November/December AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 8 2 2 a foam is defined as the effective densi- ty of the cellular hybrid divided by the density of the solid material it is made from. The relative density values cor- responded to a porosity between 50% and 95%. Focusing on the main issue of how the compressive strength depends on porosity, three scaffold materials can be plotted in a chart of these reference materials. Interestingly, all three sets of modeled materials show compressive strength comparable to those of trabec- ular bone, but falling short of matching cortical bone. This means that these scaffolds are approaching the perfor- mance of natural materials in some re- spects, but could not yet be used for load-bearing applications. Considering the requirements of a minimum porosi- ty, at least 70% is needed to allow cells, waste, and other essential substanc- es to flow through the scaffold to aid in bone regeneration. Bone tissue scaffolds are a quickly emerging application area, as demand for transplantable tissue currently out- strips supply. This is one of the main driving forces behind development of this type of implant. There is a great deal of research going on to improve the current crop of scaffolds by creating composites or coating scaffolds with biodegradable polymers that combine the mechanical properties needed to support bone regrowth with other func- tional requirements. Due to the deficiency in com- pressive strength, there are few high- ly porous scaffolds on the market, and the ones that are available tend to be used in non-load-bearing situations. One such example is the NovaBone Porous-Bone Graft Scaffold produced from bioglass by NovaBone Prod- ucts LLC. More information about the range of scaffolds on the market can be found in the ASM Medical Mater- ials Database. CASE STUDY II: SUTURE ANCHOR IMPLANTS In our second case study, candi- date materials are investigated for a su- ture anchor implant, to be fixed in bone tissue to hold soft tissue in place, via sutures, during healing. The function of a suture anchor is to aid in reattach- ment or fixing of soft tissue to bone. The implant is threaded so it screws into predrilled holes in the bone, thus anchoring the suture. A widely used material for hip replacements and dental implants is titanium. This mate- rial has been shown to be biocompat- ible for these procedures and is now routinely used in surgery. However, there are drawbacks to using metals in bone tissue. Metals are significantly stiffer and harder than bone, which may cause problems and damage to surrounding bone tissue, i.e., stress shielding. The underlying biomechanical properties of bone are critical to adjust function in vivo. This is one of the reasons that high performance polymers were developed as implant materials. Polymers such as polyetheretherketone (PEEK) have shown better mechanical compatibility with bone for applications that do not require metallic strength. Despite high material cost, PEEK is widely used in commercial implants such as suture anchors, which are fixed in cortical bone for surgeries including rotator cuff and anterior cruciate lig- ament (ACL) repair. In this case study, the PEEK implant constitutes a fixed volume situation, i.e., the selected ma- terial may be varied, but not the part geometry. In this case, the objective is simply to minimize the cost per volume of the part within constraints regarding mechanical performance, such as stiff- ness and yield strength. Again, the materials selection tools of CES EduPack were used. The number of candidates could be im- mediately limited to biomedical ma- terials, i.e., those with regulatory clearance. This resulted in nearly 370 el- igible material records in the selection. Figure 1 shows aplot of priceper volume (price/kg * density). PEEK is at the high end of the price spectrum for polymers. A good starting point for the selection proved to be considering roughly 100 of the lower-cost biomedical polymers, which will result in materials cheaper than the metals per volume. MacroForm, a collagen bioactive glass scaffold fromNovaBone, is engineered to have an open porous structure to facili- tate absorption of bone marrow aspirate. The collagen bioactive glass scaffold mixes with bone marrow to yield a porous scaffold that facilitates bone growth. Courtesy of mtfbiologics.org . Fig. 1 − A good starting point for this selection is the depicted group of ~100 polymers from the biomedical subset.