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 3 Next, biocompatible polymers were screened using various con- straints. In order to avoid the prob- lem of implants being too stiff, Young’s modulus was limited to a value lower than a typical human cortical bone in the region where the implant should function. The material needs to be stiff enough to allow it to be inserted into position via the pre-drilled hole, which can also be specified in a lim- it stage during screening of materials (3 < Young’s modulus [GPa] < 10). The other mechanical proper- ties that need to be considered during screening include yield strength (to support tensile stress from the suture as well as stress during the insertion procedure) and fracture toughness, to avoid brittle fracture in vivo. In the screenshot below, we see that the ref- erence material called Unfilled PEEK performs better than the low-cost bio- medical polymers from the previous se- lection (Fig. 2). COMPARING THE CANDIDATES After screening, the resulting screenshot shows the values of the ob- jective function for the five best can- didates in the ranked list (Fig. 2). If this objective is considered (see also Fig. 1), the two forms of polyethylene terephthalate (PET) are the cheapest alternatives. They suffer from poor ster- ilizability in dry heat/steam, which is a disadvantage. Even the most costly of these five materials, styrene meth- yl methacrylate (SMMA), is around 30 times cheaper per volume than PEEK in this comparison. Both PET and SMMA (e.g., under the trade name NAS 30) are indeed materials used for implants and would be plausible candidates to replace PEEK [3,4] . Two very interesting alternatives would be polyoxymeth- ylene copolymer (POM) for a perma- nent option and polylactide (PLA) as a degradable choice. In addition to be- ing cost effective and its current use in suture anchors, PLA is biodegradable, which is a desirable property for short- term treatments. In 1993, POM was dismissed as an option for bone contact applications due to its bio-inertness and lack of in- teraction with bone [5] . Since then it has been used for a range of medical de- vices such as replacement heart valves and catheters [6] . Based on its lack of in- teraction with bone, the concern is that the suture anchor could slip in its pre- drilled hole, leading to complications where the suture anchor must be re- placed or reinserted [5,7] . This is something that should also be considered for PET and SMMA. If POM was chosen as a new suture anchor material, it would need to be formed into a biocomposite or coated with a material that would encourage bone bonding, such as hydroxyapatite (HA) or Bioglass [8] . This would promote bone formation and fix the suture an- chor. Composites of bioinert polymers and materials that encourage bone in- teraction are currently being explored for future applications [9] . PLA has a long degradation time, in the range of 12 to 24 months, which means it can be used for short-term treatments where the anchor is not per- manent [10] . In this situation, the deg- radation rate should match the bone formation rate in order for the hole that is left behind to be filled [11] . For an adult, this process takes three to six months [12] . One option is to increase the degradation rate of the polymer, but this would reduce mechanical stabil- ity and lead to a greater probability of failure. Another option is to encourage bone bonding with the implant so that as the implant degrades, the space left behind is filled with new bone. As men- tioned previously, this can be done by introducing materials that encourage bone formation [8] . Biocomposites such as PLA/HA are already being used as alternatives to permanent suture an- chors, as they feature suitable mechan- ical properties along with the ability to degrade away [13,14] . ADVANTAGES OF BIOMEDICAL POLYMERS Suture anchors come in a range of sizes and diameters to help surgeons Fig. 2 − Screenshot of the final stage of the selection, when five candidates remain after screening.