Solid fillers, surface modifications

Adhesion interactions at the solids/polymers interfaces are first and foremost adsorption interactions between the sofid surface and polymer molecules [1—11]. After polymerization there is a low molecular-weight fraction of coupling agents, which can decrease the cohesion and adhesion of the polymer film. If the molecules from this fraction interact with the filler particles preferentially (which can be reached due to the filler surface modification) instead of with the material surface covered, then the boundary layer of the film can be free from this fraction and adhesion increases as strengthening the boundary layer of the coating leads to stronger adhesion of the coating to the covered surfaces [46]. 

This chapter will focus on fundamental concepts related to surface modification of materials utilized within polymeric biocomposites for orthopedic applications. For this chapter, orthopedic applications are defined as medical indications or procedures that benefit from utilization of polymeric biocomposites and/or additional implanted therapeutic material to aid in bone regeneration at a localized site. The term surface modification refers to the physical attachment of molecules, predominantly silanes and/or polymers, to the surface of a solid-phase material. Polymeric biocomposites are a class of biomaterials that comprises a biocompatible bulk polymer and a particulated solid phase, often referred to as a binder and a filler, respectively. As there are vast combinations of polymers and solid materials that fit this definition, this chapter highlights solely those combinations that have been utilized for orthopedic applications, in either the acadenuc or the medical industry settings. 

Polymeric biocomposites that have utilized techniques to modify the surface of the solid phase before combination with the polymer binder narrow the field. The filler material and polymer binder combinations that fit this criterion and are highlighted in this chapter are paired in Table 3.1. In addition to the surface-modification techniques, what differentiates these biocomposites from one another are the fabrication processes used to combine the polymer and bioactive components and the polymerization of the polymer itself. A general schematic of how surface-modified polymeric... 

Attachment of molecules to the surface of a solid filler in polymeric biocomposites affects a variety of innate properties, particularly those related to the surface of (he filler material. An overview of the surface modification techniques and how they alter specific filler properties is outlined in Table 3.3. The attachment of molecules affects the immediate physical and chemical composition of a surface, which can alter secondary surface properties related to surface interactions, such as wetting, zeta potential, suiface solution reactions including dissolution/degradation, as well as cellular interactions. These primary and secondary properties do not necessarily alter how the filler interacts with polymer binders in a biocomposite setting, but these properties can change the inherent overall properties of the resultant filler. 

Figure 34 Effect of surface modification on solid filler properties bioactivity, (a) Schematic of surface reaction and subsequent nucleation of apatite on bioactive fiUer surface. Images from SEM of porous poly(L-lactide) composites made with (b) unmodified BG and (c) silane-treated BG, after 7 days in SBF, showing effect on nucleation of apatite.

Modifying the surface of sohd fillers used in polymeric biocomposites controls the surface properties (both primary and secondary), which affects both the mechanical and physical properties of the resultant polymeric biocomposite as well as its ability to remodel in vivo. An overview of the surface-modification techniques and how they alter the resultant biocomposite properties is outlined in Table 3.3. The fundamental theory of composite design is to obtain physical properties that lie between those of the individual components. As previously outlined, a primary motivator to modify the surface of a solid filler is to inaease adhesion between the solid filler and polymer components, and thus the overall mechanical properties of the biocomposite. This observation has been supported by numerous studies citing an increase in tensile properties. Other overall biocomposite properties that are affected by surface modification of filler components include binding to polymer phase, solid-filler incorporation into polymer binder, water uptake, and degradation. 

The fundamental surface-modification methods applied to solid fillers in polymer biocomposites, such as those previously outlined, are based on techniques and surface chemistries that have been utilized for several decades. More recently, new methods have been applied to the surface modification of solid fillers intended for use in polymeric biocomposites for orthopedic applications. Plasma polymerization forms polymeric materials, such as nanoscale-thick polymer coatings, via partially ionized gas (plasma) (Larranaga et al., 2013 Nichols et al., 2007). This rapid and solvent-free alternative approach to the conventional wet-surface modification processes previously described has several advantages that may be particularly appealing for the... 

As an extension to this surface-modification method, researchers have utilized plasma polymerization of acrylic acid to immobilize biologically active molecules, such as recombinant human bone formation protein-2 (rhBMP-2). rhBMP-2 is a signaling molecule that promotes bone formation by osteoinduction that has been utilized for various orthopedic tissue-engineering applications (Kim et al., 2013). One research group modified a PCL scaffold surface with plasma-polymerized acrylic acid (PPAA) and rhBMP-2 via electrostatic interactions (Kim et al., 2013) (which is outside of the scope of this chapter). This interesting approach may be apphed to the surface modification of solid fillers and provide additional benefits compared to the surface-modification techniques currently utihzed in orthopedic polymeric biocomposite development. The acrylic acid and rhBMP-2-modifled surface showed improved cell attachment and adhesion compared to the surface with acrylic acid alone. The ability to modify the surface of a solid-filler particle in a polymeric biocomposite with a bioactive molecule, such as rhBMP-2, provides a delivery vehicle for the bioactive molecule to the polymeric biocomposite and the eventual implantation site of this biomaterial. Such surface-modification and immobihzation approaches may provide a method to control the release kinetics of attached molecules to the localized bone-defect site. 

Montmorillonite is usually modified with ammonium salts, in our study the selected ionic liquids were applied as modifying agents for the intercalation of montmorillonite. Surface properties of modified fillers, the zeta potential of suspended solids in the water, the oil absorption number, the impact of modifications on the tendency to agglomerate in the non-polar and polar medium were studied. 

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