Degradation of Biomaterials

The tissue engineering approach requires suitable biomaterials to serve as three-dimensional scaffolds to support cell growth and differentiation into functional tissues. Depending on the type of tissue in need of repair, a biomaterial must be designed with specific performance criteria in mind. Several excellent books and review articles (e.g., Ratner et al. (2013), Temenoff and Mikos (2008)) on biomaterials have appeared. Essential characteristics of biomaterial scaffolds for tissue engineering applications are described by Williams (2014). For instance, biomaterials used as load-bearing prostheses for hips and knees should retain their mechanical function for the lifetime of the patient. In large bone defects, where load-bearing is not critical (e.g., the skull), biomaterials—used alone or with cells as tissue engineering constructs—need not be so mechanically strong (Chapter 10). In this case, a degradable biomaterial scaffold would be ideal to allow newly formed bone tissue to gradually take the place of the implanted construct resulting in seamless bone repair and no residual material. In this way, the manner in which the biomaterial is degraded—broken down in the body—is a primary consideration. When a biomaterial is implanted in the body, it is immediately exposed to physiologic fluid and shortly after, cells whose main purpose is to clear it from the host (Chapter 15). Thus, the degradation of biomaterials involves multiple physiologic processes at the same time making it a science to its own. This chapter reviews the degradation mechanisms of the two most established classes of biomaterials—ceramics and polymers—and how these degradation properties can be beneficial in their primary application, bone tissue engineering.