Biomaterial Degradation and Resorption
Biomaterial Degradation and Resorption
Biomaterials may be permanent or degradable. The degradation process may be chemically driven or accomplished by cells. Bioresorbable implants are designed to degrade gradually over time in the biological environment and be replaced with natural tissues. The goal is to meet the requirements of strength and cell support while the regeneration of tissues is occurring. Small changes in biomaterial chemistry and structure may greatly alter the resorption rate, allowing for materials to be tailored for various applications or leading to unexpected product failure. Collagen and the lactic acid and/or glycolic acid polymers [PLLA and PGA or copolymer PLGA] are the most commonly used for resorbable applications. PLLA and PGA degrade through a process of hydrolytic degradation of the polyester bond. At low molecular weights, the implant can disintegrate and produce small fragments that elicit an immune response from macrophages. PLLA and PGA degrade in a time period of 6 months to several years depending on initial molecular weight and crystallinity.
Copolymers of the two typically degrade into fragments in a few months. The lactic acid and glycolic fragments are eventually metabolized into carbon dioxide and water. Tricalcium phosphate ceramics degrade through a surface dissolution process into calcium and phosphate salts, which are also present naturally in the body.
Biomaterial degradation may lead to chronic nonhealing wounds that are arrested at one of the normal phases of wound healing. This may happen if a biomaterial degrades too quickly and releases particulate matter that extends the inflammation stage. Persistent inflammation leads to the formation of giant multinucleated cells that continue to attempt to remove the offending material. They are the trademark of a foreign body response and may necessitate surgical removal of the implanted device. If the healing passes through to the fibrous capsule formation stage, there may still be complications. For example, a drug delivery implant may eventually no longer function due to impaired drug release by the fibrous encapsulation in response to the degrading drug delivery implant.
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