Polymeric Biomaterials for Structural Applications

Recently, significant interest has emerged in the development of polymer-based biomaterials for the fabrication of mechanically robust implants for utilization in orthopedic surgery. Traditionally, metallic alloys have been used in orthopedic implants to utilize their high elastic modulus and materials strength in a typically load bearing applications. However, introduction of metallic systems can entail numerous effects on the overall physiology. Typically, pure metals are not sufficiently strong to be directly utilized as load bearing biomaterials. To improve their material strength, metal alloys are utilized. For example, pure unalloyed iron does not possess sufficient material strength or the durability for utilization in biomedical implants. However, when alloyed to create stainless steel, pure iron is transformed into a much more suitable metallic system. However, stainless steel that contains chromium that can be gradually released into the body thus entailing potential allergic and toxic reactions.

Furthermore, the elastic modulus of say cortical bones ranges from 10 to 20 GPa [27]. In comparison, the modulus of typical metallic alloys is in the range of 100-200 GPa. When materials with dramatically different properties are in intimate contact and subjected to stresses, stress concentrations that are associated with the strain mismatch can occur. Consider for example a Ti plate with a modulus of ~100 GPa that has undergone effective osseointegration with a bone that is characterized by a modulus of ~10 GPa. When this ''composite'' system of the implant and the bone is deformed by a remote stress, the majority of the load would be accommodated by the stiffer Ti implant. This is particularly true if effective osseointegration has occurred and concomitantly there is no interfacial slip between the implant and the bone.

In accordance with the Wolff's law of stress induced bone remodeling, if a majority of the applied loads is accommodated by the implant, then over time the surrounding native bone structure may reconfigure over time and lose bone mass [28]. Furthermore, usually the implant is smaller than the surrounding bone and at the edges the mismatch in mechanical properties can in fact lead to strong stress concentrations. This scenario is very analogous to that observed during typical indentation scenarios when a flat punch deforms a substrate [29]. Cumulatively, these two effects can lead to secondary fractures as well as anomalous reconfiguration of the bone structure as a result of the highly heterogeneous stress state that is directly related to the mismatch in the mechanical properties between the implant and the native bone structure [30].

Utilization of polymer systems offers a versatile system wherein to enable the manufacture of implants with mechanical properties that match that of the bone. Furthermore, it is known that the native bone itself is characterized by anisotropic mechanical stiffness. For example, the cortical bone in the longitudinal direction is characterized by a stiff of 17 GPa and ~13 GPa in the transverse direction. Most conventional polymers however do not possess the intrinsic stiffness that matches that of the native bone. However, the primary advantage of the polymers is the control that can be exercised via controlled dispersion of second phases.

Consider the case of fiber reinforced polymers wherein the fibers are aligned in a certain direction. Figure 2.5 illustrates carbon fiber reinforced epoxy plates that have been utilized in epiphyseal fractures [31]. In such devices carbon fibers of exceptional intrinsic tensile stiffness can utilized wherein mechanical properties can be tailored by controlling the orientation of the high-modulus reinforcing phase. Carbon fibers can ensure a much greater improvement in the modulus in a direction that is parallel to their predominant direction of alignment while they may not be as effective in the transverse direction. Therefore, it is then possible to tailor via appropriate processing schemes to match the anisotropic elastic properties of the bone by controlling the dispersion and the orientation of the fibers in the composite.

Much like the case of the metallic implants, polymer composite systems are also required to ensure sufficient biocompatibility. While the aforementioned carbon fiber reinforced composites are expected to possess significant biocom-patibility, in practice however, biocompatibility has been affected due to the substantial release of carbon particles into the surrounding tissues [32]. Alternatively, it has been suggested that "bioglass" may be an effective reinforcement material that overcomes the limitations of the carbon fibers because they may remain bioactive even when in contact with the surrounding tissues unlike the carbon fibers [33].

Fig. 2.5 Carbon fiber reinforced epoxy plates for epiphyseal fractures (Courtesy: Elsevier and Orthodynamics, UK [31])

Fig. 2.5 Carbon fiber reinforced epoxy plates for epiphyseal fractures (Courtesy: Elsevier and Orthodynamics, UK [31])

Significant challenges remain in the design and manufacture of suitable biocomposites for orthopedic and restorative applications to ensure durability, biocompatibility all the while accomplishing the desired therapeutic effect.

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