction.• Metals with a more negative Standard Electrode Potential are more likely to corrode relative to other metals.• The Galvanic Series ranks the reactivity of metals in seawater.• Increasing T speeds up oxidation/reduction reactions.• Corrosion may be controlled by: -- using metals which form a protective oxide layer -- reducing T

-- adding inhibitors-- painting--using cathodic protection.

SUMMARY