The biological environment, seemingly a mild, aqueous salt solution at 37°C, is, in fact, surprisingly aggressive and can lead to rapid or gradual breakdown of many materials. Some mechanisms of biodegradation have evolved over millennia specifically to rid the living organism of invading foreign substances – these same mechanisms now attack our contemporary biomaterials. Other breakdown mechanisms have their basis in well-understood chemical and physical principles, and will occur in a living organism or in a beaker on a laboratory bench. After this introduction, four chapters (II.4.2, II.4.3, II.4.4, and II.4.5) directly address degradation. The first three of these consider breakdown in the biological environment. Chapter II.4.5 describes another type of degradation, calcification, which can lead to device failure and can exacerbate other degradation mechanisms. In addition, many of the textbook chapters address degradation in other contexts. Chapter I.2.6 reviews the chemistry of polymers designed to be biodegradable. Chapter III.1.4 addresses device failure, sometimes related to unintentional degradation. Most of the device-specific chapters consider degradation issues.
The biomaterials of medical devices are usually exposed to varying degrees of cyclic or periodic stress (humans ambulate and the cardiovascular system pumps). Abrasion and flexure may also take place. Such mechanical challenges occur in an aqueous, ionic environment that can be electrochemically active to metals, and plasticizing (softening) to polymers. It is well-known that a material under mechanical stress will degrade more rapidly than the same material that is not under load.
Specific biological mechanisms are also invoked. Proteins adsorb to the material and can enhance the corrosion rate of metals. Cells (especially macrophages) adhere to materials via those interfacial proteins, and can be activated to secrete powerful oxidizing agents and enzymes intended to digest or dissolve the material. The secreted, potent degradative agents are concentrated in the space between the adherent cell and the biomaterial upon which they act, undiluted by the surrounding aqueous medium. Also, bacteria, bacterial biofilms (Chapter II.2.8) and yeast can enhance degradation and corrosion rates.
To understand the biological degradation of implant materials, synergistic pathways must be considered. For example, cracks associated with stress crazing open up fresh surface area to reaction. Swelling and water uptake can similarly increase the number of sites for reaction, and provide an access route for degradative agents into the “core” of the biomaterial. Amorphous material at metal (and polymer) grain boundaries can degrade more rapidly, leading to increases in surface area and localized stresses. Degradation products can alter the local pH, catalyzing further reaction. Hydrolysis of hydrophobic polymers can generate hydrophilic species, leading to polymer swelling and providing an entry mechanism for degrading species to transport into the bulk of the polymer. Cracks might also serve as sites for the initiation of calcification.
Biodegradation is a term that is used in many contexts. It can be used for reactions that occur over minutes or over years. It can be engineered to happen at a specific time after implantation or it can be an unexpected long-term consequence of the severity of the biological environment. Implant materials can solubilize, crumble, become rubbery or become rigid with time. The products of degradation may be toxic or irritating to the body or they may be designed to perform a pharmacologic function.
Calcification, a process we strive for in bone healing, is undesirable in most soft tissue contexts. Calcific mineral can interfere with the mechanical function of devices, induce cracking in polymers and embolize, leading to complications downstream. Implants based on natural tissue are particularly subject to calcification, but calcification is reasonably common in synthetic polymer devices.
Here are a few interesting biomaterial degradation issues that might stimulate further thinking on this subject in conjunction with the tutorial chapters in this section.
• Consider strategies used to create materials that degrade at controlled rates, versus strategies for synthesizing biostable materials intended for long-term performance in the body.
• Consider the degradation of materials commonly used in medicine that do not have well-defined breakdown mechanisms. Some examples include poly(ethylene glycol), hydroxyapatite, and some polysaccharides. How does the body deal with these common materials?
• A new class of biomaterials is now under development that degrades on cue. The cue might be thermal, photonic or enzymatic. Ingenious chemical design principles are being applied to create such materials, but how might the body react to the products generated by a sudden breakdown of the structure?
• Learn about new strategies to stabilize materials against degradation, for example, vitamin E loading of orthopedic polymers, and incorporation of polyisobutylene segments into elastomers.
• Endovascular stents are among the most widely used of all medical devices (Chapter II.5.3.B). A new generation of biodegradable stents is expected to have huge impact on cardiovascular therapies. Consider how biodegradable poly(lactic acid) or magnesium or iron will perform in the complex intra-vascular environment.
• For a medical device intended for years of service, especially a device where failure can lead to death, how can we test and qualify the device for the expected period of service? Are there useful in vitro tests? Are there relevant and justified animal models?
• Henry Petroski and other authors have discussed the important role of failure in advancing engineering design. Consider medical device failure, past and present, associated with degradation, and how these unintended complications will lead to better medical devices. A few examples include the degradation of polyurethane pacemaker leads, the breakdown of a protective sheath on the tailstring of the Dalkon Shield IUD, and the wear debris associated with the oxidation of ultra-high molecular weight polyethylene in hip prostheses.
Degradation in biological environments is seen with metals, polymers, ceramics, and composites. It is observed to some degree in most long-term implants, and even in some medium-term and short-term implants. Often, its initiation, mechanism, and consequences are incompletely defined. Biodegradation as a subject is broad in scope, and critical to device performance. It rightfully should command considerable attention for the biomaterials scientist. This section introduces biodegradation issues for a number of classes of materials, and provides a basis for further study on this complex but critical subject.